Patent Publication Number: US-2023141486-A1

Title: Communication apparatus and communication method for channel sounding

Description:
TECHNICAL FIELD 
     The present disclosure relates to communication apparatuses and methods for channel sounding, and more particularly to communication apparatuses and methods for channel sounding in EHT WLAN (extremely high throughput wireless local area network). 
     BACKGROUND 
     In the standardization of next generation wireless local area network (WLAN), a new radio access technology necessarily having backward compatibilities with IEEE 802.11a/b/g/n/ac/ax technologies has been discussed in the IEEE 802.11 Working Group and is named IEEE 802.11be Extremely High Throughput (EHT) WLAN. 
     In 802.11be EHT WLAN, in order to provide significant peak throughput and capacity increase beyond 802.11ax high efficiency (HE) WLAN, especially for cell-edge STAs, it has been proposed to enable multiple access point (multi-AP) coordination in a multi-AP system. 
     However, there has been no much discussion on communication apparatuses and methods for channel sounding, specifically on efficient procedure on multi-AP based sounding. 
     There is thus a need for communication apparatuses and methods that provide feasible technical solutions for channel sounding in the context of EHT WLAN. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure. 
     SUMMARY 
     Non-limiting and exemplary embodiments facilitate providing communication apparatuses and communication methods for channel sounding in context of EHT WLAN. 
     In a first aspect, the present disclosure provides a communication apparatus comprising: circuitry, which, in operation, generates a first frame for a sounding procedure; and a transmitter, which, in operation, transmits the first frame to each of one or more peer communication apparatuses, the first frame comprising a first field which indicates an intended usage of the sounding procedure. 
     In a second aspect, the present disclosure provides a peer communication apparatus comprising: a receiver, which, in operation, receives a first frame for a sounding procedure from a communication apparatus; and circuitry, which, in operation, processes the first frame, the first frame comprising a first field which indicates an intended usage of the sounding procedure. 
     In a third aspect, the present disclosure provides a communication method comprising: generating a first frame for a sounding procedure; and transmitting the first frame to each of one or more peer communication apparatuses, the first frame comprising a first field which indicates an intended usage of the sounding procedure. 
     It should be noted that general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the disclosure will be better understood and readily apparent to one of ordinary skilled in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
         FIG.  1 A  depicts a schematic diagram of uplink and downlink single-user (SU) multiple input multiple output (MIMO) communication between an access point (AP) and a station (STA) in a MIMO wireless network. 
         FIG.  1 B  depicts a schematic diagram of downlink multi-user (MU) communication between an AP and multiple STAs in a MIMO wireless network. 
         FIG.  10    depicts a schematic diagram of trigger-based uplink MU communication between an AP and multiple STAs in a MIMO wireless network. 
         FIG.  1 D  depicts a schematic diagram of trigger-based downlink multi-AP communication between multiple APs and a STA in a MIMO wireless network. 
         FIG.  2 A  depicts a single-AP based sounding procedure between two STAs in an 11ax HE WLAN. 
         FIG.  2 B  depicts a single-AP based sounding procedure between an AP and multiple STAs in an 11ax HE WLAN. 
         FIG.  3 A  shows a schematic example of communication apparatus in accordance with various embodiments. The communication apparatus may be implemented as an AP or a STA and configured for channel sounding in accordance with the present disclosure. 
         FIG.  3 B  shows a flow diagram illustrating a communication method according to the present disclosure. 
         FIG.  4    depicts a flow chart illustrating a sounding setup procedure according to an embodiment. 
         FIG.  5 A  depicts an example format of an EHT Action frame. 
         FIG.  5 B  depicts an example format of the Sounding Setup Element field of the EHT Action frame. 
         FIG.  5 C  depicts another example format of the Sounding Setup Element field of the EHT Action frame. 
         FIG.  6 A  depicts a flow chart illustrating single-AP based explicit sounding procedure between two STAs in an 11be EHT WLAN according to an embodiment. 
         FIG.  6 B  depicts a flow chart illustrating single-AP based explicit sounding procedure between an AP and multiple STAs in an 11be EHT WLAN according to another embodiment. 
         FIG.  7 A  depicts a flow chart illustrating single-AP based implicit sequential sounding procedure according to an embodiment. 
         FIG.  7 B  depicts a flow chart illustrating single-AP based implicit sequential sounding procedure according to another embodiment. 
         FIG.  8    depicts a flow chart illustrating single-AP based implicit joint sounding procedure according to an embodiment. 
         FIG.  9    depicts a flow chart illustrating multi-AP based explicit sequential sounding procedure according to an embodiment. 
         FIG.  10    depicts a flow chart illustrating multi-AP based explicit joint sounding procedure according to an embodiment. 
         FIG.  11 A  depicts a flow chart illustrating multi-AP based implicit sequential sounding procedure according to an embodiment. 
         FIG.  11 B  depicts a flow chart illustrating multi-AP based implicit sequential sounding procedure according to another embodiment. 
         FIG.  12    depicts a flow chart illustrating multi-AP based implicit joint sounding procedure according to an embodiment. 
         FIG.  13 A  depicts a flow chart illustrating multi-AP based hybrid sequential sounding procedure according to an embodiment. 
         FIG.  13 B  depicts a flow chart illustrating multi-AP based hybrid sequential sounding procedure according to another embodiment. 
         FIG.  14 A  depicts a flow chart illustrating multi-AP based hybrid joint sounding procedure according to an embodiment. 
         FIG.  14 B  depicts a flow chart illustrating multi-AP based hybrid joint sounding procedure according to another embodiment. 
         FIG.  15    depicts an example format of an EHT Null Data Packet (NDP) Announcement frame. 
         FIG.  16 A  depicts an example format of a STA Feedback Info field when the Sounding Type field refers to single-AP based explicit sounding. 
         FIG.  16 B  depicts an example format of a STA Sounding Info field when the Sounding Type field refers to single-AP based implicit sounding. 
         FIG.  16 C  depicts an example format of an AP-STA Explicit Sounding Info field when the Sounding Type field refers to multi-AP based explicit sounding. 
         FIG.  17    shows a configuration of a communication device, for example an AP, according to the present disclosure. 
         FIG.  18    shows a configuration of a communication device, for example an STA, according to the present disclosure. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale. For example, the dimensions of some of the elements in the illustrations, block diagrams or flowcharts may be exaggerated in respect to other elements to help an accurate understanding of the present embodiments. 
     DETAILED DESCRIPTION 
     Some embodiments of the present disclosure will be described, by way of example only, with reference to the drawings. Like reference numerals and characters in the drawings refer to like elements or equivalents. 
     In the following paragraphs, certain exemplifying embodiments are explained with reference to an access point (AP) and a station (STA) for uplink or downlink channel sounding, especially in a multiple-input multiple-output (MIMO) wireless network. 
     In the context of IEEE 802.11 (Wi-Fi) technologies, a station, which is interchangeably referred to as a STA, is a communication apparatus that has the capability to use the 802.11 protocol. Based on the IEEE 802.11-2016 definition, a STA can be any device that contains an IEEE 802.11-conformant media access control (MAC) and physical layer (PHY) interface to the wireless medium (WM). 
     For example, a STA may be a laptop, a desktop personal computer (PC), a personal digital assistant (PDA), an access point or a Wi-Fi phone in a wireless local area network (WLAN) environment. The STA may be fixed or mobile. In the WLAN environment, the terms “STA”, “wireless client”, “user”, “user device”, and “node” are often used interchangeably. 
     Likewise, an AP, which may be interchangeably referred to as a wireless access point (WAP) in the context of IEEE 802.11 (Wi-Fi) technologies, is a communication apparatus that allows STAs in a WLAN to connect to a wired network. The AP usually connects to a router (via a wired network) as a standalone device, but it can also be integrated with or employed in the router. 
     As mentioned above, a STA in a WLAN may work as an AP at a different occasion, and vice versa. This is because communication apparatuses in the context of IEEE 802.11 (Wi-Fi) technologies may include both STA hardware components and AP hardware components. In this manner, the communication apparatuses may switch between a STA mode and an AP mode, based on actual WLAN conditions and/or requirements. 
     In a MIMO wireless network, “multiple” refers to multiple antennas used simultaneously for transmission and multiple antennas used simultaneously for reception, over a radio channel. In this regard, “multiple-input” refers to multiple transmitter antennas, which input a radio signal into the channel, and “multiple-output” refers to multiple receiver antennas, which receive the radio signal from the channel and into the receiver. For example, in an N×M MIMO network system, N is the number of transmitter antennas, M is the number of receiver antennas, and N may or may not be equal to M. For the sake of simplicity, the respective numbers of transmitter antennas and receiver antennas are not discussed further in the present disclosure. 
     In a MIMO wireless network, single-user (SU) communications and multi-user (MU) communications can be deployed for communications between communication apparatuses such as APs and STAs. MIMO wireless network has benefits like spatial multiplexing and spatial diversity, which enable higher data rates and robustness through the use of multiple spatial streams. According to various embodiments, the term “spatial stream” may be used interchangeably with the term “space-time stream” (or STS). 
       FIG.  1 A  depicts a schematic diagram of SU communication  100  between an AP  102  and a STA  104  in a MIMO wireless network. As shown, the MIMO wireless network may include one or more STAs (e.g. STA  104 , STA  106 , etc.). If the SU communication  100  in a channel is carried out over whole channel bandwidth, it is called full bandwidth SU communication. If the SU communication  100  in a channel is carried out over a part of the channel bandwidth (e.g. one or more 20 MHz subchannels within the channel is punctured), it is called punctured SU communication. In the SU communication  100 , the AP  102  transmits multiple space-time streams using multiple antennas (e.g. four antennas as shown in  FIG.  1 A ) with all the space-time streams directed to a single communication apparatus, i.e. the STA  104 . For the sake of simplicity, the multiple space-time streams directed to the STA  104  are illustrated as a grouped data transmission arrow  108  directed to the STA  104 . 
     The SU communication  100  can be configured for bi-directional transmissions. As shown in  FIG.  1 A , in the SU communication  100 , the STA  104  may transmit multiple space-time streams using multiple antennas (e.g. two antennas as shown in  FIG.  1 A ) with all the space-time streams directed to the AP  102 . For the sake of simplicity, the multiple space-time streams directed to the AP  102  are illustrated as a grouped data transmission arrow  110  directed to the AP  102 . 
     As such, the SU communication  100  depicted in  FIG.  1 A  enables both uplink and downlink SU transmissions in a MIMO wireless network. 
       FIG.  1 B  depicts a schematic diagram of downlink MU communication  112  between an AP  114  and multiple STAs  116 ,  118 ,  120  in a MIMO wireless network. The MIMO wireless network may include one or more STAs (e.g. STA  116 , STA  118 , STA  120 , etc.). The MU communication  112  can be an OFDMA (orthogonal frequency division multiple access) communications or a MU-MIMO communication. For an OFDMA communication in a channel, the AP  114  transmits multiple streams simultaneously to the STAs  116 ,  118 ,  120  in the network at different resource units (RUs) within the channel bandwidth. For a MU-MIMO communication in a channel, the AP  114  transmits multiple streams simultaneously to the STAs  116 ,  118 ,  120  at same RU(s) within the channel bandwidth using multiple antennas via spatial mapping or precoding techniques. If the RU(s) at which the OFDMA or MU-MIMO communication occurs occupy whole channel bandwidth, the OFDMA or MU-MIMO communications is called full bandwidth OFDMA or MU-MIMO communications. If the RU(s) at which the OFDMA or MU-MIMO communication occurs occupy a part of channel bandwidth (e.g. one or more 20 MHz subchannel within the channel is punctured), the OFDMA or MU-MIMO communication is called punctured OFDMA or MU-MIMO communications. For example, two space-time streams may be directed to the STA  118 , another space-time stream may be directed to the STA  116 , and yet another space-time stream may be directed to the STA  120 . For the sake of simplicity, the two space-time streams directed to the STA  118  are illustrated as a grouped data transmission arrow  124 , the space-time stream directed to the STA  116  is illustrated as a data transmission arrow  122 , and the space-time stream directed to the STA  120  is illustrated as a data transmission arrow  126 . 
     To enable uplink MU transmissions, trigger-based communication is provided to the MIMO wireless network. In this regard,  FIG.  10    depicts a schematic diagram of trigger-based uplink MU communication  128  between an AP  130  and multiple STAs  132 ,  134 ,  136  in a MIMO wireless network. 
     Since there are multiple STAs  132 ,  134 ,  136  participating in the trigger-based uplink MU communication, the AP  130  needs to coordinate simultaneous transmissions of multiple STAs  132 ,  134 ,  136 . 
     To do so, as shown in  FIG.  10   , the AP  130  transmits triggering frames  139 ,  141 ,  143  simultaneously to STAs  132 ,  134 ,  136  to indicate user-specific resource allocation information (e.g. the number of space-time streams, a starting STS number and the allocated RUs) each STA can use. In response to the triggering frames, STAs  132 ,  134 ,  136  may then transmit their respective space-time streams simultaneously to the AP  130  according to the user-specific resource allocation information indicated in the triggering frames  139 ,  141 ,  143 . For example, two space-time streams may be directed to the AP  130  from STA  134 , another space-time stream may be directed to the AP  130  from STA  132 , and yet another space-time stream may be directed to the AP  130  from STA  136 . For the sake of simplicity, the two space-time streams directed to the AP  130  from STA  134  are illustrated as a grouped data transmission arrow  140 , the space-time stream directed to the AP  130  from STA  132  is illustrated as a data transmission arrow  138 , and the space-time stream directed to the AP  130  from STA  136  is illustrated as a data transmission arrow  142 . 
     Trigger-based communication is also provided to the MIMO wireless network to enable downlink multi-AP communication. In this regard,  FIG.  1 D  depicts a schematic diagram of downlink multi-AP communication  144 , between a STA  150  and multiple APs  146 ,  148  in a MIMO wireless network. 
     Since there are multiple APs  146 ,  148  participating in the trigger-based downlink multi-AP MIMO communication, the master AP  146  needs to coordinate simultaneous transmissions of multiple APs  146 ,  148 . 
     To do so, as shown in  FIG.  1 D , the master AP  146  transmits triggering frames  147 ,  153  simultaneously to the AP  148  and the STA  150  to indicate AP-specific resource allocation information (e.g. the number of space-time streams, a starting STS stream number and the allocated RUs) each AP can use. In response to the triggering frames, the multiple APs  146 ,  148  may then transmit respective space-time streams to the STA  150  according to the AP-specific resource allocation information indicated in the triggering frame  147 ; and the STA  150  may then receive all the space-time streams according to the AP-specific resource allocation information indicated in the triggering frame  153 . For example, two space-time streams may be directed to the STA  150  from AP  146 , and another two space-time streams may be directed to the STA  150  from AP  148 . For the sake of simplicity, the two space-time streams directed to the STA  150  from AP  146  are illustrated as a grouped data transmission arrow  152 , and the two space-time streams directed to the STA  150  from the AP  148  is illustrated as a grouped data transmission arrow  154 . 
     Due to packet/PPDU (physical layer protocol data unit) based transmission and distributed MAC (medium access control) scheme in 802.11 WLAN, time scheduling (e.g. TDMA (time division multiple access)-like periodic time slot assignment for data transmission) does not exist in 802.11 WLAN. Frequency and spatial resource scheduling is performed on a packet basis. In other words, resource allocation information is on a PPDU basis. 
     According to various embodiments, EHT WLAN supports non-trigger-based communications as illustrated in  FIG.  1 A  and  FIG.  1 B  and trigger-based communications as illustrated in  FIG.  10    and  FIG.  1 D . In non-trigger-based communications, a communication apparatus transmits a PPDU to one other communication apparatus or more than one other communication apparatuses in an unsolicited manner. In trigger-based communications, a communication apparatus transmits a PPDU to one other communication apparatus or more than one other communication apparatuses only after a soliciting triggering frame is received. 
       FIG.  2 A  depicts a single-AP based sounding procedure  200  between two STAs  202 ,  204  in an 11ax HE WLAN. The single-AP based sounding procedure  200  may start when the STA 1   202 , e.g. an AP, generates a HE NDP (null data packet) Announcement frame  206  to an intended STA, e.g. STA 2   204 . The HE NDP Announcement frame  206  comprises requested sounding feedback parameters per STA such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns of compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI (channel quality indicator) feedback. When the feedback type is SU feedback or MU feedback, the requested sounding feedback information comprises compressed beamforming feedback information per subcarrier or subcarrier group. When the feedback type is CQI feedback, the requested sounding feedback information comprises CQI information per subcarrier or subcarrier group. In IEEE 802.11 networks, a short interframe spacing (SIFS) is the time spacing prior to transmission of an acknowledgement by a STA. Upon transmission of the HE NDP Announcement frame  206 , a SIFS  207  may take effect, and at  208 , the STA 1   202  may transmit a HE Sounding NDP  210  to the STA 2   204 . The HE Sounding NDP  210  may comprise a HE Long Training Field (HE-LTF) for CSI (channel state information) estimation. 
     After the last symbol of the HE Sounding NDP  210  is transmitted, a SIFS  211  may take effect, and at  212 , the STA 2   204  may transmit a HE Compressed Beamforming/CQI frame  214  comprising sounding feedback information to the STA 1   202 . In an embodiment, the sounding feedback information may be derived by the STA 2   204  from CSI which is estimated from the HE-LTF field of the HE Sounding NDP  210  and be prepared according to its requested sounding feedback parameters indicated in the HE NDP Announcement frame  206 . Based on the received sounding feedback information from the STA 2   204 , the STA 1   202  may be able to determine a steering matrix and/or allocate appropriate resource units (RUs) for subsequent transmissions to the STA 2   204 . 
       FIG.  2 B  depicts a single-AP based sounding procedure  220  between an AP  222  and multiple STAs  224 ,  226  in an 11ax HE WLAN. The single-AP based sounding procedure  220  may start when the AP  222  generates an HE NDP Announcement frame  228  to intended STAs such as STA 1   224  and STA 2   226 . The HE NDP Announcement frame  228  comprises requested sounding feedback parameters per STA such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns of compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI feedback. When the feedback type is SU feedback or MU feedback, the requested sounding feedback information comprises compressed beamforming feedback information per subcarrier or subcarrier group. When the feedback type is CQI feedback, the requested sounding feedback information comprises CQI information per subcarrier or subcarrier group. Upon transmission of the HE NDP Announcement frame  228 , a SIFS  229  may take effect, and at  230 , the AP  222  may transmit a HE Sounding NDP  232  to the STA 1   224  and STA 2   226 . 
     After the HE Sounding NDP  232  is transmitted, a SIFS  233  may take effect, and at  234 , the AP  222  may transmit a Beamforming Report Poll (BFRP) Trigger frame  236  to solicit simultaneous transmissions of sounding feedback information from the STA 1   224  and STA 2   226 . 
     After the last symbol of the BFRP Trigger frame  236  is transmitted, a SIFS  237  may take effect, and at  238 , the STA 1   224  and STA 2   226  may simultaneously transmit respective HE Compressed Beamforming/CQI frames  240 ,  242  comprising respective sounding feedback information to the AP  222 . In an embodiment, the sounding feedback information may be derived by the STA 1   224  and STA 2   226  from respective CSIs which are estimated from the HE-LTF field of the HE Sounding NDP  232  and be prepared according to respective sounding feedback parameters indicated in the HE NDP Announcement frame  228 . Based on the received sounding feedback information from the STA 1   224  and STA 2   226 , the AP  222  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   224  and STA 2   226  for subsequent transmissions to the STA 1   224  and/or STA 2   226 . 
     It is noted that in 11ax HE WLAN, AP and STA(s) engaged in a single-AP based sounding procedure belong to a single BSS (basic service set). Therefore, in order to improve throughput in 11be EHT WLAN over 11ax HE WLAN, it is an object to of present disclosure to substantially overcome the existing challenges to provide communication apparatuses and methods for channel sounding that enable multi-AP coordination in a multi-AP system. 
     According to the present disclosure, a multi-AP coordination setup procedure is carried out among APs for multi-AP coordination candidate set formation. A multi-AP coordination candidate set comprises a sharing AP and one or more shared APs. In a multi-AP coordination setup, a AP set identifier (ID) is assigned to a multi-AP coordination candidate set, where each AP in the multi-AP coordination candidate set may be assigned with a AP ID, which together with the AP set ID, is used to uniquely identify a specific AP in the multi-AP coordination candidate set. In an embodiment, in a multi-AP coordination setup, capability negotiation among APs in a multi-AP coordination candidate set may be performed. Alternatively, capability negotiation among APs in a multi-AP coordination candidate set may be performed prior to a multi-AP coordination setup procedure, for example using backhaul. In an embodiment, in a multi-AP coordination setup, intended STAs may indicate preferable APs in a multi-AP coordination candidate set for multi-AP coordination operation. 
     After a multi-AP coordination setup procedure is completed, each AP in a multi-AP coordination candidate set may indicate multi-AP coordination related information such as multi-AP coordination candidate set information, AP&#39;s capabilities including AP sounding capabilities and multi-AP coordination capabilities, etc. in Beacon frame or the like. 
     In various embodiments, prior to starting a sounding procedure for multi-AP operation, sharing AP in a multi-AP coordination candidate set may initiate a sounding setup procedure with each of shared AP(s) in the multi-AP coordination candidate set to make necessary preparations for the sounding procedure. 
       FIG.  3 A  shows a schematic, partially sectioned view of a communication apparatus  300  according to the present disclosure. The communication apparatus  300  may also be implemented as an AP or a STA. 
     As shown in  FIG.  3 A , the communication apparatus  300  may include circuitry  314 , at least one radio transmitter  302 , at least one radio receiver  304 , and at least one antenna  312  (for the sake of simplicity, only one antenna is depicted in  FIG.  3 A  for illustration purposes). The circuitry  314  may include at least one controller  306  for use in software and hardware aided execution of tasks that the at least one controller  306  is designed to perform, including control of communications with one or more other communication apparatuses in a MIMO wireless network. The circuitry  314  may furthermore include at least one transmission signal generator  308  and at least one receive signal processor  310 . The at least one controller  306  may control the at least one transmission signal generator  308  for generating MAC frames (for example EHT Action frames) and PPDUs (for example PPDUs used for non-trigger-based communications or PPDUs used for trigger-based multi-AP joint transmission if the communication apparatus  300  is an AP, and for example PPDUs used for non-trigger-based communications or PPDUs used for trigger-based uplink transmissions if the communication apparatus  300  is a STA) to be sent through the at least one radio transmitter  302  to one or more other communication apparatuses and the at least one receive signal processor  310  for processing MAC frames (for example EHT Action frames) and PPDUs (for example PPDUs used for non-trigger-based communications or PPDUs used for trigger-based uplink transmissions if the communication apparatus  300  is an AP, and for example PPDUs used for non-trigger-based communications or PPDUs used for trigger-based multi-AP joint transmission if the communication apparatus  300  is a STA) received through the at least one radio receiver  304  from the one or more other communication apparatuses under the control of the at least one controller  306 . The at least one transmission signal generator  308  and the at least one receive signal processor  310  may be stand-alone modules of the communication apparatus  300  that communicate with the at least one controller  306  for the above-mentioned functions, as shown in  FIG.  3 A . Alternatively, the at least one transmission signal generator  308  and the at least one receive signal processor  310  may be included in the at least one controller  306 . It is appreciable to those skilled in the art that the arrangement of these functional modules is flexible and may vary depending on the practical needs and/or requirements. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. In various embodiments, when in operation, the at least one radio transmitter  302 , at least one radio receiver  304 , and at least one antenna  312  may be controlled by the at least one controller  306 . 
     The communication apparatus  300 , when in operation, provides functions required for single-AP or multi-AP based channel sounding. For example, the communication apparatus  300  may be an AP (for example a sharing AP), and the circuitry  314  (for example the at least one transmission signal generator  308  of the circuitry  314 ) may, in operation, generate a first frame (for example Sounding Setup Request frame) comprising a first field which indicates an intended usage of a sounding procedure. The radio transmitter  302  may in operation, transmit the first frame to each of one or more peer communication apparatuses (for example shared APs). In an embodiment, the radio receiver  304  may in operation, receive a second frame (for example Sounding Setup Response frame) from the each of the one or more peer communication apparatuses, the second frame comprising a first field which indicates one or more recommended type of the sounding procedure. In another embodiment, the circuitry  314  (for example the at least one transmission signal generator  308  of the circuitry  314 ) may, in operation, further generate a third frame (for example EHT NDP Announcement frame) which starts the sounding procedure. 
     The communication apparatus  300  may be a peer AP (for example a shared AP), and the radio receiver  304  may, in operation, receive a first frame (for example Sounding Setup Request frame) from one other communication apparatus (e.g. a sharing AP) comprising a first field which indicates an intended usage of a sounding procedure. The circuitry  314  (for example the at least one receive signal processor  310  of the circuitry  314 ) may, in operation, process the first frame. In an embodiment, the radio transmitter  302  may in operation, transmit a second frame (for example Sounding Setup Response frame) to the one other communication apparatus, the second frame comprising a first field which indicates one or more recommended type of the sounding procedure. 
       FIG.  3 B  shows a flow diagram illustrating a communication method according to the present disclosure. In step  318 , a step of generating a first frame for a sounding procedure is carried out. In step  320 , a step of transmitting the first frame to each of one or more peer communication apparatuses is carried out, wherein the first frame comprising a first field which indicates an intended usage of the sounding procedure. 
     In an embodiment, the first frame may comprise a second field which indicates an intended type of the sounding procedure. In another embodiment, the first frame may comprise a third field which indicates one or more intended communication apparatuses (for example STAs) which will be engaged in the sounding procedure. 
       FIG.  4    depicts a flow chart illustrating a sounding setup procedure  400  between two APs, specifically between a sharing AP  402  and a shared AP  404 , according to the present disclosure. Contention based channel access procedures, e.g. enhanced distributed channel access (EDCA) procedures, is illustrated by blocks  405 ,  413 , and SIFS  409 ,  417  are illustrated. A sounding setup procedure includes Sounding Setup Request and Response frames exchanged between sharing AP and each of shared AP(s) in a multi-AP coordination candidate set. A Sounding Setup Request or Response frame is an EHT Action frame. In particular, the sharing AP  402  may generate a first frame  408 , for example EHT Action frame comprising a sounding setup request (hereinafter referred as “Sounding Setup Request frame”) to initiate the sounding setup procedure  400 . The radio transmitter of the sharing AP  402  may transmit the Sounding Setup Request frame  408  to the shared AP  404 . 
     Upon reception of the Sounding Setup Request frame  408 , a SIFS  409  may take effect, and at  410 , the shared AP  404  may transmit an Acknowledgement (Ack) frame  412  to the sharing AP  402  to indicate a successful receipt of the Sounding Setup Request frame  408 . 
     After the last symbol of the Ack frame  412  is transmitted, the shared AP  404  may generate a second frame  416 , for example an EHT Action frame comprising a sounding setup response (hereinafter referred as “Sounding Setup Request frame”) to respond to the Sounding Setup Request frame  408  and indicate whether the shared AP  404  has been ready for a subsequent sounding procedure. 
     Upon reception of the Sounding Setup Response frame  416 , a SIFS  417  may take effect, and at  418 , the sharing AP  402  may transmit an Ack frame  420  to the shared AP  404  to indicate a successful receipt of the Sounding Setup Response frame  416 . 
       FIG.  5 A  depicts an example format of an EHT Action frame  500 , which may be used as Sounding Setup Request frame  408  or Sounding Setup Response frame  416  as illustrated in  FIG.  4   . The EHT Action frame  500  may include (or consist of) a Frame Control field, a Duration field, three Address fields (Address 1, 2 and 3 respectively) a Sequence Control field, a HT (high throughput) Control field, a Frame Body field  502  and a FCS (frame check sequence) field. The Frame Control field, a Duration field, the three Address fields (Address 1, 2 and 3 respectively) the Sequence Control field, the HT Control field may be grouped as MAC header. The Frame Body field  502  may further include a Category field, an EHT Action field, a Dialog Token field, a Sounding Setup Element field  504  and other elements or fields. 
       FIG.  5 B  depicts an example format of the Sounding Setup Element field  504  of the EHT Action frame  500  when the Action Type field  506  refers to “Request” indicating a Sounding Setup Request frame. The Sounding Setup Element field  504  may include (or consist of) an Element ID field, a Length field, an Extended Element ID field, an Action Type field  506 , an AP Set ID field  508 . When the Action Type field  506  refers to “Request”, the Sounding Setup Element field  502  may further include an Intended Sounding Usage field  510 , an Intended Sounding Type field  512  and an Intended STAs field  514 . The AP Set ID field  508  identifies a multi-AP coordination candidate set including sharing AP and at least one shared AP. The Intended STAs field  514  indicate one or more STAs which belong to BSS of the sharing AP and/or BSS of the at least one shared AP and are supposed to be engaged in the following sounding procedure. It is noted that in 11be EHT WLAN, AP(s) and STA(s) engaged in a single-AP or multi-AP based sounding procedure may belong to different BSSs. In 11be EHT WLAN, an STA can be identified by BSSID (BSS identifier) of an AP with which the STA is associated and its STA ID. Alternatively, AP ID of an AP with which an STA is associated, together with AP set ID of a multi-AP coordination candidate set including the AP and STA ID of the STA, can identify the STA. Further details of the Intended Sounding Usage field  510  and the Intended Sounding Type field  512  will be elaborated in the following. 
       FIG.  5 C  depicts another example format of the Sounding Setup Element field  504  for the EHT Action frame  500  when the Action Type field  506  refers to “Response” indicating a Sound Setup Response frame. Similarly, the Sounding Setup Element field  504  may include (or consist of) an Element ID field, a Length field, an Extended Element ID field, an Action Type field  506  and an AP Set ID field  508 . When the Action Type field refers to “Response”, the Sounding Setup Element field  504  may further include a Recommended Sounding Type field  514  to indicate one or more recommended type of the sounding procedure. Further details of the Recommended Sounding Type field  514  will be elaborated in the following. 
     The sharing AP  402  may determine intended sounding parameters such as intended sounding usage and intended sounding type based on capability negotiation among APs in a multi-AP coordination candidate set which is performed prior to a sounding setup procedure. 
     The Intended Sounding Usage field  510  indicates an intended usage of a sounding procedure following the sounding setup procedure (i.e. an intended scheme of multi-AP coordination that utilizes results of the sounding procedure). An intended scheme of multi-AP coordination is one of the following:
         Coordinated spatial reuse   Coordinated orthogonal frequency-division multiple access (OFDMA)   Coordinated beamforming; and   Joint beamforming (also known as joint transmission)       

     The Intended Sounding Type field  512  indicates an intended type of a sounding procedure following the sounding setup procedure, which is one of the following:
         Single-AP based explicit sounding;   Single-AP based implicit sequential sounding;   Single-AP based implicit joint sounding;   Multi-AP based explicit sequential sounding;   Multi-AP based explicit joint sounding;   Multi-AP based implicit sequential sounding;   Multi-AP based implicit joint sounding;   Multi-AP based hybrid sequential sounding; and   Multi-AP based hybrid joint sounding       

     Further, in the Sounding Setup Response frame  416 , the Recommended Sounding Type field  514  indicates one or more recommended type of a sounding procedure following the sounding setup procedure, which is one of the following:
         Single-AP based explicit sounding;   Single-AP based implicit sequential sounding;   Single-AP based implicit joint sounding;   Multi-AP based explicit sequential sounding;   Multi-AP based explicit joint sounding;   Multi-AP based implicit sequential sounding;   Multi-AP based implicit joint sounding; and   Exemption from multi-AP based sounding is requested.       

     It is noted that the shared AP  404  should take into account the intended scheme of multi-AP coordination when the shared AP  404  makes any recommendation on the sounding type in the Recommended Sounding Type field  514 . For one example, results of multi-AP based sequential sounding cannot be used for joint beamforming. For another example, results of single-AP based sounding may be used for coordinated spatial reuse and coordinated OFDMA but cannot be used for coordinated beamforming and joint beamforming. 
     After receiving a Sounding Setup Request frame  408  from the sharing AP  402 , the shared AP  404  may determine if its transmit and receive (TX/RX) chains need to be reconfigured for the following sounding procedure according to the information on intended STA(s) indicated in the Sounding Setup Request frame  408 . 
     According to an embodiment of the present disclosure, when the shared AP  404  is not able to get ready for the intended sounding type indicated in the Sounding Setup Request frame  408 , the shared AP  404  may recommend one or more different sounding type or request to be exempted from following sounding procedure in the Sounding Setup Response frame  416 . 
     According to another embodiment of the present disclosure, when the shared AP  404  supports single-AP based implicit sounding or multi-AP based implicit sounding and its TX/RX chains are reconfigured for the following sounding procedure, a calibration procedure may be initiated by the shared AP  404  to recalibrate its TX/RX chains before it sends the Sounding Setup Response frame  416  to the sharing AP  402 . However, if the calibration procedure is not successfully completed, the shared AP  404  may not recommend any single-AP based or multi-AP based implicit sounding procedure. 
     A sounding procedure may start after the sounding setup procedure between sharing AP and each of shared AP(s) in a multi-AP coordination candidate set is completed. A sounding procedure may start with transmission of an EHT NDP Announcement frame, which indicates one of the following types of the sounding procedure:
         Single-AP based explicit sounding;   Single-AP based implicit sequential sounding;   Single-AP based implicit joint sounding;   Multi-AP based explicit sequential sounding;   Multi-AP based explicit joint sounding;   Multi-AP based implicit sequential sounding;   Multi-AP based implicit joint sounding;   Multi-AP based hybrid sequential sounding; and   Multi-AP based hybrid joint sounding.
 
The EHT NDP Announcement frame starting a multi-AP based sounding procedure may be transmitted by sharing AP.
       

       FIG.  6 A  depicts a flow chart illustrating a single-AP based explicit sounding procedure  600  between two STAs  602 ,  604  in an 11be EHT WLAN according to an embodiment. The Single-AP based explicit sounding procedure  600  may start when STA 1   602 , e.g. an AP, transmits an EHT NDP Announcement frame  606  to an intended STA, such as STA 2   604 . The EHT NDP Announcement frame  606  comprises requested sounding feedback parameters per STA such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns of compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback, CQI feedback and calibration feedback. When the feedback type is SU feedback or MU feedback, the requested sounding feedback information comprises compressed beamforming feedback information per subcarrier or subcarrier group. When the feedback type is CQI feedback, the requested sounding feedback information comprises CQI information per subcarrier or subcarrier group. When the feedback type is calibration feedback, the requested sounding feedback information comprises compressed CSI per subcarrier or subcarrier group. Upon reception of the EHT NDP Announcement frame  606 , a SIFS  607  may take effect, and at  608 , the STA 1   602  may transmit an EHT Sounding NDP  610  to STA 2   604 . The EHT Sounding NDP  610  may comprise an EHT-LTF field for CSI estimation. 
     After the last symbol of the EHT Sounding NDP  610  is transmitted, a SIFS  611  may take effect, and at  612 , the STA 2   604  may transmit an EHT Compressed Beamforming/CQI frame  614  comprising sounding feedback information to the STA 1   602 . In an embodiment, the sounding feedback information may be derived by the STA 2   604  from CSI which is estimated from the EHT-LTF field of the EHT Sounding NDP  610  and be prepared according to its sounding feedback parameters indicated in the EHT NDP Announcement frame  606 . Based on the received sounding feedback information from the STA 2   604 , the STA 1   602  may be able to determine a steering matrix and/or allocate appropriate RUs for subsequent transmissions to the STA 2   604 . 
       FIG.  6 B  depicts a flow chart illustrating a single-AP based explicit sounding procedure  620  between an AP  622  and multiple STAs  624 ,  626  in an 11be EHT WLAN according to another embodiment. The single-AP based explicit sounding procedure  620  may start when the AP  622  transmits an EHT NDP Announcement frame  628  to intended STAs such as STA 1   624  and STA 2   626 . Similarly, the EHT NDP Announcement frame  628  may comprise requested sounding feedback parameters per STA such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns of compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback, CQI feedback and calibration feedback. When the feedback type is SU feedback or MU feedback, the requested sounding feedback information comprises compressed beamforming feedback information per subcarrier or subcarrier group. When the feedback type is CQI feedback, the requested sounding feedback information comprises CQI information per subcarrier or subcarrier group. When the feedback type is calibration feedback, the requested sounding feedback information comprises compressed CSI per subcarrier or subcarrier group. Upon transmission of the EHT NDP Announcement frame  628 , a SIFS  629  may take effect, and at  630 , the AP  622  may transmit an EHT Sounding NDP  632  to the STA 1   624  and STA 2   626 . 
     After the EHT Sounding NDP  632  is transmitted, a SIFS  633  may take effect, and at  634 , the AP  622  may transmit an EHT BFRP Trigger frame  636  to solicit simultaneous transmissions of sounding feedback information from the STA 1   624  and STA 2   626 . After the last symbol of the EHT BFRP Trigger frame  636  is transmitted, a SIFS  637  may take effect, and at  638 , the STA 1   624  and STA 2   626  may simultaneously transmit respective EHT Compressed Beamforming/CQI frames  640 ,  642  comprising sounding feedback information to the AP  622 . In an embodiment, the sounding feedback information may be derived by the STA 1   624  and STA 2   626  from respective CSIs which are estimated from the EHT-LTF field of the EHT Sounding NDP  632  and be prepared according to respective sounding feedback parameters indicated in the EHT NDP Announcement frame  628 . Based on the received sounding feedback information from the STA 1   624  and STA 2   626 , the AP  622  is able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   624  and STA 2   626  for subsequent transmissions to the STA 1   624  and/or STA 2   626 . 
     According to the present disclosure, when the sounding type indicated in the EHT NDP Announcement frame  606 ,  628  is single-AP based explicit sounding and the feedback type indicated in the EHT NDP Announcement frame  606 ,  628  is SU, MU or CQI feedback, the procedure  600 ,  620  is a normal single-AP based explicit sounding procedure; whereas when the sounding type indicated in the EHT NDP Announcement frame  606 ,  628  is single-AP based explicit sounding and the feedback type indicated in the EHT NDP Announcement frame  606 ,  628  is calibration feedback, the procedure  600 ,  620  is a single-AP based calibration procedure. In other words, single-AP based calibration procedure is a variant of single-AP based explicit sounding procedure as illustrated in  FIGS.  6 A and  6 B . Advantageously, according to the present disclosure, a single procedure  600 ,  620  can be used for the purpose of both calibration and explicit sounding. 
     In various embodiments, when the procedure  620  is a single-AP based calibration procedure, the EHT NDP Announcement frame  628  may indicate calibration RU allocation and calibration spatial stream (SS) allocation for each of the STA 1   624  and STA 2   626 . In particular, data RU allocation and data SS allocation for each of the STA 1   624  and STA 2   626  are indicated in the corresponding EHT BFRP Trigger frame  636 . Calibration SSs allocated to a STA may include data SSs allocated to the STA. In other words, number of calibration SSs may be equal to or larger than number of data SSs. In addition, calibration RUs allocated to a STA may be the same as data RUs allocated to the STA. 
     In various embodiments, when the procedure  600 ,  620  is a single-AP based calibration procedure, the EHT Compressed Beamforming/CQI frame  614 ,  640 ,  642  may contain DL compressed CSI information instead of DL compressed beamforming feedback information. Further, each STA, such as STA 2   604 , STA 1   624 , STA 2   626 , transmits multiple SSs over an EHT-LTF field of an EHT PPDU containing EHT Compressed Beamforming/CQI frame  614 ,  640 ,  642  where the EHT-LTF field is used for UL CSI estimation for data demodulation and calibration. In this way, AP  602 ,  622  may be able to determine calibration coefficient for each of its TX antennas according to the DL compressed CSI information included in the EHT Compressed Beamforming/CQI frame  614 ,  640 ,  642  and UL CSI estimation. 
       FIG.  7 A  depicts a flow chart illustrating a single-AP based implicit sequential sounding procedure  700  between an AP  702  and multiple STAs  704 ,  706  in an 11be EHT WLAN according to an embodiment (option 1). The single-AP based implicit sequential sounding procedure  700  may start when the AP  702  transmits a first EHT NDP Announcement frame  708  to intended STAs such as STA 1   704  and STA 2   706 . The first EHT NDP Announcement frame  708  may indicate STA ordering in which the intended STAs, for example STA 1   704  to STA 2   706 , may transmit an EHT Sounding NDP to the AP  702 . The first EHT NDP Announcement frame  708  may also indicate sounding RU allocation and sounding SS allocation for the first STA of the STA ordering. After receiving the EHT NDP Announcement frame  708 , other STA(s) that are not the first STA of the STA ordering, for example STA 2   706 , may be switched from Awake state to Doze state for power save. At  711  after the SIFS  709 , the first STA of the STA ordering, for example STA 1   704 , may prepare a first EHT Sounding NDP  712  based on its sounding RU allocation and sounding SS allocation and transmit it to AP  702 . The AP  702  may then estimate first UL CSI from the received first EHT Sounding NDP  712  and determine corresponding DL CSI for the STA 1   704  by compensating the first UL CSI according to the calibration parameters which were obtained through a calibration procedure. 
     After the last symbol of the first EHT Sounding NDP  712  is transmitted at  713 , STA 1   704  may switch from Awake state to Doze state for power save. During the SIFS  714  at  715 , the next STA in the STA ordering, for example STA 2   706 , may switch from Doze state back to Awake state. After the SIFS  714  at  716 , the AP  702  may transmit a second EHT NDP Announcement frame  718 , which may indicate sounding RU allocation and sounding SS allocation for the next STA of the STA ordering. Upon transmission of the second EHT NDP Announcement frame  718 , a SIFS  719  may take effect, and at  720 , the STA 2   706  may prepare a second EHT Sounding NDP  722  based on its sounding RU allocation and sounding SS allocation and transmit it to AP  702 . The AP  702  may then estimate second UL CSI from the received second EHT Sounding NDP  722  and determine corresponding DL CSI for the STA 2   706  by compensating the second UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSIs for the STA 1   704  and STA 2   706 , the AP  702  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   704  and STA 2   706  for subsequent transmissions to the STA 1   704  and/or STA 2   706 . Advantageously, single-AP based implicit sequential sounding procedure as illustrated in  FIG.  7 A  may require less sounding overhead than single-AP based explicit sounding procedure as illustrated in  FIG.  6 B  since sounding feedback information needs not to be transmitted. 
       FIG.  7 B  depicts a flow chart illustrating a single-AP based implicit sequential sounding procedure  730  between an AP  732  and multiple STAs  734 ,  736  in an 11be EHT WLAN according to another embodiment (option 2). Similarly, the single-AP based implicit sequential sounding procedure  730  may start when the AP  732  transmits an EHT NDP Announcement frame  738  to intended STAs such as STA 1   734  and STA 2   736 . The EHT NDP Announcement frame  738  may indicate STA ordering in which the intended STAs, for example STA 1   734  to STA 2   736 , may transmit an EHT Sounding NDP to the AP  732 . The EHT NDP Announcement frame  738  may indicate sounding RU allocation and sounding SS allocation for each of the intended STAs. As such, the EHT-LTF field duration of an EHT Sounding NDP (equivalent to transmission time of the EHT Sounding NDP) transmitted by each STA can be determined from the sounding SS allocation for the STA. Based on the transmission time of an EHT Sounding NDP of each STA, a first EHT Sounding NDP from the first STA of the STA ordering can be followed by a second EHT Sounding NDP from the second STA of the STA ordering. 
     For example, upon transmission of the EHT NDP Announcement frame  738 , a SIFS  739  may take effect, during the SIFS  739  at  740 , other STA that are not the first STA in the STA ordering, for example STA 2   706 , may be switched from Awake state to Doze state for power save, and after the SIFS at  741 , the first STA of the STA ordering, for example STA 1   734 , may transmit a first EHT Sounding NDP  742  to AP  732 . The AP  732  may then estimate first UL CSI from the received first EHT Sounding NDP  742  and determine corresponding DL CSI for STA 1   734  by compensating the first UL CSI according to the calibration parameters which were obtained through a calibration procedure. After the last symbol of the first EHT Sounding NDP  742  is transmitted at  743 , STA 1   734  may switch from Awake state to Doze state for power save. A SIFS  744  may take effect, during the SIFS  744  at  745 , the STA 2   736  may switch from Doze state to Awake state, and after the SIFS  744  at  746 , the STA 2   736  may transmit a second EHT Sounding NDP  748  to AP  732 . The AP  732  may then estimate second UL CSI from the received EHT Sounding NDP  748  and determine corresponding DL CSI for the STA 2   736  by compensating the second UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSIs for the STA 1   734  and STA 2   736 , the AP  732  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   734  and STA 2   736  for subsequent transmissions to the STA 1   734  and/or STA 2   736 . Advantageously, single-AP based implicit sequential sounding option 2 as illustrated in  FIG.  7 B  further reduces sounding overhead than single-AP based implicit sequential sounding option 1 as illustrated in  FIG.  7 A . 
       FIG.  8    depicts a flow chart illustrating a single-AP based implicit joint sounding procedure  800  between an AP  802  and multiple STAs  804 ,  806  in an 11be EHT WLAN according to an embodiment. The single-AP based implicit joint sounding procedure  800  may start when the AP  802  transmits an EHT NDP Announcement frame  808  to intended STAs such as STA 1   804  and STA 2   806 . The EHT NDP Announcement frame  808  may indicate sounding RU allocation and sounding SS allocation for each of intended STAs. Upon transmission of the EHT NDP Announcement frame  808 , a SIFS  809  may take effect, and at  810 , STA 1   804  and STA 2   806  may transmit respective EHT Sounding NDPs  812 ,  814  to AP  808  according to respective sounding RU allocation and sounding SS allocation. The AP  802  may then estimate first UL CSI from the received EHT Sounding NDP  812  and second UL CSI from the received EHT Sounding NDP  814  and determine corresponding DL CSIs for the STA 1   804  and STA 2   806  respectively by compensating the first UL CSI and the second UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSIs for the STA 1   804  and STA 2   806 , the AP  802  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   804  and STA 2   806  for subsequent transmissions to the STA 1   804  and/or STA 2   806 . Advantageously, single-AP based implicit joint sounding as illustrated in  FIG.  8    may provide less sounding overhead than single-AP based implicit sequential sounding as illustrated in  FIG.  7 A  and  FIG.  7 B . 
       FIG.  9    depicts a flow chart illustrating a multi-AP based explicit sequential sounding procedure  900  between multiple APs  902 ,  904  and multiple STAs  906 ,  908  in an 11be EHT WLAN according to an embodiment. The multi-AP based explicit sequential sounding procedure  900  may start when a sharing AP  902  transmits a first EHT NDP Announcement frame  910  to shared AP(s) that participate into the sounding procedure  1000  and intended STAs such as STA 1   906  and STA 2   908 . The first EHT NDP Announcement frame  910  may indicate shared AP ordering in which the shared AP(s) engaged in the sounding procedure  900  may transmit an EHT NDP Announcement frame and an EHT Sounding NDP to the intended STAs such as STA 1   906  and STA 2   908 . The first EHT NDP Announcement frame  910  may indicate sounding RU allocation and sounding SS allocation for each of shared AP(s) engaged in the sounding procedure  900 . Each EHT NDP Announcement frame may indicate requested sounding feedback parameters for each STA such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns of compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback, CQI feedback and calibration feedback. When the feedback type is SU feedback or MU feedback, the requested sounding feedback information comprises compressed beamforming feedback information per subcarrier or subcarrier group. When the feedback type is CQI feedback, the requested sounding feedback information comprises CQI information per subcarrier or subcarrier group. When the feedback type is calibration feedback, the requested sounding feedback information comprises compressed CSI per subcarrier or subcarrier group. 
     Upon transmission of the first EHT NDP Announcement frame  910 , a SIFS  911  may take effect, and at  912 , the sharing AP  902  may transmit a first EHT Sounding NDP  914  to STA 1   906  and STA 2   908 . After the first EHT Sounding NDP  914  is transmitted, a SIFS  915  may take effect, and at  916 , the sharing AP  902  may transmit an EHT BFRP Trigger frame  918  to solicit simultaneous transmissions of sounding feedback information from STA 1   906  and STA 2   908 . After the last symbol of the EHT BFRP Trigger frame  918  is transmitted, a SIFS  919  may take effect, and at  920 , the STA 1   906  and STA 2   908  may simultaneously transmit respective first EHT Compressed Beamforming/CQI frames  922 ,  924  comprising sounding feedback information to the sharing AP  902 . Based on the received sounding feedback information from the STA 1   906  and STA 2   908 , the sharing AP  902  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   906  and STA 2   908  for subsequent transmissions to the STA 1   906  and/or STA 2   908 . 
     Upon transmission of the first EHT Compressed Beamforming/CQI frames  922 ,  924 , a SIFS  925  may take effect, and at  926 , the next AP in the AP ordering, for example shared AP  904 , may transmit a second EHT NDP Announcement frame  928  to STA 1   906  and STA 2   908 . Upon transmission of the second EHT NDP Announcement frame  928 , a SIFS  929  may take effect, and at  930 , the shared AP  904  may transmit a second EHT Sounding NDP  932  to STA 1   906  and STA 2   908 . Similarly, after the second EHT Sounding NDP  932  is transmitted, a SIFS  933  may take effect, and at  934 , the shared AP  904  may transmit an EHT BFRP Trigger frame  918  to solicit simultaneous transmissions of sounding feedback information from STA 1   906  and STA 2   908 . After the last symbol of the EHT BFRP Trigger frame  936  is transmitted, a SIFS  937  may take effect, and at  938 , the STA 1   906  and STA 2   908  may simultaneously transmit respective second EHT Compressed Beamforming/CQI frames  940 ,  942  comprising sounding feedback information to shared AP  904 . Based on the received sounding feedback information from the STA 1   906  and STA 2   908 , the shared AP  904  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   906  and STA 2   908  for subsequent transmissions to the STA 1   906  and/or STA 2   908 . 
     According to the present disclosure, when the sounding type indicated in the EHT NDP Announcement frame  910 ,  1010  is multi-AP based explicit sounding and the feedback type indicated in the EHT NDP Announcement frame  910 ,  1010  is SU, MU or CQI feedback, the procedure  900 ,  1000  is a normal multi-AP based explicit sounding procedure; whereas when the sounding type indicated in the EHT NDP Announcement frame  910 ,  1010  is multi-AP based explicit sounding and the feedback type indicated in the EHT NDP Announcement frame  910 ,  1010  is calibration feedback, the procedure  900 ,  1000  is a multi-AP based calibration procedure. In other words, multi-AP based calibration procedure is a variant of multi-AP based explicit sounding procedure as illustrated in  FIG.  9    and  FIG.  10   . Advantageously, according to the present disclosure, a single procedure  900 ,  1000  can be used for the purpose of both calibration and explicit sounding. 
     In various embodiments, when the procedure  900 ,  1000  is a multi-AP based calibration procedure, the EHT NDP Announcement frame  910 ,  928 ,  1010  may indicate calibration RU allocation and calibration SS allocation for each STA, such as STA 1   906 , STA 2   908 , STA 1   1006 , STA 2   1008 . In particular, data RU allocation and data SS allocation for each STA are indicated in the corresponding EHT BFRP Trigger frame  918 ,  936 ,  1020 . Calibration SSs allocated to a STA may include data SSs allocated to the STA. In other words, number of calibration SSs may be equal to or larger than number of data SSs. Calibration RUs allocated to a STA may be the same as data RUs allocated to the STA. 
     In various embodiments, when the procedure  900 ,  1000  is a multi-AP based calibration procedure, EHT Compressed Beamforming/CQI frame  922 ,  924 ,  940 ,  942 ,  1024 ,  1026  may contain DL compressed CSI instead of DL compressed beamforming feedback information. Further, each STA, such as STA 1   906 , STA 2   908 , STA 1   1006 , STA 2   1008 , transmits multiple SSs over an EHT-LTF field of an EHT PPDU containing EHT Compressed Beamforming/CQI frame  922 ,  924 ,  940 ,  942 ,  1024 ,  1026  where the EHT-LTF field is used for UL CSI estimation for data demodulation and calibration. In this way, AP  902 ,  904 ,  1002 ,  1004  may be able to determine calibration coefficient for each of its TX antennas according to the DL compressed CSI included in the EHT Compressed Beamforming/CQI frame  922 ,  924 ,  940 ,  942 ,  1024 ,  1026  and UL CSI estimation. 
     In various embodiments, there are two options for a STA to transmit an EHT Compressed Beamforming/CQI frame in a multi-AP based sequential calibration procedure: i) a STA may transmit same SSs over the EHT-LTF fields of multiple EHT PPDUs containing EHT Compressed Beamforming/CQI frames; or ii) a STA may transmit different SSs over the EHT-LTF fields of multiple EHT PPDUs containing EHT Compressed Beamforming/CQI frames, which as a result, EHT-LTF overhead may advantageously be reduced. For example, with an assumption of STA 1   906  or STA 2   908  has four TX antennas and transmits two EHT Compressed Beamforming/CQI frames, under option 1, the STA 1   904  and STA 2   908  may transmit four SSs over eight EHT-LTF symbols of EHT PPDUs containing EHT Compressed Beamforming/CQI frames  922 ,  924 ,  940 ,  942  using P 8×8  matrix; whereas under option 2, the STA 1   906  and STA 2   908  may transmit two SSs over four EHT-LTF symbols of EHT PPDUs containing first EHT Compressed Beamforming/CQI frames  922 ,  924  using P 4×4  matrix, and the STA 1   906  and STA 2   908  may transmit another two SSs over four EHT-LTF symbols of EHT PPDUs containing second EHT Compressed Beamforming/CQI frames  940 ,  942  using P 4×4  matrix. 
       FIG.  10    depicts a flow chart illustrating a multi-AP based explicit joint sounding procedure  1000  between multiple APs  1002 ,  1004  and multiple STAs  1006 ,  1008  in an 11be EHT WLAN according to an embodiment. The multi-AP based explicit joint sounding procedure  1000  may start when a sharing AP  1002  transmits an EHT NDP Announcement frame  1010  to shared AP(s) that participate into the sounding procedure  1000  and intended STAs such as STA 1   1006  and STA 2   1008 . The EHT NDP Announcement frame  1010  may indicate sounding RU allocation and sounding SS allocation for each shared AP that engaged in the sounding procedure  1000 . The EHT NDP Announcement frame  1010  may indicate requested sounding feedback parameters for each pair of STA and AP among sharing AP  1002 , shared AP  1004  and intended STAs  1006 ,  1008 . The requested sounding feedback parameters for each AP-STA pair may comprise AP dependent sounding feedback parameters such as feedback bandwidth and the number of columns of compressed beamforming feedback matrix; and AP independent sounding feedback parameters such as feedback type, subcarrier grouping and quantization resolution, where in term of sounding results used for joint beamforming, for a STA, each AP independent sounding feedback parameter may be set to a same value for all AP-STA pairs in the EHT NDP Announcement frame  1010 ; whereas each AP dependent sounding feedback parameter may be set to a different value for each AP-STA pair in the EHT NDP Announcement frame  1010 . 
     Upon transmission of the EHT NDP Announcement frame  1010 , a SIFS  1011  may take effect, and at  1012 , all APs engaged in the sounding procedure  1000  such as sharing AP  1002  and shared AP  1004  may simultaneously transmit respective EHT Sounding NDPs  1014 ,  1016  to all intended STAs such as STA 1   1006  and STA 2   1008 . After the EHT Sounding NDPs  1014 ,  1016  are transmitted, a SIFS  1017  may take effect, and at  1018 , the AP  1002  may transmit an EHT BFRP Trigger frame  1020  to solicit simultaneous transmissions of sounding feedback information from STA 1   1006  and STA 2   1008 . After the last symbol of the EHT BFRP Trigger frame  1020  is transmitted, a SIFS  1021  may take effect, and at  1022 , the STA 1   1006  and STA 2   1008  may simultaneously transmit respective EHT Compressed Beamforming/CQI frames  1024 ,  1026  comprising the requested sounding feedback information to APs  1002 ,  1004 . Based on the received sounding feedback information from the STA 1   1006  and STA 2   1008 , the AP  1002 ,  1004  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1006  and STA 2   1008  for subsequent transmissions to the STA 1   1006  and/or STA 2   1008 . Alternatively, the sharing AP  1002  may be able to determine a steering matrix and/or allocate appropriate RUs for each AP-STA pair. The sharing AP  1002  may then inform shared AP  1004  of the corresponding steering matrices and/or RU allocations for the STA 1   1006  and STA 2   1008  for subsequent transmissions to the STA 1   1006  and/or STA 2   1008 . Advantageously, multi-AP based explicit joint sounding as illustrated in  FIG.  10    has less sounding overhead than multi-AP based explicit sequential sounding as illustrated in  FIG.  9   . 
       FIG.  11 A  depicts a flow chart illustrating a multi-AP based implicit sequential sounding procedure  1100  between multiples APs  1102 ,  1104  and multiple STAs  1106 ,  1108  in an 11be EHT WLAN according to an embodiment (option 1). The multi-AP based implicit sequential sounding procedure  1100  may start when a sharing AP  1102  transmits a first EHT NDP Announcement frame  1110  to shared AP(s) that participate into the sounding procedure  1100  and intended STAs such as STA 1   1106  and STA 2   1108 . The first EHT NDP Announcement frame  1110  may indicate shared AP(s) that participate into the sounding procedure  1100 , e.g. shared AP  1104 . The first EHT NDP Announcement frame  1110  may also indicate STA ordering in which the intended STAs, for example STA 1   1106  to STA 2   1108 , may transmit an EHT Sounding NDP to the sharing AP  1102  and all the shared APs that participate into the sounding procedure  1100 . Each EHT NDP Announcement frame  1110 ,  1120  may indicate sounding RU allocation and sounding SS allocation for a corresponding STA. Upon transmission of the first EHT NDP Announcement frame  1110 , a SIFS  1111  may take effect. During the SIFS  1111  at  1112 , other STA(s) that are not the first STA of the STA ordering, for example STA 2   1108 , may be switched from Awake state to Doze state for power save, and after the SIFS at  1113 , the first STA of the STA ordering, for example STA 1   1106 , may transmit a first EHT Sounding NDP  1114  to sharing AP  1102  and shared AP  1104 . The sharing AP  1102  and shared AP  1104  may then estimate UL CSI from the received first EHT Sounding NDP  1114  and determine DL CSI for the STA 1   1106  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSI for the STA 1   1106 , the sharing AP  1102  and shared AP  1104  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA 1   1106  for subsequent transmissions to the STA 1   1106 . 
     After the last symbol of the first EHT Sounding NDP  1114  is transmitted at  1115 , STA 1   1106  may switch from Awake state to Doze state for power save. A SIFS  1116  may take effect, during the SIFS  1116  at  1117 , the next STA in the STA ordering, for example STA 2   1108 , may switch from Doze state to Awake state, and after the SIFS  1116  at  1118 , the AP  1102  may transmit a second EHT NDP Announcement frame  1120 . Upon transmission of the second EHT NDP Announcement frame  1120 , a SIFS  1121  may take effect, and at  1122 , the STA 2   1108  may transmit a second EHT Sounding NDP  1124  to sharing AP  1102  and shared AP  1104 . The sharing AP  1102  and shared AP  1104  then estimate UL CSI from the received second EHT Sounding NDP  1124  and determines DL CSI for the STA 2   1108  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSI for STA 2   1108 , the sharing AP  1102  and shared AP  1104  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA 2   1108  for subsequent transmissions to the STA 2   1108 . Advantageously, multi-AP based implicit sequential sounding procedure as illustrated in  FIG.  11 A  may require less sounding overhead than multi-AP based explicit sounding procedure as illustrated in  FIG.  9    or  FIG.  10    since sounding feedback information needs not to be transmitted. 
       FIG.  11 B  depicts a flow chart illustrating a multi-AP based implicit sequential sounding procedure  1130  between multiples APs  1132 ,  1134  and multiple STAs  1136 ,  1138  in an 11be EHT WLAN according to another embodiment (option 2). Similarly, the multi-AP based implicit sequential sounding procedure  1130  may start when the AP  1132  transmits an EHT NDP Announcement frame  1140  to shared AP(s) that participate into the sounding procedure  1130  and intended STAs such as STA 1   1136  and STA 2   1138 . The EHT NDP Announcement frame  1140  indicates shared AP(s) that participate into the sounding procedure  1130 , e.g. shared AP  1134 . The EHT NDP Announcement frame  1140  may indicate STA ordering in which the intended STAs, for example STA 1   1136  to STA 2   1138 , may transmit an EHT Sounding NDP to the AP  1132  and shared AP(s) that participate into the sounding procedure  1130 . The EHT NDP Announcement frame  1140  may further indicate sounding RU allocation and sounding SS allocation for each STA. As such, the EHT-LTF field duration of an EHT Sounding NDP (equivalent to transmission time of an EHT Sounding NDP) transmitted by each STA can be determined from the SS allocation for the STA. Based on the transmission time of an EHT Sounding NDP of each STA, a first EHT Sounding NDP from the first STA of the STA ordering can be followed by a second EHT Sounding NDP from the second STA of the STA ordering. 
     For example, upon transmission of the EHT NDP Announcement frame  1140 , a SIFS  1141  may take effect, during the SIFS  1141  at  1142 , other STA that are not the first STA in the STA ordering, for example STA 2   1138 , may be switched from Awake state to Doze state for power save, and after the SIFS at  1143 , the first STA of the STA ordering, for example STA 1   1136 , may transmit a first EHT Sounding NDP  1144  to the sharing AP  1132  and shared AP  1134 . The sharing AP  1132  and shared AP  1134  may then estimate UL CSI from the received first EHT Sounding NDP  1144  and determine DL CSI for the STA 1   1136  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure. After the last symbol of the first EHT Sounding NDP  1144  is transmitted at  1145 , STA 1   1136  may switch from Awake state to Doze state for power save. A SIFS  1148  may take effect, during the SIFS  1148  at  1147 , the STA 2   1138  may switch from Doze state back to Awake state, and after the SIFS  1146  at  1148 , the STA 2   1138  may transmit a second EHT Sounding NDP  1150  to the sharing AP  1132  and shared AP  1134 . The sharing AP  1132  and shared AP  1134  may then estimate UL CSI from the received second EHT Sounding NDP  1150  and determines DL CSI for the STA 2   1138  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSIs for the STA 1   1136  and STA 2   1138 , the sharing AP  1132  and shared AP  1134  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1136  and STA 2   1138  for subsequent transmissions to the STA 1   1136  and/or STA 2   1138 . Advantageously, multi-AP based implicit sequential sounding option 2 illustrated in  FIG.  11 B  further reduces sounding overhead than multi-AP based implicit sequential sounding option 1 illustrated in  FIG.  11 A . 
       FIG.  12    depicts a flow chart illustrating a multi-AP based implicit joint sounding procedure  1200  between multiple APs  1202 ,  1204  and multiple STAs  1206 ,  1208  in an 11be EHT WLAN according to an embodiment. The multi-AP based implicit joint sounding procedure  1200  may start when sharing AP  1202  transmits an EHT NDP Announcement frame  1210  to shared AP(s) that participate into the sounding procedure  1200  such as shared AP  1204  and intended STAs such as STA 1   1206  and STA 2   1208 . The EHT NDP Announcement frame  1210  may indicate shared AP(s) that participate into the sounding procedure  1200 , and may also indicate sounding RU allocation and sounding SS allocation for each STA. Upon transmission of the EHT NDP Announcement frame  1210 , a SIFS  1211  may take effect, and at  1212 , STA 1   1206  and STA 2   1208  may transmit respective EHT Sounding NDPs  1214 ,  1216  to sharing AP  1202  and shared AP  1204 . The sharing AP  1202  and shared AP  1204  may then estimate first UL CSI from the received EHT Sounding NDP  1214  and determine DL CSI for the STA 1   1206  by compensating the first UL CSI according to the calibration parameters which were obtained through a calibration procedure. Similarly, the sharing AP  1202  and shared AP  1204  may estimate second UL CSI from the received EHT Sounding NDP  1216  and determine DL CSI for the STA 2   1208  by compensating the second UL CSI according to the calibration parameters which were obtained through a calibration procedure. Further, based on the DL CSIs for the STA 1   1206  and STA 2   1208 , the sharing AP  1202  and shared AP  1204  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1206  and STA 2   1208  for subsequent transmissions to the STA 1   1206  and/or STA 2   1208 . Advantageously, multi-AP based implicit joint sounding as illustrated in  FIG.  12    may require less sounding overhead than multi-AP based implicit sequential sounding as illustrated in  FIG.  11 A  and  FIG.  11 B . 
     According to the present disclosure, sharing AP and shared AP(s) that are engaged in a multi-AP based hybrid sounding procedure are divided into two groups, where group 1 APs comprises AP(s) that participate into an explicit sounding portion of the multi-AP based hybrid sounding procedure and group 2 APs comprises AP(s) that participate into an implicit sounding portion of the multi-AP based hybrid sounding procedure. 
       FIG.  13 A  depicts a flow chart illustrating a multi-AP based hybrid sequential sounding procedure  1300  between multiple APs  1302 ,  1304 ,  1306  and a STA  1308  in an 11be EHT WLAN according to an embodiment. In this embodiment, sharing AP  1302  and shared AP 1   1304  are group 1 APs, whereas shared AP 2   1306  is a group 2 AP. The multi-AP base hybrid sequential sounding procedure  1300  may start when the sharing AP  1302  transmits a first EHT NDP Announcement frame  1310  to all shared AP(s) that participate into the sounding procedure  1300  (e.g. shared APs  1304 ,  1306 ) and intended STA  1308 . The first EHT NDP Announcement frame  1310  may indicate group 2 APs and ordering of group 1 APs in which each of the group 1 APs may transmit an EHT NDP Announcement frame and an EHT Sounding NDP to intended STA. The first EHT NDP Announcement frame  1310  may indicate sounding RU allocation and sounding SS allocation for each of group 1 APs. Each EHT NDP Announcement frame may indicate sounding RU allocation and sounding SS allocation for the intended STA  1308 . Each EHT NDP Announcement frame may indicate request sounding feedback parameters for the intended STA  1308  such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns in compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI feedback. Upon transmission of the first EHT NDP Announcement frame  1310 , a SIFS  1311  may take effect, and at  1312 , sharing AP  1302  may transmit a first EHT Sounding NDP  1314  to STA  1308 . After the last symbol of the first EHT Sounding NDP  1314  is transmitted, a SIFS  1315  may take effect, and at  1316 , the STA  1308  transmits a first EHT Compressed Beamforming/CQI frame  1318  comprising sounding feedback information to the sharing AP  1302 . It is noted that the EHT-LTF field of EHT PPDU containing the first EHT Compressed Beamforming/CQI frame  1318  can also be used for CSI estimation for implicit sounding with group 2 AP(s), e.g. shared AP 2   1306 . Based on the received sounding feedback information from STA  1308 , sharing AP  1302  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1308  for subsequent transmissions to the STA  1308 . 
     Upon transmission of the first EHT Compressed Beamforming/CQI frame  1318 , a SIFS  1319  may take effect, and at  1320 , the next AP in the ordering of group 1 APs, for example shared AP 1   1304 , may transmit a second EHT NDP Announcement frame  1322  to STA  1308 . Upon transmission of the second EHT NDP Announcement frame  1322 , a SIFS  1323  may take effect, and at  1324 , the shared AP 1   1304  may transmit a second EHT Sounding NDP  1326  to STA  1308 . Similarly, after the second EHT Sounding NDP  1326  is transmitted, a SIFS  1327  may take effect, and at  1328 , the STA  1308  may transmit a second EHT Compressed Beamforming/CQI frame  1330  comprising sounding feedback information to the shared AP 1   1304 . It is noted that the EHT-LTF field of EHT PPDU containing the second EHT Compressed Beamforming/CQI frame  1330  can also be used for implicit sounding with group 2 AP(s), e.g. shared AP 2   1306 . Based on the received sounding feedback information from STA  1308 , sharing AP  1304  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1308  for subsequent transmissions to the STA  1308 . Further, shared AP 2   1306  may be able to estimate UL CSI from the EHT-LTF fields of EHT PPDUs containing the first and second EHT Compressed Beamforming/CQI frame  1318 ,  1330  and determine corresponding DL CSI for the STA  1308  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then shared AP  1306  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1308  based on the DL CSI for subsequent transmissions to the STA  1308 . 
       FIG.  13 B  depicts a flow chart illustrating a multi-AP based hybrid sequential sounding procedure  1340  between multiple APs  1342 ,  1344 ,  1346  and multiple STAs  1348 ,  1350  in an 11be EHT WLAN according to an embodiment. In this embodiment, sharing AP  1342  and shared AP 1   1344  are group 1 APs, whereas shared AP 2   1346  is a group 2 AP. The multi-AP base hybrid sequential sounding procedure  1340  may start when the sharing AP  1342  transmits a first EHT NDP Announcement frame  1352  to all shared AP(s) that participate into the sounding procedure  1340  (e.g. shared APs  1344 ,  1346 ) and intended STAs  1348 ,  1350 . The first EHT NDP Announcement frame  1352  may indicate group 2 APs and ordering of group 1 APs in which each of the group 1 APs may transmit an EHT NDP Announcement frame and an EHT Sounding NDP to intended STA. The first EHT NDP Announcement frame  1352  may indicate sounding RU allocation and sounding SS allocation for each of group 1 APs. Each EHT NDP Announcement frame may indicate sounding RU allocation and sounding SS allocation for each of the STA 1   1348  and STA 2   1350 . In particular, data RU allocation and data SS allocation for each of the STA 1   1348  and STA 2   1350  are indicated in the corresponding EHT BFRP Trigger frame. Sounding SSs may include data SSs. In other words, the number of sounding SSs may be equal to or larger than number of data SSs. Data RU allocation may be the same as sounding RU allocation for each STA. Each EHT NDP Announcement frame may indicate request sounding feedback parameters for each of the intended STAs  1348 ,  1350  such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns in compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI feedback. Upon transmission of the first EHT NDP Announcement frame  1352 , a SIFS  1353  may take effect, and at  1354 , sharing AP  1342  may transmit a first EHT Sounding NDP  1356  to STA 1   1348  and STA 2   1350 . After the last symbol of the first EHT Sounding NDP  1356  is transmitted, a SIFS  1357  may take effect, and at  1358 , the sharing AP  1342  may transmit an EHT BFRP Trigger frame  1360  to solicit simultaneous transmissions of sounding feedback information from the STA 1   1348  and STA 2   1350 . Upon reception of the EHT BFRP Trigger frame  1360 , a SIFS  1361  may take effect, and at  1362 , the STA 1   1348  and STA 2   1350  simultaneously transmit respective first EHT Compressed Beamforming/CQI frames  1364 ,  1366  comprising sounding feedback information to the sharing AP  1342 . It is noted that the EHT-LTF fields of EHT PPDUs containing the first EHT Compressed Beamforming/CQI frames  1364 ,  1366  can also be used for CSI estimation for implicit sounding with group 2 AP(s), e.g. shared AP 2   1346 . Based on the received sounding feedback information from STA 1   1348  and STA 2   1350 , sharing AP  1342  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1348  and STA 2   1350  for subsequent transmissions to the STA 1   1348  and/or STA 2   1350 . 
     Upon transmission of the first EHT Compressed Beamforming/CQI frame  1364 ,  1366 , a SIFS  1367  may take effect, and at  1368 , the next AP in the ordering of group 1 APs, for example shared AP 1   1344 , may transmit a second EHT NDP Announcement frame  1370  to STA 1   1348  and STA 2   1350 . Upon transmission of the second EHT NDP Announcement frame  1370 , a SIFS  1371  may take effect, and at  1372 , the shared AP 1   1344  may transmit a second EHT Sounding NDP  1374  to STA 1   1348  and STA 2   1350 . Similarly, after the second EHT Sounding NDP  1374  is transmitted, a SIFS  1375  may take effect, and at  1376 , the sharing AP  1342  may transmit an EHT BFRP Trigger frame  1378  to solicit simultaneous transmissions of sounding feedback information from the STA 1   1348  and STA 2   1350 . Upon reception of the EHT BFRP Trigger frame  1378 , a SIFS  1379  may take effect, and at  1380 , the STA 1   1348  and STA 2   1350  simultaneously transmit respective second EHT Compressed Beamforming/CQI frames  1382 ,  1384  comprising sounding feedback information to the sharing AP  1344 . It is noted that the EHT-LTF fields of EHT PPDUs containing the second EHT Compressed Beamforming/CQI frames  1382 ,  1384  can also be used for CSI estimation for implicit sounding with group 2 AP(s), e.g. shared AP 2   1346 . Based on the received sounding feedback information from STA 1   1348  and STA 2   1350 , the shared AP 1   1344  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1348  and STA 2   1350  for subsequent transmissions to the STA 1   1348  and/or STA 2   1350 . Further, the shared AP 2   1346  may be able to estimate UL CSI from the EHT-LTF fields of EHT PPDUs containing the first and second EHT Compressed Beamforming/CQI frame  1364 ,  1382  and determine DL CSI for the STA 1   1348  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then the shared AP 2   1346  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA 1   1348  based on the DL CSI for subsequent transmissions to the STA 1   1348 . Similarly, the shared AP 2   1346  may be able to estimate UL CSI from the EHT-LTF fields of EHT PPDUs containing the first and second EHT Compressed Beamforming/CQI frame  1366 ,  1384  and determine DL CSI for the STA 2   1350  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then shared AP 2   1346  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA 2   1350  based on the DL CSI for subsequent transmissions to the STA 2   1350 . 
       FIG.  14 A  depicts a flow chart illustrating a multi-AP based hybrid joint sounding procedure  1400  between multiple APs  1402 ,  1404 ,  1406  and a STA  1408  in an 11be EHT WLAN according to an embodiment. In this embodiment, sharing AP  1402  and shared AP 1   1404  are group 1 APs, whereas shared AP 2   1406  is a group 2 AP. The multi-AP base hybrid sequential sounding procedure  1400  may start when the sharing AP  1402  transmits an EHT NDP Announcement frame  1410  to all shared AP(s) that participate into the sounding procedure  1400  (e.g. shared APs  1404 ,  1406 ) and intended STA  1408 . The EHT NDP Announcement frame  1410  may indicate group 2 APs and group 1 APs in which each of the group 1 APs may transmit an EHT Sounding NDP to intended STA. The EHT NDP Announcement frame  1410  may indicate sounding RU allocation and sounding SS allocation for each of group 1 APs. The EHT NDP Announcement frame may indicate sounding RU allocation and sounding SS allocation for the intended STA  1408 . The EHT NDP Announcement frame may indicate request sounding feedback parameters for each pair of the intended STA  1408  and group 1 AP such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns in compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI feedback. Upon transmission of the EHT NDP Announcement frame  1410 , a SIFS  1411  may take effect, and at  1412 , sharing AP  1402  and shared AP  1404  may simultaneously transmit respective EHT Sounding NDP  1414 ,  1416  to STA  1408 . After the last symbols of the EHT Sounding NDPs  1414 ,  1416  are transmitted, a SIFS  1417  may take effect, and at  1418 , the STA  1408  transmits an EHT Compressed Beamforming/CQI frame  1419  comprising sounding feedback information to the sharing AP  1402  and shared AP 1   1404 . It is noted that the EHT-LTF field of EHT PPDU containing the EHT Compressed Beamforming/CQI frame  1419  can also be used for CSI estimation for implicit sounding with group 2 AP(s), e.g. shared AP 2   1406 . Based on the received sounding feedback information from the STA  1408 , the sharing AP  1402  and shared AP 1   1404  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1408  for subsequent transmissions to the STA  1408 . Further, the shared AP 2   1406  may be able to estimate UL CSI from the EHT-LTF field of EHT PPDU containing the EHT Compressed Beamforming/CQI frame  1419  and determine DL CSI for the STA  1408  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then the shared AP 2   1406  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1408  based on the DL CSI for subsequent transmissions to the STA  1408 . 
       FIG.  14 B  depicts a flow chart illustrating a multi-AP based hybrid joint sounding procedure  1420  between multiple APs  1422 ,  1424 ,  1426  and multiple STAs  1428 ,  1430  in an 11be EHT WLAN according to an embodiment. In this embodiment, sharing AP  1422  and shared AP 1   1424  are group 1 APs, whereas shared AP 2   1426  is a group 2 AP. The multi-AP base hybrid joint sounding procedure  1420  may start when the sharing AP  1422  transmits an EHT NDP Announcement frame  1432  to all shared AP(s) that participate into the sounding procedure  1420  (e.g. shared APs  1424 ,  1426 ) and intended STAs  1428 ,  1430 . The EHT NDP Announcement frame  1432  may indicate group 2 APs and group 1 APs in which each of the group 1 APs may transmit an EHT Sounding NDP to intended STA 1   1428  and STA 2   1430 . The EHT NDP Announcement frame  1432  may indicate sounding RU allocation and sounding SS allocation for each of group 1 APs. The EHT NDP Announcement frame  1432  may indicate sounding RU allocation and sounding SS allocation for each of the STA 1   1428  and STA 2   1430 . In particular, data RU allocation and data SS allocation for each of the STA 1   1428  and STA 2   1430  are indicated in the corresponding EHT BFRP Trigger frame  1442 . Sounding SSs may include data SSs. In other words, the number of sounding SSs may be equal to or larger than number of data SSs. Data RU allocation may be the same as sounding RU allocation for each STA. The EHT NDP Announcement frame  1432  may indicate request sounding feedback parameters for each of the intended STAs  1428 ,  1430  such as feedback bandwidth, feedback type, subcarrier grouping, quantization resolution and number of columns in compressed beamforming feedback matrix. The feedback type is one of SU feedback, MU feedback and CQI feedback. Upon transmission of the EHT NDP Announcement frame  1432 , a SIFS  1433  may take effect, and at  1434 , sharing AP  1422  and shared AP 1   1424  may transmit respective EHT Sounding NDPs  1436 ,  1438  to STA 1   1428  and STA 2   1430 . After the last symbols of the EHT Sounding NDPs  1436 ,  1438  are transmitted, a SIFS  1439  may take effect, and at  1440 , the sharing AP  1422  may transmit an EHT BFRP Trigger frame  1442  to solicit simultaneous transmissions of sounding feedback information from the STA 1   1428  and STA 2   1430 . Upon reception of the EHT BFRP Trigger frame  1442 , a SIFS  1443  may take effect, and at  1444 , the STA 1   1428  and STA 2   1430  simultaneously transmit respective EHT Compressed Beamforming/CQI frames  1446 ,  1448  comprising sounding feedback information to the sharing AP  1422  and shared AP 1   1424 . It is noted that the EHT-LTF fields of EHT PPDUs containing the EHT Compressed Beamforming/CQI frames  1446 ,  1448  can also be used for CSI estimation for implicit sounding with group 2 AP(s), e.g. shared AP 2   1426 . Based on the received sounding feedback information from the STA 1   1428  and STA 2   1430 , the sharing AP  1422  and shared AP 1   1424  may be able to determine a steering matrix and/or allocate appropriate RUs for each of the STA 1   1428  and STA 2   1430  for subsequent transmissions to the STA 1   1428  and/or STA 2   1430 . Further, the shared AP 2   1426  may be able to estimate UL CSI from the EHT-LTF field of EHT PPDU containing the EHT Compressed Beamforming/CQI frame  1446  and determine DL CSI for the STA 1   1428  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then the shared AP 2   1426  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA 1   1428  based on the DL CSI for subsequent transmissions to the STA 1   1428 . Similarly, the shared AP 2   1426  may be able to estimate UL CSI from the EHT-LTF field of EHT PPDU containing the EHT Compressed Beamforming/CQI frame  1448  and determine DL CSI for the STA  1430  by compensating the UL CSI according to the calibration parameters which were obtained through a calibration procedure; and then the shared AP 2   1426  may be able to determine a steering matrix and/or allocate appropriate RUs for the STA  1430  based on the DL CSI for subsequent transmissions to the STA  1430 . 
       FIG.  15    depicts an example format of an EHT NDP Announcement frame  1500 . The EHT NDP Announcement frame  1500  may include (or consist of) a Frame Control field, a Duration field, a RA (recipient STA address) field, a TA (transmitting STA address) field, a Sounding Dialog Token field, a Sounding Type field  1502  and an AP-STA Info field  1504  and a FCS field. The Frame Control field, the Duration field, the RA field and the TA field may be grouped as MAC header. 
     When the Sounding Type field  1502  refers to single-AP based explicit sounding. The AP-STA Info field  1504 , for single-AP based explicit sounding, may comprise one or more STA Feedback Info field  1602  as shown in  FIG.  16 A . A STA Feedback Info field  1602  may include (or consist of) an Intended STA field, a Feedback Bandwidth field, a Feedback Type field an Subcarrier Grouping field, a Quantization Resolution field and a Number of Columns of Compressed Beamforming Feedback Matrix field. In an embodiment, a STA Feedback Info field is used to indicate requested sounding feedback parameters for a STA indicated in the Intended STA field. 
     When the Sounding Type field  1502  refers to single-AP based implicit sounding, the AP-STA Info field  1504 , for single-AP based implicit sounding, may comprise one or more STA Sounding Info field  1604  as shown in  FIG.  16 B . A STA Sounding Info field  1604  may include (or consist of) an Intended STA field, a STA Sounding RU Allocation field and a STA Sounding SS Allocation field. In an embodiment, a STA Sounding Info field is used to indicate sounding RU allocation and sounding SS allocation for a STA indicated in the Intended STA field. 
     When the Sounding Type field  1502  refers to multi-AP based explicit sounding, the AP-STA Info field  1504 , for multi-AP based explicit sounding, may comprise one or more AP-STA Explicit Sounding Info field  1606  as shown in  FIG.  16 C . An AP-STA Explicit Sounding Info field  1606  may include (or consist of) an AP Sounding Info field  1608  and one or more STA Feedback Info field  1602  as shown in  FIG.  16 A . The AP Sounding Info field  1608  may further include an Intended AP field, an AP Sounding RU Allocation field and an AP Sounding SS Allocation field. In an embodiment, an AP-STA Explicit Sounding Info field is used to indicate sounding RU allocation and sounding SS allocation for an AP indicated in the Intended AP field and the corresponding sounding feedback parameters for each STA. 
     When the Sounding Type field  1502  refers to multi-AP based implicit sounding, the AP-STA Info field  1504 , for multi-AP based implicit sounding, may comprise one or more Intended AP fields and one or more STA Sounding Info field  1604  as illustrated in  FIG.  16 B . In one embodiment, the one or more Intended AP fields are used to indicate shared AP(s) that participate into the multi-AP based implicit sounding and the one or more STA Sounding Info fields are used to indicate sounding RU allocation and sounding SS allocation for each intended STA. 
     Further, when the Sounding Type field  1502  refers to multi-AP based hybrid sounding, the AP-STA Info field  1504  may comprises one or more AP-STA Explicit Sounding Info field  1606  as shown in  FIG.  16 C  that indicates necessary information for explicit sounding portion of the multi-AP based hybrid sounding; one or more Intended AP fields and one or more STA Sounding Info field  1604  as shown in  FIG.  16 B  that indicate necessary information for implicit sounding portion of the multi-AP based hybrid sounding. 
       FIG.  17    shows a configuration of a communication device, for example an AP, according to the present disclosure. Similar to the schematic example of the communication apparatus  300  shown in  FIG.  3 A , the communication apparatus  1700  includes circuitry  1702 , at least one radio transmitter  1710 , at least one radio receiver  1712 , at least one antenna  1714  (for the sake of simplicity, only one antenna is depicted in  FIG.  17   ). The circuitry  1702  may include at least one controller  1708  for use in software and hardware aided execution of tasks that the controller  1708  is designed to perform communication for channel sounding. The circuitry  1702  may further include a transmission signal generator  1704  and a receive signal processor  1706 . The at least one controller  1708  may control the transmission signal generator  1704  and the receive signal processor  1706 . The transmission signal generator  1704  may include a frame generator  1722 , a control signaling generator  1724 , and a PPDU generator  1726 . The frame generator  1722  may generate MAC frames, e.g. EHT NDP Announcement frames, EHT Action frames or EHT BFRP Trigger frames. The control signaling generator  1724  may generate control signaling fields of PPDUs to be generated (e.g. EHT-SIG fields of EHT Sounding NDPs or EHT-SIG fields of EHT PPDUs comprising EHT NDP Announcement frames, EHT Action frames or EHT BFRP Trigger frames). The PPDU generator  1726  may generate PPDUs (e.g. EHT Sounding NDPs or EHT PPDUs comprising EHT NDP Announcement frames, EHT Action frames or EHT BFRP Trigger frames). 
     The receive signal processor  1706  may include a data demodulator and decoder  1734 , which may demodulate and decode data portions of the received signals (e.g. data fields of EHT PPDUs comprising EHT NDP Announcement frames, EHT Action frames or EHT BFRP Trigger frames). The receive signal processor  1706  may further include a control demodulator and decoder  1734 , which may demodulate and decode control signaling portions of the received signals (e.g. EHT-SIG fields of EHT Sounding NDPs or EHT-SIG fields of EHT PPDUs comprising EHT Compressed Beamforming/CQI frames). The at least one controller  1708  may include a control signal parser  1742  and a scheduler  1744 . The scheduler  1744  may determine RU information and user-specific allocation information for allocations of downlink SU or MU transmissions and triggering information for allocations of uplink MU transmissions. The control signal parser  1742  may analyse the control signaling portions of the received signals and the triggering information for allocations of uplink MU transmissions shared by the scheduler  1744  and assist the data demodulator and decoder  1732  in demodulating and decoding the data portions of the received signals (e.g. data fields of EHT PPDUs comprising EHT Compressed Beamforming/CQI frames). 
       FIG.  18    shows a configuration of a communication device, for example an STA, according to the present disclosure. Similar to the schematic example of communication apparatus  300  shown in  FIG.  3 A , the communication apparatus  1800  includes circuitry  1802 , at least one radio transmitter  1810 , at least one radio receiver  1812 , at least one antenna  1814  (for the sake of simplicity, only one antenna is depicted in  FIG.  18   ). The circuitry  1802  may include at least one controller  1808  for use in software and hardware aided execution of tasks that the controller  1808  is designed to perform communication for channel sounding. The circuitry  1802  may further include a receive signal processor  1804  and a transmission signal generator  1806 . The at least one controller  1808  may control the receive signal processor  1804  and the transmission signal generator  1806 . The receive signal processor  1804  may include a data demodulator and decoder  1832  and a control demodulator and decoder  1834 . The control demodulator and decoder  1834  may demodulate and decode control signaling portions of the received signals (e.g. EHT-SIG fields of EHT Sounding NDPs or EHT-SIG fields of EHT PPDUs comprising EHT NDP Announcement frames or EHT BFRP Trigger frames). The data demodulator and decoder  1032  may demodulate and decode data portions of the received signals (e.g. data fields of EHT PPDUs comprising EHT NDP Announcement frames or EHT BFRP Trigger frames) according to RU information and user-specific allocation information of its own allocations. 
     The at least one controller  1808  may include a control signal parser  1842 , and a scheduler  1844  and a trigger information parser  1846 . The control signal parser  1842  may analyse the control signaling portions of the received signals (e.g. EHT-SIG fields of EHT Sounding NDPs or EHT-SIG fields of EHT PPDUs comprising EHT NDP Announcement frames or EHT BFRP Trigger frame) and assist the data demodulator and decoder  1832  in demodulating and decoding the data portions of the received signals (e.g. data fields of EHT PPDUs comprising EHT NDP Announcement frames or EHT BFRP Trigger frames). The triggering information parser  1848  may analyse the triggering information for its own uplink allocations from the received triggering frames contained in the data portions of the received signals. The transmission signal generator  1804  may include a control signaling generator  1824 , which may generate control signaling fields of PPDUs to be generated (e.g. EHT-SIG fields of EHT Sounding NDPs or EHT-SIG fields of EHT PPDUs comprising EHT Compressed Beamforming/CQI frames). The transmission signal generator  1804  may further include a PPDU generator  1826 , which generate PPDUs (e.g. EHT Sounding NDPs or EHT PPDUs comprising EHT Compressed Beamforming/CQI frames). The transmission signal generator  1804  may further include a frame generator  1822  may generate MAC frames, e.g. EHT Compressed Beamforming/CQI frames. 
     As described above, the embodiments of the present disclosure provide an advanced communication system, communication methods and communication apparatuses for channel sounding in MIMO WLAN networks and improve spectral efficiency in MIMO WLAN networks. 
     The present disclosure can be realized by software, hardware, or software in cooperation with hardware. Each functional block used in the description of each embodiment described above can be partly or entirely realized by an LSI such as an integrated circuit, and each process described in each embodiment may be controlled partly or entirely by the same LSI or a combination of LSIs. The LSI may be individually formed as chips, or one chip may be formed so as to include a part or all of the functional blocks. The LSI may include a data input and output coupled thereto. The LSI here may be referred to as an IC, a system LSI, a super LSI, or an ultra LSI depending on a difference in the degree of integration. However, the technique of implementing an integrated circuit is not limited to the LSI and may be realized by using a dedicated circuit, a general-purpose processor, or a special-purpose processor. In addition, a FPGA (Field Programmable Gate Array) that can be programmed after the manufacture of the LSI or a reconfigurable processor in which the connections and the settings of circuit cells disposed inside the LSI can be reconfigured may be used. The present disclosure can be realized as digital processing or analogue processing. If future integrated circuit technology replaces LSIs as a result of the advancement of semiconductor technology or other derivative technology, the functional blocks could be integrated using the future integrated circuit technology. Biotechnology can also be applied. 
     The present disclosure can be realized by any kind of apparatus, device or system having a function of communication, which is referred to as a communication apparatus. 
     The communication apparatus may comprise a transceiver and processing/control circuitry. The transceiver may comprise and/or function as a receiver and a transmitter. The transceiver, as the transmitter and receiver, may include an RF (radio frequency) module including amplifiers, RF modulators/demodulators and the like, and one or more antennas. 
     Some non-limiting examples of such a communication apparatus include a phone (e.g. cellular (cell) phone, smart phone), a tablet, a personal computer (PC) (e.g. laptop, desktop, netbook), a camera (e.g. digital still/video camera), a digital player (digital audio/video player), a wearable device (e.g. wearable camera, smart watch, tracking device), a game console, a digital book reader, a telehealth/telemedicine (remote health and medicine) device, and a vehicle providing communication functionality (e.g. automotive, airplane, ship), and various combinations thereof. 
     The communication apparatus is not limited to be portable or movable, and may also include any kind of apparatus, device or system being non-portable or stationary, such as a smart home device (e.g. an appliance, lighting, smart meter, control panel), a vending machine, and any other “things” in a network of an “Internet of Things (IoT)”. 
     The communication may include exchanging data through, for example, a cellular system, a wireless LAN system, a satellite system, etc., and various combinations thereof. 
     The communication apparatus may comprise a device such as a controller or a sensor which is coupled to a communication device performing a function of communication described in the present disclosure. For example, the communication apparatus may comprise a controller or a sensor that generates control signals or data signals which are used by a communication device performing a communication function of the communication apparatus. 
     The communication apparatus also may include an infrastructure facility, such as a base station, an access point, and any other apparatus, device or system that communicates with or controls apparatuses such as those in the above non-limiting examples. 
     It will be understood that while some properties of the various embodiments have been described with reference to a device, corresponding properties also apply to the methods of various embodiments, and vice versa. 
     It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present disclosure as shown in the specific embodiments without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects illustrative and not restrictive.