Patent Publication Number: US-10321403-B2

Title: Implicit sounding for OFDMA operation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/007,020, filed on Jan. 26, 2016, now U.S. Pat. No. 9,883,490, to issue on Jan. 30, 2018, which claims priority to and the benefit of U.S. Provisional Application 62/107,963, filed on Jan. 26, 2015 and U.S. Provisional Application 62/112,095, filed on Feb. 4, 2015, the entire respective contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The technology described herein relates generally to wireless networking. More particularly, the technology relates generally to allocating frequency resources to each of a plurality of stations (STAs) in a Wireless Local Area Network (WLAN). 
     2. Description of the Related Art 
     A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012). The IEEE Std 802.11™-2012 was subsequently amended by IEEE Std 802.11ae™-2012, IEEE Std 802.11aa™-2012, IEEE Std 802.11ad™-2012, and IEEE Std 802.11ac™-2013. These standards are each hereby incorporated by reference herein in their entirety. 
     Recently, a high efficiency WLAN for enhancing system throughput in high-density scenarios is being developed by the IEEE 802.11ax task group. In such WLAN scenarios, a transmitting device may transmit an Orthogonal Frequency Division Multiple Access (OFDMA) frame having respective data for each of a plurality of receiving devices. Within the OFDMA frame, the respective data for the receiving devices may be transmitted in one or more frequency regions (sub-bands) respectively allocated to the receiving devices. Each sub-band includes a plurality of subcarriers at different frequencies. 
     The respective channels between the transmitting device and each of the receiving devices may be frequency selective, which means the channel magnitude changes over frequency. In the design of an OFDMA wireless network, it is possible to take advantage of the frequency selectivity of the channel. For instance, the transmitting device can schedule downlink (DL) transmission for several receiving devices within a downlink OFDMA PHY Protocol Data Unit (PPDU), where the information related to each receiving device is placed in the sub-band wherein the receiving device experiences higher channel magnitude. 
     The sub-bands that have a high channel magnitude for each receiving device may be determined in order to use resource units or sub-bands optimally. A process of determining the sub-bands that have a high channel magnitude for each receiving device may require the transmission of additional frames and may thereby cause a deterioration in an efficiency, a throughput, or of both on the WLAN. Reducing or eliminating that deterioration may be desirable. 
     SUMMARY 
     A method for communication in a wireless local area network comprises receiving, at a first device, a control frame from a second device, determining, by the first device using the control frame, a channel state information (CSI) for each of a plurality of sub bands of a bandwidth of the control frame, and allocating, by the first device to the second device, one or more sub bands from the plurality of sub bands based on the determination, the one or more sub bands to be allocated to the second device for transmitting data to the second device from the first device via an Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     In an embodiment, the first bandwidth is one of a 40 MHz, an 80 MHz, an 80+80 MHz, and a 160 MHz bandwidth. The control frame may be an Orthogonal Frequency Division Multiplexing (OFDM) frame. 
     In an embodiment, the control frame is an acknowledgement (ACK) frame or a block acknowledgement (BA) frame. 
     In an embodiment, the method further comprises transmitting to the second device by the first device a downlink (DL) frame prior to the first device receiving the ACK or BA frame. The first bandwidth is a widest allowed bandwidth between the first device and the second device. The widest allowed bandwidth is a widest continuous bandwidth that is available for communications between the second device and the first device. 
     In an embodiment, the DL frame may include a block acknowledgement request (BAR) frame. 
     In an embodiment, the transmitting of the DL frame is by transmitting the DL frame using a second bandwidth, the second bandwidth being greater than 20 MHz, where the first bandwidth is a widest bandwidth that is equal to or less than the second bandwidth and that is determined to be available use by the second device. 
     In an embodiment, the DL frame includes a bandwidth indicator that indicates the second bandwidth. 
     In an embodiment, the control frame is a power save (PS) poll frame. 
     A system for communication in a basic service set (BSS) comprises a radio frequency (RF) transceiver, one or more processors, and a non-transitory computer readable medium The non-transitory computer readable medium comprising computer executable instructions which, when executed by the one or more processors, causes the system to receive a control frame from a station, determine a channel strength for each of a plurality of sub bands associated with a bandwidth of the control frame, allocate to the station one or more sub bands from the plurality of sub bands based, at least in part, on the determination, the one or more sub bands to be allocated to the station for transmitting data to the station; and transmit the data using the one or more allocated sub bands and via an Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     In an embodiment, the control frame is an Orthogonal Frequency Division Multiplexing (OFDM) frame and the bandwidth is one of a 40 MHz, an 80 MHz, an 80+80 MHz, and a 160 MHz bandwidth. 
     In an embodiment, the control frame is an acknowledgement (ACK) frame, a block acknowledgement (BA) frame, or a power save (PS) poll frame. 
     A method for communication between devices in a wireless network comprises determining, by a first device, a widest allowed bandwidth for transmitting a control frame to a second device, transmitting, by the first device, the control frame to the second device using the widest allowed bandwidth, including using a plurality of sub bands of the widest allowed bandwidth to transmit the control frame, and receiving, by the first device, data from the second device via an Orthogonal Frequency Division Multiple Access (OFDMA) frame and using one or more sub bands selected from the plurality of sub-bands, the one or more sub-bands having been selected based on channel strength information (CSI) determinations made for each of the plurality of sub bands. 
     In an embodiment, the widest allowed bandwidth is a widest continuous bandwidth that is available for communications between the second device and the first device. 
     In an embodiment, the control frame is an Orthogonal Frequency Division Multiplexing (OFDM) frame having a bandwidth greater than 20 MHz. 
     In an embodiment, the control frame is a power save (PS) Poll frame, and the determining the widest allowed bandwidth includes determining whether one or more of a secondary 80 MHz channel, a secondary 40 MHz channel, and a secondary 20 channel is sensed busy during Point Coordination Function Interframe Space (PIFS) time before transmission of the PS-Poll frame, wherein the determination is made when the second device indicates that the second device permits transmission by the first device of the control frame using the widest allowed bandwidth 
     In an embodiment, the control frame is an acknowledgement (ACK) frame or a block ACK (BA) frame, and the widest allowed bandwidth is determined based on a downlink (DL) frame received from the access point. 
     In an embodiment, the determining of the widest allowed bandwidth includes determining a bandwidth of the DL frame. 
     In an embodiment, the determining of the widest allowed bandwidth includes sensing an explicit bandwidth indicator included in the DL frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a wireless network according to an embodiment. 
         FIG. 2  illustrates a schematic block diagram of a wireless device according to an embodiment. 
         FIG. 3  illustrates an Orthogonal Frequency Division Multiplexing (OFDM) frame that may be employed in an embodiment. 
         FIG. 4  illustrates an Orthogonal Frequency Division Multiple Access (OFDMA) frame that may be employed in an embodiment. 
         FIG. 5  illustrates a sequence of frames including a plurality of OFDMA frames according to an embodiment. 
         FIG. 6A  illustrates a process for communicating using an OFDMA frame according to an embodiment. 
         FIG. 6B  illustrates a process for communicating using an OFDMA frame according to an embodiment. 
         FIG. 7A  illustrates an Access Point (AP) initiated Wireless Local Area Network (WLAN) operations that include implicit channel sounding according to an embodiment. 
         FIG. 7B  illustrates AP initiated WLAN operations that include implicit channel sounding according to another embodiment. 
         FIG. 7C  illustrates AP initiated WLAN operations that include implicit channel sounding according to another embodiment. 
         FIG. 8  illustrates a frame with duplicate Acknowledge (ACK) or Block ACK (BA) frames according to an embodiment. 
         FIG. 9A  illustrates a Block Acknowledgement Request (BAR) frame according to an embodiment. 
         FIG. 9B  illustrates a Multi-Traffic Identifier (TID) BAR frame according to an embodiment. 
         FIG. 9C  illustrates the BAR Control fields of  FIGS. 10A and 10B  according to an embodiment. 
         FIG. 10  illustrates a station-initiated process using a PS-Poll frame adapted for implicit OFDMA sounding according to another embodiment. 
         FIG. 11  illustrates a frame with duplicate PS-poll frames according to an embodiment. 
         FIG. 12  illustrates relationships between primary and secondary channels of various bandwidths according to an embodiment. 
         FIG. 13  illustrates a process for communicating using an OFDMA frame according to an embodiment. 
         FIG. 14  illustrates a process for transmitting data using one or more sub-bands or resource units in an access point according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain illustrative embodiments have been illustrated and described. As those skilled in the art would realize, the described embodiments may be modified in various different ways without departing from the spirit or scope of the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
       FIG. 1  illustrates a wireless network according to an embodiment. The wireless network includes an infrastructure Basic Service Set (BSS)  100  of a Wireless Local Area Network (WLAN). In an 802.11 wireless LAN, the BSS provides the basic building-block and typically includes an access point (AP) and one or more associated stations (STAs). In  FIG. 1 , the BSS  100  includes an access point  102  wirelessly communicating with a plurality of wireless devices (or stations)  104  to  114  (also referred to as AP and STA 1  to STA 6 ). The wireless devices may include a medium access control layer (MAC) and a physical layer (PHY) according to an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. 
     The AP  102  is a station, that is, a STA configured to control and coordinate functions of the BSS  100 . The AP  102  may transmit information to a single station selected from the plurality of stations STA 1  to STA 6  in the BSS  100  using a single frame, or may simultaneously transmit information to two or more (e.g., to all six) stations STA 1  to STA 6  in the BSS  100  using either a single Orthogonal Frequency Division Multiplexing (OFDM) broadcast frame, a single OFDM Multi-User Multi-Input-Multi-Output (MU-MIMO) transmission, or a single Orthogonal Frequency Division Multiple Access (OFDMA) frame. 
     The stations STA 1  to STA 6  may transmit data to the AP  102  using a single frame, or transmit information to and receive information from another one of the stations STA 1  to STA 6  using a single frame. 
     In another embodiment, the AP  102  may be absent and the stations STA 1  to STA 6  may be in an ad-hoc network. 
     Each of the stations STA 1  to STA 6  and the AP  102  includes a processor and a transceiver, and may further include a user interface and a display device. The processor is configured to generate a frame to be transmitted through a wireless network, to process a frame received through the wireless network, and to execute protocols of the wireless network. The processor may perform some or all of its functions by executing computer programming instructions stored on a non-transitory computer-readable medium. The transceiver represents a unit functionally connected to the processor, and designed to transmit and receive a frame through the wireless network. The transceiver may be defined using a single component that performs the functions of transmitting and receiving, or two separate components each performing one of such functions. The processor and the transceiver may be implemented by respective hardware components, software components, or both that are integrated within each of the respective STA 1  to STA 6  and the AP  102 . 
     AP  102  may be or may include a WLAN router, a stand-alone Access Point, a WLAN bridge, a Light-Weight Access Point (LWAP) managed by a WLAN controller, and the like. In addition, a station such as a personal computer or cellular phone may be able to operate as the AP  102 , such as when a cellular phone is configured to operate as a wireless “hot spot.” 
     Each of STA 1  to STA 6  may be or may include a desktop computer, a laptop computer, a tablet PC, a wireless phone, a mobile phone, a smart phone, an e-book reader, a Portable Multimedia Player (PMP), a portable game console, a navigation system, a digital camera, a Digital Multimedia Broadcasting (DMB) player, a digital audio recorder, a digital audio player, a digital picture recorder, a digital picture player, a digital video recorder, a digital video player, and the like. 
     The present disclosure may be applied to WLAN systems according to IEEE 802.11 standards but is not limited thereto. 
     In IEEE 802.11 standards, frames exchanged between stations (including access points) are classified into management frames, control frames, and data frames. A management frame may be a frame used for exchanging management information that is not forwarded to a higher layer of a communication protocol stack. A control frame may be a frame used for controlling access to a medium. A data frame may be a frame used for transmitting data to be forwarded to a higher layer of the communication protocol stack. 
     Each frame&#39;s type and subtype may be identified using a type field and a subtype field included in a control field of the frame, as prescribed in the applicable standard. 
       FIG. 2  illustrates a schematic block diagram of a wireless device  200  according to an embodiment. The wireless or WLAN device  200  can represent any device in a BSS, e.g., AP  102  or any of stations  104 - 114  in  FIG. 1 . The WLAN device  200  includes a baseband processor  210 , a radio frequency (RF) transceiver  240 , an antenna unit  250 , a storage device (e.g., memory)  232 , one or more input interfaces  234 , and one or more output interfaces  236 . The baseband processor  210 , the memory  232 , the one or more input interfaces  234 , the one or more output interfaces  236 , and the RF transceiver  240  may communicate with each other via a bus  260 . 
     The baseband processor  210  performs baseband signal processing, and includes a MAC processor  212  and a PHY processor  222 . 
     In an embodiment, the MAC processor  212  includes a MAC software processing unit  214  and a MAC hardware processing unit  216 . The storage device (or memory)  232  may be a non-transitory computer readable medium that stores software (e.g., computer programing instructions) hereinafter referred to as “MAC software”. The MAC software processing unit  214  executes the MAC software to implement a first plurality of functions of the MAC layer. The MAC hardware processing unit  216  may implement a second plurality of functions of the MAC layer in special-purpose hardware, hereinafter referred to “MAC hardware.” However, the MAC processor  212  is not limited thereto. For example, the MAC processor  212  may be configured to perform the first and second plurality of functions entirely in software or entirely in hardware according to implementation. 
     The PHY processor  222  includes a transmitting signal processing unit  224  and a receiving signal processing unit  226 . The PHY processor  222  implements a plurality of functions of the PHY layer. These functions may be performed in software, hardware, or a combination thereof according to implementation. In an embodiment, the PHY processor  222  may be configured to generate channel state information (CSI) by processing uplink frames received via the RF transceiver  240 . In some cases, the uplink frames may be in the form of OFDM frames. In an embodiment, the PHY processor  222  may be configured to implement the various operations to be described herein. 
     The channel state information (CSI) may include one or more of a Received Signal Strength Indication (RS SI), a Signal to Interference and Noise Ratio (SINR), a Modulation and Coding Scheme (MCS), and a Number of Spatial Streams (NSS). CSI may be generated for one or more of a frequency block, a sub-band within the frequency block, a subcarrier within a frequency block, a receiving antenna, a transmitting antenna, and combinations of a plurality thereof. 
     The RF transceiver  240  includes an RF transmitter  242  and an RF receiver  244 . The RF transceiver  240  is configured to transmit first information received from the baseband processor  210  to the WLAN, and provide second information received from the WLAN to the baseband processor  210 . 
     The antenna unit  250  includes one or more antennas. When Multiple-Input Multiple-Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antenna unit  250  may include a plurality of antennas. 
     The one or more input interfaces  234  receive information from a user, and the one or more output interfaces  236  output information to the user. The one or more input interfaces  234  may include one or more of a keyboard, keypad, mouse, touchscreen, touch screen, microphone, and the like. The one or more output interfaces  236  may include one or more of a display device, touch screen, speaker, and the like. 
     As described herein, many functions of the WLAN device  200  may be implemented in either hardware or software, and which functions are implemented in software and which functions are implemented in hardware will vary according to constraints imposed on a design. The constraints may include one or more of design cost, manufacturing cost, time to market, power consumption, available semiconductor technology, and so on. 
     As described herein, a wide variety of electronic devices, circuits, firmware, software, and combinations thereof may be used to implement the functions of the components of the WLAN device  200 . Furthermore, the WLAN device  200  may include other components, such as application processors, storage interfaces, clock generator circuits, power supply circuits, and the like, which have been omitted in the interest of brevity. 
       FIG. 3  illustrates an Orthogonal Frequency Division Multiplexing (OFDM) frame  300  according to an embodiment. The OFDM frame  300  includes a PHY header  320  and a Physical Layer Convergence Procedure (PLCP) Service Data Unit (PSDU)  330 . The PHY header  320  includes a legacy preamble  302 , a Very High Throughput (VHT) first signal (VHT-SIG-A) field  304 , a VHT Short Training Field (VHT-STF)  306 , a VHT Long Training Field (VHT-LTF)  308 , and a VHT second signal (VHT-SIG-B) field  310 . 
     The legacy preamble  302  includes Legacy STF (L-STF) and Legacy LTF (L-LTF) fields that include symbols that are used by a receiving device to identify the start of the OFDM frame  300 , synchronize timers, and, in MIMO-capable equipment, select an antenna. The legacy preamble  302  also includes a Legacy Signal (L-SIG) field that includes information from which a legacy receiving device (for example, a device operating according to IEEE Stds 802.11a™-1999, 802.11b™-1999, or 802.11g™-2003) may determine a data rate and a length for the frame. 
     The VHT-SIG-A field  304  and VHT-SIG-B field  310  include information that the receiver uses to receive the PSDU  312 , such as a bandwidth indication, a Space-Time Block Coding (STBC) indication, a Group ID, a partial Association Identifier (AID) field, a Modulation and Coding Scheme (MCS) indication, and so on. 
     The VHT-STF  306  and VHT-LTF  308  fields include symbols that are used by the receiving device to set receiver gain and to estimate a channel between a transmitter of the OFDM frame  300  and the receiving device, respectively. 
     The PSDU  330  includes data symbols that are used to transport an aggregate MAC Protocol Data Unit (A-MPDU) including one or more MPDUs. The MPDUs in the PSDU  330  are intended either for a single receiving device when the OFDM frame  300  is not a broadcast frame, or for all the receiving devices in a group when the OFDM frame  300  is a broadcast frame wherein all the receiving devices in the group receive the same information. 
     As can be seen in  FIG. 3 , the PSDU  330  of the OFDM frame  300  uses all of the subcarriers used by the PHY header  320 . Therefore, in an embodiment, a frame using a 20 MHZ frequency segment includes the PSDU  330  using 56 subcarriers, a frame using a 40 MHZ frequency segment includes the PSDU  330  using 114 subcarriers, a frame using a 80 MHZ frequency segment includes the PSDU  330  using 242 subcarriers, and a frame using a 160 MHZ frequency segment or two 80 MHz frequency segments includes the PSDU  330  using 484 subcarriers. 
       FIG. 4  illustrates an Orthogonal Frequency Division Multiple Access (OFDMA) frame  400  that may be transmitted, received, or both in an embodiment. The OFDMA frame  400  includes a PHY header  410  and first, second, third, and fourth PSDUs  412 ,  414 ,  416 , and  418 . 
     The PHY header  410  includes a legacy preamble  402 , a High Efficiency (HE) signal portion  404 , an HE Short Training Field (HE STF)  406 , and an HE Long Training Field (HE LTF)  408 . Unlike the PHY header  320 , the PHY header  410  includes an indication that the OFDMA frame  410  is an OFDMA frame, or in some embodiments, there would be an indication whether the frame is a single-user (SU) or a multi-user (MU) frame. 
     The High Efficiency (HE) signal portion  404  includes an HE signal A (HE SIG-A) field, which is transmitted using one or more symbols, and an HE signal B (HE SIG-B) field, which is transmitted using one or more symbols. The HE SIG-A field and HE SIG-B field include information that the receiver uses to receive the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418 . In an embodiment, a single HE SIG-A field may be duplicated across a plurality of frequency blocks and may contain information common to all of the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418 . In an embodiment, the HE SIG-B field includes a unique sub-field for each of the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418 . In an embodiment, the HE SIG-B field may include resource allocation information so that the receiving stations can determine assigned resources in the received PPDU. In an embodiment, the HE SIG-B field may have a variable length. 
     The HE STF  406  and HE LTF  408  fields include symbols that are used by the receiving device to set receiver gain and to estimate a channel between a transmitter of the OFDMA frame  400  and the receiving device, respectively. 
     The first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418  occupy respective sub-bands within the frequency segment used by the OFDMA frame  400 , each sub-band including a contiguous plurality of subcarriers. Each of the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418  includes data symbols that are used to transport A-MPDUs including one or more MPDUs. 
     The MPDUs in each of the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418  may each be intended for a different receiving device, respectively.  FIG. 4  illustrates the first, second, third and fourth PSDUs  412 ,  414 ,  416 , and  418  that are associated with first through fourth stations STA 1  through STA 4 , respectively. However, embodiments are not limited thereto. 
     In an embodiment, the OFDMA frame  400  using a 20 MHZ frequency segment may include 9 PSDUs, the OFDMA frame  400  using a 40 MHZ frequency segment may include 18 PSDUs, the OFDMA frame  400  using a 80 MHZ frequency segment may include 36 PSDUs, and the OFDMA frame  400  using a 160 MHZ frequency segment or two 80 MHz frequency segments may include 72 PSDUs. 
       FIG. 5  illustrates a sequence of frames that may be transmitted by, for example, the AP of  FIG. 1  to multiple STAs according to an embodiment, illustrating an association of PSDUs with receiving devices (e.g., the stations STA 1  to STA 6  of  FIG. 1 ). The sequence of frames includes a first OFDMA frame  500 , a first acknowledgement (ACK) frame  530 , a second OFDMA frame  540 , and a second ACK frame  570 . In an embodiment, each of the frames uses a 20 MHz frequency segment, and the first and second OFDMA frames  500  and  540  each include nine sub-bands within the 20 MHz frequency segment (where considering the guard-bands in each side, each sub-channel would be 2 MHz). In an embodiment, the ACK frames  530  and  570  may be a MU ACK frame (with separate PSDUs from each STA), or may be a set of SU ACK frames that are either immediate responses (i.e., the first SU ACK frame) or are responses to polls/BARs from the AP (i.e., every subsequent SU ACK frame after the first immediate response ACK). 
     The first OFDMA frame  500  includes a PHY header  510  substantially similar to the PHY header  410  of  FIG. 4 . The first OFDMA frame  500  further includes first through ninth sub-bands  512  through  528 , each including a contiguous plurality of subcarriers. 
     In the first OFDMA frame  500 , the first and second sub-bands  512  and  514  are used to transport information to a first station STA 1 . The third, fourth, and fifth sub-bands  516 ,  518 , and  520  are used to transport data to second, third, and fourth stations STA 2 , STA 3 , and STA 4 , respectively. The sixth and seventh sub-bands  522  and  524  are used to transport information to a fifth station STA 5 . The eighth and ninth sub-bands  526  and  528  are used to transport information to a sixth station STA 6 . 
     The first OFDMA frame  500  is followed by one or more first ACK or Block ACK (BA) frames  530 . 
     In the example of  FIG. 5 , at some time after the first OFDMA frame  500  is transmitted, the second OFDMA frame  540  is transmitted. Like the first OFDMA frame  500 , the second OFDMA frame  540  includes a PHY header  550  substantially similar to the PHY header  410  of  FIG. 4  and first through ninth sub-bands  552  through  568 , each including a contiguous plurality of subcarriers. The second OFDMA frame  540  may use the same 20 MHz frequency segment that was used by the first OFDMA frame  500 . 
     The allocation of sub-bands to stations in the second OFDMA frame  540  could be different from the allocation of sub-bands to stations in the first OFDMA frame  500 . In the second OFDMA frame  540 , the first and second sub-bands  552  and  554  are again used to transport information to a first station STA 1 , and the fourth and fifth sub-bands  558  and  560  are again used to transport data to third and fourth stations STA 3  and STA 4 , respectively. 
     However, unlike in the first OFDMA frame  500 , in the second OFDMA frame  540 , the sixth and seventh sub-bands  562  and  564  are used to transport information to a seventh station STAT, the ninth sub-band  568  is used to transport information to an eighth station STAB, and the third and eight sub-bands are not used. 
     The second OFDMA frame  540  is followed by one or more second ACK or BA frames  570 . 
       FIGS. 6A and 6B  illustrate a process  600  for communicating using an OFDMA frame according to an embodiment. In the illustrated example, a BSS (e.g. the BSS  100  of  FIG. 1 ) includes an AP and first, second, and third OFDMA-capable stations STA 1 , STA 2 , and STA 3 . Note that references in the following to an access point (AP) may be in reference to an access point or to any node capable of transmitting OFDMA frames. 
     At S 610 , the AP receives OFDM frames from STA 1 , STA 2 , and STA 3  using bandwidths that are the widest bandwidths available to the stations STA 1 , STA 2 , and STA 3 . In an embodiment, the OFDM frames that may be transmitted by the stations STA 1 , STA 2 , and STA 3  may be any type of frame that a STA sends as part of its regular functions such as, for example, any one of a power-save (PS) poll frame, an ACK frame, a BA frame, and the like. The sounding process  612  shown in  FIG. 6B  generally corresponds to S 610  of  FIG. 6A . 
     At S 620 , for each of the stations STA 1 , STA 2 , and STA 3 , the AP determines sub-band channel state information (CSI) indicative of resource unit or sub-band channel conditions for a plurality of RUs or sub-bands of a bandwidth available for OFDMA communication between the AP and the stations STA 1 , STA 2 , and STA 3 . In some cases, a resource unit (RU) may comprise of a set of adjacent/continuous sub-bands and may be an assigned portion of a channel. In an embodiment, the bandwidth is a widest available bandwidth (hereinafter “widest allowed bandwidth”) that can be transmitted across the current channel by the AP, received across the current channel by the stations in the BSS, and sensed across the current channel using channel sensing methods, for example, using Clear Channel Assessment (CCA). That is, the widest available or allowable bandwidth is the widest bandwidth that can be used entirely by a first network device to transmit a frame, and to be used entirely by a second network device to receive the frame. 
     The AP may use one or more fields of the OFDM frames received at S 610  to respectively determine the sub-channel CSI, such as, for example, a VHT-LTF symbol or HE-LTF symbol. 
     In an embodiment, the determination of the CSI for the plurality of sub-bands or RUs between the AP and the stations STA 1 , STA 2 , and STA 3  may be based on OFDM frames received from the stations STA 1 , STA 2 , and STA 3  in the sounding process  612  of  FIG. 6B . However, embodiments are not limited thereto. 
     In the sounding process  5610 , the AP receives first, second, and third frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  to be used for OFDMA sub-band sounding for the stations STA 1 , STA 2 , and STA 3 , e.g., from all the stations associated within the BSS. The frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  are OFDM uplink frames in an implementation. The frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  may be transmitted at the widest allowed bandwidths available to stations STA 1 , STA 2 , and STA 3 , respectively. 
     Although the sounding process  612  involves uplink frames, as will be further described herein, the results of the sounding process  612  may be used to select the sub-carriers or sub-bands or RUs for transmitting DL frames by the AP to the stations STA 1 , STA 2 , and STA 3  as a result of reciprocity of the channels between the AP and the stations 
     STA 1 , STA 2 , and STA 3 . 
     In an embodiment, OFDMA sub-band soundings may be performed by the AP using received uplink frames (e.g., the first frame OFDM UF 1 , the second frame OFDM UF 2 , and the third frame OFDM_UF 3 ) or other previously received frames transmitted by the stations STA 1 , STA 2 , and STA 3  when the received frames each include an indication that OFDMA sub-band soundings are to be performed. 
     In another embodiment, whether OFDMA sub-band soundings may be performed by the AP using the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  may be determined according to a capability of the respective stations STA 1 , STA 2 , and STA 3  that broadcast the frame OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3 . In an embodiment, the capability of the stations STA 1 , STA 2 , and STA 3  may be communicated to the AP in a field of frames respectively transmitted by the stations STA 1 , STA 2 , and STA 3 . 
     In an embodiment, the AP determines respective OFDMA sub-band CSI for each of the stations STA 1 , STA 2 , and STA 3  using the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  received from STA 1 , STA 2 , and STA 3 , respectively. Determining the respective OFDMA sub-band CSI for each of the stations STA 1 , STA 2 , and STA 3  may include determining one or more of RSSI, a SINR, a MCS, and a NSS per a specific resource unit (RU) with a given bandwidth, sub-band or set of sub-bands for each of the transmitting antennas of each of the stations with respect to each of the receiving antennas of the AP. In an embodiment, the AP may determine the OFDMA sub-band CSI using respective LTF symbols (e.g., VHT LTF or HE-LTF symbols) of the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3 . 
     At S 614 , the first station STA 1  transmits a first frame OFDM_UF 1  adapted to support implicit OFDMA sounding to the AP. At S 616 , the second station STA 2  transmits a second frame OFDM_UF 2  adapted to support implicit OFDMA sounding to the AP. At S 616 , the third station STA 3  transmits a third frame OFDM_UF 3  adapted to support implicit OFDMA sounding to the AP. 
     In an embodiment, each of the first frame OFDM_UF 1 , the second frame OFDM_UF 2 , and the third frame OFDM_UF 3  may be adapted to support implicit OFDMA sounding by being transmitted from their respective stations STA 1 , STA 2 , and STA 3  using the widest bandwidths respectively available between the stations STA 1 , STA 2 , and STA 3  and the AP. 
     In an embodiment, one or more of the stations STA 1 , STA 2 , and STA 3  may respectively transmit the first frame OFDM_UF 1 , the second frame OFDM_UF 2 , and the third frame OFDM_UF 3 , unsolicited. 
     In an embodiment, one or more of the stations STA 1 , STA 2 , and STA 3  may send the respective frame OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  whenever the AP solicits such frames. For example, when the first frame OFDM_UF 1  is an ACK or BA frame, the first frame OFDM_UF 1  may be transmitted as a response to a data frame or a block acknowledgment request (BAR) previously transmitted to the first station STA 1  by the AP 1 . 
     In some embodiments, the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  may be a control frame such as, for example, any one of a PS-poll frame, an ACK frame, a BA frame, and the like. 
     Returning to  FIG. 6A , at S 620  the AP determines sub-band channel state information (CSI) corresponding to conditions or states of sub-bands or RUs in the channels between each of the stations STA 1 , STA 2 , and STA 3  and the AP by processing the uplink frames (i.e., the first frame OFDM_UF 1 , the second frame OFDM_UF 2 , and the third frame OFDM_UF 3 ) received from station STA 1 , station STA 2 , and station STA 3 , respectively. In an embodiment, one or more of an RSSI, an SINR, an MCS, and an NSS of the sub-bands or RUs used to transmit the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  may be determined in order to obtain the CSI for each sub-band or RU between the AP and the stations STA 1 , STA 2 , and STA 3 . 
     The AP measures one or more characteristics of the respective channel between the AP and the stations STA 1 , STA 2 , and STA 3  using one or more fields of the respective uplink frame transmitted through the respective channel, such as, for example, an LTF symbol, e.g., VHT-LTF or HE-LTF symbols. 
     At S 630 , the AP determines an allocation of sub-bands to the stations STA 1 , STA 2 , and STA 3  in the BSS according to the sub-band (or RU) CSI respectively determined at S 620  for each of the stations STA 1 , STA 2 , and STA 3 . 
     In an embodiment, allocating sub-bands or RUs to stations includes determining a number of sub-bands or RUs to allocate to each station and assigning sub-bands or RUs to each station until the allocated number of sub-bands or RUs in the channel is assigned to each station. Each sub-band or RU may be assigned to a station, from among the stations having less than the allocated number of sub-bands or RUs currently assigned to them, for which the sub-band or RU has the most suitable CSI, e.g., a highest channel magnitude or among the sub-bands or RUs with highest channel magnitude. 
     In some embodiments, the AP may be configured to use other processes for allocating sub-bands or RUs to stations according to the sub-band or RU CSI for each station, such as water filling, call admission control, and the like. 
     At S 650 , the AP prepares an OFDMA frame OFDMA_DF including first, second, and third information intended for delivery to the first, second, and third stations STA 1 , STA 2 , and STA 3 , respectively. The first to third information may include data from buffers within the AP, each buffer being respectively associated with an intended station of the first to third stations STA 1  to STA 3 . The first information is disposed in one or more sub-bands or RUs assigned to the first station STA 1 , the second information is disposed in one or more sub-bands assigned to the second station STA 2 , and so on. 
     At S 670 , the AP transmits the OFDMA frame OFDMA DF to the stations STA 1 , STA 2 , and STA 3  according to the sub-band or RU allocation performed in S 630 . 
     Although not illustrated in  FIGS. 6A and 6B , in some embodiments, the frames OFDM_UF 1 , OFDM_UF 2 , and OFDM_UF 3  used in the sounding process S 610  may have been transmitted in response to one more frames transmitted by the AP to the stations STA 1 , STA 2 , and STA 3 . The one or more frames transmitted by the AP may be an OFDMA frame (for example, a DL OFDMA data frame), one or more OFDM frames (for example, one or more OFDM BAR frames), or the like. 
       FIGS. 7A and 7B  illustrates Access Point (AP) initiated WLAN operations  700  and  700 ′, respectively, that include implicit channel sounding according to some embodiments. In  FIG. 7A  and  FIG. 7B , the AP transmits one or more downlink (DL) data frames to stations STA 1  and STA 2  and receives corresponding ACK or BA responses from the stations STA 1  and STA 2 . The one or more DL frames may be separate OFDM frames such as illustrated in  FIG. 7B , or may be a multi-user frame, such as a broadcast frame, MU MIMO frame, or OFDMA frame as illustrated in  FIG. 7A . In both cases, the AP expects to receive ACK or BA frames from the intended stations as respective responses to the one or more DL frames, including stations STA 1  and STA 2 , addressed in a previously sent multiuser frame (e.g., frame DL_F 1  in  FIG. 7A ), or addressed in multiple OFDM frames (e.g., frames DL_F 1 ′ and DL_F 2 ′ of  FIG. 7B ). 
     In a process disclosed herein, the stations STA 1  and STA 2  may send their respective ACK or BA frames using specific formats that may be used by the AP to determine information (e.g., channel state information (CSI) for various sub-bands or RUs between the AP and the stations STA 1  and STA 2 ). The sub-band or RU CSI can be used to configure the transmission of subsequent DL or UL OFDMA frames scheduled by the AP for a group of stations including stations STA 1  and STA 2 . 
     In some cases, the stations STA 1  and STA 2  may respond with an ACK or BA frame that is transmitted in the same bandwidth as the preceding DL frame. The stations STA 1  and STA 2  may respond with ACK or BA frames in non-HT duplicate format such as that shown in  FIG. 8 , which is shown for 80 MHz bandwidth. The duplication structure of  FIG. 8  can be applied for non-HT duplicate formats with bandwidths of 40 MHz, 80 MHz, 160 MHz or 80+80 MHz. However, embodiments are not limited to that shown in  FIG. 8 , and the stations may respond with ACK or BA frames in HT, VHT or HE formats with a bandwidth of 40 MHz, 80 MHz, 160 MHz or 80+80 MHz. 
     Referring to  FIG. 7A , a BSS includes an AP and first and second OFDMA-capable stations STA 1  and STA 2 . An OFDMA-capable station is a station that is (at least) capable of receive and decoding data transmitted to the station in an OFDMA frame. 
     At S 702 , the first and second stations STA 1  and STA 2  receive a first DL frame DL_F 1  from the AP. The first DL frame DL_F 1  may include one or more of a MAC Control Frame, a MAC Data frame, or a MAC Management frame. In some embodiments, the first DL frame DL_F 1  may be an OFDM or OFDMA frame. The first DL frame DL_F 1  may be transmitted with a widest allowed bandwidth, as previously described. 
     In an embodiment, the first DL frame DL_F 1  upon reception by a station (STA 1  or STA 2 ) prompts the station to transmit a frame, for example, an ACK frame or Block ACK frame, adapted for implicit OFDMA sounding. 
     At S 706 , the first station STA 1  transmits a first ACK frame ACK 1  in response to the first DL frame DL_F 1 . In an embodiment, the first ACK frame ACK 1  is an OFDM frame with a bandwidth that mirrors the bandwidth of the first DL frame DL_F 1 . In an embodiment, the first DL frame DL_F 1  will have a bandwidth that is greater than 20 MHz. 
     For example, when the first DL frame DL_F 1  has a bandwidth of 40 MHz, 80 MHz, 160 MHz, or 80+80 MHz, the first ACK frame ACK 1  may have a bandwidth of up to 40 MHz, 80 MHz, 160 MHz, or 80+80 MHz, respectively. The bandwidth of the first ACK frame ACK 1  may be less than the bandwidth of the first DL frame DL_F 1  when the first station STA 1  senses that one or more secondary channels within the bandwidth of the first DL frame DL_F 1  are in use. 
     In an embodiment, the AP transmits the first DL frame DL_F 1  using the widest allowed bandwidth available between the AP and station STA 1 . 
     In response, the first station STA 1  transmits the first ACK frame ACK 1  at the widest allowed bandwidth between the first station STA 1  and the AP and that mirrors (i.e., that is less than or equal to) the bandwidth of the first DL frame DL_F 1 . 
     When the widest allowed bandwidth (e.g., 80 MHz such as illustrated in  FIG. 8 ) is wider than a minimum bandwidth (e.g., 20 MHz) of a frequency segment (e.g., a 5 GHz band), the first ACK frame ACK 1  may include information duplicated across the widest allowed bandwidth. 
     In an embodiment, the first ACK frame ACK 1  may include a bandwidth indicator that indicates the bandwidth of the ACK frame ACK 1 . 
     In an embodiment, the first ACK frame ACK 1  may be in the form of a non-HT duplicated frame such as illustrated in  FIG. 8 , which is an illustrative non-HT duplicate frame with a bandwidth of 80 MHz. In another embodiment, the first ACK frame ACK1 may be in the form of a non-HT duplicate frame having any of a 40 MHz bandwidth, a 160 MHz bandwidth, and an 80+80 MHz bandwidth. In another embodiment, the first ACK frame ACK 1  may be in the form of a HT, VHT, or HE formats with a bandwidth of 40 MHz, 80 MHz, 160 MHz, or 80+80 MHz. 
     In an embodiment, the AP determines first OFDMA sub-band CSI for a first sub-channel of a channel between the AP and the first station STA 1  by using the first ACK frame ACK 1 , such as by respectively determining one or more of an RSSI, a SINR, and MCS, and an NSS for each of a plurality of resource units or sub-bands of the first ACK frame ACK 1 . The OFDMA sub-band or resource unit with best CSI may be used by the AP for exchanging subsequent UL and DL OFDMA frames with the first station STA 1 . 
     At S 714 , the second station STA 2  transmits a second ACK frame ACK 2 . In an embodiment, the second ACK frame ACK 2  is an OFDM frame. The second station STA 2  transmits the second ACK frame ACK 2  with a bandwidth that mirrors the bandwidth of the first DL frame DL_F 1 . 
     In an embodiment, the second ACK frame ACK 2  may be in the form of a non-HT duplicated frame such as illustrated in  FIG. 8 , which is an illustrative non-HT duplicate frame having 80 MHz bandwidth. In an embodiment, the second ACK frame ACK 2  may be in the form of a non-HT duplicate frame having any one of a 40 MHz bandwidth, a 160 MHz bandwidth, and an 80+80 MHz bandwidth. In an embodiment, the second ACK frame ACK 2  may have one of a HT, VHT and HE formats, and may have a bandwidth of any one of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. 
     In an embodiment, the AP determines second OFDMA sub-band CSI for a second sub-channel of the channel that includes the first sub-channel between the AP and the second station STA 2  by using the second ACK frame ACK 2 , such as by respectively determining one or more of an RSSI, a SINR, and MCS, and an NSS for each of a plurality of sub-bands or RUs of the second ACK frame ACK 2 . 
     The AP then allocates sub-bands or RUs to the stations STA 1  and STA 2  based on the first and second OFDMA sub-band CSI. At S 720  the AP transmits a DL OFDMA frame OFDMA_DL based on the allocation of the sub-bands or RUs, so that the DL OFDMA frame OFDMA_DL includes 1) first data intended for delivery to station STA 1  and transmitted using one or more sub-bands or RUs that have been allocated to first station STA 1 , and 2) second data intended for delivery to the second station STA 2  and transmitted using one or more sub-bands or RUs that have been allocated to the second station STA 2 . 
     In another embodiment, shown in  FIG. 7B , the second station STA 2  may transmit the second frame ACK 2  after receiving a second DL frame DL_F 2 ′ from the AP at S 712 ′. The second DL frame DL_F 2 ′ may include one or more of a MAC Control Frame, a MAC Data frame, or a MAC Management frame. The second DL frame DL_F 2 ′may be transmitted at the widest allowed bandwidth, as previously described. In an embodiment, the second DL frame DL_F 2 ′ may include an indication that implicit OFDMA sub-band sounding is requested. Upon reception of the second DL frame DL F 2 ′, the second station STA 2  transmits to the AP a second ACK frame ACK 2  with a bandwidth that mirrors the bandwidth of the second DL frame DL_F 2 ′. In some cases, the bandwidth used to transmit the second ACK frame BA 2  may be the widest allowed bandwidth between the AP and the second station STA 2 . In an embodiment, the widest allowed bandwidth is greater than a bandwidth of 20 MHz, for example a bandwidth of any one of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. 
     The AP may determine respective CSI of the sub-bands or RUs of the respective channels between the AP and the stations STA 1  and STA 2  using the first ACK frame ACK 1  and the second ACK frame ACK 2 , respectively. Based on the determined CSI, the AP allocates the sub-bands or RUs to the stations STA 1  and STA 2 , as well as to other stations that may be part of the same BSS. At S 720 ′, the AP transmits an DL OFDMA frame OFDMA_DL′ based on the allocation of the sub-bands or RUs, so that the DL OFDMA frame OFDMA_DL′ includes 1) first data intended for delivery to station STA 1  and transmitted using one or more sub-bands or RUs that have been allocated to first station STA 1 , and 2) second data intended for delivery to the second station STA 2  and transmitted using one or more sub-bands or RUs that have been allocated to the second station STA 2 . 
       FIG. 7C  illustrates an AP initiated WLAN operation  700 ″ that include implicit channel sounding according to another embodiment.  FIG. 7C  illustrates a station STA 1  transmitting a response frame ACK/BA 1 ″ in response to receiving a Block Acknowledgement Request (BAR) frame BAR. 
     In an embodiment, the response frame ACK/BA 1 ″ may be a Block ACK (BA) frame. In an embodiment, the response frame ACK/BA 1 ″ may be an ACK frame. 
     At S 702 ″, the AP transmits a DL frame DL_F 1 ″ to the station STA 1 . In some embodiments, the DL frame DL_F 1 ″ may be an OFDM frame or an OFDMA frame. After transmitting the DL frame DL_F″, at S 704 ″ the AP transmits the BAR frame BAR to the station STA 1 . 
     In an embodiment, the AP may transmit the BAR frame to the station STA 1  using the widest allowed bandwidth available between the AP and the station STA 1 . In an embodiment, the widest allowed bandwidth is a bandwidth that is greater than 20 MHz, such as a bandwidth of 40 MHz, 80 MHz, 160 MHz, or 80+80 MHz. 
     Upon receiving the BAR frame BAR, at S 706 ″ the station STA 1  transmits the response frame ACK/BA 1 ″ with a bandwidth that mirrors the bandwidth of the BAR frame, which as previously described, may be the widest allowed bandwidth. In an embodiment, the response frame ACK/BA 1 ″ may be an OFDM frame. By using the widest allowed bandwidth for transmitting the response frame ACK/BA 1 , the AP has an opportunity to determine the respective CSI of the widest range of sub-bands or RUs that may be available in the channel or channels between the AP and the station STA 1 . Based on the CSI of the sub-bands or RUs between the AP and the station STA 1 , the AP allocates one or more sub-bands or RUs to the station STA 1 . At S 720 ″ the AP transmits a DL OFDMA frame OFDMA_DL including data intended for delivery to the station STA 1 , wherein the data intended for delivery to the station STA 1  is transmitted using the one or more sub-bands or RUs that have been allocated to the station STA 1 . 
     When the AP and station STA are engaged in Single User (SU) transmission then process  700 ″ happens between the AP and the station STA 1 . The AP transmits a BAR frame BAR in a specific format described herein. The station STA 1  responds with the response frame ACK/BA 1  in a specific format and that is transmitted in the same bandwidth as the BAR frame BAR. 
     When the AP and several stations are engaged in a DL MU transmission (such as an 802.11ac DL MU MIMO transmission or an 802.11ax DL OFDMA transmission) then this process happens between the AP, which sends a BAR frame for each station in a specific format described here, and each of the stations that responds with an ACK or BA frame in a specific format that mirrors the initial BAR frame. 
       FIG. 8  illustrates a frame  800  suitable for use in an OFDMA sub-band sounding process, such as the OFDMA sub-band sounding performed as part of the respective WLAN operations  700 ,  700 ′, and  700 ″of  FIGS. 7A, 7B, and 7C , according to an embodiment. The frame  800  may be an OFDM uplink frame. 
     The frame  800  is transmitted at a widest available bandwidth, which in the example shown in  FIG. 8  is 80 MHz. The frame  800  includes an ACK or BA frame  818  having a 20 MHz bandwidth and first, second, and third duplicate frames  812 ,  814 , and  816  each occupying additional respective 20 MHz bandwidths. The first, second, and third duplicate frames  812 ,  814 , and  816  that are included in frame  800  are identical in content to the ACK or BA frame  818 , and are used to duplicate the ACK or BA frame  818  across the 80 MHz widest allowed bandwidth of the frame  800 . 
     In an embodiment, the frame  800  may be in a legacy non-HT duplicate format. In an embodiment, the frame  800  may be in a VHT or HE format, in which case no explicit duplication is needed. 
     In an embodiment, when a BA frame is sent as an immediate response to either an implicit BA request of a previous frame or to a BAR frame included in the previous frame, and the previous frame is a non-HT PPDU, the BA frame has one of a non-HT PPDU format, a HT PPDU format, a VHT PPDU format, and an HE PPDU format, and the rate and bandwidth of the BA frame is selected as follows. 
     A primary rate is defined to be a highest rate in a BSSBasicRateSet parameter that is less than or equal to a rate of the previous frame. When no rate in the BSSBasicRateSet parameter meets these conditions, the primary rate is defined to be a highest mandatory rate of an attached PHY that is less than or equal to the rate of the previous frame. The BSSBasicRateSet is the list of data rates that must be supported by any station wishing to join a BSS network. 
     The station that is about to send the responsive BA frame may set the CH_BANDWIDTH parameter of the BA frame based on the CH_BANDWIDTH parameter value of the DL frame that includes the data PPDU or based on the CH_BANDWIDTH parameter value of the BAR frame of the data PPDU (e.g., data PPDU of the DL frame of  FIGS. 7 a , 7 b , and 7 c   ) that was received previously. 
     For example, in an embodiment, the CH_BANDWIDTH parameter value of the TXVECTOR of the BA frame to be sent by the station may be set to equal to the CH_BANDWIDTH parameter value of the DL frame that includes the non-HT PPDU. In another embodiment, when the AP transmits a BAR frame, the CH_BANDWIDTH parameter value of the TXVECTOR of the BA frame to be sent by the station may be set to equal to the CH_BANDWIDTH parameter value of the RXVECTOR of the BAR frame, however the format of the BAR frame may vary. The TXVECTOR represents a list of parameters that the MAC sublayer provides to the local PHY entity in order to transmit a PSDU. The RXVECTOR represents a list of parameters that the PHY provides the local MAC entity upon receipt of a valid PLCP header or upon receipt of the last PSDU data bit in the received frame. 
     In an embodiment the BAR frame may be sent by the AP in non-HT duplicate format. In an embodiment, when a CH_BANDWIDTH_IN_NON_HT parameter of the TXVECTOR at the AP is set to larger than 20 MHz, the scrambling sequence carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT, and the TA field value is the address of the STA that will be transmitting the BA frame or a bandwidth signaling TA. In the case where the BA frame to be transmitted by the STA is in a non-HT or non-HT duplicate format and where the scrambling sequence carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT, such as the case described above, the TA field value is a bandwidth signaling TA. The scrambling sequence is generated when the PLCP DATA scrambler scrambles transmit data during the PPDU encoding process. 
     In another embodiment, the BAR frame may be sent in HT format when the CH_BANDWIDTH parameter value of the RXVECTOR is 40 MHz. In yet another embodiment, the BAR frame may be sent in VHT format when the CH_BANDWIDTH parameter value of the RXVECTOR is 40 MHz, 80 Mhz, 160 Mhz, or 80+80 MHz. In another embodiment, the BAR frame may be sent in HE format when the CH_BANDWIDTH parameter value of the RXVECTOR is 40 MHz, 80 Mhz, 160 Mhz, or 80+80 Mhz. 
     Note that although the above-described procedures for selecting the rate and bandwidth was explained with respect to BA frames, the same procedures may be employed for selecting rate and bandwidths for an ACK frame. In such implementations, it may be assumed that the AP and the STA are engaged in a SU frame exchange and the STA sends ACK frame immediately after the DL data frame. While the DL data frame may be a non-OFDMA frame, the procedure described here would allow the AP to gather information that later can be used for determining sub-band or RU allocations for exchanging subsequent DL or UL OFDMA frames. 
     In an embodiment, when an ACK control response frame is carried in a non-HT PPDU, the primary rate may be defined as the highest rate in the BSSBasicRateSet parameter that is less than or equal to the rate of the previous frame. When no rate in the 
     BSSBasicRateSet parameter meets these conditions, the primary rate may be defined as the highest mandatory rate of the attached PHY that is less than or equal to the rate of the previous frame. The STA shall transmit the non-HT PPDU ACK control response frame at either the primary rate, or an alternate rate, when one exists. For example, an alternate rate may be selected provided that all of the following conditions are met: 1) the duration of the frame at the alternate rate is the same as the duration of the frame at the primary rate determined by section 9.7.6.5.2 of IEEE Std 802.11™-2012; 2) the alternate rate is in either the BSSBasicRateSet parameter or is a mandatory rate of the attached PHY; and 3) the modulation class of the frame at the alternate rate is the same class as that of the primary rate selected by section 9.7.6.5.2 of IEEE Std 802.11™-2012. 
     In an embodiment, the CH_BANDWIDTH parameter of the ACK frame sent in response to a data PPDU may be based on the CH_BANDWIDTH parameter value of the data PPDU. The CH_BANDWIDTH parameter value of the RXVECTOR of the data PPDU frame may be equal to the CH_BANDWIDTH parameter value of the TXVECTOR of the ACK frame. However the format of the ACK frame may vary. 
     For example, in an embodiment, the ACK frame may be sent in a non-HT duplicate format. In this embodiment, when the CH_BANDWIDTH _N_NON_HT parameter of the TXVECTOR is set to larger than 20 MHz, the scrambling sequence carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT, and the transmitter address (TA) field value is the address of the STA transmitting the frame or a bandwidth signaling TA. In the ACK frame transmitted by the STA in a non-HT or non-HT duplicate format and where the scrambling sequence carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT, such as in the case described above, the TA field value is a bandwidth signaling TA. 
     In an embodiment, the ACK frame may be sent in an HT format when the CH_BANDWIDTH parameter value of the RXVECTOR is 40 MHz. In another embodiment, the ACK frame may be sent in VHT format when the CH_BANDWIDTH parameter value of the RXVECTOR is one of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. In another embodiment, the ACK frame may be sent in HE format when the CH_BANDWIDTH parameter value of the RXVECTOR is one of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. 
     In the above described embodiments, the STA may respond to the BAR frame or a DL data frame sent by the AP by sending a BA or ACK frame that has the same bandwidth as the bandwidth of the BAR frame or the DL data frame. 
     In some embodiments, an indicator may be included in the BAR frame to indicate a requested bandwidth for the BA or ACK frame. When the BAR frame is in a non-HT duplicate format, the bandwidth indicator may be included in the structure of the non-HT duplicated frame. When the BAR frame is in VHT or HE format, the bandwidth indicator may be in the VHT or HE frame. 
     In an embodiment, the bandwidth indicator may be compliant with an IEEE 802.11 standard (e.g., the 802.11ac or 802.11ax standards). Using the bandwidth indicator, a STA may determine a bandwidth of the BAR frame and prepare an appropriate response frame, as described above. 
     However, in an embodiment, the bandwidth indicator may be provided in the payload of the BAR frame, as will be described herein. 
     In an embodiment, a BAR frame may be provided by the AP that provide indications related to implicit sounding in the BAR Control and Starting Sequence Control fields.  FIGS. 9A, 9B, and 9C  show a BAR control portion in accordance with embodiments, in which a two-bit wide Wideband BA Response subfield  928  is included in the BAR control field  910 .″ In an embodiment, the Wideband BA Response field may be encoded as described below. 
     The Wideband BA Response subfield  928  having a value of 00 indicates that a BA response frame should be sent with 20 MHz bandwidth, i.e., a CH_BANDWIDTH_IN_NON_HT or a CH_BANDWIDTH parameter of the TXVECTOR is set to 20 MHz. 
     The Wideband BA Response subfield  928  having a value of 01 indicates that the BA response frame should be sent with 40 MHz bandwidth, i.e., the CH_BANDWIDTH_IN_NON_HT or CH_BANDWIDTH parameter of the TXVECTOR is set to 40 MHz. 
     The Wideband BA Response subfield  928  having a value of 10 indicates that the BAR frame should be sent with 80 MHz bandwidth, i.e., the CH_BANDWIDTH_IN_NON_HT or CH_BANDWIDTH parameter of the TXVECTOR is set to 80 MHz. 
     The Wideband BA Response subfield  928  having a value of 11 then indicates that the BAR frame should be sent with 160 MHz or 80+80 MHz bandwidth, i.e., the CH_BANDWIDTH_IN_NON_HT or CH_BANDWIDTH parameter of the TXVECTOR is set to 160 MHz or 80+80 MHz. 
     Referring to  FIG. 9A , which illustrates a BAR (Block ACK Request) frame  900  and its components including a Frame Control field  902 , a Duration field  904 , a Receiver Address (RA) field  906 , a Transmitter Address (TA) field  908 , a BAR Control Field  910 , a Block ACK Starting Sequence Control field  912 , and a Frame Check Sequence (FCS) field  914 . 
     The Frame Control field  902  is used to indicate, for example, the protocol version being used, the frame type or sub-type (e.g., BAR frame) that the frame is a control frame, and so forth. The Duration field  904  is used to reserve the medium for a fixed time period. The RA field  906  is the address of the receiver that is to receive the BAR frame, which in this case is a station. The TA field  908  is the address of the transmitter of the BAR frame, which in this case is the AP. 
     The BAR Control field  910  is 16 bits long and may be used to provide various information including, for example, indicating whether an immediate acknowledgement is or is not expected (as indicated by B 0 : BAR ACK of  FIG. 9C ), to determine which of three possible BlockAckReq frame variants is represented (as indicated by the B 1 : Multi-TID and B 2 : Compressed Bitmap), and so forth. The Block ACK Starting Sequence Control field  912  contains the sequence number of the first MSDU or A-MSDU for which the BAR frame is sent. The FCS field  914  is a 32-bit field containing a 32-bit cyclic redundancy code (CRC). 
       FIG. 9B  illustrates a Multi-Traffic Identifier (TID) BAR frame  920 . The Multi-TID BAR frame  920  includes the same components as the BAR frame  900  of  FIG. 9A  except that the Multi-TID BAR frame  920  includes one or more Starting Sequence Control fields  912  and respective Per TID Info fields  916 . The Per TID Info field  916  includes 12 bits that are reserved and 4 bits for TID value. 
     Both the BAR frame  900  and the Multi-TID BAR frame  920  have the BAR Control field  910 . In an embodiment, the BAR Control field  910  may include the Wideband BA Response subfield  928  for indicating a preferred bandwidth for the BA response frame that a responding station may subsequently send.  FIG. 9C  illustrates the contents of the BAR Control fields  910  of  FIGS. 9A and 9B  including the Wideband BA Response subfield  928  according to some embodiments. 
     The BAR Control field  910 , in addition to the Wideband BA Response subfield  928 , includes a BAR ACK subfield  922 , a Multi-TID subfield  924 , a Compressed Bitmap subfield  926 , a Reserved subfield  930 , and a TID/NumTIDs subfield  932 . The BAR ACK subfield  922  indicates whether the recipient of the BAR is required to transmit an immediate acknowledgement. The values of the Multi-TID subfield  924  and the Compressed Bitmap subfield  926  determines which of three possible BAR frame variants is represented (i.e., Basic BlockAckReq, Compressed BlockAckReq, or Multi-TID BlockAckReq). Seven bits of the 16 bit BAR Control field  910  are included in the Reserved subfield  930 , and four bits are for the TID/NumTIDs subfield  932 . 
     With respect to the Wideband BA Response subfield  928 , there are two bits reserved for the Wideband BA Response subfield  928 . Thus, one of at least four different values may be provided in the Wideband BA Response subfield  928  to indicate one of at least four different bandwidths (e.g., any one of 20 MHz, 40 MHz, 80 MHz, and 160 MHz). 
     In an embodiment, a STA that receives a BAR frame with the Wideband BA Response field may prepare the BA frame in accordance with 802.11 standards. However, the bandwidth of the BA frame, i.e. CH_BANDWIDTH_IN_NON_HT or CH_BANDWIDTH parameter of the TXVECTOR may be set according to the value of the Wideband BA Response field. The format of the BA frame, i.e. whether it should be sent in non-HT duplicate or VHT or HE formats may be decided at the STA as described above. 
     In an embodiment, the above-described behaviors of the AP and a STA may be based on the capability of the STA. In other words, a STA might perform as described above only when its capability allows, otherwise it may send the BA or ACK frames according to the existing rules in the 802.11 specifications (e.g., 802.11ac, 802.11ax, etc.). Thus, in some embodiments a capability field may be included into certain uplink (UL) frames, such as management frames, transmitted by the STA in order to “announce” the capabilities of the STA. 
     In an embodiment, the capability field that may be included in UL frames is described in the following. Since such capabilities may be applied to the next-generation of 802.11 devices (such 802.11ax or HE devices), the capability field may be a HE Capabilities element. There are several fields that may be included in the HE Capabilities element that specifies the capabilities of the device. In an embodiment, a Wideband ACK Capability field may be included into UL frames, such as management frames, which in one implementation has one bit width where it is set as follows: 
     For AP devices the Wideband ACK Capability field may be reserved. For STA devices the Wideband ACK Capability field may be set to 1 when the STA is capable of sending ACK or BA frames with bandwidth wider than 20 MHz, otherwise the Wideband ACK Capability field may be set to zero. Also note that the Wideband ACK capability field may be assigned to other HE elements, but its usage should be as described above. 
     In another embodiment, the Wideband ACK Capability field might have two or more bits where its setting is as follows: 
     The Wideband ACK Capability field may be set to 1 when the STA is capable of sending ACK or BA frames with a bandwidth of 40 MHz. The Wideband ACK Capability may be set to 2 when the STA is capable of sending ACK or BA frames with a bandwidth of 80 MHz. The Wideband ACK Capability may be set to 3 when the STA is capable of sending ACK or BA frames with a bandwidth of 160 MHz or 80+80 Mhz. The 
     Wideband ACK Capability may be set to 0 when the STA is only capable of sending ACK or BA frames with a 20 MHz bandwidth. 
       FIG. 10  illustrates a station-initiated process  1000  using a PS-Poll frame adapted for implicit OFDMA sounding, according to an embodiment. 
     At S 1002 , a station STA transitions out of power save (PS) mode and sends PS-Poll frame in HE format. In some embodiments, a PS-Poll frame or a variant of PS-Poll frame may carry an HE Control (HEC) field. Such a variant of PS-Poll frame might be denoted as enhanced PS-Poll frame or the like, and may be carried in an HE PPDU format, or any PPDU format that is capable of carrying the HEC field or any control field with the following attributes. 
     HEC field is an extension of HT Control field, and may be placed after the address fields in the MAC header of the PS-Poll frame. Within the HEC field, the STA places additional sub-fields that carry useful information for the AP that the STA is associated. 
     In some embodiments, the rate adaptation sub-field is carried in the HEC field where it carries CSI, SNR, MCS, or NSS for one or more 20 MHz sub-channels that the STA operates at, or carries CSI, SNR, MCS, or NSS for one or more resource-units or sub-bands. In some embodiments, a sub-field is carried in HEC of the PS-Poll frame where it indicates the transmission power of the STA or the transmission power that the STA is expected to operate at the next one or more units of time, such as Beacon Interval (BI). 
     In some embodiments, a sub-field is carried in HEC of the PS-Poll frame where it indicates the level of AWGN noise or interference that the STA experienced in an immediate past interval, such as one or more BIs. In some embodiments, a sub-field is carried in HEC of the PS-Poll frame where it indicates the buffer status of one or more access categories (ACs) or Traffic Categories (TCs) or Traffic Indicators (TIDs). In some embodiments, a sub-field is carried in HEC of the PS-Poll frame where it indicates the receive operating mode indication which indicates any change in receiver operation, either in bandwidth, number of spatial streams or both. 
     At S 1104  the station STA determines the widest allowed bandwidth that may be employed in order to transmit a subsequent frame such as a PS-Poll frame. In an embodiment, the widest allowed bandwidth is a bandwidth that is greater than 20 MHz. 
     After determining the widest allowed bandwidth, and as a result of coming out of the PS mode, the station STA may transmit to the AP a frame at S 1006 . In an embodiment, the frame may include an IEEE Std 802.11-2012 PS-Poll frame. Upon receiving the PS-Poll frame PS-Poll, the AP at S 1008  determines CSI of the sub-bands or sub-carriers (or RUs). 
     The AP may use the determined sub-band CSI (or RU CSI) to select one or more sub-bands or RUs for transmitting data to the station STA using an DL OFDMA frame or for receiving data from the station using an UL OFDMA frame. For example, the AP may use the selected one or more sub-bands or RUs for transmitting the data to the AP via a DL frame OFDMA_DL at S 1010 . 
     In the embodiments related to  FIG. 10  and  FIG. 11 , the STA may transmit to the AP a PS-Poll frame using the widest bandwidth that both the AP and the STA are configured to operate over and that the condition of the wireless medium allows (that is, the widest allowed bandwidth). For the embodiments, after the STA exits a power save mode, the STA sends the PS-Poll frame to the AP. The PS-Poll frame indicates to the AP that the STA is out of sleep mode and ready to receive one or more DL payloads associated with the STA. The AP acknowledges the reception of the PS-Poll frame and may subsequently send the DL payloads, if any, to the STA. In some cases, the AP may send the DL payload immediately after the reception of the PS-Poll frame. In an embodiment, the DL payload may be transmitted to the STA via an OFDMA_DL frame. 
     In the PS-Poll procedure shown in  FIG. 10 , the STAs send PS-Poll frames in specific formats. The formats of the PS-Poll frames enable the AP to collect sub-carrier information, sub-band or RU information, or both for the widest bandwidth possible (e.g., “widest allowed bandwidth” or “widest possible bandwidth”). 
     In an embodiment, a STA may send a PS-Poll frame in a non-HT duplicate format such as shown in  FIG. 11 , which is shown for 80 MHz bandwidth. However, embodiments are not limited thereto, and the duplication structure of  FIG. 11  may be applied for non-HT duplicate formats with bandwidths of any of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. In another embodiment, the PS-Poll frame may be sent in a HT, VHT, or HE format with a bandwidth of any of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. As previously described, regardless of whether the PS-Poll frame is in a non-HT duplicate format, an HT format, a VHT format, or an HE format, the PS-poll frame should be sent using the widest allowed bandwidth. 
     For example, in an embodiment, a STA may send the PS-Poll frame in non-HT duplicate format with a CH_BANDWIDTH_IN_NON_HT parameter of the TXVECTOR set to a 160 MHz (or 80+80 MHz) bandwidth when both the AP and the STA operate with the 160 MHz (or 80+80 MHz) bandwidth and the secondary 20 MHz, the secondary 40 MHz, and the secondary 80 MHz channels are not sensed BUSY by the STA at a Point Coordination Function (PCF) Interframe Space (PIFS) time before transmission of the PS-Poll frame. In this case, the widest allowed bandwidth is 160 MHz (or 80+80 MHz) bandwidth. 
     In an embodiment, the STA may send the PS-Poll frame in non-HT duplicate format with a CH_BANDWIDTH_IN_NON_HT parameter of the TXVECTOR set to 80 MHz when either (a) 80 MHz is the maximum bandwidth at which the STA operates (i.e., the operational bandwidth of the STA at the time that the STA associated with the AP or a narrower bandwidth when that was announced in an Operating Mode Notification sent by the STA or the AP), or (b) both the AP and the STA operate at a wider than 80 MHz bandwidth (that is, both the STA and the AP can operate using a 160 MHz or a 80+80 MHz bandwidth) and the secondary 80 MHz channel is sensed BUSY by the STA at a PIFS time before transmission of the frame. For this case, the widest allowed bandwidth is 80 MHz. 
     In another embodiment, the STA may send the PS-Poll frame in non-HT duplicate format with a CH_BANDWIDTH_IN_NON_HT parameter of the TXVECTOR set to 40 MHz when either (a) 40 MHz is the maximum bandwidth at which the STA operates (i.e. the operational bandwidth at the time that the STA associated with the AP or a narrower bandwidth (e.g., 20 MHz) that was announced in an Operating Mode Notification sent by the STA or the AP), or (b) both AP and the STA operate in wider than 40 MHz bandwidth (e.g., 80 MHz bandwidth) and the secondary 40 MHz channel is sensed BUSY by the STA at PIFS time before transmission of the frame. For this case, the widest allowed bandwidth is 40 MHz. 
     In addition, when the CH_BANDWIDTH_IN_NON_HT parameter of the TXVECTOR is set to larger than 20 MHz, the TA field value may be an address of the STA transmitting the PS-polling frame or a bandwidth signaling TA. When the PS-Poll frame is transmitted by a VHT or HE STA in a non-HT or non-HT duplicate format and a scrambling sequence of the PS-Poll frame carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT (such as the cases described above), the TA field value of the PS-Poll frame may be the bandwidth signaling TA. When the ACK or BA frame is transmitted by the STA in a non-HT or non-HT duplicate format and the scrambling sequence carries the TXVECTOR parameter CH_BANDWIDTH_IN_NON_HT, then the TA field value may be the bandwidth signaling TA. 
     In an embodiment, when the PS-poll frame is to be sent in non-HT duplicate format, the PS-poll frame may be transmitted by the STA using a rate no higher than the highest rate in the BSSBasicRateSet parameter that is less than or equal to the rate or non-HT reference rate of the previously transmitted frame that was directed to the same receiving STA. When no rate in the BSSBasicRateSet parameter meets these conditions, the control frame (e.g., PS-Poll frame) may be transmitted at a rate no higher than the highest mandatory rate of the attached PHY that is less than or equal to the rate or non-HT reference rate of the previously transmitted frame that was directed to the same receiving STA. 
       FIG. 11  illustrates a frame  1100  that may be used in a station-initiated OFDMA sub-band sounding process according to an embodiment. The frame  1100  may be used as an uplink PS-Poll frame in the station-initiated process  1000  of  FIG. 10 . 
     The frame  1100  is transmitted at a widest available bandwidth, which in the embodiment of  FIG. 11  is 80 MHz. The frame  1100  includes a PS-Poll frame  1118  having a bandwidth of 20 MHz, and first, second, and third duplicate PS-Poll frames  1112 ,  1114 , and  1116 . 
     The first, second, and third duplicate PS-Poll frames  1112 ,  1114 , and  1116  are substantially identical in content to the PS-Poll frame  1118 , and are used to duplicate the PS-Poll frame  1118  across the 80 MHz widest allowed bandwidth of the frame  1100 . 
       FIG. 12  illustrates an illustrative relationship between various primary and secondary channels of various bandwidths in accordance with 802.11 standards such as 802.11ac standards. Each of the various bandwidths (e.g., 40 MHz, 80 MHz, 80+80 MHz, and 160 MHz) comprises of at least a primary channel and a secondary channel. For example, the 40 MHz bandwidth  1202  comprises a 20 MHz primary channel (e.g., channel  60 ) and a secondary 20 MHz channel (e.g., channel  64 ). Similarly, the 80 MHz bandwidth  1204  comprises a 40 MHz primary channel (e.g., channels  60  and  64 ) and a 40 MHz secondary channel (e.g., channels  52  and  56 ). 
     The 40 MHz primary channel for the  80  bandwidth  1204  comprises the 20 MHz primary channel (e.g., channel  60 ) and the 20 MHz secondary channel (e.g., channel  64 ) of the 40 MHz bandwidth  1202 . In order to use the entire bandwidth of the 40 MHz bandwidth  1202  both the 20 MHz primary channel and the 20 MHz secondary channel must be free or IDLE (e.g., not BUSY). Similarly in order to use the entire bandwidth of the 80 MHz bandwidth  1204 , the 40 MHz secondary channel as well as the 20 MHz secondary channel (which, along with the 20 MHz primary channel, comprises the 40 MHz primary channel) must be free (e.g., not BUSY). Note further that both the 80+80 MHz bandwidth  1206  as well as the 160 MHz bandwidth  1208  is comprised of eight 20 MHz channels (e.g., channels  36 ,  40 ,  44 ,  48 ,  52 ,  56 ,  60 , and  64 ). Thus, in order to use the entire bandwidth of the 80+80 MHz bandwidth  1206  or the 160 MHz bandwidth  1208 , the 80 MHz secondary channel, the 40 MHz secondary channel, and the 20 MHz secondary channel must not be BUSY. 
     In an embodiment, the STA may transmit the PS-poll frame in HT, VHT, or HE format with widest allowed bandwidths of 40 MHz, 80 MHz, 80+80 MHz, or 160 MHz. In particular, the bandwidth of the PS-poll frame when the PS-poll frame is in HT, VHT, or HE format may be as follows. 
     When the STA sends the PS-Poll frame in HT PPDU, VHT PPDU or HE PPDU formats, then in some embodiments the CH_BANDWIDTH parameter of the TXVECTOR is set according to the maximum bandwidth that both the AP and the STA operate (i.e. a bandwidth at the time that the STA associated with the AP or a narrower bandwidth when the narrower bandwidth was announced in an Operating Mode Notification sent by the STA or the AP) and whether the secondary 20 MHz, the secondary 40 MHz, and the secondary 80 MHz channels are sensed BUSY according to 802.11 rules. 
     More specifically, the CH_BANDWIDTH parameter of the TXVECTOR may be set to 40 MHz when both the AP and the STA are currently (e.g., at or just before the PS-Poll frame being transmitted) operating at a maximum bandwidth of 40 MHz, or when both the AP and the STA operate wider than the 40 MHz bandwidth but the secondary 40 MHz channel is sensed BUSY by the STA at PIFS time before transmission of the PS-poll frame. The CH_BANDWIDTH parameter of the TXVECTOR may be set to 80 MHz when both the AP and the STA are currently operating at maximum bandwidth of 80 MHz, or when both the AP and the STA operate in wider than 80 MHz bandwidth but the secondary 80 MHz channel is sensed BUSY by the STA at PIFS time before transmission of the PS-poll frame. The CH_BANDWIDTH parameter of the TXVECTOR may be set to 160 MHz (or 80+80 MHz) when both AP and STA are currently operating with bandwidth of 160 MHz (or 80+80 MHz), and the secondary 80 MHz channel is not sensed BUSY by the STA at PIFS time before transmission of the PS-poll frame. The channel sensing rule for the primary 20 MHz channel, the primary 40 MHz channel, and the primary 80 MHz channel may be in accordance with rules specified in 802.11 specifications (e.g., 802.11ac or 802.11ax). 
       FIG. 13  illustrates a multi-station initiated process  1300  using PS-polling frames adapted to implicit OFDMA sub-band or RU sounding according to some embodiments. For ease of illustration and for the sake of brevity, the process  1300  is described below with respect to only two stations, but embodiments are not limited thereto. Thus, the process  1300  may be implemented with more than two stations. 
     At S 1302 , a first station STA 1  transmits to the AP a first PS-poll frame PS-Poll 1  after coming out of PS mode. The first PS-poll frame PS-Poll 1  is transmitted using the widest allowed bandwidth available between the first station STA 1  and the AP. 
     At S 1304 , a second station STA 2  transmits to the AP a second PS-poll frame PS-Poll 2  after also coming out of PS mode. The second PS-poll frame PS-Poll 2  is transmitted using the widest allowed bandwidth available between the second station STA 2  and the AP. 
     At S 1306 , the AP determines CSI of one or more sub-bands or RUs with respect to stations STA 1  and STA 2  by measuring certain characteristics (e.g., one or more of an RSSI, an SINR, an MCS, and an NSS) of the received PS-poll frames PS-Poll 1  and PS-Poll 2 . Based on the determined CSI, the AP may allocate sub-bands or RUs to stations STA 1  and STA 2 . 
     At S 1308 , the AP sends a DL OFDMA frame OFDMA_DL to the stations STA 1  and STA 2 . The DL OFDMA frame OFDMA_DL includes first data addressed to the first station STA 1  and transmitted using one or more first sub-bands or RUs that were allocated to the first station STA 1 . The DL OFDMA frame OFDMA_DL also includes second data addressed to the second station STA 2  and transmitted using one or more second sub-bands or RUs that were allocated to the second station STA 2 . 
     A STA may perform as described above only when its capability allows, otherwise it may send the PS-Poll frames according to the existing rules in the 802.11 specifications (e.g., 802.11ac). In an embodiment, a capability field that indicates the capability to transmit wideband control frame (e.g., ACK/BA, PS-poll frame, and the like) as described above may be included in UL frames (or DL frames) such as management frames. By including such capability fields in UL or DL frames, stations or APs can announce their capabilities to, for example, the BSS that they are affiliated with. 
     In an embodiment, a capability field may include an Implicit Wideband Operation field that has one bit width where it is set as described below. 
     For AP devices this field is reserved, for STA devices the Implicit Wideband Operation field may be set to 1 when the STA is capable of sending PS-Poll frames with a bandwidth wider than 20 MHz, otherwise this field may be set to zero. Note further that the Implicit Wideband Operation field may be assigned to other HE elements, but its usage should be as described above. 
     In another embodiment, the Implicit Wideband Operation field may have two or more bits where its setting is as described below. 
     The Implicit Wideband Operation may be set to 1 when the STA is capable of sending PS-Poll frames with bandwidth of 40 MHz. When the STA is capable of sending PS-Poll frames with a bandwidth of 80 MHz, the Implicit Wideband Operation field may be set to 2. When the STA is capable of sending PS-Poll frames with a bandwidth of 160 MHz or 80+80 MHz, the Implicit Wideband Operation field may be set to 3. Finally, when the STA is only capable of sending PS-Poll frames with a 20 MHz bandwidth, the Implicit Wideband Operation field may be set to zero. 
       FIG. 14  illustrates a process  1400  of determining OFDMA CSI by an Access Point (AP) according to an embodiment. The process  1400  may be employed in a system using one or more of the processes  700 ,  700 ′,  700 ″,  1000 , and  1300  of respective  FIGS. 7A, 7B, 7C, 11, and 14 . 
     At S 1402 , the AP may implicitly or explicitly indicate to a station STA that the station STA is to use a particular bandwidth to transmit an uplink (UL) frame. For example, the AP may transmit to the station STA a frame that indicates a specified bandwidth that is relatively wide (e.g., bandwidth greater than 20 MHz) is to be used or that indicates that the widest allowed bandwidth respectively available between the station STA and the AP is to be used. 
     In an embodiment, in order to provide an explicit indication, the AP may transmit a downlink (DL) frame that may include a field (e.g., the Wideband BA Response subfield as described above) that directs the one or more STAs to transmit one or more respective UL frames using the respective widest allowed bandwidth. 
     In an embodiment, an implicit indication may be provided by the AP by, for example, transmitting a DL frame in a particular format. For example, a DL frame, such as a DL control frame, that is typically transmitted with a bandwidth of 20 MHz may instead be transmitted with a bandwidth that is greater than 20 MHz. When a STA detects the wider bandwidth of the received DL frame, the STA may determine to transmit an uplink frame using a widest allowed bandwidth, which in some cases, may be a bandwidth that mirrors the bandwidth of the received DL frame. 
     In another embodiment where the STA does not need to be prompted by the AP to transmit UL frames that support implicit OFDMA sub-band sounding (such as the PS-polling sounding implementations described above), the AP does not implicitly or explicitly indicate the bandwidth (e.g., widest allowable bandwidth) to be used for transmitting a subsequent UL frame. That is, in some embodiments, S 1402 , as well as one or more of the other operations illustrated in  FIG. 14 , may not be implemented. 
     At S 1404 , the AP receives an UL frame suitable for use in performing implicit OFDMA sub-band sounding (such as, for examples, one of the frames illustrated in  FIGS. 3, 8, and 11 ) from a station. The received UL frame is a frame transmitted by the station at a maximum allowed bandwidth. In an embodiment, the received UL frame having a bandwidth that is greater than 20 MHz. For example, the received UL frame may have a bandwidth of any of 40 MHz, 80 MHz, 160 MHz, and 80+80 MHz. The UL frame may be any one of a PS-polling frame, an ACK frame, a BA frame, and the like. 
     While receiving the UL frame, the AP may perform one or more of a RS SI measurement, a SSINR measurement, an estimate of a best MCS, and an estimate of a best NSS for each sub-band of the maximum allowed bandwidth or for each combination of the sub-band and a transmitting antenna, a receiving antenna, or both. In an embodiment, measurements of various characteristics for each of a plurality of subcarriers of each sub-band may be combined to determine respective measurements for each sub band. 
     The measurements for each sub-band may be taken for each of a plurality of transmitting antennas when the UL frame is transmitted using the plurality of transmitting antennas. The measurements for each sub-band may be taken for each of a plurality of receiving antennas when the UL frame is received using the plurality of receiving antennas. The measurements for each sub-band may be taken for each combination of a transmitting antenna and a receiving antenna when the UL frame is transmitted using the plurality of transmitting antennas and received using the plurality of receiving antennas. 
     At S 1406 , the AP determines relative sub-band strengths for the station for each sub-band or for each sub-band/antenna combination using one or more of the measured RSSI, the measured SINR, the estimated best MCS, and the estimated best NSS. In an embodiment, the channel strength measures for each measured sub-band are determined for one or more of each transmitting antenna, each receiving antenna, and each combination of a transmitting antenna and a receiving antenna. 
     Determining the channel strength measures for each measured sub-band may include computing a metric using one or more of an RSSI, an SINR, an MCS, and an NSS. The metric may be normalized to provide relative channel information between the measured sub-bands. 
     In an embodiment, computing an RSSI, an SINR, or both of a sub-band may include computing an average of RSSIs or SINRs, respectively, of the subcarriers that belong to the sub-band. In an embodiment, computing an MCS of a sub-band may include computing a highest bit rate MCS that meets a Packet Error Rate (PER) requirement. In an embodiment, computing an NSS of a sub-band may include computing a highest NSS that meets the PER requirement. 
     At S 1408 , the AP determines allocations of one or more sub-bands to the station and transmission parameters for an OFDMA DL frame to be sent to the station according to the relative subband strengths for the station and the measured data. In an embodiment, the AP allocates a sub-band in the OFDMA DL frame to the station when a relative sub-band strength of the sub-band for the station is higher than a relative sub-band strength of the sub-band for one or more other stations. 
     At S 1410 , the AP transmits data to the STA using one or more sub-bands or RUs allocated to STA. In particular, the data may be transmitted in the one or more allocated sub-bands or RUs of a DL_OFDMA frame to be transmitted to the STA as well as to other STAs. 
     In above explanations and figures, illustrative embodiments were provided to allow a person of ordinary skill in the art to understand and implement embodiments of the disclosure. However, embodiments are not limited thereto, and are therefore not limited to the number of STAs, specific identifications, specific formats, specific number of STAs per identifications, or other specifics of the illustrative embodiments. Furthermore, while in the description and related figures the reference has made to one or more IEEE Std 802.11 standards, the design is not limited to 802.11, and a person of ordinary skill in the art in light of the teachings and disclosures herein would understand how the present disclosures apply to any next-generation wireless operation that operates in licensed or unlicensed bands. 
     Embodiments of the present disclosure include electronic devices configured to perform one or more of the operations described herein, which include operations for generating, decoding, or both, the frames described herein. The electronic devices may include wireless devices such as wireless device  200  shown in  FIG. 2 , including the AP  103  and stations  104  to  114  illustrated in  FIG. 1 . However, embodiments are not limited thereto. 
     Embodiments of the present disclosure may further include systems configured to operate using the processes and frames described herein. The systems may include basic service sets (BSSs) such as the BSS  100  of  FIG. 1 , but embodiments are not limited thereto. 
     Embodiments of the present disclosure may be implemented in the form of program instructions executable through various computer means and recorded in a non-transitory computer-readable medium. The non-transitory computer-readable medium may include one or more of program instructions, data files, data structures, and the like. The program instructions may be adapted to execute the processes and to generate and decode the frames described herein when executed on a device such as the wireless device  200  of  FIG. 2 . 
     In an embodiment, the non-transitory computer-readable medium may include a read only memory (ROM), a random access memory (RAM), or a flash memory. In an embodiment, the non-transitory computer-readable medium may include a magnetic, optical, or magneto-optical disc such as a hard disk drive, a floppy disc, a CD_ROM, and the like. 
     While this invention has been described in connection with what is presently considered to be practical embodiments, embodiments are not limited to the disclosed embodiments, but, on the contrary, may include various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Further, two or more embodiments may be combined.