Patent Publication Number: US-2022224383-A1

Title: Systems and Methods for Extremely High Throughput Sounding Process Indication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/856,652, filed on Apr. 23, 2020, which claims the benefit of U.S. Provisional Application No. 62/844,562, filed on May 7, 2019, both of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates to wireless communication systems having a plurality of wireless transmitting communication devices that collaborate to transmit information to one or more wireless receiving communication devices. 
     BACKGROUND 
     Wireless local area network (WLAN) communication systems include wireless communication devices that transmit and receive signals, including access points (APs), that function as an interface between the WLAN and one or more further networks, and stations (STA) that exchange signals with the APs. 
     In some cases, a plurality of APs may work in collaboration to communicate with one or more STAs in a WLAN. For example, the proposed IEEE 802.11be standard is being developed to support a next generation of Extremely High Throughput (EHT) WLAN. EHT communication may be achieved with AP collaboration, which can utilize coordination between multiple APs to minimize interference and improve quality of service for communications with STAs. 
     Improvements are desired to facilitate the coordination of multiple APs in a collaboration mode to communicate with one or more STAs. 
     SUMMARY 
     According to one example aspect, there is a method of communication between a plurality of collaborating Access Points (APs) and a receiving station (STA) operating in a wireless local area network (WLAN). The method includes the steps of: transmitting, by an AP in the plurality of collaborating APs, a control signal for the receiving STA, the control signal comprising a null data packet announcement (NDPA) frame including a type indicator that indicates a type of channel sounding process. 
     In some embodiments, the type indicator includes a type indication subfield. 
     In some embodiments, the type indication subfield has a length of 2 bits. 
     In some embodiments, the type indication subfield is in a sounding dialog token field, the sounding dialog token field having a length of 16 bits. 
     In some embodiments, the sounding dialog token field has a second subfield, the second subfield including information representative of a total number of the collaborating APs. 
     In some embodiments, the type indication subfield is in a STA information field that is different from a sounding dialog token field. 
     In some embodiments, the STA information field has a second subfield, the second subfield including information representative of a total number of the collaborating APs. 
     In some embodiments, the type indication subfield includes a binary value representative of at least one of the following sounding process: High Efficiency (HE), Very High Throughput (VHT), and Extremely High Throughput (EHT). 
     In some embodiments, the type indicator includes a Transmitter&#39;s Address (TA) field, the TA field comprising a Basic Service Set identifier (BSSID) subfield having a pre-defined value to indicate an AP collaboration mode. 
     According to another example aspect, there is a method of communication between a plurality of collaborating Access Point (APs) and a receiving station (STA) operating in a wireless local area network (WLAN). The method includes the steps of: transmitting, by each AP in the plurality of collaborating APs, a data frame to the receiving STA; wherein each data frame includes a preamble portion, the preamble portion including one or more training fields. 
     In some embodiments, the method may include transmitting the preamble portion to the receiving STA based on a beamforming steering matrix that is used to transmit a payload portion of the data frame. 
     In some embodiments, a SIG field in the preamble portion includes a first subfield including information representative of a total number of signal streams transmitted by the plurality of collaborating APs. 
     In some embodiments, the SIG field in the preamble portion includes a second subfield including information representative of a total number of signal streams transmitted by the corresponding AP to the receiving STA. 
     In some embodiments, the SIG field in the preamble portion includes a third subfield including information representative of a total number of Long Training Fields (LTF) symbols based on the information in the first subfield. 
     In some embodiments, the total number of LTF symbols is equivalent to the total number of signal streams transmitted by the plurality of collaborating APs. 
     In some embodiments, each of the signal streams carries a spatial stream. 
     According to one example aspect, there is a wireless transmitting device configured to perform any of the above methods. 
     According to another example aspect, there is a method of communication between a plurality of collaborating Access Point (APs) and a receiving station (STA) operating in a wireless local area network (WLAN). The method includes the steps of: transmitting, by a receiving STA from the plurality of receiving STAs to one or more APs in the plurality of collaborating APs, a beamforming control signal, the beamforming control signal comprising a compressed beamforming action (CBA) frame including a first subfield decodable by the one or more wireless APs; wherein the beamforming control signal includes one or more compressed beamforming reports (CBRs), each of the one or more compressed beamforming reports corresponding to an AP in the plurality of collaborating APs; and wherein the first subfield includes information represented by a bit pattern, the bit pattern configured to indicate the correspondence between the one or more compressed beamforming reports and the one or more APs. 
     In some embodiments, a length of the first subfield in bits is equivalent to the total number of the plurality of collaborating APs, and each bit in the bit pattern corresponds to an AP in the plurality of collaborating APs. 
     In some embodiments, when an n th  bit in the bit pattern includes a string of “0”, the control signal does not have a compressed beamforming report for the corresponding n th  wireless transmitting device. 
     In some embodiments, the beamforming control signal includes two or more compressed beamforming reports, and at least two of the two or more compressed beamforming reports are separated by a delimiter. 
     According to yet another example aspect, there is a method of communication between a plurality of collaborating APs and a receiving STA operating in a wireless local area network (WLAN). The method includes the steps of: transmitting, by the receiving STA to one or more APs in the plurality of collaborating APs, a beamforming control signal, the beamforming control signal comprising a compressed beamforming action frame decodable by the one or more APs; wherein the beamforming control signal includes one or more compressed beamforming reports (CBRs), each of the one or more compressed beamforming reports corresponding to an AP in the one or more APs; and wherein the beamforming control signal includes an Nr Index subfield including information representative of a number of rows of a feedback matrix for and placed ahead of each report of the one or more compressed beamforming reports. 
     According to one example aspect, there is a method of communication between a plurality of collaborating Access Point (APs) and a receiving station (STA) operating in a wireless local area network (WLAN). The method includes the steps of: transmitting, by each AP in the plurality of collaborating APs, a respective a null data packet (NDP) frame to the receiving STA; wherein each NDP frame includes a SIG field, the SIG field including information representative of a Basic Service Set identifier (BSSID) configured to be used by the receiving STA to determine a correct NDP frame for estimating a feedback matrix during a sounding process between the respective AP and the receiving STA. 
     In some embodiments, the BSSID includes a BSS color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings which show example embodiments of the present application, and in which: 
         FIG. 1  is a block diagram illustrating communications between multiple Access Points (APs) and multiple stations (STAs); 
         FIG. 2  is a block diagram illustrating communications including a sounding process between an AP and a STA; 
         FIG. 3A  illustrates a null-data packet announcement (NDPA) frame format based on the IEEE 802.11ax standard; 
         FIG. 3B  illustrates a NDPA frame format according to an example embodiment; 
         FIG. 4A  illustrates a sounding dialog token field format based on the IEEE 802.11ax standard; 
         FIG. 4B  illustrates a sounding dialog token field frame format according to an example embodiment; 
         FIG. 5  illustrates a compressed beamforming action frame (CBAF) format according to an example embodiment; 
         FIG. 6  illustrates an example data frame format according to an example embodiment; and 
         FIG. 7  is a block diagram illustrating an example electronic device that may act as an AP or receiving STA. 
     
    
    
     Similar reference numerals may have been used in different figures to denote similar components. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  shows a wireless local area network (WLAN) that includes a plurality of wireless communication devices configured to transmit and receive wireless signals according to example embodiments. The wireless communication devices include access points (APs)  102  and stations (STAs)  104 . In example embodiments, APs  102  are configured to function as an interface between one or more STAs  104  and one or more further networks  103 . Each STA  104  is associated through a registration procedure with one or more of the APs  102 . In example embodiments, APs  102  may operate collaboratively as a group in an AP collaboration mode. For example, APs  102  in group of collaborating APs  102  can communicate with one or more stations (STAs  104 ) using a Beamforming (BF) technology such as Coordinated BF (CBF) or Joint Transmission (JT). In some example embodiments, each AP  102  in a group of collaborating APs  102  collects channel state information (CSI) information for all the STAs  104  that are associated with any of the collaborating APs  102 . For example, each AP  102  may collect CSI information for the STAs  104  that the AP  102  is associated with as well as CSI information for STAs  104  that are associated with the other APs  102  in the AP collaboration. 
     There has been interest in including support in the IEEE 802.11 family of standards to enable extremely high throughput (EHT) wireless communications (being studied by the Task Group referred to as TGbe). For example, EHT operations beyond 160 MHz bandwidth, such as operations in bandwidths in the range of 180 MHz to 320 MHz (e.g., 180 MHz, 200 MHz, 220 MHz, 240 MHz, 260 MHz, 280 MHz, 300 MHz, or 320 MHz bandwidths) may be considered. 
     In one or more example embodiments described herein, frame formats specified in the IEEE 802.11ax and IEEE 802.11ac standards (that respectively support High Efficiency (HE) WLAN communications and Very High Throughput (VHT) WLAN communications) are modified to support AP collaboration in EHT WLAN communications. For example, one or more of the NDPA frame, NDP frame, and Compressed Beamforming Report Action frame formats specified in the IEEE 802.11ax standard are modified to facilitate coordination of APs  102  and STAs  104  in an AP collaboration mode that supports EHT WLAN communications. 
     As shown in  FIG. 1 , multiple collaborating APs  102  may communicate with multiple STAs  104  in a WLAN. In some embodiments, collaborating APs may synchronize their data transmissions to reduce signal interference. For example, the collaborating APs  102  may use coordinated orthogonal frequency-division multiple access (OFDMA) to synchronize data transmissions in a collaborating mode. In other embodiments, two or more collaborating APs  102  may perform a joint data transmission to or reception from multiple STAs reusing the same time/frequency resources. The collaborating APs  102  are configured to jointly process both the data and the channel state information (CSI) of all STAs  104  involved in the data transmission or reception. In this case, an AP  102  in the collaborating APs  102  may act as a Master AP, whose job is to coordinate and facilitate over-the-air synchronization of the collaborating APs  102  using inter-AP control signals. In some instances, the APs  102  may communicate using a backhaul network, such as network  103 . 
     Because each AP  102  may have multiple antennas, a STA  104  may receive, simultaneously or near simultaneously, multiple signal streams from one or more APs  102 . In order to adapt data transmissions to current channel conditions, it is necessary for each AP  102  to know various channel properties of a communication link between the AP  102  and a particular STA  104 . The channel properties of the communication link may be referred to as channel state information (CSI). The CSI describes how a signal propagates from the transmitter (which may be either the AP  102  or the STA  104 ) to the receiver (which may be either the STA  104  or the AP  102 ) and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI information may include a feedback matrix that is included in a compressed beamforming report (CBR) sent by a STA  104  to an AP  102 . In some embodiments, the feedback matrix may be calculated by the STA  104  based at least in part on information sent to the STA  104  by the AP  102 . The process to learn and estimate such channel properties is known as a channel sounding process or simply a sounding process. 
       FIG. 2  illustrates an example schematic diagram showing an AP  102  in communication with a STA  104 . An example AP  102  includes at least a processor  109  and a wireless local area network (WLAN) transceiver circuit  110 . An example STA  104  includes at least a processor  105  and a WLAN transceiver circuit  106  (e.g., a Wi-Fi transceiver) for exchanging signals through a wireless interface with the WLAN transceiver circuit  110  of AP  102 . 
       FIG. 2  also illustrates a set of example signals that may be communicated for performing a channel sounding process between an AP  102  and a STA  104 . Each of the APs  102  intending to communicate with a STA  104  initiates a channel sounding process by sending a null data packet announcement (NDPA) frame  152  to the STA  104 , as described below. 
     To initiate the channel sounding process, the AP  102  sends a control signal, carrying a null data packet announcement (NDPA) frame  152  (described further below) to all the STAs  104  with which the AP  102  intends to communicate (“the target STA(s)  104 ”). The NDPA frame  152  begins the channel sounding process and its primary purpose is to carry a STA info field for each target STA  104 . If an AP  102  intends to communicate with multiple target STAs  104 , the AP can broadcast the same NDPA frame  152  to all the STAs  104  in the WLAN, where the NDPA frame  152  includes some fields common and applicable to all the target STAs  104 , and a separate STA information field for each of the target STAs  104 . Each AP  102  may send a NDPA frame  152  to all target STAs  104 . It is possible that different APs  102  may each have a different set of target STA(s)  104  in a sounding process, and the NDPA frame  152  sent by each AP  102  identifies the target STAs  104  of the transmitting AP  102 . A STA  104  in the network may receive a NDPA frame  152  but determine that it is not an intended target STA based on the information in the NDPA frame  152 . The STAs  104  not identified by a NDPA frame  152  sent by a particular AP  102  may simply defer channel access for the particular AP  102  until the sounding process is completed. 
     Following the NDPA frame  152  and a short interframe space (SIFS) period, the AP  102  transmits a further control signal, which carries a null data packet (NDP) frame  154 , to each of the target STAs  104 , further described below. The NDP frame  154  generally has no data field, and includes training fields that contain symbols which will be used by the target STA(s)  104  to generate a compressed beamforming action (CBA) frame  156  that includes one or more compressed beamforming reports (CBRs). If a STA  104  is designated as a target STA  104  by a single AP  102 , then the CBA frame  156  may include only a single CBR for the AP  102 . If the STA  104  is designated as a target STA  104  by a plurality of collaborating APs  102 , then the CBA frame  156  may include multiple CBRs, with each CBR including a feedback matrix estimated by the target STA  104  for a corresponding AP  102  in example embodiments. The feedback matrix is calculated by the STA  104  based on information received in the NDPA frame  152  and NDP frame  154 , and may take the form of a sequence of angles in a compressed form. Once the STA  104  has generated the CBA frame  156 , it transmits the CBA frame  156  back to the collaborating APs  102 . Appropriate information exchange  158  between the AP  102  and the STA  104  may then take place, for each AP-STA communication pair. 
     In some embodiments, after gathering all the CBA frames  156 , each collaborating AP  102  can transmit a data frame  157 , which may be a beamformed data frame, with multiple data streams to the target STAs  104 . If the STAs  104  correctly decode the data in the data frame  157 , the STAs  104  send back an acknowledgement (ACK) (not shown in  FIG. 2 ) frame to the AP  102 , after which the AP  102  may begin information exchange  158 . 
     In one example embodiment, in order to facilitate collaboration of multiple APs  102  with each other in order to transmit and receive signals with at least one STA  104 , the format of NDPA frame  152 , which can be transmitted by one or more APs  102 , such as a Master AP  102  in the group of collaborating APs  102 , is modified from the NDPA frame format specified in the IEEE 802.11ax standard. 
       FIG. 3A  shows an example NDPA frame in accordance with the IEEE 802.11ax standard. As shown, the frame may include a MAC header including subfields such as frame control (2 bytes), duration (2 bytes), RA (Recipient Address, 6 bytes), and TA (Transmitter Address, 6 bytes). The frame may further include subfields including a sounding dialog token field (1 byte), one or more STA info fields (4 bytes each), with each STA info field intended for a separate STA  104 , and a FCS (frame check sequence) field (4 bytes).  FIG. 4A  shows an example format for a sounding dialog token field format of the NDPA frame of  FIG. 3A  in accordance with the IEEE 802.11ax standard, which has a reserved field (1 bit), a HE field (1 bit), and a sounding dialog token number (6 bits). 
     In some embodiments, the NDPA frame includes a type indicator used to indicate a type of the sounding process between an AP  102  and a STA  104 . The type indicator can indicate, for example, if the sounding process is a single AP sounding process, or an AP collaboration sounding process. For another example, the type indicator can indicate if the sounding process is HE, VHT, or EHT, as further described below. 
     In example embodiments, the NDPA frame illustrated in  FIG. 3A  is adapted for EHT, for use either for a single AP sounding or for AP collaboration sounding. In this regard,  FIG. 3B  illustrates the format of an EHT NDPA frame  152  according to example embodiments. The format of NDPA frame  152  is similar to the NDPA frame format shown in  FIG. 3A , except that the sounding dialog token field  302  of NDPA frame  152  is 2 bytes and includes additional information.  FIG. 4B  shows an example format for the sounding dialog token field  302  of NDPA frame  152 , which includes a type indication subfield  402  (2 bits), a sounding dialog token number subfield  404  (6 bits), an Overlapped Basic Service Set (OBSS) CSI subfield  406  (2 bits) and a reserved subfield  408  (6 bits). 
     In an example embodiment, the type indication subfield  402  may be set to indicate a type of sounding process. A target STA  104  can generate a compressed beamforming report that corresponds to the type of sounding process in the subfield  402 . For example, the type indication field  402  may indicate if the sounding process is a HE sounding process, a VHT sounding process, or an EHT sounding process. 
     In some example embodiments, the two bits of the type indication subfield  402  in  FIG. 4B  may be set to have the following type definitions:
         type indication subfield=10: EHT Sounding Process;   type indication subfield=01: HE Sounding Process;   type indication subfield=00: VHT Sounding Process.       

     In some example embodiments, the OBSS CSI subfield  406  may be used to indicate a number of NDP frames  154  that are to be transmitted immediately following the NDPA frame  152 . The target STAs  104  who need to participate in the sounding process to generate the CBRs are listed in the STA Info subfields of NDPA frame as seen in  FIG. 3B . In this sense, the OBSS CSI subfield  406  represents a total number of APs  102  that are in collaboration and transmitting signals in the WLAN communication network. For example, the 2 bits of the OBSS CSI subfield  406  in can be set have the following definitions:
         OBSS CSI subfield=00: No AP collaboration, i.e., Single AP sounding;   OBSS CSI subfield=01: 2 AP collaboration, i.e., 2 NDP frames will follow successively right after the NDPA transmission;   OBSS CSI subfield=10: 3 AP collaboration, i.e., 3 NDP frames will follow successively right after the NDPA transmission;   OBSS CSI subfield=11: 4 AP collaboration, i.e., 4 NDP frames will follow successively right after the NDPA transmission.       

     In example embodiments, the length of the OBSS CSI subfield  406  can be varied to support a greater number of collaborating APs. For example, a 3 bit OBSS CSI subfield  406  can be used to support up to eight APs  102  in collaboration. In such embodiments, the length of the reserved subfield  408  can be adjusted accordingly (e.g. from 6 bits to 5 bits) to maintain the length of the sounding dialog token field  302  at 2 bytes. 
     In alternative example embodiments, the number of collaborating AP&#39;s can be indicated in NDPA frame  152  by a different means than including OBSS CSI subfield  406  in the sounding dialog token field  302 . For example, in an alternative frame format, a subfield in a STA Info field of the NDPA frame  152  could be used to indicate the total number of collaborating APs. In some example embodiments, the STA Info field could include a second subfield, the second subfield including information representative of a total number of APs in collaboration. 
     In alternative example embodiments, rather than using specific type indication subfield  402  in NDPA frame  152 , the TA field  304  of the IEEE 802.11ax NDPA frame of  FIG. 3A  may be implemented as a type indicator to indicate a type of the sounding process, including for example to indicate whether the NDPA frame is for a single AP sounding process or an AP collaboration sounding process. The TA field  304  may be used to determine if the sounding process initiated by the NDPA frame is a single AP sounding process or an AP collaboration sounding process. For example, if the Transmitter&#39;s Address in the TA field  304  is the same as a STA&#39;s associated Basic Service Set identifier (BSSID), the NDPA frame indicates a single AP sounding process as contemplated by the IEEE 802.11ax standard. However, if the Transmitter&#39;s Address in the TA field  304  is different from each STA&#39;s associated BSSID, the NDPA frame indicates an AP collaboration sounding process. The STA&#39;s associated BSSID can be included in the STA information field  306  in the NDPA frame. In some examples, the TA field  304  can be set to a MAC address of the AP  102  that transmits the NDPA frame. In some examples, the TA field  304  can include a Basic Service Set identifier (BSSID) subfield having a pre-defined value to represent the Transmitter&#39;s Address. In such an embodiment, the size of the sounding dialog token subfield can be 1 byte. In some embodiments, when the NDPA frame indicates an AP collaboration sounding process, the STA  104  may assume that the type of sounding process is EHT. In some embodiments, when the NDPA frame indicates a single AP sounding process, the STA  104  may look at an additional subfield in the sounding dialog token field  401  as shown in  FIG. 4A  to determine a specific type of the single AP sounding process. 
       FIG. 4A  shows an example sounding dialog token field  401  in accordance with IEEE 802.11ax standard. The sounding dialog token field  401  may include a subfield  403  further indicating a type of a sounding process: e.g. a HE or non-HE sounding process. An example of the non-HE sounding process may be a VHT sounding process. For example, the 1 bit of the subfield  403  in  FIG. 4A  may be set to have the following type definitions:
         type indication subfield=1: HE Sounding Process   type indication subfield=0: Non-HE Sounding Process       

     As noted above, following the NDPA frame  152 , one or more NDP frames  154  are transmitted from an AP or APs  102  to one or more target STAs  104 . Each STA  104  can analyze the training fields (e.g. Long Training Fields) included in its respective NDP frame  154  to calculate a specific channel response. The STA  104  can then use the specific channel response information to determine the appropriate CSI to include in a CBR for the NDP frame  154 . The CBR is then included in a CBA frame  156  and transmitted by the STA  104  to provide CSI feedback (FB) to the AP  102 . For example, the CBR included in CBA frame  156  can be extracted and used by the AP  102  to calculate a BF steering matrix to direct transmission towards the specific STA  104  during the data transmission  158 . 
     In some embodiments, a signal field or SIG field in each NDP frame  154  can include a subfield to indicate which NDP frame  154  should be used to estimate for sounding for a particular STA  104 . For example, each NDP frame  154  that follows the NDPA frame  152  may include a subfield, the subfield including information representative of an identifier for a Basic Service Set (BSS) configured to be used by the STA  104  to generate a sounding feedback for the corresponding AP  102 . The identifier for a BSS used in the subfield may be a BSS color. 
     In some embodiments, after gathering all the CBA frames  156 , each collaborating AP  102  can transmit a data frame  157 , which may be a beamformed data frame  157 , with multiple data streams to the target STAs  104 . The data frame  157  can include a preamble portion and a payload portion. In case of a transmission in an AP Collaboration mode when multiple collaborating APs  102  are transmitting, a preamble portion which includes one or more signal (SIG) fields can be beamformed with the same beamforming steering matrix as applied to the payload portion. 
     When the CBR is received as part of a CBA frame  156  and processed by an AP  102 , the scheduling information such as the number of LTF symbols (represented by the subfield N_LTF), the number of transmitter (TX) streams, beamforming steering matrix, and so on, may be independent of the other collaborating APs  102 , once the CSI information is collected from all the participant STAs  104 . However, in this case, the leakage interferences from other APs  102  in the collaboration may degrade the performance. 
     Hence, a data frame  157  sent from each collaborating AP  102  may include an indication for the number of all the TX streams including the interferences coming from the other APs  102  in collaboration. For example, the scheduling information such as the number of streams for its own BSS, the corresponding stream index for resolvable LTFs and the number of STAs in case of MU-MIMO in its own BSS, may be shared among collaborating APs  102  in advance, and the number of total TX streams including interferences and the new number of LTFs may be indicated in the SIG field. 
     In some embodiments, the APs  102  that are collaborating in collaboration mode each shares Coordinated Beamforming (CBF) scheduling information with the other AP  102 &#39;s in the collaboration. For example, each AP  102  may indicate the number of streams transmitted by the AP  102  to the STAs  104  that the AP  102  is associated with, the corresponding stream index for resolvable LTFs, and the total number of STAs  104  in case there are multiple STAs to schedule per BSS associated with the AP  102  for which the AP  102  must schedule transmissions. This information can be shared between the collaborating APs  102  in advance before the actual data transmissions from multiple APs  102  takes place. 
       FIG. 6  shows an example data frame  157  transmitted from one collaborating AP  102  among a plurality of collaborating APs  102  to a receiving STA  104 , in accordance with some embodiments. The data frame  157  includes a preamble portion  610  and may include a payload portion  630  after the preamble portion  610 . The payload portion  630  may carry payload data. The preamble portion  610  may include a signal (SIG) field such as EHT_SIG fields  620 . Each EHT_SIG field  620  may include one or more subfields  621 ,  623 ,  625  as described herein. 
     In some embodiments, a “N_STS in collaboration” subfield (or in short “N_STS_CBF”)  621  may be included in an EHT SIG field  620  (e.g., an EHT_SIG_A or EHT_SIG_B field). The N_STS_CBF subfield  621  may include information representative of the total number of signal streams transmitted by all the APs  102  in collaboration. This information can be used by each STA  104  that is associated with an AP  102  to optimize MIMO detection using the resolvable LTFs. In some example embodiments, the EHT SIG field  620  can include a further subfield  623  to indicate the total number of spatial streams used by each respective AP  102  for associated STAs  104 , which can be a subfield denoted as N_STS subfield  623 . The N_STS subfield  623  can enable a receiving STA  104  to recover its own corresponding streams and can discard the unwanted streams which correspond to the other STAs  104  in the OBSS. The received data frame  157  can be considered to be a single AP transmission when the N_STS_CBF subfield  621  has a value that is equivalent to the value in the N_STS subfield  623 , otherwise the received frame can be considered to be sent in an AP collaboration mode. In this embodiment, there is therefore no need for a separate bit to indicate AP collaboration mode in the EHT SIG field. 
     In example embodiments, the total number of LTF symbols, as represented by a value in an optional N_LTF subfield  625  in the EHT_SIG field  620 , is determined based on the N_STS_CBF according to the IEEE 802.11ax standard. For example, N_LTF generally is equal to N_STS_CBF, because the total number of LTF symbols is equivalent to the total number of signal streams transmitted by the plurality of APs  102 . 
       FIG. 5  shows an example of a CBA frame  156  in accordance with some example embodiments. A CBA frame  156  may be sent by a STA  104  to one or more APs  102 . As shown, the CBA frame  156  includes a category field  502  (e.g. EHT, 1 byte), an EHT Action field  504  (1 byte), an EHT MIMO control field  506  (7 byte), and one or more compressed beamforming report (CBR) fields  508  separated by delimiters. There may be a CBR field  508  for each AP  102  collaborating in the collaboration mode. The EHT MIMO Control field  506  may include a subfield, namely, NDP index subfield  510 , that is used to indicate the presence or absence of CSI feedback (FB) in the CBA frame  156  for the APs  102 . In this regard, in example embodiments, the NDP index subfield  510  may use pre-defined bit patterns to associate the CBRs included in the CBA frame  156  with the NDP frames  154  (and corresponding APs  102 ) that the CBRs have been generated in respect of. In example embodiments, the sizes of NDP index subfield  510  and a following reserved subfield  512  are adjusted depending on the size of AP collaboration limit. For example, if the total number of APs  102  collaborating in AP collaboration mode is 4, the size NDP index subfield  510  is 4 bits, and the size of the following reserved subfield  510  is 4 bits. For any bit in this 4-bit NDP Index subfield  510 , a value of “1” indicates a positive presence of a CBR corresponding to an NDP frame  154  sent by an associated AP  102  that corresponds to the bit position, and a value of “0” indicates a negative presence of a CBR to the corresponding NDP frame  154  and AP  102 . For example, the bit pattern 1111 represents that the four CBRs in included in CBR fields  508  are being sent in response to respective NDP frames  514  sent by all four collaborating APs  102 . In a further example, the bit pattern 1011 represents that a CBR for a NDP frame  154  sent by the second AP  102  is missing from the CBR fields  508  of CBA frame  156 , and that all other CBRs are present. 
     When the total number of APs  102  participating in the AP collaboration mode is 8, the size of this NDP index subfield  510  is 8 bits, and thus, the reserved subfield  512  which follows is not present. For any bit in this 8-bit NDP Index subfield, a value of “1” indicates a positive presence of a CBR for the corresponding NDP frame  154  and AP  102 , and a value of “0” indicates a negative presence of a CBR for the corresponding NDP frame  154  and AP  102 . For example, the bit pattern 11111111 represents that the following eight CBRs are the CSI reports in response to those NDP packets, which have come from all eight APs in the AP collaboration mode. For another example, the bit pattern 11011111 represents that the CBR to the 3 rd  NDP packet is missing in the CBRs that followed the NDP index subfield, and that all other CBRs are present. 
     In some embodiments, in a CBA frame  156  sent by a STA  104 , a delimiter  518  is used to separate the CBR generated in response to an NDP frame  154  from the CBR generated in response to another NDP frame  154 . A delimiter  518  can be a subfield having a pre-defined value that indicates an end of an immediately preceding CBR  508  in the CBA frame  156 . 
     In example embodiments, the CBA frame  156  includes an Nc Index subfield  514  and Nr Index subfields  516  that describe the size of the feedback matrix used for CBRs in terms of the number of columns and the number of rows, respectively. In some embodiments, the Nc Index subfield  514  will include the same value for the CBRs  508  included in the CBA frame  156 , and is included in the EHT MIMO Control field  506 . However, because the Nr Index (e.g. row number) is an AP dependent parameter, the content of the Nr index subfield  516  may be different for different CBRs depending on the AP  102  that the CBR is generated in respect of. Accordingly, in CBA frame  156  that is used to support AP collaboration mode, a respective Nr Index subfield  506  is included before each CBR field  508 . In some examples, a respective AP ID subfield (which may for example include a BSS color value) may be included before each CBR field  508 . 
       FIG. 7  shows an example electronic device, which can be a transceiving apparatus, that may act as an AP  102  or a receiving STA  104 , includes at least one processing unit  250 , at least one transmitter  252 , at least one receiver  254 , one or more antennas  256 , at least one memory  258 , and one or more input/output devices or interfaces  266 . The processing unit  250  (which may include processor  105  in the case of STA  104 , or processor  109  in the case of AP  102 ) implements various processing operations of the AP  102  or the receiving STA  104 , such as signal coding, data processing, power control, input/output processing, or any other functionality. The processing unit  250  can also be configured to implement some or all of the functionality and/or embodiments described herein. Each processing unit  250  includes any suitable processing or computing device configured to perform one or more operations. Each processing unit  250  can, for example, include a microprocessor, microcontroller, digital signal processor, field programmable gate array, or application specific integrated circuit. 
     The transmitter  252  and receiver  254  may collectively be used to implement WLAN transceiver circuit  106  in the case of STA  104  or WLAN transceiver circuit  110  in the case of AP  102 . Each transmitter  252  includes any suitable structure for generating signals for wireless or wired transmission. Each receiver  254  includes any suitable structure for processing signals received wirelessly or by wire. Although shown as separate components, at least one transmitter  252  and at least one receiver  254  could be combined into a transceiver. Each antenna  256  includes any suitable structure for transmitting and/or receiving wireless or wired signals. Although a common antenna  256  is shown here as being coupled to both the transmitter  252  and the receiver  254 , one or more antennas  256  could be coupled to the transmitter(s)  252 , and one or more separate antennas  256  could be coupled to the receiver(s)  254 . In some examples, one or more antennas  256  may be an antenna array, which may be used for beamforming and beam steering operations. Each memory  258  includes any suitable volatile and/or non-volatile storage and retrieval device(s). Any suitable type of memory may be used, such as random access memory (RAM), read only memory (ROM), hard disk, optical disc, subscriber identity module (SIM) card, memory stick, secure digital (SD) memory card, and the like. The memory  258  stores instructions and data used, generated, or collected by the AP  102  or the receiving STA  104 . For example, the memory  258  could store software instructions or modules configured to implement some or all of the functionality and/or embodiments described herein and that are executed by the processing unit(s)  250 . 
     Each input/output device/interface  266  permits interaction with a user or other devices in the network. Each input/output device/interface  266  includes any suitable structure for providing information to or receiving/providing information from a user, including network interface communications. 
     Although the present disclosure describes methods and processes with steps in a certain order, one or more steps of the methods and processes may be omitted or altered as appropriate. One or more steps may take place in an order other than that in which they are described, as appropriate. 
     Although the present disclosure is described, at least in part, in terms of methods, a person of ordinary skill in the art will understand that the present disclosure is also directed to the various components for performing at least some of the aspects and features of the described methods, be it by way of hardware components, software or any combination of the two. Accordingly, the technical solution of the present disclosure may be embodied in the form of a software product. A suitable software product may be stored in a pre-recorded storage device or other similar non-volatile or non-transitory computer readable medium, including DVDs, CD-ROMs, USB flash disk, a removable hard disk, or other storage media, for example. The software product includes instructions tangibly stored thereon that enable a processing device (e.g., a personal computer, a server, or a network device) to execute examples of the methods disclosed herein. The machine-executable instructions may be in the form of code sequences, configuration information, or other data, which, when executed, cause a machine (e.g., a processor or other processing device) to perform steps in a method according to examples of the present disclosure. 
     The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. The described example embodiments are to be considered in all respects as being only illustrative and not restrictive. Selected features from one or more of the above-described embodiments may be combined to create alternative embodiments not explicitly described, features suitable for such combinations being understood within the scope of this disclosure. 
     All values and sub-ranges within disclosed ranges are also disclosed. Also, although the systems, devices and processes disclosed and shown herein may comprise a specific number of elements/components, the systems, devices and assemblies could be modified to include additional or fewer of such elements/components. For example, although any of the elements/components disclosed may be referenced as being singular, the embodiments disclosed herein could be modified to include a plurality of such elements/components. The subject matter described herein intends to cover and embrace all suitable changes in technology. 
     Although a combination of features is shown in the illustrated embodiments, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system or method designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     Although this disclosure has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the disclosure, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.