Devices and methods for improved channel sounding in a WLAN

Abstract

An access point, AP, for communication with one or more non-AP stations associated with the AP in a wireless local area network, the AP being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, the AP being configured to: select in which of the said modes to operate for communication with a non-AP station; form in dependence on that selection a station information, STA Info, field for a null data physical protocol data unit announcement, NDPA, frame, the STA Info field comprising an indication of the selected mode; and transmit to the non-AP station an NDPA frame comprising the formed STA Info field. There is also provided a corresponding non-AP station device. There is also provided a method for operating the AP and non-AP.

Description

[0001] DEVICES AND METHODS FOR IMPROVED CHANNEL SOUNDING IN A WLAN

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications. More specifically, the present disclosure relates to devices and methods for improved channel sounding in a wireless local area network, WLAN, in particular a WLAN (also referred to as Wi-Fi network) according to the IEEE 802.11 framework of standards.

[0004] BACKGROUND

[0005] IEEE 802.11-based wireless local area networks, WLANs, (also referred to as Wi-Fi networks) have become popular at an unprecedented rate. Coordinated Beamforming, CoBF, is a Multi-AP, M-AP, coordination scheme where two or more APs transmit concurrently using the same channel bandwidth by utilizing beamforming techniques to minimize mutual interference. Beamforming depends on channel sounding to determine how to radiate energy in a preferred direction. Many factors may influence how to steer a beam in a particular direction.

[0006] SUMMARY OF THE INVENTION

[0007] It is an objective of the present disclosure to provide devices and methods for improved channel sounding in a wireless local area network, WLAN, in particular a WLAN (also referred to as Wi-Fi network) according to the IEEE 802.11 framework of standards.

[0008] The foregoing and other objectives are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

[0009] According to one aspect there is provided an access point, AP, for communication with one or more non-AP stations associated with the AP in a wireless local area network, the AP being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, the AP being configured to: select in which of the said modes to operate for communication with a non-AP station; form in dependence on that selection a station information, STA Info, field for a null data physical protocol data unit announcement, NDPA, frame, the STA Info field comprising an indication of the selected mode; and transmit to the non-AP station an NDPA frame comprising the formed STA Info field.

[0010] In an embodiment, the indication is at least partially given by one or more reserved bits of the STA info field. In an embodiment, the indication may be at least partially given by one or more of bits 11 to 24 and 29 to 31 of the STA Info field as defined according to WLAN in 802.1 Ibe. In an embodiment, the AP may be configured to set the state of one bit of the STA Field in dependence on whether the selected mode is coordinated beamforming.

[0011] In an embodiment, the AP may be configured to, in dependence on the selected mode being sequential sounding or joint sounding, set the state of three bits of the STA Field to indicate (i) that the selected mode is sequential sounding or (ii) a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

[0012] In an embodiment, the AP may be configured to set the three bits to one of the values 0 and 7 to indicate that the selected mode is sequential sounding. In an embodiment, the AP may be configured to, if the selected mode is sequential sounding, select whether a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP and to determine in dependence on that selection whether to set the three bits to the value 0 or 7.

[0013] In an embodiment, the AP may be configured to set the three bits to either of two values to indicate that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP. In an embodiment, the two values may be 0 or 7.

[0014] In an embodiment, the AP may be configured to select a precoder matrix with a number of columns, Nc, for each of BSS AP and OBSS AP and to determine in dependence on that selection a value for four bits in an Nc index field of the STA Field so as to indicate that selection.

[0015] In an embodiment, the AP may be configured to set two bits of the STA Field to a first value in dependence on the selected mode being regular sounding.

[0016] In an embodiment, the AP may be configured to set the two bits of the STA Field to a second value different from the first value, in dependence on the selected mode being joint sounding.

[0017] In an embodiment, the AP may be configured to set the two bits of the STA Field to a third value different from the first and second values, in dependence on the selected mode being sequential sounding with BSS AP.

[0018] In an embodiment, the AP may be configured to, set the two bits of the STA Field to a fourth value different from the first to third values, in dependence on the selected mode being sequential sounding with OBSS AP. In an embodiment, the first value may be zero. In an embodiment, one of the two bits may have the same state to represent the third and fourth values.

[0019] According to another aspect there is provided a method for operating an access point, AP, for communication with one or more non-AP stations associated with the AP in a wireless local area network, WLAN, the AP and non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, wherein the method comprises: sending a Null Data Physical Protocol Data Unit Announcement, NDPA, frame to the one or more non-AP stations, the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode; and receiving one or more beamforming feedback report, BFR, frames from the one or more non-AP stations, wherein each BFR frame comprises feedback information based on a subset used in a Sounding Null Data Packet NDP according to the selected mode.

[0020] According to another aspect there is provided a non-access point, non-AP, station for communication with one or more APs in a wireless local area network, the non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, the non-AP station being configured to: receive, from an AP of the one or more APs which is associated with the non-AP station, a null data physical protocol data unit announcement, NDPA, frame comprising a station information, STA Info, field, the STA Info field comprising an indication of the selected mode; and process a received Sounding Null Data Packet, NDP, based on the selected mode to generate feedback information; form a beamforming report, BFR, frame comprising the feedback information; and transmit the BFR frame to the AP.

[0021] In an embodiment, the indication may be at least partially given by one or more reserved bits of the STA info field.

[0022] In an embodiment, the indication may be at least partially given by one or more of bits 11 to 24 and 29 to 31 of the STA Info field as defined according to WLAN in 802.1 Ibe. In an embodiment, the non-AP may be configured to interpret the state of one bit of the STA Field and determine that the selected mode is coordinated beamforming.

[0023] In an embodiment, the non-AP station may be configured to, in dependence on the selected mode being sequential sounding or joint sounding, interpret the state of three bits of the STA Field to indicate (i) that the selected mode is sequential sounding or (ii) a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

[0024] In an embodiment, the non-AP station may be configured to interpret the three bits being one of the values 0 and 7 to indicate that the selected mode is sequential sounding.

[0025] In an embodiment, the non-AP station may be configured to interpret the three bits being one of the value 0 or 7 as indicating that the selected mode is sequential sounding, and a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP.

[0026] In an embodiment, the non-AP may be configured to interpret the three bits being either of two values as indicating that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP. In an embodiment, the two values may be 0 or 7.

[0027] In an embodiment, the non-AP station may be configured to interpret a value for four bits of the STA Field as indicating a number of columns, Nc, for each of the BSS AP and the OBSS AP of a precoder matrix.

[0028] In an embodiment, the non-AP station may be configured to interpret a first value of two bits of the STA Field as indicating that the selected mode is regular sounding.

[0029] In an embodiment, the non-AP station may be configured to interpret a second value of the two bits of the STA Field, the second value different from the first value, as indicating that the selected mode is joint sounding.

[0030] In an embodiment, the non-AP station may be configured to interpret a third value of the two bits of the STA Field, the third value different from the first and second values, as indicating that the selected mode is sequential sounding with BSS AP.

[0031] In an embodiment, the non-AP station may be configured to interpret a fourth value of the two bits of the STA Field, the fourth value different from the first to third values, as indicating that the selected mode is sequential sounding with OBSS AP. In an embodiment, the first value may be zero. In an embodiment, the third and fourth values may be represented with one of the two bits having the same state.

[0032] According to another aspect there is provided a method for operating a non-access point, non-AP, station for communication with an AP associated with the non-AP station in a wireless local area network, WLAN, the AP and non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, wherein the method comprises: receiving a Null Data Physical Protocol Data Unit Announcement, NDPA, frame from the AP associated with the non-AP station, the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode; and sending a beamforming feedback report, BFR, frame to the AP, wherein the BFR frame comprises feedback information based on a plurality of subsets used in a Sounding Null Data Packet NDP according to the selected mode.

[0033] According to another aspect there is provided a computer program product comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method of claim 17 or the method of claim 34 when the program code is executed by the computer or the processor. BRIEF DESCRIPTION OF THE FIGURES

[0034] The present disclosure will now be described by way of example with reference to the accompanying drawings. In the drawings:

[0035] Fig. 1 shows a schematic diagram illustrating a WLAN including an AP according to an embodiment and a further OBSS AP in communication with a plurality of non-AP stations according to an embodiment.

[0036] Fig. 2 illustrates a sounding procedure with a joint Sounding NDP implemented by an AP, an OBSS AP and a plurality of non-AP stations according to an embodiment.

[0037] Fig. 3 illustrates a sounding procedure with several sounding sequences implemented by an AP, an OBSS AP and a plurality of non-AP stations according to an embodiment.

[0038] Fig. 4 shows an example STA Info field.

[0039] Fig. 5 shows a flow diagram illustrating steps of a method for operating the AP.

[0040] Fig. 6 shows a flow diagram illustrating steps of a method for operating a non-access point, non-AP, station.

[0041] DETAILED DESCRIPTION OF THE INVENTION

[0042] In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the present disclosure or specific aspects in which embodiments of the present disclosure may be used. It is understood that embodiments of the present disclosure may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

[0043] For instance, it is to be understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and / or aspects described herein may be combined with each other, unless specifically noted otherwise.

[0044] Fig. 1 shows a wireless communication system 100, in particular a wireless local area network, WLAN, in accordance with the IEEE 802.11 framework of standards (also referred to as a Wi-Fi network 100). The WLAN 100 comprises a first access point, AP, 110a according to an embodiment in communication with a plurality of associated non-AP stations 110a, b according to an embodiment. The WLAN 100 may further comprise at least one further AP 110b in communication with a further plurality of non-AP stations 120c, d associated with the further AP 110b. By way of example, the non-AP stations 120a, b and the further non-AP stations 120c, d may comprise smartphones, laptop computers, tablet computers, desktop computers or other types of wireless devices. As indicated, the stations, i.e. APs 110a, b and non-AP stations 120a-d, shown in Fig. 1 may be multi-antenna stations and configured for beamforming, in particular coordinated beamforming, CoBF. To this end, by way of example, the AP 110a comprises a plurality of antennas 114a, the further OBSS AP 110b comprises a plurality of antennas 114b, and, as a representative exemplary, the non-AP station 120a comprises a plurality of antennas 124a.

[0045] As further illustrated in Fig . 1 , the AP 110a (as well as the further AP 110b) may comprise a processing circuitry I l la and a communication interface 113a, in particular a wireless communication interface 113a enabling communication in accordance with the IEEE 802.11 framework of standards. The processing circuitry Il la may be implemented in hardware and / or software and may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors. The AP 110a may further comprise a memory 115a configured to store executable program code which, when executed by the processing circuitry I lla, causes the AP 110a to perform the functions and methods described herein.

[0046] Likewise, the non-AP station 120a (as well as the other non-AP stations 120b-d shown in Fig. 1) may comprise a processing circuitry 121a and a communication interface 123a, in particular a wireless communication interface 123a enabling communication in accordance with the IEEE 802.11 framework of standards. The processing circuitry 121a may be implemented in hardware and / or software and may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors. The non-AP 120a may further comprise a memory 125a configured to store executable program code which, when executed by the processing circuitry 121a, causes the non-AP station 120a to perform the functions and methods described herein.

[0047] Before describing detailed embodiments of the AP 110a according to an embodiment and the non-AP station 120a according to an embodiment, in the following some technical background as well as terminology will be introduced making use of one or more of the following abbreviations and / or acronyms:

[0048] A-MPDU Aggregate MPDU

[0049] A-MSDU Aggregate MSDU

[0050] AP Access Point

[0051] BFR Beamforming Feedback Report

[0052] BSS Basic Service Set

[0053] BW Bandwidth

[0054] CoBF Coordinated Beamforming

[0055] CSI Channel State Information

[0056] DL Downlink

[0057] EHT Extremely High Throughput

[0058] E VM Error Vector Magnitude

[0059] FCS Forward Correction Sequence

[0060] LAN Local Area Network

[0061] LLC Logical Link Control

[0062] MAC Medium Access Control

[0063] M-AP Multi Access Point

[0064] MIMO Multiple In Multiple Out

[0065] ML Multi-link

[0066] MPDU MAC Protocol Data Unit

[0067] NDP Null Data PPDU NDPA NDP Announcement

[0068] OBSS Overlapping BSS

[0069] PHY Physical (layer)

[0070] PPDU Physical Layer Protocol Data Unit

[0071] RSSI Received Signal Strength Indicator

[0072] RU Resource Unit

[0073] SNR Signal to Noise Ratio

[0074] STA Station

[0075] TB Trigger Based

[0076] TF Trigger Frame

[0077] TXOP Transmit Opportunity

[0078] UL Uplink

[0079] WLAN Wireless Local Area Network

[0080] UHR Ultra-High Reliability

[0081] As used herein, an access point, AP, such as the APs 110a, b illustrated in Fig. 1 , is a wireless station, STA, that provides access to other networks. An AP can support many connected, i.e. associated non-AP STAs, such as the non-AP stations 120a-d illustrated in Fig. 1. APs use control information to control traffic flow over the wireless medium among all associated non-AP STAs within a BSS.

[0082] There are three main types of frames communicated between WLAN STAs, namely data, management, and control frames. Data traffic is exchanged between two or more stations, STAs, in a WLAN to facilitate communication, one of which is typically an AP. The data frames are either generated by the two or more STAs and / or by an external network. This traffic is delivered in a secured manner over the WLAN when the AP and the corresponding non-AP STAs negotiate a cryptographic encapsulation method and keys to encrypt the data traffic. Management frames are exchanged between the AP and one or more non-AP STAs in a BSS to establish and maintain state of data communications. Security can be negotiated to encrypt or authorize data and / or management frames. Control frames are exchanged between the AP and the non-AP STAs in a BSS to control the flow of the data frame exchange.

[0083] As already described above, according to embodiments disclosed herein, the stations, i.e. APs 110a,b and non-AP stations 120a-d, shown in Fig. 1 may be multi-antenna stations and configured for beamforming, in particular coordinated beamforming, CoBF. CoBF is a Multi-AP, M-AP, coordination scheme where the two (or more) APs 110a, b transmit concurrently using the same channel bandwidth by utilizing beamforming techniques to minimize mutual interference. In this coordination scheme each AP 110a, b exchanges data frames with its BSS non-AP stations (in Fig. 1 referred to as STAs) only. For example, in the scenario illustrated in Fig. 1, STA1 120a and STA2 120b are associated with API 110a, while STA3 120c and STA4 120d are associated with AP2 110b. Each AP 110a,b exchanges the data frames only with its associated STAs, but each AP 110a,b applies a precoder (usually represented by a precoder matrix) such that the interference to the OBSS STAs is minimized when both APs transmit concurrently. In other words, the actual power of the signals (illustrated by the straight dashed arrows in Fig. 1 ) at the receiver side is small enough to allow successful detection of the desired signal (illustrated by the curved solid arrows in Fig. 1).

[0084] In order to allow the APs 110a,b to construct precoders that maximize the performance of BSS STAs and minimize the interference to OBSS STAs a sounding procedure is performed that allows the estimation of the channels between all APs 110a, b and all STAs 120a-d. This means that each AP 110a, b has to transmit a training signal that will be received by its BSS STAs and the relevant OBSS STAs to allow estimating each of the relevant channels. Several different sounding procedures are known, which may be implemented by the APs 110a, b of Fig. 1 according to an embodiment. A sounding procedure for multiple STAs defined by the current IEEE 802.11 framework of standards comprises the transmission of the following PPDUs: a PPDU carrying the NDPA frame which indicates the parameters of the upcoming training signal and the required feedback parameters; a Sounding NDP as the “training” signal to be measured by each solicited non-AP STA (either BSS STA or OBSS STA); a PPDU carrying the BFRP frame defining a request poll to initiate the transmission of the feedback report; and a PPDU carrying the BFR frame, i.e. a feedback transmission, including a MIMO Control Field which describes the provided feedback and the precoder matrices information.

[0085] A Multi-AP, M-AP, sounding procedure implemented by the APs 110a, b may include a joint transmission of the PPDU carrying the Sounding NDP, where the APs 110a,b transmit concurrently. Another option, which may be implemented by the APs 110a,b, is a sequential sounding procedure, where each AP 110a, b performs its sounding procedure independently.

[0086] For the sounding procedure, where each AP 110a,b transmits the PPDU carrying the NDPA frame independently and the Sounding NDP is transmitted concurrently by one or more of the APs 110a,b, the non-AP stations 120a-d have to measure the Sounding NDP and send their feedback for the measured Sounding NDPs only to the associated AP 110a,b. The non-AP stations 120a-d are not aware of which AP 110a,b actually transmits each Sounding NDP and just estimate the channels from all the transmitting sources (antennas).

[0087] As will be described in the following under further reference to Fig. 2, which illustrates a sounding procedure with a joint NDP, the API 110a is configured to send a Null Data Physical Protocol Data Unit, NDP, announcement, NDPA, frame 201a to its associated non-AP station 120a.

[0088] In an embodiment, the AP 110a is configured to send the joint Sounding NDP 203 concurrently with the AP 110b to the non- AP station 120a. The joint Sounding NDP 203 is a single PPDU typically comprising two subsets which correspond to the plurality of the transmit antennas 114a, b of the AP 110a and OBSS AP 110b. In other words, in an embodiment, the first subset 203a used in the joint Sounding NDP 203 corresponds to one or more of the plurality of transmission antennas 114a of the AP 110a and the second subset 203b of the joint Sounding NDP203 corresponds to one or more of the plurality of transmission antennas 114b of the OBSS AP 110b. For instance, in the example shown in Fig. 1, the first subset 203a of the joint Sounding NDP 203 corresponds to transmission antennas with the indices 1 to 4 of the AP 110a, while the second subset 203b of the joint Sounding NDP 203 corresponds to transmission antennas with the indices 5 to 8 of the OBSS AP 110b.

[0089] In response to sending a BFRP frame 205a to its associated non-AP station 120a for initiating the transmission of the feedback, the AP 110a is further configured to receive one or more beamforming feedback report, BFR, frames 207 from its associated non-AP station 120a. As illustrated in Fig. 2 and as will be described in more detail below, each BFR frame 207 comprises a first feedback information 207a of a first M-AP feedback information type based on the first subset 203a used in the joint Sounding NDP 203 transmitted by API 110a and the OBSS AP 110b and a second feedback information 207b of a second M- AP feedback information type (different from the first M-AP feedback information type) based on the second subset 203b used in the joint Sounding NDP203. More specifically, in the example shown in Fig. 2, the first feedback information 207a is based on the first subset 203a of the joint Sounding NDP 203 corresponding to the transmission antennas with the indices 1 to 4, while the second feedback information 207b is based on the second subset 203b of the joint Sounding NDP 203 corresponding to the transmission antennas with the indices 5 to 8.

[0090] Thus, due to the NDPA frame’s 201a indication of the first and second subset 203a, b of the joint Sounding NDP 203, each non-AP station 120a, b may include the first feedback information 207a (based on the first subset) of the first feedback type and the second feedback information 207b (based on the second subset) of the second feedback type in the BFR frame 207. As will be appreciated, based on the different M-AP feedback information types, the AP 110a and the OBSS AP 110b may better adjust their coordinated beamforming for reducing mutual interference to the non-AP STA1 120a. As further illustrated in Fig. 2, the same sounding procedure as described above may be performed by the AP 110b and its associated non-AP station 120c. More specifically, the AP 110b may send an NDPA frame 211b and a joint Sounding NDP 213 concurrently with the AP 110a to its associated non-AP station 120c, wherein the joint Sounding NDP 213 comprises a single subset 213b, including at least a first subset 213b, and wherein the preceding NDPA frame 211b is indicative, i.e. comprises an indication of the plurality of subsets 213a,b used in the following joint Sounding NDP 213. In an embodiment, the first subset 213b used in the j oint Sounding NDP 213 corresponds to one or more of the plurality of transmission antennas 114b of the OBSS AP 110b and the second subset 213a of the joint Sounding NDP 213, transmitted by API 110a, corresponds to one or more of the plurality of transmission antennas 114a of the API 110a. For instance, in the example shown in Fig. 1, the first subset 213b of the joint Sounding NDP 213 corresponds to transmission antennas with the indices 1 to 4, while the second subset 213a of the joint Sounding NDP 213 corresponds to transmission antennas with the indices 5 to 8. In response to sending a BFRP frame 215b to its associated non-AP station 120c for initiating the transmission of the feedback, the AP 110b is further configured to receive one or more beamforming feedback report, BFR, frames 217 from its associated non-AP station 120c, wherein each BFR frame 217 comprises a first feedback information 217a of a first M-AP feedback information type based on the first subset 213b used in the joint Sounding NDP 213 and a second feedback information 217b of a second M-AP feedback information type (different to the first M-AP feedback information type) based on the second subset 213a used in the joint Sounding NDP 213.

[0091] Thus, according to embodiments disclosed herein the non-AP stations 120a-d are informed by the APs 110a, b by means of the NDPAs 201 a, 211 b that the respective j oint Sounding NDP 203 , 213 is composed of several subsets that may require different feedback reports for each subset and in order to compute the precoder for each subset independently. In addition, the required feedback for each subset may require a different format.

[0092] The sounding procedure illustrated in Fig. 2, which includes the transmission of a joint Sounding NDP 203, 213, may be used, for instance, in the case, when the total number of transmission antennas of all the APs, i.e. the AP 110a and the AP 110b, does not exceed the maximum number of allowed spatial streams, e.g. 8 spatial streams. Another case is when the total number of transmission antennas exceeds the number of allowed spatial streams. In this case, the sounding sequence initiated by each AP 110a, b may be repeated several times and at least one Sounding NDP may be transmitted by each AP, as illustrated in Fig. 3.

[0093] More specifically, in the embodiment shown in Fig. 3, the AP 110a transmits a NDPA frame 301 which indicates for a first subset 203a and a following Sounding NDP 303 to its associated non-AP station(s) 120a, b and receives, in response to transmitting a BFRP trigger frame 305, a BFR frame 307. In a further stage illustrated in Fig. 3, the AP 110a transmits a further NDPA frame 311 which indicates for a second subset 203b, but the following Sounding NDP 313 is transmitted by the OBSS AP 110b. In response to transmitting a BFRP frame 315, the AP 110a receives a BFR frame 317 from its associated non-AP station 120a. In a further stage illustrated in Fig. 3, the AP 110b transmits a NDPA frame 321 which indicates for a first subset 203a and a following Sounding NDP 323 to its associated non-AP station 120c and receives, in response to transmitting a BFRP frame 325, a BFR frame 327. In a further stage illustrated in Fig. 3, the AP 110b transmits a further NDPA frame 331 which indicates for a second subset 203b, but the Sounding NDP 333 is transmitted by the OBSS AP 110a. In response to transmitting a BFRP frame 335, the AP 110b receives a BFR frame 337 from its associated non-AP station 120c. As will be appreciated, similar to different M-AP feedback information types corresponding to different subsets (as in the embodiment shown in Fig. 2), for the embodiment shown in Fig. 3 each feedback may include a different M-AP feedback information type. More specifically, a M-AP feedback information type of BFR frame 307 and BFR frame 317 may differ, as well as of BFR frame 327 and BFR frame 337.

[0094] As already mentioned above, the stations, i.e. APs 110a, b and non-AP stations 120a-d, shown in Fig. 1 may be multi-antenna stations and configured for beamforming, in particular coordinated beamforming, CoBF. It has been proposed that in order to ensure a minimized mutual interference between the APs 110a, b and the OBSS STAs 120a-d in CoBF, the precoder should be computed differently for BSS and OBSS APs 110a, b. In an embodiment, each non-AP station 120a-d may compute a regular, SVD based feedback for the channel estimation of AP 110a, b, while for the channel estimation of the OBSS AP 110a, b, each non-AP station 120a-d may apply the U matrix of the BSS AP channel and then compute the orthonormal basis of U*HoBssfor each OBSS AP channel. In other words, when a joint Sounding NDP is transmitted, a different M-AP feedback type is obtained for each subset 203a, b of the joint Sounding NDP (in the scenario illustrated in Fig. 3 each NDP 303, 313, 323, 333 consists of a single subset and the M-AP feedback type differs between NDPs). As will be appreciated according to embodiments disclosed herein each non-AP station 120a-d knows which type of M-AP feedback is to be provided for each subset, such as the subsets 203a, b.

[0095] As already described above, according to embodiments disclosed herein, an NDPA frame, such as the NDPA frames 201a, 211b of Fig. 2, comprises an indication of several subsets of the ensuing joint Sounding NDP(s).

[0096] The NDPA frame 201a may include an indication indicative of the presence of several subsets 203a, b in the following joint Sounding NDP. Each subset 203a, b may correspond to specific transmission antenna indices.

[0097] As described above, in order to ensure minimized mutual interference between APs and OBSS STAs the precoder should be computed differently for BSS and OBSS APs. In particular, STA needs to compute a regular, SVD based feedback for BSS AP channel. For OBSS AP channel, the STA field should apply the U matrix of the BSS AP channel and then compute the orthonormal basis of U*HOBSS for OBSS AP channel. It can be assumed that there will be only one new type of feedback which will be used for OBSS AP. Thus, in order to ensure the STA obtains and provides the correct feedback with the correct precoder there is a need to indicate in the STA field the relevant information.

[0098] The relevant information may include, as required, which type of CoBF Sounding Procedure is applied, an indication of the first and second subset, in particular an indication of the Tx antennas / NSTS allocated for OBSS AP in the case of Joint Sounding, and an indication of which AP (BSS or OBSS) transmits the NDP in the case of Sequential Sounding.

[0099] Although existing approaches provide the information that is required to support different feedback types for OBSS AP, there are several significant drawbacks in those approaches.

[0100] For example, in existing approaches the STA is required to process one or more new fields that are dedicated to Shared AP. This means that a change in the behaviour of the STA is needed and this requires new functionality. Further, if a dynamically allocated AP ID is used in these new fields, this requires Sharing AP to indicate all the STAs on the AP IDs that should be parsed, which means additional overhead and higher complexity. Even if implemented in the general case, the STA is still required to process a new field, which means new functionality and implies device design modifications.

[0101] Therefore, the existing approaches may support different feedback types for OBSS AP but at the cost of requiring STA design changes.

[0102] There is therefore a need to develop a way to support the additional functionality while also using existing designs for the NDPA and STA Info Field to support this different type of feedback for OBSS and minimize the required changes in STA design.

[0103] The idea proposed herein is to use reserved bits of the STA Info Field, or existing bits which may not be in use, to indicate all the required information to support different feedback type for OBSS AP. Accordingly, the STA would be required to process only STA Info Field where STA ID matches its own ID. Each STA Info Field dedicated to the STA may therefore include the following information. A CoBF Indication, which indicates that different feedback should be used for OBSS AP. The BSS and OBSS AP phase for Sequential Sounding. The starting Nss of the OBSS AP for Joint Sounding. The Nc for BSS and OBSS APs, which is only needed for Joint Sounding.

[0104] There is therefore provided herein an access point for communication with one or more non-access point stations associated with the AP in a wireless local area network. The AP is capable of supporting a set of sounding modes comprising regular sounding and coordinated beamforming, where coordinated beamforming may use sequential sounding or joint sounding. The AP is configured to select in which of the said modes to operate for communication with a non-AP station and form in dependence on that selection a station information, STA Info, field for a null data physical protocol data unit announcement, NDPA, frame. The STA Info field comprises an indication of the selected mode. The AP is then configured to transmit to the non-AP station an NDPA frame comprising the formed STA Info field.

[0105] Similarly, there is provided a non-access point, non-AP, station for communication with one or more APs in a wireless local area network. The non-AP station being capable of supporting a set of sounding modes comprising regular sounding and coordinated beamforming, where coordinated beamforming may use sequential sounding or joint sounding. The non-AP station is configured to receive from an AP of the one or more APs which is associated with the non-AP STA a null data physical protocol data unit announcement, NDPA, frame comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode. The non-AP station is further configured to process a received Sounding Null Data Packet, NDP, based on the selected mode to generate feedback information, form a beamforming report, BFR, frame comprising the feedback information, and transmit the BFR frame to the AP. The non-AP STA is typically associated with a single AP. It also typically communicates with that single AP. However, during CoBF sounding, it may need to process a signal that is transmitted by the OBSS AP, which the non-AP STA is not typically associated with.

[0106] All the indications needed can be located in the existing reserved bits or those bits that may be unused in the NDPA during CoBF sounding. It is assumed for the embodiments described herein below that CoBF scheme is limited to 2 APs and that there is only a single feedback type for OBSS AP. That is, that there is no need to indicate the feedback type explicitly.

[0107] As mentioned above, it is necessary to indicate the starting Nss of OBSS AP for Joint Sounding. However, it can be assumed that in the case of Joint Sounding that the BSS AP always starts with Nss index 0. There is also a need to allocate at least two Nss indices to each AP, as CoBF requires at least 2 transmit antennas at each AP.

[0108] Thus, assuming 802.1 Ibn will be able to support up to 8 SS, indices 0 and 7 will not be used in the case of Joint Sounding. In this embodiment it can also be assumed that Nc is identical for BSS and OBSS AP and is therefore indicated by the existing Nc Index field.

[0109] Based on the above, up to four bits may be needed to provide all the required information to the STA. A single bit is needed to indicate either CoBF Sounding or Regular Sounding. A further three bits may then be needed to provide one of the indications as follows. ‘0’ for indicating Sequential Sounding where NDP is transmitted by BSS AP. ‘7’ for indicating Sequential Sounding where NDP is transmitted by OBSS AP. Any of ‘2’ to ’6’ for indicating the starting index of OBSS AP for Joint Sounding.

[0110] Fig. 4 shows an example STA Info field 400. The example STA Info field is configured to the specifications as defined in WLAN, specifically in 802.1 Ibe (WiFi 7). The STA Info field 400 comprises 31 bits. The numbers 402 across the top of each subfield prefixed with a ‘B’ indicate the first and last bit within that subfield as ordered within the total bits of the STA Info field. The numbers across the bottom of the STA Info field indicate the total number of bits in each subfield.

[0111] The AP may be configured to set the state of one bit of the STA Field in dependence on whether the selected mode is coordinated beamforming. The AP may be configured to, in dependence on the selected mode being sequential sounding or joint sounding, set the state of three bits of the STA Field to indicate either that the selected mode is sequential sounding or a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

[0112] Similarly, the non-AP may be configured to interpret the state of one bit of the STA Field and determine that the selected mode is coordinated beamforming. The non-AP station may be configured to, in dependence on the selected mode being sequential sounding or joint sounding, interpret the state of three bits of the STA Field to indicate either that the selected mode is sequential sounding or a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

[0113] The AP may be configured to set the three bits to one of the values 0 and 7 to indicate that the selected mode is sequential sounding. The AP may be configured to, if the selected mode is sequential sounding, select whether a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP and to determine in dependence on that selection whether to set the three bits to the value 0 or 7.

[0114] That is, the AP may be configured to set the three bits to either of two values to indicate that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP. The two values may be 0 or 7.

[0115] Accordingly, the non-AP station may be configured to interpret the three bits being one of the values 0 and 7 to indicate that the selected mode is sequential sounding. The non-AP station may be configured to interpret the three bits being one of the value 0 or 7 as indicating that the selected mode is sequential sounding, and a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP. That is, the non-AP may be configured to interpret the three bits being either of two values as indicating that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP. The two values may be 0 or 7.

[0116] There are two options to allocate these indications in the STA Info Field. One option is to allocate reserved bit B20 to indicate either CoBF Sounding or Regular Sounding and other reserved bits B29-B31 to indicate codes ‘0’ - ‘7’.

[0117] Thus, there is proposed the option of using reserved bits to support the different feedback types. Specifically, where bit 20, 406 of the STA Info field is designated as a reserved bit. Further, where bits 29, 30, and 31 408 are also designated as reserved bits of the STA Info field.

[0118] The indication may therefore be at least partially given by one or more reserved bits of the STA info field.

[0119] There is also the option of using 4 bits of the Partial BW Info subfield 410 to provide the indications above. The Partial BW Info subfield 410 comprises bit Bl 1 to B19 according to the definition of the STA Info Field as defined in WLAN, specifically in 802.1 Ibe (WiFi 7). As the Partial BW Info subfield 410 might be unused in CoBF sounding, it is possible to use any 4 bits of those 9 bits to indicate the information above.

[0120] A combination of the two options for indicating the information as described above, where part of the unused reserved bits and part of the unused Partial BW Info subfield, may also be used. Therefore, the indication may be at least partially given by one or more of bits 11 to 24 and 29 to 31 of the STA Info field as defined according to WLAN in 802.1 Ibe.

[0121] Above it is assumed that the number of columns in the precoder (Nc) is identical for BSS and OBSS AP. However, different APs may transmit over a different number of spatial streams for BSS STAs, so there may be a need to compute a precoder matrix of a different size per subset. In particular, the number of columns in the precoder matrix (Nc) may differ. However, it may be assumed that the number of columns that the STA may be required to provide is up to 4. Thus, it is proposed to reuse an existing Nc Index subfield to indicate the Nc for BSS and OBSS Aps. For example, in an Nc Index subfield 412 specified as defined in WLAN, e.g. specifically in 802.1 Ibe (WiFi 7), bits 21 to 22 may indicate Nc Index for BSS AP, and bits 23 to 24 will indicate Nc Index for OBSS AP.

[0122] The AP may be configured to request a precoder feedback matrix with a number of columns, Nc, for each of BSS AP and OBSS AP and to determine in dependence on that selection a value for four bits in an Nc index field of the STA Field so as to indicate that selection. Accordingly, the non-AP station may be configured to interpret a value for four bits of the STA Field as indicating a number of columns, Nc, for each of the BSS AP and the OBSS AP of a requested precoder feedback matrix.

[0123] In another scenario it may also be assumed that there is no flexibility in CoBF sounding parameters and both APs have the same number of Tx antennas. In this case, only two bits are needed as ‘00’ can indicate regular Sounding, i.e. not CoBF, ‘01’ can indicate joint CoBF Sounding, ‘10’ can indicate sequential CoBF Sounding (BSS AP), and ‘11’ can indicate sequential CoBF Sounding (OBSS AP). It should be understood that any combination of the two bits may be chosen to indicate any of the four possible states. For this solution, reserved bits, for example bits 29 to 30408 or the Partial BW Info 410 bits may also be used to provide the indication.

[0124] Therefore, the AP may be configured to set two bits of the STA Field to a first value in dependence on the selected mode being regular sounding. The AP may be configured to set the two bits of the STA Field to a second value different from the first value, in dependence on the selected mode being joint sounding. The AP may be configured to set the two bits of the STA Field to a third value different from the first and second values, in dependence on the selected mode being sequential sounding with BSS AP. The AP may be configured to set the two bits of the STA Field to a fourth value different from the first to third values, in dependence on the selected mode being sequential sounding with OBSS AP. The first value may be zero. One of the two bits may have the same state to represent the third and fourth values.

[0125] Accordingly, the non-AP station may be configured to interpret a first value of two bits of the STA Field as indicating that the selected mode is regular sounding. The non-AP station may be configured to interpret a second value of the two bits of the STA Field, the second value different from the first value, as indicating that the selected mode is joint sounding. The non-AP station may be configured to interpret a third value of the two bits of the STA Field, the third value different from the first and second values, as indicating that the selected mode is sequential sounding with BSS AP. The non-AP station may be configured to interpret a fourth value of the two bits of the STA Field, the fourth value different from the first to third values, as indicating that the selected mode is sequential sounding with OBSS AP. The first value may be zero. The third and fourth values may be represented with one of the two bits having the same state.

[0126] Fig. 5 shows a flow diagram illustrating steps of a method 500 for operating the AP 110a of Fig. 1 for communication with the non-AP stations 120a, b associated with the AP 110a in the WLAN 100. The AP and non-AP station being capable of supporting a set of sounding modes comprising regular sounding and coordinated beamforming using sequential sounding or joint sounding. The method 500 comprises a first step 502 of sending a Null Data Physical Protocol Data Unit Announcement, NDPA, frame 201a to the one or more non-AP stations 120a, b, the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode. The method comprises a further step 504 of receiving one or more beamforming feedback report, BFR, frames 207 from the one or more non-AP stations 120a,b, wherein each BFR frame 207 comprises feedback information 207a, 207b based on a subset 203a, 203b used in a Sounding Null Data Packet NDP 203 according to the selected mode.

[0127] As the method 500 can be implemented by the AP 110a as well as the further OBSS AP 110b, further features of the method 500 result directly from the functionality of the AP 110a as well as its different embodiments described above and below.

[0128] Fig. 6 shows a flow diagram illustrating steps of a method 600 for operating a non-access point, non-AP, station 120a for communication with an AP 110a associated with the non-AP station 120a in a wireless local area network, WLAN, 100. The AP and non-AP station being capable of supporting a set of sounding modes comprising regular sounding and coordinated beamforming, the coordinated beamforming using sequential sounding or joint sounding. The method 600 comprises a first step 602 of receiving a Null Data Physical Protocol Data Unit Announcement, NDPA, frame 201a from the AP 110a associated with the non-AP station 120a, the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode. The method comprises a further step 604 of sending a beamforming feedback report, BFR, frame 207 to the AP 110a, wherein the BFR frame 207 comprises feedback information 207a, 207b based on a plurality of subsets 203a, 203b used in a Sounding Null Data Packet NDP 203 according to the selected mode.

[0129] As the method 600 can be implemented by the non-AP station 120a as well as the other non-AP stations 120b-d, further features of the method 600 result directly from the functionality of the non-AP station 120a as well as its different embodiments described above and below.

[0130] There is also provided herein according to the embodiments of the methods and apparatus described above, a computer program product comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method described in reference to Fig. 5 or the method described in reference to Fig. 6 when the program code is executed by the computer or the processor.

[0131] The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments of the present disclosure (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments (unit = step).

[0132] In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described embodiment of an apparatus is merely exemplary. For example, the unit division is merely logical function division and may be another division in an actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms.

[0133] The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments.

[0134] In addition, functional units in the embodiments of the disclosure may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit.

[0135] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present disclosure may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure.

Claims

CLAIMS1. An access point, AP, for communication with one or more non-AP stations associated with the AP in a wireless local area network, the AP being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, the AP being configured to: select in which of the said modes to operate for communication with a non-AP station; form in dependence on that selection a station information, STA Info, field for a null data physical protocol data unit announcement, NDPA, frame, the STA Info field comprising an indication of the selected mode; and transmit to the non-AP station an NDPA frame comprising the formed STA Info field.

2. The AP according to claim 1 , wherein the indication is at least partially given by one or more reserved bits of the STA info field.

3. The AP according to claim 1 or 2, wherein the indication is at least partially given by one or more of bits 11 to 24 and 29 to 31 of the STA Info field as defined according to WLAN in 802.1 Ibe.

4. The AP according to any preceding claim, wherein the AP is configured to set the state of one bit of the STA Field in dependence on whether the selected mode is coordinated beamforming.

5. The AP according to any preceding claim, wherein the AP is configured to, in dependence on the selected mode being sequential sounding or joint sounding, set the state of three bits of the STA Field to indicate (i) that the selected mode is sequential sounding or (ii) a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

6. The AP according to claim 5, wherein the AP is configured to set the three bits to one of the values 0 and 7 to indicate that the selected mode is sequential sounding.

7. The AP according to claim 5 or 6, wherein the AP is configured to, if the selected mode is sequential sounding, select whether a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP and to determine in dependence on that selection whether to set the three bits to the value 0 or 7.

8. The AP according to claim 5, wherein the AP is configured to set the three bits to either of two values to indicate that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP.

9. The AP according to claim 8, wherein the two values are 0 or 7.

10. The AP according to any preceding claim, wherein the AP is configured to select a precoder matrix with a number of columns, Nc, for each of BSS AP and OBSS AP and to determine in dependence on that selection a value for four bits in an Nc index field of the STA Field so as to indicate that selection.

11. The AP according to claim 1 or 2, wherein the AP is configured to set two bits of the STA Field to a first value in dependence on the selected mode being regular sounding.

12. The AP according to claim 11, wherein the AP is configured to set the two bits of the STA Field to a second value different from the first value, in dependence on the selected mode being joint sounding.

13. The AP according to any of claims 11 or 12, wherein the AP is configured to set the two bits of the STA Field to a third value different from the first and second values, in dependence on the selected mode being sequential sounding with BSS AP.

14. The AP according to any of claims 11 to 13, wherein the AP is configured to, set the two bits of the STA Field to a fourth value different from the first to third values, in dependence on the selected mode being sequential sounding with OBSS AP.

15. The AP according to any of claims 11 to 14, wherein the first value is zero.

16. The AP according to any of claims 11 to 15, wherein one of the two bits has the same state to represent the third and fourth values.

17. A method (500) for operating an access point, AP, (110a) for communication with one ormore non-AP stations (120a, b) associated with the AP (110a) in a wireless local area network, WLAN, (100), the AP and non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, wherein the method (500) comprises: sending (502) a Null Data Physical Protocol Data Unit Announcement, NDPA, frame (201a) to the one or more non- AP stations (120a,b), the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode; and receiving (504) one or more beamforming feedback report, BFR, frames (207) from the one or more non-AP stations (120a, b), wherein each BFR frame (207) comprises feedback information (207a, 207b) based on a subset (203a, 203b) used in a Sounding Null Data Packet NDP (203) according to the selected mode.

18. A non-access point, non-AP, station (120) for communication with one or more APs (110) in a wireless local area network, the non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, the non-AP station being configured to: receive from an AP of the one or more APs which is associated with the non-AP station a null data physical protocol data unit announcement, NDPA, frame (201) comprising a station information, STA Info, field, the STA Info field comprising an indication of the selected mode; and process a received Sounding Null Data Packet, NDP, (203) based on the selected mode to generate feedback information; form a beamforming report, BFR, frame (207) comprising the feedback information; and transmit the BFR frame to the AP.

19. The non-AP station according to claim 18, wherein the indication is at least partially given by one or more reserved bits of the STA info field.

20. The non-AP station according to claim 18 or 19, wherein the indication is at least partially given by one or more of bits 11 to 24 and 29 to 31 of the STA Info field as defined according to WLAN in 802.1 Ibe.

21. The non-AP station according to any of claims 18 to 20, wherein the non-AP is configured to interpret the state of one bit of the STA Field and determine that the selected mode is coordinated beamforming.

22. The non-AP station according to any of claims 18 to 21, wherein the non-AP station is configured to, in dependence on the selected mode being sequential sounding or joint sounding, interpret the state of three bits of the STA Field to indicate(i) that the selected mode is sequential sounding or (ii) a starting index of Overlapping Basic Service Set, OBSS, AP for joint sounding.

23. The non-AP station according to claim 22, wherein the non-AP station is configured to interpret the three bits being one of the values 0 and 7 to indicate that the selected mode is sequential sounding.

24. The non-AP station according to claim 22 or 23, wherein the non-AP station is configured to interpret the three bits being one of the value 0 or 7 as indicating that the selected mode is sequential sounding, and a null data physical layer protocol, NDP, frame is to be transmitted by a Basic Service Set, BSS, AP or an OBSS AP.

25. The non-AP station according to any of claims 22 to 24, wherein the non-AP is configured to interpret the three bits being either of two values as indicating that the selected mode is sequential sounding where a null data physical layer protocol, NDP, frame is transmitted by a Basic Service Set, BSS, AP or an OBSS AP.

26. The non-AP station according to claim 25, wherein the two values are 0 or 7.

27. The non-AP station according to any of claims 18 to 24, wherein the non-AP station is configured to interpret a value for four bits of the STA Field as indicating a number of columns, Nc, for each of the BSS AP and the OBSS AP of a precoder matrix.

28. The non-AP station according to claim 18 or 19, wherein the non-AP station is configured to interpret a first value of two bits of the STA Field as indicating that the selected mode is regular sounding.

29. The non-AP according to claim 28, wherein the non-AP station is configured to interpret a second value of the two bits of the STA Field, the second value different from the first value, as indicating that the selected mode is joint sounding.

30. The non-AP station according to claim 28 or 29, wherein the non-AP station is configured to interpret a third value of the two bits of the STA Field, the third value different from the first and second values, as indicating that the selected mode is sequential sounding with BSS AP.

31. The non-AP station according to any of claims 28 to 30, wherein the non-AP station is configured to interpret a fourth value of the two bits of the STA Field, the fourth value different from the first to third values, as indicating that the selected mode is sequential sounding with OBSS AP.

32. The non-AP station according to any of claims 28 to 31 , wherein the first value is zero.

33. The non-AP station according to any of claims 28 to 32, wherein the third and fourth values are represented with one of the two bits having the same state.

34. A method (600) for operating a non-access point, non-AP, station (120a) for communication with an AP (110a) associated with the non-AP station (120a) in a wireless local area network, WLAN, (100), the AP and non-AP station being capable of supporting a set of sounding modes comprising (i) regular sounding and (ii) coordinated beamforming using (a) sequential sounding or (b) joint sounding, wherein the method (600) comprises:16receiving (602) a Null Data Physical Protocol Data Unit Announcement, NDPA, frame (201a) from the AP (110a) associated with the non-AP station (120a), the NDPA comprising a station information, STA Info, field, where the STA Info field comprises an indication of the selected mode; and sending (604) a beamforming feedback report, BFR, frame (207) to the AP (110a), wherein the BFR frame (207) comprises feedback information (207a, 207b) based on a plurality of subsets (203a, 203b) used in a Sounding NullData Packet NDP (203) according to the selected mode.

35. A computer program product comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method (500) of claim 17 or the method (600) of claim 34 when the program code is executed by the computer or the processor.

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