Patent Publication Number: US-10313976-B2

Title: OFDMA sounding for WLAN system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Application No. 62/142,712, entitled “OFDMA SOUNDING FOR HE WLAN,” filed Apr. 3, 2015, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present description relates in general to wireless communication systems and methods, and more particularly to, for example, without limitation, orthogonal frequency division multiple access (OFDMA) sounding for wireless local area network (WLAN) system. 
     BACKGROUND 
     Wireless local area network (WLAN) devices are deployed in diverse environments. These environments are generally characterized by the existence of access points and non-access point stations. Increased interference from neighboring devices gives rise to performance degradation. Additionally, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices. 
     The description provided in the background section should not be assumed to be prior art merely because it is mentioned in or associated with the background section. The background section may include information that describes one or more aspects of the subject technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of an example of a wireless communication network. 
         FIG. 2  illustrates a schematic diagram of an example of a wireless communication device. 
         FIG. 3A  illustrates a schematic block diagram of an example of a transmitting signal processor in a wireless communication device. 
         FIG. 3B  illustrates a schematic block diagram of an example of a receiving signal processor in a wireless communication device. 
         FIG. 4  illustrates an example of a high efficiency (HE) frame. 
         FIGS. 5A and 5B  illustrate examples of sounding protocols with a beamformer and more than one beamformee. 
         FIG. 6  illustrates an example of a non-data packet announcement frame. 
         FIG. 7A  illustrates an example of a beamforming feedback frame. 
         FIG. 7B  illustrates an example of the MIMO Control field shown in  FIG. 7A . 
         FIG. 8  illustrates an example of a numerology for a 20 MHz channel bandwidth. 
         FIG. 9  illustrates an example of a numerology for a 40 MHz channel bandwidth. 
         FIG. 10  illustrates an example of a numerology for a 80 MHz channel bandwidth. 
         FIGS. 11, 12A, and 12B  illustrate examples of an order of priority for allocating the states in the case of a 20 MHz channel bandwidth. 
         FIG. 13  illustrates an example of an order of priority for allocating the states in the case of a 40 MHz channel bandwidth. 
         FIG. 14  illustrates an example of an order of priority for allocating the states in the case of an 80 MHz channel bandwidth. 
         FIGS. 15A, 15B, and 15C  illustrate flow charts of examples of methods for facilitating wireless communication for multi-user transmission. 
     
    
    
     In one or more implementations, not all of the depicted components in each figure may be required, and one or more implementations may include additional components not shown in a figure. Variations in the arrangement and type of the components may be made without departing from the scope of the subject disclosure. Additional components, different components, or fewer components may be utilized within the scope of the subject disclosure. 
     DETAILED DESCRIPTION 
     The detailed description set forth below is intended as a description of various implementations and is not intended to represent the only implementations in which the subject technology may be practiced. As those skilled in the art would realize, the described implementations may be modified in various different ways, all without departing from the scope of the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. 
     In one or more implementations, the subject technology may support a sounding protocol between a beamformer and one or more beamformees. In one or more implementations, the subject technology may support non-data packet (NDP) sounding. The sounding protocol may allow a beamformer to request and retrieve beamforming information from one or more beamformees and compute a beamforming matrix based on beamforming information fed back by the beamformees to the beamformer. The beamforming information may be referred to as beamforming report information or beamforming feedback information and may include signal-to-noise ratio (SNR) and/or beamforming feedback vector/matrix information. The beamformer may utilize the beamforming information to generate a beamforming matrix and generate beamformed data packets (using the beamforming matrix) to be transmitted to the beamformees. The beamformees may receive the beamformed data packets. In some aspects, beamformees may compress their respective feedback matrices to reduce overhead associated with the sounding protocol. Such feedback matrices may be referred to as compressed feedback matrices. 
     In some aspects, the beamformer may be referred to as an initiator of the sounding protocol, and each of the beamformees may be referred to as a responder of the sounding protocol. In some aspects, the beamformer may be an access point, and the receivers may be stations (e.g., non-AP stations). 
     In one or more aspects, uplink (UL) multi-user (MU) operation may allow the beamformer to solicit (e.g., using a trigger frame) immediate simultaneous response frames from the beamformees. In such aspects, the beamformees may transmit their response frames using UL MU transmission technology such as UL MU orthogonal frequency division multiple access (OFDMA) and/or UL MU multi-input multi-output (MIMO). 
     A trigger frame may be a frame sent by an AP that seeks data, control, or management frame response(s) from stations that participate in a subsequent UL MU frame. The trigger frame may be utilized to initiate the simultaneous MU transmission in OFDMA. In an aspect, a trigger frame may include, for example, some or all of the following features: (a) a list of stations (STAs) that an access point (AP) seeks a response from; (b) resource allocation information for each STA (e.g., a subband assigned to each STA); and/or (c) attributes of the expected UL MU frame, such as the duration, bandwidth, etc., among other features. In other words, the trigger frame may be used to allocate resource for UL MU transmission and to solicit an UL MU transmission from the participating stations in response to the trigger frame. The term “resource” may refer to, for example, a bandwidth, time/duration that the STAs expect to occupy a transmission medium, and/or possibly a number of spatial streams that the STAs may use. 
     The beamforming feedback vector/matrix computed by each beamformee may be referred to as a beamforming feedback matrix or a feedback matrix for simplicity. The feedback matrix may be represented as a V matrix. The beamforming vector/matrix computed by the beamformer may be referred to as a beamforming matrix for simplicity. The beamforming matrix may also be referred to as a steering matrix or pre-coding matrix and may be represented as a Q matrix. In one aspect, the beamforming matrix and feedback matrix may change from tone to tone. A tone may be referred to as subcarrier. Each tone may be associated with or otherwise identified by a tone index or a subcarrier index. A tone index may be referred to as a subcarrier index. 
     A sounding protocol may be referred to as a sounding procedure, sounding feedback sequence, sounding protocol sequence, channel sounding protocol, channel measurement protocol, channel calibration protocol, channel state information (CSI) sounding protocol, beamforming protocol, channel calibration protocol, or variants thereof (e.g., CSI feedback sequence). 
     In one or more implementations, a sounding protocol may be utilized in orthogonal frequency division multiple access (OFDMA) communication. In OFDMA, feedback information (e.g., average SNR values) in the unit of subband may be helpful. The unit of subband may be a portion of a channel bandwidth. In an aspect, the unit of subband may include, without limitation, 26 tones, 52 tones, 106 tones, 242 tones, and 484 tones. In an aspect, a respective average SNR value computed over each subband may be utilized for OFDMA allocation. For example, the beamformer may utilize the feedback information from the beamformees to determine portions of the channel bandwidth to be allocated to each of the beamformees. 
     In one or more implementations, the subject technology may provide subband-wise non-data packet announcement (NDPA or NDP-A) schemes and relevant feedback methods. In some aspects, modifications and/or additions to the very high throughput (VHT) sounding protocol utilized in the Institute of Electrical and Electronics Engineers (IEEE) 802.11ac standard may be implemented to facilitate NDP sounding in OFDMA communication. In this regard, in some aspects, modifications and/or additions may be made with respect to NDPA frames and/or feedback report frames utilized in IEEE 802.11ac. In some aspects, the subject technology facilitates informing of feedback granularity associated with beamforming information to relevant stations and sending of feedback according to the informed feedback granularity. 
       FIG. 1  illustrates a schematic diagram of an example of a wireless communication network  100 . In the wireless communication network  100 , such as a wireless local area network (WLAN), a basic service set (BSS) includes a plurality of wireless communication devices (e.g., WLAN devices). In one aspect, a BSS refers to a set of STAs that can communicate in synchronization, rather than a concept indicating a particular area. In the example, the wireless communication network  100  includes wireless communication devices  111 - 115 , which may be referred to as stations (STAs). 
     Each of the wireless communication devices  111 - 115  may include a media access control (MAC) layer and a physical (PHY) layer according to an IEEE 802.11 standard. In the example, at least one wireless communication device (e.g., device  111 ) is an access point (AP). An AP may be referred to as an AP STA, an AP device, or a central station. The other wireless communication devices (e.g., devices  112 - 115 ) may be non-AP STAs. Alternatively, all of the wireless communication devices  111 - 115  may be non-AP STAs in an Ad-hoc networking environment. 
     An AP STA and a non-AP STA may be collectively called STAs. However, for simplicity of description, in some aspects, only a non-AP STA may be referred to as a STA. An AP may be, for example, a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, a network adapter, a network interface card (NIC), a router, or the like. A non-AP STA (e.g., a client device operable by a user) may be, for example, a device with wireless communication capability, a terminal, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile terminal, a mobile subscriber unit a laptop, a non-mobile computing device (e.g., a desktop computer with wireless communication capability) or the like. In one or more aspects, a non-AP STA may act as an AP (e.g., a wireless hotspot). 
     In one aspect, an AP is a functional entity for providing access to a distribution system, by way of a wireless medium, for an associated STA. For example, an AP may provide access to the internet for one or more STAs that are wirelessly and communicatively connected to the AP. In  FIG. 1 , wireless communications between non-AP STAs are made by way of an AP. However, when a direct link is established between non-AP STAs, the STAs can communicate directly with each other (without using an AP). 
     In one or more implementations, OFDMA-based 802.11 technologies are utilized, and for the sake of brevity, a STA refers to a non-AP high efficiency (HE) STA, and an AP refers to an HE AP. In one or more aspects, a STA may act as an AP. 
       FIG. 2  illustrates a schematic diagram of an example of a wireless communication device. The wireless communication device  200  includes a baseband processor  210 , a radio frequency (RF) transceiver  220 , an antenna unit  230 , a memory  240 , an input interface unit  250 , an output interface unit  260 , and a bus  270 , or subsets and variations thereof. The wireless communication device  200  can be, or can be a part of, any of the wireless communication devices  111 - 115 . 
     In the example, the baseband processor  210  performs baseband signal processing, and includes a medium access control (MAC) processor  211  and a PHY processor  215 . The memory  240  may store software (such as MAC software) including at least some functions of the MAC layer. The memory may further store an operating system and applications. 
     In the illustration, the MAC processor  211  includes a MAC software processing unit  212  and a MAC hardware processing unit  213 . The MAC software processing unit  212  executes the MAC software to implement some functions of the MAC layer, and the MAC hardware processing unit  213  may implement remaining functions of the MAC layer as hardware (MAC hardware). However, the MAC processor  211  may vary in functionality depending on implementation. The PHY processor  215  includes a transmitting (TX) signal processing unit  280  and a receiving (RX) signal processing unit  290 . The term TX may refer to transmitting, transmit, transmitted, transmitter or the like. The term RX may refer to receiving, receive, received, receiver or the like. 
     The PHY processor  215  interfaces to the MAC processor  211  through, among others, transmit vector (TXVECTOR) and receive vector (RXVECTOR) parameters. In one or more aspects, the MAC processor  211  generates and provides TXVECTOR parameters to the PHY processor  215  to supply per-packet transmit parameters. In one or more aspects, the PHY processor  215  generates and provides RXVECTOR parameters to the MAC processor  211  to inform the MAC processor  211  of the received packet parameters. 
     In some aspects, the wireless communication device  200  includes a read-only memory (ROM) (not shown) or registers (not shown) that store instructions that are needed by one or more of the MAC processor  211 , the PHY processor  215  and/or other components of the wireless communication device  200 . 
     In one or more implementations, the wireless communication device  200  includes a permanent storage device (not shown) configured as a read-and-write memory device. The permanent storage device may be a non-volatile memory unit that stores instructions even when the wireless communication device  200  is off. The ROM, registers and the permanent storage device may be part of the baseband processor  210  or be a part of the memory  240 . Each of the ROM, the permanent storage device, and the memory  240  may be an example of a memory or a computer-readable medium. A memory may be one or more memories. 
     The memory  240  may be a read-and-write memory, a read-only memory, a volatile memory, a non-volatile memory, or a combination of some or all of the foregoing. The memory  240  may store instructions that one or more of the MAC processor  211 , the PHY processor  215 , and/or another component may need at runtime. 
     The RF transceiver  220  includes an RF transmitter  221  and an RF receiver  222 . The input interface unit  250  receives information from a user, and the output interface unit  260  outputs information to the user. The antenna unit  230  includes one or more antennas. When multi-input multi-output (MIMO) or multi-user MIMO (MU-MIMO) is used, the antenna unit  230  may include more than one antenna. 
     The bus  270  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal components of the wireless communication device  200 . In one or more implementations, the bus  270  communicatively connects the baseband processor  210  with the memory  240 . From the memory  240 , the baseband processor  210  may retrieve instructions to execute and data to process in order to execute the processes of the subject disclosure. The baseband processor  210  can be a single processor, multiple processors, or a multi-core processor in different implementations. The baseband processor  210 , the memory  240 , the input interface unit  250 , and the output interface unit  260  may communicate with each other via the bus  270 . 
     The bus  270  also connects to the input interface unit  250  and the output interface unit  260 . The input interface unit  250  enables a user to communicate information and select commands to the wireless communication device  200 . Input devices that may be used with the input interface unit  250  may include any acoustic, speech, visual, touch, tactile and/or sensory input device, e.g., a keyboard, a pointing device, a microphone, or a touchscreen. The output interface unit  260  may enable, for example, the display or output of videos, images, audio, and data generated by the wireless communication device  200 . Output devices that may be used with the output interface unit  260  may include any visual, auditory, tactile, and/or sensory output device, e.g., printers and display devices or any other device for outputting information. One or more implementations may include devices that function as both input and output devices, such as a touchscreen. 
     One or more implementations can be realized in part or in whole using a computer-readable medium. In one aspect, a computer-readable medium includes one or more media. In one or more aspects, a computer-readable medium is a tangible computer-readable medium, a computer-readable storage medium, a non-transitory computer-readable medium, a machine-readable medium, a memory, or some combination of the foregoing (e.g., a tangible computer-readable storage medium, or a non-transitory machine-readable storage medium). In one aspect, a computer is a machine. In one aspect, a computer-implemented method is a machine-implemented method. 
     A computer-readable medium may include storage integrated into a processor and/or storage external to a processor. A computer-readable medium may be a volatile, non-volatile, solid state, optical, magnetic, and/or other suitable storage device, e.g., RAM, ROM. PROM, EPROM, a flash, registers, a hard disk, a removable memory, or a remote storage device. 
     In one aspect, a computer-readable medium comprises instructions stored therein. In one aspect, a computer-readable medium is encoded with instructions. In one aspect, instructions are executable by one or more processors (e.g.,  210 ,  211 ,  212 ,  213 ,  215 ,  280 ,  290 ) to perform one or more operations or a method. Instructions may include, for example, programs, routines, subroutines, data, data structures, objects, sequences, commands, operations, modules, applications, and/or functions. Those skilled in the art would recognize how to implement the instructions. 
     A processor (e.g.,  210 ,  211 ,  212 ,  213 ,  215 ,  280 ,  290 ) may be coupled to one or more memories (e.g., one or more external memories such as the memory  240 , one or more memories internal to the processor, one or more registers internal or external to the processor, or one or more remote memories outside of the device  200 ), for example, via one or more wired and/or wireless connections. The coupling may be direct or indirect. In one aspect, a processor includes one or more processors. A processor, including a processing circuitry capable of executing instructions, may read, write, or access a computer-readable medium. A processor may be, for example, an application specific integrated circuit (ASIC), a digital signal processor (DSP), or a field programmable gate array (FPGA). 
     In one aspect, a processor (e.g.,  210 ,  211 ,  212 ,  213 ,  215 ,  280 ,  290 ) is configured to cause one or more operations of the subject disclosure to occur. In one aspect, a processor is configured to cause an apparatus (e.g., a wireless communication device  200 ) to perform operations or a method of the subject disclosure. In one or more implementations, a processor configuration involves having a processor coupled to one or more memories. A memory may be internal or external to the processor. Instructions may be in a form of software, hardware or a combination thereof. Software instructions (including data) may be stored in a memory. Hardware instructions may be part of the hardware circuitry components of a processor. When the instructions are executed or processed by one or more processors, (e.g.,  210 ,  211 ,  212 ,  213 ,  215 ,  280 ,  290 ), the one or more processors cause one or more operations of the subject disclosure to occur or cause an apparatus (e.g., a wireless communication device  200 ) to perform operations or a method of the subject disclosure. 
       FIG. 3A  illustrates a schematic block diagram of an example of a transmitting signal processing unit  280  in a wireless communication device. The transmitting signal processing unit  280  of the PHY processor  215  includes an encoder  281 , an interleaver  282 , a mapper  283 , an inverse Fourier transformer (IFT)  284 , and a guard interval (GI) inserter  285 . 
     The encoder  281  encodes input data. For example, the encoder  281  may be a forward error correction (FEC) encoder. The FEC encoder may include a binary convolutional code (BCC) encoder followed by a puncturing device, or may include a low-density parity-check (LDPC) encoder. The interleaver  282  interleaves the bits of each stream output from the encoder  281  to change the order of bits. In one aspect, interleaving may be applied only when BCC encoding is employed. The mapper  283  maps the sequence of bits output from the interleaver  282  into constellation points. 
     When MIMO or MU-MIMO is employed, the transmitting signal processing unit  280  may use multiple instances of the interleaver  282  and multiple instances of the mapper  283  corresponding to the number of spatial streams (N SS ). In the example, the transmitting signal processing unit  280  may further include a stream parser for dividing outputs of the BCC encoders or the LDPC encoder into blocks that are sent to different interleavers  282  or mappers  283 . The transmitting signal processing unit  280  may further include a space-time block code (STBC) encoder for spreading the constellation points from the number of spatial streams into a number of space-time streams (N STS ) and a spatial mapper for mapping the space-time streams to transmit chains. The spatial mapper may use direct mapping, spatial expansion, or beamforming depending on implementation. When MU-MIMO is employed, one or more of the blocks before reaching the spatial mapper may be provided for each user. 
     The IFT  284  converts a block of the constellation points output from the mapper  283  or the spatial mapper into a time domain block (e.g., a symbol) by using an inverse discrete Fourier transform (IDFT) or an inverse fast Fourier transform (IFFT). If the STBC encoder and the spatial mapper are employed, the IFT  284  may be provided for each transmit chain. 
     When MIMO or MU-MIMO is employed, the transmitting signal processing unit  280  may insert cyclic shift diversities (CSDs) to prevent unintentional beamforming. The CSD insertion may occur before or after the inverse Fourier transform operation. The CSD may be specified per transmit chain or may be specified per space-time stream. Alternatively, the CSD may be applied as a part of the spatial mapper. 
     The GI inserter  285  prepends a GI to the symbol. The transmitting signal processing unit  280  may optionally perform windowing to smooth edges of each symbol after inserting the GI. The RF transmitter  221  converts the symbols into an RF signal and transmits the RF signal via the antenna unit  230 . When MIMO or MU-MIMO is employed, the GI inserter  285  and the RF transmitter  221  may be provided for each transmit chain. 
       FIG. 3B  illustrates a schematic block diagram of an example of a receiving signal processing unit  290  in a wireless communication device. The receiving signal processing unit  290  of the PHY processor  215  includes a GI remover  291 , a Fourier transformer (FT)  292 , a demapper  293 , a deinterleaver  294 , and a decoder  295 . 
     The RF receiver  222  receives an RF signal via the antenna unit  230  and converts the RF signal into one or more symbols. In some aspects, the GI remover  291  removes the GI from the symbol. When MIMO or MU-MIMO is employed, the RF receiver  222  and the GI remover  291  may be provided for each receive chain. 
     The FT  292  converts the symbol (e.g., the time domain block) into a block of the constellation points by using a discrete Fourier transform (DFT) or a fast Fourier transform (FFT) depending on implementation. In one or more implementations, the FT  292  is provided for each receive chain. 
     When MIMO or MU-MIMO is employed, the receiving signal processing unit  290  may further include a spatial demapper for converting the Fourier transformed receiver chains to constellation points of the space-time streams, and a STBC decoder (not shown) for despreading the constellation points from the space-time streams into the spatial streams. 
     The demapper  293  demaps the constellation points output from the FT  292  or the STBC decoder to the bit streams. If the LDPC encoding is used, the demapper  293  may further perform LDPC tone demapping before the constellation demapping. The deinterleaver  294  deinterleaves the bits of each stream output from the demapper  293 . In one or more implementations, deinterleaving may be applied only when BCC decoding is used. 
     When MIMO or MU-MIMO is employed, the receiving signal processing unit  290  may use multiple instances on the demapper  293  and multiple instances of the deinterleaver  294  corresponding to the number of spatial streams. In the example, the receiving signal processing unit  290  may further include a stream deparser for combining the streams output from the deinterleavers  294 . 
     The decoder  295  decodes the streams output from the deinterleaver  294  and/or the stream deparser. For example, the decoder  295  may be an FEC decoder. The FEC decoder may include a BCC decoder or an LDPC decoder. 
       FIG. 4  illustrates an example of a high efficiency (HE) frame  400 . The HE frame  400  is a physical layer convergence procedure (PLCP) protocol data unit (or PPDU) format. An HE frame may be referred to as an OFDMA frame, a PPDU, a PPDU format, an OFDMA PPDU, an MU PPDU, another similar term, or vice versa. An HE frame may be simply referred to as a frame for convenience. In one or more implementations, an AP may transmit a frame for downlink (DL) using a frame format shown in this figure or a variation thereof (e.g., without any or some portions of an HE header). A STA may transmit a frame for uplink (UL) using a frame format shown in this figure or a variation thereof (e.g., without any or some portions of an HE header). 
     Referring to  FIG. 4 , the HE frame  400  contains a header and a data field. The header includes a legacy header comprised of a legacy short training field (L-STF), a legacy long training field (L-LTF), and a legacy signal (L-SIG) field. These legacy fields contain symbols based on an early design of an IEEE 802.11 specification. The L-STF, L-LTF, and L-SIG fields may be 8 μs, 8 μs, and 4 μs, respectively. Presence of these symbols would make any new design compatible with the legacy designs and products. The legacy header may be referred to as a legacy preamble. In one or more aspects, the term header may be referred to as a preamble. 
     In one or more implementations, the legacy STF, LTF, and SIG symbols are modulated/carried with FFT size of 64 on a 20 MHz sub-channel and are duplicated every 20 MHz if the frame has a channel bandwidth wider than 20 MHz (e.g., 40 MHz, 80 MHz, 160 MHz). Therefore, the legacy field (i.e., the STF, LTF, and SIG fields) occupies the entire channel bandwidth of the frame. The L-STF field may be utilized for packet detection, automatic gain control (AGC), and coarse frequency-offset (FO) correction. In one aspect, the L-STF field does not utilize frequency domain processing (e.g., FFT processing) but rather utilizes time domain processing. Thus, in one aspect, the L-STF field is not affected by the channel dispersion. The L-LTF field may be utilized for channel estimation, fine frequency-offset correction, and symbol timing. The L-SIG field includes one orthogonal frequency division multiplexing (OFDM) symbol. Thus, in one aspect, the term L-SIG field may be used interchangeably with L-SIG symbol. In one or more aspects, the L-SIG field may contain information indicative of a data rate and a length (e.g., in bytes) associated with the HE frame  400 , which may be utilized by a receiver of the HE frame  400  to calculate a time duration of a transmission of the HE frame  400 . 
     The header may also include an HE header comprised of an HE-SIG-A field and an HE-SIG-B field. The HE-SIG-A field may sometimes be referred to simply as a SIG-A field. These fields contain symbols that carry control information that may be vital regarding each PLCP service data unit (PSDU) and regarding the radio frequency (RF), PHY, and MAC properties of a PPDU. Several sub-fields may be located either in the HE-SIG-A and/or HE-SIG-B fields. In one aspect, the HE-SIG-A field can be carried/modulated using an FFT size of 64 on a 20 MHz basis. The HE-SIG-B field can be carried/modulated using an FFT size of e.g., 64 or 256 on a 20 MHz basis depending on implementation. The HE-SIG-A and HE-SIG-B fields may occupy the entire channel bandwidth of the frame. In some aspects, the size of the HE-SIG-A field and/or the HE-SIG-B field is variable. In other words, the number of symbols contained in the HE-SIG-A field and/or HE-SIG-B field can vary from frame to frame. An HE-SIG-B field is not always present in all frames. In some cases, single user (SU) packets and UL trigger-based packets do not contain the HE-SIG-B field. To facilitate decoding of the HE frame  400  by a receiver, the size of (e.g., number of symbols contained in) the HE-SIG-B field may be indicated in the HE-SIG-A field. In some aspects, the HE header also includes a repeated L-SIG (RL-SIG) field, whose content is the same as the L-SIG field. 
     For a 20 MHz channel, an FFT size of 64 is associated with a discrete Fourier transform (DFT) period of 3.2 μs and a subcarrier spacing of 312.5 kHz. For a 20 MHz channel, an FFT size of 256 is associated with a DFT period of 12.8 μs and a subcarrier spacing of 78.125 kHz. The DFT period may also be referred to as an inverse DFT period (IDFT) or an IDFT/DFT period. The DFT period may be denoted as T DFT . The subcarrier spacing may be referred to as a subcarrier frequency spacing and may be denoted as Δ F . The subcarrier spacing may be obtained by dividing the channel bandwidth by the FFT size. The subcarrier spacing is the reciprocal of the DFT period. 
     The HE header may further include HE-STF and HE-LTF fields, which contain symbols used to perform necessary RF and PHY processing for each PSDU and/or for the whole PPDU. The HE-LTF symbols may be modulated/carried with an FFT size of 256 for 20 MHz bandwidth and modulated over the entire bandwidth of the frame. Thus, the HE-LTF field may occupy the entire channel bandwidth of the frame. In one aspect, an HE-LTF sequence may be utilized by a receiver to estimate MIMO channel between the transmitter and the receiver. Channel estimation may be utilized to decode data transmitted and compensate for channel properties (e.g., effects, distortions). For example, when a preamble is transmitted through a wireless channel, various distortions may occur, and a training sequence in the HE-LTF field is useful to reverse the distortion. This may be referred to as equalization. To accomplish this, the amount of channel distortion is measured. This may be referred to as channel estimation. In one aspect, channel estimation is performed using an HE-LTF sequence, and the channel estimation may be applied to other fields that follow the HE-LTF sequence. 
     The HE-STF symbols may have a fixed pattern and a fixed duration. For example, the HE-STF symbols may have a predetermined repeating pattern. In one aspect, the HE-STF symbols do not require FFT processing. The HE frame  400  may include the data field, represented as HE-DATA, that contains data symbols. The data field may also be referred to as a payload field, data, payload or PSDU. 
     In one or more aspects, additional one or more HE-LTF fields may be included in the header. For example, an additional HE-LTF field may be located after a first HE-LTF field. The HE-LTF fields may be, for example, modulated/carried with FFT size of 64 on a 20 MHz channel and may be included as part of the first part of the HE frame  400 . In one or more implementations, a TX signal processing unit  280  (or an IFT  284 ) illustrated in  FIG. 3A  may carry out the modulation described in this paragraph as well as the modulations described in other paragraphs above. In one or more implementations, an RX signal processing unit  290  (or an FT  292 ) may perform demodulation for a receiver. 
     In one or more implementations, the subject technology supports sounding protocols that include non-data packet (NDP) sounding and and/or explicit feedback from beamformees to a beamformer that initiates the sounding protocol. The NDP sounding may involve the exchanging between the beamformees and beamformer of non-data packet announcement (NDPA) frame(s), NDP frame(s), beamforming report poll frame(s), and beamforming report frame(s). 
       FIG. 5A  illustrates an example of a sounding protocol with a beamformer and more than one beamformee. In an aspect, the sounding protocol may be referred to as an NDP sounding protocol. Although  FIG. 5A  illustrates an example of a sounding protocol with multiple beamformees, the sounding protocol may involve one beamformer and one beamformee in some cases. 
     The beamformer may initiate the sounding protocol by sending an NDPA frame  510  followed by an NDP frame  512 . The NDPA frame  510  may be utilized by the beamformer to identify the beamformees being included by the beamformer in the sounding protocol and indicate to these beamformees that the beamformer requests (e.g., expects) them to prepare (e.g., measure, generate) beamforming information to be fed back to the beamformer. 
     The non-data packet frame may be referred to as a null data packet frame. The non-data packet announcement frame may be referred to as a null data packet announcement frame. A sounding protocol that utilizes NDPA frames and NDP frames may be referred to as an NDP sounding protocol or NDP-based sounding protocol. 
     The NDPA frame  510  may include one or more fields to identify the beamformees. In some aspects, the NDPA frame  510  may include one Station Information (STA Info) field for each beamformee. The NDPA frame  510  may include a STA Info 1 field, STA Info 2 field, and STA Info 3 field that are associated with beamformees 1, 2, and 3, respectively. Each STA Info field may include an Association Identifier (AID) field that identifies a respective beamformee. An example of an NDPA frame will be described further below with respect to  FIG. 6 . 
     The NDPA frame  510  is generally immediately followed by the NDP frame  512 . Upon receipt of the NDP frame  512 , each of the beamformees identified in the NDPA frame  510  may generate beamforming information (e.g., average SNR value(s), feedback matrix/matrices) based on the NDP frame  512 . In some aspects, the NDP frame  512  may be the HE frame  400 , except without the HE-DATA field or with an empty HE-DATA field. For example, the NDP frame  512  may include only the header (e.g., the legacy and HE headers) of the HE frame  400 . In some aspects, the beamformees may compute feedback matrices and/or SNR values based on the NDP frame  512 . The beamforming information may be based on analysis of, for example, the training fields (e.g., L-STF, L-LTF, HE-STF, HE-LTF) contained in the NDP frame  512 . For example, the beamformees may perform measurements (e.g., power measurements) on the NDP frame  512  at various tones. 
     In response to the NDPA frame  510 , beamformee 1 may transmit a beamforming feedback frame  514 . The beamforming feedback frame  514  includes beamforming information generated by beamformee 1. To retrieve the beamforming information from the remaining beamformees, the beamformer may transmit a beamforming report poll frame  516 , whose intended recipient may be designated in a Receiver Address (RA) field of the beamforming report poll frame  516 . The intended recipient of the beamforming report poll frame  516  is the beamformee whose beamforming information is being requested by (e.g., retrieved by) the beamformer. When the beamforming report poll frame  516  identifies beamformee 2 in its RA field, beamformee 2 may transmit a beamforming feedback frame  518  to the beamformer in response to the beamforming report poll frame  516 . The beamformer may transmit a beamforming report poll frame  520  whose RA field is designated as beamformee 3. Beamformee 3 may transmit a beamforming feedback frame  522  to the beamformer in response to the beamforming report poll frame  520 . The beamforming feedback frames  518  and  522  contain the beamforming information generated by beamformees 2 and 3, respectively. An example of a beamforming feedback frame will be described further below with respect to  FIGS. 7A and 7B . 
     In some aspects, beamformees may compress their respective beamforming information (e.g., feedback matrices) to reduce overhead associated with the sounding protocol. A beamforming feedback frame that contains compressed feedback matrices may be referred to as a compressed beamforming feedback frame. The compressed feedback matrices may be referred to as compressed V matrices and their elements may be referred to as compressed-V beamforming weights. In an aspect, the compression and/or format of the beamforming information may be indicated by the beamformer in the NDPA frame. The disclosure may refer to compressed versions of the beamforming information, feedback matrices, and the beamforming feedback frames for simplicity, although non-compressed versions of the beamforming information, feedback matrices, and beamforming feedback frames may be utilized. 
     In an aspect, as shown in  FIG. 5A , the beamformer may retrieve beamforming information in the order associated with the index of the STA Info field. For example, the beamformee associated with STA Info 1 field may transmit a beamforming report frame to the beamformer upon receipt of the NDP frame  512 , while the remaining beamformees (e.g., beamformee 2 and beamformee 3) need to be polled prior to the remaining beamformees transmitting their respective beamforming report frames to the beamformer. The remaining beamformees may be polled such that the beamformee associated with the STA Info 2 field is polled and then the beamformee associated with the STA Info 3 field is polled. Other manners by which the beamformer determines an order in which beamforming information is retrieved from the beamformees may be utilized. In another aspect, the beamformer may retrieve beamforming information simultaneously from one or more beamformees associated with STA Info fields (e.g. STA Info 1. STA Info 2 and STA Info 3) using UL multi-user transmission technology such as UL MU OFDMA and UL MU-MIMO. An example of such an aspect will be described further below with respect to  FIG. 5B . 
     Upon retrieving the beamforming information from the beamformees, the beamformer may generate a beamforming matrix to be utilized for generating beamformed data packets for beamformees 1, 2, and 3. In one aspect, the time period between any two adjacent frames  510 ,  512 ,  514 ,  516 ,  518 ,  520 , and  522  may be a short interface space (SIFS). 
       FIG. 5B  illustrates another example of a sounding protocol with a beamformer and more than one beamformee. The description from  FIG. 5A  generally applies to  FIG. 5B , with examples of differences between  FIG. 5A  and  FIG. 5B  and other description provided herein for purposes of clarity and simplicity. 
     The sounding protocol (e.g., HE sounding protocol) may be initiated by the beamformer sending the NDPA frame  510  followed by the NDP frame  512 . In an aspect the NDP frame  512  may be transmitted a SIFS after the end of the NDPA frame  510  (e.g., after the end of the PPDU carrying the NDPA frame  510 ). Upon receipt of the NDP frame  512 , each of the beamformees identified in the NDPA frame  510  may generate beamforming information based on the NDP frame  512 . 
     The beamformer may use (e.g., send) a trigger frame  530  to solicit (e.g., retrieve) beamforming feedback from the beamformees. In an aspect, the trigger frame  530  may be utilized by the beamformer to retrieve beamforming information simultaneously from the beamformees 1, 2, and 3. The trigger frame  530  may indicate resource allocation information for the beamformees. The resource allocation information may include a subband (or a frequency subchannel) assigned to each respective one of the beamformees. In some aspects, the resource allocation information may also include scheduling information regarding when a respective one of the beamformees may transmit using its assigned subband, and/or may include the number of spatial streams that the beamformees may use. 
     In response to the trigger frame  530 , beamformees 1, 2, and 3 may transmit (e.g., simultaneously transmit) a beamforming feedback frame  532 ,  534 , and  536 , respectively. The beamforming feedback frame  532 ,  534 , and  536  may contain the beamforming information generated by beamformees 1, 2, and 3, respectively. Upon retrieving the beamforming information from the beamformees, the beamformer may generate a beamforming matrix to be utilized for generating beamformed data packets for beamformees 1, 2, and 3. 
     In one or more implementations, the frames  510 ,  512 ,  514 ,  516 ,  518 ,  520 ,  522 ,  530 ,  532 ,  534 , and  536  illustrated in  FIGS. 5A and 5B  may represent PPDUs. In some aspects, the frames  514 ,  516 ,  518 ,  520 ,  522 ,  530 ,  532 ,  534 , and  536  are Media Access Control (MAC) Protocol Data Units (MPDUs) (e.g., MAC frames). The MPDUs may be a payload(s) of a PPDU. The PPDU may have the format of the HE frame  400  shown in  FIG. 4 . 
       FIG. 6  illustrates an example of an NDPA frame  600 . In some aspects, the NDPA frame  510  may be, may include, or may be a part of, the NDPA frame  600 . In some aspects, the NDPA frame  600  may be a MAC frame that forms at least a part of the payload of the HE frame  400 . The NDPA frame  600  may include a Frame Control field, Duration field, Receiver Address (RA) field, Transmitter Address (TA) field. Station Information (STA Info) 1 field, STA Info n field, and Frame Check Sequence (FCS) field. It is noted that the ellipses between the STA Info 1 field and STA Info n field indicate that one or more additional STA Info fields or no STA Info fields are present between the STA Info 1 field and STA Info n field. Each STA Info field is associated with one station. Although the NDPA frame  600  includes at least a STA Info 1 field and a STA Info n field, an NDPA frame may include a single STA Info field. 
     In some aspects, the TA field may be set to the address of the transmitter of the NDPA frame  600 . The transmitter may be the beamformer that initiates a sounding protocol. In some aspects, when the NDPA frame  600  includes more than one STA Info field, the RA field of the NDPA frame  600  may be set to a broadcast address. In some aspects, the broadcast address may be a MAC sublayer address. The broadcast address may be a distinguished, predefined group (e.g., multidestination) address that is utilized to denote a set of all stations on a given network (e.g., LAN). As an example, with reference to  FIG. 6 , the TA field may include the address of the beamformer, and the RA field may include a broadcast address associated with beamformees 1, 2, and 3. In some aspects (not shown), if an NDPA frame includes a single STA Info field, the RA field of the NDPA frame may be set to an address (e.g., MAC address) of the single beamformee associated with the single STA Info field. 
     With reference to  FIG. 6  and Table 1 below, each STA Info field may include an Association Identifier (AID) field, a Feedback Type field, and an Nc Index field. The AID field in each STA Info field may contain an AID value that identifies a beamformee. An AID field of the STA Info 1, STA Info 2, and STA Info 3 fields may be set to an AID value associated with beamformee 1, 2, and 3, respectively. In some aspects, the AID field may be referred to as the AID12 field, such as in the case that the AID field includes 12 bits (e.g., 12 least significant bits) of the AID value. 
     The Feedback Type field includes a value indicative of a type of feedback (e.g., SU-feedback, MU feedback) requested by the beamformer. In the MU feedback case, the Nc Index field may be used to indicate the number of columns in the compressed beamforming feedback matrix to be provided by the beamformee to the beamformer. In some aspects, in the SU feedback case, the Nc Index field is not used. The beamformees may generate beamforming information in accordance with the Feedback Type field and/or the Nc Index field of the NDPA frame  600 . 
     Table 1 below provides an example of fields that may be contained in each of the STA Info fields of the NDPA frame  600  (e.g., STA Info 1 field, STA Info n field, etc.). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example of fields in a STA Info field 
               
            
           
           
               
               
            
               
                 Field 
                 Description 
               
               
                   
               
               
                 AID12 
                 Contains the 12 least significant bits of the AID of a 
               
               
                   
                 STA expected to process the following NDP and 
               
               
                   
                 prepare the sounding feedback. Equal to 0 if the STA 
               
               
                   
                 is an AP, mesh STA or STA that is a member of an 
               
               
                   
                 independent basic service set (IBSS). 
               
               
                 Feedback Type 
                 Indicates the type of feedback requested. 
               
               
                   
                  Set to 0 for SU. 
               
               
                   
                  Set to 1 for MU. 
               
               
                 Nc Index 
                 If the Feedback Type field indicates MU, then Nc 
               
               
                   
                 Index indicates the number of columns, Nc, in the 
               
               
                   
                 Compressed Beamforming Feedback Matrix subfield 
               
               
                   
                 minus 1: 
               
               
                   
                  Set to 0 to request Nc = 1 
               
               
                   
                  Set to 1 to request Nc = 2 
               
               
                   
                  . . . 
               
               
                   
                  Set to 7 to request Nc = 8 
               
               
                   
                 Reserved if the Feedback Type field indicates SU. 
               
               
                   
               
            
           
         
       
     
       FIG. 7A  illustrates an example of a beamforming feedback frame  700 . The beamforming feedback frame may also be referred to as a beamforming feedback report frame, beamforming report frame, or report frame. The beamforming feedback frame  700  may be a MAC frame. In an aspect, the beamforming feedback frame  700  may be a payload of the HE frame  400 . The beamforming feedback frame  700  may be a compressed beamforming feedback frame (e.g., when beamforming information contained in the beamforming feedback frame  700  is compressed). The beamforming feedback frame  700  may be any one of beamforming feedback frames  514 ,  518 , and  522 . 
     The beamforming feedback frame  700  includes a Category field. Action field, MIMO Control field, Beamforming Report field, and MU Exclusive Beamforming Report field. In an aspect, the MIMO Control field may be considered a header or portion thereof of the beamforming feedback frame  700  whereas the Beamforming Report field and the MU Exclusive Beamforming Report field may be considered a payload or portion thereof of the beamforming feedback frame. The MIMO Control field may contain information indicative of the format of the Beamforming Report field and the MU Exclusive Beamforming Report field. 
     In a non-MU case (e.g., SU case), the beamforming feedback frame does not include the MU Exclusive Beamforming Report field. In some aspects, the Beamforming Report field may be referred to as a Compressed Beamforming Report field and used to include compressed beamforming information. The Beamforming Report field may contain SU feedback information or MU feedback information depending on the Feedback Type field of the NDPA frame (e.g.,  600  in  FIG. 6 ) from the beamformer. A portion of the Beamforming Report field may include average SNR values and another portion of the Beamforming Report field may include beamforming feedback matrices. The beamformer can calculate a beamforming matrix Q based on the SU and/or MU feedback information. 
       FIG. 7B  illustrates an example of the MIMO Control field shown in  FIG. 7A . The MIMO Control field includes an Nc Index field, Nr Index field, Channel Width field, Grouping field, Codebook Information field, Feedback Type field, Remaining Feedback Segments field, First Feedback Segment field, and Sounding Dialog Token Number field. These fields may be referred to as subfields of the MIMO Control field. The Nc Index field and the Nr Index field may indicate a number of columns and rows, respectively, in the feedback matrices (e.g., compressed feedback matrices). The Grouping field may indicate a subcarrier grouping Ng, where one compressed feedback matrix is provided every Ng subcarriers. For example, if Ng is 4, a compressed feedback matrix may be provided every fourth subcarrier. The Feedback Type field may indicate the type of feedback (e.g., SU or MU). The Feedback Type field of the MIMO Control field may have the same value as the Feedback Type field of the NDPA frame from the beamformer. 
     In an aspect, if the feedback type is SU, the Beamforming Report field may contain the average SNR values over all reported data subcarriers of space-time (ST) streams from 1 to Nc. An example of beamforming report information included in the Beamforming Report field is illustrated in Table 2. In this regard, Table 2 provides an example of an order in which information is provided in the Beamforming Report field. For example, in the Beamforming Report field, an “Average SNR of Space-Time Stream 1” field may be followed by an “Average SNR of Space-Time Stream 2” field. In an aspect, Table 2 illustrates VHT compressed beamforming report information. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Example of Compressed Beamforming Report information 
               
            
           
           
               
               
               
            
               
                 Field 
                 Size (bits) 
                 Meaning 
               
               
                   
               
               
                 Average SNR of  
                 8 
                 Signal-to-noise ratio at the 
               
               
                 Space-Time Stream 1 
                   
                 beamformee for space-time 
               
               
                   
                   
                 stream 1 averaged over all data 
               
               
                   
                   
                 subcarriers. See Table 8-53h 
               
               
                   
                   
                 (Average SNR of Space-Time 
               
               
                   
                   
                 Stream i subfield). 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Average SNR of  
                 8 
                 Signal-to-noise ratio at the 
               
               
                 Space-Time Stream Nc 
                   
                 beamformee for space-time 
               
               
                   
                   
                 stream Nc averaged over  
               
               
                   
                   
                 all data subcarriers. 
               
               
                   
                   
                 See Table 8-53h 
               
               
                   
                   
                 (Average SNR of Space-Time 
               
               
                   
                   
                 Stream i subfield). 
               
               
                 Compressed Beamforming  
                 N α ×  
                 Compressed beamforming 
               
               
                 Feedback Matrix V for  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 subcarrier k = scidx(0) 
                   
                 Table 8-53d (Order of angles  
               
               
                   
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 Compressed Beamforming  
                 N α ×  
                 Compressed beamforming 
               
               
                 Feedback Matrix V for  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 subcarrier k = scidx(1) 
                   
                 Table 8-53d (Order of angles  
               
               
                   
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 Compressed Beamforming  
                 N α ×  
                 Compressed beamforming 
               
               
                 Feedback Matrix V for  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined  
               
               
                 subcarrier k = scidx(2) 
                   
                 in Table 8-53d (Order of  
               
               
                   
                   
                 angles in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Compressed Beamforming  
                 N α ×  
                 Compressed beamfoming 
               
               
                 Feedback Matrix V for  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 subcarrier k = scidx  
                   
                 Table 8-53d (Order of angles  
               
               
                 (Ns − 1) 
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                   
               
               
                 NOTE 
               
               
                 scidx( ) is defined in Table 8-53g (Subcarriers for which a Compressed Beamfoming Feedback Matrix subfield is sent back) 
               
            
           
         
       
     
     In an aspect, if the feedback type is MU, the beamforming feedback frame  700  may include additional beamforming report information on top of the information provided for SU feedback (e.g., in the Beamforming Report field). The additional beamforming report information may include delta SNR (ΔSNR) for space-time stream from 1 to Nc for each reported subcarrier. Table 3 illustrates an example of MU Exclusive Beamforming Report information. In this regard, Table 3 provides an example of an order in which information is provided in the MU Exclusive Beamforming Report field. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Example of MU Exclusive Beamforming Report information 
               
            
           
           
               
               
               
            
               
                   
                 Size 
                   
               
               
                 Field 
                 (Bits) 
                 Meaning 
               
               
                   
               
               
                 Delta SNR for space-time stream 1 for 
                 4 
                 ΔSNR sscidx(0), 1   
               
               
                 subcarrier k = sscidx(0) 
                   
                   
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Delta SNR for space-time stream Nc for 
                 4 
                 ΔSNR sscidx(0), Nc   
               
               
                 subcarrier k = sscidx(0) 
                   
                   
               
               
                 Delta SNR for space-time stream 1 for 
                 4 
                 ΔSNR sscidx(1), 1   
               
               
                 subcarrier k = sscidx(1) 
                   
                   
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Delta SNR for space-time stream Nc for 
                 4 
                 ΔSNR sscidx(1), Nc   
               
               
                 subcarrier k = sscidx(1) 
                   
                   
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Delta SNR for space-time stream 1 for 
                 4 
                 ΔSNR sscidx(Ns’−1), 1   
               
               
                 subcarrier k = sscidx(Ns’ − 1) 
                   
                   
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Delta SNR for space-time stream Nc for 
                 4 
                 ΔSNR sscidx(Ns’−1), Nc   
               
               
                 subcarrier k = sscidx(Ns’ − 1) 
               
               
                   
               
               
                 NOTE 
               
               
                 sscidx( ) is defined in Table 8-53j (Number of subcarriers and subcarrier mapping). 
               
            
           
         
       
     
     In an aspect, an example of the various variables in Table 3 is provided as follows: 
               Δ   ⁢           ⁢     SNR     k   ,   i         =     min   (       max   (       round   (       10   ⁢       log   10     (                H   k     ⁢     V     k   ,   i              2     N     )       -       SNR   _     i       )     ,     -   8       )     ,   7     )           
where
         k is the subcarrier index in the range of sscidx(0), . . . , sscidx(Ns′−1)   i is the space-time stream index in the range of 1, . . . . Nc;   H k  is the estimated MIMO channel for subcarrier k;   V k,i  is column i of the beamforming matrix V for subcarrier k;   N is the average noise plus interference power, measured at the beamformee, that was used to calculate  SNR   i ; and     SNR   i  is the average SNR of space-time stream i reported in the Compressed Beamforming Report information (Average SNR in Space-Time Stream i field).       

     It is noted that the ellipses in Tables 2 and 3 may indicate that one or more additional fields or no additional fields are present between fields adjacent to the ellipses. 
     In one or more implementations, the sounding protocol may be utilized in OFDMA communication. In OFDMA, information associated with overall channel condition from each station&#39;s perspective (e.g., each station&#39;s beamforming condition) may facilitate beamforming (e.g., computation of the beamforming matrix Q by the beamformer). In some aspects, feedback for one or more subbands smaller than or equal to the operating channel bandwidth (e.g., 20 MHz, 40 MHz, 80 MHz, 160 MHz) may be utilized for beamforming in OFDMA operations. In an aspect, finer granularity for the beamforming information may incur more beamforming feedback overhead than coarser granularity. 
     In OFDMA, in one aspect, an access point may allocate different portions of a channel bandwidth to different stations. In one aspect, a portion of a channel bandwidth is allocated to a station. In one aspect, a portion of a channel bandwidth may be a resource unit (RU). In another aspect, a portion of a channel bandwidth may be one or more resource units. In yet another aspect, a portion of a channel bandwidth may be one or more blocks of a channel bandwidth. In an aspect, the access point may be the beamformer of  FIGS. 5A and 5B  and the stations that are allocated different portions of the channel bandwidth may be the beamformees of  FIGS. 5A and 5B . In an aspect, the access point may utilize the sounding protocol to determine portion(s) of the channel bandwidth to be allocated to each of the beamformees. 
     In one or more implementations, the NDPA frame (e.g.,  510  in  FIGS. 5A and 5B ) may include subband-related information. In an aspect, the subband-related information may identify subbands of a bandwidth over which beamforming information is being requested by the beamformer from the beamformees. In an aspect, such a bandwidth may be referred to as a bandwidth to be reported by a beamforming report. In another aspect, such a bandwidth may be referred to as a bandwidth to be reported by the Beamforming Report field of a beamforming feedback frame. In another aspect, such a bandwidth may be referred to as a bandwidth to be reported by a beamforming feedback frame. In another aspect, such a bandwidth may be referred to as a bandwidth to be reported by a frame comprising a beamforming report. In another aspect, such a bandwidth may be referred to as a bandwidth to be reported by a station (e.g., beamformee 1, beamformee 2, and/or beamformee 3 in  FIG. 5A or 5B ) responding to an AP (e.g., a beamformer in  FIG. 5A or 5B ). 
     In an aspect, the subbands identified for a beamformee may encompass (i) less than an entirety of the bandwidth to be reported or (ii) less than the channel bandwidth (e.g., a channel bandwidth of a frame  510 ,  512 ,  514 ,  518 ,  522 ,  530 ,  532 ,  534 , or  536  of  FIG. 5A or 5B ). In an aspect, a bandwidth to be reported by a beamforming report is smaller than a channel bandwidth of a frame (e.g., a channel bandwidth of a frame  510 ,  512 ,  514 ,  518 ,  522 ,  530 ,  532 ,  534 , or  536  of  FIG. 5A or 5B ). In an aspect, each subband is smaller than a bandwidth to be reported. In an aspect, each subband is smaller than a channel bandwidth. 
     In an aspect, a minimum size of each subband is 26 tones when a beamforming feedback frame (e.g., a frame  514 ,  518 ,  522 ,  532 ,  534 , or  536  in  FIG. 5A or 5B ) is for OFDMA transmission. In an aspect, the 26 tones correspond to 26 tones in a smallest resource unit of a numerology of OFDMA transmission associated with a channel bandwidth of a frame (e.g., a channel bandwidth of a frame  510 ,  512 ,  514 ,  518 ,  522 ,  530 ,  532 ,  534 , or  536  of  FIG. 5A or 5B ). In an aspect, a beamforming report utilizes one or more subbands designated by at least one of the AP&#39;s frames (e.g., a frame  510 ,  512  and/or  530  of  FIG. 5A or 5B ), one or more subbands are one or more resource units, and each of the one or more resource units has 26 tones. 
     Based on the subband-related information included in the NDPA frame, each beamformee may calculate an average SNR value for each subband identified in the NDPA frame and feed back the calculated average SNR value to the beamformer (e.g., in the Beamforming Report field of a beamforming feedback frame). The Beamforming Report (e.g., Compressed Beamforming Report) information may include the average SNR value for the subbands. In an aspect, VHT compressed beamforming report information may be modified to include such average SNR values for the subbands. In an aspect, the Compressed Beamforming Report information provided in Table 2 may be modified to include such average SNR values for the subbands (e.g., rather than one average SNR value averaged over all data subcarriers for each space-time stream). 
     In some aspects, the NDPA frame may include the same subband information for all the stations (e.g., beamformees) identified in the sounding protocol. In some aspects, the NDPA frame may include different subband information for different stations (e.g., different beamformees). In these aspects, the beamformer may request that one station provide average SNR values for a first set of subbands and may request that another station provide average SNR values for a second set of subbands, where at least one subband in the first set is not contained in the second set of subbands. 
     In one or more implementations, the subband information associated with each beamformee may be contained in the Nc Index field of the STA Info field associated with the beamformee. With reference to  FIG. 6  and Table 1, the Nc index may be interpreted differently depending on whether SU feedback or MU feedback is utilized. In Table 1, if the feedback type is SU, then the Nc Index field is not used in OFDM transmission. In some aspects. MU-MIMO is supported only in a downstream direction. In some aspects, OFDMA and MU may not be supported simultaneously. 
     In one or more implementations, if the feedback type is SU, the Nc Index field, previously unused, may be utilized to include subband-related information for the sounding protocol. The subband-related information may be utilized by the beamformer to request beamforming information for individual subband(s) identified in the Nc Index field. In an aspect, a minimum size of each subband is 26 tones. In an aspect, the beamformer may request beamforming information for individual subbands each of which includes 26 tones. 
     Table 4 provides an example of Compressed Beamforming Report information in which beamforming information is generated in accordance with subband-related information. A Compressed Beamforming Report information may be referred to as a beamforming report. The Compressed Beamforming Report information may be utilized in HE-based communication, and thus may be referred to as HE Compressed Beamforming Report information or HE Compressed OFDMA Beamforming Report. In Table 4, the Compressed Beamforming Report information includes an average SNR for each of Nsb subbands in each of Nc space-time streams to be fed back to the beamformer. The Nsb is the total number of subbands for this sounding protocol. In an aspect, Nsb may be determined by subband information in the Nc Index field in the NDPA frame. The subbands may be indicated by the beamformer to the beamformees in the NPDA frame. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Example of HE Compressed OFDMA  
               
               
                 Beamforming Report information 
               
            
           
           
               
               
               
            
               
                 Field 
                 Size (bits) 
                 Meaning 
               
               
                   
               
               
                 Average SNR of  
                 8 
                 Signal-to-noise ratio at the 
               
               
                 Space-Time Stream  
                   
                 beamformee for space-time 
               
               
                 1 in subband 1 
                   
                 stream 1 averaged over all data 
               
               
                   
                   
                 subcarriers in subband 1. See 
               
               
                   
                   
                 Table 8-53h. 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Average SNR of  
                 8 
                 Signal-to-noise ratio at 
               
               
                 Space-Time Stream  
                   
                 beamformee for space-time 
               
               
                 1 in subband Nsb 
                   
                 stream 1 averaged over all data 
               
               
                   
                   
                 subcarriers in subband Nsb.  
               
               
                   
                   
                 See Table 8-53h. 
               
               
                 Average SNR of  
                   
                 Signal-to-noise ratio at the 
               
               
                 Space-Time Stream  
                   
                 beamformee for space-time 
               
               
                 2 in subband 1 
                   
                 stream 2 averaged over all data 
               
               
                   
                   
                 subcarriers in subband 1. See 
               
               
                   
                   
                 Table 8-53h. 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Average SNR of  
                   
                 Signal-to-noise ratio at the 
               
               
                 Space-Time Stream  
                   
                 beamformee for space-time 
               
               
                 Nc in subband Nsb 
                   
                 stream Nc averaged over all  
               
               
                   
                   
                 data subcarriers in subband  
               
               
                   
                   
                 Nsb. See Table 8-53h. 
               
               
                 Compressed  
                 N α ×  
                 Compressed beamforming 
               
               
                 Beamforming Feedback  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 Matrix V for subcarrier  
                   
                 Table 8-53d (Order of angles  
               
               
                 k = scidx(0) 
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 Compressed  
                 N α ×  
                 Compressed beamforming 
               
               
                 Beamforming Feedback  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 Matrix V for subcarrier  
                   
                 Table 8-53d (Order of angles  
               
               
                 k = scidx(1) 
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 Compressed  
                 N α ×  
                 Compressed beamforming 
               
               
                 Beamforming Feedback  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined in 
               
               
                 Matrix V for subcarrier  
                   
                 Table 8-53d (Order of angles  
               
               
                 k = scidx(2) 
                   
                 in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 Compressed  
                 N α ×  
                 Compressed beamforming 
               
               
                 Beamforming Feedback  
                 (b ψ  + b ϕ )/2 
                 feedback matrix as defined  
               
               
                 Matrix V for subcarrier  
                   
                 in Table 8-53d (Order of  
               
               
                 k = scidx(Ns − 1) 
                   
                 angles in the Compressed  
               
               
                   
                   
                 Beamforming Feedback Matrix  
               
               
                   
                   
                 subfield) and Table 8-53e  
               
               
                   
                   
                 (Quantization of angles). 
               
               
                   
               
               
                 NOTE 
               
               
                 scidx( ) is defined in Table 8-53g (Subcarriers for which a Compressed Beamforming Feedback Matrix subfield is sent back) 
               
            
           
         
       
     
     In some aspects, each average SNR value may be n bits in size. Therefore, the total number of bits for all the average SNR values for each subband of each space-time stream may be given by Nsb×n×Nc. As an example, n may be 8 bits. It is noted that Nsb is the total number of subbands for a given sounding protocol. Nsb may be determined by subband information provided in the NDPA frame (e.g., in the Nc Index field in some aspects). Each average SNR value of a subband may be calculated by averaging over the SNR values obtained for each reported subcarrier in the subband. In an aspect, pilot subcarriers are not included in the calculation of the average SNR value. The reported subcarriers in a subband may be different from all subcarriers in the subband because of the pilot subcarriers. In an aspect, the reported subcarriers include only the subcarriers for data in the subband, and DC tones are not included in the average SNR values. In an aspect, the average SNR value for a subband (or each subband) may be averaged over fewer than all of the data subcarriers in the subband (or in each such subband). For instance, the SNR value may be determined for half of the data subcarriers (e.g., SNR value determined for every other subcarrier) in a subband (or each subband), and the average SNR value for the subband may be determined based on these determined SNR values. 
     In some aspects, some of the average SNR values or the beamforming feedback matrices may be omitted from the beamforming report. For instance, the beamforming report may include one or more average SNR values but not the beamforming feedback matrices. 
     In some aspects, the information to be included in the beamforming report may be different for each station. The beamformer may indicate the information to be included in the beamforming report in a STA Info field associated with each station. In this regard, with reference to Table 4, each station may have a respective number of subbands Nsb and a respective number of space-time streams Nc over which to generate beamforming information (e.g., average SNR values). The number of subbands Nsb for one station may be the same or may be different from the number of subbands Nsb for another station. The number of space-time streams Nc for one station may be the same or may be different from the number of space-time streams Nc for another station. In an aspect, the beamforming information may be generated for every space-time stream 1 through Nc, inclusive. In an aspect, the beamforming information is skipped for some of the space-time streams 1 through Nc. For example, the beamforming information may be provided for every other space-time stream. The subband(s) for each station may be defined differently. For example, the tone(s) in subband 1 of beamformee 1 may be different from the tone(s) in subband 1 of beamformee 2. 
     In one or more implementations, the subband-related information may be based on resource units that may be allocated to stations. In an aspect, the resource units that may be allocated for a channel bandwidth may be provided by a numerology. In an aspect, the beamformer is the access point that may allocate or has allocated the resource unit(s) to the beamformees. In an aspect, the beamformer and the beamformees are aware of the resource unit(s) assigned to the beamformees and/or the channel bandwidth. 
       FIG. 8  illustrates an example of a numerology for a 20 MHz channel bandwidth. The numerology provides different manners by which to allocate resources for the 20 MHz channel bandwidth into individual resource units. A resource unit contains tones, where each tone may be a data tone or a pilot tone. For a 20 MHz HE PPDU transmission, the 20 MHz may be divided into 256 tones with a 78.125 kHz spacing between tones. In the OFDMA case, tone indices −3, −2, −1, 0, +1, +2, and +3 may be direct current (DC) tones (i.e., 7 DC tones). In the non-OFDMA case (e.g., SU case), tone indices −1, 0, and +1 may be DC tones (i.e., 3 DC tones), and a signal may be transmitted on tone indices −122 to −2 and 2 to 122. Hence, there may be up to a total of 242 usable tones for the 20 MHz HE PPDU transmission, which do not include DC tones. The remaining 11 tones may be guard tones, where 6 tones may be for one edge of the bandwidth, and 5 tones may be for the other edge of the bandwidth. (See  801  and  802  in  FIG. 8 .) In one aspect, usable tones do not include DC tones or guard tones. In one aspect, DC tones are not allocated to any station. 
     A first row  805  illustrates an example of usable tones for a 20 MHz channel bandwidth. In one aspect, usable tones include data/pilot tones and any reserved tones. A data/pilot tone is a data tone or a pilot tone. A data/pilot tone is a tone that can be utilized as a data tone or a pilot tone. A reserved tone may be referred to as a null tone or left over tone. In an aspect, the reserved tone may have zero energy. 
     A second row  810  illustrates a resource allocation of the 20 MHz bandwidth into multiple resource units. In some aspects, the 20 MHz bandwidth may be allocated into 9 resource units. Each non-center resource unit includes 26 data/pilot tones. A center resource unit includes 26 data/pilot tones and 7 DC tones. 
     A third row  815  illustrates a resource allocation of the 20 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the second row  810 . In some aspects, the 20 MHz bandwidth may be allocated into 5 resource units in the following manner: 4 resource units (each including 52 data/pilot tones) and one center resource unit (including 26 data/pilot tones and 7 DC tones). 
     A fourth row  820  illustrates a resource allocation of the 20 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the third row  815 . In some aspects, as shown in the fourth row  820 , the 20 MHz bandwidth may be allocated into 2 resource units (each including 106 data/pilot tones) and one center resource unit (including 26 data/pilot tones and 7 DC tones). In these aspects, no reserved tones are utilized. In some aspects (not shown), the 20 MHz bandwidth may be allocated into 3 resource units in the following manner: 2 resource units (each including 106 data/pilot tones) and one center resource unit (including 26 data/pilot tones and 3 DC tones). 
     For each of the resource allocations illustrated in the second row  810 , third row  815 , and fourth row  820 , one station may be allocated to one or more of the resource units. For example, for the fourth row  820 , a first station may be allocated to the leftmost resource unit containing 106 data/pilot tones, a second station may be allocated to the center resource unit containing 26 data/pilot tones, and/or a third station may be allocated to the rightmost resource unit containing 106 data/pilot tones. In some aspects, a station that is allocated to the center resource unit may not be allocated to any of the other resource units. 
     A fifth row  825  illustrates a resource allocation of the 20 MHz bandwidth into a single 242 tone resource unit. For example, the resource allocation may include, in order from the lowest usable tone index to the highest usable tone index: 121 data/pilot tones and 121 data/pilot tones. The allocation may be utilized for a non-OFDMA case, in which all the data/pilot tones (e.g., the 242 data/pilot tones) are allocated to a single STA. 
       FIG. 9  illustrates an example of a numerology for a 40 MHz channel bandwidth. For a 40 MHz HE PPDU transmission, the 40 MHz may be divided into 512 tones with a 78.125 kHz spacing between tones. In an aspect, a signal may be transmitted on tone indices −244 to −3 and +3 to +244, with the tone indices between −2 and +2, inclusive, being DC tones (i.e., 5 DC tones). In another aspect, a signal may be transmitted within a subset of the tone indices between −244 to −3 and +3 to +244. Hence, there may be up to a total of 484 usable tones (not including 5 DC tones). The remaining tones may be guard tones (e.g., 12 guard tones on the left edge and 11 guard tones on the right edge of the bandwidth). A first row  905  illustrates an example of usable tones spanning tone indices −244 to +244. 
     A second row  910  illustrates a resource allocation of the 40 MHz bandwidth into multiple resource units. In some aspects, the 40 MHz bandwidth may be allocated into 18 resource units that include 26 data/pilot tones each. 
     A third row  915  illustrates a resource allocation of the 40 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the second row  910 . In some aspects, the 40 MHz bandwidth may be allocated into a total of 10 resource units, having (a) 8 resource units that include 52 data/pilot tones each and (b) 2 resource units that include 26 data/pilot tones each. 
     A fourth row  920  illustrates a resource allocation of the 40 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the third row  915 . In some aspects, the 40 MHz bandwidth may be allocated into a total of 6 resource units, having (a) 4 resource units that include 106 data/pilot tones each and (b) 2 resource units that include 26 data/pilot tones each. In such aspects, no reserved tones are utilized. 
     A fifth row  925  illustrates a resource allocation of the 40 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the fourth row  920 . In some aspects, the 40 MHz bandwidth may be allocated into 2 resource units that include 242 data/pilot tones each. In such implementations, no reserved tones are utilized. 
     For each of the allocations illustrated in the second through fifth rows of  FIG. 9 , one STA may be allocated to one or more of the resource units. For example, for the fifth row  925 , one STA may be allocated to one of the 242-tone resource unit, and another STA may be allocated to the other of 242-tone resource unit. 
     A sixth row  930  illustrates a resource allocation of the 40 MHz bandwidth into a single resource unit with 484 non-DC tones. For example, the single resource unit may include, in order from the lowest usable tone index to the highest usable tone index: 242 data/pilot tones and 242 data/pilot tones. The single resource unit may be utilized for a non-OFDMA case, in which all the data/pilot tones are allocated to a single STA. 
       FIG. 10  illustrates an example of a numerology for an 80 MHz channel bandwidth. For an 80 MHz HE PPDU transmission, the 80 MHz may be divided into 1024 tones with a 78.125 kHz spacing between tones. In some aspects, the number of usable tones for the OFDMA case may be different from the number of usable tones for the non-OFDMA case. In some aspects, the number of DC tones for the OFDMA case may be different from the number of DC tones for the non-OFDMA case. 
     For an 80 MHz HE PPDU transmission for the OFDMA case, in an aspect, a signal may be transmitted on tones −500 to −4 and +4 to +500, with the tones between −3 and +3, inclusive, being DC tones (i.e., 7 DC tones). In this example, the number of usable tones may be 994, not including the DC tones. The remaining tones may be guard tones (e.g., 12 guard tones on the left edge and 11 guard tones on the right edge). In another aspect, a signal may be transmitted within a subset of the tone indices between −500 to −4 and +4 to +500. 
     For an 80 MHz HE PPDU transmission for the non-OFDMA case, a signal may be transmitted on tones −500 to −3 and +3 to +500, with the tones between −2 and +2, inclusive, being DC tones (i.e., 5 DC tones). In this example, the number of usable tones may be 996 (excluding the DC tones). The remaining tones may be guard tones (e.g., 12 guard tones on the left edge and 11 guard tones on the right edge). 
     A first row  1005  illustrates usable tones. The usable tones may span tone indices −500 to +500 for the OFDMA case (excluding −3 to +3 for 7 DC tones) and tone indices −500 to +500 for the non-OFDMA case (excluding −2 to +2 for 5 DC tones). 
     A second row  1010  illustrates a resource allocation of the 80 MHz bandwidth into multiple resource units. In one or more implementations, the 80 MHz bandwidth may be allocated into a total of 37 resource units, having (a) 36 resource units that include 26 data/pilot tones each and (b) a center resource unit that includes 26 data/pilot tones. In one or more implementations, a station that is allocated to the center resource unit may not be allocated to any of the other resource units. 
     A third row  1015  illustrates a resource allocation of the 80 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the second row  1010 . In one or more implementations, the 80 MHz bandwidth may be allocated into a total of 21 resource units, having (a) 16 resource units that include 52 data/pilot tones each, (b) 4 resource units that include 26 data/pilot tones each, and (c) a center resource unit that includes 26 data/pilot tones. 
     A fourth row  1020  illustrates a resource allocation of the 80 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the third row  1015 . In some aspects, the 80 MHz bandwidth may be allocated into a total of 13 resource units, having (a) 8 resource units that include 106 data/pilot tones each, (b) 4 resource units that include 26 data/pilot tones each, and (c) a center resource unit that includes 26 data/pilot tones. 
     A fifth row  1025  illustrates a resource allocation of the 80 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the fourth row  1020 . In some aspects, the 80 MHz bandwidth may be allocated into a total of 5 resource units, having (a) 4 resource units that include 242 data/pilot tones each and (b) a center resource unit that includes 26 data/pilot tones. In such aspects, no reserved tones are utilized. 
     A sixth row  1030  illustrates a resource allocation of the 80 MHz bandwidth into fewer, but generally larger, resource units than the resource allocation illustrated in the fifth row  1025 . In some aspects, the 80 MHz bandwidth may be allocated into a total of 3 resource units, having (a) 2 resource units that include 484 data/pilot tones each and (b) a center resource unit that includes 26 data/pilot tones. In such aspects, no reserved tones are utilized. 
     A seventh row  1035  illustrates a resource allocation of the 80 MHz bandwidth into a single resource unit (e.g., non-OFDMA case). In some aspects, the non-OFDMA case utilizes 5 DC tones, and thus the single resource unit contains 996 non-DC tones. 
     In some aspects, with reference to  FIGS. 8-10 , the 26 data/pilot tone resource unit may be a 24 data tone based resource unit, since a 26-tone resource unit may include 24 data tones and 2 pilot tones. Similarly, in some aspects, the 52 data/pilot tone resource unit may be a 48 data tone based resource unit, the 106 data/pilot tone resource unit may be a 102 data tone based resource unit when 4 pilot tones are utilized per resource unit, the 106 data/pilot tone resource unit may be a 102 data tone based resource unit when 4 pilot tones are utilized per resource unit and the 242 data/pilot tone resource unit may be a 234 data tone based resource unit, and the 484 data/pilot tone resource unit may be a 468 data tone based resource unit. 
     In one or more implementations, a resource allocation for a 160 MHz channel bandwidth may be obtained through multiple (e.g., eight) duplications of the resource allocation for 20 MHz channel bandwidth, through multiple (e.g., four) duplications of the resource allocation for 40 MHz channel bandwidth, or through duplications (e.g., two) of the resource allocation for 80 MHz channel bandwidth. 
     In one or more implementations, a resource allocation for 80+80 MHz channel bandwidths can be obtained through multiple (e.g., eight) duplications of the resource allocation for 20 MHz channel bandwidth, through multiple (e.g., four) duplications of the resource allocation for 40 MHz channel bandwidth, or through duplications (e.g., two) of the resource allocation for 80 MHz channel bandwidth. 
     In one or more implementations, the subband-related information may be based on the numerologies provided in  FIGS. 8, 9, and 10 . For example, the subband-related information may be indicative of one or more resource units in a numerology. Although  FIGS. 8, 9, and 10  provide numerologies that may be utilized for allocation of a 20 MHz, 40 MHz, and 80 MHz channel bandwidth, respectively, into resource units, other manners (e.g., numerologies) by which to allocate the channel bandwidth may be utilized. 
     In one or more implementations, the subband-related information associated with each beamformee may be contained in the Nc Index field of the STA Info field associated with the beamformee. In other words, the beamformer may signal the subband-related information to each beamformee using the Nc Index field of the respective STA Info field associated with each beamformee. In other implementations, the subband information may be contained in another part of an NDPA frame and/or in another frame (e.g., not in the NDPA frame). In some cases, the Nc Index field may be 3 bits, which may allow for up to 8 different states (e.g., values) that can be contained in the Nc Index field. In other cases, the Nc Index field may be fewer or more than 3 bits, allowing for fewer than or more than 8 different states. For simplicity of description, the subband information is described as being included in the Nc Index field. 
     In one aspect, the Nc Index field may be used to indicate full band feedback or subband-unit feedback. For example, for full band feedback, the station may provide average SNR values over the entire operating channel bandwidth (e.g., 20 MHz, 40 MHz, 80 MHz, 160 MHz). For subband-unit feedback, the station may provide average SNR values over one or more subband units. The subband-unit may be a minimum size of a subband. With reference to the numerologies of  FIGS. 8-10 , the minimum size may be a 26-tone (e.g., 2 MHz) subband, since the smallest resource unit, minimum bandwidth of channel allocation for the OFDMA case, is 26 tones. In an aspect, an AP (e.g., beamformer) may signal to the station subbands for which the AP is requesting feedback from the station in terms of subband-units. For instance, the AP may signal the station to provide average SNR values for the four left-most subband units (e.g., 4 resource units of 26 tones each). In an aspect, the station may provide average SNR values over each subband unit contained in the entire operating channel bandwidth. As an example, in the first row  805  of  FIG. 8 , the 20 MHz bandwidth may be divided into 9 subband units (e.g., 9 resource units of 26 tones each), and the station may provide one average SNR value for each of the 9 subband units. The Nc Index field may be set to one value (e.g., 7) for full band feedback and another value (e.g., 0) for subband-unit feedback. 
     In an aspect, the state (e.g., value) of the Nc Index field may be used to indicate a granularity (e.g., size of subband) for feedback. For example, with reference to the numerologies illustrated in  FIGS. 8-10 , the Nc Index field may be set to one of the following values:
         Set to 0 for 24-data tone based RU (26-tone based RU=24 data+2 pilot);   Set to 1 for 48-data tone based RU (52-tone based RU=48 data+4 pilot);   Set to 2 for 102-data tone based RU (106 or 108-tone based RU depending on number of pilots per RU);   Set to 3 for 234-data tone based RU (242-tone based RU);   Set to 4 for 468-data tone based RU (484-tone based RU);   . . .   Set to 7 for full band feedback.       

     In an aspect, some states are not used with some channel bandwidths. For example, for a 20 MHz channel bandwidth, the Nc Index field may be set to 0, 1, 2, 3, and 7, but not 4, 5, and 6 (e.g., 4, 5, and 6 may be unspecified or otherwise have no meaning for 20 MHz channel bandwidth). 
     As another example, the Nc Index field may be set to one of the following values to indicate the size of a subblock for feedback:
         Set to 0 for 26-tone based feedback (e.g., 2 MHz subblock);   Set to 1 for 52-tone based feedback (e.g., 4 MHz subblock);   Set to 2 for 102+(4 or 6 pilots)-tone based feedback (e.g., 8 MHz);   Set to 3 for 242-tone based feedback (e.g., 20 MHz);   Set to 4 for 484-tone based feedback (e.g., 40 MHz);   . . .   Set to 7 for full band feedback.       

     In an aspect, each state of the Nc Index field may correspond to a type in a given OFDMA numerology. For example, each type may refer to a row of the numerology. For 20 MHz channel bandwidth:
         Set to 0 for a first type (8×26-tone RU+1 center 26-tone RU) provided in the second row  810  of  FIG. 8 ;   Set to 1 for a second type (2×52-tone RU+1 center 26-tone RU) provided in the third row  815  of  FIG. 8     Set to 2 for a third type (2×(106 or 108)-tone+1 center 26-tone RU) provided in the fourth row  820  of  FIG. 8     . . .   Set to 7 for full band feedback       

     For 40 MHz channel bandwidth:
         Set to 0 for 2×(8×26-tone RU+26-tone RU) provided in the second row  910  of  FIG. 9 ;   Set to 1 for 2×(2×52-tone RU+26-tone RU) provided in the third row  915  of  FIG. 9 ;   Set to 2 for 2×(2×(106 or 108)-tone RU+26-tone RU) provided in the fourth row  920  of  FIG. 9 ;   Set to 3 for 2×242-tone RU provided in the fifth row  925  of  FIG. 9 ;   . . .   Set to 7 for full band feedback       

     For 80 MHz channel bandwidth:
         Set to 0 for 2×(18×26-tone RU)+1 center 26-tone RU provided in the second row  1010  of  FIG. 10 ;   Set to 1 for 4×(4×52-tone RU+26-tone RU)+1 center 26-tone RU provided in the third row  1015  of  FIG. 10 ;   Set to 2 for 4×242 tone RU+1 center 26-tone RU provided in the fifth row  1025  of  FIG. 10 ;   Set to 3 for 2×484 tone RU+1 center 26-tone RU provided in the sixth row  1030  of  FIG. 10 ;   . . .   Set to 7 for full band feedback       

     It is noted that the ellipses separating a state of the Nc Index field from the full band feedback state in the examples above may indicate that one or more additional states or no state are present between the states. 
     In an aspect, each state of the Nc Index field may be a preferred band information. For a given channel bandwidth, each beamformee (e.g., non-AP station) may have a preferred subband(s) from the perspective of the beamformer (e.g., AP). In such cases, when the beamformer sends an NDPA frame (e.g.,  512  in  FIG. 5 ) to beamformee(s), the beamformer may indicate preferred subband(s) for each beamformee in the Nc Index field of the STA Info field corresponding to the beamformee. In one aspect, with this approach, the size of feedback may be reduced relative to a case in which subbands span the entire channel bandwidth since the beamformer is requesting feedback for less than the entire channel bandwidth. For example, the number of elements of the compressed beamforming feedback matrix V for subcarrier k (e.g., shown in Tables 2 and 4) can be reduced. In other words, in this approach, a station does not need to send feedback matrices V for all subcarrier index k in the channel bandwidth or for all subcarrier groupings (e.g., the Grouping field of the MIMO Control field shown in  FIG. 7A ), rather the station may send back feedback matrices V only for subcarriers or subcarrier groupings within the preferred subbands informed by the beamformer via the NDPA frame. 
     In one or more implementations, an order of priority may be allocated to different states of the Nc Index field. In the examples provided above, the Nc Index field is assumed to be at least three bits to accommodate up to eight states. If there is a lack of available bits in the Nc Index field (e.g., fewer than three bits and/or more than eight states), states may be allocated in a descending order of subband size and/or the center 26-tone RU may have the lowest order in the mapping. In an aspect, a wider subband may have higher priority than narrower subbands, and the center 26-tone may have lower priority than a normal (e.g., non-center) 26-tone subband. 
       FIG. 11  illustrates an example of an order of priority for allocating the states in the case of a 20 MHz channel bandwidth. In particular,  FIG. 11  illustrates the numerology of  FIG. 8  together with a priority for each of the possible resource units defined in the numerology. If the NDPA frame allows for at least four bits for the subband-related information, each of the resource units labeled 0 through 15 in  FIG. 11  may be represented using four bits. In a case where the NDPA frame has fewer than four bits for the subband-related information (e.g., the Nc Index field is three bits), some of the states may be excluded. For example, if three bits are utilized for the subband-related information, the NDPA frame may identify the RUs labeled 0 through 7 using the subband-related information but not the RUs 8 through 15. 
       FIGS. 12A and 12B  illustrate other examples of an order of priority for allocating the states in in the case of a 20 MHz channel bandwidth. In  FIG. 12A , the order excludes the non-OFDMA allocation. If there are three available bits for the subband information, then the indication may be limited to indicating 52-tone RUs. The 26-tone RU may be excluded except for the center 26-tone, which may be included as a special RU. In  FIG. 12B , the order includes the non-OFDMA allocation. Other mapping between the indications and preferred subbands may be utilized. 
     Similar descending order of preferred band mapping may also be applied to other channel bandwidths.  FIG. 13  illustrates an example of an order of priority for allocating the states in the case of a 40 MHz channel bandwidth. In such an example, three available bits may be utilized in the subband-related information. In  FIG. 13 , the non-OFDMA allocation is included in this indication, and the two 26-tone RUs are indicated using the same value (e.g., 7). In an aspect, the importance of these two 26-tone RUs may be less than that of the other indicated subbands (e.g., 0 through 6).  FIG. 14  illustrates an example of an order of priority for allocating the states in the case of an 80 MHz channel bandwidth. 
     Although  FIGS. 11, 12A, 12B, 13, and 14  provide examples of order of priority for allocating the states for different channel bandwidths, other mapping between the indications and preferred subbands may be utilized. If there are more available bits for the subband-related information, the indications may be extended to allow finer granularity in the subbands. 
     In an aspect, common station information in which a subband size is configured may be included in the NDPA frame, and all related stations of a sounding protocol may send back relevant information based on the subband size indicated in the common station information. In an aspect, the NDPA frame may indicate the subband size to be a smallest resource unit size (e.g., 26-tone resource unit), which is associated with minimum granularity. 
     In an aspect, a combination of different subband sizes may be provided using K assignments. The number of assignments K may be indicated by the NDPA frame, where K is the number of assignment blocks in a given PPDU. As shown in Table 5, for example, when K=3, 4, 5, there is only one assignment of a 26-tone resource unit, which is located in the center of the channel bandwidth. Since the center 26-tone resource unit might have worse PHY performance (e.g., due to lack of LTF tones and/or tone erasures), feedback information for larger sized subbands may be more valuable than feedback information for the center 26-tone resource unit. In Table 5, 1×26 is a 26-tone resource unit, 2×26 is 52-tone resource unit, and 102+P is a 102 data tone based resource unit with P pilot tones. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Example of K factor 
               
               
                 and assignment configuration 
               
            
           
           
               
               
               
               
               
            
               
                   
                 K (assign- 
                   
                   
                   
               
               
                   
                 ments) 
                 1 × 26 
                 2 × 26 
                 102 + P 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 3 
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                 2 
               
               
                   
                 4 
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                 7 
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     The subject disclosure may be utilized in connection with “802.11ac-2013—IEEE Standard for Information technology—Telecommunications and information exchange between systems-Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications-Amendment 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz,” published Dec. 18, 2013 (IEEE Standard), which is incorporated herein by reference in its entirety and includes, for example, IEEE Standard&#39;s Tables 8-53d, 8-53e, 8-53g, 8-53h, 8-53i, and 8-53j, which are referenced above in this disclosure. 
     It should be noted that like reference numerals may designate like elements. These components with the same reference numerals have certain characteristics that are the same, but as different figures illustrate different examples, the same reference numeral does not indicate that a component with the same reference numeral has the exact same characteristics. While the same reference numerals are used for certain components, examples of differences with respect to a component are described throughout this disclosure. 
       FIGS. 15A, 15B and 15C  illustrate flow charts of examples of methods for facilitating wireless communication. For explanatory and illustration purposes, the example processes  1510 ,  1520  and  1530  may be performed by the wireless communication devices  111 - 115  of  FIG. 1  and their components such as a baseband processor  210 , a MAC processor  211 , a MAC software processing unit  212 , a MAC hardware processing unit  213 , a PHY processor  215 , a transmitting signal processing unit  280  and/or a receiving signal processing unit  290 ; however, the example processes  1510 ,  1520  and  1530  are not limited to the wireless communication devices  111 - 115  of  FIG. 1  or their components, and the example processes  1510 ,  1520  and  1530  may be performed by some of the devices shown in  FIG. 1 , or other devices or components. Further for explanatory and illustration purposes, the blocks of the example processes  1510 ,  1520  and  1530  are described herein as occurring in serial or linearly. However, multiple blocks of the example processes  1510 ,  1520  and  1530  may occur in parallel. In addition, the blocks of the example processes  1510 ,  1520  and  1530  need not be performed in the order shown and/or one or more of the blocks/actions of the example processes  1510 ,  1520  and  1530  need not be performed. 
     Various examples of aspects of the disclosure are described below as clauses for convenience. These are provided as examples, and do not limit the subject technology. As an example, some of the clauses described below are illustrated in  FIGS. 15A, 15B and 15C . 
     Clause A. A station for facilitating communication in a wireless network for multi-user transmission, the station comprising: one or more memories; and one or more processors coupled to the one or more memories, the one or more processors configured to cause: receiving a first frame; receiving a second frame associated with the first frame; generating a beamforming report, wherein the beamforming report comprises a plurality of average signal-to-noise ratio (SNR) values, and each of the plurality of average SNR values is associated with a respective subband of a bandwidth to be reported; and providing a third frame for transmission, wherein the third frame comprises the beamforming report. 
     Clause B. An access point for facilitating communication in a wireless network for multi-user transmission, the access point comprising: one or more memories; and one or more processors coupled to the one or more memories, the one or more processors configured to cause: transmitting a first frame to one or more stations; transmitting a second frame to the one or more stations; receiving a third frame from a first station of the one or more stations, wherein the third frame comprises a first plurality of average signal-to-noise ratio (SNR) values, and each of the first plurality of average SNR values is associated with a respective subband of a bandwidth; and generating a beamforming matrix based at least on the third frame. 
     Clause C. A computer-implemented method of facilitating communication in a wireless network for multi-user transmission, the method comprising: receiving a first frame; receiving a second frame subsequent to receiving the first frame, the second frame is associated with the first frame; generating a beamforming report, wherein the beamforming report comprises one or more of average signal-to-noise ratio (SNR) values, and each of the one or more of the average SNR values is associated with a respective subband of a bandwidth, and transmitting a third frame comprising the beamforming report. 
     In one or more aspects, additional clauses are described below. 
     A method comprising one or more methods or operations described herein. 
     An apparatus or a station comprising one or more memories (e.g.,  240 , one or more internal, external or remote memories, or one or more registers) and one or more processors (e.g.,  210 ) coupled to the one or more memories, the one or more processors configured to cause the apparatus to perform one or more methods or operations described herein. 
     An apparatus or a station comprising one or more memories (e.g.,  240 , one or more internal, external or remote memories, or one or more registers) and one or more processors (e.g.,  210  or one or more portions), wherein the one or more memories store instructions that, when executed by the one or more processors, cause the one or more processors to perform one or more methods or operations described herein. 
     An apparatus or a station comprising means (e.g.,  210 ) adapted for performing one or more methods or operations described herein. 
     A computer-readable storage medium (e.g.,  240 , one or more internal, external or remote memories, or one or more registers) comprising instructions stored therein, the instructions comprising code for performing one or more methods or operations described herein. 
     A computer-readable storage medium (e.g.,  240 , one or more internal, external or remote memories, or one or more registers) storing instructions that, when executed by one or more processors (e.g.,  210  or one or more portions), cause the one or more processors to perform one or more methods or operations described herein. 
     In one aspect, a method may be an operation, an instruction, or a function and vice versa. In one aspect, a clause may be amended to include some or all of the words (e.g., instructions, operations, functions, or components) recited in other one or more clauses, one or more sentences, one or more phrases, one or more paragraphs, and/or one or more claims. 
     To illustrate the interchangeability of hardware and software, items such as the various illustrative blocks, modules, components, methods, operations, instructions, and algorithms have been described generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application. 
     A reference to an element in the singular is not intended to mean one and only one unless specifically so stated, but rather one or more. For example, “a” module may refer to one or more modules. An element proceeded by “a,” “an,” “the,” or “said” does not, without further constraints, preclude the existence of additional same elements. 
     Headings and subheadings, if any, are used for convenience only and do not limit the invention. The word exemplary is used to mean serving as an example or illustration. To the extent that the term include, have, or the like is used, such term is intended to be inclusive in a manner similar to the term comprise as comprise is interpreted when employed as a transitional word in a claim. Relational terms such as first and second and the like may be used to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. 
     Phrases such as an aspect, the aspect, another aspect, some aspects, one or more aspects, an implementation, the implementation, another implementation, some implementations, one or more implementations, an embodiment, the embodiment, another embodiment, some embodiments, one or more embodiments, a configuration, the configuration, another configuration, some configurations, one or more configurations, the subject technology, the disclosure, the present disclosure, other variations thereof and alike are for convenience and do not imply that a disclosure relating to such phrase(s) is essential to the subject technology or that such disclosure applies to all configurations of the subject technology. A disclosure relating to such phrase(s) may apply to all configurations, or one or more configurations. A disclosure relating to such phrase(s) may provide one or more examples. A phrase such as an aspect or some aspects may refer to one or more aspects and vice versa, and this applies similarly to other foregoing phrases. 
     A phrase “at least one of” preceding a series of items, with the terms “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list. The phrase “at least one of” does not require selection of at least one item; rather, the phrase allows a meaning that includes at least one of any one of the items, and/or at least one of any combination of the items, and/or at least one of each of the items. By way of example, each of the phrases “at least one of A, B, and C” or “at least one of A, B, or C” refers to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C. 
     It is understood that the specific order or hierarchy of steps, operations, or processes disclosed is an illustration of exemplary approaches. Unless explicitly stated otherwise, it is understood that the specific order or hierarchy of steps, operations, or processes may be performed in different order. Some of the steps, operations, or processes may be performed simultaneously. The accompanying method claims, if any, present elements of the various steps, operations or processes in a sample order, and are not meant to be limited to the specific order or hierarchy presented. These may be performed in serial, linearly, in parallel or in different order. It should be understood that the described instructions, operations, and systems can generally be integrated together in a single software/hardware product or packaged into multiple software/hardware products. 
     The disclosure is provided to enable any person skilled in the art to practice the various aspects described herein. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology. The disclosure provides various examples of the subject technology, and the subject technology is not limited to these examples. Various modifications to these aspects will be readily apparent to those skilled in the art, and the principles described herein may be applied to other aspects. 
     All structural and functional equivalents to the elements of the various aspects described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using a phrase means for or, in the case of a method claim, the element is recited using the phrase step for. 
     The title, background, brief description of the drawings, abstract, and drawings are hereby incorporated into the disclosure and are provided as illustrative examples of the disclosure, not as restrictive descriptions. It is submitted with the understanding that they will not be used to limit the scope or meaning of the claims. In addition, in the detailed description, it can be seen that the description provides illustrative examples and the various features are grouped together in various implementations for the purpose of streamlining the disclosure. The method of disclosure is not to be interpreted as reflecting an intention that the claimed subject matter requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed configuration or operation. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separately claimed subject matter. 
     The claims are not intended to be limited to the aspects described herein, but are to be accorded the full scope consistent with the language claims and to encompass all legal equivalents. Notwithstanding, none of the claims are intended to embrace subject matter that fails to satisfy the requirements of the applicable patent law, nor should they be interpreted in such a way.