Protocol and frame format for coordinated beamforming

Abstract

To provide a method and an apparatus for coodinated beamforming.SOLUTION: A method includes transmitting, by a first access point (AP) of multiple collaborating APs to one or more stations (STAs) associated with the multiple collaborating APs, a request for channel state information (CSI) associated with the one or more STAs and the first AP. The method further includes receiving a response including the CSI associated with the one or more STAs and the first AP. The method further includes transmitting, to a second AP of the multiple collaborating APs, a sounding request indicating that the second AP requests, from the one or more STAs, CSI associated with the one or more STAs and the second AP.The method further includes sharing CSI information between the first AP and the second AP.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of communication networks, and in particular to procedures and frame structures for coordinated beamforming (CoBF). [Background technology]

[0002] cross reference This application claims the benefit of priority to U.S. Non-Provisional Patent Application No. 17 / 484,709, entitled "Protocol and Frame Format for Coordinated Beamforming," filed September 24, 2021, the entire contents of which are incorporated herein by reference.

[0003] CoBF is likely to be a key feature of the multi-access point (M-AP) coordination topic in 802.11be Release 2 (R2). There are several issues yet to be resolved when introducing CoBF into IEEE 802.11. The first issue is determining how selected user information can be shared among cooperating APs. Existing protocols do not provide adequate procedures for sharing selected user information among cooperating APs. The second issue is related to the precoder calculation for APs participating in CoBF. Currently, precoder calculation is vendor-specific. Vendor-specific precoder calculation is not appropriate for CoBF due to the interference that can result from such calculation.

[0004] Therefore, there is a need for procedures and frame structures for CoBF that overcome or mitigate one or more limitations of the prior art.

[0005] This background information is provided to identify information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should it be construed, that any of the preceding information constitutes prior art against the present invention. Summary of the Invention

[0006] The present disclosure provides methods and apparatus related to cooperative beamforming. A first aspect of the present disclosure provides a method. The method includes transmitting, by a first access point (AP) of multiple cooperating APs, a request for channel state information (CSI) associated with one or more stations (STAs) associated with the multiple cooperating APs. The method further includes receiving, by the first AP, a response from the one or more STAs, the response including the CSI associated with the one or more STAs and the first AP. The method further includes transmitting, by the first AP, a sounding request to a second AP of the multiple cooperating APs, indicating that the second AP requests, from the one or more STAs, CSI associated with the one or more STAs and the second AP. The method further includes receiving, by the first AP, a first AP-AP sharing message from the second AP, the first AP including CSI associated with the one or more STAs and the second AP. The method further includes transmitting, by the first AP, a second AP-AP sharing message to the second AP, the second AP including the one or more STAs and a CSI associated with the first AP. The method may provide a medium access control (MAC) protocol for sharing information between APs participating in the CoBF.

[0007] In some embodiments of the first aspect, the second AP-AP sharing message includes a frame including one or more fields indicating one or more of: an identifier of the first AP, an indication that the frame is an AP-AP sharing frame, one or more STAs associated with the first AP, CSI associated with the one or more STAs and the first AP, one or more streams per STA for the one or more STAs associated with the first AP, and precoder information associated with the second AP. The method may further provide a frame format for sharing information between APs participating in a CoBF.

[0008] In some embodiments of the first aspect, the method further includes calculating, by the first AP, a plurality of precoders, each calculated precoder corresponding to a respective AP of the plurality of cooperating APs and based on CSI information associated with one or more STAs and the respective AP. The method may further provide calculating precoder information for APs participating in CoBF.

[0009] In some embodiments of the first aspect, transmitting a request for CSI associated with one or more STAs and the first AP includes transmitting a Null Data Packet Announcement (NDPA) to each of the one or more STAs. In some embodiments, transmitting a request for CSI associated with one or more STAs and the first AP further includes transmitting a Null Data Packet (NDP) to each of the one or more STAs a shortest inter-frame space (SIFS) time unit after transmitting the NDPA. In some embodiments, transmitting a request for CSI associated with one or more STAs and the first AP further includes transmitting a beamforming report frame from each of the one or more STAs a SIFS time unit after transmitting the NDP.

[0010] In some embodiments of the first aspect, the precoder information indicates a calculated precoder among multiple precoders. In some embodiments, the precoder information indicates a precoder index that indicates how the precoder is calculated. In some embodiments, CSI associated with one or more STAs and the first AP is indicated in a selected user field of the frame. In some embodiments, an identifier of the first AP is indicated in a transmitter address (TA) field of a medium access control (MAC) header of the frame. The method may further provide for information sharing between cooperating APs that may be necessary for CoBF.

[0011] A second aspect of the present disclosure provides another method. The method includes receiving, by a first access point (AP), from a second AP, a first sharing message including channel state information (CSI) associated with the second AP, wherein the first AP and the second AP cooperate. The method further includes transmitting, by the first AP to the second AP, a second sharing message including a frame including one or more fields indicating one or more of an identifier of the first AP, an indication that the frame is an AP-AP sharing frame, one or more stations (STAs) associated with the first AP, the CSI associated with the first AP, one or more streams per STA for the one or more STAs associated with the first AP, and precoder information associated with the second AP. The method may further provide a frame format for sharing information between APs participating in a CoBF.

[0012] In some embodiments of the second aspect, the method further includes calculating, by the first AP, a precoder associated with the second AP based on CSI associated with the second AP. In some embodiments, the precoder information associated with the second AP indicates a calculated precoder associated with the second AP. In some embodiments, the precoder information associated with the second AP indicates a precoder index that indicates how the precoder is calculated. In some embodiments, the CSI associated with the first AP is indicated in a selected users field of the frame. In some embodiments, an identifier of the first AP is indicated in a transmitter address (TA) field of a medium access control (MAC) header of the frame. The method may further provide for sharing of information between cooperating APs that may be required for CoBF.

[0013] A third aspect of the present disclosure provides another method. The method includes transmitting, by a first access point (AP) of multiple cooperating APs to one or more stations (STAs) associated with the multiple cooperating APs, a request for channel state information (CSI) associated with one or more STAs and the first AP. The method further includes receiving, by the first AP, a response from the one or more STAs, the response including the CSI associated with the one or more STAs and the first AP. The method further includes transmitting, by the first AP to a second AP of the multiple cooperating APs, a sounding request indicating that the second AP requests, from the one or more STAs, the CSI associated with the one or more STAs and the second AP. The method further includes transmitting, by the first AP to the second AP, a first AP-AP share message including the CSI associated with the one or more STAs and the first AP. The method further includes receiving, by the first AP, a second AP-AP share message from the second AP, the CSI including the one or more STAs and the CSI associated with the second AP. The method may provide an alternative MAC protocol for sharing information between APs participating in a CoBF.

[0014] In some embodiments of the third aspect, the method further includes calculating, by the first AP, an end time for the multiple cooperating APs to collect corresponding CSI.

[0015] In some embodiments of the third aspect, the second AP-AP sharing message comprises a frame including one or more fields indicating one or more of: an identifier of the second AP; an indication that the frame is an AP-AP sharing frame; one or more STAs associated with the second AP; CSI associated with the one or more STAs and the second AP; and precoder information associated with the first AP. The method may further provide a frame format for sharing information between APs participating in CoBF. The method may further provide a frame format for sharing information between APs participating in CoBF. The method may further provide for sharing information between cooperating APs that may be required for CoBF.

[0016] A fourth aspect of the present disclosure provides another method. The method includes receiving, by a first access point (AP) of a plurality of cooperating APs, a sounding request from a second AP of the plurality of cooperating APs, indicating that the first AP requests channel state information (CSI) from one or more stations (STAs) associated with the plurality of cooperating APs. The method further includes transmitting, by the first AP, a request to the one or more STAs for CSI associated with the one or more STAs and the first AP. The method further includes receiving, by the first AP, a response from the one or more STAs comprising CSI associated with the one or more STAs and the first AP. The method further includes receiving, by the first AP, a first AP-AP Share message from the second AP including CSI associated with the one or more STAs and the second AP. The method further includes transmitting, by the first AP, a second AP-AP Share message to the second AP including CSI associated with the one or more STAs and the first AP. The method may provide a MAC protocol for sharing information between APs participating in CoBF.

[0017] In some embodiments of the fourth aspect, the second AP-AP sharing message comprises a frame including one or more fields indicating one or more of: an identifier of the first AP, an indication that the frame is an AP-AP sharing frame, one or more STAs associated with the first AP, CSI associated with the one or more STAs and the first AP, and precoder information associated with the second AP. The method may further provide a frame format for sharing information between APs participating in a CoBF.

[0018] In some embodiments of the fourth aspect, the method further includes calculating, by the first AP, a plurality of precoders, each calculated precoder corresponding to a respective AP of a plurality of cooperating APs and based on CSI information associated with one or more STAs and the respective AP. In some embodiments, the precoder information is a calculated precoder of the plurality of precoders. The method may further provide for sharing of information between cooperating APs that may be required for CoBF.

[0019] A fifth aspect of the present disclosure provides another method. The method includes receiving, by a first access point (AP) of a plurality of cooperating APs, a sounding request from a second AP of the plurality of cooperating APs, indicating that the first AP requests channel state information (CSI) from one or more stations (STAs) associated with the plurality of cooperating APs. The method further includes transmitting, by the first AP, a request to the one or more STAs for CSI associated with the one or more STAs and the first AP. The method further includes receiving, by the first AP, a response from the one or more STAs comprising CSI associated with the one or more STAs and the first AP. The method further includes transmitting, by the first AP, a first AP-AP Share message to the second AP, the first AP including the CSI associated with the one or more STAs and the first AP. The method further includes receiving, by the first AP, a first AP-AP Share message from the second AP, the first AP including the CSI associated with the one or more STAs and the second AP. In some embodiments of the fifth aspect, the second AP-AP sharing message comprises a frame including one or more fields indicating one or more of: an identifier of the second AP; an indication that the frame is an AP-AP sharing frame; one or more STAs associated with the second AP; CSI associated with the one or more STAs and the second AP; and precoder information associated with the first AP. The method may provide a MAC protocol for sharing information between APs participating in a CoBF. The method may further provide a frame format for sharing information between APs participating in a CoBF.

[0020] A sixth aspect of the present disclosure provides another method. The method includes transmitting, by a first access point (AP) of multiple cooperating APs, a request for channel state information (CSI) associated with one or more stations (STAs) associated with the multiple cooperating APs. The method further includes receiving, by the first AP, a response from the one or more STAs comprising the CSI associated with the one or more STAs and the first AP. The method further includes transmitting, by the first AP, a sounding request to a second AP of the multiple cooperating APs indicating that the second AP will send, to the one or more STAs, a request for CSI associated with the one or more STAs and the second AP. The method further includes receiving, by the first AP, a first AP-AP sharing message from the second AP including the one or more STAs and the CSI associated with the second AP. The method further includes receiving, by the second AP, the sounding request from the first AP. The method further includes transmitting, by the second AP to one or more STAs, a request for CSI associated with the one or more STAs and the second AP. The method further includes receiving, by the second AP, a response from the one or more STAs comprising CSI associated with the one or more STAs and the second AP. The method further includes transmitting, by the second AP, a first AP-AP sharing message to the first AP. The method may provide a MAC protocol for sharing information between APs participating in the CoBF.

[0021] According to a seventh aspect, there is provided an apparatus, the apparatus including modules configured to perform a method according to the various aspects described herein.

[0022] According to an eighth aspect, there is provided an apparatus, the apparatus including: a memory configured to store a program; and a processor configured to execute the program stored in the memory, wherein when the program stored in the memory is executed, the processor is configured to perform the method in the various aspects described herein.

[0023] According to a ninth aspect, a computer-readable medium is provided, the computer-readable medium storing program code executed by a device, the program code being used to perform the methods in the various aspects described herein.

[0024] According to a tenth aspect, there is provided a chip, the chip including a processor and a data interface, wherein the processor reads instructions stored in a memory by using the data interface to perform various aspects described herein.

[0025] Other aspects of the present disclosure provide apparatuses and systems configured to implement methods according to the various aspects disclosed herein. For example, wireless stations and access points can be configured with machine-readable memories that include instructions that, when executed by processors of these devices, configure the devices to perform the methods disclosed herein.

[0026] Although embodiments have been described above in relation to aspects of the present invention, they may also be implemented based on those embodiments. Those skilled in the art will understand that an embodiment may be implemented in conjunction with the described aspect, but may also be implemented with other embodiments of that aspect. It will be apparent to those skilled in the art if the embodiments are mutually exclusive or incompatible with each other. Some embodiments may be described in relation to one aspect, but may also be applicable to other aspects, as will be apparent to those skilled in the art. [Brief explanation of the drawings]

[0027] Further features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.

[0028] [Figure 1] FIG. 1 illustrates multi-AP cooperation according to one embodiment of the present disclosure. [Figure 2] FIG. 1 illustrates a protocol for inter-AP sharing in CoBF according to one embodiment of the present disclosure. [Figure 3] FIG. 1 illustrates an alternative protocol for inter-AP sharing in CoBF according to one embodiment of the present disclosure. [Figure 4] FIG. 10 is a diagram illustrating a frame format of an AP-AP shared frame according to one embodiment of the present disclosure. [Figure 5] FIG. 1 illustrates a 2-AP CoBF with one selected station (STA) at each participating AP, according to one embodiment of the present disclosure. [Figure 6] 1 is a schematic diagram of an electronic device that may perform some or all of the operations of the above methods and functions explicitly or implicitly described herein, according to different embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] It should be noted that throughout the accompanying drawings, like features are identified by like reference numerals.

[0030] The channel state information (CSI) feature was first introduced in 802.11n in the context of multiple-input multiple-output (MIMO). As can be understood by those skilled in the art, the CSI training sequence can be designed to measure the channel characteristics between the transmitter and receiver. CSI can represent how an electromagnetic signal propagates from the transmitter to the receiver and the combined effects of scattering, fading, and power attenuation with distance of the signal.

[0031] CSI may reflect the wireless signal propagation characteristics of the link from the transmitter to the receiver at a particular carrier frequency. CSI measurements may include information on when the wireless signal is transmitted through surrounding objects and people in the time, frequency, and spatial domains. CSI measurements may include amplitude variations of the CSI in the time domain, phase shifts of the CSI in the spatial and frequency domains (e.g., transmit / receive antennas and carrier frequencies), and phase shifts of the CSI in the time domain.

[0032] As mentioned above, Coordinated Beamforming (CoBF) is likely to be a key feature of the multi-access point (M-AP) cooperation topic in 802.11be R2. Introducing CoBF into IEEE 802.11 poses several standard issues. The first standard issue is that selected user information needs to be shared over the air between the coordinated APs. The second standard issue is determining the coordinating AP (which may be called the master AP in some circumstances) and the coordinated AP (which may be called the slave AP in some circumstances). As can be appreciated by those skilled in the art, the second standard issue can be resolved during the M-AP setup phase. The third standard issue is determining how to configure or define an interference coordination precoder between the coordinated APs. The embodiments described herein may address the first and third standard issues.

[0033] 1 illustrates multi-AP cooperation according to one embodiment of the present disclosure. In one embodiment, multi-AP cooperation system 100 includes a first AP—AP1 102—and a second AP—AP2 112. AP1 102 and AP2 112 simultaneously transmit their frames to one or more associated STAs, indicating that AP1 102 and AP2 112 are cooperating (which may be referred to as M-AP cooperation). While two APs are shown, those skilled in the art will understand that an M-AP cooperation system may include three or more APs.

[0034] Each AP may be associated with one or more STAs. For example, AP1 102 is associated with STA1 104 and STA1-U 106. Similarly, AP2 112 is associated with STA2 114 and STA2-U 116. While multiple STAs may be associated with an AP, the AP may select one or more STAs for scheduling. For example, AP1 102 has selected STA1 104 for scheduling, but has not selected STA1-U 106. Similarly, AP2 has selected STA2 114 for scheduling, but has not selected STA2-U 116. Thus, the channel

[0035]

number

[0036] ,

[0037]

number

[0038] ,

[0039]

number

[0040] , and

[0041]

number

[0042] is formed between selected STAs, STA1 104 and STA2 114, and their associated APs, AP1 102 and AP2 112, for frame transmission. As shown,

[0043]

number

[0044] is the channel between STA1 104 and AP1 102.

[0045]

number

[0046]

number

[0047] is the channel between STA2 114 and AP1 102,

[0048]

number

[0049] is the channel between STA1 104 and AP2 112,

[0050]

number

[0051] is the channel between STA2 114 and AP2 112.

[0052] In the context of beamforming, there can be two schemes for M-AP cooperation. The first scheme is CoBF, which means that beamforming is coordinated between the cooperated APs (e.g., AP1 102 and AP2 112). The second scheme is joint transmission, which means that the cooperated APs jointly transmit frames. Joint transmission may indicate that each of the cooperated APs shares data for its selected STAs with the other APs.

[0053] As described herein, each cooperating AP may select one or more STAs from among its associated STAs. In the embodiment of FIG. 1, AP1 102 selects STA1 104, and AP2 112 selects STA2 114. After selecting one or more STAs, each cooperating AP shares information about one or more selected STAs with other cooperating APs. Embodiments described herein may provide a protocol for sharing selected STAs between cooperating APs.

[0054] The embodiments described herein may provide for the calculation of an interference-adjusted precoder. Referring to FIG. 1, as can be understood by those skilled in the art,

[0055]

number

[0056] and

[0057]

number

[0058] is the interference,

[0059]

number

[0060] and

[0061]

number

[0062] is used for the actual information data. Therefore, to reduce interference,

[0063]

number

[0064] and

[0065]

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[0066] It is desirable to minimize as much as possible. Therefore, interference needs to be managed accordingly. As part of interference management, one or more interference-adjusted precoders may be computed and shared among cooperating APs.

[0067] As can be appreciated by those skilled in the art, during the sounding process 260 (see FIG. 2), cooperating APs, e.g., AP1 102 and AP2 112, may collect CSI information from all participating STAs.

[0068]

number

[0069] 102, AP2 112 may overhear the feedback information associated with channel

[0070]

number

[0071] 102, AP1 102 may eavesdrop on the feedback information to AP2 112 associated with AP1 102.

[0072] The method for calculating a precoder based on collected CSI information has not yet been standardized, and each vendor calculates its own precoder (because when a beamforming packet is transmitted by an AP to a STA, the STA does not need to know what type of beamforming is applied). However, for cooperating APs, having each AP independently calculate its own precoder is likely to lead to interference, and thus packets transmitted by the AP may not reach their destination (e.g., one or more receiving STAs). Therefore, the calculation of precoders for cooperating APs needs to be managed to minimize potential interference. Managing the calculation of precoders may include sharing precoder information over the air between cooperating APs before transmitting one or more beamforming packets.

[0073] The embodiments described herein may provide protocols and frame formats associated with the CoBF scheme. As can be understood by those skilled in the art, CoBF limits information sharing between coordinated APs, but such information sharing may not be avoided entirely. The embodiments described herein may define what information is shared between coordinated APs and, therefore, may provide the necessary protocols and their frame formats based on the defined information.

[0074] 2 illustrates a protocol for inter-AP sharing in CoBF according to one embodiment of the present disclosure. Protocol 200 is based on serial sounding in which NDPA and NDP are continuously transmitted by each of the cooperating APs. In protocol 200, there are two cooperating APs (AP1 102 and AP2 112). The cooperating APs include a coordinating AP, e.g., AP1 102, and one or more coordinated APs, e.g., AP2 112.

[0075] As can be appreciated by those skilled in the art, the sounding packet comprises one or more of an NDPA, an NDP, and a beamforming report (BFRP) trigger frame, which is used to simultaneously receive CSI reports from each of the participating STAs when there are multiple participating STAs.

[0076] In one embodiment, each cooperating AP transmits sounding packets continuously. For example, a cooperating AP, AP1 202, simultaneously transmits 212 an NDPAl 210 to each of the participating STAs, e.g., STA11 through STA1N 206 and one or more of STA21 through STA2M 208. STA11 through STA1N 206 refer to one or more STAs associated with AP1 202, and similarly, STA21 through STA2M 208 refer to one or more STAs associated with AP2 204.

[0077] One shortest inter-frame spacing (SIFS) time unit after transmitting NDPAl 210, the cooperating AP, AP1 202, simultaneously transmits (216) NDP1 214 to each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208.

[0078] Each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may calculate its CSI between the STA and AP1 202. For example, STA11 may calculate its CSI between STA11 and AP1 202, and STA1N may calculate its CSI between STA1N and AP1 202. Similarly, STA21 may calculate its CSI between STA21 and AP1 202, and STA2N may calculate its CSI between STA2N and AP1 202.

[0079] In an embodiment in which one more STA is participating in protocol 200, SIFS time units after transmitting NDP1 214, the cooperating AP, AP1 202, simultaneously transmits (220) a BFRP trigger frame 218 to the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208.

[0080] SIFS time units after receiving the BFRP trigger frame 218, one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may then each transmit their calculated CSI report to AP1 202 (226). The participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, simultaneously transmit their CSI reports 222 and 224 to AP1 202 (226).

[0081] SIFS time units after receiving one or more CSI reports 222 and 224 from the participating STAs, a cooperating AP, e.g., AP1 202, may transmit a sounding request frame 228 to the next cooperating AP (e.g., AP2 204) according to the sequence of the sounding process 260.

[0082] A SIFS time unit after receiving the sounding request frame 228, the coordinated AP, AP2 204, may transmit an acknowledgment frame 230 to the cooperating AP, AP1 202. A subsequent SIFS time unit later, the coordinated AP, AP2 204, simultaneously transmits (234) an NDPA2 232 to each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208. A SIFS time unit after transmitting NDPA2 232, the coordinated AP, AP2 204, simultaneously transmits (238) an NDP2 236 to each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208.

[0083] Each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may calculate its CSI between the STA and AP2 204. For example, STA11 may calculate its CSI between STA11 and AP2 204, and STA1N may calculate its CSI between STA1N and AP1 202. Similarly, STA21 may calculate its CSI between STA21 and AP2 204, and STA2N may calculate its CSI between STA2N and AP2 204.

[0084] In embodiments where multiple STAs are participating in protocol 200, SIFS time units after transmitting NDP2 236, the coordinated AP, AP2 204, simultaneously transmits (242) a BFRP trigger frame 240 to the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208. As can be appreciated by those skilled in the art, in embodiments where only one STA is participating, a BFRP trigger frame need not be transmitted.

[0085] SIFS time units after receiving the BFRP trigger frame 240, one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may then each transmit their calculated CSI report to AP2 204 (248). The participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, simultaneously transmit their CSI reports 244 and 246 to AP2 204 (248).

[0086] Each of the cooperating APs (including one or more coordinated APs, e.g., AP2 204, and a cooperating AP, e.g., AP1 202) may have multiple BFRP trigger phases depending on the number of STAs participating in the sounding procedure. For example, if there are too many associated STAs per participating AP to poll for CSI reports in a single BFRP trigger frame transmission, it may be necessary to poll for CSI reports multiple times per participating AP using multiple BFRP TF transmissions.

[0087] 2 shows two cooperating APs (cooperating AP, AP1 202 and coordinated AP, AP2 204), those skilled in the art will understand that there may be three or more cooperating APs. In the case of multiple coordinated APs (e.g., AP2 204 and AP3 (not shown)), SIFS time units after AP2 204 receives CSI reports 244 and 246 from the participating STAs, AP2 204 transmits a sounding request frame to the next coordinated AP, e.g., AP3. AP3 may send an Ack to AP2 204 (SIFS time units after receiving the sounding request frame). A subsequent SIFS time unit later, AP3 may transmit an NDPA simultaneously to each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208. After a SIFS unit time, AP3 may simultaneously transmit an NDP to each of one or more of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208. Each of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may calculate its CSI between the STA and AP3. SIFS unit time after transmitting the NDP, AP3 may simultaneously transmit a BFRP trigger frame to each of one or more of the participating STAs. SIFS time units after receiving the BFRP trigger frame, each of the participating STAs, e.g., STA11 through STA1N 206 and STA21 through STA2M 208, may then simultaneously transmit its calculated CSI report to AP3.

[0088] When the last coordinated AP (which can set up the serial sounding sequence during the setup phase) finishes the sounding process 260 (the CSI report action frame is collected by the last AP), an AP-AP sharing frame needs to be transmitted, i.e., the AP-AP sharing frame also implies the end of the serial sounding process 260.

[0089] The AP-AP shared frame transmission sequence may be in reverse order from the sounding sequence. In FIG. 2, the sounding sequence begins with the coordinating AP, AP1 202, and then with the coordinated AP, AP2 204. Thus, the AP-AP shared frame transmission process 262 begins with the coordinated AP2 204, and then with the coordinating AP, AP1 202. In the embodiment shown in FIG. 2, SIFS time units after receiving CSI reports 244 and 246, coordinated AP2 204 transmits (252) an inter-AP shared frame 250 to its coordinating AP, e.g., coordinating AP1 202. SIFS units after receiving the AP-AP shared frame 250, coordinating AP1 202 transmits (256) an inter-AP shared frame 254 to its coordinating AP, e.g., coordinated AP2 204.

[0090] In the case of multiple coordinated APs (e.g., AP2 204 and AP3 (not shown)), the sequence of AP-AP shared frame transmissions may be AP3, AP2 204, and AP1 202. Thus, SIFS time units after receiving CSI reports from one or more participating STAs, AP3 may transmit an AP-AP shared frame to the last cooperating AP (e.g., AP1 202) according to the sequence of the AP-AP shared frame transmission process. SIFS time units after that, AP2 204 may transmit an AP-AP shared frame to the last cooperating AP (e.g., AP1 202) according to the sequence of the AP-AP shared frame transmission process. SIFS time units after collecting all AP-AP shared frames from the coordinated APs (AP2 204 and AP3), the cooperating AP1 202 may transmit the intended AP-AP shared frame to all coordinated APs (in this case, AP2 204 and AP3).

[0091] The AP-AP shared frame transmission process 262 ends when a frame (e.g., frame 254) from the last cooperating AP (in this embodiment, cooperating AP, AP1 202) has been received by all participating APs according to the sequence of the AP-AP shared frame transmission process 262. The frame AP-AP shared frame 254 may include user selection information from the cooperating APs, as described further herein.

[0092] As described herein, the cooperating AP and one or more coordinated APs (including the sequence of the cooperating APs for successively transmitting sounding packet frames) are determined in the setup phase.

[0093] 3 illustrates an alternative protocol for inter-AP sharing in CoBF according to one embodiment of the present disclosure. In FIG. 3, the sounding sequence process in FIG. 3 is similar to the sounding sequence process 260 in FIG. 2. However, the AP-AP shared frame transmission process 362 is in the reverse order from the AP-AP shared frame transmission process 262. Therefore, the AP-AP shared frame transmission process 362 is in the same order as the sounding sequence process 260, e.g., AP1 initiates process 362 and AP2 terminates process 362.

[0094] According to one embodiment, the cooperating AP, AP1 202, sets the end time of the sounding process 260 with the last coordinated AP (e.g., AP2 204 in FIG. 3 ). The cooperating AP, AP1 202, may calculate the end time of the sounding process 260 based on one or more of the cooperating APs, the SIFS time unit, and the participating STAs. A SIFS time unit after the last coordinated AP, in this embodiment, AP2 204, receives the CSI reports 244 and 246 from the participating STAs, the cooperating AP, AP1 202, transmits (302) the AP-AP shared frame 254 to the last cooperating AP (in this embodiment, AP2 204) according to the sequence of the AP-AP shared frame transmission process 362. SIFS time units after receiving the AP-AP sharing frame 254, the coordinated AP, AP2 204, transmits 304 the AP-AP sharing frame 250 to its cooperating AP (eg, AP1 202).

[0095] In the case of multiple coordinated APs (e.g., AP2 204 and AP3 (not shown)), the sequence of AP-AP shared frame transmission may be AP1 202, AP2 204, and AP3. According to one embodiment, the AP-AP shared frame transmission process may be as follows: AP1 202 sets the end time of the sounding process 260 by the last coordinated AP (e.g., AP3). SIFS time units after the last coordinated AP, e.g., AP3, receives a CSI report from the joining STA, the cooperating AP, AP1 202, transmits an AP-AP shared frame to the last cooperating AP (e.g., AP3) according to the sequence of the AP-AP shared frame transmission process. SIFS time units after receiving the AP-AP shared frame, the coordinated AP, AP2 204, transmits an AP-AP shared frame to the last cooperating AP (e.g., AP3) according to the sequence of the AP-AP shared frame transmission process. After collecting all AP-AP shared frames from the participating APs (in this case AP1 202 and AP2 204), SIFS time units later, the last AP (AP3) may transmit the target AP-AP shared frame to all participating APs (in this case AP1 202 and AP2 204) according to the AP-AP shared frame transmission process sequence.

[0096] The AP-AP shared frame transmission process ends when the frame from the last cooperating AP has been received by all participating APs according to the sequence of the AP-AP shared frame transmission process. The last AP-AP shared frame may include user selection information from the cooperating AP, as described further herein.

[0097] The embodiments described with reference to FIGS. 2 and 3 may provide a medium access control (MAC) protocol for APs participating in a CoBF to share necessary information with each other.

[0098] FIG. 4 illustrates a frame format of an AP-AP shared frame according to one embodiment of the present invention.

[0099] 2 and 3 (e.g., AP-AP shared frames 250 and 254). The frame format may comprise one or more of a PHY header field 402, a MAC header field 404, an AP identifier (ID) field 406 indicating the identity of the transmitting AP, a selected user field 408, a precoder index field 410, and a frame check sequence (FCS) field 412.

[0100] In one embodiment, the PHY header 402 and the MAC header 404 may share the same format as that of an NDPA frame. The MAC header 404 may indicate that the frame is an AP-AP shared frame. The MAC header 404 may include a subframe type to indicate that the frame is an AP-AP shared frame. In another embodiment, the AP-AP shared frame may be indicated through an NDPA variant frame that may be implemented in future designs.

[0101] In another embodiment, the AP ID field may not be present, and the AP ID information may be indicated in the MAC header field 404 via a transmitter address (TA) field (within the MAC header field). In other embodiments, the AP ID information may be indicated via the AP ID field 406.

[0102] The selected users field 408 may indicate a list of selected users (one or more selected STAs) of the participating AP indicated in the AP ID field 406. As described with reference to Figure 1, each of the cooperating APs may select one or more STAs from their associated STAs. The selected STAs are then shared between the cooperating APs via an AP-to-AP shared frame transmission process according to the embodiments described with reference to Figures 2 and 3.

[0103] The selected users field 408 may further indicate the number of streams per selected user. The selected users field 408 may further indicate a user ID (which may be a MAC address or associated ID (AID)). The selected users field 408 may further indicate a bandwidth (BW), a modulation and coding system (MCS), and other related information.

[0104] CSI information between the corresponding AP and each selected STA may also be included in the selected user field 408. If the precoder calculation is not standardized, i.e., implementation specific, the AP-AP shared frame may need to include an interference coordination (IA) CoBF precoder field 411. Thus, the precoder index field 410 may need to be replaced by the IA CoBF precoder field 411 if the precoder calculation is not standardized.

[0105] In embodiments where the AP-AP shared frame includes the IA CoBF precoder field 411, the IA CoBF precoder field may be indicated by the last cooperating AP in the AP-AP shared transmission process, e.g., 262 and 362. For example, in the embodiment following FIG. 2, the IA CoBF precoder field 411 may be indicated by the cooperating AP, AP1 202, e.g., via the AP-AP shared frame 254, and in the embodiment following FIG. 3, the IA CoBF precoder field 411 may be indicated by the coordinated AP, AP2 204, e.g., via the AP-AP shared frame 250.

[0106] The selected user may be determined by the corresponding participating AP from among the associated STAs. For example, the selected user for a cooperating AP, e.g., AP1 202, is determined or selected by the cooperating AP from among the associated STAs of the cooperating AP, and similarly, the selected user for a coordinated AP, e.g., AP2 204, is determined or selected by the coordinated AP from among the associated STAs of the coordinated AP. In the embodiment following FIG. 2, if the AP ID field 406 indicates a cooperating AP (e.g., AP1 202), the AP-AP shared transmission procedure 262 may end (e.g., if the AP-AP shared frame 252 indicates the cooperating AP ID in the AP ID field). In the embodiment of FIG. 2, the cooperating AP, AP1 202, may further indicate a precoder index in the precoder index field 410, where the precoder is standardized for CoBF.

[0107] As described herein, for cooperating APs, it is desirable to manage interference that may be present. Interference management may include calculating a precoder index. The calculation of the precoder index may be standardized. The precoder index field 410 may indicate a method for calculating the precoder index.

[0108] As can be appreciated by those skilled in the art, CSI information received during the sounding sequence process 260 (e.g., CSI report frames 222, 224, 244, 246) may be required for precoder calculations. Thus, during the AP-AP shared transmission process 262 and 363, CSI information is shared between cooperating APs. The CSI information may be indicated in the selected user field 408.

[0109] In some embodiments, precoders for all cooperating APs may be calculated by the last AP in the AP-AP shared transmission process 262 and 363 and shared among the cooperating APs. For example, in the embodiment of Figure 2, the last AP in the AP-AP shared transmission process 262 is the cooperating AP, AP1 202, which may calculate precoders for all cooperating APs and share the calculated precoders with the corresponding cooperating APs. Similarly, in the embodiment of Figure 3, the last AP in the AP-AP shared transmission process 362 is the coordinated AP, AP2 204, which may calculate precoders for all cooperating APs and share the calculated precoders with the corresponding cooperating APs.

[0110] 5 illustrates a 2-AP CoBF with one selected STA at each participating AP, according to one embodiment of the present disclosure. FIG. 5 is similar to FIG. 1 and illustrates the cooperating APs and the selected STAs. The cooperating APs perform simultaneous cooperative beamforming (BF) transmissions. As described herein, to minimize or cancel the resulting interference between the cooperating APs,

[0111]

number

[0112] and

[0113]

number

[0114] It is desirable to eliminate channels, and to do so, interference must be managed.

[0115] In one embodiment, managing interference may include applying zero-forcing beamforming (ZF-BF), in which the channels at each cooperating AP may be aggregated and reconstructed. For example, at AP1 102, the channel is

[0116]

number

[0117] and

[0118]

number

[0119] Therefore, the aggregated channels at AP1 are

[0120]

number

[0121] and can be denoted as C1. Similarly, the channel at AP2 112 is

[0122]

number

[0123] and

[0124]

number

[0125] Therefore, the aggregated channels at AP2 112 are

[0126]

number

[0127] and can be denoted as C2.

[0128] The ZF-BF-based IA precoder in AP1 202 may be obtained by taking the pseudo-inverse of the C1 matrix and then taking the first K1 columns. The ZF-BF-based IA precoder in AP2 112 may be obtained by taking the pseudo-inverse of the C2 matrix and then taking the last K2 columns, where K1 is the rank of the transmission by AP1 and K2 is the rank of the transmission by AP2.

[0129] Upon completion of the inter-AP sharing process 262 or 362, all participating APs know the selected STAs (including the number of scheduled streams for each STA) that will be scheduled for the CoBF. As described above, each participating AP may calculate an aggregated channel based on the channels formed between the participating AP and the selected STAs, and then obtain the pseudo-inverse of the aggregated channel. In the case of two-AP cooperation, the cooperating AP may obtain the first K1 columns of the aggregated channel of the precoder, where K1 represents the size of the transmission rank of the cooperating AP. The cooperated AP may obtain the last K2 columns of the aggregated channel of the precoder, where K2 represents the size of the transmission rank of the cooperated AP.

[0130] In some embodiments, the last participating AP in an AP-AP sharing process may calculate precoders for all cooperating APs according to ZF-BF and share the calculated precoders with their corresponding APs via the AP-AP sharing frame. For example, in the embodiment of FIG. 2, cooperating AP1 202 may calculate its ZF-BF IA precoder and a ZF-BF IA precoder for a coordinated AP, e.g., AP2 204, and share the calculated precoders with their corresponding APs via the AP-AP sharing frame 254. Similarly, in the embodiment of FIG. 3, coordinated AP2 204 may calculate its ZF-BF IA precoder and a ZF-BF IA precoder for a coordinated AP (in the case of multiple coordinated APs) and a cooperating AP, e.g., AP1 202, and share the calculated precoders with their corresponding APs via the AP-AP sharing frame 250.

[0131] As described herein, all cooperating APs share their CSI information via sounding sequence processing 260. Thus, the AP responsible for computing the precoders for all cooperating APs (e.g., AP1 202 in FIG. 2 and AP2 in FIG. 3) has knowledge of the CSI information received at each cooperating AP and can therefore compute the precoder. After computing the precoder, the responsible AP may transmit an AP-AP shared frame to its cooperating APs, included in the AP-AP shared frame, indicated via the IA CoBF precoder field 411, the computed precoder.

[0132] After receiving the calculated precoder via the AP-AP shared frame, the receiving AP (e.g., AP2 204 in FIG. 2 and AP1 202 in FIG. 3) and the serving AP may cooperate using the calculated precoder to simultaneously transmit CoBF packets.

[0133] In some embodiments, for example, in accordance with Federal Communications Commission (FCC) regulations, transmit or transmitter (TX) power may need to be kept constant below a certain level regardless of the number of TX chains, and therefore the precoder may need to be normalized according to, for example, the MIMO configuration.

[0134] In one embodiment, the precoder at AP1 is denoted as P1, the precoder at AP2 is denoted as P2, and then the new normalized precoders are denoted as P1, P2, P3, P4, P5, P6, P7, P8, P9, P10, P11, P12, P13, P14, P15, P16, P17, P18, P19, P19, P20, P21, P22, P23, P24, P25, P26, P27, P28, P29 ...

[0135]

number

[0136] and

[0137]

number

[0138] It can be written as:

[0139] "||P||E" may represent the Euclidean norm, where the magnitude of each P matrix element is squared and summed over all elements, and then a square root operation is applied. Thus, the Euclidean norm is obtained using the square root of the sum of the magnitudes of all matrix elements. In some embodiments, TX power normalization for each precoder of a participating AP may be performed individually. Thus, each participating AP may perform its own TX power normalization.

[0140] The precoder information may vary for each subcarrier in one or more participating STAs. Preferably, the phase precoder information for each subcarrier is continuous. However, because not all subcarriers of one or more STAs have reference signals for channel estimation, one or more STAs may rely on interpolation or smoothing for channel estimation.

[0141] When beamforming (BF) is applied to a frame, phase information (of the precoder between subcarriers) may become discontinuous between tones, making channel estimation via interpolation difficult. Therefore, in some embodiments, phase-continuous processing may be applied during beamforming to avoid the phase-discontinuity problem. In some embodiments, channel estimation during the sounding process to measure CSI information may be performed through 802.11 long training field (LTF)-based reference signals. In such embodiments, CSI information may be obtained for all numbers of groups of tones (Ng) and eventually for all tones.

[0142] In one embodiment, P0, P1, P2, ... may represent the column vectors of the precoding matrix for each tone 0, 1, 2, etc. based on the CSI information during the sounding process. Thus, the continuous phase-based precoding column vectors of the precoding matrix are P0, P1, P2, ...

[0143]

number

[0144] ,

[0145]

number

[0146] , where

[0147]

number

[0148] ,

[0149]

number

[0150] and T1 may represent a phase-continuous update, i.e.,

[0151]

number

[0152] The procedure explained (

[0153]

number

[0154] ,

[0155]

number

[0156] and T1 may represent a phase-continuous update, i.e.,

[0157]

number

[0158] ) is repeated until the tones of each OFDM symbol are completed. Phase-continuous processing for each precoder of a participating AP may be performed independently. Thus, each participating AP may perform its own phase-continuous processing during beamforming.

[0159] The embodiments described herein may provide procedures for AP-to-AP shared transmissions involving cooperating APs. The embodiments described herein may further provide a format for an AP-to-AP shared frame.

[0160] 6 is a schematic diagram of an electronic device 600 that can perform some or all of the operations of the above methods and functions explicitly or implicitly described herein according to different embodiments of the present invention. For example, a computer with network capabilities can be configured as electronic device 600. In some embodiments, electronic device 600 can be a UE, an AP, a STA, etc., as will be understood by those skilled in the art.

[0161] As shown, electronic device 600 may include a processor 610, such as a central processing unit (CPU), or a dedicated processor such as a graphics processing unit (GPU), or other such processor unit, memory 620, non-transitory mass storage 630, an input-output interface 640, a network interface 650, and a transceiver 660, all communicatively coupled via a bidirectional bus 670. According to a particular embodiment, any or all of the illustrated elements may be utilized, or only a subset of the elements may be utilized. Furthermore, electronic device 600 may include multiple instances of a particular element, such as multiple processors, memories, or transceivers. Elements of a hardware device may also be directly connected to other elements without the use of a bidirectional bus. In addition to, or in place of, the processor and memory, other electronic devices, such as integrated circuits, may be used to perform the necessary logical operations.

[0162] The memory 620 may include any type of non-transitory memory, such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), or any combination thereof. The mass storage element 630 may include any type of non-transitory storage device, such as a solid-state drive, a hard disk drive, a magnetic disk drive, an optical disk drive, a USB drive, or any computer program product configured to store data and machine-executable program code. According to particular embodiments, the memory 620 or mass storage 630 may record statements and instructions executable by the processor 610 to perform any of the foregoing method operations described above.

[0163] Embodiments of the present invention can be implemented using electronic hardware, software, or a combination thereof. In some embodiments, the present invention is implemented by one or more computer processors executing program instructions stored in memory. In some embodiments, the present invention is implemented partially or fully in hardware, for example, using one or more field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICs) to rapidly perform processing operations.

[0164] While specific embodiments of the technology have been described herein for purposes of illustration, it will be understood that various modifications can be made without departing from the scope of the technology. Accordingly, the specification and drawings should be considered merely as illustrative of the invention as defined by the appended claims, and it is intended to cover any and all modifications, variations, combinations, or equivalents that fall within the scope of the invention. In particular, it is within the scope of the technology to provide a computer program product or program element, or a program storage or memory device, such as a magnetic or optical wire, tape, or disk, for storing machine-readable signals for controlling the operation of a computer in accordance with the methods of the technology and / or for configuring some or all of its components in accordance with the systems of the technology.

[0165] The operations associated with the methods described herein may be implemented as coded instructions in a computer program product. In other words, the computer program product is a computer-readable medium having recorded thereon software code for performing the methods when the computer program product is loaded into a memory and executed on a microprocessor of a wireless communication device.

[0166] Furthermore, each operation of the method may be performed according to one or more program elements, modules, or objects created from any programming language such as C++, Java, etc. on any computing device such as a personal computer, a server, a PDA, etc. Furthermore, each operation, or a file or object implementing each operation, etc., may be performed by dedicated hardware or a circuit module designed for that purpose.

[0167] Through the description of the foregoing embodiments, the present invention can be implemented by using hardware alone or by using software and a required universal hardware platform. Based on such understanding, the technical solutions of the present invention can be embodied in the form of a software product. The software product can be stored in a non-volatile or non-transitory storage medium, which can be a compact disc read-only memory (CD-ROM), a USB flash disk, or a removable hard disk. The software product includes a number of instructions that enable a computer device (a personal computer, a server, or a network device) to execute the methods provided in the embodiments of the present invention. For example, such execution can correspond to the simulation of the logical operations described herein. The software product can additionally or alternatively include a number of instructions that enable a computer device to perform operations for configuring or programming a digital logic device according to the embodiments of the present invention.

[0168] While the invention has been described with reference to particular features and embodiments thereof, it will be apparent that various modifications and combinations can be made without departing from the invention. Accordingly, the specification and drawings should be considered merely as illustrative of the invention as defined by the appended claims, and it is intended to cover any modifications, variations, combinations or equivalents that fall within the scope of the invention.

Claims

1. transmitting, by a first AP of a plurality of cooperating access points (APs), to one or more stations (STAs) associated with the plurality of cooperating APs, a request for channel state information (CSI) associated with the one or more STAs and the first AP; receiving, by the first AP, a response from the one or more STAs comprising the CSI associated with the one or more STAs and the first AP; transmitting, by the first AP, a sounding request to a second AP of the plurality of cooperating APs, indicating that the second AP requests CSI from the one or more STAs associated with the one or more STAs and the second AP; receiving, by the first AP, from the second AP, a first AP-AP sharing message including the CSI associated with the one or more STAs and the second AP; sending, by the first AP, a second AP-AP sharing message to the second AP, the second AP including the CSI associated with the one or more STAs and the first AP; A method comprising:

2. 2. The method of claim 1 , wherein the second AP-AP sharing message comprises the frame including one or more fields indicating one or more of: an identifier of the first AP; an indication that the frame is an AP-AP sharing frame; one or more STAs associated with the first AP; the CSI associated with the one or more STAs and the first AP; one or more streams per STA for the one or more STAs associated with the first AP; and precoder information associated with the second AP.

3. and further comprising: computing, by the first AP, a plurality of precoders, each computed precoder comprising: corresponding to each AP of the plurality of cooperating APs, The method of claim 2 , based on CSI information associated with the one or more STAs and the respective APs.

4. The method of claim 2 or 3, wherein the precoder information indicates a calculated precoder of the plurality of precoders.

5. The method according to claim 2 , wherein the precoder information indicates a precoder index indicating a precoder calculation method.

6. The method of claim 2 , wherein the CSI associated with the one or more STAs and the first AP is indicated in a selected users field of the frame.

7. The method of claim 2 , wherein the identifier of the first AP is indicated in a transmitter address (TA) field of a medium access control (MAC) header of the frame.

8. receiving, by a first access point (AP), from a second AP, a first sharing message including channel state information (CSI) associated with the second AP, wherein the first AP and the second AP are associated; transmitting, by the first AP to the second AP, a second sharing message comprising a frame including one or more fields indicating one or more of: an identifier of the first AP, an indication that the frame is an AP-AP shared frame, one or more stations (STAs) associated with the first AP, CSI associated with the first AP, one or more streams per STA for the one or more STAs associated with the first AP, and precoder information associated with the second AP; A method comprising:

9. The method of claim 8 , further comprising: calculating, by the first AP, a precoder associated with the second AP based on the CSI associated with the second AP.

10. The method of claim 9 , wherein the precoder information associated with the second AP indicates the calculated precoder associated with the second AP.

11. The method of claim 9 or 10, wherein the precoder information associated with the second AP indicates a precoder index that indicates a precoder calculation method.

12. The method of claim 9 , wherein the CSI associated with the first AP is indicated in a selected users field of the frame.

13. The method of any one of claims 9 to 12, wherein the identifier of the first AP is indicated in a transmitter address (TA) field of a medium access control (MAC) header of the frame.

14. at least one processor; and at least one machine-readable medium storing executable instructions, the executable instructions, when executed by the at least one processor, transmitting, to one or more stations (STAs) associated with a plurality of cooperating access points (APs), a request for channel state information (CSI) associated with the one or more STAs and a first AP; receiving a response from the one or more STAs comprising the CSI associated with the one or more STAs and the first AP; transmitting a sounding request to a second AP of the plurality of cooperating APs, indicating that the second AP requests CSI from the one or more STAs associated with the one or more STAs and the second AP; receiving a first AP-AP sharing message from the second AP, the first AP-AP sharing message including the CSI associated with the one or more STAs and the second AP; sending a second AP-AP sharing message to the second AP, the second AP including the CSI associated with the one or more STAs and the first AP; and configuring the first AP of the plurality of cooperating APs to perform:

15. 15. The apparatus of claim 14, wherein the second AP-AP sharing message comprises a frame including one or more fields indicating one or more of: an identifier of the first AP; an indication that the frame is an AP-AP sharing frame; one or more STAs associated with the first AP; the CSI associated with the one or more STAs and the first AP; one or more streams per STA for the one or more STAs associated with the first AP; and precoder information associated with the second AP.

16. The executable instructions, when executed by the at least one processor, Further configuring the first AP to compute a plurality of precoders, each computed precoder comprising: corresponding to each AP of the plurality of cooperating APs, The apparatus of claim 15 , based on CSI information associated with the one or more STAs and the respective APs.

17. The apparatus of claim 16 , wherein the precoder information indicates a calculated precoder of the plurality of precoders.

18. The apparatus according to claim 15 , wherein the precoder information indicates a precoder index indicating a precoder calculation method.

19. The apparatus of claim 15 , wherein the CSI associated with the one or more STAs and the first AP is indicated in a selected users field of the frame.

20. 20. The apparatus of claim 15, wherein the identifier of the first AP is indicated in a transmitter address (TA) field of a medium access control (MAC) header of the frame.

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