Patent Publication Number: US-9419849-B2

Title: Method and apparatus for generating a PHY data unit

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/044,710, entitled “Control Mode PHY for WLAN,” filed Oct. 2, 2013, now issued as U.S. Pat. No. 9,178,745 on Nov. 3, 2015, which claims the benefit of U.S. Provisional Patent Application No. 61/708,871, filed Oct. 2, 2012 and which is also a continuation-in-part of U.S. patent application Ser. No. 13/366,064, entitled “Control Mode PHY for WLAN,” filed Feb. 3, 2012, now issued as U.S. Pat. No. 9,130,727 on Sep. 8, 2015, which claims the benefit of the following U.S. Provisional Patent Applications:
         U.S. Provisional Patent Application No. 61/439,777, entitled “11ah OFDM Low Rate PHY,” filed on Feb. 4, 2011;   U.S. Provisional Patent Application No. 61/440,804, entitled “11ah OFDM Low Rate PHY,” filed on Feb. 8, 2011;   U.S. Provisional Patent Application No. 61/454,444, entitled “11ah OFDM Low Rate PHY,” filed on Mar. 18, 2011;   U.S. Provisional Patent Application No. 61/477,076, entitled “11ah OFDM Low Rate PHY,” filed on Apr. 19, 2011;   U.S. Provisional Patent Application No. 61/480,238, “11ah OFDM Low Rate PHY,” filed on Apr. 28, 2011;   U.S. Provisional Patent Application No. 61/490,447, entitled “11ah OFDM Low Rate PHY,” filed on May 26, 2011;   U.S. Provisional Patent Application No. 61/492,464, entitled “11ah OFDM Low Rate PHY,” filed on Jun. 2, 2011;   U.S. Provisional Patent Application No. 61/499,964, entitled “11ah OFDM Low Rate PHY,” filed on Jun. 22, 2011;   U.S. Provisional Patent Application No. 61/500,505, entitled “11ah OFDM Low Rate PHY,” filed on Jun. 23, 2011;   U.S. Provisional Patent Application No. 61/515,244, entitled “11ah OFDM Low Rate PHY,” filed on Aug. 4, 2011; and   U.S. Provisional Patent Application No. 61/524,263, entitled “11ah OFDM Low Rate PHY,” filed on Aug. 16, 2011.       

     The disclosures of all of the above-referenced patent applications are hereby incorporated by reference herein in their entireties. 
     Additionally, the present application is related to U.S. patent application Ser. No. 13/366,038, filed Feb. 3, 2012, entitled “Control mode PHY for WLAN,” which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks and, more particularly, to long range low power wireless local area networks. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     When operating in an infrastructure mode, wireless local area networks (WLANs) typically include an access point (AP) and one or more client stations. WLANs have evolved rapidly over the past decade. Development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps, and the IEEE 802.11ac Standard specifies a single-user peak throughput in the Gbps range. 
     Work has begun on a two new standards, IEEE 802.11 ah and IEEE 802.11 af, each of which will specify wireless network operation in sub-1 GHz frequencies. Lowe frequency communication channels are generally characterized by better propagation qualities and extended propagation ranges compared to transmission at higher frequencies. In the past, sub-1 GHz ranges have not been utilized for wireless communication networks because such frequencies were reserved for other applications (e.g., licensed TV frequency bands, radio frequency band, etc.). There are few frequency bands in the sub 1-GHz range that remain unlicensed, with different specific unlicensed frequencies in different geographical regions. The IEEE 802.11ah Standard will specify wireless operation in available unlicensed sub-1 GHz frequency bands. The IEEE 802.11af Standard will specify wireless operation in TV White Space (TVWS), i.e., unused TV channels in sub-1 GHz frequency bands. 
     SUMMARY 
     In one embodiment, a method for generating a physical layer (PHY) data unit for transmission via a communication channel includes encoding a plurality of information bits using a forward error correction (FEC) encoder, and encoding the plurality of information bits according to a block coding scheme, comprising including in the plurality of information bits m copies of each bit in the plurality of information bits, wherein one or more bits in the m copies of each bit are flipped. The method also includes mapping the plurality of information bits to a plurality of constellation symbols, and generating a plurality of orthogonal frequency division multiplexing (OFDM) symbols to include the plurality of constellation symbols. The method further includes generating the PHY data unit to include the plurality of OFDM symbols. 
     In another embodiment, an apparatus for generating a PHY data unit for transmission via a communication channel comprises a network interface device including a forward error correction (FEC) encoder to encode a plurality of information bits to be included in the PHY data unit, and a constellation mapper to map the plurality of information bits to a plurality of constellation symbols. The network interface device is configured to encode the plurality of information bits according to a block coding scheme, comprising including m copies of each bit in the plurality of information bits, wherein one or more bits in the m copies of each bit are flipped, generate a plurality of OFDM symbols to include the plurality of constellation symbols, and generate the PHY data unit to include the plurality of orthogonal frequency division multiplexing (OFDM) symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment; 
         FIGS. 2A and 2B  are diagrams of a prior art orthogonal frequency division multiplexing (OFDM) short range data unit; 
         FIG. 3  is a diagram of another prior art OFDM short range data unit; 
         FIG. 4  is a diagram of another OFDM short range data unit; 
         FIG. 5  is a diagram of another prior art OFDM short range data unit; 
         FIG. 6  is a set of diagrams illustrating modulation of various preamble fields as defined by the IEEE 802.11n Standard; 
         FIG. 7  is a set of diagrams illustrating modulation used to modulate symbols of a prior art data unit; 
         FIG. 8  is a diagram of a prior art single carrier (SC) short range data unit; 
         FIG. 9  is a diagram of a normal mode OFDM long range data unit that, according to an embodiment; 
         FIG. 10  is a diagram of a normal mode OFDM long range data unit that, according to another embodiment; 
         FIG. 11  is a diagram of a normal mode OFDM long range data unit that, according to another embodiment; 
         FIG. 12  is a diagram of a normal mode OFDM long range data unit that, according to another embodiment; 
         FIG. 13  is a diagram of a normal mode OFDM long range data unit that, according to another embodiment; 
         FIG. 14  is a block diagram of an example PHY processing unit for generating normal mode data units, according to an embodiment; 
         FIG. 15A  is a block diagram of an example PHY processing unit for generating control mode data units, according to an embodiment; 
         FIG. 15B  is a block diagram of an example PHY processing unit for generating control mode data units, according to another embodiment; 
         FIG. 16A  is a diagram of an example signal field format, according to an embodiment; 
         FIG. 16B  is a diagram of a block encoded signal field of  FIG. 16A , according an embodiment; 
         FIG. 17A  is a diagram of an another example signal field format, according to another embodiment; 
         FIG. 17B  is a diagram of a block encoded signal field of  FIG. 17A , according an embodiment; 
         FIG. 17C  is a diagram of an another example signal field format, according to yet another embodiment; 
         FIG. 17D  is a diagram of an another example signal field format, according to still another embodiment; 
         FIG. 18A  is a block diagram of an example PHY processing unit for generating control mode data units, according to another embodiment; 
         FIG. 18B  is a block diagram of an example PHY processing unit for generating control mode data units, according to another embodiment; 
         FIG. 19A  is a block diagram of an example PHY processing unit for generating control mode data units, according to yet another embodiment; 
         FIG. 19B  is a block diagram of an example PHY processing unit for generating control mode data units, according to another embodiment; 
         FIG. 20A  is a diagram of a signal field of a control mode data unit, according to an embodiment; 
         FIG. 20B  is a diagram of a signal field of a control mode data unit, according to another embodiment; 
         FIG. 20C  is a diagram of a data portion of a control mode data unit, according to an embodiment; 
         FIG. 20D  is a diagram of a data portion of a control mode data unit, according to another embodiment; 
         FIG. 21  is a diagram of an example control mode data unit which includes a longer preamble compared to a preamble included in a normal mode data unit, according to an embodiment; 
         FIG. 22A  is a diagram of a long training field of a normal mode data unit, according to an embodiment; 
         FIG. 22B  is a diagram a long training field of a control mode data unit, according to an embodiment; 
         FIG. 23  is a diagram of a control mode OFDM data unit, according to an embodiment; 
         FIG. 24  is a diagram of a control mode OFDM data unit, according to another embodiment; 
         FIGS. 25A and 25B  are diagrams of a short training sequence subfield and a short training sequence subfield included in a control preamble and a normal mode preamble, respectively, according to an embodiment; 
         FIG. 25C  illustrates frequency domain values corresponding to a short training sequence of  FIG. 25A , according to an embodiment; 
         FIGS. 26A-26B  are diagrams of a control mode short training sequence and a normal mode short training sequence, respectively, according to an embodiment; 
         FIG. 26C  illustrates frequency domain values corresponding to the short training sequence of  FIG. 26A , according to an embodiment; 
         FIGS. 27A and 27B  are diagrams of a control mode data unit and a normal mode data unit, respectively, according to an embodiment; 
         FIGS. 28A-28B  are diagrams of a control mode data unit and a normal mode data unit, respectively, according to another embodiment; 
         FIGS. 29A-29B  are diagrams of a control mode data unit and a normal mode data unit, respectively, according to another embodiment; 
         FIGS. 30A-30B  are diagrams of a control mode data unit and a normal mode data unit, respectively, according to another embodiment; 
         FIGS. 31A-31B  are diagrams of a control mode preamble and a normal mode preamble, respectively, according to another embodiment; 
         FIGS. 32A-32B  are diagrams of a control mode preamble and a normal mode preamble, respectively, according to another embodiment; 
         FIGS. 33A-33B  are diagrams of a normal mode preamble and a control mode preamble, respectively, according to another embodiment; 
         FIG. 33C  is a diagram of a control mode preamble, according to another embodiment; 
         FIGS. 34A-34B  are diagrams of a control mode preamble and a normal mode preamble, respectively, according to another embodiment; 
         FIG. 35  is a flow diagram of an example method for generating a data unit, according to an embodiment; 
         FIG. 36  is a flow diagram of an example method for generating data units, according to an embodiment; 
         FIG. 37  is a flow diagram of another example method for generating a data unit, according to another embodiment; and 
         FIG. 38  is a flow diagram of another example method for generating a data unit, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In embodiments described below, a wireless network device such as an access point (AP) of a wireless local area network (WLAN) transmits data streams to one or more client stations. The AP is configured to operate with client stations according to at least a first communication protocol. The first communication protocol defines operation in a sub 1 GHz frequency range, and is typically used for applications requiring long range wireless communication with relatively low data rates. The first communication protocol (e.g., IEEE 802.11 of or IEEE 802.11ah) is referred to herein as a “long range” communication protocol. In some embodiments, the AP is also configured to communicate with client stations according to one or more other communication protocols which define operation in generally higher frequency ranges and are typically used for closer-range communications with higher data rates. The higher frequency communication protocols (e.g., IEEE 802.11a, IEEE 802.11n, and/or IEEE 802.11ac) are collectively referred to herein as “short range” communication protocols. 
     In some embodiments, the physical layer (PHY) data units conforming to the long range communication protocol (“long range data units”) are the same as or similar to data units conforming to a short range communication protocol (“short range data units”), but are generated using a lower clock rate. To this end, in an embodiment, the AP operates at a clock rate suitable for short range operation, and down-clocking is used to generate a clock to be used for the sub 1 GHz operation. As a result, in this embodiment, a data unit that conforms to the long range communication protocol (“long range data unit”) maintains a physical layer format of a data unit that conforms to a short range communication protocol (short range data unit”), but is transmitted over a longer period of time. In addition to this “normal mode” specified by the long range communication protocol, in some embodiments, the long range communication protocol also specifies a “control mode” with a reduced data rate compared to the lowest data rate specified for the normal mode. Because of the lower data rate, the control mode further extends communication range and generally improves receiver sensitivity. In some embodiments, the AP utilizes the control mode in signal beacon or association procedures and/or in transmit beamforming training operations, for example. Additionally or alternatively, the AP utilizes the control mode in situations in which longer range transmission is needed and a lower data rate is acceptable, such as, for example, to communicate with a smart meter or a sensor which periodically transmits small amounts of data (e.g., measurement readings) over a long distance. 
       FIG. 1  is a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment. An AP  14  includes a host processor  15  coupled to a network interface  16 . The network interface  16  includes a medium access control (MAC) processing unit  18  and a physical layer (PHY) processing unit  20 . The PHY processing unit  20  includes a plurality of transceivers  21 , and the transceivers  21  are coupled to a plurality of antennas  24 . Although three transceivers  21  and three antennas  24  are illustrated in  FIG. 1 , the AP  14  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  21  and antennas  24  in other embodiments. 
     The WLAN  10  includes a plurality of client stations  25 . Although four client stations  25  are illustrated in  FIG. 1 , the WLAN  10  can include different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations  25  in various scenarios and embodiments. At least one of the client stations  25  (e.g., client station  25 - 1 ) is configured to operate at least according to the long range communication protocol. In some embodiments, at least one of the client stations  25  (e.g., client station  25 - 4 ) is a short range client station that is configured to operate at least according to one or more of the short range communication protocols. 
     The client station  25 - 1  includes a host processor  26  coupled to a network interface  27 . The network interface  27  includes a MAC processing unit  28  and a PHY processing unit  29 . The PHY processing unit  29  includes a plurality of transceivers  30 , and the transceivers  30  are coupled to a plurality of antennas  34 . Although three transceivers  30  and three antennas  34  are illustrated in  FIG. 1 , the client station  25 - 1  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  30  and antennas  34  in other embodiments. 
     In an embodiment, one or both of the client stations  25 - 2  and  25 - 3 , has a structure the same as or similar to the client station  25 - 1 . In an embodiment, the client station  25 - 4  has a structure similar to the client station  25 - 1 . In these embodiments, the client stations  25  structured the same as or similar to the client station  25 - 1  have the same or a different number of transceivers and antennas. For example, the client station  25 - 2  has only two transceivers and two antennas, according to an embodiment. 
     In various embodiments, the PHY processing unit  20  of the AP  14  is configured to generate data units conforming to the long range communication protocol and having formats described hereinafter. The transceiver(s)  21  is/are configured to transmit the generated data units via the antenna(s)  24 . Similarly, the transceiver(s)  24  is/are configured to receive the data units via the antenna(s)  24 . The PHY processing unit  20  of the AP  14  is configured to process received data units conforming to the long range communication protocol and having formats described hereinafter, according to various embodiments. 
     In various embodiments, the PHY processing unit  29  of the client device  25 - 1  is configured to generate data units conforming to the long range communication protocol and having formats described hereinafter. The transceiver(s)  30  is/are configured to transmit the generated data units via the antenna(s)  34 . Similarly, the transceiver(s)  30  is/are configured to receive data units via the antenna(s)  34 . The PHY processing unit  29  of the client device  25 - 1  is configured to process received data units conforming to the long range communication protocol and having formats described hereinafter, according to various embodiments. 
     In some embodiments, the AP  14  is configured to operate in dual band configurations. In such embodiments, the AP  14  is able to switch between short range and long range modes of operation. According to one such embodiment, when operating in short range mode, the AP  14  transmits and receives data units that conform to one or more of the short range communication protocols. When operating in a long range mode, the AP  14  transmits and receives data units that conform to the long range communication protocol. Similarly, the client station  25 - 1  is capable of dual frequency band operation, according to some embodiments. In these embodiments, the client station  25 - 1  is able to switch between short range and long range modes of operation. In other embodiments, the AP  14  and/or the client station  25 - 1  is dual band device that is able to switch between different low frequency bands defined for long range operations by the long range communication protocol. In yet another embodiment, the AP  14  and/or the client station  25 - 1  is single band device configured to operate in only one long range frequency band. 
       FIG. 2A  is a diagram of a prior art OFDM short range data unit  200  that the AP  14  is configured to transmit to the client station  25 - 4  via orthogonal frequency division multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 4  is also configured to transmit the data unit  200  to the AP  14 . The data unit  200  conforms to the IEEE 802.11a Standard and occupies a 20 Megahertz (MHz) band. The data unit  200  includes a preamble having a legacy short training field (L-STF)  202 , generally used for packet detection, initial synchronization, and automatic gain control, etc., and a legacy long training field (L-LTF)  204 , generally used for channel estimation and fine synchronization. The data unit  200  also includes a legacy signal field (L-SIG)  206 , used to carry certain physical layer (PHY) parameters of with the data unit  200 , such as modulation type and coding rate used to transmit the data unit, for example. The data unit  200  also includes a data portion  208 .  FIG. 2B  is a diagram of example data portion  208  (not low density parity check encoded), which includes a service field, a scrambled physical layer service data unit (PSDU), tail bits, and padding bits, if needed. The data unit  200  is designed for transmission over one spatial or space-time stream in single input a single output (SISO) channel configuration. 
       FIG. 3  is a diagram of a prior art OFDM short range data unit  300  that the AP  14  is configured to transmit to the client station  25 - 4  via orthogonal frequency domain multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 4  is also configured to transmit the data unit  300  to the AP  14 . The data unit  300  conforms to the IEEE 802.11n Standard, occupies a 20 MHz band, and is designed for mixed mode situations, i.e., when the WLAN includes one or more client stations that conform to the IEEE 802.11a Standard but not the IEEE 802.11n Standard. The data unit  300  includes a preamble having an L-STF  302 , an L-LTF  304 , an L-SIG  306 , a high throughput signal field (HT-SIG)  308 , a high throughput short training field (HT-STF)  310 , and M data high throughput long training fields (HT-LTFs)  312 , where M is an integer which generally corresponds to a number of spatial streams used to transmit the data unit  300  in a multiple input multiple output (MIMO) channel configuration. In particular, according to the IEEE 802.11n Standard, the data unit  300  includes two HT-LTFs  312  if the data unit  300  is transmitted using two spatial streams, and four HT-LTFs  312  is the data unit  300  is transmitted using three or four spatial streams. An indication of the particular number of spatial streams being utilized is included in the HT-SIG field  308 . The data unit  300  also includes a data portion  314 . 
       FIG. 4  is a diagram of a prior art OFDM short range data unit  400  that the AP  14  is configured to transmit to the client station  25 - 4  via orthogonal frequency domain multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 4  is also configured to transmit the data unit  400  to the AP  14 . The data unit  400  conforms to the IEEE 802.11n Standard, occupies a 20 MHz band, and is designed for “Greenfield” situations, i.e., when the WLAN does not include any client stations that conform to the IEEE 802.11a Standard but not the IEEE 802.11n Standard. The data unit  400  includes a preamble having a high throughput Greenfield short training field (HT-GF-STF)  402 , a first high throughput long training field (HT-LTF 1 )  404 , a HT-SIG  406 , and M data HT-LTFs  408 , where M is an integer which generally corresponds to a number of spatial streams used to transmit the data unit  400  in a multiple input multiple output (MIMO) channel configuration. The data unit  400  also includes a data portion  410 . 
       FIG. 5  is a diagram of a prior art OFDM short range data unit  500  that the client station AP  14  is configured to transmit to the client station  25 - 4  via orthogonal frequency domain multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 4  is also configured to transmit the data unit  500  to the AP  14 . The data unit  500  conforms to the IEEE 802.11ac Standard and is designed for “Mixed field” situations. The data unit  500  occupies a 20 MHz or a 40 MHz bandwidth channel. In other embodiments or scenarios, a data unit similar to the data unit  500  occupies a channel of a different bandwidth, such as an 80 MHz, or a 160 MHz bandwidth channel, for example. The data unit  500  includes a preamble having an L-STF  502 , an L-LTF  504 , an L-SIG  506 , a first very high throughput signal field (VHT-SIG-A)  508 , a very high throughput short training field (VHT-STF)  510 , M very high throughput long training fields (VHT-LTFs)  512 , where M is an integer, and a second very high throughput signal field (VHT-SIG-B)  512 . The data unit  500  also includes a data portion  514 . In some embodiments, the data unit  500  is a multi-user data unit which carries information to more than one of the client stations  25  simultaneously. In such embodiments or scenarios, the first VHT-SIG-A includes information common to all of the intended client stations, and VHT-SIG-B includes user-specific information for each of the intended client stations. 
       FIG. 6  is a set of diagrams illustrating modulation of the L-SIG, HT-SIG 1 , and HT-SIG 2  fields as defined by the IEEE 802.11n Standard. The L-SIG field is modulated according to binary phase shift keying (BPSK), whereas the HT-SIG 1  and HT-SIG 2  fields are modulated according to BPSK, but on the quadrature axis (Q-BPSK). In other words, the modulation of the HT-SIG 1  and HT-SIG 2  fields is rotated by 90 degrees as compared to the modulation of the L-SIG field. As illustrated in  FIG. 6 , such modulation allows a receiving device to determine or auto-detect, without decoding the entire preamble, that the data unit conforms to the IEEE802.11n Standard rather than the IEE802.11a Standard. 
       FIG. 7  is a set of diagrams illustrating modulation of the L-SIG field, the first symbol of the VHT-SIG-A field, the second symbol of the VHT-SIG-A field, and VHT-SIG-B as defined by the IEEE 802.11ac Standard. The L-SIG is modulated according to binary phase shift keying (BPS K). Similarly, first symbol of the VHT-SIGA field is modulated according to BPSK. On the other hand, the second symbol of the VHT-SIG-A field is modulated according to BPSK, but on the quadrature axis (Q-BPSK). The VHT-SIG-B field is modulated according to BPSK, similar to the L-SIG-field and the first symbol of the VHT-SIG-A field. Similar to the 802.11n auto-detect feature discussed above, such modulation allows a receiving device to determine or auto-detect, without decoding the entire preamble, that the data unit conforms to the IEEE802.11ac Standard rather than either one of the IEE802.11a Standard or the IEEE802.11n Standard. 
       FIG. 8  is a diagram of a single carrier (SC) short range data unit  800  that the client station AP  14  configured to transmit to the client station  25 - 4  via a single carrier channel, according to an embodiment. In an embodiment, the client station  25 - 4  is also configured to transmit the data unit  800  to the AP  14 . The data unit  800  includes a SYNC field  802  that allows a receiver to detect presence of a data unit and begin synchronizing with the incoming signal. The data  800  also includes a start frame delimiter (SFD) field  804  that signals the beginning of a frame. The SYNC field  802  and the SFD field  804  form the preamble portion of the data unit  800 . The data unit  800  also includes a header portion having a signal field (SIGNAL)  806 , a service field (SERVICE)  808 , a length field (LENGTH)  810 , and a cyclic check redundancy check field (CRC)  812 . The data unit  800  also includes a physical layer service data unit (PSDU), i.e., the data portion  814 . 
     In various embodiments and/or scenarios, data units that conform to a long range communication protocol (e.g., the IEEE 802.11af or 802.11ah Standard) are formatted at least substantially the same as defined by the IEEE 802.11a Standard, the 802.11n Standard (mixed mode or Greenfield), or the 802.11ac Standard, as described and shown above in connection with  FIGS. 2-5 , but are transmitted at a lower frequency (e.g., sub-1 GHz) and using a slower clock rate. In some such embodiments, a transmitting device (e.g., the AP  14 ) down-clocks by a factor of N the clock rate used for generating the short range data units, to a lower clock rate to be used for generating the long range data units. The long range data unit is therefore generally transmitted over a longer time, and occupies a smaller bandwidth, than the corresponding short range data unit. The down-clocking factor N is different according to different embodiments and/or scenarios. In one embodiment, the down-clocking factor N is equal to 10. In other embodiments, other suitable down-clocking factor (N) values are utilized, and transmission times and bandwidths of long range data units are scaled accordingly. In some embodiments, the down-clocking factor N is a power of two (e.g., N=8, 16, 32, etc.). In some embodiments, in addition to down-clocked data units, which correspond to the “normal”, long range communication protocol also specifies a “control” mode (and a corresponding “control mode” data unit format) with a reduced data rate compared to the lowest data rate specified for the normal mode. Examples of normal mode data units generated using down-clocking, according to some such embodiments, are described blow with reference to  FIGS. 9-13  and are also described in U.S. patent application Ser. No. 13/359,336, filed on Jan. 26, 2012, which is hereby incorporated by reference herein in its entirety. 
       FIG. 9  is a diagram of an example normal mode OFDM long range data unit  900  that the AP  14  is configured to transmit to the client station  25 - 1  via orthogonal frequency domain multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 1  is also configured to transmit the data unit  900  to the AP  14 . The data unit  900  is similar to the data unit  500  of  FIG. 5  except that the data unit  900  is transmitted using a clock rate that is down-clocked from the short range clock rate by a down-clocking factor N. As a result, symbol duration of each OFDM symbol of the data unit  900  is N times longer compared to symbol duration of an OFDM symbol included in the data unit  500 . In the embodiment of  FIG. 9 , N is equal to 10. Accordingly, each OFDM symbol included in the data unit  900  is 10 times longer compared to an OFDM symbol included in the data unit  500 . In other embodiments, other suitable down-clocking factors are utilized. 
       FIG. 10  is a diagram of an example normal mode OFDM long range data unit  1000  that the AP  14  is configured to transmit to the client station  25 - 1  via orthogonal frequency domain multiplexing (OFDM) modulation, according to an embodiment. In an embodiment, the client station  25 - 1  is also configured to transmit the data unit  1000  to the AP  14 . The data unit  1000  is similar to the “green-field” data unit  400  of  FIG. 4 , except that the data unit  1000  is transmitted using a clock rate that is down-clocked from the short range clock rate by a down-clocking factor N. As a result, symbol duration of each OFDM symbol of the data unit  1000  is N times longer compared to symbol duration of an OFDM symbol included in the data unit  400 . In the embodiment of  FIG. 10 , N is equal to 10. Accordingly, each OFDM symbol included in the data unit  1000  is 10 times longer compared to an OFDM symbol included in the data unit  400 . In other embodiments, other suitable down-clocking factors are utilized. 
       FIG. 11  is a diagram of an example normal mode OFDM long range data unit  1100  that the AP  14  is configured to transmit to the client station  25 - 1  via orthogonal frequency domain multiplexing (OFDM) modulation when operating in a long range mode, according to an embodiment. In an embodiment, the client station  25 - 1  is also configured to transmit the data unit  1100  to the AP  14 . The data unit  1100  is similar to the data unit  500  of  FIG. 5 , except that the data unit  1100  is transmitted using a clock rate that is down-clocked from the short range clock rate by a down-clocking factor N. As a result, symbol duration of each OFDM symbol of the data unit  1100  is N times longer compared to symbol duration of an OFDM symbol included in the data unit  500 . In the embodiment of  FIG. 10 , N is equal to 10. Accordingly, each OFDM symbol included in the data unit  1100  is 10 times longer compared to an OFDM symbol included in the data unit  500 . In other embodiments, other suitable down-clocking factors are utilized. 
       FIG. 12  is a diagram of an example normal mode OFDM long range data unit  1200  that the AP  14  is configured to transmit to the client station  25 - 1  via orthogonal frequency domain multiplexing (OFDM) modulation when operating in a long range mode, according to an embodiment. The data unit  1200  is similar to the data unit  1100  of  FIG. 11  except that that the legacy portion of the preamble (i.e., L-STF  1102 , L-LTF  1104 , L-SIG  1106 ) is omitted from the data unit  1200 . In one embodiment, the VHT-SIG-B field  1214  is omitted from the data unit  1200 . Further, bit allocations for some or all fields of the data unit  1200  are different from the bit allocations defined by a short range communication protocol in some embodiments. 
       FIG. 13  is a diagram of an example normal mode OFDM long range data unit  1300  that the AP  14  is configured to transmit to the client station  25 - 1  via orthogonal frequency domain multiplexing (OFDM) modulation when operating in a long range mode, according to an embodiment. In an embodiment, the client station  25 - 1  is also configured to transmit the data unit  1300  to the AP  14 . The data unit  1300  is similar to the data unit  200  of  FIG. 2 , except that the data unit  1300  is transmitted using a clock rate that is down-clocked from the short range clock rate by a down-clocking factor N. As a result, symbol duration of each OFDM symbol of the data unit  1300  is N times longer compared to symbol duration of an OFDM symbol included in the data unit  200 . In the embodiment of  FIG. 10 , N is equal to 10. Accordingly, each OFDM symbol included in the data unit  1300  is 10 times longer compared to an OFDM symbol included in the data unit  200 . In other embodiments, other suitable down-clocking factors are utilized. 
       FIG. 14  is a block diagram of an example PHY processing unit  1400  for generating normal mode data units, according to an embodiment. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1400 . In various embodiments and/or scenarios, the PHY processing unit  1400  generates long range data units such as one of the data units of  FIGS. 9-13 , for example. 
     The PHY processing unit  1400  includes a scrambler  1402  that generally scrambles an information bit stream to reduce the occurrence of long sequences of ones or zeros. An FEC encoder  1406  encodes scrambled information bits to generate encoded data bits. In one embodiment, the FEC encoder  1406  includes a binary convolutional code (BCC) encoder. In another embodiment, the FEC encoder  1406  includes a binary convolutional encoder followed by a puncturing block. In yet another embodiment, the FEC encoder  1406  includes a low density parity check (LDPC) encoder. An interleaver  1410  receives the encoded data bits and interleaves the bits (i.e., changes the order of the bits) to prevent long sequences of adjacent noisy bits from entering a decoder at the receiver. A constellation mapper  1414  maps the interleaved sequence of bits to constellation points corresponding to different subcarriers of an OFDM symbol. More specifically, for each spatial stream, the constellation mapper  1414  translates every bit sequence of length log 2 (M) into one of M constellation points. 
     The output of the constellation mapper  1414  is operated on by an inverse discrete Fourier transform (IDFT) unit  1418  that converts a block of constellation points to a time-domain signal. In embodiments or situations in which the PHY processing unit  1400  operates to generate data units for transmission via multiple spatial streams, the cyclic shift diversity (CSD) unit  1422  inserts a cyclic shift into all but one of the spatial streams to prevent unintentional beamforming. The output of the CSD unit  1422  is provided to the guard interval (GI) insertion and windowing unit  1426  that prepends, to an OFDM symbol, a circular extension of the OFDM symbol and smooths the edges of each symbol to increase spectral decay. The output of the GI insertion and windowing unit  1426  is provided to the analog and radio frequency (RF) unit  1430  that converts the signal to analog signal and upconverts the signal to RF frequency for transmission. 
     In various embodiments, control mode corresponds to the lowest data rate MCS of the normal mode and introduces redundancy or repetition of bits into at least some fields of the data unit to further reduce the data rate. For example, control mode introduces redundancy into the data portion and/or the signal field of a control mode data unit according to one or more repetition and coding schemes described below, in various embodiments and/or scenarios. As an example, according to an embodiment, data units in normal mode are generated according a particular modulation and coding scheme (MCS), e.g., and MCS selected from a set of MCSs, such as MCS 0  (binary phase shift keying (BPSK) modulation and coding rate of ½) to MCS 9  (quadrature amplitude modulation (QAM) and coding rate of ⅚), with higher order MCSs corresponding to higher data rates. Control mode data units, in one such embodiment, are generated using modulation and coding as defined by MCS 0  and with added bit repletion or block encoding that further reduce the data rate. 
       FIG. 15A  is a block diagram of an example PHY processing unit  1500  for generating control mode data units, according to an embodiment. In some embodiments, the PHY processing unit  1500  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1500 . 
     The PHY processing unit  1500  is similar to the PHY processing unit  1400  of  FIG. 14  except that the PHY processing unit  1500  includes a block coding unit  1504  coupled to the scrambler  1502 . In an embodiment, the block coding unit  1504  reads incoming (scrambled) information bits one block at a time, generates a number of copies of each block (or each bit in a block), interleaves the resulting bits according to a coding scheme and outputs the interleaved bits for further encoding by the BCC encoder  1506 . Generally, each block contains the number of information bits that, after having been encoded by the block coding unit  1504  and by the BCC encoder  1506 , fill the data tones of a single OFDM symbol, according to an embodiment. As an example, in one embodiment, the block coding unit  1504  generates two copies (2× repetition) of each block of 12 information bits to generate 24 bits to be included in an OFDM symbol. The 24 bits are then encoded by the BCC encoder  1506  at the coding rate of ½ to generate 48 bits that modulate 48 data tones of an OFDM symbol (e.g., using BPSK modulation). As another example, in another embodiment, the block coding unit  1504  generates four copies (4× repetition) of each block of 6 information bits to generate 24 bits which are then encoded by the BCC encoder  1506  at the coding rate of ½ to generate 48 bits that modulate 48 data tones of an OFDM symbol. As yet another example, in another embodiment, the block coding unit  1504  generates two copies (2× repetition) of each block of 13 information bits to generate 26 bits which are then encoded by the BCC encoder  1506  at the coding rate of ½ to generate 52 bits that modulate 52 data tones of an OFDM symbol. 
     In some embodiments, the block coding unit  1504  applies a 4× repetition scheme when generating a data (or a signal) field as defined by MCS 0  as specified in the IEEE 802.11n Standard for 20 MHz channel, i.e. with 52 data tones per OFDM symbol. In this case, according to an embodiment, the block coding unit  1504  generates four copies of each block of 6 information bits to generate 24 bits and then adds two padding bits (i.e., two bits of a predetermined values) to provide the specified number of bits (i.e., 26 bits for 52 data tones) to the BCC encoder which encoded the 26 bits using the coding rate of ½ to generate 52 coded bits for modulating the 52 data tones. 
     In one embodiment, the block coding unit  1504  utilizes a “block level” repetition scheme in which each block of n bits is repeated m consecutive times. As an example, if m is equal to 4 (4× repetitions), the block coding unit  1504  generates a sequence [C, C, C, C], where C is a block of n bits, according to an embodiment. In another embodiment, the block coding unit  1504  utilizes a “bit level” repetition scheme in which each incoming bit is repeated m consecutive times. In this case, in an embodiment, if m is equal to 4 (4× repetitions), the block coding unit  1504  generates the sequence [b 1  b 1  b 1  b 1  b 2  b 2  b 2  b 2  b 3  b 3  b 3  b 3  . . . ], where b 1  is the first bit in the block of bits, b 2  is the second bit, and so on. In yet another embodiment, the block coding unit  1504  generates m number of copies of the incoming bits and interleaves the resulting bit stream according to any suitable code. Alternatively, in still another embodiment, the block coding unit  1504  encodes incoming bits or incoming blocks of bits using any suitable code, e.g., a Hamming block code with the coding rate of a ½, ¼, etc., or any other block code with the coding rate of ½, ¼, etc. (e.g., (1, 2) or (1, 4) block code, (12, 24) block code or (6, 24) block code, a (13, 26) block code, etc.). 
     According to an embodiment, the effective coding rate corresponding to a combination of the coding performed by the block coding unit  1504  and coding performed by the BCC encoder  1506  the product of the two coding rates. For example, in an embodiment in which the block coding unit  1504  utilizes 4× repetition (or coding rate of ¼) and the BCC encoder  1506  utilizes a coding rate of ½, the resulting effective coding rate is equal to ⅛. As a result of the reduced coding rate compared to the coding rate used to generate a similar normal mode data unit, data rate in control mode is effectively reduced by a factor corresponding to the number the coding rate applied by the block coding unit  1504  (e.g., a factor of 2, a factor of 4, etc.), according to an embodiment. 
     According to some embodiments, the block coding unit  1504  utilizes the same block coding scheme for generating the signal field of a control mode data unit as the block coding scheme used for generating the data portion of the control mode data unit. For instance, in an embodiment, an OFDM symbol of the signal field and an OFDM symbol of the data portion each includes 48 data tones, and in this embodiment, the block coding unit  1504  applies a 2× repetition scheme to blocks of 12 bits for the signal field and the data portion, for example. In another embodiment, the data portion and the signal field of a control mode data unit are generated using different block coding schemes. For example, in an embodiment, the long range communication protocol specifies a different number of data tones per OFDM symbol in the signal field compared to the number of data tones per OFDM symbol in the data portion. Accordingly, in this embodiment, the block coding unit  1504  utilizes a different block size and, in some embodiments, a different coding scheme, when operating on the signal field compared to the block size and the coding scheme used for generating the data portion. For example, if the long range communication protocol specifies 52 data tones per OFDM symbol of the signal field and 48 data tones per OFDM tones of the data portion, the block coding unit  1504  applies a 2× repetition scheme to blocks of 13 bits of the signal field and a 2× repetition scheme to blocks of 12 bits of the data portion, according to one embodiment. 
     The BCC encoder  1506  encodes the block coded information bits, according to an embodiment. In an embodiment, BCC encoding is performed continuously over the entire field being generated (e.g., the entire data field, the entire signal field, etc.). Accordingly, in this embodiment, information bits corresponding to the field being generated are partitioned into blocks of a specified size (e.g., 6 bits, 12 bits, 13 bits, or any other suitable number of bits), each block is processed by the block coding unit  1504 , and the resulting data stream is then provided to the BCC encoder  1506  which continuously encodes the incoming bits. 
     Similar to the interleaver  1410  of  FIG. 14 , in various embodiments, the interleaver  1510  changes the order of bits in order to provide diversity gain and reduce the chance that consecutive bits in a data stream will become corrupted in the transmission channel. In some embodiments, however, the block coding unit  1504  provides sufficient diversity gain and the interleaver  1510  is omitted. 
     In some embodiments, information bits in the data portion of a control mode data unit are be padded (i.e., a number of bits of a known value is added to the information bits) so that the data unit occupies an integer number of OFDM symbols, for example. Referring to  FIG. 1 , in some embodiments, padding is implemented in the MAC processing unit  18 ,  28  and/or the PHY processing unit  20 ,  29 . In some such embodiments, the number of padding bits is determined according to padding equations provided in a short range communication protocol (e.g., the IEEE 802.11a Standard, the IEEE 802.11n Standard, the IEEE 802.11ac Standard, etc.). In general, these padding equations involve computing a number of padding bits based, in part, on a number of data bits per OFDM symbol (N DBPS ) and/or a number coded data bits per symbol (N CBPS ). In control mode, according to an embodiment, the number of padding bits is determined based on the number of information bits in an OFDM symbol (e.g., 6 bits, 12 bits, 13 bits, etc.) before the information bits are block encoded by the block coding unit  1504  and BCC encoded by the BCC encoder  1506 . Accordingly, the number of padding bits in a control mode data unit is generally different from the number of padding bits in the corresponding normal mode data (or in the corresponding short range data unit). On the other hand, according to an embodiment, the number of coded bits per symbol is the same as the number of coded bits per symbol in normal mode data unit (or in the corresponding short range data unit), e.g., 24, 48, 52, etc. coded bits per OFDM. 
       FIG. 15B  is a block diagram of an example PHY processing unit  1550  for generating control mode data units, according to another embodiment. In some embodiments, the PHY processing unit  1550  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1550 . 
     The PHY processing unit  1550  is similar to the PHY processing unit  1500  of  FIG. 15 , except that in the PHY processing unit  1550 , the BCC encoder  1506  is replaced by the LDPC encoder  1556 . Accordingly, in this embodiment, the output of the block coding unit  1504  is provided for further block encoding by the LDPC encoder  1556 . In an embodiment, the LDPC encoder  1556  utilizes a block code corresponding to a coding rate of ½, or a block code corresponding to another suitable coding rate. In the illustrated embodiment, the PHY processing unit  1550  omits the interleaver  1510  because adjacent bits in an information stream are generally spread out by the LDPC code itself and no further interleaving is needed. Additionally, in an embodiment, further frequency diversity is provided by the LDPC tone remapping unit  1560 . According to an embodiment, the LDPC tone remapping unit  1560  reorders coded information bits or blocks of coded information bits according to a tone remapping function. The tone remapping function is generally defined such that consecutive coded information bits or blocks of information bits are mapped onto nonconsecutive tones in the OFDM symbol to facilitate data recovery at the receiver in cases in which consecutive OFDM tones are adversely affected during transmission. In some embodiments, the LDPC tone remapping unit  1560  is omitted. Referring again to  FIG. 15A , in various embodiments, a number of tail bits are typically added to each field of a data unit for proper operation of the BCC encoder  1506 , e.g., to ensure that the BCC encoder, after having encoded each field, is brought back to zero state. In one embodiment, for example, six tail bits are inserted at the end of the data portion before the data portion is provided to the BCC encoder  1506  (e.g., after the bits are processed by the block coding unit  1504 ). 
     Similarly, in the case of a signal field, tail bits are inserted into the signal field before the signal field is provided to the BCC encoder  1506 , in various embodiments.  FIG. 16A  is a diagram of an example signal field  1600 , according to an embodiment. The signal field  1600  includes a four bit rate subfield  1604 , a twelve bit length subfield  1604  and one bit parity bit  1606 . The signal field  1600  also includes a five bit reserved subfield  1608 . In this embodiment, bit allocation of the signal field  1600  does not include bits allocated for tail bits.  FIG. 16B  is a diagram of a block encoded signal field  1650 , according an embodiment. In an embodiment, the block coding unit  1504  of  FIG. 15  generates the signal field  1650  by encoding the signal field  1600  of  FIG. 16A . The block coded signal field  1650  includes four OFDM symbols  1652 , wherein each OFDM symbol  1652  includes four repetitions of a block of six bits of the signal field  1600 . As illustrated, six tail bits and two padding bits are added at the end of the last OFDM of the block encoded signal field (OFDM symbol  1652 - 4 ). 
     Alternatively, tail bits are inserted into a signal field before the signal field is block encoded, in another embodiment. Accordingly, in this case, the inserted tail bits are repeated or otherwise encoded (e.g., by the block coding unit  1504  of  FIG. 15 ).  FIG. 17A  is a diagram of a signal field  1700  in which six tail bits are inserted at the end of the signal field before block encoding, according to an embodiment. Signal field  1700  includes a four bit rate subfield  1702 , a one bit reserved subfield  1704 , a twelve bit length subfield  1706 , followed by one parity bit ( 1708 ) and six tail bits ( 1710 ).  FIG. 17B  is a diagram of a block encoded signal field  1750 , according an embodiment. In an embodiment, the block coding unit  1504  of  FIG. 15  generates the signal field  1750  by encoding the signal field  1700  of  FIG. 17A . As in the example of the block coded signal field  1650  of  FIG. 16 , the signal field  1750  includes four OFDM symbols. In this embodiment, however, the block coded signal field  1750  includes multiple repetitions (four repetitions, in this case) of the tail bits. 
     In some embodiments, the signal field of a control mode data unit has a different format compared to the signal field format of a normal mode data unit. In some such embodiment, the signal field of control mode data units is shorter compared to a signal field of a normal mode data unit. For example, only one modulation and coding scheme is used in control mode, according to an embodiment, and therefore less information (or no information) regarding modulation and coding needs to be communicated in the control mode signal field. Similarly, in an embodiment, maximum length of a control mode data unit is shorter compared to a maximum length of a normal mode data unit and, in this case, less bits are needed for the length subfield of the control mode signal field. As an example, in one embodiment, a control mode signal field is formatted according to the IEEE 802.11n Standard but omits certain subfields (e.g., the low density parity check (LDPC) subfield, the space time block coding (STBC) subfield, etc.). Additionally or alternatively, in some embodiments, a control mode signal field includes a shorter CRC subfield compared to the cyclic redundancy check (CRC) subfield of a normal mode signal field (e.g., less than 8 bits). In general, in control mode, certain signal field subfields are omitted or modified and/or certain new information is added, according to some embodiments. 
       FIG. 17C  is diagram of a control mode signal field  1760 , according to one such embodiment. The control mode signal field  1760  is similar to the signal field  1700  of  FIG. 17A , but the rate subfield is omitted from the signal field  1760  (e.g., because only one rate is specified for control mode). Additionally, the signal field  1760  includes a number of space time streams (Nsts) subfield  1764  for signaling null data packets in a channel sounding procedure, for example. 
       FIG. 17D  is diagram of a control mode signal field  1770 , according to another embodiment. The signal field  1770  is similar to the signal field  1700  of  FIG. 17A , but omits a rate field, includes a null data packet field (NDP)  1772  for signaling null data packets in a channel sounding procedure, for example. The signal field  1770  also includes an indication of the number of space time streams in the length field  1774 . 
       FIG. 18A  is a block diagram of an example PHY processing unit  1800  for generating control mode data units, according to another embodiment. In some embodiments, the PHY processing unit  1800  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1800 . 
     The PHY processing unit  1800  is similar to the PHY processing unit  1500  of  FIG. 15 , except that in the PHY processing unit  1800  the block coding unit  1809  is located after the BCC encoder ( 1806 ). Accordingly, in this embodiment, information bits are first encoded by the BCC encoder  1806  and the BCC coded bits are then replicated or otherwise block encoded by the block coding unit  1808 . As in the example embodiment of the PHY processing unit  1500 , in an embodiment, processing by the BCC encoder  1806  is performed continuously over the entire field being generated (e.g., the entire data portion, the entire signal field, etc.). Accordingly, in this embodiment, information bits corresponding to the field being generated are first encoded by the BCC encoder  1806  and the BCC coded bits are then partitioned into blocks of a specified size (e.g., 6 bits, 12 bits, 13 bits, or any other suitable number of bits). Each block is then processed by the block coding unit  1808 . As an example, in one embodiment, the BCC encoder  1806  encodes 12 information bits per OFDM symbol using the coding rate of ½ to generate 24 BCC coded bits and provides the BCC coded bits to the block coding unit  1808 . In an embodiment, the block coding unit  1808  generates two copies of each incoming block and interleaves the generated bits according to a coding scheme to generate 48 bits to be included in an OFDM symbol. In one such embodiment, the 48 bits correspond to 48 data tones generated using a Fast Fourier Transform (FFT) of size 64 at the IDFT processing unit  1818 . As another example, in another embodiment, the BCC encoder  1806  encodes 6 information bits per OFDM symbol using the coding rate of ½ to generate 12 BCC coded bits and provides the BCC coded bits to the block coding unit  1808 . In an embodiment, the block coding unit  1808  generates two copies of each incoming block and interleaves the generated bits according to a coding scheme to generate 24 bits to be included in an OFDM symbol. In one such embodiment, the 24 bits correspond to 24 data tones generated using an FFT of size 32 at the IDFT processing unit  1818 . 
     Similar to the block coding unit  1504  of  FIG. 15 , the repetition and coding scheme used by the block coding unit  1808  to generate the signal field of a control mode data unit, depending on an embodiment, is the same as or different from the repetition and coding scheme used by the block coding unit  1808  to generate the data portion of the control mode data unit. In various embodiments, the block coding unit  1808  implements a “block level” repetition scheme or a “bit level” repetition scheme as discussed above in regard to the block coding unit  1504  of  FIG. 15 . Similarly, in another embodiment, the block coding unit  1808  generates m number of copies of the incoming bits and interleaves the resulting bit stream according to a suitable code, or otherwise encodes incoming bits or incoming blocks of bits using any suitable code, e.g., a Hamming block code with the coding rate of a ½, ¼, etc., or any other block code with the coding rate of ½, ¼, etc. (e.g., (1, 2) or (1, 4) block code, (12, 24) block code or (6, 24) block code, a (13, 26) block code, etc.). 
     The effective coding rate for data units generated by the PHY processing unit  1800  is a product of the coding rate used by the BCC encoder  1806  and the number of repetitions (or the coding rate) used by the block coding unit  1808 , according to an embodiment. 
     In an embodiment, the block coding unit  1808  provides sufficient diversity gain such that no further interleaving of coded bits is needed, and the interleaver  1810  is omitted. One advantage of omitting the interleaver  1810  is that in this case OFDM symbols with 52 data tones can be generated using 4× or a 6× repetition schemes even though in some such situations the number of data bits per symbol is not an integer. For example, in one such embodiment, the output of the BCC encoder  1806  is partitioned into blocks of 13 bits and each block is repeated four times (or block encoded with a rate of ¼) to generate 52 bits to be included in an OFDM symbol. In this case, if the BCC encoder utilizes a coding rate of ½, the number of data bits per symbol is equal 6.5. In an example embodiment utilizing 6× repetition, the BCC encoder  1806  encodes information bits using a coding rate of ½ and the output is partitioned into blocks of four bits. The block coding unit  1808  repeats each four bit block six times (or block encodes each block using a coding rate of ⅙) and adds four padding bits to generate 52 bits to be included in an OFDM symbol. 
     As in the example of the PHY processing unit  1500  of  FIG. 15  discussed above, if padding is used by the PHY processing unit  1800 , the number of data bits per symbol (N DBPS ) used for padding bit computations is the actual number of non-redundant data bits in an OFDM symbol (e.g., 6 bits, 12 bits, 13 bits as in the example above, or any other suitable number of bits). The number of coded bits per symbol (N CBPS ) used in padding bit computations is equal to the number of bits actually included in an OFDM symbol (e.g., 24 bits, 48 bits, 52 bits, or any other suitable number of bits included in an OFDM symbol). 
     Also as in the example of the PHY processing unit  1500  of  FIG. 15 , a number of tail bits are typically inserted into each field of a data unit for proper operation of the BCC encoder  1806 , e.g., to ensure that the BCC encoder, after having encoded each field, is brought back to zero state. In one embodiment, for example, six tail bits are inserted at the end of the data portion before the data portion is provided to the BCC encoder  1806  (i.e., after processing by the block coding unit  1504  is performed). Similarly, in the case of a signal field, tail bits are inserted at the end of the signal field before the signal field is provided to the BCC encoder  1806 , according to an embodiment. In an example embodiment in which the block coding unit  1808  utilizes a 4× repetition scheme (or another block code with the coding rate of ¼), the BCC encoder  1806  utilizes the coding rate of ½, and the signal field includes 24 information bits (including tail bits), the 24 signal field bits are BCC encoded to generate 48 BCC encoded bits which are then partitioned into four blocks of 12 bits each for further encoding by the block coding unit  1808 . Accordingly, in this embodiment, the signal field is transmitted over four OFDM symbols each of which includes 6 information bits of the signal field. 
     As discussed above with regard to  FIGS. 15-17 , in some embodiments, the signal field of a control mode data unit is shorter (i.e. includes less bits) compared to a signal field of a normal mode data unit. Accordingly, in these embodiments, less OFDM symbols are generally needed to transmit the signal field compared to embodiments in which a longer signal field is generated using the same repetition and coding scheme. 
     Further, in some embodiments, the PHY processing unit  1800  generates OFDM symbols with 52 data tones according to the MCS 0  specified in the IEEE 802.11n Standard or the IEEE 802.11ac Standard and the block coding unit  1808  utilizes a 4× repetition scheme. In some such embodiments, extra padding is used to ensure that the resulting encoded data stream to be included in an OFDM symbol includes 52 bits. In one such embodiment, padding bits are added to coded information the bits after the bits have been processed by the block coding unit  1808 . 
     In the embodiment of  FIG. 18A , the PHY processing unit  1800  also includes a peak to average power ratio (PAPR) reduction unit  1809 . In an embodiment, the peak to average power ratio unit reduction  1809  flips bits in some or all repeated blocks to reduce or eliminate the occurrence of the same bit sequences at different frequency locations in an OFDM symbol thereby reducing the peak to average power ratio of the output signal. In general, bit flipping involves changing the bit value of zero to the bit value of one and changing the bit vale of one to the bit value of zero. According to an embodiment, the PAPR reduction unit  1809  implements bit flipping using an XOR operation. For example, in an embodiment utilizing 4× repetition of a block of coded bits, if a block of coded bits to be included in an OFDM symbols is denoted as C and if C′=C XOR  1  (i.e., block C with bits flipped), then some possible bit sequences at the output of the PAPR reduction unit  1809 , according to some embodiments, are [C C′ C′ C′], [C′ C′ C′ C], [C C′ C C′], [C C C C′], etc. In general, any combination of block with bits flipped and blocks with bits not flipped can be used. In some embodiments, the PAPR unit  1809  is omitted. 
     In an embodiment, the PAPR reduction unit  1809  is configured to include in the output of the PAPR reduction unit  1809  blocks D, where each block D corresponds to a respective block C exclusive ORed (XORed) with a sequence M, where M is a suitable sequence of bits chosen from all possible sequences M such that, when utilized, the sequence M minimizes PAPR. In another embodiment, M is a suitable sequence that includes both ones and zeros and that, when utilized, reduces PAPR as compared to XORing each block C with only ones. 
     M is a sequence having the same length as the block C. As discussed above, each block D is equal to a respective block C XORed with M, in some embodiments. The PAPR reduction unit  1809  is configured to include the blocks D in the output of the PAPR reduction unit  1809 , in some embodiments. For example, in an embodiment utilizing 2× repetition of a block C of coded bits, two possible bit sequences at the output of the PAPR reduction unit  1809 , according to some embodiments, are [C D] and [D C]. As another example, in an embodiment utilizing 4× repetition of a block C of coded bits, possible bit sequences at the output of the PAPR reduction unit  1809 , according to some embodiments, include [C C C D], [C D C D], [D C D C], [D D D C], etc. In one embodiment, each block C has a length of 12-bits, each block D has a length of 12-bits, and each block M has a length of 12-bits, and 2× repetition is utilized. In other embodiments, other suitable block lengths, and/or other suitable repetition rates (e.g.,  4   x  repetition) are utilized. 
       FIG. 18B  is a block diagram of an example PHY processing unit  1850  for generating control mode data units, according to another embodiment. In some embodiments, the PHY processing unit  1850  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1850 . 
     The PHY processing unit  1850  is similar to the PHY processing unit  1800  of  FIG. 18A , except that in the PHY processing unit  1850 , the BCC encoder  1506  is replaced by the LDPC encoder  1856 . Accordingly, in this embodiment, information bits are first encoded by the LDPC encoder  1856  and the LDPC coded bits are then replicated or otherwise block encoded by the block coding unit  1808 . In an embodiment, the LDPC encoder  1856  utilizes a block code corresponding to a coding rate of ½, or a block code corresponding to another suitable coding rate. In the illustrated embodiment, the PHY processing unit  1850  omits the interleaver  1810  because adjacent bits in an information stream are generally spread out by the LDPC code itself and, according to an embodiment, no further interleaving is needed. Additionally, in an embodiment, further frequency diversity is provided by the LDPC tone remapping unit  1860 . According to an embodiment, the LDPC tone remapping unit  1860  reorders coded information bits or blocks of coded information bits according to a tone remapping function. The tone remapping function is generally defined such that consecutive coded information bits or blocks of information bits are mapped onto nonconsecutive tones in the OFDM symbol to facilitate data recovery at the receiver in cases in which consecutive OFDM tones are adversely affected during transmission. In some embodiments, the LDPC tone remapping unit  1860  is omitted. 
       FIG. 19A  is a block diagram of an example PHY processing unit  1900  for generating control mode data units, according to another embodiment. In some embodiments, the PHY processing unit  1900  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1900 . 
     The PHY processing unit  1900  is similar to the PHY processing unit  1800  of  FIG. 18A  except that in the PHY processing unit  1900  the block coding unit  1916  is located after the constellation mapper  1914 . Accordingly, in this embodiment, BCC encoded information bits, after having been processed by the interleaver  1910 , are mapped to constellation symbols and the constellation symbols are then replicated or otherwise block encoded by the block coding unit  1916 . According to an embodiment, processing by the BCC encoder  1906  is performed continuously over the entire field being generated (e.g., the entire data field, the entire signal field, etc.). In this embodiment, information bits corresponding to the field being generated are first encoded by the BCC encoder  1806  and the BCC coded bits are then mapped to constellation symbols by the constellation mapper  1914 . The constellation symbols are then partitioned into blocks of a specified size (e.g., 6 symbols, 12 symbols, 13 symbols, or any other suitable number of symbols) and each block is then processed by the block coding unit  1916 . As an example, in an embodiment utilizing 2× repetition, the constellation mapper  1914  generates 24 constellation symbols and the block coding unit  1916  generates two copies of the 24 symbols to generate 48 symbols corresponding to 48 data tones of an OFDM symbol (e.g., as specified in the IEEE 802.11a Standard). As another example, in an embodiment utilizing 4× repetition, the constellation mapper  1914  generates 12 constellation symbols and the block coding unit  1916  generates four copies of the 12 constellation symbols to generate 48 symbols corresponding to 48 data tones of an OFDM symbol (e.g., as specified in the IEEE 802.11a Standard). As yet another example, in an embodiment utilizing 2× repetition, the constellation mapper  1914  generates 26 constellation symbols and the block coding unit  1916  repeats the 26 symbols (i.e., generates two copies of the 26 symbols) to generate 52 symbols corresponding to 52 data tones of an OFDM symbol (e.g., as specified in the IEEE 802.11n Standard or the IEEE 802.11ac Standard). In general, in various embodiments and/or scenarios, the block coding unit  1916  generates any suitable number of copies of blocks of incoming constellation symbols and interleaves the generated symbols according to any suitable coding scheme. Similar to the block coding unit  1504  of  FIG. 15  and the block coding unit  1808  of  FIG. 18 , the repetition and coding scheme used by the block coding unit  1916  to generate a signal field (or signal fields) of a control mode data unit is, depending on the embodiment, the same as or different from the repetition and coding scheme used by the block coding unit  1916  to generate the data portion of the control mode data unit. 
     The effective coding rate for data units generated by the PHY processing unit  1900  is a product of the coding rate used by the BCC encoder  1906  and the number of repetitions (or the coding rate) used by the block coding unit  1916 , according to an embodiment. 
     According to an embodiment, because redundancy in this case is introduced after the information bits have been mapped to constellation symbols, each OFDM symbol generated by the PHY processing unit  1900  includes less non-redundant data tones compared to OFDM data tones included in a normal mode data units. Accordingly, the interleaver  1910  is designed to operate on less tones per OFDM symbol compared to the interleaver used in the normal mode (such as the interleaver  1410  of  FIG. 14 ), or the interleaver used in generating the corresponding short range data unit. For example, in an embodiment with 12 non-redundant data tones per OFDM symbol, the interleaver  1910  is designed using the number of columns (Ncol) of 6 and the number of rows (Nrow) of 2*the number of bits per subcarrier (Nbpscs). In another example embodiment with 12 non-redundant data tones per OFDM symbol, the interleaver  1910  is designed using Ncol of 4 and Nrow of 3*Nbpscs. In other embodiments, other interleaver parameters different from interleaver parameter used in the normal mode are utilized for the interleaver  1910 . Alternatively, in an embodiment, the block coding unit  1916  provides sufficient diversity gain such that no further interleaving of coded bits is needed, and the interleaver  1910  is omitted. In this case, as in the example embodiment utilizing the PHY processing unit  1800  of  FIG. 18 , OFDM symbols with 52 data tones can be generated using 4× or a 6× repetition schemes even though in some such situations the number of data bits per symbol is not an integer. 
     As in the example embodiment of the PHY processing unit  1500  of  FIG. 15A  or the PHY processing unit  1800  of  FIG. 18A  discussed above, if padding is used by the PHY processing unit  1900 , the number of data bits per symbol (N DBPS ) used for padding bit computations is the actual number of non-redundant data bits in an OFDM symbol. (e.g., 6 bits, 12 bits, 13 bits as in the example above, or any other suitable number of bits). The number of coded bits per symbol (N CBPS ) used in padding bit computations is equal to the number of non-redundant bits included in an OFDM symbol which, in this case, corresponds to number of bits in the block of constellation symbols processed by the block coding unit  1916  (e.g., 12 bits, 24 bits, 26 bits, etc.). 
     Also as in the example embodiment of the PHY processing unit  1500  of  FIG. 15  discussed above, a number of tail bits are typically inserted into each field of a data unit for proper operation of the BCC encoder  1906 , e.g., to ensure that the BCC encoder, after having encoded each field, is brought back to zero state. In one embodiment, for example, six tail bits are inserted at the end of the data portion before the data portion is provided to the BCC encoder  1906 . Similarly, in the case of a signal field, tail bits are inserted at the end of the signal field before the signal field is provided to the BCC encoder  1906 , according to an embodiment. In an example embodiment in which the block coding  1916  utilizes a 4× repetition scheme (or another block code with the coding rate of ¼), the BCC encoder  1906  utilizes the coding rate of ½, and the signal field includes 24 information bits (including tail bits), the 24 signal field bits are BCC encoded to generate 48 BCC encoded bits which are then mapped to constellation points by the constellation mapper  1914 . The constellation points are then partitioned into four blocks of 12 bits each for further processing by the block coding unit  1808 . Accordingly, in this embodiment, the signal field is transmitted over four OFDM symbols each of which includes 6 information bits of the signal field. 
     As discussed above with regard to  FIGS. 15-17 , in some embodiments, the signal field of a control mode data unit is shorter (i.e., includes less bits) compared to a signal field of a normal mode data unit. Accordingly, in these embodiments, less OFDM symbols are generally needed to transmit the signal field compared to a longer signal field generated using the same repetition and coding scheme. 
     In some embodiments, the PHY processing unit  1900  generates OFDM symbols with 52 data tones according to the MCS 0  specified in the IEEE 802.11n Standard or the IEEE 802.11as Standard and the block coding unit  1916  utilizes a 4× repetition scheme. In some such embodiments, extra padding is used to ensure that the resulting encoded data stream to be included in an OFDM symbol includes 52 bits. In one such embodiment, padding bits are added to coded information the bits after the bits have been processed by the block coding unit  1808 . 
     In the embodiment of  FIG. 19A , the PHY processing unit  1900  includes a peak to average power ratio (PAPR) reduction unit  1917 . In an embodiment, the peak to average power ratio unit  1917  adds a phase shift to some of the data tones modulated with repeated constellations. For example, in one embodiment the added phase shift is 180 degrees. The 180 degree phase shift corresponds to a sign flip of the bits that modulate the data tones for which phase shifts are implemented. In another embodiment, the PAPR reduction unit  1917  adds a phase shift that is different than 180 degrees (e.g., a 90 degree phase shift or any other suitable phase shift). As an example, in an embodiment utilizing 4× repetition, if a block of 12 constellation symbols to be included in an OFDM symbols is denoted as C and if simple block repetition is performed, the resulting sequence is [C C C C]. In some embodiments, the PAPR reduction unit  1917  introduces a 180 degree phase shift (i.e., −C) or a 90 degree phase shift (i.e., j*C) for some of the repeated blocks. In some such embodiments, the resulting sequence is, for example, [C −C −C −C], [−C −C −C −C], [C −C C −C], [C C C −C], [C j*C, j*C, j*C], or any other combination of C, −C, j*C, and −j*C. In general, any suitable phase shift can be introduced in any repeated block in various embodiments and/or scenarios. In some embodiments, the PAPR reduction unit  1809  is omitted. 
     In an embodiment, the PAPR reduction unit  1917  is configured to include in the output of the PAPR reduction unit  1917  blocks D, where each block D corresponds to a respective block C phase shifted according to a sequence M of phase shift factors, where M is a suitable sequence of phase shift factors chosen from possible sequences M such that, when utilized, the sequence M minimizes PAPR. In another embodiment, M is a suitable sequence that includes different phase shift factors and that, when utilized, reduces PAPR as compared to a sequence M consisting of equal phase shift factors. 
     M is a sequence having the same length as the block C. As discussed above, each block D is equal to a respective constellation symbols in a respective block C multiplied with respective phase shift factors in M, in some embodiments. For example, if C=[C 1 , C 2 , . . . Cm] and M=[e jθ1 , e jθ2 , . . . , e jθm ], then D=[C 1 ·e jθ1 , C 2 ·e jθ2 , . . . Cm·e jθm ]. The PAPR reduction unit  1917  is configured to include the blocks D in the output of the PAPR reduction unit  1917 , in some embodiments. For example, in an embodiment utilizing 2× repetition of a block C of constellation symbols, two possible sequences at the output of the PAPR reduction unit  1917 , according to some embodiments, are [C D] and [D C]. As another example, in an embodiment utilizing 4× repetition of a block C of constellation symbols, possible bit sequences at the output of the PAPR reduction unit  1917 , according to some embodiments, include [C C C D], [C D C D], [D C D C], [D D D C], etc. In one embodiment, each block C has a length of 12-symbols, each block D has a length of 12-symbols, and each block M has a length of 12-symbols, and 2× repetition is utilized. In other embodiments, other suitable block lengths, and/or other suitable repetition rates (e.g., 4× repetition) are utilized. 
     In some embodiments, the PHY processing unit  1900  generates OFDM symbols with 52 data tones according to the MCS 0  specified in the IEEE 802.11n Standard or the IEEE 802.11ac Standard and the block coding  1916  utilizes a 4× repetition scheme. In some such embodiments, extra pilot tones are inserted to ensure that the resulting number of data and pilot tones in an OFDM symbol is equal to 56 as specified in the short range communication protocol. As an example, in an embodiment, six information bits are BCC encoded at the coding rate of ½ and the resulting 12 bits are mapped to 12 constellation symbols (BPSK). The 12 constellation symbols modulate 12 data tones which are then repeated four times the generated 48 data tones. Four pilot tones are added as specified in the IEEE 802.1n Standard and 4 extra pilot tones are added to generate 56 data and pilot tones. 
       FIG. 19B  is a block diagram of an example PHY processing unit  1950  for generating control mode data units, according to another embodiment. In some embodiments, the PHY processing unit  1950  generates signal and/or data fields of control mode data units. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  1950 . 
     The PHY processing unit  1950  is similar to the PHY processing unit  1900  of  FIG. 19A , except that in the PHY processing unit  1950 , the BCC encoder  1906  is replaced by the LDPC encoder  1956 . Accordingly, in this embodiment, LDPC encoded information bits are mapped to constellation symbols by the constellation mapper  1914  and the constellation symbols are then replicated or otherwise block encoded by the block coding unit  1916 . In an embodiment, the LDPC encoder  1956  utilizes a block code corresponding to a coding rate of ½, or a block code corresponding to another suitable coding rate. In the illustrated embodiment, the PHY processing unit  1950  omits the interleaver  1910  because adjacent bits in an information stream are generally spread out by the LDPC code itself and, according to an embodiment, no further interleaving is needed. Additionally, in an embodiment, further frequency diversity is provided by the LDPC tone remapping unit  1960 . According to an embodiment, the LDPC tone remapping unit  1960  reorders coded information bits or blocks of coded information bits according to a tone remapping function. The tone remapping function is generally defined such that consecutive coded information bits or blocks of information bits are mapped onto nonconsecutive tones in the OFDM symbol to facilitate data recovery at the receiver in cases in which consecutive OFDM tones are adversely affected during transmission. In some embodiments, the LDPC tone remapping unit  1960  is omitted. 
     In the embodiments described above with regard to  FIGS. 15-19 , the control mode introduces redundancy by repeating bits in frequency domain. Alternatively, in some embodiments, OFDM symbol repetition of the signal and/or data fields of control mode data units is performed in time domain. For example,  FIG. 20A  illustrates a 2× repetition of each OFDM symbol of HT-SIG 1  and HT-SIG 2  fields in a preamble of a control mode data unit, according to an embodiment. Similarly,  FIG. 20B  illustrates a 2× repetition of each OFDM symbol of the L-SIG field in a preamble of a control mode data unit, according to an embodiment. 
       FIG. 20C  illustrates a time domain repetition scheme for OFDM symbols in the data portion of a control mode data unit, according to one embodiment.  FIG. 20D  illustrates a repetition scheme for OFDM symbols in the data portion, according to another embodiment. As illustrated, in the embodiment of  FIG. 20C  OFDM symbol repetitions are output continuously, while in the embodiment of  FIG. 20D  OFDM symbol repetitions are interleaved. In general, OFDM symbol repetitions are interleaved according to any suitable interleaving scheme, in various embodiments and/or scenarios. 
     According to an embodiment utilizing a time domain repetition scheme, pilot tone signs are changed from one tone to the next as specified in the IEEE 802.11a or the IEEE 802.11n, for example. Accordingly, in such embodiments, pilot tones in a repeated OFDM symbol are not the same as in the incoming (original) OFDM symbol in at least some situations. 
     In some embodiments, the preamble used for a control mode data unit (“control mode preamble”) is different compared to the preamble used for a normal mode data unit (“normal mode preamble”). For instance, a control mode preamble includes a longer long training sequence for better channel estimation or a longer short training sequence for better packet detection and synchronization at a receiving device, in some embodiments. In some embodiments, a control mode preamble includes an extra preamble in addition or the normal mode preamble. Further, in some embodiments in which the control mode preamble is different from the normal mode preamble, the control mode preamble is generated such that a receiver is able to determine or auto-detect whether the incoming data unit corresponds to the control mode or to the normal mode and therefore is able to properly decode the data unit. 
       FIG. 21  is a diagram of an example control mode data unit  2100  which includes a longer preamble compared to a preamble included in a normal mode data unit, according to an embodiment. The control PHY data unit  2100  is similar to the normal PHY data unit  1000  of  FIG. 10 , except that HT-LTF 2  field  2108  is longer than each HT-LTF field  1012  of the normal PHY data unit  1000 . More specifically, the data unit  2100  includes an HT-LTF 2  field  2108  that occupies two OFDM symbols and is transmitted over 80 μs, compared to one OFDM symbol and 40 μs used to transmit each HT-LTF field  1012  of the normal PHY data unit  1000 . 
     In an embodiment, a long training field is extended (e.g., by introducing a number of repetitions of the long training sequence used in the normal mode) to span any suitable number of OFDM symbols.  FIGS. 22A and 22B  illustrate an example embodiment in which more repetitions of a long training sequence are included in a control mode preamble than in a normal mode preamble. In particular,  FIG. 22A  illustrates a long training field  2200  of a normal mode data unit, according to an embodiment. The long training field  2200  includes a double guard interval (DGI)  2202  and two repetitions of a long training sequence  2204 . In one embodiment, DGI is equal to 2×0.8 μs, for example. In another embodiment, a DGI of another suitable value is utilized. As indicated in  FIG. 22A , in the illustrated embodiment, duration of the long training field of a normal mode data unit is 8N μs, where N is the down-clock ratio used to generate the data unit. 
       FIG. 22B  is a diagram a long training field  2500  of a control mode data unit, according to an embodiment. The long training field  2500  includes a new guard interval (New GI) field  2502  and eight repetitions of a long training sequence  2504 . In one embodiment, the New GI duration is greater than the normal mode DGI duration in the data unit  2200 . In another embodiment, New GI duration is the same as the DGI duration in the data unit  2200 . As indicated in  FIG. 22B , in the illustrated embodiment, the duration of the control mode long training field  2500  is 64N μs. In other embodiments, other suitable numbers of repetition of a long training sequence are utilized and accordingly long training fields of other suitable lengths are generated in normal mode and/or in control mode, where a control mode LTF field is longer than a normal mode LTF field by any suitable factor. 
     In another embodiment, a control mode preamble includes an extra preamble in addition to a regular preamble included in the normal mode data units.  FIG. 23  is a diagram of a control mode OFDM data unit  2300 , according to one such embodiment. The data unit  2300  is similar to the normal mode data unit  1300  of  FIG. 13 , except that the data unit  2300  includes a single carrier (SC) extra preamble portion  2302 . The extra SC preamble  2202  includes a SYNC field  2204  and a SFD field  2206 . In an embodiment, the SC preamble of the data unit  2300  is at least substantially the same as PLCP preamble  814  of the data unit  800  ( FIG. 8 ), down-clocked by the same down-clocking factor N as the down-clocking factor used to generate the OFDM portion  2312 . In another embodiment, a down-clocking factor used for the SC preamble portion  2302  is different from the down-clocking factor used for the OFDM portion  2212 . 
       FIG. 24  is a diagram of a control mode OFDM data unit  2400  utilizing an extra preamble in control mode, according to another embodiment. The data unit  2400  is similar to the data unit  2200  of  FIG. 22 , except that the SYNC field  2204  is replace by the Golay code field  2304  in  FIG. 24 . In some embodiments, the Golay code field includes a number of repetitions of a Golay complementary sequence (GCS), for example. The number of repetitions is determined based on the particular Golay sequence length utilized and the overall preamble length of the data unit  1600 , in an embodiment. In some embodiments, Golay sequences of length 16, 32, 64, 128, or any other suitable length are utilized. In some embodiments, the long range communication protocol defines a long preamble and a short preamble, each consisting of a different number of Golay sequence repetitions. In one such embodiment, different complimentary sequences are utilized for the long and the short preamble cases (e.g., Ga sequence for the long preamble, and Gb sequence for the short preamble) to allow a receiver determine which type of preamble a received data unit includes. Generally, the two complementary sequences Ga and Gb have correlation properties suitable for detection at a receiving device. For example, the complementary spreading sequences Ga and Gb may be selected so that the sum of corresponding out-of-phase aperiodic autocorrelation coefficients of the sequences Ga and Gb is zero. In some embodiments, the complementary sequences Ga and Gb have a zero or almost-zero periodic cross-correlation. In another aspect, the sequences Ga and Gb may have aperiodic cross-correlation with a narrow main lobe and low-level side lobes, or aperiodic autocorrelation with a narrow main lobe and low-level side lobes. 
     In some embodiments which include an SC extra preamble portion, for wider bandwidth OFDM data units (e.g., 40 MHz, 80 MHz, 160 MHz, etc.), the SC preamble is repeated in each down-clocked 20 MHz sub-band. In some embodiments, particularly when the single carrier extra preamble is generated using a clock rate that is different from the clock rate used to generated the OFDM portion of the data preamble, the SC/OFDM boundary requirement is defined as specified in the IEEE-802.11g Standard for direct sequence spread spectrum (DSSS) and OFDM boundary requirement. In an embodiment, the start frame delimiter (SFD) field (e.g., SFD field  2306  in  FIG. 23 , SFD field  2406  in  FIG. 24 ) is used to determine the end of the extra SC preamble. In some embodiments, the STF field (i.e., field  2308  in  FIG. 23 , filed  2408  in  FIG. 24 ) is omitted. 
     In some embodiments, the control mode preamble includes an extended short training field of a normal mode preamble. The longer short training field is used to improve packet detection and better syncing at the receiver, for example. In one such embodiment, the number of periods included in a short training field of a control mode preamble is greater than the number of periods included in a normal mode preamble. In another embodiment, duration of each period in a short training field of a control mode preamble is longer compared to the duration of each period in a short training field of a normal mode preamble. In another embodiment, the number of periods included in a short training field of a control mode preamble is greater than the number of periods included in a normal mode preamble and each period in the control mode short training field is longer compared to each period in the normal mode short training field. 
       FIGS. 25A and 25B  are diagrams of a short training sequence subfield  2500  and a short training sequence subfield  2550  included in a control preamble and a normal mode preamble, respectively, according to an embodiment. As illustrated, in this embodiment, duration of each period of the short training field  2500  is greater than duration of each period of the short training field  2550  by a factor of two. In an embodiment, the longer short training sequence is generated by including more non-zero tones in each OFDM symbol of a control mode short training sequence compared to the number of non-zero tones included in each OFDM symbol of a normal mode short training field. In general, each period of a short training sequence does not include a repetition pattern. Therefore, in an embodiment, the longer short training sequence of a control mode preamble will not trigger a correlator that is used for detecting normal mode data units at a receiving device. In other words, this format allows a receiving device to differentiate between the two modes based on repetition pattern detection in an Inverse Fast Fourier Transform (IFFT) window at the receiving device. 
       FIG. 25C  illustrates frequency domain values corresponding to the short training sequence  2500 , according to an embodiment. As illustrated in  FIG. 25C , in this embodiment, each OFDM symbol of the short training field  2550  includes non-zero values corresponding to the +/−8 OFDM tone indices. In one embodiment, the values of the two non-zero tones are such that p(−8)!=p(8) and p(−8)!=−p(8). In general, any values for which these criteria are satisfied are used as specific non-zero tone values, in various embodiments and/or scenarios. As an example, in one embodiment, the specific values for the two non-zero tones are defined as:
 
[ p (8), p (−8)]= a *[sqrt(2),1+ j]   Equation 1
 
where a is a scaling factor.
 
       FIGS. 26A-26B  are diagrams of a control mode short training sequence  2600  and a normal mode short training sequence  2650 , respectively, according to an embodiment. In the embodiment of  FIGS. 26A-26B , a control mode preamble includes a short training sequence of the same period as the short training sequence of a normal mode preamble, but includes more repetitions of the sequence compared to the number of repetitions included in the normal mode preamble. In an embodiment, the short training sequence in the training field  2650  are encoded according to a predefined code (e.g., alternating +/−1 for consecutive repetitions, or any other suitable predetermined code) thereby allowing a receiving device to distinguish between control mode and normal mode data units based on the short training field. 
       FIG. 26D  illustrates frequency domain values corresponding to the short training sequence  2600  of  FIG. 26A  in an embodiment in which alternating +1/−1 is applied across the short training sequence repetitions of the short training field  2600 , according to an embodiment. In the illustrated embodiment, each OFDM symbol of the short training field  2600  includes non-zero values corresponding to the −12, −4, 4, and 12 OFDM tone indices. In various embodiments, the specific values of the non-zero tones are such that the values are not equal to each other in absolute value (i.e., non-periodic). As an example, in one embodiment, the specific values of the four non-zero tones are defined as:
 
[ p (−12), p (−4), p (4), p (12)]=α*[−1− j,− 1− j, 1+ j,− 1− j]   Equation 2
 
where a is a scaling factor.
 
     In another embodiment in which alternating +1/−1 is applied across the short training sequence repetitions of the short training field, the alternating signs in consecutive training sequence repetitions in control mode are generated by shifting non-zero tones in the sequence to the right or to the left in frequency domain by two tones, for example. As an example, if non-zero tone values are at tones [4, 8, 12, . . . , 24, −4, −8, . . . , −24] in a normal mode short training sequence, then a control mode short training sequence is generated by shifting the normal mode non-zero tones by two with new non-zero tone locations of [6, 10, 14, . . . , 26, −2, −6, . . . , −22] or shifted to the left with new non-zero tone locations of [2, 6, 10, 14, . . . , 22, −6, −10, . . . , −22, −26]. In another embodiment, in addition to shifting non-zero tones, extra non-zero tones are added in the control mode short training sequence to generate a longer sequence. For example, two extra non-zero tones are added at tone locations 2 and −26, in the example in which non-zero tones are shifted to the right, or at tone locations 26, −2 in the example in which non-zero tones are shifted to the left. In other embodiments, different suitable number of non-zero tones is added in a control mode short training sequence (e.g., 4, 6, 8, etc.), and the added non-zero tones are at other suitable tone locations. Further, in various embodiments, the non-zero tones are of any suitable values. 
     In an embodiment in which the control mode utilizes a single carrier extra preamble in control mode (e.g., data unit  2300  of  FIG. 23 , data unit  2400  of  FIG. 24 ), a receiver auto-detects whether an incoming data unit is a control mode or a normal mode data unit based on whether the initial signal of the preamble is a single carrier signal or an OFDM signal. In some embodiments, however, a normal mode data unit as well as a control mode data unit includes an extra single carrier preamble.  FIGS. 27A and 27B  illustrate a control mode data unit  2700  and a normal mode data unit  2750 , respectively, according to one such embodiment. In this embodiment, auto-detection at the receiver is based on the different Golay code sequences (GCS) in the control mode and the normal mode data unit. For example, in an embodiment, a length-16 Ga sequence is used for the single carrier preamble portion  2706  of the control mode data unit  2700 , while a length-16 Gb sequence is used for the single carrier preamble portion  2754  of the normal mode data unit  2750 . 
       FIGS. 28A-28B  illustrate a control mode data unit  2800  and a normal mode data unit  2850 , respectively, according to another embodiment in which a single carrier extra preamble is utilized in the control mode as well as in the normal mode. In this embodiment, each the SC preamble portions  2806  and the SC preamble portion  2854  is generated using the same GSC sequence (e.g., Ga), and auto-detection is based on the cover code applied to the corresponding delimiter field  2806  and  2856  (e.g., a sign flip is applied or a different sequence is used for the control mode than for the normal mode). 
       FIGS. 29A-29B  illustrate a control mode data unit  2900  and a normal mode data unit  2950 , respectively, according to an embodiment in which a receiving device can distinguish between the two modes based on modulation of the long training field. To this end, in the control mode data unit  2900 , the phase of each tone in the field second OFDM symbol of the LTF ( 2906 - 2 ) is shifted (e.g., by pi, pi/2, −pi/2, or any other suitable factor) compared to the phase of the corresponding OFDM tone of the first OFDM symbol of the LTF ( 2956 - 1 ). On the other hand, no phase shift is introduced for the second symbol of the LTF field ( 2956 - 2 ) of the normal mode data unit  2950 . 
     In some embodiments, mode auto-detection at a receiving device is based on the modulation of certain preamble fields.  FIGS. 30A-30B  illustrate a control mode preamble  3000  and a normal mode preamble  3050 , respectively, according to one such embodiment. The control mode preamble  3000  includes a short training field (STF)  3002 , a long training field (LTF)  3004 , a first signal field (SIG 1 )  3006 , and a second signal field (SIG 2 )  3008 . Similarly, the normal mode preamble  3050  includes a short training field (STF)  3052 , a long training field (LTF)  3054  and a first signal field (SIG 1 )  3056  and a second signal field (SIG 2 )  3058 . In this embodiment, the control mode preamble STF field  3002  is longer than the normal mode preamble STF field  3052 . In another embodiment, the control mode preamble STF field  3002  is the same length as the normal mode preamble STF field  3052 . Similarly, the control mode preamble LTF field  3004 , depending on the embodiment, is longer than or the same length as the normal mode preamble LTF field  3504 . 
     With continued reference to  FIGS. 30A-30B , each of the two control mode preamble signal fields is generated using one of the block coding schemes or a time domain repetition scheme discussed above. As a result, in this embodiment, each signal field in the control mode preamble (SIG 1   3006  and SIG 2   3008 ) spans two OFDM symbols, while each signal field in the normal mode preamble (SIG 1   3056  and SIG 2   3058 ) spans one OFDM symbol. In another embodiment, each of the control mode signal fields  3006  and  3008  spans four OFDM symbols or any other suitable number of OFDM symbols. In the embodiment of  FIGS. 30A-30B , each signal field of the normal mode preamble (SIG 1   3056  and SIG 2   3058 ) is modulated using QPSK modulation. The second symbol of the first signal filed of the control mode preamble, which is received at the same time after the long training field as the second signal field of the normal mode preamble, is modulated using BPSK modulation. Accordingly, in this embodiment, a receiving device is able to auto-detect whether a data unit corresponds to the control mode or to the normal mode based on the modulation of second OFDM symbol after the LTF field. In another embodiment, the first OFDM symbol of the first signal field is modulated according to a modulation technique that is different from the modulation technique used for modulating the signal fields of a normal mode preamble. 
       FIGS. 31A-31B  illustrate a control mode preamble  3100  and a normal mode preamble  3150 , respectively, according to another embodiment. In this embodiment, data units are generated using a short range communication protocol mixed mode preamble format (e.g., data unit  900  of  FIG. 9 ), and mode auto-detecting is based on the different modulation techniques corresponding to the control mode preamble LSIG field  3106  (in this embodiment, QPSK) and the normal mode preamble LSIG field  3156  (in this embodiment, BPSK). 
       FIGS. 32A-32B  illustrate a control mode preamble  3200  and a normal mode preamble  3250 , respectively, according to another embodiment. In the illustrated embodiment, normal mode data units are generated using a short range communication protocol green field preamble format (e.g., IEEE 802.11n Standard greenfield preamble format) and control mode data unit are generated using the IEEE 802.11a Standard preamble format utilizing a block coding or a repetition scheme described above. In this embodiment mode auto-detecting is based on the different modulation techniques corresponding to the control mode preamble LSIG field  3206  (in this embodiment, BPSK) and the normal mode preamble HTSIG field  3256  (in this embodiment, QPSK). 
       FIGS. 33A-33B  illustrate a normal mode preamble  3300  and a control mode preamble  3350 , respectively, according to another embodiment. In the illustrated embodiment, the normal mode preamble  3350  includes more repetitions of a long training sequence than the normal mode preamble  3300 . In particular, in this embodiment, the normal mode preamble  3300  includes two repetitions of the long training sequence  3304  while the control mode preamble  3354  includes three or more repetitions of long training sequence  3554 . Accordingly, in this embodiment, the signal field  3306  of the normal mode preamble is received at the same point in time (relative to the beginning of the preamble) as the third LTS  3554 - 3  of the control mode preamble. In the illustrated embodiment, the signal field  3306  is modulated using QBPSK modulation, and the LTS  3554 - 4  is modulated using BPSK modulation allowing a receiver to auto-detect the mode based on the different modulation techniques. 
       FIG. 33C  is a diagram of another control mode preamble  3370 , according to another embodiment. The control mode preamble  3370  is similar to the control mode preamble  3350  of  FIG. 33B , but includes a guard interval before each repeated long training sequence. In an embodiment, in this case, the same mode auto-detection scheme as discussed above in respect to  FIGS. 33A-33B  is used to distinguish between the normal mode preamble  3300  and the control mode preamble  3370 . 
       FIGS. 34A-34B  illustrate a control mode preamble  3400  and a normal mode preamble  3450 , respectively, according to another embodiment. In this embodiment, the control mode preamble  3450  includes a short training sequence with a different period compared to the period of the short training sequence of the normal mode preamble  3400 , and also includes more repetitions of the long training sequence as in the control mode preamble  3350  of  FIG. 33B  or the control mode preamble  3370  of  FIG. 3C . As illustrated, mode auto-detection in this embodiment is based on the modulation of the signal field  3408  and the long training sequence  3454 - 3  in the manner described above with respect to  FIGS. 33 a   - 33 C. 
       FIG. 35  is a flow diagram of an example method  3500  for generating a data unit, according to an embodiment. With reference to  FIG. 1 , the method  3500  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  3500 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  3500 . With continued reference to  FIG. 1 , in yet another embodiment, the method  3500  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  1700  is implemented by other suitable network interfaces. 
     At block  3502  information bits to be included in the data unit are encoded according to a block code. In one embodiment, information bits are encoded using a block level or a bit level repetition scheme described above with respect to the block coding unit  1504  of  FIG. 15 , for example. At block  3604 , the information bits are encoded using an FEC encoder, such as the BCC encoder  1506  of  FIG. 15A , or the LDPC encoder  1556  of  FIG. 15B , for example. At block  3506 , information bits are mapped to constellation symbols. At block  3508 , a plurality OFDM symbols is generated to include the constellation points. The data unit is then generated to include the OFDM symbols at block  3510 . 
     In one embodiment, as illustrated in  FIG. 35 , information bits are encoded using a block encoder first (block  3502 ) and the block coded bits are then encoded using a BCC encoder (block  3504 ), such as described above with respect to  FIG. 15A , for example. In another embodiment, the order of blocks  3502  and  3504  is interchanged. Accordingly, in this embodiment, information bits are BCC encoded first and the BCC encoded bits are encoded according to a block coding scheme, such as described above with respect to  FIG. 18A , for example. In yet another embodiment, block  3502  is positioned after block  3506 . In this embodiment, information bits are BCC encoded at block  3504 , the BCC encoded bits are mapped to constellation symbols at block  3506 , and the constellation symbols are then encoded according to a block coding or repetition scheme, such as described above with respect to  FIG. 19A , for example, at block  3502 . 
       FIG. 36  is a flow diagram of an example method  3600  for generating data units, according to an embodiment. With reference to  FIG. 1 , the method  3600  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  3600 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  3600 . With continued reference to  FIG. 1 , in yet another embodiment, the method  3600  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  1700  is implemented by other suitable network interfaces. 
     At block  3602 , a first preamble for a first data unit is generated. In an embodiment, a normal mode preamble is generated. In an embodiment, the normal mode preamble  3050  of  FIG. 30B  or the normal mode preamble  3300  of  FIG. 33A  is generated, for example. In another embodiment, a preamble conforming to another suitable format is generated. In an embodiment, the first preamble includes a long training field and a signal field. The signal field is modulated according to a modulation technique (e.g., BPSK, QPSK, or another suitable modulation technique.) 
     At block  3606 , the second data unit is generated according to the second data unit format (e.g., a normal mode data unit). 
     At block  3606 , a second preamble is generated. In an embodiment, a control mode preamble is generated. For example, the control mode preamble  3050  of  FIG. 30A , the control mode preamble  3350  of  FIG. 33B , or the control mode preamble  3370  of  FIG. 33C  is generated. In another embodiment, a preamble conforming to another suitable format is generated. In an embodiment, the second preamble includes a control mode signal field, such as the signal field  1600  of  FIG. 16A , the signal field  1700  of  FIG. 17C , or the signal field  1780  of  FIG. 17C , for example, or another suitable signal field. In an embodiment, the second signal field is generated according to the format of the first signal field and using one of the repetition or block coding schemes discussed above. As a result, duration of the second signal field is longer than the duration of the first signal field. The second preamble also includes a second long training field which includes a number of repetitions of a long training sequence. In some embodiment, the number of repetitions of the long training sequence is greater that the number of long training sequence repetitions in the first long training field (generated at block  3602 ). 
     In an embodiment, a portion of the second signal field is modulated according to a modulation technique different from the modulation technique used for generating the first signal field (at block  3602 ) so that a receiver is able to auto-detect that the second data unit (e.g., control mode data unit) is formatted according to a second data unit format (e.g., control mode data unit format). In another embodiment, the second long training field is modulated according to a modulation technique different from the modulation technique used for generating the first signal field (at block  3602 ) so that a receiver is able to auto-detect that the second data unit (e.g., control mode data unit) is formatted according to a second data unit format (e.g., normal mode data unit format). 
     At block  3606 , the second data unit is generated according to the second data unit format (e.g., a control mode data unit). 
       FIG. 37  is a flow diagram of another example method  3700  for generating a data unit, according to an embodiment. With reference to  FIG. 1 , the method  3700  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  3700 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  3700 . With continued reference to  FIG. 1 , in yet another embodiment, the method  3700  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  3700  is implemented by other suitable network interfaces. 
     At block  3702 , information bits to be included in the data unit are encoded using an FEC encoder, such as the BCC encoder  1806  of  FIG. 18A , or the LDPC encoder  1856  of  FIG. 18B , for example. 
     At block  3704  the information bits are encoded according to a block code. In one embodiment, information bits are encoded using a block level or a bit level repetition scheme described above with respect to the block coding unit  1808  and the PAPR reduction unit  1809  of  FIGS. 18A and 18B  for example. For example, in an embodiment, the information bits are encoded such that m copies of each bit in the information bits are included, wherein one or more bits in the m copies of each bit are flipped. 
     In one embodiment, block  3704  comprises including in the information bits a plurality of blocks D of bits, wherein each block D corresponds to an exclusive OR of a respective block C of bits in information bits with a sequence M of bits. In some embodiments, M consists of ones. In other embodiments, M includes multiple zeros and multiple ones. In an embodiment, M is a suitable sequence of bits chosen from a possible sequences M such that, when utilized, the sequence M minimizes PAPR. In another embodiment, M is a suitable sequence that includes both ones and zeros and that, when utilized, reduces PAPR as compared to XORing each block C with only ones. 
     At block  3706 , information bits are mapped to constellation symbols. At block  3508 , a plurality OFDM symbols is generated to include the constellation symbols. The data unit is then generated to include the OFDM symbols at block  3710 . 
       FIG. 38  is a flow diagram of another example method  3800  for generating a data unit, according to an embodiment. With reference to  FIG. 1 , the method  3800  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  3800 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  3800 . With continued reference to  FIG. 1 , in yet another embodiment, the method  3800  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  3800  is implemented by other suitable network interfaces. 
     At block  3802 , information bits to be included in the data unit are encoded using an FEC encoder, such as the BCC encoder  1906  of  FIG. 19A , or the LDPC encoder  1956  of  FIG. 19B , for example. 
     At block  3804 , information bits are mapped to constellation symbols. 
     At block  3806  the constellation symbols are encoded according to a block code. In one embodiment, constellation symbols are encoded using a block level or symbol level repetition scheme described above with respect to the block coding unit  1916  and the PAPR reduction unit  1917  of  FIGS. 19A and 19B  for example. For example, in an embodiment, the constellation symbols are encoded such that m copies of each symbol in the constellation symbols are included, wherein one or more symbols in the m copies of each symbol are phase shifted. 
     In one embodiment, block  3806  comprises including in the constellation symbols a plurality of blocks D of symbols, wherein each block D corresponds to respective constellation symbols in a respective block C multiplied with respective phase rotation factors in a sequence M of phase rotation factors. In some embodiments, M consists of equal phase rotation factors. In other embodiments, M includes different phase rotation factors. In an embodiment, M is a suitable sequence of phase shift factors chosen from possible sequences M such that, when utilized, the sequence M minimizes PAPR. In another embodiment, M is a suitable sequence that includes different phase shift factors and that, when utilized, reduces PAPR as compared to a sequence M consisting of equal phase shift factors. 
     At block  3808 , a plurality OFDM symbols is generated to include the constellation symbols. The data unit is then generated to include the OFDM symbols at block  3810 . 
     In an embodiment, a method for generating a physical layer (PHY) data unit for transmission via a communication channel includes encoding a plurality of information bits using a forward error correction (FEC) encoder; encoding the plurality of information bits according to a block coding scheme, comprising including in the plurality of information bits m copies of each bit in the plurality of information bits, wherein one or more bits in the m copies of each bit are flipped; mapping the plurality of information bits to a plurality of constellation symbols; generating a plurality of orthogonal frequency division multiplexing (OFDM) symbols to include the plurality of constellation symbols; and generating the PHY data unit to include the plurality of OFDM symbols. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     Encoding the plurality of information bits according to the block coding scheme comprises including in the plurality of information bits a plurality of blocks D of bits in the plurality of information bits, wherein each block D corresponds to an exclusive OR of a respective block C of bits in the plurality of information bits with a sequence M of bits. 
     The sequence M consists of ones. 
     The sequence M includes multiple zeros and multiple ones. 
     Encoding the plurality of information bits according to the block coding scheme comprises repeating each bit of the plurality of information bits so that each bit is included in the plurality of information bits m consecutive times. 
     Encoding the plurality of information bits according to the block coding scheme occurs after encoding the plurality of information bits using the FEC encoder; and encoding the plurality of information bits according to the block coding scheme comprises repeating each block of n bits so that m consecutive copies of each block of n bits are included in the plurality of information bits. 
     Encoding the plurality of information bits according to the block coding scheme comprises, in each set of m consecutive copies, exclusive ORing at least one of the m copies with a sequence M of bits. 
     The sequence M includes multiple zeros and multiple ones. 
     The method further comprises interleaving the plurality of information bits according to a predetermined code. 
     Encoding the plurality of information bits according to the block code is performed after encoding the information bits using the FEC encoder. 
     Encoding the plurality of information bits using the FEC encoder comprises encoding the plurality of information bits using a binary convolutional coding (BCC) encoder. 
     In another embodiment, an apparatus for generating a PHY data unit for transmission via a communication channel comprises a network interface device including: a forward error correction (FEC) encoder to encode a plurality of information bits to be included in the PHY data unit; and a constellation mapper to map the plurality of information bits to a plurality of constellation symbols. The network interface device is configured to: encode the plurality of information bits according to a block coding scheme, comprising including m copies of each bit in the plurality of information bits, wherein one or more bits in the m copies of each bit are flipped, generate a plurality of OFDM symbols to include the plurality of constellation symbols, and generate the PHY data unit to include the plurality of orthogonal frequency division multiplexing (OFDM) symbols. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The network interface device is configured to encode the plurality of information bits according to the block coding scheme at least by including in the plurality of information bits a plurality of blocks D of bits in the plurality of information bits, wherein each block D corresponds to an exclusive OR of a respective block C of bits in the plurality of information bits with a sequence M of bits. 
     The sequence M consists of ones. 
     The sequence M includes multiple zeros and multiple ones. 
     The network interface device is configured to encode the plurality of information bits according to the block coding scheme at least by repeating each bit of the plurality of information bits so that each bit is included in the plurality of information bits m consecutive times. 
     The network interface device is configured to encode the plurality of information bits according to the block coding scheme after encoding of the plurality of information bits by the FEC encoder, and encode the plurality of information bits according to the block coding scheme at least by repeating each block of n bits so that m consecutive copies of each block of n bits are included in the plurality of information bits. 
     The network interface device is configured to encode the plurality of information bits according to the block coding scheme at least by, in each set of m consecutive copies, exclusive ORing at least one of the m copies with a sequence M of bits. 
     The sequence M includes multiple zeros and multiple ones. 
     The network interface is further configured to interleave the plurality of information bits in each block of n bits. 
     The network interface is configured to encode the plurality of information bits according to the block code after encoding of the plurality of information bits by the FEC encoder. 
     The FEC encoder includes a binary convolutional coding (BCC) encoder. 
     In another embodiment, a method for generating a physical layer (PHY) data unit for transmission via a communication channel includes encoding a plurality of information bits using a forward error correction (FEC) encoder; mapping the plurality of information bits to a plurality of constellation symbols; encoding the plurality of constellation symbols according to a block coding scheme, comprising including in the plurality of constellation symbols m copies of each constellation symbol in the plurality of constellation symbols, wherein one or more constellation symbol in the m copies of each constellation symbol are phase shifted; generating a plurality of orthogonal frequency division multiplexing (OFDM) symbols to include the plurality of constellation symbols; and generating the PHY data unit to include the plurality of OFDM symbols. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     Encoding the plurality of constellation symbols according to the block coding scheme comprises including in the plurality of constellation symbols a plurality of blocks D of constellation symbols in the plurality of constellation symbols, wherein each block D corresponds to respective constellation symbols in a respective block C multiplied with respective phase rotation factors in a sequence M of phase rotation factors. 
     The sequence M consists of equal phase rotation factors. 
     The sequence M includes different phase rotation factors. 
     Encoding the plurality of constellation symbols according to the block coding scheme comprises repeating each block C of n constellation symbols so that m consecutive copies of each block C are included in the plurality of constellation symbols. 
     Encoding the plurality of constellation symbols according to the block coding scheme comprises, in each set of m consecutive copies of C, multiplying, for at least one of the m copies of C, respective constellation symbols in the block C with respective phase rotation factors in a sequence M of phase rotation factors. 
     The sequence M includes different phase rotation factors. 
     The method further comprises interleaving the plurality of information bits according to a predetermined code. 
     Encoding the plurality of information bits using the FEC encoder comprises encoding the plurality of information bits using a binary convolutional coding (BCC) encoder. 
     In another embodiment, an apparatus for generating a PHY data unit for transmission via a communication channel comprises a network interface device including: a forward error correction (FEC) encoder to encode a plurality of information bits to be included in the PHY data unit; and a constellation mapper to map the plurality of information bits to a plurality of constellation symbols. The network interface device is configured to: encode the plurality of constellation symbols according to a block coding scheme, at least by including in the plurality of constellation symbols m copies of each constellation symbol in the plurality of constellation symbols, wherein one or more constellation symbol in the m copies of each constellation symbol are phase shifted, generate a plurality of OFDM symbols to include the plurality of constellation symbols, and generate the PHY data unit to include the plurality of orthogonal frequency division multiplexing (OFDM) symbols. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The network interface device is configured to encode the plurality of constellation symbols according to the block coding scheme at least by including in the plurality of constellation symbols a plurality of blocks D of constellation symbols in the plurality of constellation symbols, wherein each block D corresponds to respective constellation symbols in a respective block C multiplied with respective phase rotation factors in a sequence M of phase rotation factors. 
     The sequence M consists of equal phase rotation factors. 
     The sequence M includes different phase rotation factors. 
     The network interface device is configured to encode the plurality of constellation symbols according to the block coding scheme at least by repeating each block C of n constellation symbols so that m consecutive copies of each block C are included in the plurality of constellation symbols. 
     The network interface device is configured to encode the plurality of constellation symbols according to the block coding scheme at least by, in each set of m consecutive copies of C, multiplying, for at least one of the m copies of C, respective constellation symbols in the block C with respective phase rotation factors in a sequence M of phase rotation factors. 
     The sequence M includes different phase rotation factors. 
     The network interface is further configured to interleave the plurality of information bits according to a predetermined code. 
     The FEC encoder includes a binary convolutional coding (BCC) encoder. 
     At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. The software or firmware instructions may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the claims.