Patent Publication Number: US-11652594-B2

Title: Pilot transmission and reception for orthogonal frequency division multiple access

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 17/334,290, filed May 28, 2021, now U.S. Pat. No. 11,418,307 issued Aug. 16, 2022, which is a continuation of application Ser. No. 16/816,092, filed Mar. 11, 2020, now U.S. Pat. No. 11,050,539 issued Jun. 29, 2021, which is a continuation of application Ser. No. 16/443,683, filed Jun. 17, 2019, now U.S. Pat. No. 10,630,444 issued Apr. 21, 2020, which is a continuation of application Ser. No. 15/444,188, filed Feb. 27, 2017, now U.S. Pat. No. 10,361,828 issued Jul. 23, 2019, which is a continuation of application Ser. No. 15/150,127, filed May 9, 2016, now U.S. Pat. No. 9,621,311 issued Apr. 11, 2017, which claims the benefit of U.S. Provisional Application No. 62/159,187, filed May 8, 2015, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The technology described herein relates generally to wireless networking. More particularly, the technology relates to the transmission and reception of symbols (such as symbols in Orthogonal Frequency Division Multiple Access (OFDMA) frames) that include pilots transmitted using pilot tones. 
     2. Description of the Related Art 
     Wireless LAN (WLAN) devices are currently being deployed in diverse environments. Some of these environments have large numbers of access points (APs) and non-AP stations in geographically limited areas. In addition, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices. 
     A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012) (hereinafter, IEEE Std 802.11). The IEEE Std 802.11 was subsequently amended by IEEE Std 802.11ae™-2012, IEEE Std 802.11aa™-2012, IEEE Std 802.11ad™-2012, and IEEE Std 802.11ac™-2013 (hereinafter, IEEE 802.11ac). 
     Recently, an amendment focused on providing a high efficiency (HE) WLAN in high-density scenarios is being developed by the IEEE 802.11ax task group. The 802.11ax amendment focuses on improving metrics that reflect user experience, such as average per station throughput, the 5th percentile of per station throughput of a group of stations, and area throughput. Improvements will be made to support environments such as wireless corporate offices, outdoor hotspots, dense residential apartments, and stadiums. 
     An HE WLAN supports Orthogonal Frequency Division Multiple Access (OFDMA) communications. In the OFDMA communications, an Access Point (AP) may communicate simultaneously with a plurality of stations by allocating respective Resource Units (RUs) (that is, groups of subchannels) within the stations. 
     An HE WLAN also supports the use of longer symbols in data fields of an HE frame. For example, while a preamble of the HE frame may include Orthogonal Frequency Division Multiplexing (OFDM) symbols have respective durations, exclusive of Cyclic Prefixes (CPs), of 3.2 microseconds, a data field of the HE frame may include OFDM symbols have respective durations, exclusive of CPs, of 12.8 microseconds. 
     A duration of an OFDM symbol may be determined according to a number of input elements of a Fourier Transform (FT) or Inverse Fourier Transform (iFT) respectively used to decode or generate the OFDM symbol. An OFDM symbol having a duration, exclusive of CP, of 3.2 microseconds in a 20 MHz bandwidth may be generated using an iFT having 64 input elements (i.e. a 64-element iFT) and decoded using a FT having 64 input elements (i.e. a 64-element FT). An OFDM symbol having a duration, exclusive of CP, of 12.8 microseconds in a 20 MHz bandwidth may be generated using an iFT having 256 input elements (i.e. a 256-element iFT) and decoded using a FT having 256 input elements (i.e. a 256-element FT). A number of input elements of an FT or iFT may be referred to as a size of the FT or iFT. 
     Pilots are used in 802.11 systems for performing channel estimation and for performing carrier frequency offset (CFO) tracking. Pilots used for channel estimation may be included in a training field, such as a Long Training Field (LTF). 
     CFO may occur, for example, because of a frequency mismatch between oscillators of a transmitter and a receiver or because of the Doppler Effect due to relative motions of the transmitter and receiver. Even if the channel state does not change over a duration of a received frame, a residual CFO may changeover the duration. Because the CFO may change during the duration, pilots for CFO tracking may be included in symbols of data fields. Such pilots may be carried by pilot tones located at pilot tone positions of the symbols. 
     Ideally, pilots are included in all OFDM symbols, and span the entire frequency bandwidth of the transmitted signal so that CFO tracking performance may be improved by the inclusion of frequency diversity. The positioning of pilots tones carrying the pilots may vary between symbols in training fields and symbols in data fields, and between symbols generated using different Fourier Transform (FT) sizes. 
     SUMMARY 
     In an embodiment, a method of a wireless device for transmitting a frame comprises determining, by the wireless device, a plurality of Resource Units (RUs) of the frame, providing a first plurality of pilots into a first RU of the frame at a first set of positions, respectively, providing a second plurality of pilots into a second RU of the frame at a second set of positions, respectively, and transmitting the frame. The first set of positions is different from the second set of positions. 
     In an embodiment, the frame is an Orthogonal Frequency Division Multiple Access (OFDMA) frame and the plurality of RUs include respective pluralities of subcarriers. 
     In an embodiment, the first RU includes a lowest subcarrier having an odd-numbered index, and the second RU includes a lowest subcarrier having an even-numbered index. 
     In an embodiment, the first RU is a 52-subcarrier RU and the second RUs is a 52-subcarrier RU. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the first RU, a second pilot tone position separated by thirteen subcarriers from the first pilot tone position, a third pilot tone position separated by eleven subcarriers from the second pilot tone position, and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and six subcarriers away from a highest-indexed subcarrier of the first RU. The pilot tones of the second RU include a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the first RU, a sixth pilot tone position separated by thirteen subcarriers from the fifth pilot tone position, a seventh pilot tone position separated by eleven subcarriers from the sixth pilot tone position, and an eighth pilot tone position separated by thirteen subcarriers from the seventh pilot tone position and five subcarriers away from a highest-indexed subcarrier of the second RU. 
     In an embodiment, the first RU is a 26-subcarrier RU, and the second RU is a 26-subcarrier RU. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the first RU and a second pilot tone position separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from a highest-indexed subcarrier of the first RU. The the second set of positions includes a third pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the second RU and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and five subcarriers away from a highest-indexed subcarrier of the second RU. 
     In an embodiment, the method further comprises providing a third plurality of pilots in a third RU of the frame at a third set of positions, respectively. The second set of positions is different from the third set of positions, the third RU is a 26-subcarrier RU, and the third RU is a center RU that is split into 13 positive-indexed subcarriers and 13 negative-indexed subcarriers by DC tones. 
     In an embodiment, the third set of positions includes a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the third RU and a sixth pilot tone position spaced six subcarriers away from a highest-indexed subcarrier of the third RU 
     In an embodiment, the first set of positions is a mirror image of the second set of positions. 
     In an embodiment, the frame includes a 2× High Efficiency (HE) Long Training Field (HE-LTF). The first set of positions respectively correspond to locations of non-null subcarriers of symbols of the 2×HE-LTF. The second set of positions respectively correspond to the locations of the non-null subcarriers of the symbols of the 2×HE-LTF. 
     In an embodiment, pilot tone positions for all RUs in a lower half of a 20 MHz channel are mirror symmetric to pilot tone positions for corresponding mirrored RUs in an upper half of the 20 MHz channel. 
     In an embodiment, pilot tone positions for all RUs in a lower half of a 40 MHz channel are mirror symmetric to pilot tone positions for corresponding mirrored RUs in an upper half of the 40 MHz channel. 
     In an embodiment, a method of a wireless device for transmitting a frame comprises providing pilots in a resource unit, and transmitting the frame including the resource unit. When a lowest subcarrier of the resource unit has an odd-numbered index, a plurality of pilots are included at a first set of positions in the resource unit, respectively. When the a lowest subcarrier of the resource unit has an even-numbered index, a plurality of pilots are included at a second set of positions in the resource unit, respectively. The second set of positions is different from the first set of positions. 
     In an embodiment, the resource unit is a 52-subcarrier resource unit. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit, a second pilot tone position separated by thirteen subcarriers from the first pilot tone position, a third pilot tone position separated by eleven subcarriers from the second pilot tone position, and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and six subcarriers away from a highest-indexed subcarrier of the resource unit. The second set of positions include a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit, a sixth pilot tone position separated by thirteen subcarriers from the fifth pilot tone position, a seventh pilot tone position separated by eleven subcarriers from the sixth pilot tone position, and an eighth pilot tone position separated by thirteen subcarriers from the seventh pilot tone position and five subcarriers away from a highest-indexed subcarrier of the resource unit. 
     In an embodiment, the resource unit is a 26-subcarrier resource unit. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit and a second pilot tone position separated by thirteen subcarriers from the first pilot tone position and six subcarriers away from a highest-indexed subcarrier of the resource unit. The second set of positions include includes a third pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and five subcarriers away from a highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit is a center resource unit that is split into 13 negative-indexed subcarriers and 13 positive-indexed subcarriers by DC tones, a plurality of pilots are included at a third set of positions in the resource unit, respectively. The third set of positions is different from the second set of positions. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit and a second pilot tone position separated by thirteen subcarriers from the first pilot tone position and six subcarriers away from a highest-indexed subcarrier of the resource unit. The second set of positions includes a third pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and five subcarriers away from a highest-indexed subcarrier of the resource unit. The third set of positions includes a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a sixth pilot tone position spaced six subcarriers away from a highest-indexed subcarrier of the resource unit. 
     In an embodiment, a method of a wireless device for receiving a frame comprises receiving the frame. The frame includes a resource unit. The resource unit includes pilots. The method further comprises processing the pilots. When a lowest subcarrier of the resource unit has an odd-numbered index, a plurality of pilots are included at a first set of positions in the resource unit, respectively. When the lowest subcarrier of the resource unit has an even-numbered index, a plurality of pilots are included at a second set of positions in the resource unit, respectively. The second set of positions is different from the first set of positions. 
     In an embodiment, the resource unit is a 52-subcarrier resource unit. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit, a second pilot tone position separated by thirteen subcarriers from the first pilot tone position, a third pilot tone position separated by eleven subcarriers from the second pilot tone position, and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and six subcarriers away from a highest-indexed subcarrier of the resource unit. The second set of positions include a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit, a sixth pilot tone position separated by thirteen subcarriers from the fifth pilot tone position, a seventh pilot tone position separated by eleven subcarriers from the sixth pilot tone position, and an eighth pilot tone position separated by thirteen subcarriers from the seventh pilot tone position and spaced five subcarriers away from a highest-indexed subcarrier of the resource unit. 
     In an embodiment, the resource unit is a 26-subcarrier resource unit. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit and a second pilot tone position separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from a highest-indexed subcarrier of the resource unit. The second set of positions includes a third pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from a highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit is a center resource unit that is split into 13 negative-indexed subcarriers and 13 positive-indexed subcarriers by DC tones, a plurality of pilots are included at a third set of positions in the resource unit, respectively. The third set of positions is different from the second set of positions. 
     In an embodiment, the first set of positions include a first pilot tone position spaced five subcarriers away from a lowest-indexed subcarrier of the resource unit and a second pilot tone position separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from a highest-indexed subcarrier of the resource unit, The second set of positions includes a third pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a fourth pilot tone position separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from a highest-indexed subcarrier of the resource unit. The third set of positions includes a fifth pilot tone position spaced six subcarriers away from a lowest-indexed subcarrier of the resource unit and a sixth pilot tone position spaced six subcarriers away from a highest-indexed subcarrier of the resource unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a wireless network according to an embodiment. 
         FIG.  2    is a schematic diagram of a wireless device according to an embodiment. 
         FIG.  3 A  illustrates components of a wireless device configured to transmit data according to an embodiment. 
         FIG.  3 B  illustrates components of a wireless device configured to receive data according to an embodiment. 
         FIG.  4 A  illustrates an Orthogonal Frequency Division Multiple Access (OFDMA) frame including a High Efficiency (HE) Long Training Field (LTF) according to an embodiment. 
         FIG.  4 B  illustrates positions in an even-aligned Resource Unit (RU) of tones corresponding to tones of a 2×LTF having an even mapping, according to an embodiment. 
         FIG.  4 C  illustrates positions in an even-aligned RU of tones corresponding to tones of the 2×LTF having an odd mapping, according to an embodiment. 
         FIG.  4 D  illustrates positions in an RU of tones corresponding to tones of the 4×LTF according to an embodiment. 
         FIG.  5 A  illustrates even and odd RUs in a 20 MHz bandwidth according to an embodiment. 
         FIG.  5 B  is a table listing subcarrier indexes of the RUs of  FIG.  5   . 
         FIG.  6    illustrates even and odd RUs in a 40 MHz bandwidth according to an embodiment. 
         FIG.  7    illustrates even and odd RUs in an 80 MHz bandwidth according to an embodiment. 
         FIG.  8 A  illustrates even and odd RUs in a 20 MHz bandwidth according to another embodiment. 
         FIG.  8 B  is a table listing subcarrier indexes of the RUs of  FIG.  8   . 
         FIG.  9    illustrates even and odd RUs in a 40 MHz bandwidth according to another embodiment. 
         FIG.  10    illustrates an RU according to an embodiment. 
         FIG.  11    illustrates an RU having mirror symmetry to the RU of  FIG.  10   , according to an embodiment. 
         FIG.  12    illustrates a 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  13    illustrates a 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  14    illustrates a 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  15    illustrates a 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  16    illustrates a center 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  17    illustrates a center 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  18    illustrates a center 26-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  19    illustrates a 52-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  20    illustrates a 52-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  21    illustrates a 52-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  22    illustrates a 52-subcarrier RU of alternative 1 of design A, according to an embodiment. 
         FIG.  23    includes a Table 3 that indicates pilot tone positions for embodiments of 106-subcarrier RUs of alternative 1 of design A. 
         FIG.  24    includes a Table 4 that indicates pilot tone positions for embodiments of 108-subcarrier RUs of alternative 1 of design A. 
         FIG.  25    includes a Table 5 that indicates pilot tone positions for embodiments of 242-subcarrier RUs of alternative 1 of design A. 
         FIG.  26    includes a Table 6 that indicates pilot tone positions for embodiments of 242-subcarrier RUs of alternative 1 of design A. 
         FIG.  27    includes a Table 7 that indicates pilot tone positions for embodiments of center 242-subcarrier RUs of alternative 1 of design A. 
         FIG.  28    illustrates a 26-subcarrier RU of alternative 2 of design A, according to an embodiment. 
         FIG.  29    illustrates a 26-subcarrier RU of alternative 2 of design A, according to an embodiment. 
         FIG.  30    illustrates a center 26-subcarrier RU of alternative 2 of design A, according to an embodiment. 
         FIG.  31    illustrates a 52-subcarrier RU of alternative 2 of design A, according to an embodiment. 
         FIG.  32    illustrates a 52-subcarrier RU of alternative 2 of design A, according to an embodiment. 
         FIG.  33 A  illustrates pilot tone positions for RUs of a 20 MHz bandwidth (BW) of Case 1, according to an embodiment. 
         FIG.  33 B  includes a Table 9 showing pilot tone relative positions in the 20 MHz BW, according to an embodiment. 
         FIG.  34    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  35    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  36    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  37    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  38    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  39    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  40    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  41    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  42    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  43    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  44    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  45    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 1, according to an embodiment. 
         FIG.  46 A  illustrates pilot tone positions for RUs of a 40 MHz bandwidth (BW) of Case 1, according to an embodiment. 
         FIG.  46 B  includes a Table 10 showing pilot tone relative positions in the 40 MHz BW according to an embodiment. 
         FIG.  47    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  48    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  49    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  50    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  51    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  52    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  53    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 1, according to an embodiment. 
         FIG.  54 A  illustrates pilot tone positions for RUs of a 20 MHz bandwidth (BW) of Case 2, according to an embodiment. 
         FIG.  54 B  includes a Table 11 showing pilot tone relative positions in the 20 MHz BW according to an embodiment. 
         FIG.  55    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 2, according to an embodiment. 
         FIG.  56    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 2, according to an embodiment. 
         FIG.  57    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 2, according to an embodiment. 
         FIG.  58    illustrates pilot tone positions for RUs of the 20 MHz BW in of Case 2, according to an embodiment. 
         FIG.  59 A  illustrates pilot tone positions for RUs of a 40 MHz bandwidth (BW) in Case 2, according to an embodiment. 
         FIG.  59 B  includes a Table 12 showing pilot tone relative positions in the 40 MHz BW according to an embodiment. 
         FIG.  60    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  61    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  62    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  63    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  64    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  65    illustrates pilot tone positions for RUs of the 40 MHz BW in of Case 2, according to an embodiment. 
         FIG.  66    illustrates an option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  67    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  68    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  69    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  70    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  71    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  72    illustrates another option for pilot tone positions in Case 1, according to embodiments. 
         FIG.  73    illustrates an option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  74    illustrates another option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  75    illustrates another option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  76    illustrates another option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  77    illustrates another option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  78    illustrates another option for pilot tone positions in Case 2, according to embodiments. 
         FIG.  79    illustrates a process for transmitting a frame in a wireless network, according to an embodiment. 
         FIG.  80    illustrates another process for transmitting a frame in a wireless network, according to an embodiment. 
         FIG.  81    illustrates a process for transmitting a frame, according to an embodiment. 
         FIG.  82    illustrates a sub-process for providing pilots, according to an embodiment. 
         FIG.  83    illustrates a process for receiving a frame, according to an embodiment. 
         FIG.  84    illustrates a sub-process for obtaining pilots, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure relate generally to wireless networking, and more particularly, to providing and processing pilots in Orthogonal Frequency Division Multiplexing (OFDM) symbols to support same and different FFT size for symbols in a Long Training Field (LTF) and OFDM symbols in a data field of a frame, the frame being a frame transmitted in a wireless network. 
     In the following detailed description, certain illustrative embodiments have been illustrated and described. As those skilled in the art would realize, these embodiments may be modified in various different ways without departing from the scope of the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements in the specification. 
       FIG.  1    illustrates a wireless network according to an embodiment. The wireless network includes an infrastructure Basic Service Set (BSS)  100  of a Wireless Local Area Network (WLAN). In an 802.11 wireless LAN, the BSS provides the basic building-block and typically includes an access point (AP) and one or more associated stations (STAs). In  FIG.  1   , the BSS  100  includes an Access Point  102  (also referred to as AP) wirelessly communicating with first, second, third, and fourth wireless devices (or stations)  104 ,  106 ,  108 , and  110  (also referred to as STA 1 , STA 2 , STA 3 , and STA 4 , respectively). The wireless devices may each include a medium access control layer (MAC) and a physical layer (PHY) according to an IEEE 802.11 standard. 
     Although the example of  FIG.  1    shows only the BSS  100  including only the first to fourth stations STA 1  to STA 4 , embodiments are not limited thereto and may comprise BSSs including any number of STAs. 
     The AP  102  is a station, that is, a STA, configured to control and coordinate functions of the BSS  100 . The AP  102  may transmit information to a single station selected from the plurality of stations STA 1  to STA 4  in the BSS  100  using a single frame, or may simultaneously transmit information to two or more of the stations STA 1  to STA 4  in the BSS  100  using either a single Orthogonal Frequency Division Multiplexing (OFDM) broadcast frame, a single OFDM Multi-User Multi-Input-Multi-Output (MU-MIMO) transmission, or a single Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     The stations STA 1  to STA 4  may each transmit data to the AP  102  using a single frame, or transmit information to and receive information from each other using a single frame. Two or more of the stations STA 1  to STA 4  may simultaneously transmit data to the AP  102  using an Uplink (UL) OFDMA frame. When the BSS  100  supports Spatial Division Multiple Access (SDMA), two or more of the stations STA 1  to STA 4  may simultaneously transmit data to the AP  102  using an UL MU-MIMO frame. 
     In another embodiment, the AP  102  may be absent and the stations STA 1  to STA 4  may be in an ad-hoc network. 
     Each of the stations STA 1  to STA 4  and the AP  102  includes a processor and a transceiver, and may further include a user interface and a display device. 
     The processor is configured to generate a frame to be transmitted through a wireless network, to process a frame received through the wireless network, and to execute protocols of the wireless network. The processor may perform some or all of its functions by executing computer programming instructions stored on a non-transitory computer-readable medium. The transceiver represents a unit functionally connected to the processor, and designed to transmit and receive a frame through the wireless network. 
     The transceiver may include a single component that performs the functions of transmitting and receiving, or two separate components each performing one of such functions. The processor and the transceiver may be implemented in each of the stations STA 1  to STA 4  and the AP  102  using respective hardware components, software components, or both. 
     The AP  102  may be or may include a WLAN router, a stand-alone Access Point, a WLAN bridge, a Light-Weight Access Point (LWAP) managed by a WLAN controller, and the like. In addition, a device such as a personal computer, tablet computer, or cellular phone may be able to operate as the AP  102 , such as when a cellular phone is configured to operate as a wireless “hot spot.” 
     Each of the stations STA 1  to STA 4  may be or may include a desktop computer, a laptop computer, a tablet PC, a wireless phone, a mobile phone, a smart phone, an e-book reader, a Portable Multimedia Player (PMP), a portable game console, a navigation system, a digital camera, a Digital Multimedia Broadcasting (DMB) player, a digital audio recorder, a digital audio player, a digital picture recorder, a digital picture player, a digital video recorder, a digital video player, and the like. 
     The present disclosure may be applied to WLAN systems according to IEEE 802.11 standards but is not limited thereto. 
     In IEEE 802.11 standards, frames exchanged between stations (including access points) are classified into management frames, control frames, and data frames. A management frame may be a frame used for exchanging management information that is not forwarded to a higher layer of a communication protocol stack. A control frame may be a frame used for controlling access to a medium. A data frame may be a frame used for transmitting data to be forwarded to the higher layer of the communication protocol stack. 
     Each frame&#39;s type and subtype may be identified using a type field and a subtype field included in a control field of the frame, as prescribed in the applicable standard. 
       FIG.  2    illustrates a schematic block diagram of a wireless device  200  according to an embodiment. The wireless or WLAN device  200  can represent any device in a BSS, e.g., the AP  102  or any of the stations STA 1  to STA 4  in  FIG.  1   . The WLAN device  200  includes a baseband processor  210 , a radio frequency (RF) transceiver  240 , an antenna unit  250 , a storage device (e.g., memory)  232 , one or more input interfaces  234 , and one or more output interfaces  236 . The baseband processor  210 , the memory  232 , the input interfaces  234 , the output interfaces  236 , and the RF transceiver  240  may communicate with each other via a bus  260 . 
     The baseband processor  210  performs baseband signal processing, and includes a MAC processor  212  and a PHY processor  222 . The baseband processor  210  may utilize the storage device  232 , which may include a non-transitory computer readable medium having software (e.g., computer programing instructions) and data stored therein. 
     In an embodiment, the MAC processor  212  includes a MAC software processing unit  214  and a MAC hardware processing unit  216 . The MAC software processing unit  214  may implement a first plurality of functions of the MAC layer by executing MAC software, which may be included in the software stored in the storage device  232 . The MAC hardware processing unit  216  may implement a second plurality of functions of the MAC layer in special-purpose hardware, hereinafter referred to as “MAC hardware.” However, the MAC processor  212  is not limited thereto. For example, the MAC processor  212  may be configured to perform the first and second plurality of functions entirely in software or entirely in hardware according to an implementation. 
     The PHY processor  222  includes a transmitting signal processing unit  224  and a receiving signal processing unit  226 . The PHY processor  222  implement a plurality of functions of the PHY layer. These functions may be performed in software, hardware, or a combination thereof according to implementation. 
     Functions performed by the transmitting signal processing unit  224  may include one or more of Forward Error Correction (FEC) encoding, stream parsing into one or more spatial streams, diversity encoding of the spatial streams into a plurality of space-time streams, spatial mapping of the space-time streams to transmit chains, inverse Fourier Transform (iFT) computation, Cyclic Prefix (CP) insertion to create a Guard Interval (GI), and the like. 
     The RF transceiver  240  includes an RF transmitter  242  and an RF receiver  244 . The RF transceiver  240  is configured to transmit first information received from the baseband processor  210  to the WLAN, and provide second information received from the WLAN to the baseband processor  210 . 
     The antenna unit  250  includes one or more antennas. When Multiple-Input Multiple-Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antenna unit  250  may include a plurality of antennas. In an embodiment, the antennas in the antenna unit  250  may operate as a beam-formed antenna array. In an embodiment, the antennas in the antenna unit  250  may be directional antennas, which may be fixed or steerable. 
     The input interfaces  234  receive information from a user, and the output interfaces  236  output information to the user. The input interfaces  234  may include one or more of a keyboard, keypad, mouse, touchscreen, touch screen, microphone, and the like. The output interfaces  236  may include one or more of a display device, touch screen, speaker, and the like. 
     As described herein, many functions of the WLAN device  200  may be implemented in either hardware or software. Which functions are implemented in software and which functions are implemented in hardware will vary according to constraints imposed on a design. The constraints may include one or more of design cost, manufacturing cost, time to market, power consumption, available semiconductor technology, and so on. 
     As described herein, a wide variety of electronic devices, circuits, firmware, software, and combinations thereof may be used to implement the functions of the components of the WLAN device  200 . Furthermore, the WLAN device  200  may include other components, such as application processors, storage interfaces, clock generator circuits, power supply circuits, and the like, which have been omitted in the interest of brevity. 
       FIG.  3 A  illustrates components of a wireless device configured to transmit data according to an embodiment, including a Transmitting (Tx) Signal Processing Unit (TxSP)  324 , an RF transmitter  342 , and an antenna  352 . In an embodiment, the TxSP  324 , the RF transmitter  342 , and the antenna  352  correspond to the transmitting signal processing unit  224 , the RF transmitter  242 , and an antenna of the antenna unit  250  of  FIG.  2   , respectively. 
     The TxSP  324  includes an encoder  300 , an interleaver  302 , a mapper  304 , an inverse Fourier transformer (IFT)  306 , and a guard interval (GI) inserter  308 . 
     The encoder  300  receives and encodes input data. In an embodiment, the encoder  300  includes a forward error correction (FEC) encoder. The FEC encoder may include a binary convolutional code (BCC) encoder followed by a puncturing device. The FEC encoder may include a low-density parity-check (LDPC) encoder. 
     The TxSP  324  may further include a scrambler for scrambling the input data before the encoding is performed by the encoder  300  to reduce the probability of long sequences of 0s or 1s. When the encoder  300  performs the BCC encoding, the TxSP  324  may further include an encoder parser for demultiplexing the scrambled bits among a plurality of BCC encoders. If LDPC encoding is used in the encoder, the TxSP  324  may not use the encoder parser. 
     The interleaver  302  interleaves the bits of each stream output from the encoder  300  to change an order of bits therein. The interleaver  302  may apply the interleaving only when the encoder  300  performs the BCC encoding, and otherwise may output the stream output from the encoder  300  without changing the order of the bits therein. 
     The mapper  304  maps the sequence of bits output from the interleaver  302  to constellation points. If the encoder  300  performed LDPC encoding, the mapper  304  may also perform LDPC tone mapping in addition to the constellation mapping. 
     When the TxSP  324  performs a MIMO or MU-MIMO transmission, the TxSP  324  may include a plurality of interleavers  302  and a plurality of mappers  304  according to a number NSS of spatial streams of the transmission. The TxSP  324  may further include a stream parser for dividing the output of the encoder  300  into blocks and may respectively send the blocks to different interleavers  302  or mappers  304 . The TxSP  324  may further include a space-time block code (STBC) encoder for spreading the constellation points from the spatial streams into a number NSTS of space-time streams and a spatial mapper for mapping the space-time streams to transmit chains. The spatial mapper may use direct mapping, spatial expansion, or beamforming. 
     The IFT  306  converts a block of the constellation points output from the mapper  304  (or, when MIMO or MU-MIMO is performed, the spatial mapper) to a time domain block (i.e., a symbol) by using an inverse discrete Fourier transform (IDFT) or an inverse fast Fourier transform (IFFT). If the STBC encoder and the spatial mapper are used, the IFT  306  may be provided for each transmit chain. 
     When the TxSP  324  performs a MIMO or MU-MIMO transmission, the TxSP  324  may insert cyclic shift diversities (CSDs) to prevent unintentional beamforming. The TxSP  324  may perform the insertion of the CSD before or after the IFT  306 . The CSD may be specified per transmit chain or may be specified per space-time stream. Alternatively, the CSD may be applied as a part of the spatial mapper. 
     When the TxSP  324  performs a MIMO or MU-MIMO transmission, some blocks before the spatial mapper may be provided for each user. 
     The GI inserter  308  prepends a GI to each symbol produced by the IFT  306 . Each GI may include a Cyclic Prefix (CP) corresponding to a repeated portion of the end of the symbol the GI precedes. The TxSP  324  may optionally perform windowing to smooth edges of each symbol after inserting the GI. 
     The RF transmitter  342  converts the symbols into an RF signal and transmits the RF signal via the antenna  352 . When the TxSP  324  performs a MIMO or MU-MIMO transmission, the GI inserter  308  and the RF transmitter  342  may be provided for each transmit chain. 
       FIG.  3 B  illustrates components of a wireless device configured to receive data according to an embodiment, including a Receiver (Rx) Signal Processing Unit (RxSP)  326 , an RF receiver  344 , and an antenna  354 . In an embodiment, the RxSP  326 , RF receiver  344 , and antenna  354  may correspond to the receiving signal processing unit  226 , the RF receiver  244 , and an antenna of the antenna unit  250  of  FIG.  2   , respectively. 
     The RxSP  326  includes a GI remover  318 , a Fourier transformer (FT)  316 , a demapper  314 , a deinterleaver  312 , and a decoder  310 . 
     The RF receiver  344  receives an RF signal via the antenna  354  and converts the RF signal into symbols. The GI remover  318  removes the GI from each of the symbols. When the received transmission is a MIMO or MU-MIMO transmission, the RF receiver  344  and the GI remover  318  may be provided for each receive chain. 
     The FT  316  converts each symbol (that is, each time domain block) into a frequency domain block of constellation points by using a discrete Fourier transform (DFT) or a fast Fourier transform (FFT). The FT  316  may be provided for each receive chain. 
     When the received transmission is the MIMO or MU-MIMO transmission, the RxSP  326  may include a spatial demapper for converting the respective outputs of the FTs  316  of the receiver chains to constellation points of a plurality of space-time streams, and an STBC decoder for despreading the constellation points from the space-time streams into one or more spatial streams. 
     The demapper  314  demaps the constellation points output from the FT  316  or the STBC decoder to bit streams. If the received transmission was encoded using the LDPC encoding, the demapper  314  may further perform LDPC tone demapping before performing the constellation demapping. 
     The deinterleaver  312  deinterleaves the bits of each stream output from the demapper  314 . The deinterleaver  312  may perform the deinterleaving only when the received transmission was encoded using the BCC encoding, and otherwise may output the stream output by the demapper  314  without performing deinterleaving. 
     When the received transmission is the MIMO or MU-MIMO transmission, the RxSP  326  may use a plurality of demappers  314  and a plurality of deinterleavers  312  corresponding to the number of spatial streams of the transmission. In this case, the RxSP  326  may further include a stream deparser for combining the streams output from the deinterleavers  312 . 
     The decoder  310  decodes the streams output from the deinterleaver  312  or the stream deparser. In an embodiment, the decoder  312  includes an FEC decoder. The FEC decoder may include a BCC decoder or an LDPC decoder. 
     The RxSP  326  may further include a descrambler for descrambling the decoded data. When the decoder  310  performs the BCC decoding, the RxSP  326  may further include an encoder deparser for multiplexing the data decoded by a plurality of BCC decoders. When the decoder  310  performs the LDPC decoding, the RxSP  326  may not use the encoder deparser. 
       FIG.  4 A  illustrates an Orthogonal Frequency Division Multiple Access (OFDMA) frame  400  according to an embodiment. The OFDMA frame  400  may be a Down-Link (DL) Multi-User (MU) OFDMA frame transmitted by the AP  102  or an Up-Link (UL) MU OFDMA frame transmitted by a one or more of the stations STA 1  to STA 4  of the WLAN BSS  100  of  FIG.  1   . In the embodiment shown in  FIG.  4   , the OFDMA frame  400  is transmitted used a 20 MHZ bandwidth, but embodiments are not limited thereto. Embodiments of the OFDMA frame  400  may be transmitted using any of a 40 MHz bandwidth, an 80 MHz bandwidth, an 80+80 MHz bandwidth, and a 160 MHz bandwidth. 
     The OFDMA frame  400  includes one or more data payloads, represented by first, second, and third data payloads  410 ,  412 , and  414  respectively. The data payloads  410 ,  412 , and  414  may be intended for respective first, second, and third stations. 
     A bandwidth of the OFDMA frame  400  is divided into one or more Resource Units (RUs), and each of the data payloads  410 ,  412 , and  414  is allocated one or more of the RUs. In an embodiment, an RU is allocated to no more than one data payload. 
     The OFDMA frame  400  may further include a High Efficient (HE) Signal A (HE-SIG-A) field  402 , an HE Signal B (HE-SIG-B) field  404 , an HE Short Training Field (HE-STF)  406 , and an HE Long Training Field (HE-LTF)  408 . 
     The HE-SIG-A field  402  may include information for interpreting the OFDMA frame  400 . The HE-SIG-B field  404 , when present, may include information concerning the allocation of Resource Units (RUs) within the OFDMA frame  400  and may include information used by specific stations (STAs) to decode respective payloads intended for them. The HE-STF  406  may include information for use in automatic gain control in a device receiving the OFDMA frame  400 . 
     The HE-LTF  408  includes information for estimating a channel between a WLAN device transmitting the OFDMA frame  400  and a WLAN device receiving the OFDMA frame  400 . The information in the HE-LTF  408  may also be used by the receiving WLAN device to track phase and frequency offsets. In an embodiment, the OFDMA frame  400  includes a plurality of HE-LTFs  408 . 
     Pilots exist in OFDM symbols of the data payloads  410 ,  412 , and  414  and the symbols of the HE-LTF  408 . The pilots in the HE-LTF  408  may be used to compensate for CFO drift and to reduce channel estimation error for transmissions having a high number of spatial streams. 
     802.11ax systems may support a first type and a second type of HE-LTF OFDM symbols. The first type may have a same number of subcarriers (i.e. tones) as data/pilot subcarriers. The second type may have approximately half of the number of information-carrying subcarriers (that is, carriers with energy) as data/pilot subcarriers. The first type of HE-LTF may be denoted as a 4×HE-LTF design and the second type of HE-LTF (with approximately half of the number of information-carrying subcarriers) may be denoted as a 2× HE-LTF design. 
     In an embodiment of the second type of HE-LTF, information is carried in every even tone, except for the DC tones. In another embodiment of the second type of HE-LTF, information is carried in every odd tone, except for the DC tones. 
     Accordingly, in an embodiment, the HE-LTF  408  includes a 2×HE-LTF  408 -2× according to the 2×LTF design. The 2×HE-LTF  408 -2× includes an OFDM symbol  422  and a Guard Interval (GI)  424 . The OFDM symbol  422  may have a duration of 6.4 microseconds. 
     As shown in  FIG.  4 B , the OFDM symbol  422  of the 2×HE-LTF  408 -2× may have an even mapping, that is, the OFDM symbol  422  only carry information in even-numbered tones (other than DC tones) within a bandwidth of the OFDM symbol  422 , that is, tones having an index equal to 2n, where n is an integer. Arrows such as an arrow  430  indicate tones of the OFDM symbol  422  that may carry information. Tones without an arrow do not carry information. 
     As shown in  FIG.  4 C , the OFDM symbol  422  of the 2×HE-LTF  408 -2× may have an odd mapping, that is, the OFDM symbol  422  may only carry information in odd-numbered tones (other than DC tones) within a bandwidth of the OFDM symbol  422 , that is, tones having an index equal to 2n+1, where n is an integer. Arrows such as an arrow  432  indicate tones of the OFDM symbol  422  that may carry information. Tones without an arrow do not carry information. 
     In another embodiment, the HE-LTF  408  includes a 4×HE-LTF  408 -4× according to the 4×LTF design. The 4×HE-LTF  408 -4× includes an OFDM symbol  426  and a Guard Interval (GI)  428 . The OFDM symbol  426  may have a duration of 12.8 microseconds. 
     As shown in  FIG.  4 D , the OFDM symbol  426  of the 2×HE-LTF  408 -4× may carry information in all tones (other than DC tones) within a bandwidth of the OFDM symbol  426 . Arrows such as an arrow  434  indicate tones of the OFDM symbol  426  that may carry information. 
     Embodiments of the present disclosure include methods of providing pilots that support same and different FFT sizes for HE-LTF and data payload OFDM symbols. 
     In OFDMA operations, the operational bandwidth is divided up into resource units (RUs). There may be different RU sizes within the operational bandwidth, each RU size including a different number of subcarriers (i.e. tones). Examples of possible RUs for various bandwidths are shown in  FIGS.  5 A,  6 ,  7 ,  8 A, and  9   , described below. Depending on how the RUs are defined, RUs may start with an even numbered subcarrier (that is, RUs may be even RUs) or an odd numbered subcarrier (that is, RUs may be odd RUs). 
       FIG.  5 A  illustrates RUs definitions in a 20 MHz bandwidth  500  according to an embodiment. Within the 20 MHz bandwidth  500  may be defined first to ninth 26-subcarrier RUs  502 ,  504 ,  506 ,  508 ,  510 ,  512 ,  514 ,  516 , and  518 . Within the 20 MHz bandwidth  500  may be also defined first to fourth 52-subcarrier RUs  522 ,  524 ,  526  and  528 . A 242-subcarrier RU  540  may also be defined within the 20 MHz bandwidth  500 . 
     In an embodiment, first and second 106-subcarrier RUs  532  and  534  may be defined in the 20 MHz bandwidth  500 . In another embodiment, first and second 108-subcarrier RUs  536  and  538  may be defined in the 20 MHz bandwidth  500 . 
       FIG.  5 B  includes a Table 1 indicating the size and lowest and highest subcarrier (SC) indexes of the RUs that may be defined in the 20 MHz bandwidth  500 . Table 1 also indicates whether an RU is considered an odd RU (that is, one that has a lowest SC index that is an odd integer) or an even RU (that is, one that has a lowest SC index that is an even integer.) Odd and even RUs are also indicated in  FIG.  5 A . 
     The fifth 26-subcarrier RU  510 , at the center of the 20 MHz bandwidth  500 , is not to be considered an odd RU or an even RU. 
       FIG.  6    illustrates RUs definitions in a 40 MHz bandwidth  600  according to an embodiment. Within the 40 MHz bandwidth  600  may be defined first to eighteenth 26-subcarrier RUs  602 ,  604 ,  606 ,  608 ,  610 ,  612 ,  614 ,  616 ,  618 ,  642 ,  644 ,  646 ,  648 ,  650 ,  652 ,  654 ,  656 , and  658 . Also defined may be first to eighth 62-subcarrier RUs  622 ,  624 ,  626 ,  628 ,  662 ,  664 ,  666  and  668 . First and second 242-subcarrier RUs  640  and  680  and a 484-subcarrier RU  682  may also be defined within the 40 MHz bandwidth  600 . 
     In an embodiment, first to fourth second 106-subcarrier RUs  632 ,  634 ,  662  and  664  may be defined in the 40 MHz bandwidth  600 . In another embodiment, first to fourth 108-subcarrier RUs  636 ,  638 ,  666 , and  668  may be defined in the 40 MHz bandwidth  600 . 
     In an embodiment, lowest and highest SC indexes of the RUs  602  to  638  of  FIG.  6    are the same, within a first 20 MHz subchannel of the 40 MHz bandwidth  600 , as the lowest and highest SC indexes of the similarly numbered RUs  502  to  538  of  FIGS.  5 A and  5 B , respectively. Lowest and highest SC indexes of the RUs  642  to  668  of  FIG.  6    are the same, within a second 20 MHz subchannel of the 40 MHz bandwidth  600 , as the lowest and highest SC indexes of the corresponding RUs  502  to  538  of  FIGS.  5 A and  5 B , respectively. 
     The odd and even RUs of the 40 MHz bandwidth  600  are indicated by labels of the RUs. 
     Within this disclosure, the definitions of the RUs illustrated in  FIGS.  5 A and  6    are denoted as case 1. 
       FIG.  7    illustrates RUs defined within an 80 MHZ bandwidth  700 . The defined RUs include thirty-six 26-subcarrier RUs  702  to  721  and  742  to  761 , sixteen 52-subcarrier RUs  722  to  729  and  762  to  769 , four 242-subcarrier RUs  740 ,  741 ,  780 , and  781 , two 484-subcarrier RUs  782  and  783 , and a 996-subcarrier RU  784 . 
     The defined RUs in the 80 MHZ bandwidth  700  also include eight RUs that may be 106-subcarrier RUs in an embodiment or 108-subcarrier RUs in another embodiment. The eight 106-or-108-subcarrier RUs are designated by reference characters  732  to  735  and  772  to  775 . 
       FIGS.  8 A and  9    illustrate alternative RU definitions for 20 and 40 MHz bandwidths, respectively. Within this disclosure, the definitions of the RUs illustrated in  FIGS.  8 A and  9    are denoted as case 2. 
       FIG.  8 A  illustrates RUs definitions in a 20 MHz bandwidth  800  according to another embodiment. Within the 20 MHz bandwidth  800  are defined first to ninth 26-subcarrier RUs  802 ,  804 ,  806 ,  808 ,  810 ,  812 ,  814 ,  816 , and  818 . Also defined are first to fourth 52-subcarrier RUs  822 ,  824 ,  826  and  828 . A 242-subcarrier RU  840  may also be defined within the 20 MHz bandwidth  800 . 
     In an embodiment, first and second 106-subcarrier RUs  832  and  834  may be defined in the 20 MHz bandwidth  800 . In another embodiment, first and second 108-subcarrier RUs  836  and  838  may be defined in the 20 MHz bandwidth  800 . 
       FIG.  8 B  includes a Table 2 indicating the size and lowest and highest subcarrier (SC) indexes of the RUs that may be defined in the 20 MHz bandwidth  800 . Table 1 also indicates whether an RU is considered an odd RU (that is, one that has a lowest SC index that is an odd integer) or an even RU (that is, one that has a lowest SC index that is an even integer.) Odd and even RUs are also indicated in  FIG.  8 A . 
     The fifth 26-subcarrier RU  810 , at the center of the 20 MHz bandwidth  800 , is not to be considered an odd RU or an even RU. 
       FIG.  9    illustrates RUs definitions in a 40 MHz bandwidth  900  according to another embodiment. Within the 40 MHz bandwidth  900  are defined first to eighteenth 26-subcarrier RUs  902 ,  904 ,  906 ,  908 ,  910 ,  912 ,  914 ,  916 ,  918 ,  942 ,  944 ,  946 ,  948 ,  950 ,  952 ,  954 ,  956 , and  958 . Also defined are first to eighth 52-subcarrier RUs  922 ,  924 ,  926 ,  928 ,  962 ,  964 ,  966  and  968 . First and second 242-subcarrier RUs  940  and  980  and a 484-subcarrier RU  982  may also be defined within the 40 MHz bandwidth  900 . 
     In an embodiment, first to fourth second 106-subcarrier RUs  932 ,  934 ,  962  and  964  may be defined in the 40 MHz bandwidth  900 . In another embodiment, first to fourth 108-subcarrier RUs  936 ,  938 ,  966 , and  968  may be defined in the 40 MHz bandwidth  900 . 
     In an embodiment, lowest and highest SC indexes of the RUs  902  to  938  of  FIG.  9    are the same, within a first 20 MHz subchannel of the 40 MHz bandwidth  900 , as the lowest and highest SC indexes of the similarly numbered RUs  802  to  838  of  FIGS.  8 A and  8 B , respectively. Lowest and highest SC indexes of the RUs  942  to  968  of  FIG.  9    are the same, within a second 20 MHz subchannel of the 40 MHz bandwidth  900 , as the lowest and highest SC indexes of the corresponding RUs  802  to  838  of  FIGS.  8 A and  8 B , respectively. 
     The odd and even RUs of the 40 MHz bandwidth  900  are indicated by labels of the RUs. 
     1. Pilot Tone Mapping Symmetry Between Even and Odd Resource Units 
     In an embodiment, even and odd RUs may have pilot tone positions that exhibit mirror symmetry.  FIG.  10    illustrates positions of pilot tone positions in an even RU  1000  according to an embodiment.  FIG.  11    illustrates positions of pilot tone positions in an odd RU  1100  according to an embodiment, the positions of the pilot tone positions in the odd RU  1100  having mirror symmetry relative to the positions of the pilot tone positions of the even RU  1000 . The even RU  1000  and the odd RU  1100  have a same size, that is, a same number ( 26 ) of subcarriers. 
     In  FIGS.  10  and  11   , hash marks along the horizontal access correspond to odd subcarriers that respectively do not correspond to subcarriers having energy of a 2×HE-LTF. Upward pointing arrows along the horizontal access respectively correspond to even subcarriers that correspond to the subcarriers having energy of the 2×HE-LTF. 
     The even RU  1000  includes first and second pilot tone positions  1004  and  1006 . The first pilot tone position  1004  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the even RU  1000 . The second pilot tone position  1006  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  1004  and spaced 7 subcarriers away from a highest-indexed subcarrier of the even RU  1000 . 
     The odd RU  1100  includes third and fourth pilot tone positions  1104  and  1106  which are mirror-symmetric to the first and second pilot tone positions  1004  and  1006 , respectively. Accordingly, the third pilot tone position  1104  corresponds to a subcarrier spaced 6 subcarriers away from a highest-indexed subcarrier of the odd RU  1100 . The fourth pilot tone position  1106  corresponds to a subcarrier separated by 11 subcarriers from the third pilot tone position  1104  and spaced 7 subcarriers away from a lowest-indexed subcarrier of the odd RU  1100 . 
     This produces mirror symmetric pilot tone positions between an even RU and an odd RU of the same size, when the even RU and the odd RU are adjacent to each other. 
     2. Nested and Non-Nested Pilot Structure 
     In an embodiment, pilot tone positions may have a nested pilot structure. In another embodiment, pilot tone positions may have a non-nested pilot structure. 
     In the nested pilot structure, for a given operation bandwidth (such as 40 MHz or 80 MHz) the pilot tone positions for RUs having larger number of subcarriers are physically aligned (in frequency domain) with pilot tone positions for RUs having smaller numbers of subcarriers. The nested pilot structure may simplify a Carrier Frequency Offset (CFO) tracking algorithm of a receiver configured to receive frames including RU allocations having varied sizes and varied positions. 
     In the non-nested pilot structure, pilot tone positions are defined for each RU size in such a manner as to maximize CFO tracking performance. The non-nested pilot structure may have a uniform spacing of pilots within an RU. 
     3. Design A: Non-Nested Pilot Structure 
     In embodiment according to a design-A principle having a non-nested pilot structure, cases may occur in which pilots are mapped to locations that correspond to null tones (that is, tones not carrying information) of a 2×HE-LTF, since information in the 2×HE-LTF sequence is only mapped to either even or odd tones in a given OFDM symbol. 
     Embodiments of the design-A principle include alternative 1 and alternative 2. Alternative 1 defines the pilot tone positions such that the pilot tone positions never correspond to locations of null tones of an HE-LTF OFDM symbol. 
     Alternative 2 defines the pilot tone positions such that pilot spacing is uniform, without regards to whether the pilot tone positions correspond to locations of null tones of the HE-LTF OFDM symbol. As a result, an HE-LTF OFDM symbol according to Alternative 2 of Design A may lack one or more pilots present in the data payload OFDM symbols. 
     4. Design a, Alternative 1: Pilots are not Mapped to Null LTF Tones 
       FIGS.  10  and  11    both illustrates a general concept for embodiments of alternative 1 of Design-A. In order to ensure that pilots are always mapped to non-null tones of an HE-LTF of the 2×LTF design, the tone spacing between two consecutive pilot tone positions (that is, the number of tones/subcarriers between two consecutive pilot tone positions) should be an odd number. 
     Accordingly in  FIG.  10    the second pilot tone position  1006  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  1004 , and in  FIG.  11    the third pilot tone position  1104  corresponds to a subcarrier separated by 11 subcarriers from the fourth pilot tone position  1106 . That is, the tone spacing between the pilot tone positions in each of  FIGS.  10  and  11    is 11 subcarriers. 
     This ensures that pilot tone positions are mapped to 2×HE-LTF sequence mapping positions (i.e. subcarriers that carry information in a 2×LTF design). 
     In all of  FIGS.  12 - 22   , the upward arrows extending from the horizontal axis represents subcarriers corresponding to potential 2×HE-LTF sequence mapping positions from the 2×LTF design, and the hash marks on the axis represent subcarriers that do not corresponding to potential 2×HE-LTF sequence mapping positions from the 2×LTF design. The dash-dotted lines on the most left and right part of the figure represent the boundaries of the RU. 
       FIGS.  12  and  13    illustrate RUs having a spacing between pilot subcarrier positions of 13 subcarriers according to an embodiment of alternative 1 of Design A. 
     In an embodiment of alternative 1 of design A that includes an even tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  12    illustrates pilot tone positions for an even 26-subcarrier RU  1200 , and  FIG.  13    illustrates pilot tone positions for an odd 26-subcarrier RU  1300 . 
     In the embodiment including the even tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the even RU  1200  is equal to 2n, and a lowest subcarrier index f 0 ′ of the odd RU  1300  is equal to 2n+1, where n is an integer. In an embodiment, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. In an embodiment for 20 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of − 68 , −42, +70, and +96 when even and one of − 121 , −95, +17, and +43 when odd. In an embodiment for 40 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −136, 4, 30, 58, 84, 138, 164, 192, and 218 when even and one of −243, −217, −189, −163, −109, −83, −55, −29, and 111 when odd. In an embodiment for 80 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −392, −150, +18, +44, +72, +98, +152, +178, +206, +232, +260, +286, +314, +340, +394, +420, +448, and +474 when even and one of −499, −473, −445, −419, −365, −339, −311, −285, −257, −231, −203, −177, −123, −97, −69, −43, +125, and +367 when odd. In an embodiment for 160 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −1011, −985, −957, −931, −877, −851, −823, −797, −769, −743, −715, −689, −635, −609, −581, −555, −387, −145, +13, +39, +67, +93, +147, +173, +201, +227, +255, +281, +309, +335, +389, +415, +443, +469, +637, and +879 when even and one of −904, −662, −494, −468, −440, −414, −360, −334, −306, −280, −252, −226, −198, −172, −118, −92, −64, −38, +120, +362, +530, +556, +584, +610, +664, +690, +718, +744, +772, +798, +826, +852, +906, +932, +960, +986 when odd. 
     A first pilot tone position  1204  of the even RU  1200  is spaced 6 subcarriers away from a lowest subcarrier of the even RU  1200 . A second pilot tone position  1206  of the even RU  1200  is separated by 13 subcarriers from the first pilot tone position  1204  and 5 subcarriers away from a highest subcarrier of the even RU  1200 . If the lowest subcarrier of the even RU  1200  is equal to f0, two pilots are located at the (f0+6)-th subcarrier and the (f0+20)-th subcarrier. 
     A first pilot tone position  1304  of the odd RU  1300  is spaced 5 subcarriers away from a lowest subcarrier of the odd RU  1300 . A second pilot tone position  1306  of the odd RU  1300  is separated by 13 subcarriers from the first pilot tone position  1304  and 6 subcarriers away from a highest subcarrier of the odd RU  1300 . If the lowest subcarrier of the even RU  1200  is equal to f0, two pilots are located at the (f0+5)-th subcarrier and the (f0+19)-th subcarrier. 
     In another embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  12    illustrates pilot tone positions for an odd 26-subcarrier RU, and  FIG.  13    illustrates pilot tone positions for an even 26-subcarrier RUs. 
     In the embodiment including the odd tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the odd RU  1200  is equal to 2n+1, and a lowest subcarrier index f 0 ′ of the even RU  1300  is equal to 2n, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  1200  and  1300  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  1204  of the odd RU  1200  is spaced 6 subcarriers away from a lowest subcarrier of the odd RU  1200 . A second pilot tone position  1206  of the odd RU  1200  is separated by 13 subcarriers from the first pilot tone position  1204  and spaced 5 subcarriers away from a highest subcarrier of the odd RU  1200 . 
     A first pilot tone position  1304  of the even RU  1300  is spaced 5 subcarriers away from a lowest subcarrier of the even RU  1300 . A second pilot tone position  1306  of the even RU  1300  is separated by 13 subcarriers from the first pilot tone position  1304  and spaced 6 subcarriers away from a highest subcarrier of the even RU  1300 . 
       FIGS.  14  and  15    illustrate RUs having a spacing between pilot subcarrier positions of 11 subcarriers according to an embodiment of alternative 1 of Design A. 
     In an embodiment of alternative 1 of design A that includes an even tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  14    illustrates pilot tone positions for an even 26-subcarrier RU  1400 , and  FIG.  15    illustrates pilot tone positions for an odd 26-subcarrier RU  1500 . 
     In the embodiment including the even tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the even RU  1400  is equal to 2n, and a lowest subcarrier index f 0 ′ of the odd RU  1500  is equal to 2n+1, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  1400  and  1500  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  1404  of the even RU  1400  is spaced 6 subcarriers away from a lowest subcarrier of the even RU  1400 . A second pilot tone position  1406  of the even RU  1400  is separated by 11 subcarriers from the first pilot tone position  1404  and spaced 7 subcarriers away from a highest subcarrier of the even RU  1400 . 
     A first pilot tone position  1504  of the odd RU  1500  is spaced 7 subcarriers away from a lowest subcarrier of the odd RU  1500 . A second pilot tone position  1506  of the odd RU  1500  is separated by 11 subcarriers from the first pilot tone position  1504  and spaced 6 subcarriers away from a highest subcarrier of the odd RU  1500 . 
     In another embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  14    illustrates pilot tone positions for an odd 26-subcarrier RU, and  FIG.  15    illustrates pilot tone positions for an even 26-subcarrier RUs. 
     In the embodiment including the odd tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the odd RU  1400  is equal to 2n+1, and a lowest subcarrier index f 0 ′ of the even RU  1500  is equal to 2n, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  1400  and  1500  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  1404  of the odd RU  1400  is spaced 6 subcarriers away from a lowest subcarrier of the odd RU  1400 . A second pilot tone position  1406  of the odd RU  1400  is separated by 11 subcarriers from the first pilot tone position  1404  and spaced 7 subcarriers away from a highest subcarrier of the odd RU  1400 . 
     A first pilot tone position  1504  of the even RU  1500  is spaced 7 subcarriers away from a lowest subcarrier of the even RU  1500 . A second pilot tone position  1506  of the even RU  1500  is separated by 11 subcarriers from the first pilot tone position  1504  and spaced 6 subcarriers away from a highest subcarrier of the even RU  1500 . 
       FIG.  16    illustrates a center 26-subcarrier RU  1600  according to an embodiment of alternative 1 of design A that includes an even tone mapping of an HE-LTF sequence in a 2×LTF design. The center RU  1600  is split into a left 13-subcarrier unit and a right 13-subcarrier unit by one of 3, 5, and 7 Direct Current (DC) tones within a central portion  1602 . 
     When the center RU  1600  includes 3 DC tones, a lowest subcarrier index f 0  of the left 13-subcarrier unit may be −14 and a lowest subcarrier index f 1  of the right 13-subcarrier unit may be +2. When the center RU  1600  includes 5 DC tones, the left lowest subcarrier index f 0  may be −15 and the right lowest subcarrier index f 1  of the center RU  1600  may be +3. When the center RU  1600  includes 7 DC tones, the left lowest subcarrier index f 0  may be −16 and the right lowest subcarrier index f 1  of the center RU  1600  may be +4. 
     A first pilot tone position  1604  of the center RU  1600  is spaced 6 subcarriers away from a lowest subcarrier of the center RU  1600  and 6 subcarriers away from the central portion  1602 . A second pilot tone position  1606  of the center RU  1600  is spaced 6 subcarriers away from a highest subcarrier of the center RU  1600  and 6 subcarriers away from the central portion  1602 . Two pilots are located at the (f0+6)-th subcarrier and the (f1+6)-th subcarrier. 
     The first pilot tone position  1604  is located at a center of 13 subcarriers of a left portion of the center RU  1600 . The second pilot tone position  1606  is located at a center of 13 subcarriers of a right portion of the center RU  1600 . 
       FIG.  17    illustrates a center 26-subcarrier RU  1700  according to an embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design. The center RU  1700  may include one of 3, 5, and 7 Direct Current (DC) tones within a central portion  1702 . 
     When the center RU  1700  includes 3 DC tones, a left lowest subcarrier index f 0  may be −14 and a right lowest subcarrier index f 1  of the center RU  1700  may be +2. When the center RU  1700  includes 5 DC tones, the left lowest subcarrier index f 0  may be −15 and the right lowest subcarrier index f 1  of the center RU  1700  may be +3. When the center RU  1700  includes 7 DC tones, the left lowest subcarrier index f 0  may be −16 and the right lowest subcarrier index f 1  of the center RU  1700  may be +4. 
     A first pilot tone position  1704  of the center RU  1700  is spaced 5 subcarriers away from a lowest subcarrier of the even RU  1700  and 7 subcarriers away from the central portion  1702 . A second pilot tone position  1706  of the center RU  1700  is spaced 5 subcarriers away from a highest subcarrier of the center RU  1700  and 7 subcarriers away from the central portion  1702 . 
       FIG.  18    illustrates a center 26-subcarrier RU  1800  according to an embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design. The center RU  1800  may include one of 3, 5, and 7 Direct Current (DC) tones within a central portion  1802 . 
     When the center RU  1800  includes 3 DC tones, a left lowest subcarrier index f 0  may be −14 and a right lowest subcarrier index f 1  of the center RU  1800  may be +2. When the center RU  1800  includes 5 DC tones, the left lowest subcarrier index f 0  may be −15 and the right lowest subcarrier index f 1  of the center RU  1800  may be +3. When the center RU  1800  includes 7 DC tones, the left lowest subcarrier index f 0  may be −16 and the right lowest subcarrier index f 1  of the center RU  1800  may be +4. 
     A first pilot tone position  1804  of the center RU  1800  is spaced 7 subcarriers away from a lowest subcarrier of the even RU  1800  and 5 subcarriers away from the central portion  1802 . A second pilot tone position  1806  of the center RU  1800  is spaced 7 subcarriers away from a highest subcarrier of the center RU  1800  and 5 subcarriers away from the central portion  1802 . 
       FIGS.  19  and  20    illustrate 52-subchannel RUs having a spacing between first, second, third, and fourth pilot subcarrier positions of 13, 11, and 13 subcarriers, respectively, according to an embodiment of alternative 1 of Design A. 
     In an embodiment of alternative 1 of design A that includes an even tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  19    illustrates pilot tone positions for an even 52-subcarrier RU  1900 , and  FIG.  20    illustrates pilot tone positions for an odd 52-subcarrier RU  2000 . 
     In the embodiment including the even tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the even RU  1900  is equal to 2n, and a lowest subcarrier index f 0 ′ of the odd RU  2000  is equal to 2n+1, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  1900  and  2000  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. In an embodiment for 20 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −68 and +70 when even and one of −121 and +17 when odd. In an embodiment for 40 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of 4, 58, 138, and 192 when even and one of −243, −189, −109, and −55 when odd. In an embodiment for 80 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of +18, +72, +152, +206, +260, +314, +394 and +448 when even and one of −499, −445, −365, −311, −257, −203, −123 and −69 when odd. In an embodiment for the 160 MHz bandwidth, the lowest subcarrier index f 0  of either of RU  1200  and  1300  may be one of −494, −440, −360, −306, −252, −198, −118, −64, +530, +584, +664, +718, +772, +826, +906 and +960 when even and one of −1011, −957, −877, −823, −769, −715, −635, −581, +13, +67, +147, +201, +255, +309, +389 and +443 when odd. 
     A first pilot tone position  1904  of the even RU  1900  is spaced 6 subcarriers away from a lowest subcarrier of the even RU  1900 . A second pilot tone position  1906  of the even RU  1900  is separated by 13 subcarriers from the first pilot tone position  1904 . A third pilot tone position  1908  of the even RU  1900  is separated by 11 subcarriers from the second pilot tone position  1906 . A fourth pilot tone position  1910  of the even RU  1900  is separated by 13 subcarriers from the third pilot tone position  1908  and spaced 5 subcarriers away from a highest subcarrier of the even RU  1900 . If the lowest subcarrier of the even RU  1900  is equal to f 0 , four pilots are located at the (f 0 +6)-th subcarrier, the (f 0 +20)-th subcarrier, the (f 0 +32)-th subcarrier and the (f 0 +46)-th subcarrier, respectively. 
     A first pilot tone position  2004  of the odd RU  2000  is spaced 5 subcarriers away from a lowest subcarrier of the odd RU  2000 . A second pilot tone position  2006  of the odd RU  2000  is separated by 13 subcarriers from the first pilot tone position  2004 . A third pilot tone position  2008  of the odd RU  2000  is separated by 11 subcarriers from the second pilot tone position  2006 . A fourth pilot tone position  2010  of the odd RU  2000  is separated by 13 subcarriers from the third pilot tone position  2008  and spaced 6 subcarriers away from a highest subcarrier of the odd RU  2000 . If the lowest subcarrier of the odd RU  1900  is equal to f 0 ′, four pilots are located at the (f 0 ′+6)-th subcarrier, the (f 0 ′+20)-th subcarrier, the (f 0 ′+32)-th subcarrier and the (f 0 ′+46)-th subcarrier, respectively. 
     In another embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  19    illustrates pilot tone positions for an odd 52-subcarrier RU  1900 , and  FIG.  20    illustrates pilot tone positions for an even 52-subcarrier RU  2000 . 
     In the embodiment including the odd tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the odd RU  1900  is equal to 2n+1, and a lowest subcarrier index f 0 ′ of the even RU  2000  is equal to 2n, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  1900  and  2000  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  1904  of the odd RU  1900  is spaced 6 subcarriers away from a lowest subcarrier of the odd RU  1900 . A second pilot tone position  1906  of the odd RU  1900  is separated by 13 subcarriers from the first pilot tone position  1904 . A third pilot tone position  1908  of the odd RU  1900  is separated by 11 subcarriers from the second pilot tone position  1906 . A fourth pilot tone position  1910  of the odd RU  1900  is separated by 13 subcarriers from the third pilot tone position  1908  and spaced 5 subcarriers away from a highest subcarrier of the odd RU  1900 . 
     A first pilot tone position  2004  of the even RU  2000  is spaced 5 subcarriers away from a lowest subcarrier of the even RU  2000 . A second pilot tone position  2006  of the even RU  2000  is separated by 13 subcarriers from the first pilot tone position  2004 . A third pilot tone position  2008  of the even RU  2000  is separated by 11 subcarriers from the second pilot tone position  2006 . A fourth pilot tone position  2010  of the even RU  2000  is separated by 13 subcarriers from the third pilot tone position  2008  and spaced 6 subcarriers away from a highest subcarrier of the even RU  2000 . 
       FIGS.  21  and  22    illustrate 52-subchannel RUs having a spacing between first, second, third, and fourth pilot subcarrier positions of 11, 13, and 11 subcarriers, respectively, according to an embodiment of alternative 1 of Design A. 
     In an embodiment of alternative 1 of design A that includes an even tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  21    illustrates pilot tone positions for an even 52-subcarrier RU  2100 , and  FIG.  22    illustrates pilot tone positions for an odd 52-subcarrier RU  2200 . 
     In the embodiment including the even tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the even RU  2100  is equal to 2n, and a lowest subcarrier index f 0 ′ of the odd RU  2200  is equal to 2n+1, where n is an integer. In an embodiment, the lowest subcarrier index f 0  of either of RU  2100  and  2200  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  2104  of the even RU  2100  is spaced 6 subcarriers away from a lowest subcarrier of the even RU  2100 . A second pilot tone position  2106  of the even RU  2100  is separated by 11 subcarriers from the first pilot tone position  2104 . A third pilot tone position  2108  of the even RU  2100  is separated by 13 subcarriers from the second pilot tone position  2106 . A fourth pilot tone position  2110  of the even RU  2100  is separated by 11 subcarriers from the third pilot tone position  2108  and spaced 7 subcarriers away from a highest subcarrier of the even RU  2100 . 
     A first pilot tone position  2204  of the odd RU  2200  is spaced 7 subcarriers away from a lowest subcarrier of the odd RU  2200 . A second pilot tone position  2206  of the odd RU  2200  is separated by 11 subcarriers from the first pilot tone position  2204 . A third pilot tone position  2208  of the odd RU  2200  is separated by 13 subcarriers from the second pilot tone position  2206 . A fourth pilot tone position  2210  of the odd RU  2200  is separated by 11 subcarriers from the third pilot tone position  2208  and spaced 6 subcarriers away from a highest subcarrier of the odd RU  2200 . 
     In another embodiment of alternative 1 of design A that includes an odd tone mapping of an HE-LTF sequence in a 2×LTF design,  FIG.  21    illustrates pilot tone positions for an odd 52-subcarrier RU, and  FIG.  22    illustrates pilot tone positions for an even 52-subcarrier RUs. 
     In the embodiment including the odd tone mapping of the HE-LTF sequence, a lowest subcarrier index f 0  of the odd RU  2100  is equal to 2n+1, and a lowest subcarrier index f 0 ′ of the even RU  2200  is equal to 2n, where n is an integer. In an embodiment, the lowest subcarrier indexes f 0  and f 0 ′ of either of RU  2100  and  2200  is one of −122, −68, +16, and +70 when even and one of −95, −41, +43, and +97 when odd. 
     A first pilot tone position  2104  of the odd RU  2100  is spaced 6 subcarriers away from a lowest subcarrier of the odd RU  2100 . A second pilot tone position  2106  of the odd RU  2100  is separated by 11 subcarriers from the first pilot tone position  2104 . A third pilot tone position  2108  of the odd RU  2100  is separated by 13 subcarriers from the second pilot tone position  2106 . A fourth pilot tone position  2110  of the odd RU  2100  is separated by 11 subcarriers from the third pilot tone position  2108  and spaced 7 subcarriers away from a highest subcarrier of the odd RU  2100 . 
     A first pilot tone position  2204  of the even RU  2200  is spaced 7 subcarriers away from a lowest subcarrier of the even RU  2200 . A second pilot tone position  2206  of the even RU  2200  is separated by 11 subcarriers from the first pilot tone position  2204 . A third pilot tone position  2208  of the even RU  2200  is separated by 13 subcarriers from the second pilot tone position  2206 . A fourth pilot tone position  2210  of the even RU  2200  is separated by 11 subcarriers from the third pilot tone position  2208  and spaced 6 subcarriers away from a highest subcarrier of the even RU  2200 . 
       FIG.  23    includes a Table 3 that indicates pilot tone positions for six embodiments of alternative 1 of design A for 106-subcarrier RUs. The embodiments include three spacing options a, b, and c for each of two mappings (even or odd) of an HE-LTF sequence in a 2×LTF design. 
     Table 3 includes three columns corresponding to the spacing options a, b, and c, respectively. Each column is composed of two subcolumns, i) an “EVEN/EVEN or ODD/ODD” subcolumn indicating positions in either even RUs when an even mapping of the HE-LTF is used or odd RUs when an odd mapping of the HE-LTF is used, and ii) an “ODD/EVEN or EVEN/ODD” subcolumn indicating positions in either the even RUs when the odd mapping of the HE-LTF is used or the odd RUs when the even mapping of the HE-LTF is used. 
     In each row, each column indicates a subcarrier position (SC) of an element (such as an edge carrier or a pilot tone) corresponding to the row and a spacing (or Gap) between the element corresponding to the row and the element corresponding to the next row. 
     In each case, the edge carriers of the RU includes a lowest subcarrier (low SC) having an index of 0, and a highest subcarrier (high SC) having an index of 105. 
     For a first example, in an embodiment of option a of alternative 1 of design A having an even tone mapping of an HE-LTF sequence in the 2×LTF design, first to fourth pilots of even RUs are respectively located at indexes of +14, +40, +66, and +92, respectively. First to fourth pilots of odd RUs are respectively located at indexes of +13, +39, +65, and +91, respectively. 
     In the first example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 14, 25, 25, 25, and 13 in the even RUs and 13, 25, 25, 25, and 14 in the odd RUs. 
     For a second example, in an embodiment of option b of alternative 1 of design A having an odd tone mapping of an HE-LTF sequence in the 2×LTF design, first to fourth pilots of even RUs are respectively located at indexes of +15, +41, +67, and +93, respectively. First to fourth pilots of odd RUs are respectively located at indexes of +12, +38, +64, and +90, respectively. 
     As in the first example, in the second example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 15, 25, 25, 25, and 12 in the even RUs and 12, 25, 25, 25, and 15 in the odd RUs. 
     Table 3 illustrates that in every combination of i) one of option a, b, and c, and ii) one of an even and odd tone mapping of an HE-LTF sequence in the 2×LTF design, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. 
       FIG.  24    includes a Table 4 that indicates pilot tone positions for six embodiments of alternative 1 of design A for 108-subcarrier RUs. The embodiments include three spacing options a, b, and c for each of two mappings (even or odd) of an HE-LTF sequence in a 2×LTF design. 
     Table 4 includes three columns corresponding to the spacing options a, b, and c, respectively. Each column is composed of two subcolumns, i) an “EVEN/EVEN or ODD/ODD” subcolumn indicating positions in either even RUs when an even mapping of the HE-LTF is used or odd RUs when an odd mapping of the HE-LTF is used, and ii) an “ODD/EVEN or EVEN/ODD” subcolumn indicating positions in either the even RUs when the odd mapping of the HE-LTF is used or the odd RUs when the even mapping of the HE-LTF is used. 
     In each row, each column indicates a subcarrier position (SC) of an element (such as an edge carrier or a pilot tone) corresponding to the row and a spacing (or Gap) between the element corresponding to the row and the element corresponding to the next row. 
     In each case, the edge carriers of the RU includes a lowest subcarrier (low SC) having an index of 0, and a highest subcarrier (high SC) having an index of 107. 
     For a first example, in an embodiment of option a of alternative 1 of design A having an even tone mapping of an HE-LTF sequence in the 2×LTF design, first to sixth pilots of even RUs are respectively located at indexes of +8, +26, +44, +62, +80, and +98, respectively. First to sixth pilots of odd RUs are respectively located at indexes of +9, +27, +45, +63, +81, and +99, respectively. 
     In the first example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 8, 17, 17, 17, 17, 17, and 9 in the even RUs and 9, 17, 17, 17, 17, 17, and 8 in the odd RUs. 
     For a second example, in an embodiment of option b of alternative 1 of design A having an odd tone mapping of an HE-LTF sequence in the 2×LTF design, first to sixth pilots of even RUs are respectively located at indexes of +7, +25, +45, +61, +79, and +97, respectively. First to sixth pilots of odd RUs are respectively located at indexes of +10, +28, +46, +64, 82, and +100, respectively. 
     As in the first example, in the second example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 7, 17, 17, 17, 17, 17, and 10 in the even RUs and 10, 17, 17, 17, 17, 17, and 7 in the odd RUs. 
     Table 4 illustrates that in every combination of i) one of option a, b, and c, and ii) one of an even and odd tone mapping of an HE-LTF sequence in the 2×LTF design, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. 
       FIG.  25    includes a Table 5 that indicates pilot tone positions for four embodiments of alternative 1 of design A for 242-subcarrier RUs. The embodiments include two spacing options a and b for each of two mappings (even or odd) of an HE-LTF sequence in a 2×LTF design. 
     Table 5 includes a left set of columns for a 242-subcarrier RUs in a 20 MHz bandwidth, wherein the RU is always an even RU. In the set of columns for the 20 MHz channel bandwidth, three sub-columns correspond to pilot tone positions for option a for an even mapping of the HE-LTF, pilot tone positions for option b for the even mapping of the HE-LTF, and pilot tone positions for an odd mapping of the HE-LTF for both options a and b, respectively. 
     In each row on the left side, three columns indicates respective subcarrier positions (SCs) of an element (such as an edge carrier or a pilot tone) corresponding to the row for the spacing option and LTF mapping, and a fourth columns indicates spacing (or Gap) between the element corresponding to the row and the element corresponding to the next row, which does not vary between the options a and b and the LTF mapping being even or odd. 
     Table 5 includes a right set of columns for 242-subcarrier RUs in any of a 40 MHz, an 80 MHz, or a 160 MHz channel bandwidth, wherein an RU can be an even or an odd RU. Within the right set, a column is provided for a spacing option a and a spacing option b. 
     In the right set of columns, Each column for spacing option a orb is composed of two subcolumns, i) an “EVEN/EVEN or ODD/ODD” subcolumn indicating positions in either even RUs when an even mapping of the HE-LTF is used or odd RUs when an odd mapping of the HE-LTF is used, and ii) an “ODD/EVEN or EVEN/ODD” subcolumn indicating positions in either the even RUs when the odd mapping of the HE-LTF is used or the odd RUs when the even mapping of the HE-LTF is used. 
     In each row on the right side, each column indicates a subcarrier position (SC) of an element (such as an edge carrier or a pilot tone) corresponding to the row and a spacing (or Gap) between the element corresponding to the row and the element corresponding to the next row. 
     In the embodiments within the 20 MHz channel bandwidth, the edge carriers of the RU includes a lowest subcarrier (low SC) having an index of −122, and a highest subcarrier (high SC) having an index of 122. In the embodiments within the 40, 80, or 160 MHz bandwidth, the edge carriers of the RUs includes a lowest subcarrier (low SC) having an index of 0, and a highest subcarrier (high SC) having an index of 241. 
     For a first example, in an embodiment of option a of alternative 1 of design A having an even tone mapping of an HE-LTF sequence in the 2×LTF design in a 20 MHz bandwidth, first to eighth pilots of the 242-subcarrier RU are respectively located at indexes of −104, −76, −40, −12, +12, +40, +76, +104, respectively. 
     For a second example, in an embodiment of option b of alternative 1 of design A having an odd tone mapping of an HE-LTF sequence in the 2×LTF design for a 40, 80, or 160 MHz channel bandwidth, first to eighth pilots of even RUs are respectively located at indexes of 19, 47, 83, 111, 131, 159, 195, and 223, respectively. First to eighth pilots of odd RUs are respectively located at indexes of 18, 46, 82, 110, 130, 158, 194, and 222, respectively. 
     In the second example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 19, 27, 35, 27, 19, 27, 35, 27, and 18 in the even RUs and 18, 27, 35, 27, 19, 27, 35, 27, and 19 in the odd RUs. 
     Table 5 illustrates that, for channel bandwidths of 40, 80 and 160 MHz, in every combination of i) one of option a and b, and ii) one of an even and odd tone mapping of an HE-LTF sequence in the 2×LTF design, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. In a 20 MHz channel bandwidth, there is only one 242-subcarrier RU. 
       FIG.  26    includes a Table 6 that indicates pilot tone positions for six embodiments of alternative 1 of design A for 242-subcarrier RUs. The embodiments include three spacing options a, b, and c for each of two mappings (even or odd) of an HE-LTF sequence in a 2×LTF design. 
     Table 6 includes three columns corresponding to the spacing options a, b, and c, respectively. Each column is composed of two subcolumns, i) an “EVEN/EVEN or ODD/ODD” subcolumn indicating positions in either even RUs when an even mapping of the HE-LTF is used or odd RUs when an odd mapping of the HE-LTF is used, and ii) an “ODD/EVEN or EVEN/ODD” subcolumn indicating positions in either the even RUs when the odd mapping of the HE-LTF is used or the odd RUs when the even mapping of the HE-LTF is used. 
     In each row, each column indicates a subcarrier position (SC) of an element (such as an edge carrier or a pilot tone) corresponding to the row and a spacing (or Gap) between the element corresponding to the row and the element corresponding to the next row. 
     In each case, the edge carriers of the RU includes a lowest subcarrier (low SC) having an index of 0, and a highest subcarrier (high SC) having an index of 241. 
     For example, in an embodiment of option a of alternative 1 of design A having an even tone mapping of an HE-LTF sequence in the 2×LTF design, first to eighth pilots of even RUs are respectively located at indexes of +16, +46, +76, +106, +136, +166, +196, and +226, respectively. First to eighth pilots of odd RUs are respectively located at indexes of +15, +45, +75, +105, +135, +165, +195, and +225, respectively. 
     In the example, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. That is, the spacing between the pilot tone positions and edge subcarriers, starting from the lowest subcarrier, are 16, 29, 29, 29, 29, 29, 29, 29, and 15 in the even RUs and 15, 29, 29, 29, 29, 29, 29, 29, and 16 in the odd RUs. 
     Table 6 illustrates that in every combination of i) one of option a, b, and c, and ii) one of an even and odd tone mapping of an HE-LTF sequence in the 2×LTF design, the positions of the pilot tone positions in the even and odd RUs are mirror symmetric. 
       FIG.  27    includes a Table 7 that indicates pilot tone positions for six embodiments of alternative 1 of design A for center 242-subcarrier RUs. The embodiments include three spacing options a, b, and c for each of two mappings (even or odd) of an HE-LTF sequence in a 2×LTF design. 
     Table 6 includes three columns corresponding to the spacing options a, b, and c, respectively. Each column is composed of two subcolumns, i) an “EVEN” subcolumn indicating positions in the center RU when an even mapping of the HE-LTF is used, and ii) an “ODD” subcolumn indicating positions in the center RUs when the odd mapping of the HE-LTF is used. 
     In each row, each column indicates a subcarrier position of an element (such a pilot tone) corresponding to the row. The subcarrier positions that are below the center of the RU are given relative to a negative offset −f 0 . The subcarrier positions that are above the center of the RU are given relative to a positive offset +f 0 . The negative and positive offsets −f 0  and +f 0  correspond to the first non-DC subcarriers below and above the center of the RU respectively, and the values of the negative and positive offsets −f 0  and +f 0  are determined by a number of DC subcarriers at the center of the RU. 
     In the embodiments shown in Table 7, pilot within each of an upper and lower half of the center RU are separated by 29 tones. 
     Table 7 illustrates that in every combination of i) one of option a, b, and c, and ii) one of an even and odd tone mapping of an HE-LTF sequence in the 2×LTF design, the positions of the pilot tone positions below and above the center of the center RU are mirror symmetric. 
     5. Design A, Alternative 2: Pilot can be mapped to null LTF tones 
       FIG.  28    illustrates pilot tone positions for an even RU block  2800  with 26 subcarriers when an even tone mapping of an LTF sequence in a 2×LTF design is used, or for an odd RU block  2800  with 26 subcarriers and when an odd tone mapping of the LTF sequence in the 2×LTF design is used, according to an embodiment of alternative 2 of design A.  FIG.  29    illustrates pilot tone positions for an odd RU block  2900  with 26 subcarriers when an even tone mapping of the LTF sequence in the 2×LTF design is used, or for an even RU block  2900  with 26 subcarriers when an odd tone mapping of the LTF sequence in the 2×LTF design is used, according to an embodiment of alternative 2 of design A. 
     Locations of tones in the RU blocks  2800  and  2900  are given relative to an offset f0 within a 20 MHz channel. The offset f0 can be any of −122, −95, −68, −41, +16, +43, +70, and +97. 
     Alternative 2 defines the tone spacing between the pilots (that is, between first and second pilots  2804  and  2806  of the RU block  2800  and between first and second pilots  2904  and  2906  of the RU block  2900 ) to be 12 subcarriers and includes 6 subcarriers between outer edges of the RU and respective nearest pilot tone positions. In the 2×LTF design, the LTF OFDM symbol will only carry one pilot within the 26 subcarrier RU (because the LTF tones of the 2×LTF design, indicated by upward pointing arrows, only coincide with one pilot tone position in each of the RU blocks  2800  and  2900 ), however for the 4×LTF design, the LTF OFDM symbol will carry two pilots within the 26 subcarrier RU. 
       FIG.  30    illustrates pilot tone positions for an center RU block  3000  with 26 subcarriers when odd tone mapping of an LTF sequence in a 2×LTF design are used. Locations of tones in the center RU block  3000  are given relative to a first offset f0 and a second offset f 1  relative to a center subcarrier of the center RU block  3000 . The first, second, and third embodiment, the first offset f0 can respectively be −14, −15, and −16 and the second offset f 1  can respectively be +2, +3, and +4. Tones that are lower than the center subcarrier of the center RU block  3000  have position indicated relative to the first offset f 0 . Tones that are higher than the center subcarrier of the center RU block  3000  have position indicated relative to the second offset f1. 
     Alternative 2 defines the tone spacing between the first and second pilots  3004  and  3006  to be 12 subcarriers (not including any of the DC subcarriers  3002 ) and 6 subcarriers to the outer edge of the RU from pilot tone positions. This results in no pilots to be carried in the central 26 subcarrier RU of the LTF OFDM symbol in case of 2×LTF design because the LTF tones of the 2×LTF design, indicated by upward pointing arrows, do not correspond with either of the first pilot tone position  3004  and the second pilot tone position  3006 . 
       FIG.  31    illustrates pilot tone positions for an even RU block  3100  with 52 subcarriers when an even tone mapping of an LTF sequence in a 2×LTF design is used, or for an odd RU block  3100  with 52 subcarriers and when an odd tone mapping of the LTF sequence in the 2×LTF design is used, according to an embodiment of alternative 2 of design A.  FIG.  32    illustrates pilot tone positions for an odd RU block  3200  with 56 subcarriers when the even tone mapping of the LTF sequence in the 2×LTF design is used, or for an even RU block  3100  with 26 subcarriers when the odd tone mapping of the LTF sequence in the 2×LTF design is used, according to an embodiment of alternative 2 of design A. 
     Alternative 2 defines the tone spacing between the any two pilots to be 12 subcarriers and 6 subcarriers to the outer edge of the RU from pilot tone positions. Accordingly, RU Block  3100  has first, second, third, and fourth pilots  3104 ,  3106 ,  3108 , and  3110  at locations  6 ,  19 ,  32 , and  45 , respectively, and RU Block  3200  has first, second, third, and fourth pilots  3204 ,  3206 ,  3208 , and  3210  at locations  6 ,  19 ,  32 , and  45 , respectively. 
     When the 2×LTF design is used, the LTF OFDM symbol will only carry two pilots within the 52 subcarrier RU, as shown in  FIGS.  31  and  32    by only two of the pilot tone positions coinciding with the positions (indicated by upward pointing arrows) of tones in the 2×LTF design. For the 4×LTF design, the LTF OFDM symbol will carry four pilots within the 52 subcarrier RU. 
     6. Design B: Nested Pilot Structure 
     In a nested pilot structure, the pilot tone positions between different RUs having different number of subcarriers share the same physical frequency position. One or more pilot tone positions for a smallest resource size unit is determined, and pilot tone positions of a larger resource size unit are chosen from among the pilot tone positions of the smallest resource size unit. Embodiment of pilot tone position design are disclosed herein for two different cases. 
     Case 1 embodies RU definition design with 26 subcarrier RUs, 52 subcarriers RUs, 106 or 108 subcarrier RUs, 242 subcarrier RUs, 484 subcarrier RUs, and 994 or 996 subcarrier RUs, wherein there are one or more null or reserved tones between each 26 subcarrier RU and any adjacent 26 subcarrier RUs. Case 2 embodies RU definition design with 26 subcarrier RUs, 52 subcarriers RUs, 106 or 108 subcarrier RUs, 242 subcarrier RUs, 484 subcarrier RUs, and 994 or 996 subcarrier RUs, wherein pairs of consecutive 26 subcarrier RUs line up (frequency wise) with 52 subcarrier RUs, except for the central 26 subcarrier RU within each 20 MHz bandwidth. 
     In  FIGS.  33  to  45   , potential pilot tone positions in a 20 MHz bandwidth are indicated by dotted lines extending vertically through the figures. Actual pilot tone positions for each RU are indicated by upward pointing arrows. A solid upward pointing arrow indicates a pilot tone position with a fixed position. A dashed upward pointing arrow indicates a pilot tone position having one of two positions, includes a first position to the left of a reference subcarrier and a second position to the right of a reference subcarrier, according to a design choice. In an embodiment, the reference subcarriers are center subcarriers of 13-subcarrier halves of 26-subcarrier RUs. 
     In one embodiment, each potential pilot tone position aligns with a pilot tone position in an HE-LTF symbol of the frame. In some embodiments, two pilot tone positions are present in each 26 subcarrier RU for a given bandwidth. For example, as shown in  FIGS.  12 ,  13 , and  16   , the pilot tone positions in each 26 subcarrier RU may be represented by the tones  1204  and  1206  (in relation to  FIG.  12   ), tones  1304  and  1306  (in relation to  FIG.  13   ), and tones  1604  and  1606  (in relation to  FIG.  16   ). The pilot tone positions in 26 subcarrier RUs become the potential pilot tone positions for RUs with greater number of subcarriers (e.g., 52, 106, 242, 484, 996, and 2×996 subcarriers). In other word, the potential pilot tone positions are aggregation of pilot tone positions used for each 26 subcarrier RU for a given bandwidth. 
       FIG.  33 A  illustrates a first option for pilot tone positions of a 20 MHz channel  3300  in Case 1. In Case 1, at least one null or reserved tone is present between each 26-subcarrier RU. Pilot tone positions for each RU are selected from first to eighteenth potential pilot tone positions c1 to c18.  FIG.  33 B  shows Table 9, which lists the positions of each of the potential pilot tone positions c1 to c18 according to an embodiment, but embodiments are not limited thereto, and any of the pilot tone positions disclosed for 26-subchannel RUs in Design A, above, may be used for the potential pilot tone positions c1 to c18.  FIGS.  33 A and  33 B  show that the 20 MHz channel  3300  has a total number of potential pilot tone positions of 18. 
     In  FIG.  33 A  and in  FIGS.  34  to  25   , numbers appearing immediately below each RU indicate indices of lowest and highest subcarriers of the RU. The indices are relative to a central (0 th ) DC subcarrier of the 20 MHz channel  3300 . For example, in  FIG.  33 A , the numbers appearing directly below a first 26-subcarrier RU  3302  indicates that the index of a lowest subcarrier of the first 26-subcarrier RU  3302  is −122 and that the index of a highest subcarrier of the first 26-subcarrier RU  3302  is −97. The positions shown in Table  33 B are also relative to the central (0 th ) DC subcarrier of the 20 MHz channel  3300 . 
       FIG.  33 A  shows a first 26-subcarrier RU  3302  having pilot tone positions at potential pilot tone positions c1 and c2, a second 26-subcarrier RU  3304  having pilot tone positions at potential pilot tone positions c3 and c4, a third 26-subcarrier RU  3306  having pilot tone positions at potential pilot tone positions c5 and c6, a fourth 26-subcarrier RU  3308  having pilot tone positions at potential pilot tone positions c7 and c8, a fifth (center) 26-subcarrier RU  3310  having pilot tone positions at potential pilot tone positions c9 and c10, a sixth 26-subcarrier RU  3312  having pilot tone positions at potential pilot tone positions c11 and c12, a seventh 26-subcarrier RU  3314  having pilot tone positions at potential pilot tone positions c13 and c14, an eighth 26-subcarrier RU  3316  having pilot tone positions at potential pilot tone positions c15 and c16, and a ninth 26-subcarrier RU  3318  having pilot tone positions at potential pilot tone positions c17 and c18. 
       FIG.  33 A  further shows a first 52-subcarrier RU  3322  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, a second 52-subcarrier RU  3324  having pilot tone positions at potential pilot tone positions c5, c6, c7, and c8, a third 52-subcarrier RU  3326  having pilot tone positions at potential pilot tone positions c11, c12, c13, and c14, and a fourth 52-subcarrier RU  3328  having pilot tone positions at potential pilot tone positions c15, c16, c17, and c18. 
       FIG.  33 A  further shows a first 106-subcarrier RU  3332  having pilot tone positions at potential pilot tone positions c1, c4, c5, and c8, a second 106-subcarrier RU  3334  having pilot tone positions at potential pilot tone positions c11, c14, c15, and c18, and a 242-subcarrier RU  3336  having pilot tone positions at potential pilot tone positions c1, c4, c5, c8, c11, c14, c15, and c18. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  33 A  has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. 
     Furthermore, each RU in a lower 10 MHz of the 20 MHz channel  3300  has pilot tone positions that are mirror symmetric with a corresponding mirrored RU (that is, an RU with a same number of subcarriers and a same offset from the center of the 20 MHz channel  3300 ) of an upper 10 MHz of the 20 MHz channel  3300 . For example, the pilot tone positions of the first 26-subcarrier RU  3302  are positioned to be mirror-symmetric to the pilot tone positions of the ninth 26-subcarrier RU  3318 , the pilot tone positions of the second 52-subcarrier RU  3324  are positioned to be mirror-symmetric to the pilot tone positions of the third 52-subcarrier RU  3326 , and so on. 
       FIG.  34    illustrates a second option for pilot tone positions in Case 1. In an embodiment, the second option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto, and any of the pilot tone positions disclosed for 26-subchannel RUs in Design A, above, may be used for the potential pilot tone positions c1 to c18. 
       FIG.  34    shows a first, second, third, and fourth 26-subcarrier RUs  3402 ,  3404 ,  3406 , and  3408  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RUs  3410  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RUs  3412 ,  3414 ,  3416 , and  3418  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. In each of the 26-subcarrier RUs  3402 ,  3404 ,  3406 ,  3408 ,  3410 ,  3412 ,  3414 ,  3416 , and  3418 , respective first and second pilot tone positions correspond to potential pilot tone positions covered by the respective 26 subcarriers. 
       FIG.  34    further shows a first, second, third, and fourth 52-subcarrier RUs  3422 ,  3424 ,  3426 , and  3428  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. In each of the 52-subcarrier RUs  3422 ,  3424 ,  3426 , and  3428 , respective first, second, third, and fourth pilot tone positions correspond to potential pilot tone positions covered by the respective 52 subcarriers. 
       FIG.  34    further shows a first 106-subcarrier RU  3432  having pilot tone positions at potential pilot tone positions c1, c3, c6, and c8, a second 106-subcarrier RU  3434  having pilot tone positions at potential pilot tone positions c11, c13, c16, and c18. In each of the 106-subcarrier RUs  3433  and  3434 , a first pilot tone position has an index corresponding to a lowest index among potential pilot tone positions covered by the RU, a second pilot tone position is spaced two potential pilot tone positions away from the first pilot tone position, a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, and a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position. 
       FIG.  34    further shows a 242-subcarrier RU  3436  having pilot tone positions at potential pilot tone positions c1, c3, c6, c8, c11, c13, c16, and c18. The 242-subcarrier RU  3436  has a first pilot tone position having a lowest index among potential pilot tone positions covered by the 242 subcarriers, a second pilot tone position spaced two potential pilot tone positions away from the first pilot tone position, a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position, a fifth pilot tone position spaced three potential pilot tone positions away from the fourth pilot tone position, a sixth pilot tone position spaced two potential pilot tone positions away from the fifth pilot tone position, a seventh pilot tone position spaced three potential pilot tone positions away from the sixth pilot tone position, and an eighth pilot tone position spaced two potential pilot tone positions away from the seventh pilot tone position. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  34    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  34    illustrates the mirror symmetry described with respect to  FIG.  33 A . Therefore, pilot tone positions meet the following relationship: c1=−c18, c2=−c17, c3=−c16, c4=−c15, c5=−c14, c6=−c13, c7=−c12, c8=−al, and c9=−c10 (c18&gt;c17&gt;c16&gt;c15&gt;c14&gt;c13&gt;c12&gt;c11&gt;c10&gt;c9&gt;c8&gt;c7&gt;c6&gt;c5&gt;c4&gt;c3&gt;c2&gt;c1). 
       FIG.  35    illustrates a third option for pilot tone positions in Case 1. In an embodiment, the third option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto, and any of the pilot tone positions disclosed for 26-subchannel RUs in Design A, above, may be used for the potential pilot tone positions c1 to c18. 
       FIG.  35    shows a first, second, third, and fourth 26-subcarrier RU  3502 ,  3504 ,  3506 , and  3508  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  3510  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  3512 ,  3514 ,  3516 , and  3518  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  35    further shows a first, second, third, and fourth 52-subcarrier RU  3522 ,  3524 ,  3526 , and  3528  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  35    further shows a first 106-subcarrier RU  3532  having pilot tone positions at potential pilot tone positions c1, c3, c5, and c7, a second 106-subcarrier RU  3534  having pilot tone positions at potential pilot tone positions c12, c14, c16, and c18, and a 242-subcarrier RU  3536  having pilot tone positions at potential pilot tone positions c1, c3, c5, c7, c12, c14, c16, and c18. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  35    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  35    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  36    illustrates a fourth option for pilot tone positions in Case 1. In an embodiment, the fourth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  36    shows a first, second, third, and fourth 26-subcarrier RU  3602 ,  3604 ,  3606 , and  3608  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  3610  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  3612 ,  3614 ,  3616 , and  3618  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  36    further shows a first, second, third, and fourth 52-subcarrier RU  3622 ,  3624 ,  3626 , and  3628  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  36    further shows a first 106-subcarrier RU  3632  having pilot tone positions at potential pilot tone positions c2, c4, c6, and c8, a second 106-subcarrier RU  3634  having pilot tone positions at potential pilot tone positions c11, c13, c15, and c17, and a 242-subcarrier RU  3636  having pilot tone positions at potential pilot tone positions c2, c4, c6, c8, c11, c13, c15, and c17. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  36    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  36    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
     The options 1, 2, 3, and 4 of Case 1 shown in  FIGS.  33 A,  34 ,  35 , and  36   , respectively illustrate pilot tone positions for a 20 MHz bandwidth and when 106 subcarrier RUs are used instead of 108 subcarrier RUs. The pilot tone positions within the 26 subcarrier RU can be the suggested pilot tone positions embodiments described in the Design A section. 
     Resource Units (RUs) with larger sizes will have pilot tone positions that are the same as or a subset of the pilot tone positions for aggregated 26-subcarrier RUs occupying the same frequencies. The 106-subcarrier RUs have 4 pilots within its RU allocation from among the eight potential pilot tone positions available for the 106-subcarrier RU. The options 1 to 4 are different alternatives for the four pilot tone positions of the 106-subcarrier RUs and corresponding eight pilot tone positions of a 242-subcarrier RU. The pilot tone positions for 106 subcarrier RU and 242 subcarrier RU are chosen from the set of potential positions stemming from the pilot tone positions of the 26 subcarrier RUs such that frequency diversity can be maximized (that is, so that the spacing between the pilots are large). 
       FIG.  37    illustrates a fifth option for pilot tone positions in Case 1. In an embodiment, the fifth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  37    shows a first, second, third, and fourth 26-subcarrier RU  3702 ,  3704 ,  3706 , and  3708  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  3710  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  3712 ,  3714 ,  3716 , and  3718  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  37    further shows a first, second, third, and fourth 52-subcarrier RU  3722 ,  3724 ,  3726 , and  3728  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  37    further shows a first 108-subcarrier RU  3732  having pilot tone positions at potential pilot tone positions c1, c2, c4, c5, c7, and c8, a second 108-subcarrier RU  3734  having pilot tone positions at potential pilot tone positions c11, c12, c14, c15, c17, and c18, and a 242-subcarrier RU  3736  having pilot tone positions at potential pilot tone positions c1, c4, c5, c8, c11, c14, c15, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  37    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  37    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  38    illustrates a sixth option for pilot tone positions in Case 1. In an embodiment, the sixth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  38    shows a first, second, third, and fourth 26-subcarrier RU  3802 ,  3804 ,  3806 , and  3808  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  3810  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  3812 ,  3814 ,  3816 , and  3818  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  38    further shows a first, second, third, and fourth 52-subcarrier RU  3822 ,  3824 ,  3826 , and  3828  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  38    further shows a first 108-subcarrier RU  3832  having pilot tone positions at potential pilot tone positions c1, c2, c4, c5, c7, and c8, a second 108-subcarrier RU  3834  having pilot tone positions at potential pilot tone positions c11, c12, c14, c15, c17, and c18, and a 242-subcarrier RU  3836  having pilot tone positions at potential pilot tone positions c1, c4, c7, c8, c11, c12, c15, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  38    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  38    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  39    illustrates a seventh option for pilot tone positions in Case 1. In an embodiment, the seventh option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  39    shows a first, second, third, and fourth 26-subcarrier RU  3902 ,  3904 ,  3906 , and  3908  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  3910  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  3912 ,  3914 ,  3916 , and  3918  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  39    further shows a first, second, third, and fourth 52-subcarrier RU  3922 ,  3924 ,  3926 , and  3928  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  39    further shows a first 108-subcarrier RU  3932  having pilot tone positions at potential pilot tone positions c1, c2, c4, c5, c7, and c8, a second 108-subcarrier RU  3934  having pilot tone positions at potential pilot tone positions c11, c12, c14, c15, c17, and c18, and a 242-subcarrier RU  3936  having pilot tone positions at potential pilot tone positions c1, c2, c5, c8, c11, c14, c17, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  39    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  39    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  40    illustrates an eighth option for pilot tone positions in Case 1. In an embodiment, the eighth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  40    shows a first, second, third, and fourth 26-subcarrier RU  4002 ,  4004 ,  4006 , and  4008  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4010  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4012 ,  4014 ,  4016 , and  4018  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  40    further shows a first, second, third, and fourth 52-subcarrier RU  4022 ,  4024 ,  4026 , and  4028  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  40    further shows a first 108-subcarrier RU  4032  having pilot tone positions at potential pilot tone positions c1, c3, c4, c5, c6, and c8, a second 108-subcarrier RU  4034  having pilot tone positions at potential pilot tone positions c11, c13, c14, c15, c16, and c18, and a 242-subcarrier RU  4036  having pilot tone positions at potential pilot tone positions c1, c4, c5, c8, c11, c14, c15, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  40    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  40    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  41    illustrates a ninth option for pilot tone positions in Case 1. In an embodiment, the ninth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  41    shows a first, second, third, and fourth 26-subcarrier RU  4102 ,  4104 ,  4106 , and  4108  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4110  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4112 ,  4114 ,  4116 , and  4118  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  41    further shows a first, second, third, and fourth 52-subcarrier RU  4122 ,  4124 ,  4126 , and  4128  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  41    further shows a first 108-subcarrier RU  4132  having pilot tone positions at potential pilot tone positions c1, c3, c4, c5, c6, and c8, a second 108-subcarrier RU  4134  having pilot tone positions at potential pilot tone positions c11, c13, c14, c15, c16, and c18, and a 242-subcarrier RU  4136  having pilot tone positions at potential pilot tone positions c1, c3, c6, c8, c11, c13, c16, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  41    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  41    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  42    illustrates a tenth option for pilot tone positions in Case 1. In an embodiment, the tenth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  42    shows a first, second, third, and fourth 26-subcarrier RU  4202 ,  4204 ,  4206 , and  4208  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4210  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4212 ,  4214 ,  4216 , and  4218  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  42    further shows a first, second, third, and fourth 52-subcarrier RU  4222 ,  4224 ,  4226 , and  4228  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  42    further shows a first 108-subcarrier RU  4232  having pilot tone positions at potential pilot tone positions c1, c3, c4, c5, c6, and c8, a second 108-subcarrier RU  4234  having pilot tone positions at potential pilot tone positions c11, c13, c14, c15, c16, and c18, and a 242-subcarrier RU  4236  having pilot tone positions at potential pilot tone positions c1, c3, c5, c8, c11, c14, c16, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  42    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  42    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  43    illustrates an eleventh option for pilot tone positions in Case 1. In an embodiment, the eleventh option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  43    shows a first, second, third, and fourth 26-subcarrier RU  4302 ,  4304 ,  4306 , and  4308  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4310  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4312 ,  4314 ,  4316 , and  4318  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  43    further shows a first, second, third, and fourth 52-subcarrier RU  4322 ,  4324 ,  4326 , and  4328  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  43    further shows a first 108-subcarrier RU  4332  having pilot tone positions at potential pilot tone positions c1, c3, c4, c5, c6, and c8, a second 108-subcarrier RU  4334  having pilot tone positions at potential pilot tone positions c11, c13, c14, c15, c16, and c18, and a 242-subcarrier RU  4336  having pilot tone positions at potential pilot tone positions c1, c4, c6, c8, c11, c13, c15, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  43    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  43    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  44    illustrates a twelfth option for pilot tone positions in Case 1. In an embodiment, the twelfth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  44    shows a first, second, third, and fourth 26-subcarrier RU  4402 ,  4404 ,  4406 , and  4408  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4410  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4412 ,  4414 ,  4416 , and  4418  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  44    further shows a first, second, third, and fourth 52-subcarrier RU  4422 ,  4424 ,  4426 , and  4428  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  44    further shows a first 108-subcarrier RU  4432  having pilot tone positions at potential pilot tone positions c1, c2, c3, c6, c7, and c8, a second 108-subcarrier RU  4434  having pilot tone positions at potential pilot tone positions c11, c12, c13, c16, c17, and c18, and a 242-subcarrier RU  4436  having pilot tone positions at potential pilot tone positions c1, c3, c6, c8, c11, c13, c16, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  44    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  44    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
       FIG.  45    illustrates a thirteenth option for pilot tone positions in Case 1. In an embodiment, the thirteenth option may use the first to eighteenth potential pilot tone positions c1 to c18 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  45    shows a first, second, third, and fourth 26-subcarrier RU  4502 ,  4504 ,  4506 , and  4508  having pilot tone positions at potential pilot tone positions c1 and c2, c3 and c4, c5 and c6, and c7 and c8, respectively, a fifth (center) 26-subcarrier RU  4510  having pilot tone positions at potential pilot tone positions c9 and c10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  4512 ,  4514 ,  4516 , and  4518  having pilot tone positions at potential pilot tone positions c11 and c12, c13 and c14, c15 and c16, and c17 and c18, respectively. 
       FIG.  45    further shows a first, second, third, and fourth 52-subcarrier RU  4522 ,  4524 ,  4526 , and  4528  having pilot tone positions at potential pilot tone positions c1, c2, c3, and c4, potential pilot tone positions c5, c6, c7, and c8, potential pilot tone positions c11, c12, c13, and c14, and potential pilot tone positions c15, c16, c17, and c18, respectively. 
       FIG.  45    further shows a first 108-subcarrier RU  4532  having pilot tone positions at potential pilot tone positions c1, c3, c4, c5, c7, and c8, a second 108-subcarrier RU  4534  having pilot tone positions at potential pilot tone positions c11, c12, c14, c15, c16, and c18, and a 242-subcarrier RU  4536  having pilot tone positions at potential pilot tone positions c1, c3, c5, c7, c12, c14, c16, and c18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  45    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Further, the pilot tone positions shown in  FIG.  41    illustrates the mirror symmetry described with respect to  FIG.  33 A . 
     The options 5, 6, 7, 8, 9, 10, 11, 12, and 13 of Case 1 shown in  FIGS.  37 ,  38 ,  39 ,  40 ,  41 ,  42 ,  43 ,  44 , and  45   , respectively, illustrate pilot tone positions for a 20 MHz bandwidth and when 108-subcarrier RUs are used instead of 106-subcarrier RUs. The pilot tone positions within the 26 subcarrier RU can be the suggested pilot tone positions embodiments described in the Design A section. 
     Resource Units (RUs) with larger sizes will have pilot frequency positions that are the same as or a subset of the pilot tone positions for aggregated 26-subcarrier RUs occupying the same frequencies. The 108-subcarrier RUs have 6 pilots within their respective RU allocation from among the eight potential pilot tone positions available for the 108-subcarrier RUs. The options 5 to 13 are different alternatives for the six pilot tone positions of the 108-subcarrier RUs and corresponding eight pilot tone positions of a 242-subcarrier RU. The pilot tone positions for 108 subcarrier RU and 242 subcarrier RU are chosen from the set of potential positions stemming from the pilot tone positions of the 26 subcarrier RUs such that frequency diversity can be maximized (that is, so that the spacing between the pilots are large). 
     7. Larger (Greater than 20 MHz) Bandwidth 
     In an embodiment, pilot tone positions defined for a 20 MHz bandwidth are reused in the 40, 80, and 160 MHz bandwidths. 
     The 40 MHz bandwidth RU definitions are defined using two logical aggregated 20 MHz RU definitions. Except for the central RUs, the left 20 MHz of the 40 MHz bandwidth will have the RU definitions and pilot tone positions corresponding to a 20 MHz embodiment described above and the right 20 MHz of the 40 MHz bandwidth will also have the RU definitions and pilot tone positions corresponding to a 20 MHz embodiment. 
     Because the DC tones in the middle of a 20 MHz bandwidth does not exist in the left 20 MHz or right 20 MHz of the 40 MHz bandwidth, a central 26-subcarrier RUs does not exist in the left 20 MHz or right 20 MHz of the 40 MHz bandwidth. Accordingly, an even 26 subcarrier RU or an odd 26 subcarrier RU occupies a central portion of the left 20 MHz portion and the right 20 MHz portion of the 40 MHz bandwidth, said central portion corresponding to the area occupied by the central 26-subcarrier RU of a 20 MHz bandwidth. Whether an even or odd 26-subcarrier RU is used in the central portion depends on the precise location of the RU. 
     Similarly, for the 80 MHz bandwidth RU definitions, two 40 MHz RU definitions and associated (relative) pilot tone positions are used. A center 26-subcarrier RU may be disposed between the two 40 MHz RU definitions. 
     For the 160 MHz bandwidth RU definitions, two 80 MHz RU definitions and associated (relative) pilot tone positions are used. 
       FIGS.  46 A to  53    illustrate embodiments of pilot tone positions for a 40 MHz bandwidth for Case 1. 
     In  FIGS.  46 A to  54   , potential pilot tone positions are indicated by dotted lines extending vertically through the figures. Actual pilot tone for each RU are indicated by upward pointing arrows. A solid upward pointing arrow indicates a pilot tone with a fixed position. A dashed upward pointing arrow indicates a pilot tone having one of two positions, includes a first position to the left of a reference subcarrier and a second position to the right of a reference subcarrier, according to a design choice. In an embodiment, the reference subcarriers are center subcarriers of 13-subcarrier halves of 26-subcarrier RUs. 
       FIG.  46 A  illustrates a first option for pilot tone positions in Case 1 for a 40 MHz channel  4600 , wherein at least one null or reserved tones is present between each 26-subcarrier RU. Pilot tone positions for each RU are selected from first to thirty-sixth potential pilot tone positions d1 to d36.  FIG.  46 B  shows Table 10, which lists positions of each of the potential pilot tone positions d1 to d36 according to an embodiment, but embodiments are not limited thereto, and any of the pilot tone positions disclosed for 26-subchannel RUs in Design A, above, may be duplicated for use as the potential pilot tone positions d1 to d36. The order of the potential pilot tone positions d1 to d36 in Table 10 goes down the left sub-table and up the right sub-table to better illustrate the mirror symmetry of the potential pilot tone positions d1 to d36. 
       FIG.  46 A  shows, on the left side, a first 26-subcarrier RU  4602  having pilot tone positions at potential pilot tone positions d1 and d2, a second 26-subcarrier RU  4604  having pilot tone positions at potential pilot tone positions d3 and d4, a third 26-subcarrier RU  4606  having pilot tone positions at potential pilot tone positions d5 and d6, a fourth 26-subcarrier RU  4608  having pilot tone positions at potential pilot tone positions d7 and d8, a fifth 26-subcarrier RU  4612  having pilot tone positions at potential pilot tone positions d9 and d10, a sixth 26-subcarrier RU  4614  having pilot tone positions at potential pilot tone positions d11 and d12, a seventh 26-subcarrier RU  4616  having pilot tone positions at potential pilot tone positions d13 and d14, an eighth 26-subcarrier RU  4618  having pilot tone positions at potential pilot tone positions d15 and d16, and a ninth 26-subcarrier RU  4620  having pilot tone positions at potential pilot tone positions d17 and d18. 
       FIG.  46 A  shows, on the right side, a tenth 26-subcarrier RU  4622  having pilot tone positions at potential pilot tone positions d19 and d20, an eleventh 26-subcarrier RU  4624  having pilot tone positions at potential pilot tone positions d21 and d22, a twelfth 26-subcarrier RU  4626  having pilot tone positions at potential pilot tone positions d23 and d24, a thirteenth 26-subcarrier RU  4628  having pilot tone positions at potential pilot tone positions d25 and d26, a fourteenth 26-subcarrier RU  4632  having pilot tone positions at potential pilot tone positions d27 and d28, a fifteenth 26-subcarrier RU  4634  having pilot tone positions at potential pilot tone positions d29 and d30, a sixteenth 26-subcarrier RU  4636  having pilot tone positions at potential pilot tone positions d31 and d32, a seventeenth 26-subcarrier RU  4638  having pilot tone positions at potential pilot tone positions d33 and d34, and an eighteenth 26-subcarrier RU  4640  having pilot tone positions at potential pilot tone positions d35 and d36. 
       FIG.  46 A  further shows, on the left side, a first 52-subcarrier RU  4642  having pilot tone positions at potential pilot tone positions d1, d2, d3, and d4, a second 52-subcarrier RU  4644  having pilot tone positions at potential pilot tone positions d5, d6, d7, and d8, a third 52-subcarrier RU  4646  having pilot tone positions at potential pilot tone positions d11, d12, d13, and d14, and a fourth 52-subcarrier RU  4648  having pilot tone positions at potential pilot tone positions d15, d16, d17, and d18. 
       FIG.  46 A  further shows, on the right side, a fifth 52-subcarrier RU  4652  having pilot tone positions at potential pilot tone positions d19, d22, d21, and d22 a sixth 52-subcarrier RU  4654  having pilot tone positions at potential pilot tone positions d23, d24, d25, and d26, a seventh 52-subcarrier RU  4656  having pilot tone positions at potential pilot tone positions d29, d30, d31, and d32, and an eighth 52-subcarrier RU  4658  having pilot tone positions at potential pilot tone positions d33, d34, d35, and d36. 
       FIG.  46 A  further shows a first 106-subcarrier RU  4662  having pilot tone positions at potential pilot tone positions d1, d3, d5, and d7, a second 106-subcarrier RU  4664  having pilot tone positions at potential pilot tone positions d11, d13, d15, and d17, a third 106-subcarrier RU  4666  having pilot tone positions at potential pilot tone positions d20, d22, d24, and d26, and a fourth 106-subcarrier RU  4668  having pilot tone positions at potential pilot tone positions d30, d32, d34, and d36. 
       FIG.  46 A  further shows a first 242-subcarrier RU  4672  having pilot tone positions at potential pilot tone positions d1, d3, d5, d7, d11, d13, d15, and d17, a second 242-subcarrier RU  4674  having pilot tone positions at potential pilot tone positions d20, d22, d24, d26, d30, d32, d34, and d36, and a 484-subcarrier RU  4676  having pilot tone positions at potential pilot tone positions d1, d3, d5, d7, d11, d13, d15, d17, d20, d22, d24, d26, d30, d32, d34, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  46 A  has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. 
     Furthermore, each RU in a lower 20 MHz of the 40 MHz channel  4600  has pilot tone positions that are mirror symmetric with a corresponding mirrored RU (that is, an RU with a same number of subcarriers and a same offset from the center of the 40 MHz channel  4600 ) of an upper 20 MHz of the 40 MHz channel  4600 . For example, the pilot tone positions of the first 26-subcarrier RU  4602  are positioned to be mirror-symmetric to the pilot tone positions of the eighteenth 26-subcarrier RU  4640 , the pilot tone positions of the second 52-subcarrier RU  4644  are positioned to be mirror-symmetric to the pilot tone positions of the seventh 52-subcarrier RU  4656 , and so on. 
       FIG.  47    illustrates a second option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the second option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto, and any of the pilot tone positions disclosed for 26-subchannel RUs in Design A, above, may be duplicated for use as the potential pilot tone positions d1 to d36. 
       FIG.  47    shows first to eighteenth 26-subcarrier RUs  4702  to  4740  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  47    shows first to eighth 52-subcarrier RUs  4742  to  4758  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  47    further shows a first 106-subcarrier RU  4762  having pilot tone positions at potential pilot tone positions d2, d4, d6, and d8, a second 106-subcarrier RU  4764  having pilot tone positions at potential pilot tone positions d12, d14, d16, and d18, a third 106-subcarrier RU  4766  having pilot tone positions at potential pilot tone positions d19, d21, d23, and d25, and a fourth 106-subcarrier RU  4768  having pilot tone positions at potential pilot tone positions d29, d31, d33, and d35. 
       FIG.  47    further shows a first 242-subcarrier RU  4772  having pilot tone positions at potential pilot tone positions d2, d4, d6, d8, d12, d14, d16, and d18, a second 242-subcarrier RU  4774  having pilot tone positions at potential pilot tone positions d19, d21, d23, d25, d29, d31, d33, and d35, and a 484-subcarrier RU  4776  having pilot tone positions at potential pilot tone positions d2, d4, d6, d8, d12, d14, d16, d18, d19, d21, d23, d25, d29, d31, d33, and d35. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  47    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  47    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  48    illustrates a third option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the second option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  48    shows first to eighteenth 26-subcarrier RUs  4802  to  4840  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  48    shows first to eighth 52-subcarrier RUs  4842  to  4858  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  48    further shows a first 106-subcarrier RU  4862  having pilot tone positions at potential pilot tone positions d1, d3, d5, and d7, a second 106-subcarrier RU  4864  having pilot tone positions at potential pilot tone positions d12, d14, d16, and d18, a third 106-subcarrier RU  4866  having pilot tone positions at potential pilot tone positions d19, d21, d23, and d25, and a fourth 106-subcarrier RU  4868  having pilot tone positions at potential pilot tone positions d30, d32, d34, and d36. 
       FIG.  48    further shows a first 242-subcarrier RU  4872  having pilot tone positions at potential pilot tone positions d1, d3, d5, d7, d12, d14, d16, and d18, a second 242-subcarrier RU  4874  having pilot tone positions at potential pilot tone positions d19, d21, d23, d25, d30, d32, d34, and d36, and a 484-subcarrier RU  4876  having pilot tone positions at potential pilot tone positions d1, d3, d5, d7, d12, d14, d16, d18, d19, d21, d23, d25, d30, d32, d34, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  48    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  48    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  49    illustrates a fourth option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the fourth option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  49    shows first to eighteenth 26-subcarrier RUs  4902  to  4940  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  49    shows first to eighth 52-subcarrier RUs  4942  to  4958  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  49    further shows a first 108-subcarrier RU  4962  having pilot tone positions at potential pilot tone positions d1, d3, d4, d5, d6, and d8, a second 108-subcarrier RU  4964  having pilot tone positions at potential pilot tone positions d11, d13, d14, d15, d16, and d18, a third 108-subcarrier RU  4966  having pilot tone positions at potential pilot tone positions d19, d21, d22, d23, d24, and d26, and a fourth 108-subcarrier RU  4968  having pilot tone positions at potential pilot tone positions d29, d31, d32, d33, d34, and d36. 
       FIG.  49    further shows a first 242-subcarrier RU  4972  having pilot tone positions at potential pilot tone positions d1, d3, d6, d8, d11, d13, d16, and d18, a second 242-subcarrier RU  4974  having pilot tone positions at potential pilot tone positions d19, d21, d24, d26, d29, d31, d34, and d36, and a 484-subcarrier RU  4976  having pilot tone positions at potential pilot tone positions d1, d3, d6, d8, d11, d13, d16, d18, d19, d21, d24, d26, d29, d31, d34, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  49    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  49    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  50    illustrates a fifth option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the fifth option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  50    shows first to eighteenth 26-subcarrier RUs  5002  to  5040  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  50    shows first to eighth 52-subcarrier RUs  5042  to  5058  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  50    further shows a first 108-subcarrier RU  5062  having pilot tone positions at potential pilot tone positions d1, d3, d4, d5, d6, and d8, a second 108-subcarrier RU  5064  having pilot tone positions at potential pilot tone positions d11, d13, d14, d15, d16, and d18, a third 108-subcarrier RU  5066  having pilot tone positions at potential pilot tone positions d19, d21, d22, d23, d24, and d26, and a fourth 108-subcarrier RU  5068  having pilot tone positions at potential pilot tone positions d29, d31, d32, d33, d34, and d36. 
       FIG.  50    further shows a first 242-subcarrier RU  5072  having pilot tone positions at potential pilot tone positions d1, d4, d5, d8, d11, d14, d15, and d18, a second 242-subcarrier RU  5074  having pilot tone positions at potential pilot tone positions d19, d22, d23, d26, d29, d32, d33, and d36, and a 484-subcarrier RU  5076  having pilot tone positions at potential pilot tone positions d1, d4, d5, d8, d11, d14, d15, d18, d19, d22, d23, d26, d29, d32, d33, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  50    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  50    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  51    illustrates a sixth option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the sixth option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  51    shows first to eighteenth 26-subcarrier RUs  5102  to  5140  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  51    shows first to eighth 52-subcarrier RUs  5142  to  5158  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  51    further shows a first 108-subcarrier RU  5162  having pilot tone positions at potential pilot tone positions d1, d2, d4, d5, d7, and d8, a second 108-subcarrier RU  5164  having pilot tone positions at potential pilot tone positions d11, d12, d14, d15, d17, and d18, a third 108-subcarrier RU  5166  having pilot tone positions at potential pilot tone positions d19, d20, d22, d23, d25, and d26, and a fourth 108-subcarrier RU  5168  having pilot tone positions at potential pilot tone positions d29, d30, d32, d33, d35, and d36. 
       FIG.  51    further shows a first 242-subcarrier RU  5172  having pilot tone positions at potential pilot tone positions d1, d3, d4, d7, d12, d15, d17, and d18, a second 242-subcarrier RU  5174  having pilot tone positions at potential pilot tone positions d19, d20, d22, d25, d30, d33, d35, and d36, and a 484-subcarrier RU  5176  having pilot tone positions at potential pilot tone positions d1, d3, d4, d7, d12, d15, d17, d18, d19, d20, d22, d25, d30, d33, d35, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  51    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  51    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  52    illustrates a seventh option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the seventh option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  52    shows first to eighteenth 26-subcarrier RUs  5202  to  5240  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  52    shows first to eighth 52-subcarrier RUs  5242  to  5258  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  52    further shows a first 108-subcarrier RU  5262  having pilot tone positions at potential pilot tone positions d1, d2, d4, d5, d7, and d8, a second 108-subcarrier RU  5264  having pilot tone positions at potential pilot tone positions d11, d12, d14, d15, d17, and d18, a third 108-subcarrier RU  5266  having pilot tone positions at potential pilot tone positions d19, d20, d22, d23, d25, and d26, and a fourth 108-subcarrier RU  5268  having pilot tone positions at potential pilot tone positions d29, d30, d32, d33, d35, and d36. 
       FIG.  52    further shows a first 242-subcarrier RU  5272  having pilot tone positions at potential pilot tone positions d1, d2, d5, d8, d11, d14, d17, and d18, a second 242-subcarrier RU  5274  having pilot tone positions at potential pilot tone positions d19, d20, d23, d26, d29, d32, d35, and d36, and a 484-subcarrier RU  5276  having pilot tone positions at potential pilot tone positions d1, d2, d5, d8, d11, d14, d17, d18, d19, d20, d23, d26, d29, d32, d35, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  52    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  52    exhibit the mirror symmetry described above for  FIG.  46 A . 
       FIG.  53    illustrates an eighth option for pilot tone positions in Case 1 for a 40 MHz bandwidth. In an embodiment, the eighth option may use the first to thirty-sixth potential pilot tone positions d1 to d36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  53    shows first to eighteenth 26-subcarrier RUs  5302  to  5340  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  4602  to  4640  of  FIG.  46 A , respectively. 
       FIG.  53    shows first to eighth 52-subcarrier RUs  5342  to  5358  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  4642  to  4658  of  FIG.  46 A , respectively. 
       FIG.  53    further shows a first 108-subcarrier RU  5362  having pilot tone positions at potential pilot tone positions d1, d2, d4, d5, d7, and d8, a second 108-subcarrier RU  5364  having pilot tone positions at potential pilot tone positions d11, d12, d14, d15, d17, and d18, a third 108-subcarrier RU  5366  having pilot tone positions at potential pilot tone positions d19, d20, d22, d23, d25, and d26, and a fourth 108-subcarrier RU  5368  having pilot tone positions at potential pilot tone positions d29, d30, d32, d33, d35, and d36. 
       FIG.  53    further shows a first 242-subcarrier RU  5372  having pilot tone positions at potential pilot tone positions d1, d4, d5, d8, d11, d14, d15, and d18, a second 242-subcarrier RU  5374  having pilot tone positions at potential pilot tone positions d19, d22, d23, d26, d29, d32, d33, and d36, and a 484-subcarrier RU  5376  having pilot tone positions at potential pilot tone positions d1, d4, d5, d8, d11, d14, d15, d18, d19, d22, d23, d26, d29, d32, d33, and d36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  53    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  53    exhibit the mirror symmetry described above for  FIG.  46 A . 
     Case 2 embodies RU definition design with 26 subcarrier RUs, 52 subcarriers RUs, 106 or 108 subcarrier RUs, 242 subcarrier RUs, 484 subcarrier RUs, and 994 or 996 subcarrier RUs, wherein pairs of consecutive 26 subcarrier RUs line up (frequency wise) with 52 subcarrier RUs, except for the central 26 subcarrier RU within each 20 MHz bandwidth. 
     In  FIGS.  54 A to  58   , potential pilot tone positions are indicated by dotted lines extending vertically through the figures. Actual pilot tone for each RU are indicated by upward pointing arrows. A solid upward pointing arrow indicates a pilot tone with a fixed position. A dashed upward pointing arrow indicates a pilot tone having one of two positions, includes a first position to the left of a reference subcarrier and a second position to the right of a reference subcarrier, according to a design choice. In an embodiment, the reference subcarriers are center subcarriers of 13-subcarrier halves of 26-subcarrier RUs. 
       FIG.  54 A  illustrates a first option for pilot tone positions for a 20 MHz channel  5400  in Case 2. Pilot tone positions for each RU are selected from first to eighteenth potential pilot tone positions e1 to e18.  FIG.  54 B  shows Table 11, which lists the positions of each of the potential pilot tone positions e1 to e18 according to an embodiment. 
       FIG.  54 A  shows a first 26-subcarrier RU  5402  having pilot tone positions at potential pilot tone positions e1 and e2, a second 26-subcarrier RU  5404  having pilot tone positions at potential pilot tone positions e3 and e4, a third 26-subcarrier RU  5406  having pilot tone positions at potential pilot tone positions e5 and e6, a fourth 26-subcarrier RU  5408  having pilot tone positions at potential pilot tone positions e7 and e8, a fifth (center) 26-subcarrier RU  5410  having pilot tone positions at potential pilot tone positions e9 and e10, a sixth 26-subcarrier RU  5412  having pilot tone positions at potential pilot tone positions e11 and e12, a seventh 26-subcarrier RU  5414  having pilot tone positions at potential pilot tone positions e13 and e14, an eighth 26-subcarrier RU  5416  having pilot tone positions at potential pilot tone positions e15 and e16, and a ninth 26-subcarrier RU  5418  having pilot tone positions at potential pilot tone positions e17 and e18. 
       FIG.  54 A  further shows a first 52-subcarrier RU  5422  having pilot tone positions at potential pilot tone positions e1, e2, e3, and e4, a second 52-subcarrier RU  5424  having pilot tone positions at potential pilot tone positions e5, e6, e7, and e8, a third 52-subcarrier RU  5426  having pilot tone positions at potential pilot tone positions e11, e12, e13, and e14, and a fourth 52-subcarrier RU  5428  having pilot tone positions at potential pilot tone positions e15, e16, e17, and e18. 
       FIG.  54 A  further shows a first 106-subcarrier RU  5432  having pilot tone positions at potential pilot tone positions e1, e3, e5, and e7, a second 106-subcarrier RU  5434  having pilot tone positions at potential pilot tone positions e12, e14, e16, and e18, and a 242-subcarrier RU  5436  having pilot tone positions at potential pilot tone positions e1, e3, e5, e7, e12, e14, e16, and e18. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  54 A  has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. 
     Furthermore, each RU in a lower 10 MHz of the 20 MHz channel  5400  has pilot tone positions that are mirror symmetric with a corresponding mirrored RU (that is, an RU with a same number of subcarriers and a same offset from the center of the 20 MHz channel  5400 ) of an upper 10 MHz of the 20 MHz channel  5400 . For example, the pilot tone positions of the first 26-subcarrier RU  5402  are positioned to be mirror-symmetric to the pilot tone positions of the ninth 26-subcarrier RU  5418 , the pilot tone positions of the second 52-subcarrier RU  5424  are positioned to be mirror-symmetric to the pilot tone positions of the third 52-subcarrier RU  5426 , and so on. 
       FIG.  55    illustrates a second option for pilot tone positions in Case 2. In an embodiment, the second option may use the first to eighteenth potential pilot tone positions e1 to e18 as shown in Table 11 of  FIG.  54 B , but embodiments are not limited thereto. 
       FIG.  55    shows a first, second, third, and fourth 26-subcarrier RU  5502 ,  5504 ,  5506 , and  5508  having pilot tone positions at potential pilot tone positions e1 and e2, e3 and e4, e5 and e6, and e7 and e8, respectively, a fifth (center) 26-subcarrier RU  5510  having pilot tone positions at potential pilot tone positions e9 and e10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  5512 ,  5514 ,  5516 , and  5518  having pilot tone positions at potential pilot tone positions e11 and e12, e13 and e14, e15 and e16, and e17 and e18, respectively. 
       FIG.  55    further shows a first, second, third, and fourth 52-subcarrier RU  5522 ,  5524 ,  5526 , and  5528  having pilot tone positions at potential pilot tone positions e1, e2, e3, and e4, potential pilot tone positions e5, e6, e7, and e8, potential pilot tone positions e11, e12, e13, and e14, and potential pilot tone positions e15, e16, e17, and e18, respectively. 
       FIG.  55    further shows a first 106-subcarrier RU  5532  having pilot tone positions at potential pilot tone positions e1, e3, e6, and e8, a second 106-subcarrier RU  5534  having pilot tone positions at potential pilot tone positions e11, e13, e16, and e18, and a 242-subcarrier RU  5536  having pilot tone positions at potential pilot tone positions e1, e3, e6, e8, e11, e13, e16, and e18. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  55    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  55    exhibit the mirror symmetry described above for  FIG.  54 A . 
       FIG.  56    illustrates a third option for pilot tone positions in Case 2. In an embodiment, the third option may use the first to eighteenth potential pilot tone positions e1 to e18 as shown in Table 11 of  FIG.  54 B , but embodiments are not limited thereto. 
       FIG.  56    shows a first, second, third, and fourth 26-subcarrier RU  5602 ,  5604 ,  5606 , and  5608  having pilot tone positions at potential pilot tone positions e1 and e2, e3 and e4, e5 and e6, and e7 and e8, respectively, a fifth (center) 26-subcarrier RU  5610  having pilot tone positions at potential pilot tone positions e9 and e10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  5612 ,  5614 ,  5616 , and  5618  having pilot tone positions at potential pilot tone positions e11 and e12, e13 and e14, e15 and e16, and e17 and e18, respectively. 
       FIG.  56    further shows a first, second, third, and fourth 52-subcarrier RU  5622 ,  5624 ,  5626 , and  5628  having pilot tone positions at potential pilot tone positions e1, e2, e3, and e4, potential pilot tone positions e5, e6, e7, and e8, potential pilot tone positions e11, e12, e13, and e14, and potential pilot tone positions e15, e16, e17, and e18, respectively. 
       FIG.  56    further shows a first 106-subcarrier RU  5632  having pilot tone positions at potential pilot tone positions e2, e4, e6, and e8, a second 106-subcarrier RU  5634  having pilot tone positions at potential pilot tone positions e11, e13, e15, and e17, and a 242-subcarrier RU  5636  having pilot tone positions at potential pilot tone positions e2, e4, e6, e8, e11, e13, e15, and e17. 
     Each of the 52-, 106-, and 242-subcarrier RUs in  FIG.  56    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  56    exhibit the mirror symmetry described above for  FIG.  54 A . 
     Options 1, 2, and 3 for Case 2 as shown in  FIGS.  54 A,  55 , and  56   , respectively, represent pilot tone positions for a 20 MHz bandwidth and when a 106-subcarrier RU is used instead of a 108-subcarrier RU. The pilot tone positions within the 26-subcarrier RU can be the suggested pilot tone positions embodiments described in Design A section. 
     Resource Units (RUs) with larger sizes will have identical pilot frequency positions as the pilot tone positions for aggregated 26 subcarrier RUs. 
     The 106-subcarrier RUs respectively have 4 pilots within their RU allocations. However, there are potentially 8 pilot tone positions available for the 106 subcarrier RU. The options 1, 2, and 3 shown in  FIGS.  54 A,  55 , and  56   , respectively, are different alternatives to the pilot tone positions of the 106 subcarrier RUs and 242 subcarrier RU. The pilot tone positions for 106 subcarrier RU and 242 subcarrier RU are chosen from the set of potential positions stemming from the pilot tone positions of the 26 subcarrier RUs such that frequency diversity can be maximized (e.g. spacing between pilots are large). In case 2, the edge of the 106 subcarrier RU is lined up with 26 subcarrier and 52 subcarrier RUs. 
       FIG.  57    illustrates a fourth and a fifth option for pilot tone positions in Case 2. In an embodiment, the fourth and fifth options may each use the first to eighteenth potential pilot tone positions e1 to e18 as shown in Table 11 of  FIG.  54 B , but embodiments are not limited thereto. 
     The fourth and fifth option respectively have identical pilot tone positions for 26-, 52-, and 108-subcarrier RUs. Pilot tone positions for an option-four 242-subcarrier RU  5736 A of option four are different from pilot tone positions for an option-five 242-subcarrier RU  5736 B of option five. 
       FIG.  57    shows a first, second, third, and fourth 26-subcarrier RU  5702 ,  5704 ,  5706 , and  5708  having pilot tone positions at potential pilot tone positions e1 and e2, e3 and e4, e5 and e6, and e7 and e8, respectively, a fifth (center) 26-subcarrier RU  5710  having pilot tone positions at potential pilot tone positions e9 and e10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  5712 ,  5714 ,  5716 , and  5718  having pilot tone positions at potential pilot tone positions e11 and e12, e13 and e14, e15 and e16, and e17 and e18, respectively. 
       FIG.  57    further shows a first, second, third, and fourth 52-subcarrier RU  5722 ,  5724 ,  5726 , and  5728  having pilot tone positions at potential pilot tone positions e1, e2, e3, and e4, potential pilot tone positions e5, e6, e7, and e8, potential pilot tone positions e11, e12, e13, and e14, and potential pilot tone positions e15, e16, e17, and e18, respectively. 
       FIG.  57    further shows a first 108-subcarrier RU  5732  having pilot tone positions at potential pilot tone positions e1, e3, e4, e5, e6, and e8, and a second 108-subcarrier RU  5734  having pilot tone positions at potential pilot tone positions e11, e13, e14, e15, e16, and e18. 
       FIG.  57    further shows the option-four 242-subcarrier RU  5736 A of option four having pilot tone positions at potential pilot tone positions e1, e3, e6, e8, e11, e13, e16, and e18, and the option-five 242-subcarrier RU  5736 A of option five having pilot tone positions at potential pilot tone positions e1, e4, e5, e8, e11, e14, e15, and e18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  57    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  57    exhibit the mirror symmetry described above for  FIG.  54 A . 
       FIG.  58    illustrates a sixth option for pilot tone positions in Case 2. In an embodiment, the sixth option may use the first to eighteenth potential pilot tone positions e1 to e18 as shown in Table 11 of  FIG.  54 B , but embodiments are not limited thereto. 
       FIG.  58    shows a first, second, third, and fourth 26-subcarrier RU  5802 ,  5804 ,  5806 , and  5808  having pilot tone positions at potential pilot tone positions e1 and e2, e3 and e4, e5 and e6, and e7 and e8, respectively, a fifth (center) 26-subcarrier RU  5810  having pilot tone positions at potential pilot tone positions e9 and e10, and sixth, seventh, eighth, and ninth 26-subcarrier RU  5812 ,  5814 ,  5816 , and  5818  having pilot tone positions at potential pilot tone positions e11 and e12, e13 and e14, e15 and e16, and e17 and e18, respectively. 
       FIG.  58    further shows a first, second, third, and fourth 52-subcarrier RU  5822 ,  5824 ,  5826 , and  5828  having pilot tone positions at potential pilot tone positions e1, e2, e3, and e4, potential pilot tone positions e5, e6, e7, and e8, potential pilot tone positions e11, e12, e13, and e14, and potential pilot tone positions e15, e16, e17, and e18, respectively. 
       FIG.  58    further shows a first 108-subcarrier RU  5832  having pilot tone positions at potential pilot tone positions e1, e2, e4, e5, e7, and e8, a second 108-subcarrier RU  5834  having pilot tone positions at potential pilot tone positions e11, e12, e14, e15, e11, and e18, and a 242-subcarrier RU  5836  having pilot tone positions at potential pilot tone positions e1, e4, e5, e8, e11, e14, e15, and e18. 
     Each of the 52-, 108-, and 242-subcarrier RUs in  FIG.  58    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  58    exhibit the mirror symmetry described above for  FIG.  54 A . 
     Options 4, 5, and 6 of Case 2 shown in  FIGS.  57 ,  57 , and  58   , respectively, represent pilot tone positions for a 20 MHz bandwidth and when a 108-subcarrier RU is used instead of a 106-subcarrier RU. The pilot tone positions within the 26-subcarrier RUs can be the suggested pilot tone positions embodiments described in Design A section. 
     Resource Units (RUs) with larger sizes will have identical pilot frequency positions as the pilot tone positions for aggregated 26 subcarrier RUs. 108-subcarrier RUs have 6 pilots within their respective RU allocation and the 242-subcarrier RU has 8 pilots within its RU allocation. However, there are potentially 8 potential pilot tone positions available for the 108-subcarrier RU and 18 potential pilot tone positions for the 242 subcarrier RU. 
     The options 4, 5, and 6 are different alternatives for the pilot tone positions of the 108-subcarrier RUs and 242-subcarrier RU. The pilot tone positions for 108 subcarrier RU and 242 subcarrier RU are chosen from the set of potential positions stemming from the pilot tone positions of the 26 subcarrier RUs such that frequency diversity can be maximized (e.g. spacing between pilots are large). 
     Pilot tone positions of 40, 80, and 160 MHz bandwidths for case 2 are defined using the concatenated (relative) pilot tone positions of the 20 MHz bandwidth pilot tone positions. As in Case 1, central 26-subcarrier RUs in left and right 20 MHz portions of a 40 MHz bandwidth are respectively replaced by an even or odd 26-subcarrier RU. An 80 MHz bandwidth will use two concatenated 40 MHz pilot tone positions with a single central 26-subcarrier RU in the middle.  FIGS.  59  to  65    illustrate embodiments of pilot tone positions for a 40 MHz bandwidth for Case 2. 
     In  FIGS.  59  to  65   , potential pilot tone positions are indicated by dotted lines extending vertically through the figures. Actual pilot tone for each RU are indicated by upward pointing arrows. A solid upward pointing arrow indicates a pilot tone with a fixed position. A dashed upward pointing arrow indicates a pilot tone having one of two positions, includes a first position to the left of a reference subcarrier and a second position to the right of a reference subcarrier, according to a design choice. In an embodiment, the reference subcarriers are center subcarriers of 13-subcarrier halves of 26-subcarrier RUs. 
       FIG.  59 A  illustrates a first option for pilot tone positions in Case 2 for a 40 MHz channel  5900 . Pilot tone positions for each RU are selected from first to thirty-sixth potential pilot tone positions f1 to f36.  FIG.  59 B  shows Table 12, which lists positions of each of the potential pilot tone positions f1 to f36 according to an embodiment, but embodiments are not limited thereto. The order of the potential pilot tone positions f1 to f36 in Table 10 goes down the left sub-table and up the right sub-table to better illustrate the mirror symmetry of the potential pilot tone positions f1 to f36. Therefore, the pilot tone positions f1 to f36 meets the following relationship: f1=−f36, f2=435, f3=−f34, f4=−f33, f5=−f32, f6=−f31, f7=−f30, f8=−f29, f9=−f28, f10=−f27, f11=−f26, f12=−f25, f13=−f24, f14=−f23, f15=−f22, f16=−f21, f17=−f20, and f18=−f19 (f36&gt;f35&gt;f34&gt;f33&gt;f32&gt;f31&gt;f30&gt;f29&gt;f28&gt;f27&gt;f26&gt;f25&gt;f24&gt;f23&gt;f22&gt;f21&gt;f20&gt;f19&gt;f18&gt;f17&gt;f16&gt;f15&gt;f14&gt;f13&gt;f12&gt;f11&gt;f10&gt;f9&gt;f8&gt;f7&gt;f6&gt;f5&gt;f4&gt;f3&gt;f2&gt;f1). 
       FIG.  59 A  shows, on the left side, a first 26-subcarrier RU  5902  having pilot tone positions at potential pilot tone positions f1 and f2, a second 26-subcarrier RU  5904  having pilot tone positions at potential pilot tone positions f3 and f4, a third 26-subcarrier RU  5906  having pilot tone positions at potential pilot tone positions f5 and f6, a fourth 26-subcarrier RU  5908  having pilot tone positions at potential pilot tone positions f7 and f8, a fifth 26-subcarrier RU  5912  having pilot tone positions at potential pilot tone positions f9 and f10, a sixth 26-subcarrier RU  5914  having pilot tone positions at potential pilot tone positions f11 and f12, a seventh 26-subcarrier RU  5916  having pilot tone positions at potential pilot tone positions f13 and f14, an eighth 26-subcarrier RU  5918  having pilot tone positions at potential pilot tone positions f15 and f16, and a ninth 26-subcarrier RU  5920  having pilot tone positions at potential pilot tone positions f17 and f18. 
       FIG.  59 A  shows, on the right side, a tenth 26-subcarrier RU  5922  having pilot tone positions at potential pilot tone positions f19 and f20, an eleventh 26-subcarrier RU  5924  having pilot tone positions at potential pilot tone positions f21 and f22, a twelfth 26-subcarrier RU  5926  having pilot tone positions at potential pilot tone positions f23 and f24, a thirteenth 26-subcarrier RU  5928  having pilot tone positions at potential pilot tone positions f25 and f26, a fourteenth 26-subcarrier RU  5932  having pilot tone positions at potential pilot tone positions f27 and f28, a fifteenth 26-subcarrier RU  5934  having pilot tone positions at potential pilot tone positions f29 and f30, a sixteenth 26-subcarrier RU  5936  having pilot tone positions at potential pilot tone positions f31 and f32, a seventeenth 26-subcarrier RU  5938  having pilot tone positions at potential pilot tone positions f33 and f34, and an eighteenth 26-subcarrier RU  5940  having pilot tone positions at potential pilot tone positions f35 and f36. 
     In each of the 26-subcarrier RUs  5902  to  5940 , respective first and second pilot tone positions correspond to potential pilot tone positions covered by the respective 26 subcarriers. 
       FIG.  59 A  further shows, on the left side, a first 52-subcarrier RU  5942  having pilot tone positions at potential pilot tone positions f1, f2, f3, and f4, a second 52-subcarrier RU  5944  having pilot tone positions at potential pilot tone positions f5, f6, f7, and f8, a third 52-subcarrier RU  5959  having pilot tone positions at potential pilot tone positions f11, f12, f13, and f14, and a fourth 52-subcarrier RU  5948  having pilot tone positions at potential pilot tone positions f15, f16, f17, and f18. 
       FIG.  59 A  further shows, on the right side, a fifth 52-subcarrier RU  5952  having pilot tone positions at potential pilot tone positions f19, f22, f21, and f22 a sixth 52-subcarrier RU  5954  having pilot tone positions at potential pilot tone positions f23, f24, f25, and f26, a seventh 52-subcarrier RU  5956  having pilot tone positions at potential pilot tone positions f29, f30, f31, and f32, and an eighth 52-subcarrier RU  5958  having pilot tone positions at potential pilot tone positions f33, f34, f35, and f36. 
     In each of the 52-subcarrier RUs  5942  to  5958 , respective first, second, third, and fourth pilot tone positions correspond to potential pilot tone positions covered by the respective 52 subcarriers. 
       FIG.  59 A  further shows a first 106-subcarrier RU  5962  having pilot tone positions at potential pilot tone positions f1, f3, f6, and f8, a second 106-subcarrier RU  5964  having pilot tone positions at potential pilot tone positions f11, f13, f16, and f18, a third 106-subcarrier RU  5966  having pilot tone positions at potential pilot tone positions f19, f21, f24, and f26, and a fourth 106-subcarrier RU  5968  having pilot tone positions at potential pilot tone positions f29, f31, f34, and f36. In each of the 106-subcarrier RUs  5962  to  5968 , a first pilot tone position has an index corresponding to a lowest index among potential pilot tone positions covered by the RU, a second pilot tone position is spaced two potential pilot tone positions away from the first pilot tone position, a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, and a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position. 
       FIG.  59 A  further shows a first 242-subcarrier RU  5972  having pilot tone positions at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, and f18, a second 242-subcarrier RU  5974  having pilot tone positions at potential pilot tone positions f19, f21, f24, f26, f29, f31, f34, and f36. In each of the first and second 242-subcarrier RUs  5972  and  5964 , a first pilot tone position has a lowest index among potential pilot tone positions covered by the 242 subcarriers, a second pilot tone position is spaced two potential pilot tone positions away from the first pilot tone position, a third pilot tone position is spaced three potential pilot tone positions away from the second pilot tone position, a fourth pilot tone position is spaced two potential pilot tone positions away from the third pilot tone position, a fifth pilot tone position is spaced three potential pilot tone positions away from the fourth pilot tone position, a sixth pilot tone position is spaced two potential pilot tone positions away from the fifth pilot tone position, a seventh pilot tone position is spaced three potential pilot tone positions away from the sixth pilot tone position, and an eighth pilot tone position is spaced two potential pilot tone positions away from the seventh pilot tone position. 
       FIG.  59 A  further shows a 484-subcarrier RU  5976  having pilot tone positions at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, f18, f19, f21, f24, f26, f29, f31, f34, and f36. The 484-subcarrier RU  5976  has a first pilot tone position having a lowest index among potential pilot tone positions covered by the 242 subcarriers, and second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, and sixteenth pilot tone positions spaced 2, 3, 2, 3, 2, 3, 2, 1, 2, 3, 2, 3, 2, 3, 2 potential pilot tone positions away from the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, and fifteenth pilot tone position, respectively. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  59 A  has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. 
     Furthermore, each RU in a lower 20 MHz of the 40 MHz channel  5900  has pilot tone positions that are mirror symmetric with a corresponding mirrored RU (that is, an RU with a same number of subcarriers and a same offset from the center of the 40 MHz channel  4900 ) of an upper 20 MHz of the 40 MHz channel  4900 . For example, the pilot tone positions of the first 26-subcarrier RU  5902  are positioned to be mirror-symmetric to the pilot tone positions of the eighteenth 26-subcarrier RU  5940 , the pilot tone positions of the second 52-subcarrier RU  5944  are positioned to be mirror-symmetric to the pilot tone positions of the seventh 52-subcarrier RU  5956 , and so on. 
       FIG.  60    illustrates a second option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the second option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  60    shows first to eighteenth 26-subcarrier RUs  6002  to  6040  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  60    shows first to eighth 52-subcarrier RUs  6042  to  6058  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  60    further shows a first 106-subcarrier RU  6062  having pilot tone positions at potential pilot tone positions f1, f3, f5, and f7, a second 106-subcarrier RU  6064  having pilot tone positions at potential pilot tone positions f11, f13, f15, and f17, a third 106-subcarrier RU  6066  having pilot tone positions at potential pilot tone positions f20, f22, f24, and f26, and a fourth 106-subcarrier RU  6068  having pilot tone positions at potential pilot tone positions f30, f32, f34, and f36. 
       FIG.  60    further shows a first 242-subcarrier RU  6072  having pilot tone positions at potential pilot tone positions f1, f3, f5, f7, f11, f13, f15, and f17, a second 242-subcarrier RU  6074  having pilot tone positions at potential pilot tone positions f20, f22, f24, f26, f30, f32, f34, and f36, and a 484-subcarrier RU  6076  having pilot tone positions at potential pilot tone positions f1, f3, f5, f7, f11, f13, f15, f17, f20, f22, f24, f26, f30, f32, f34, and f36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  60    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  60    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIG.  61    illustrates a third option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the third option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  61    shows first to eighteenth 26-subcarrier RUs  6102  to  6140  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  61    shows first to eighth 52-subcarrier RUs  6142  to  6158  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  61    further shows a first 106-subcarrier RU  6162  having pilot tone positions at potential pilot tone positions f2, f4, f6, and f8, a second 106-subcarrier RU  6164  having pilot tone positions at potential pilot tone positions f12, f14, f16, and f18, a third 106-subcarrier RU  6166  having pilot tone positions at potential pilot tone positions f19, f21, f23, and f25, and a fourth 106-subcarrier RU  6168  having pilot tone positions at potential pilot tone positions f29, f31, f33, and f35. 
       FIG.  61    further shows a first 242-subcarrier RU  6172  having pilot tone positions at potential pilot tone positions f2, f4, f6, f8, f12, f14, f16, and f18, a second 242-subcarrier RU  6174  having pilot tone positions at potential pilot tone positions f19, f21, f23, f25, f29, f31, f33, and f35, and a 484-subcarrier RU  6176  having pilot tone positions at potential pilot tone positions f2, f4, f6, f8, f12, f14, f16, f18, f19, f21, f23, f25, f29, f31, f33, and f35. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  61    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  61    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIG.  62    illustrates a fourth option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the fourth option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  62    shows first to eighteenth 26-subcarrier RUs  6202  to  6240  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  62    shows first to eighth 52-subcarrier RUs  6242  to  6258  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  62    further shows a first 106-subcarrier RU  6262  having pilot tone positions at potential pilot tone positions f2, f4, f6, and f8, a second 106-subcarrier RU  6264  having pilot tone positions at potential pilot tone positions f11, f13, f15, and f17, a third 106-subcarrier RU  6266  having pilot tone positions at potential pilot tone positions f20, f22, f24, and f26, and a fourth 106-subcarrier RU  6268  having pilot tone positions at potential pilot tone positions f29, f31, f33, and f35. 
       FIG.  62    further shows a first 242-subcarrier RU  6272  having pilot tone positions at potential pilot tone positions f2, f4, f6, f8, f11, f13, f15, and f17, a second 242-subcarrier RU  6274  having pilot tone positions at potential pilot tone positions f20, f22, f24, f26, f29, f31, f33, and f35, and a 484-subcarrier RU  6276  having pilot tone positions at potential pilot tone positions f2, f4, f6, f8, f11, f13, f15, f17, f20, f22, f24, f26, f29, f31, f33, and f35. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  62    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  62    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIG.  63    illustrates a fifth option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the fifth option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  63    shows first to eighteenth 26-subcarrier RUs  6302  to  6340  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  63    shows first to eighth 52-subcarrier RUs  6342  to  6358  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  63    further shows a first 108-subcarrier RU  6362  having pilot tone positions at potential pilot tone positions f1, f3, f4, f5, f6, and f8, a second 108-subcarrier RU  6364  having pilot tone positions at potential pilot tone positions f11, f13, f14, f15, f16, and f18, a third 108-subcarrier RU  6366  having pilot tone positions at potential pilot tone positions f19, f21, f22, f23, f24, and f26, and a fourth 108-subcarrier RU  6368  having pilot tone positions at potential pilot tone positions f29, f31, f32, f33, f34, and f36. 
       FIG.  63    further shows a first 242-subcarrier RU  6372  having pilot tone positions at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, and f18, a second 242-subcarrier RU  6374  having pilot tone positions at potential pilot tone positions f19, f21, f24, f26, f29, f31, f34, and f36, and a 484-subcarrier RU  6376  having pilot tone positions at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, f18, f19, f21, f24, f26, f29, f31, f34, and f36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  63    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  63    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIG.  64    illustrates a sixth option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the sixth option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  64    shows first to eighteenth 26-subcarrier RUs  6402  to  6440  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  64    shows first to eighth 52-subcarrier RUs  6442  to  6458  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  64    further shows a first 108-subcarrier RU  6462  having pilot tone positions at potential pilot tone positions f1, f3, f4, f5, f6, and f8, a second 108-subcarrier RU  6464  having pilot tone positions at potential pilot tone positions f11, f13, f14, f15, f16, and f18, a third 108-subcarrier RU  6466  having pilot tone positions at potential pilot tone positions f19, f21, f22, f23, f24, and f26, and a fourth 108-subcarrier RU  6468  having pilot tone positions at potential pilot tone positions f29, f31, f32, f33, f34, and f36. 
       FIG.  64    further shows a first 242-subcarrier RU  6472  having pilot tone positions at potential pilot tone positions f1, f4, f5, f8, f11, f14, f15, and f18, a second 242-subcarrier RU  6474  having pilot tone positions at potential pilot tone positions f19, f22, f23, f26, f29, f32, f33, and f36, and a 484-subcarrier RU  6476  having pilot tone positions at potential pilot tone positions f1, f4, f5, f8, f11, f14, f15, f18, f19, f22, f23, f26, f29, f32, f33, and f36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  64    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  64    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIG.  65    illustrates a seventh option for pilot tone positions in Case 2 for a 40 MHz bandwidth. In an embodiment, the seventh option may use the first to thirty-sixth potential pilot tone positions f1 to f36 as shown in Table 9 of  FIG.  33 B , but embodiments are not limited thereto. 
       FIG.  65    shows first to eighteenth 26-subcarrier RUs  6502  to  6540  having pilot tone positions at the same potential pilot tone positions as the first to eighteenth 26-subcarrier RU  5902  to  5940  of  FIG.  59 A , respectively. 
       FIG.  65    shows first to eighth 52-subcarrier RUs  6542  to  6558  having pilot tone positions at the same potential pilot tone positions as the first to eighth 52-subcarrier RUs  5942  to  5958  of  FIG.  59 A , respectively. 
       FIG.  65    further shows a first 108-subcarrier RU  6562  having pilot tone positions at potential pilot tone positions f1, f2, f4, f5, f7, and f8, a second 108-subcarrier RU  6564  having pilot tone positions at potential pilot tone positions f11, f12, f14, f15, f17, and f18, a third 108-subcarrier RU  6566  having pilot tone positions at potential pilot tone positions f19, f20, f22, f23, f25, and f26, and a fourth 108-subcarrier RU  6568  having pilot tone positions at potential pilot tone positions f29, f30, f32, f33, f35, and f36. 
       FIG.  65    further shows a first 242-subcarrier RU  6572  having pilot tone positions at potential pilot tone positions f1, f4, f5, f8, f11, f14, f15, and f18, a second 242-subcarrier RU  6574  having pilot tone positions at potential pilot tone positions f19, f22, f23, f26, f29, f32, f33, and f36, and a 484-subcarrier RU  6576  having pilot tone positions at potential pilot tone positions f1, f4, f5, f8, f11, f14, f15-f18, f19, f22, f23, f26, f29, f32, f33, and f36. 
     Each of the 52-, 106-, 242-, and 484-subcarrier RUs in  FIG.  65    has pilot tone positions selected from among the pilot tone positions used by the RUs having fewer subcarriers that occupy the same bandwidth. Furthermore, the pilot tone positions of  FIG.  65    exhibit the mirror symmetry described above for  FIG.  59 A . 
       FIGS.  66  to  78    represent example embodiments of the relative pilot tone positions within the 52 subcarrier RU for different cases and options. The pilot tone position positions shown in  FIGS.  66  to  78    can be used in both Design A and B, although it was drawn to represent an example of the nested pilot tone positions (i.e. design B) for the 52 subcarrier RUs. 
     In  FIGS.  66  to  78   , hash marks along the horizontal access correspond to subcarriers that respectively do not correspond to subcarriers having energy in a 2×HE-LTF. Upward pointing arrows along the horizontal access respectively correspond to subcarriers that correspond to the subcarriers having energy in the 2×HE-LTF. 
       FIG.  66    illustrates an option for pilot tone positions in Case 1, according to embodiments.  FIG.  66    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  66    illustrates a first embodiment including a 52-subcarrier RU  6610  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  66    also illustrates pilot tone positions of a first 26-subcarrier RU  6600 A and a second 26-subcarrier RU  6600 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6610  and that would be even and odd RUs, respectively, in the first embodiment. 
       FIG.  66    illustrates a second embodiment including a 52-subcarrier RU  6610  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  66    also illustrates pilot tone positions of a first 26-subcarrier RU  6600 A and a second 26-subcarrier RU  6600 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6610  and that would be odd and even RUs, respectively, in the second embodiment. 
     In  FIG.  66   , a lowest-indexed subcarrier of the 52-subcarrier RU  6610  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  6600 A. A null or reserved subcarrier  6620  is disposed between the first and second 26-subcarrier RUs  6600 A and  6600 B. 
     The first 26-subcarrier RU  6600 A includes first and second pilot tone positions  6602  and  6604 . The first pilot tone position  6602  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the first 26-subcarrier RU  6600 A. The second pilot tone position  6604  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6602  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  6600 A. 
     The second 26-subcarrier RU  6600 B includes third and fourth pilot tone positions  6606  and  6608 . The third pilot tone position  6606  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  6600 B. The fourth pilot tone position  6608  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6606  and spaced 6 subcarriers away from a highest-indexed subcarrier of the second 26-subcarrier RU  6600 B. 
     The 52-subcarrier RU  6610  includes fifth, sixth, seventh, and eighth pilot tone positions  6612 ,  6614 ,  6616 , and  6618 . The fifth, sixth, seventh, and eighth pilot tone positions  6612 ,  6614 ,  6616 , and  6618  of the 52-subcarrier RU  6610  respectively correspond to the first, second, third, and fourth pilot tone positions  6602 ,  6604 ,  6606 , and  6608  of the first and second 26-subcarrier RUs  6600 A and  6600 B. 
     The fifth pilot tone position  6612  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  6610 . The sixth pilot tone position  6614  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  6612 . The seventh pilot tone position  6614  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  6612 . The eighth pilot tone position  6618  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  6616  and spaced 5 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  6610 . 
       FIG.  66    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  6600 A (which may be odd or even) and the second 26-subcarrier RU  6600 B (which may be even or odd, respectively).  FIG.  66    also illustrates a nested design in which the 52-subcarrier RU  6610  uses the same pilot tone positions as the first and second RUs  6600 A and  6600 B, but embodiments are not limited thereto. 
       FIG.  67    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  67    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  67    illustrates a first embodiment including a 52-subcarrier RU  6710  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  67    also illustrates pilot tone positions of a first 26-subcarrier RU  6700 A and a second 26-subcarrier RU  6700 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6710  and that would be odd and even RUs, respectively, in the first embodiment. 
       FIG.  67    illustrates a second embodiment including a 52-subcarrier RU  6710  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  67    also illustrates pilot tone positions of a first 26-subcarrier RU  6700 A and a second 26-subcarrier RU  6700 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6710  and that would be even and odd RUs, respectively, in the second embodiment. 
     In  FIG.  67   , a highest-indexed subcarrier of the 52-subcarrier RU  6710  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  6700 B. A null or reserved subcarrier  6720  is disposed between the first and second 26-subcarrier RUs  6700 A and  6700 B. 
     The first 26-subcarrier RU  6700 A includes first and second pilot tone positions  6702  and  6704 . The first pilot tone position  6702  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  6700 A. The second pilot tone position  6704  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6702  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  6700 A. 
     The second 26-subcarrier RU  6700 B includes third and fourth pilot tone positions  6706  and  6708 . The third pilot tone position  6706  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  6700 B. The fourth pilot tone position  6708  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6706  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  6700 B. 
     The 52-subcarrier RU  6710  includes fifth, sixth, seventh, and eighth pilot tone positions  6712 ,  6714 ,  6716 , and  6718 . The fifth, sixth, seventh, and eighth pilot tone positions  6712 ,  6714 ,  6716 , and  6718  of the 52-subcarrier RU  6710  respectively correspond to the first, second, third, and fourth pilot tone positions  6702 ,  6704 ,  6706 , and  6708  of the first and second 26-subcarrier RUs  6700 A and  6700 B. 
     The fifth pilot tone position  6712  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the 52-subcarrier RU  6710 . The sixth pilot tone position  6714  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  6712 . The seventh pilot tone position  6714  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  6712 . The eighth pilot tone position  6718  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  6716  and spaced 6 subcarriers away from the highest-indexed subcarrier of the 52-subcarrier RU  6710 . 
       FIG.  67    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  6700 A (which may be odd or even) and the second 26-subcarrier RU  6700 B (which may be even or odd, respectively).  FIG.  67    also illustrates a nested design in which the 52-subcarrier RU  6710  uses the same pilot tone positions as the first and second RUs  6700 A and  6700 B, but embodiments are not limited thereto. 
       FIG.  68    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  68    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  68    illustrates a first embodiment including a 52-subcarrier RU  6810  being an even RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  68    also illustrates pilot tone positions of a first 26-subcarrier RU  6800 A and a second 26-subcarrier RU  6800 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6810  and that would be even and odd RUs, respectively, in the first embodiment. 
       FIG.  68    may also be seen illustrating a second embodiment including a 52-subcarrier RU  6810  being an odd RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  68    also illustrates pilot tone positions of a first 26-subcarrier RU  6800 A and a second 26-subcarrier RU  6800 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6810  and that would be odd and even RUs, respectively, in the second embodiment. 
     In  FIG.  68   , a lowest-indexed subcarrier of the 52-subcarrier RU  6810  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  6800 A. A null or reserved subcarrier  6820  is disposed between the first and second 26-subcarrier RUs  6800 A and  6800 B. 
     The first 26-subcarrier RU  6800 A includes first and second pilot tone positions  6802  and  6804 . The first pilot tone position  6802  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the first 26-subcarrier RU  6800 A. The second pilot tone position  6804  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6802  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  6800 A. 
     The second 26-subcarrier RU  6800 B includes third and fourth pilot tone positions  6806  and  6808 . The third pilot tone position  6806  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  6800 B. The fourth pilot tone position  6808  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6806  and spaced 6 subcarriers away from a highest-indexed subcarrier of the second 26-subcarrier RU  6800 B. 
     The 52-subcarrier RU  6810  includes fifth, sixth, seventh, and eighth pilot tone positions  6812 ,  6814 ,  6816 , and  6818 . The fifth, sixth, seventh, and eighth pilot tone positions  6812 ,  6814 ,  6816 , and  6818  of the 52-subcarrier RU  6810  respectively correspond to the first, second, third, and fourth pilot tone positions  6802 ,  6804 ,  6806 , and  6808  of the first and second 26-subcarrier RUs  6800 A and  6800 B. 
     The fifth pilot tone position  6812  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  6810 . The sixth pilot tone position  6814  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  6812 . The seventh pilot tone position  6814  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  6812 . The eighth pilot tone position  6818  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  6816  and spaced 5 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  6810 . 
       FIG.  68    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  6800 A (which may be odd or even) and the second 26-subcarrier RU  6800 B (which may be even or odd, respectively).  FIG.  68    also illustrates a nested design in which the 52-subcarrier RU  6810  uses the same pilot tone positions as the first and second RUs  6800 A and  6800 B, but embodiments are not limited thereto. 
       FIG.  69    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  69    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  69    illustrates a first embodiment including a 52-subcarrier RU  6910  being an even RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  69    also illustrates pilot tone positions of a first 26-subcarrier RU  6900 A and a second 26-subcarrier RU  6900 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6910  and that would be odd and even RUs, respectively, in the first embodiment. 
       FIG.  69    illustrates a second embodiment including a 52-subcarrier RU  6910  being an odd RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  69    also illustrates pilot tone positions of a first 26-subcarrier RU  6900 A and a second 26-subcarrier RU  6900 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  6910  and that would be even and odd RUs, respectively, in the second embodiment. 
     In  FIG.  69   , a highest-indexed subcarrier of the 52-subcarrier RU  6910  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  6900 B. A null or reserved subcarrier  6920  is disposed between the first and second 26-subcarrier RUs  6900 A and  6900 B. 
     The first 26-subcarrier RU  6900 A includes first and second pilot tone positions  6902  and  6904 . The first pilot tone position  6902  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  6900 A. The second pilot tone position  6904  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6902  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  6900 A. 
     The second 26-subcarrier RU  6900 B includes third and fourth pilot tone positions  6906  and  6908 . The third pilot tone position  6906  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  6900 B. The fourth pilot tone position  6908  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  6906  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  6900 B. 
     The 52-subcarrier RU  6910  includes fifth, sixth, seventh, and eighth pilot tone positions  6912 ,  6914 ,  6916 , and  6918 . The fifth, sixth, seventh, and eighth pilot tone positions  6912 ,  6914 ,  6916 , and  6918  of the 52-subcarrier RU  6910  respectively correspond to the first, second, third, and fourth pilot tone positions  6902 ,  6904 ,  6906 , and  6908  of the first and second 26-subcarrier RUs  6900 A and  6900 B. 
     The fifth pilot tone position  6912  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the 52-subcarrier RU  6910 . The sixth pilot tone position  6914  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  6912 . The seventh pilot tone position  6914  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  6912 . The eighth pilot tone position  6918  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  6916  and spaced 6 subcarriers away from the highest-indexed subcarrier of the 52-subcarrier RU  6910 . 
       FIG.  69    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  6900 A (which may be odd or even) and the second 26-subcarrier RU  6900 B (which may be even or odd, respectively).  FIG.  69    also illustrates a nested design in which the 52-subcarrier RU  6910  uses the same pilot tone positions as the first and second RUs  6900 A and  6900 B, but embodiments are not limited thereto. 
       FIG.  70    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  70    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  70    illustrates a first embodiment including a 52-subcarrier RU  7010  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  70    also illustrates pilot tone positions of a first 26-subcarrier RU  7000 A and a second 26-subcarrier RU  7000 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7010  and that would be odd and even RUs, respectively, in the first embodiment. 
       FIG.  70    illustrates a second embodiment including a 52-subcarrier RU  7010  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  70    also illustrates pilot tone positions of a first 26-subcarrier RU  7000 A and a second 26-subcarrier RU  7000 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7010  and that would be even and odd RUs, respectively, in the second embodiment. 
     In  FIG.  70   , a highest-indexed subcarrier of the 52-subcarrier RU  7010  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7000 B. A null or reserved subcarrier  7020  is disposed between the first and second 26-subcarrier RUs  7000 A and  7000 B. 
     The first 26-subcarrier RU  7000 A includes first and second pilot tone positions  7002  and  7004 . The first pilot tone position  7002  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7000 A. The second pilot tone position  7004  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7002  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7000 A. 
     The second 26-subcarrier RU  7000 B includes third and fourth pilot tone positions  7006  and  7008 . The third pilot tone position  7006  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7000 B. The fourth pilot tone position  7008  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7006  and spaced 5 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7000 B. 
     The 52-subcarrier RU  7010  includes fifth, sixth, seventh, and eighth pilot tone positions  7012 ,  7014 ,  7016 , and  7018 . The fifth, sixth, seventh, and eighth pilot tone positions  7012 ,  7014 ,  7016 , and  7018  of the 52-subcarrier RU  7010  respectively correspond to the first, second, third, and fourth pilot tone positions  7002 ,  7004 ,  7006 , and  7008  of the first and second 26-subcarrier RUs  7000 A and  7000 B. 
     The fifth pilot tone position  7012  corresponds to a subcarrier spaced 4 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7010 . The sixth pilot tone position  7014  corresponds to a subcarrier separated by 13 subcarriers from the fifth pilot tone position  7012 . The seventh pilot tone position  7014  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  7012 . The eighth pilot tone position  7018  corresponds to a subcarrier separated by 13 subcarriers from the seventh pilot tone position  7016  and spaced 5 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7010 . 
       FIG.  70    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  7000 A (which may be odd or even) and the second 26-subcarrier RU  7000 B (which may be even or odd, respectively).  FIG.  70    also illustrates a nested design in which the 52-subcarrier RU  7010  uses the same pilot tone positions as the first and second RUs  7000 A and  7000 B, but embodiments are not limited thereto. 
       FIG.  71    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  71    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  71    illustrates a first embodiment including a 52-subcarrier RU  7110  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  71    also illustrates pilot tone positions of a first 26-subcarrier RU  7100 A and a second 26-subcarrier RU  7100 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7110  and that would be even and odd RUs, respectively, in the first embodiment. 
       FIG.  71    illustrates a second embodiment including a 52-subcarrier RU  7110  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  71    also illustrates pilot tone positions of a first 26-subcarrier RU  7100 A and a second 26-subcarrier RU  7100 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7110  and that would be odd and even RUs, respectively, in the second embodiment. 
     In  FIG.  71   , a lowest-indexed subcarrier of the 52-subcarrier RU  7110  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7100 A. A null or reserved subcarrier  7120  is disposed between the first and second 26-subcarrier RUs  7100 A and  7100 B. 
     The first 26-subcarrier RU  7100 A includes first and second pilot tone positions  7102  and  7104 . The first pilot tone position  7102  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the first 26-subcarrier RU  7100 A. The second pilot tone position  7104  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7102  and spaced 5 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7100 A. 
     The second 26-subcarrier RU  7100 B includes third and fourth pilot tone positions  7106  and  7108 . The third pilot tone position  7106  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7100 B. The fourth pilot tone position  7108  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7106  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7100 B. 
     The 52-subcarrier RU  7110  includes fifth, sixth, seventh, and eighth pilot tone positions  7112 ,  7114 ,  7116 , and  7118 . The fifth, sixth, seventh, and eighth pilot tone positions  7112 ,  7114 ,  7116 , and  7118  of the 52-subcarrier RU  7110  respectively correspond to the first, second, third, and fourth pilot tone positions  7102 ,  7104 ,  7106 , and  7108  of the first and second 26-subcarrier RUs  7100 A and  7100 B. 
     The fifth pilot tone position  7112  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7110 . The sixth pilot tone position  7114  corresponds to a subcarrier separated by 13 subcarriers from the fifth pilot tone position  7112 . The seventh pilot tone position  7114  corresponds to a subcarrier separated by 11 subcarriers from the sixth pilot tone position  7112 . The eighth pilot tone position  7118  corresponds to a subcarrier separated by 13 subcarriers from the seventh pilot tone position  7116  and spaced 5 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7110 . 
       FIG.  71    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  7100 A (which may be odd or even) and the second 26-subcarrier RU  7100 B (which may be even or odd, respectively).  FIG.  71    also illustrates a nested design in which the 52-subcarrier RU  7110  uses the same pilot tone positions as the first and second RUs  7100 A and  7100 B, but embodiments are not limited thereto. 
       FIG.  72    illustrates another option for pilot tone positions in Case 1, according to embodiments.  FIG.  72    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  72    illustrates a first embodiment including a 52-subcarrier RU  7210  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  72    also illustrates pilot tone positions of a first 26-subcarrier RU  7200 A and a second 26-subcarrier RU  7200 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7210  and that would be odd and even RUs, respectively, in the first embodiment. 
       FIG.  72    illustrates a second embodiment including a 52-subcarrier RU  7210  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  72    also illustrates pilot tone positions of a first 26-subcarrier RU  7200 A and a second 26-subcarrier RU  7200 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7210  and that would be even and odd RUs, respectively, in the second embodiment. 
     In  FIG.  72   , a highest-indexed subcarrier of the 52-subcarrier RU  7210  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7200 B. A null or reserved subcarrier  7220  is disposed between the first and second 26-subcarrier RUs  7200 A and  7200 B. 
     The first 26-subcarrier RU  7200 A includes first and second pilot tone positions  7202  and  7204 . The first pilot tone position  7202  corresponds to a subcarrier spaced 7 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7200 A. The second pilot tone position  7204  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7202  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7200 A. 
     The second 26-subcarrier RU  7200 B includes third and fourth pilot tone positions  7206  and  7208 . The third pilot tone position  7206  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7200 B. The fourth pilot tone position  7208  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7206  and spaced 7 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7200 B. 
     The 52-subcarrier RU  7210  includes fifth, sixth, seventh, and eighth pilot tone positions  7212 ,  7214 ,  7216 , and  7218 . The fifth, sixth, seventh, and eighth pilot tone positions  7212 ,  7214 ,  7216 , and  7218  of the 52-subcarrier RU  7210  respectively correspond to the first, second, third, and fourth pilot tone positions  7202 ,  7204 ,  7206 , and  7208  of the first and second 26-subcarrier RUs  7200 A and  7200 B. 
     The fifth pilot tone position  7212  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7210 . The sixth pilot tone position  7214  corresponds to a subcarrier separated by 11 subcarriers from the fifth pilot tone position  7212 . The seventh pilot tone position  7214  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  7212 . The eighth pilot tone position  7218  corresponds to a subcarrier separated by 11 subcarriers from the seventh pilot tone position  7216  and spaced 7 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7210 . 
       FIG.  72    illustrates mirror symmetric pilot tone positions between the first 26-subcarrier RU  7200 A (which may be odd or even) and the second 26-subcarrier RU  7200 B (which may be even or odd, respectively).  FIG.  72    also illustrates a nested design in which the 52-subcarrier RU  7210  uses the same pilot tone positions as the first and second RUs  7200 A and  7200 B, but embodiments are not limited thereto. 
       FIG.  73    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  73    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  73    illustrates a first embodiment including a 52-subcarrier RU  7310  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  73    also illustrates pilot tone positions of a first 26-subcarrier RU  7300 A and a second 26-subcarrier RU  7300 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7310  and that would be even RUs in the first embodiment. 
       FIG.  73    illustrates a second embodiment including a 52-subcarrier RU  7310  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  73    also illustrates pilot tone positions of a first 26-subcarrier RU  7300 A and a second 26-subcarrier RU  7300 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7310  and that would be odd RUs in the second embodiment. 
     In  FIG.  73   , a lowest-indexed subcarrier of the 52-subcarrier RU  7310  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7300 A and a highest-indexed subcarrier of the 52-subcarrier RU  7310  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7300 B. 
     The first 26-subcarrier RU  7300 A includes first and second pilot tone positions  7302  and  7304 . The first pilot tone position  7302  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7300 A. The second pilot tone position  7304  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  7302  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7300 A. 
     The second 26-subcarrier RU  7300 B includes third and fourth pilot tone positions  7306  and  7308 . The third pilot tone position  7306  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7300 B. The fourth pilot tone position  7308  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  7306  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7300 B. 
     The 52-subcarrier RU  7310  includes fifth, sixth, seventh, and eighth pilot tone positions  7312 ,  7314 ,  7316 , and  7318 . The fifth, sixth, seventh, and eighth pilot tone positions  7312 ,  7314 ,  7316 , and  7318  of the 52-subcarrier RU  7310  respectively correspond to the first, second, third, and fourth pilot tone positions  7302 ,  7304 ,  7306 , and  7308  of the first and second 26-subcarrier RUs  7300 A and  7300 B. 
     The fifth pilot tone position  7312  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7310 . The sixth pilot tone position  7314  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  7312 . The seventh pilot tone position  7314  corresponds to a subcarrier separated by 12 subcarriers from the sixth pilot tone position  7312 . The eighth pilot tone position  7318  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  7316  and spaced 6 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7310 . 
       FIG.  73    illustrates a nested design in which the 52-subcarrier RU  7310  uses the same pilot tone positions as the first and second RUs  7300 A and  7300 B, but embodiments are not limited thereto. 
       FIG.  74    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  74    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  74    illustrates a first embodiment including a 52-subcarrier RU  7410  being an even RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  74    also illustrates pilot tone positions of a first 26-subcarrier RU  7400 A and a second 26-subcarrier RU  7400 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7410  and that would be even RUs in the first embodiment. 
       FIG.  74    illustrates a second embodiment including a 52-subcarrier RU  7410  being an odd RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  74    also illustrates pilot tone positions of a first 26-subcarrier RU  7400 A and a second 26-subcarrier RU  7400 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7410  and that would be odd RUs in the second embodiment. 
     In  FIG.  74   , a lowest-indexed subcarrier of the 52-subcarrier RU  7410  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7400 A and a highest-indexed subcarrier of the 52-subcarrier RU  7410  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7400 B. 
     The first 26-subcarrier RU  7400 A includes first and second pilot tone positions  7402  and  7404 . The first pilot tone position  7402  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7400 A. The second pilot tone position  7404  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  7402  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7400 A. 
     The second 26-subcarrier RU  7400 B includes third and fourth pilot tone positions  7406  and  7408 . The third pilot tone position  7406  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7400 B. The fourth pilot tone position  7408  corresponds to a subcarrier separated by 12 subcarriers from the first pilot tone position  7406  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7400 B. 
     The 52-subcarrier RU  7410  includes fifth, sixth, seventh, and eighth pilot tone positions  7412 ,  7414 ,  7416 , and  7418 . The fifth, sixth, seventh, and eighth pilot tone positions  7412 ,  7414 ,  7416 , and  7418  of the 52-subcarrier RU  7410  respectively correspond to the first, second, third, and fourth pilot tone positions  7402 ,  7404 ,  7406 , and  7408  of the first and second 26-subcarrier RUs  7400 A and  7400 B. 
     The fifth pilot tone position  7412  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7410 . The sixth pilot tone position  7414  corresponds to a subcarrier separated by 12 subcarriers from the fifth pilot tone position  7412 . The seventh pilot tone position  7414  corresponds to a subcarrier separated by 12 subcarriers from the sixth pilot tone position  7412 . The eighth pilot tone position  7418  corresponds to a subcarrier separated by 12 subcarriers from the seventh pilot tone position  7416  and spaced 6 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7410 . 
       FIG.  74    illustrates a nested design in which the 52-subcarrier RU  7410  uses the same pilot tone positions as the first and second RUs  7400 A and  7400 B, but embodiments are not limited thereto. 
       FIG.  75    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  75    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  75    illustrates a first embodiment including a 52-subcarrier RU  7510  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  75    also illustrates pilot tone positions of a first 26-subcarrier RU  7500 A and a second 26-subcarrier RU  7500 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7510  and that would be even RUs in the first embodiment. 
       FIG.  75    illustrates a second embodiment including a 52-subcarrier RU  7510  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  75    also illustrates pilot tone positions of a first 26-subcarrier RU  7500 A and a second 26-subcarrier RU  7500 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7510  and that would be odd RUs in the second embodiment. 
     In  FIG.  75   , a lowest-indexed subcarrier of the 52-subcarrier RU  7510  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7500 A and a highest-indexed subcarrier of the 52-subcarrier RU  7510  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7500 B. 
     The first 26-subcarrier RU  7500 A includes first and second pilot tone positions  7502  and  7504 . The first pilot tone position  7502  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7500 A. The second pilot tone position  7504  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7502  and spaced 7 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7500 A. 
     The second 26-subcarrier RU  7500 B includes third and fourth pilot tone positions  7506  and  7508 . The third pilot tone position  7506  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7500 B. The fourth pilot tone position  7508  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7506  and spaced 7 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7500 B. 
     The 52-subcarrier RU  7510  includes fifth, sixth, seventh, and eighth pilot tone positions  7512 ,  7514 ,  7516 , and  7518 . The fifth, sixth, seventh, and eighth pilot tone positions  7512 ,  7514 ,  7516 , and  7518  of the 52-subcarrier RU  7510  respectively correspond to the first, second, third, and fourth pilot tone positions  7502 ,  7504 ,  7506 , and  7508  of the first and second 26-subcarrier RUs  7500 A and  7500 B. 
     The fifth pilot tone position  7512  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7510 . The sixth pilot tone position  7514  corresponds to a subcarrier separated by 11 subcarriers from the fifth pilot tone position  7512 . The seventh pilot tone position  7514  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  7512 . The eighth pilot tone position  7518  corresponds to a subcarrier separated by 11 subcarriers from the seventh pilot tone position  7516  and spaced 7 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7510 . 
       FIG.  75    illustrates a nested design in which the 52-subcarrier RU  7510  uses the same pilot tone positions as the first and second RUs  7500 A and  7500 B, but embodiments are not limited thereto. 
       FIG.  76    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  76    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  76    illustrates a first embodiment including a 52-subcarrier RU  7610  being an even RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  76    also illustrates pilot tone positions of a first 26-subcarrier RU  7600 A and a second 26-subcarrier RU  7600 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7610  and that would be even RUs in the first embodiment. 
       FIG.  76    illustrates a second embodiment including a 52-subcarrier RU  7610  being an odd RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  76    also illustrates pilot tone positions of a first 26-subcarrier RU  7600 A and a second 26-subcarrier RU  7600 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7610  and that would be odd RUs in the second embodiment. 
     In  FIG.  76   , a lowest-indexed subcarrier of the 52-subcarrier RU  7610  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7600 A and a highest-indexed subcarrier of the 52-subcarrier RU  7610  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7600 B. 
     The first 26-subcarrier RU  7600 A includes first and second pilot tone positions  7602  and  7604 . The first pilot tone position  7602  corresponds to a subcarrier spaced 7 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7600 A. The second pilot tone position  7604  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7602  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7600 A. 
     The second 26-subcarrier RU  7600 B includes third and fourth pilot tone positions  7606  and  7608 . The third pilot tone position  7606  corresponds to a subcarrier spaced 7 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7600 B. The fourth pilot tone position  7608  corresponds to a subcarrier separated by 11 subcarriers from the first pilot tone position  7606  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7600 B. 
     The 52-subcarrier RU  7610  includes fifth, sixth, seventh, and eighth pilot tone positions  7612 ,  7614 ,  7616 , and  7618 . The fifth, sixth, seventh, and eighth pilot tone positions  7612 ,  7614 ,  7616 , and  7618  of the 52-subcarrier RU  7610  respectively correspond to the first, second, third, and fourth pilot tone positions  7602 ,  7604 ,  7606 , and  7608  of the first and second 26-subcarrier RUs  7600 A and  7600 B. 
     The fifth pilot tone position  7612  corresponds to a subcarrier spaced 7 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7610 . The sixth pilot tone position  7614  corresponds to a subcarrier separated by 11 subcarriers from the fifth pilot tone position  7612 . The seventh pilot tone position  7614  corresponds to a subcarrier separated by 13 subcarriers from the sixth pilot tone position  7612 . The eighth pilot tone position  7618  corresponds to a subcarrier separated by 11 subcarriers from the seventh pilot tone position  7616  and spaced 6 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7610 . 
       FIG.  76    illustrates a nested design in which the 52-subcarrier RU  7610  uses the same pilot tone positions as the first and second RUs  7600 A and  7600 B, but embodiments are not limited thereto. 
       FIG.  77    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  77    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  77    illustrates a first embodiment including a 52-subcarrier RU  7710  being an even RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  77    also illustrates pilot tone positions of a first 26-subcarrier RU  7700 A and a second 26-subcarrier RU  7700 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7710  and that would be even RUs in the first embodiment. 
       FIG.  77    illustrates a second embodiment including a 52-subcarrier RU  7710  being an odd RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  77    also illustrates pilot tone positions of a first 26-subcarrier RU  7700 A and a second 26-subcarrier RU  7700 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7710  and that would be odd RUs in the second embodiment. 
     In  FIG.  77   , a lowest-indexed subcarrier of the 52-subcarrier RU  7710  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7700 A and a highest-indexed subcarrier of the 52-subcarrier RU  7710  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7700 B. 
     The first 26-subcarrier RU  7700 A includes first and second pilot tone positions  7702  and  7704 . The first pilot tone position  7702  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7700 A. The second pilot tone position  7704  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7702  and spaced 5 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7700 A. 
     The second 26-subcarrier RU  7700 B includes third and fourth pilot tone positions  7706  and  7708 . The third pilot tone position  7706  corresponds to a subcarrier spaced 6 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7700 B. The fourth pilot tone position  7708  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7706  and spaced 5 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7700 B. 
     The 52-subcarrier RU  7710  includes fifth, sixth, seventh, and eighth pilot tone positions  7712 ,  7714 ,  7716 , and  7718 . The fifth, sixth, seventh, and eighth pilot tone positions  7712 ,  7714 ,  7716 , and  7718  of the 52-subcarrier RU  7710  respectively correspond to the first, second, third, and fourth pilot tone positions  7702 ,  7704 ,  7706 , and  7708  of the first and second 26-subcarrier RUs  7700 A and  7700 B. 
     The fifth pilot tone position  7712  corresponds to a subcarrier spaced 6 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7710 . The sixth pilot tone position  7714  corresponds to a subcarrier separated by 13 subcarriers from the fifth pilot tone position  7712 . The seventh pilot tone position  7714  corresponds to a subcarrier separated by 11 subcarriers from the sixth pilot tone position  7712 . The eighth pilot tone position  7718  corresponds to a subcarrier separated by 13 subcarriers from the seventh pilot tone position  7716  and spaced 5 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7710 . 
       FIG.  77    illustrates a nested design in which the 52-subcarrier RU  7710  uses the same pilot tone positions as the first and second RUs  7700 A and  7700 B, but embodiments are not limited thereto. 
       FIG.  78    illustrates another option for pilot tone positions in Case 2, according to embodiments.  FIG.  78    can be interpreted as illustrating two embodiments, according to whether a reference subcarrier index f 0  is even or odd. 
       FIG.  78    illustrates a first embodiment including a 52-subcarrier RU  7810  being an even RU and having pilot tone positions such as may be used with an odd mapping of a HE-LTF sequence in a 2×LTF design, wherein a reference subcarrier index f 0  has a value equal to 2×N, where N is an integer.  FIG.  78    also illustrates pilot tone positions of a first 26-subcarrier RU  7800 A and a second 26-subcarrier RU  7800 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7810  and that would be even RUs in the first embodiment. 
       FIG.  78    illustrates a second embodiment including a 52-subcarrier RU  7810  being an odd RU and having pilot tone positions such as may be used with an even mapping of a HE-LTF sequence in a 2×LTF design, wherein the reference subcarrier index f 0  has a value equal to 2×N+1, where N is an integer.  FIG.  78    also illustrates pilot tone positions of a first 26-subcarrier RU  7800 A and a second 26-subcarrier RU  7800 B that occupy a plurality of subcarriers occupied by the 52-subcarrier RU  7810  and that would be odd RUs in the second embodiment. 
     In  FIG.  78   , a lowest-indexed subcarrier of the 52-subcarrier RU  7810  is aligned with a lowest-indexed subcarrier of the first 26-subcarrier RU  7800 A and a highest-indexed subcarrier of the 52-subcarrier RU  7810  is aligned with a highest-indexed subcarrier of the second 26-subcarrier RU  7800 B. 
     The first 26-subcarrier RU  7800 A includes first and second pilot tone positions  7802  and  7804 . The first pilot tone position  7802  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the first 26-subcarrier RU  7800 A. The second pilot tone position  7804  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7802  and spaced 6 subcarriers away from a highest-indexed subcarrier of the first 26-subcarrier RU  7800 A. 
     The second 26-subcarrier RU  7800 B includes third and fourth pilot tone positions  7806  and  7808 . The third pilot tone position  7806  corresponds to a subcarrier spaced 5 subcarriers away from a lowest-indexed subcarrier of the second 26-subcarrier RU  7800 B. The fourth pilot tone position  7808  corresponds to a subcarrier separated by 13 subcarriers from the first pilot tone position  7806  and spaced 6 subcarriers away from the highest-indexed subcarrier of the second 26-subcarrier RU  7800 B. 
     The 52-subcarrier RU  7810  includes fifth, sixth, seventh, and eighth pilot tone positions  7812 ,  7814 ,  7816 , and  7818 . The fifth, sixth, seventh, and eighth pilot tone positions  7812 ,  7814 ,  7816 , and  7818  of the 52-subcarrier RU  7810  respectively correspond to the first, second, third, and fourth pilot tone positions  7802 ,  7804 ,  7806 , and  7808  of the first and second 26-subcarrier RUs  7800 A and  7800 B. 
     The fifth pilot tone position  7812  corresponds to a subcarrier spaced 5 subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier RU  7810 . The sixth pilot tone position  7814  corresponds to a subcarrier separated by 13 subcarriers from the fifth pilot tone position  7812 . The seventh pilot tone position  7814  corresponds to a subcarrier separated by 11 subcarriers from the sixth pilot tone position  7812 . The eighth pilot tone position  7818  corresponds to a subcarrier separated by 13 subcarriers from the seventh pilot tone position  7816  and spaced 6 subcarriers away from a highest-indexed subcarrier of the 52-subcarrier RU  7810 . 
       FIG.  78    illustrates a nested design in which the 52-subcarrier RU  7810  uses the same pilot tone positions as the first and second RUs  7800 A and  7800 B, but embodiments are not limited thereto. 
     Embodiments include processes for transmitting and receiving frames including pilot signals within allocated resources of an OFDMA transmission wherein locations of the pilots are chosen that maximize frequency diversity gain and improve carrier frequency offset tracking performance. 
       FIG.  79    illustrates a process  7900  for transmitting a frame in a wireless network, according to an embodiment. The process  7900  may be performed by a wireless device configured to transmit frames. 
     At S 7902 , Resource Units (RU) of the frame are determined. In an embodiment, the RUs are RUs of a payload portion of an Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     At S 7904 , pilots are included into one or more odd 26-subcarrier RUs of the frame at a first set of positions relative to respective lowest subcarriers of the odd 26-subcarrier RUs. An odd RU is an RU wherein the lowest subcarrier has an odd index. 
     At S 7906 , pilots are included into one or more even 26-subcarrier RUs of the frame at a second set of positions relative to respective lowest subcarriers of the even 26-subcarrier RUs. An even RU is an RU wherein the lowest subcarrier has an even index. 
     In an embodiment, the first set of positions is different than the second set of positions. In an embodiment, the first set of positions is a mirror image of the second set of positions. 
     At S 7908 , pilots are included into a center 26-subcarrier RU of the frame at a third set of positions relative to respective lowest subcarriers of the center 26-subcarrier RUs. A center RU is an RU including at least one center-most non-DC subcarrier of the subchannel including the RU. 
     In an embodiment, the first set of positions is different than the third set of positions and the second set of positions is different than the third set of positions. 
     At S 7910 , the wireless device transmits the frame. 
       FIG.  80    illustrates another process  8000  for transmitting a frame in a wireless network, according to an embodiment. The process  8000  may be performed by a wireless device configured to transmit frames. 
     At S 8002 , Resource Units (RU) of the frame are determined. In an embodiment, the RUs are RUs of a payload portion of an Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     At S 8004 , pilots are included into one or more 52-subcarrier RUs of a first plurality of RUs of the frame at a first set of positions relative to respective lowest subcarriers of the 52-subcarrier RUs. In an embodiment, the first plurality of RUs are even RUs. 
     At S 8006 , pilots are included into one or more 52-subcarrier RUs of a second plurality of RUs of the frame at a second set of positions relative to respective lowest subcarriers of the 52-subcarrier RUs. In an embodiment, the first plurality of RUs are odd RUs. 
     In an embodiment, the first set of positions is different than the second set of positions. In an embodiment, the first set of positions is a mirror image of the second set of positions. 
     At S 8010 , the wireless device transmits the frame. 
       FIG.  81    illustrates a process  8100  for transmitting a frame in a wireless network, according to an embodiment. The process  8100  may be performed by a wireless device configured to transmit frames. 
     At S 8102 , the process  8100  inserts a first plurality of pilots into a Resource Unit (RU) for a High Efficiency Long Training Field (HE-LTF) symbol. The process  8100  determines respective positions of the first plurality of pilots based on i) a size of the RU, ii) whether a lowest subcarrier of the RU has an odd-numbered index or an even numbered index, and iii) whether the RU is a center RU. 
     At S 8104 , the process  8100  inserts a second plurality of pilots into a Resource Unit (RU) for a data symbol. The process  8100  determines respective positions of the second plurality of pilots based on i) a size of the RU, ii) whether a lowest subcarrier of the RU has an odd-numbered index or an even numbered index, and iii) whether the RU is a center RU. 
     At S 8106 , the process  8100  transmits a frame. The frame includes the HE-LTF symbol and the data symbol. 
       FIG.  82    illustrates a sub-process  8200  for inserting pilots, according to an embodiment. The sub-process  8200  may be included in S 8102 , S 8104 , or both of process  8100  of  FIG.  81   . 
     At S 8202 , the sub-process  8200  determines whether an RU includes 26 or 52 subcarriers. When the RU includes 52 subcarriers, at S 8202  the sub-process  8200  proceeds to S 8204 . Otherwise, when the RU includes 26 subcarriers, at S 8202  the sub-process  8200  proceeds to S 8210 . 
     At S 8204 , the sub-process  8200  determines whether a lowest subcarrier of the RU has an odd index (i.e. the RU is an odd RU) or an even index (i.e. the RU is an even RU). When the RU is the odd RU, at S 8204  the sub-process  8200  proceeds to S 8206 . Otherwise, when the RU is the even RU, at S 8204  the sub-process  8200  proceeds to S 8208 . 
     At S 8206 , sub-process  8200  inserts a plurality of pilots into a symbol at a first set of predetermined positions, respectively. 
     At S 8208 , sub-process  8200  inserts a plurality of pilots into a symbol at a second set of predetermined positions, respectively. 
     At S 8210 , the sub-process  8200  determines whether the RU has a center RU. When the RU is the center RU, at S 8210  the sub-process  8200  proceeds to S 8214 . Otherwise, at S 8210  the sub-process  8200  proceeds to S 8212 . 
     At S 8212 , the sub-process  8200  determines whether a lowest subcarrier of the RU has an odd index (i.e. the RU is an odd RU) or an even index (i.e. the RU is an even RU). When the RU is the odd RU, at S 8212  the sub-process  8200  proceeds to S 8216 . Otherwise, when the RU is the even RU, at S 8212  the sub-process  8200  proceeds to S 8218 . 
     At S 8214 , sub-process  8200  inserts a plurality of pilots into a symbol at a third set of predetermined positions, respectively. 
     At S 8216 , sub-process  8200  inserts a plurality of pilots into a symbol at a fourth set of predetermined positions, respectively. 
     At S 8218 , sub-process  8200  inserts a plurality of pilots into a symbol at a fifth set of predetermined positions, respectively. 
     In an embodiment, each of the first, second, third, fourth, and fifth sets of predetermined positions is different from every other of the first, second, third, fourth, and fifth sets of predetermined positions. 
       FIG.  83    illustrates a process  8300  for receiving a frame in a wireless network, according to an embodiment. The process  8300  may be performed by a wireless device configured to receive frames. 
     At S 8302 , the process  8300  receives a frame. The frame includes High Efficiency Long Training Field (HE-LTF) symbols and data symbols. 
     At S 8304 , the process  8300  obtains a first plurality of pilots from a Resource Unit (RU) for a HE-LTF symbol of the frame. The process  8300  determines respective positions of the first plurality of pilots based on i) a size of the RU, ii) whether a lowest subcarrier of the RU has an odd-numbered index or an even numbered index, and iii) whether the RU is a center 
     RU. 
     At S 8306 , the process  8300  obtains a second plurality of pilots from a Resource Unit (RU) for a data symbol of the frame. The process  8300  determines respective positions of the second plurality of pilots based on i) a size of the RU, ii) whether a lowest subcarrier of the RU has an odd-numbered index or an even numbered index, and iii) whether the RU is a center RU. 
     At S 8308 , the process  8400  processes the HE-LTF symbol. 
     At S 8310 , the process  8400  processes the data symbol. 
       FIG.  84    illustrates a sub-process  8400  for obtaining pilots, according to an embodiment. The sub-process  8400  may be included in S 8304 , S 8306 , or both of process  8300  of  FIG.  83   . 
     At S 8402 , the sub-process  8400  determines whether an RU includes 26 or 52 subcarriers. When the RU includes 52 subcarriers, at S 8402  the sub-process  8400  proceeds to S 8404 . Otherwise, when the RU includes 26 subcarriers, at S 8402  the sub-process  8400  proceeds to S 8410 . 
     At S 8404 , the sub-process  8400  determines whether a lowest subcarrier of the RU has an odd index (i.e. the RU is an odd RU) or an even index (i.e. the RU is an even RU). When the RU is the odd RU, at S 8404  the sub-process  8400  proceeds to S 8406 . Otherwise, when the RU is the even RU, at S 8404  the sub-process  8400  proceeds to S 8408 . 
     At S 8406 , sub-process  8400  obtains a plurality of pilots from a first set of predetermined positions within a symbol of the RU, respectively. 
     At S 8408 , sub-process  8400  obtains a plurality of pilots from a second set of predetermined positions within the symbol, respectively. 
     At S 8410 , the sub-process  8400  determines whether the RU has a center RU. When the RU is the center RU, at S 8410  the sub-process  8400  proceeds to S 8414 . Otherwise, at S 8410  the sub-process  8400  proceeds to S 8412 . 
     At S 8412 , the sub-process  8400  determines whether a lowest subcarrier of the RU has an odd index (i.e. the RU is an odd RU) or an even index (i.e. the RU is an even RU). When the RU is the odd RU, at S 8412  the sub-process  8400  proceeds to S 8416 . Otherwise, when the RU is the even RU, at S 8412  the sub-process  8400  proceeds to S 8418 . 
     At S 8414 , sub-process  8400  obtains a plurality of pilots from a third set of predetermined positions within the symbol, respectively. 
     At S 8416 , sub-process  8400  obtains a plurality of pilots from a fourth set of predetermined positions within the symbol, respectively. 
     At S 8418 , sub-process  8400  obtains a plurality of pilots from a fifth set of predetermined positions within the symbol, respectively. 
     In an embodiment, each of the first, second, third, fourth, and fifth sets of predetermined positions is different from every other of the first, second, third, fourth, and fifth sets of predetermined positions. 
     The above explanation and figures are applied to an HE receiver, an HE frame, an HE PPDU, an HE-SIG field and the like of the IEEE 802.11ax amendment, but they can also applied to a receiver, a frame, PPDU, a SIG field, and the like of the next amendment of IEEE 802.11. 
     Further aspects of the present disclosure relate to one or more of the following clauses. 
     In an embodiment, a method of a wireless device for transmitting a frame comprises providing pilots in a resource unit at a plurality of pilot tone positions, and transmitting a frame including the resource unit. The frame has a plurality of potential pilot tone positions. The plurality of pilot tone positions are a subset of the plurality of potential pilot tone positions. Half of the potential pilot tone positions are mirror-symmetrical with an other half of the potential pilot tone positions about a DC tone of the frame. 
     In an embodiment, when the resource unit has 106 subcarriers, the plurality of pilot tone positions include 1) a first pilot tone position having a lowest index among potential pilot tone positions covered by the 106 subcarriers, 2) a second pilot tone position spaced two potential pilot tone positions away from the first pilot tone position, 3) a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, and 4) a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position. 
     In an embodiment, when the resource unit has 242 subcarriers, the plurality of pilot tone positions include: 1) a first pilot tone position having a lowest index among potential pilot tone positions covered by the 242 subcarriers, 2) a second pilot tone position spaced two potential pilot tone positions away from the first pilot tone position, 3) a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, 4) a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position, 5) a fifth pilot tone position spaced three potential pilot tone positions away from the fourth pilot tone position, 6) a sixth pilot tone position spaced two potential pilot tone positions away from the fifth pilot tone position, 7) a seventh pilot tone position spaced three potential pilot tone positions away from the sixth pilot tone position, and 8) an eighth pilot tone position spaced two potential pilot tone positions away from the seventh pilot tone position. 
     In an embodiment, when the resource unit has 52 subcarriers, the plurality of pilot tone positions include a first pilot tone position, a second pilot tone position, a third pilot tone position, and a fourth pilot tone position which correspond to potential pilot tone positions covered by the 52 subcarriers. 
     In an embodiment, when a lowest-indexed subcarrier of the 52-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. When the lowest-indexed subcarrier of the 52-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the RU has 26 subcarriers, the plurality of pilot tone positions include a first pilot tone position and a second pilot tone position which correspond to potential pilot tone positions covered by the 26 subcarriers. 
     In an embodiment, when a lowest-indexed subcarrier of the 26-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. When a lowest-indexed subcarrier of the 26-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the 26-subcarrier resource unit is a center resource unit that is split into 13 positive-indexed subcarriers and 13 negative-indexed subcarriers by DC tones, the first pilot tone position is spaced six subcarriers away from a lowest-indexed subcarrier of the 26-subcarrier resource unit and spaced six subcarriers away from a highest-indexed subcarrier of the 13 negative-indexed subcarriers, and the second pilot tone position is spaced six subcarriers away from a lowest-indexed subcarrier of 13 positive-indexed subcarriers and spaced six subcarriers away from a highest-indexed subcarrier of the 26-subcarrier resource unit. When the 26-subcarrier resource unit is not the center resource unit and a lowest-indexed subcarrier of the 26-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. When the 26-subcarrier resource unit is not the center resource unit and a lowest-indexed subcarrier of the 26-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. 
     In an embodiment, the frame is transmitted on a 20 MHz channel, and a total number of the potential pilot tone positions is 18. 
     In an embodiment, the frame is transmitted on a 40 MHz channel, and a total number of the potential pilot tone positions is 36. 
     In an embodiment, a method of a wireless device for receiving a frame comprises receiving a frame including a resource unit including pilots which are included at a plurality of pilot tone positions, and processing the pilots. The frame has a plurality of potential pilot tone positions. A half of the potential pilot tone positions is mirror-symmetrical with an other half of the potential pilot tone positions. 
     In an embodiment, when the resource unit has 106 subcarriers, the plurality of pilot tone positions include a 1) first pilot tone position having a lowest index among potential pilot tone positions covered by the 106 subcarriers, 2) a second pilot tone position spaced two potential pilot tone positions away from the first pilot tone position, 3) a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, and 4) a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position. 
     In an embodiment, when the resource unit has 242 subcarriers, the plurality of pilot tone positions include: 1) a first pilot tone position having a lowest index among potential pilot tone positions covered by the 242 subcarriers, 2) a second pilot tone position spaced two potential pilot tone positions away from the first pilot tone position, 3) a third pilot tone position spaced three potential pilot tone positions away from the second pilot tone position, 4) a fourth pilot tone position spaced two potential pilot tone positions away from the third pilot tone position, 5) a fifth pilot tone position spaced three potential pilot tone positions away from the fourth pilot tone position, 6) a sixth pilot tone position spaced two potential pilot tone positions away from the fifth pilot tone position, 7) a seventh pilot tone position spaced three potential pilot tone positions away from the sixth pilot tone position, and 8) an eighth pilot tone position spaced two potential pilot tone positions away from the seventh pilot tone position. 
     In an embodiment, when the resource unit has 52 subcarriers, the plurality of pilot tone positions include a first pilot tone position, a second pilot tone position, a third pilot tone position, and a fourth pilot tone position which correspond to potential pilot tone positions covered by the 52 subcarriers. 
     In an embodiment, when a lowest-indexed subcarrier of the 52-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the 52-subcarrier resource unit. When a lowest-indexed subcarrier of the 52-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the 52-subcarrier resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the 52-subcarrier resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers, the plurality of pilot tone positions include a first pilot tone position and a second pilot tone position which correspond to potential pilot tone positions covered by the 26 subcarriers. 
     In an embodiment, when a lowest-indexed subcarrier of the 26-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. When a lowest-indexed subcarrier of the 26-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. 
     In an embodiment, when the 26-subcarrier resource unit is a center resource unit that is split into 13 positive-indexed subcarriers and 13 negative-indexed subcarriers by DC tones, the first pilot tone position is spaced six subcarriers away from a lowest-indexed subcarrier of the 26-subcarrier resource unit and spaced six subcarriers away from a highest-indexed subcarrier of the 13 negative-indexed subcarriers, and the second pilot tone position is spaced six subcarriers away from a lowest-indexed subcarrier of 13 positive-indexed subcarriers and spaced six subcarriers away from a highest-indexed subcarrier of the 26-subcarrier resource unit. When the 26-subcarrier resource unit is not the center resource unit and a lowest-indexed subcarrier of the 26-subcarrier resource unit has an even index, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. When the resource unit is not the center resource unit and a lowest-indexed subcarrier of the 26-subcarrier resource unit has an odd index, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the 26-subcarrier resource unit, and the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the 26-subcarrier resource unit. 
     In an embodiment, the frame is transmitted on a 20 MHz channel, and a total number of the potential pilot tone positions is 18. 
     In an embodiment, the frame is transmitted on a 40 MHz channel, and a total number of the potential pilot tone positions is 36. 
     In an embodiment, potential pilot tone positions are aggregation of pilot tone positions used for each 26 subcarrier RU on a given bandwidth. 
     Embodiments include frames transmitted on a 20 MHz channel and having a nested pilot structure. 
     In an embodiment, a method of a wireless device for transmitting a frame comprises providing pilots in a resource unit (RU), and transmitting a frame including the RU on a 20 MHz channel. The frame has 18 potential pilot tone positions c1 to c18 corresponding to subcarrier indices. For the respective subcarrier indices, c18&gt;c17&gt;c16&gt;c15&gt;c14&gt;c13&gt;c12&gt;c11&gt;c10&gt;c9&gt;c8&gt;c7&gt;c6&gt;c5&gt;c4&gt;c3&gt;c2&gt;c1. 
     In an embodiment, for the respective subcarrier indices, c1=−c18, c2=−c17, c3=−c16, c4=−c15, c5=−c14, c6=−c13, c7=−c12, c8=−c11, and c9=−c10. 
     In an embodiment, when the RU has a first set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11, c12, c13 and c14. When the RU has a second set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions c15, c16, c17 and c18. 
     In an embodiment, when a RU has 106 positive-indexed subcarriers including the first set of 52 positive-indexed subcarriers and the second set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11, c13, c16 and c18. 
     In an embodiment, when the RU has a first set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11 and c12. When the RU has a second set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c13 and c14. When the RU has a third set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c15 and c16. When the RU has a fourth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c17 and c18. 
     In an embodiment, when the RU has a first set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1, c2, c3 and c4. When the RU has a second set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions c5, c6, c7 and c8. 
     In an embodiment, when the RU has 106 negative-indexed subcarriers including the first set of 52 negative-indexed subcarriers and the second set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1, c3, c6 and c8. 
     In an embodiment, when the RU has a first set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1 and c2. When the RU has a second set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c3 and c4. When the RU has a third set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c5 and c6. When the RU has a fourth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c7 and c8. 
     In an embodiment, when the RU has 26 subcarriers that are split into 13 positive-indexed subcarriers and 13 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions c9 and c10. 
     In an embodiment, when the RU has a set of 242 subcarriers that are split into 121 positive-indexed subcarriers and 121 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions c1, c3, c6, c8, c11, c13, c16 and c18. 
     In an embodiment, a method of a wireless device for receiving a frame comprises receiving, on a 20 MHz channel, a frame including a resource unit including pilots, and processing the pilots in the resource unit. The frame has 18 potential pilot tone positions c1 to c18 corresponding to subcarrier indices. For the respective subcarrier indices, c18&gt;c17&gt;c16&gt;c15&gt;c14&gt;c13&gt;c12&gt;c11&gt;c10&gt;c9&gt;c8&gt;c7&gt;c6&gt;c5&gt;c4&gt;c3&gt;c2&gt;c1. 
     In an embodiment, for the respective subcarrier indices, c1=−c18, c2=−c17, c3=−c16, c4=−c15, c5=−c14, c6=−c13, c7=−c12, c8=−c11, and c9=−c10. 
     In an embodiment, when the RU has a first set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11, c12, c13 and c14. When the RU has a second set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions c15, c16, c17 and c18. 
     In an embodiment, when a RU has 106 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11, c13, c16 and c18. 
     In an embodiment, when the RU has a first set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c11 and c12. When the RU has a second set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c13 and c14. When the RU has a third set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c15 and c16. When the RU has a fourth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions c17 and c18. 
     In an embodiment, when the RU has a first set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1, c2, c3 and c4. When the RU has a second set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions c5, c6, c7 and c8. 
     In an embodiment, when the RU has 106 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1, c3, c6 and c8. 
     In an embodiment, when the RU has a first set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c1 and c2. When the RU has a second set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c3 and c4. When the RU has a third set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c5 and c6. When the RU has a fourth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions c7 and c8. 
     In an embodiment, when the RU has 26 subcarriers that are split into 13 positive-indexed subcarriers and 13 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions c9 and c10. 
     In an embodiment, when the RU has a set of 242 subcarriers that are split into 121 positive-indexed subcarriers and 121 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions c1, c3, c6, c8, c11, c13, c16 and c18. 
     Embodiments include frames transmitted on a 40 MHz channel and having a nested pilot structure. 
     In an embodiment, a method of a wireless device for transmitting a frame comprises providing pilots in the resource unit, and transmitting a frame including a resource unit (RU) on a 40 MHz channel. The frame has 36 potential pilot tone positions f1 to f36 corresponding to subcarrier indices. For the respective indices, f36&gt;f35&gt;f34&gt;f33&gt;f32&gt;f31&gt;f30&gt;f29&gt;f28&gt;f27&gt;f26&gt;f25&gt;f24&gt;f23&gt;f22&gt;f21&gt;f20&gt;f19&gt;f18&gt;f17&gt;f16&gt;f15&gt;f14&gt;f13&gt;f12&gt;f11&gt;f10&gt;f9&gt;f8&gt;f7&gt;f6&gt;f5&gt;f4&gt;f3&gt;f2&gt;f1. 
     In an embodiment, for the respective indices, f1=−f36, f2=−f35, f3=−f34, f4=−f33, f5=−f32, f6=−f31, f7=−f30, f8=−f29, f9=−f28, f10=−f27, f11=−f26, f12=−f25, f13=−f24, f14=−f23, f15=−f22, f16=−f21, f17=−f20, and f18=−f19. 
     In an embodiment, when the RU has a first set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, f20, f21 and f22. When the RU has a second set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f23, f24, f25 and f26. When the RU has a third set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f29, f30, f31 and f32. When the RU has a fourth set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f33, f34, f35 and f36. 
     In an embodiment, when the RU has a first set of 106 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, c21, c24 and c26. When the RU has a second set of 106 positive-indexed subcarriers, pilots are included at potential pilot tone positions c29, c31, c34 and c36. 
     In an embodiment, when the RU has a first set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19 and f20. When the RU has a second set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f21 and f22. When the RU has a third set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f23 and f24. When the RU has a fourth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f25 and f26. When the RU has a fifth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f27 and f28. When the RU has a sixth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f29 and f30. When the RU has a seventh set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f31 and f32. When the RU has an eighth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f33 and f34. When the RU has a ninth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f35 and f36. 
     In an embodiment, when the RU has a first set of 106 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f3, f6 and f8. When the RU has a second set of 106 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11, f13, f16 and f18. 
     In an embodiment, when the RU has a first set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f2, f3 and f4. When the RU has a second set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f5, f6, f7 and f8. When the RU has a third set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11, f12, f13 and f14. When the RU has a fourth set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f15, f16, f17 and f18. 
     In an embodiment, when the RU has a first set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1 and f 2 . When the RU has a second set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f3 and f4. When the RU has a third set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f5 and f6. When the RU has a fourth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f7 and f8. When the RU has a fifth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f9 and f10. When the RU has a sixth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11 and f12. When the RU has a seventh set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f13 and f14. When the RU has an eighth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f15 and f16. When the RU has a ninth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f17 and f18. 
     In an embodiment, when the RU has 242 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, f21, f24, f26, f29, f31, f34 and f36. When the RU has 242 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, and f18. 
     In an embodiment, when the RU has 484 subcarriers that are split into 242 positive-indexed subcarriers and 242 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, f18, f19, f21, f24, f26, f29, f31, f34 and f36. 
     In an embodiment, a method of a wireless device for receiving a frame comprises receiving, on 40 MHz channel, a frame including a resource unit including pilots, and processing pilots in the resource unit. The frame has 36 potential pilot tone positions f1 to f36 corresponding to subcarrier indices. For the respective subcarrier indices, f36&gt;f35&gt;f34&gt;f33&gt;f32&gt;f31&gt;f30&gt;f29&gt;f28&gt;f27&gt;f26&gt;f25&gt;f24&gt;f23&gt;f22&gt;f21&gt;f20&gt;f19&gt;f18&gt;f17&gt;f16&gt;f15&gt;f14&gt;f13&gt;f12&gt;f11&gt;f10&gt;f9&gt;f8&gt;f7&gt;f6&gt;f5&gt;f4&gt;f3&gt;f2&gt;f1. 
     In an embodiment, for the respective subcarrier indices, f1=−f36, f2=−f35, f3=−f34, f4=−f33, f5=−f32, f6=−f31, f7=−f30, f8=−f29, f9=−f28, f10=−f27, f11=−f26, f12=−f25, f13=−f24, f14=−f23, f15=−f22, f16=−f21, f17=−f20, and f18=−f19. 
     In an embodiment, when the RU has a first set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, f20, f21 and f22. When the RU has a second set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f23, f24, f25 and f26. When the RU has a third set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f29, f30, f31 and f32. When the RU has a fourth set of 52 positive-indexed subcarriers, pilots are included at potential pilot tone positions f33, f34, f35 and f36. 
     In an embodiment, when the RU has a first set of 106 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, c21, c24 and c26. When the RU has a second set of 106 positive-indexed subcarriers, pilots are included at potential pilot tone positions c29, c31, c34 and c36. 
     In an embodiment, when the RU has a first set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19 and f20. When the RU has a second set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f21 and f22. When the RU has a third set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f23 and f24. When the RU has a fourth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f25 and f26. When the RU has a fifth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f27 and f28. When the RU has a sixth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f29 and f30. When the RU has a seventh set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f31 and f32. When the RU has an eighth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f33 and f34. When the RU has a ninth set of 26 positive-indexed subcarriers, pilots are included at potential pilot tone positions f35 and f36. 
     In an embodiment, when the RU has a first set of 106 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f3, f6 and f8. When the RU has a second set of 106 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11, f13, f16 and f18. 
     In an embodiment, when the RU has a first set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f2, f3 and f4. When the RU has a second set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f5, f6, f7 and f8. When the RU has a third set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11, f12, f13 and f14. When the RU has a fourth set of 52 negative-indexed subcarriers, pilots are included at potential pilot tone positions f15, f16, f17 and f18. 
     In an embodiment, when the RU has a first set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1 and f2. When the RU has a second set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f3 and f4. When the RU has a third set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f5 and f6. When the RU has a fourth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f7 and f8. When the RU has a fifth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f9 and f10. When the RU has a sixth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f11 and f12. When the RU has a seventh set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f13 and f14. When the RU has an eighth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f15 and f16. When the RU has a ninth set of 26 negative-indexed subcarriers, pilots are included at potential pilot tone positions f17 and f18. 
     In an embodiment, when a RU has 242 positive-indexed subcarriers, pilots are included at potential pilot tone positions f19, f21, f24, f26, f29, f31, f34 and f36. When the RU has 242 negative-indexed subcarriers, pilots are included at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, and f18. 
     In an embodiment, when the RU has 484 subcarriers that are split into 242 positive-indexed subcarriers and 242 negative-indexed subcarriers by DC tones, pilots are included at potential pilot tone positions f1, f3, f6, f8, f11, f13, f16, f18, f19, f21, f24, f26, f29, f31, f34 and f36. 
     Embodiments further include frames having a nested pilot structure between 26-subchannel resource units and 52-subchannel resource units. 
     In an embodiment, a method of a wireless device for transmitting a frame comprises providing pilots in a resource unit, and transmitting a frame including the resource unit. When the resource unit has 52 subcarriers whose lowest index number is f0, pilots are included at a first pilot tone position, a second pilot tone position, a third pilot tone position, and a fourth pilot tone position, respectively. When the resource unit has 26 subcarriers whose lowest index number is equal to f0, pilots are included at a fifth pilot tone position, and a sixth pilot tone position, respectively. The fifth pilot tone position is the same as the first pilot tone position, and the sixth pilot tone position is the same as the second pilot tone position. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26), pilots are included at a seventh pilot tone position, and an eighth pilot tone position, respectively. The seventh pilot tone position is the same as the third pilot tone position, and the eighth pilot tone position is the same as the fourth pilot tone position. 
     In an embodiment, when the resource unit has 52 subcarriers whose lowest index number is equal to f0 and is an even number, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 52 subcarriers whose lowest index number is equal to f0 and is an odd number, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to f0 and is an even number, the fifth pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit and the sixth pilot tone position is separated by thirteen subcarriers from the fifth pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to f0 and is an odd number, the fifth pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit and the sixth pilot tone position is separated by thirteen subcarriers from the fifth pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26) and is an even number, the seventh pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit and the eighth pilot tone position is separated by thirteen subcarriers from the seventh pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26) and is an odd number, the seventh pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit and the eighth pilot tone position is separated by thirteen subcarriers from the seventh pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, a method of a wireless device for receiving a frame comprises receiving a frame including a resource unit including pilots and processing pilots in the resource unit. When the resource unit has 52 subcarriers whose lowest index number is f0, pilots are included at a first pilot tone position, a second pilot tone position, a third pilot tone position, and a fourth pilot tone position, respectively. When the resource unit has 26 subcarriers whose lowest index number is f0, pilots are included at a fifth pilot tone position, and a sixth pilot tone position, respectively. The fifth pilot tone position is the same as the first pilot tone position. The sixth pilot tone position is the same as the second pilot tone position. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26), pilots are included at a seventh pilot tone position, and an eighth pilot tone position, respectively. The seventh pilot tone position is the same as the third pilot tone position. The eighth pilot tone position is the same as the fourth pilot tone position. 
     In an embodiment, when the resource unit has 52 subcarriers whose lowest index number is equal to f0 and is an even number, the first pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 52 subcarriers whose lowest index number is equal to f0 and is an odd number, the first pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, the second pilot tone position is separated by thirteen subcarriers from the first pilot tone position, the third pilot tone position is separated by eleven subcarriers from the second pilot tone position, and the fourth pilot tone position is separated by thirteen subcarriers from the third pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to f0 and is an even number, the fifth pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, and the sixth pilot tone position is separated by thirteen subcarriers from the fifth pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to f0 and is an odd number, the fifth pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, and the sixth pilot tone position is separated by thirteen subcarriers from the fifth pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26) and is an even number, the seventh pilot tone position is spaced six subcarriers away from the lowest-indexed subcarrier of the resource unit, and the eighth pilot tone position is separated by thirteen subcarriers from the seventh pilot tone position and spaced five subcarriers away from the highest-indexed subcarrier of the resource unit. 
     In an embodiment, when the resource unit has 26 subcarriers whose lowest index number is equal to (f0+26) and is an odd number, the seventh pilot tone position is spaced five subcarriers away from the lowest-indexed subcarrier of the resource unit, and the eighth pilot tone position is separated by thirteen subcarriers from the seventh pilot tone position and spaced six subcarriers away from the highest-indexed subcarrier of the resource unit. 
     Embodiments of the present disclosure include electronic devices configured to perform one or more of the operations described herein. However, embodiments are not limited thereto. 
     Embodiments of the present disclosure may further include systems configured to operate using the processes described herein. The systems may include basic service sets (BSSs) such as the BSSs  100  of  FIG.  1   , but embodiments are not limited thereto. 
     Embodiments of the present disclosure may be implemented in the form of program instructions executable through various computer means, such as a processor or microcontroller, and recorded in a non-transitory computer-readable medium. The non-transitory computer-readable medium may include one or more of program instructions, data files, data structures, and the like. The program instructions may be adapted to execute the processes and to generate and decode the frames described herein when executed on a device such as the wireless devices shown in  FIG.  1   . 
     In an embodiment, the non-transitory computer-readable medium may include a read only memory (ROM), a random access memory (RAM), or a flash memory. In an embodiment, the non-transitory computer-readable medium may include a magnetic, optical, or magneto-optical disc such as a hard disk drive, a floppy disc, a CD-ROM, and the like. 
     While this invention has been described in connection with what is presently considered to be practical embodiments, embodiments are not limited to the disclosed embodiments, but, on the contrary, may include various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The order of operations described in a process is illustrative and some operations may be re-ordered. Further, two or more embodiments may be combined.