TECHNICAL FIELD

The present disclosure relates to wireless communications. More specifically, the present disclosure relates to devices and methods for sounding for coordinated beamforming.

BACKGROUND

IEEE-802.11 -based WLANs have become popular at an unprecedented rate. WLANs support a variety of data transfer modes including (but not only) file transfer, emails, web browsing and real-time applications such as audio and video applications. For efficiently supporting high throughputs, the evolving IEEE 802.11 standards specify several transmission (TX) schemes. Particularly useful for increasing the link throughput are TX schemes which deploy multiple TX antennas (some, but not all, also requiring multiple RX antennas on the receiver side), which are so called MIMO modes. Multiple TX antennas can be utilized in different advantageous ways, such as spatial TX diversity for improving the link reliability and performance, beamforming (BF), i.e. focusing the radiated power in the direction(s) of target receiver(s) and/or suppressing it in undesirable directions, for reducing unwanted interference to non-targeted receivers, and/or spatial multiplexing (SM), i.e. sending multiple data streams simultaneously over the same time-frequency resources, either to the same receiver or to different ones. Coordinated beamforming (C-BF) is a cooperation scheme where multiple access points (APs) cooperate such that when an AP transmits to its associated stations (STAs), i.e. to the ST As of its basic service set (BSS) it creates a null towards the STAs of an overlapping BSS (OBSS). In this way, the OBSS STAs are not interfered by the AP’s transmission.

SUMMARY

It is an objective of the present disclosure to provide improved devices and methods for sounding for coordinated beamforming (C-BF).

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.

According to a first aspect, a multi-antenna non-AP station configured to communicate with a first BSS (basic service set) multi-antenna AP is provided. The multi-antenna non-AP station comprises a communication interface with a plurality of antennas configured to receive one or more first pilot signals from the first BSS multi-antenna AP and one or more second pilot signals from a second OBSS (overlapping basic service set) multi-antenna AP.

Moreover, the multi-antenna non-AP station comprises a processing circuitry configured to estimate a first channel matrix H _1,1 representing the channel between the multi-antenna non- AP station and the first multi-antenna AP based on the one or more first pilot signals and a second channel matrix H _1,2 representing the channel between the multi-antenna non-AP station and the second multi-antenna AP based on the one or more second pilot signals. The processing circuitry is further configured to determine, based on the first channel matrix H _1,1 , an interference rejection combiner matrix W ₁ for suppressing a multi-user interference caused by the first BSS multi-antenna AP.

The communication interface is further configured to send first feedback information based on the first channel matrix to the first BSS multi-antenna AP and second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 to the second OBSS multi-antenna AP. Based on the second feedback information the second OBSS multi-antenna AP may use an improved beamformer causing less multi-user interference.

In a further possible implementation form, the second feedback information is based on a basis (i.e. a set of basis vectors), in particular an orthonormal basis of the matrix product of the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 , i.e. a basis of W _{1 ▪} H _1,2 (which is herein denoted as B = W _{1 ▪} H _1,2 ).

In a further possible implementation form, the processing circuitry is configured to generate the basis, in particular the orthonormal basis of B = W _{1 ▪} H _1,2 based on a QR decomposition of B ^* (i.e. the complex conjugate of the matrix B = W ₁ ▪ H _1,2 ) in the form Q ▪ R.

In a further possible implementation form, the processing circuitry is configured to generate the basis, in particular the orthonormal basis of B = W ₁ ▪ H _1,2 based on a Gram-Schmidt factorization.

In a further possible implementation form, the processing circuitry is configured to generate the basis, in particular the orthonormal basis of B = W ₁ ▪ H _1,2 based on a Cholesky factorization or a singular value decomposition (SVD). In a further possible implementation form, the processing circuitry is configured to generate the second feedback information based on one or more orthonormal vectors of the basis of the row space of the matrix B = W ₁ ▪ H _{1 ,2} .

In a further possible implementation form, the plurality of antennas defines a maximum number of spatial communication streams between the multi-antenna non-AP station and the first BSS multi-antenna AP, wherein the communication interface is configured to communicate with the first BSS multi-antenna AP using less than the maximum number of spatial communication streams.

In a further possible implementation form, for determining the interference rejection combiner matrix W ₁ the processing circuitry is configured to decompose a singular value decomposition of the first channel matrix into a first component due the used spatial communication streams and a second component due to the unused spatial communication streams, i.e. wherein the interference rejection combiner matrix W ₁ is given by wherein denotes the complex conjugate of the matrix

In a further possible implementation form, the processing circuitry is configured to generate the first feedback information by determining one or more columns of the matrix

In a further possible implementation form, the multi-antenna non-AP station further comprises a memory configured to store the interference rejection combiner matrix Wi. Storing the interference rejection combiner matrix W ₁ in a memory of the multi-antenna non- AP station allows using the interference rejection combiner matrix W ₁ for efficiently generating the feedback information for a further OBSS multi-antenna AP.

In a further possible implementation form, the processing circuitry is further configured to determine a signal strength of the one or more second pilot signals from the second OBSS multi-antenna AP, wherein the communication interface is further configured to send the second feedback information to the second OBSS multi-antenna AP depending on the signal strength of the one or more second pilot signals.

In a further possible implementation form, the communication interface is configured to receive a null data packet announcement, NDPA, frame and a null data packet, NDP, frame from the first BSS multi-antenna AP and/or the second OBSS multi-antenna AP, wherein the NDP frame comprises the one or more first pilot signals and/or the one or more second pilot signals.

In a further possible implementation form, the communication interface is further configured to, in response to receiving a beamforming report poll, BFRP, from the first BSS multiantenna AP and/or the second OBSS multi-antenna AP, to send the first feedback information to the first BSS multi-antenna AP and/or the second feedback information to the second OBSS multi-antenna AP.

In a further possible implementation form, the processing circuitry is configured to determine the first feedback information based on the first channel matrix H _1,1 , in response to receiving the one or more first pilot signals, and to determine the second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 in response to receiving the one or more first pilot signals and the one or more second pilot signals.

In a further possible implementation form, the communication interface is configured to send the first feedback information to the first multi-antenna AP, in response to receiving a first null data packet, NDP, including the one or more first pilot signals, and a first beamforming report poll, BFRP, from the first multi-antenna AP, and the communication interface is further configured to send the second feedback information to the second multi-antenna AP, in response to receiving a second BFRP from the second multi-antenna AP, wherein the second feedback information is based on the one or more first pilot signals of the first NDP and the one or more second pilot signals of a second NDP, wherein the second NDP was transmitted at an earlier stage by the second multi-antenna AP to a further multi-antenna non-AP station and also received by the multi-antenna non-AP station.

In a further possible implementation form, the communication interface is configured to receive the one or more first pilot signals and the one or more second pilot signals substantially simultaneously from the first multi-antenna AP and the second multi-antenna AP and the communication interface is further configured to send the first feedback information and the second feedback information substantially simultaneously to the first multi-antenna AP and the second multi-antenna AP.

In a further possible implementation form, the multi-antenna non-AP station is a multiantenna non-AP station in accordance with the standard IEEE 802.11. According to a second aspect a method for operating a multi-antenna non-AP station configured to communicate with a first BSS multi-antenna AP is provided. The method comprises the steps of: receiving by a plurality of the antennas of the multi-antenna non-AP station one or more first pilot signals from the first BSS multi-antenna AP and one or more second pilot signals from a second OBSS multi-antenna AP; estimating a first channel matrix H _1,1 representing the channel between the multi-antenna non-AP station and the first BSS multi-antenna AP based on the one or more first pilot signals and a second channel matrix H _{1 ,2} representing the channel between the multi- antenna non-AP station and the second OBSS multi-antenna AP based on the one or more second pilot signals; determining, based on the first channel matrix H _1,1 , an interference rejection combiner matrix W ₁ for suppressing a multi-user interference caused by the first BSS multi-antenna AP; and sending first feedback information based on the first channel matrix to the first BSS multi-antenna AP and second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _{1 ,2} to the second OBSS multi-antenna AP.

In a further possible implementation form, the method further comprises the steps of: determining the first feedback information based on the first channel matrix H _1,1 in response to receiving the one or more first pilot signals; and determining second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _{1 ,2} in response to receiving the one or more first pilot signals and the one or more second pilot signals.

In a further possible implementation form, the first feedback information is sent by the multi- antenna non-AP station, in response to receiving a first null data packet, NDP, including the one or more first pilot signals, and a first beamforming report poll, BFRP, from the first multi- antenna AP, and the second feedback information is sent, in response to receiving a second BFRP from the second multi-antenna AP, wherein the second feedback information is based on the one or more first pilot signals of the first NDP and the one or more second pilot signals of a second NDP, wherein the second NDP was transmitted at an earlier stage by the second OBSS multi-antenna AP to a further multi-antenna non-AP station and also received by the multi-antenna non-AP station.

In a further possible implementation form, the one or more first pilot signals and the one or more second pilot signals are received by the multi-antenna non-AP station substantially simultaneously from the first BSS multi-antenna AP and the second OBSS multi-antenna AP and the first feedback information and the second feedback information are sent by the multi- antenna non-AP station substantially simultaneously to the first BSS multi-antenna AP and the second OBSS multi-antenna AP.

The method according to the second aspect of the present disclosure can be performed by the multi-antenna non-AP station according to the first aspect of the present disclosure.

Thus, further features of the method according to the second aspect of the present disclosure result directly from the functionality of the multi-antenna non-AP station according to the first aspect of the present disclosure as well as its different implementation forms described above and below.

According to a third aspect a computer program product is provided, comprising program code which causes a computer or a processor to perform the method according to the second aspect, when the program code is executed by the computer or the processor.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present disclosure are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 shows a wireless communication network, including a first AP associated with a first non-AP station according to an embodiment forming a first BSS and a second AP associated with a second non-AP station forming a second BSS;

Fig. 2 shows an exemplary wireless communication network, including a multi-antenna AP and three multi-antenna non-AP stations; Fig. 3 shows an exemplary wireless communication network, including a multi-antenna AP and three multi-antenna non-AP stations;

Fig. 4 shows an exemplary wireless communication network, including two multi-antenna APs and four multi-antenna non-AP stations;

Fig. 5 shows the PER as a function of the SNR for the exemplary scenarios shown in figures 2 to 4;

Fig. 6 shows a wireless communication network, including a first and a second multi-antenna AP and a plurality of multi-antenna non-AP stations, including a multi-antenna non-AP station according to an embodiment;

Fig. 7 shows the PER as a function of the SNR for different configurations of the wireless communication network shown in figure 6;

Fig. 8 shows a sequence diagram illustrating a sounding procedure implemented by the wireless communication network of figure 6;

Fig. 9 shows a sequence diagram illustrating a further sounding procedure implemented by the wireless communication network of figure 6;

Fig. 10 shows a sequence diagram illustrating a further sounding procedure implemented by the wireless communication network of figure 6; and

Fig. 11 is a flow diagram illustrating a method of operating a multi-antenna wireless non-AP station according to an embodiment.

In the following, identical reference signs refer to identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

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.

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.

Figure 1 shows an exemplary wireless communication network 100 configured for coordinated beamforming (C-BF). The wireless communication network 100 comprises a first multi-antenna AP 110a (referred to as "AP1" in figure 1) communicating within its BSS1 with the multi-antenna non-AP station 120a, i.e. its associated station 120a (referred to as "STA 1" in figure 1) and a second multi-antenna AP 110b (referred to as "AP2" in figure 1) communicating within its BSS2 with the associated station 120b (referred to as "STA 2" in figure 1). As will be appreciated, for the first multi-antenna AP 110a and its associated station 120a the BSS2 constitutes an OBSS (and vice versa). In an embodiment, the wireless communication network 100 and the elements thereof may be configured to comply with one or more of the standards of the IEEE 802.11 family of standards.

As illustrated in figure 1 , when the first multi-antenna AP 110a transmits to its associated station 120a, it creates a null towards the OBSS station 120b, and, when the second multiantenna AP 110b transmits to its associated station 120b, it creates a null towards the OBSS station 120a. This allows both APs 110a, 110b to transmit to their respective stations 120a, 120b simultaneously without degrading the SNR or SINR at the receiver of each station 120a, 120b. In order to support C-BF, each AP 110a, 110b needs to precode its data such that the SNR for its associated STA is sufficiently high while creating a null towards OBSS STAs (such that the OBSS STAs cannot "hear" the AP’s transmission). This means each AP needs to know either the channels or the precoders for their associated STAs as well as the OBSS STAs which require nulling. This information may be obtained by means of a sounding procedure.

Before describing several embodiments of the multi-antenna non-AP station 120a in more detail, in the following some technical background as well as terminology concerning wireless networks and devices in accordance with the IEEE 802.11 WLAN standard will be introduced under reference to figures 2 to 5. In the following one or more of the following acronyms may be used:

ACK Acknowledgment

AP Access Point

BA block-ACK - in 802.11 , typically a receiver is expected to respond to a data transmission by responding with an ACK; if multiple frames are transmitted within a single PPDU, each has to be acknowledged

BFR Beamforming Feedback Report

BFRP Beamforming Feedback Report Poll

BSS Basic Service Set

DoF Degrees of Freedom

DL Downlink

MAC Medium Access Control

MIMO Multiple Input Multiple Output

MPDU MAC Protocol Data Unit

MUI Multi-User Interference

MU-MIMO Multi-User MIMO

NDP Null Data Packet

NDPA Null Data Packet Announcement

OBSS Overlapping Basic Service Set

OFDMA Orthogonal Frequency Division Multiple Access

PHY Physical Layer

PPDU PHY Protocol Data Unit

Rx Receive

SINR Signal to Interference and Noise Ratio

SNR Signal to Noise Ratio

STA Station (in general, can be either an AP STA or a non-AP STA) SVD Singular Value Decomposition

TF Trigger Frame - in 802.11ax, the Trigger Frame was introduced as a means to trigger a STA or multiple STAs to transmit simultaneously and in sync to the triggering AP Tx Transmit

UL Uplink

Figure 2 shows an exemplary wireless communication network 10 comprising a single multiantenna AP 11 a and a plurality of multi-antenna STAs 12a-c. When the single AP 11 a wants to transmit to the multiple STAs 12a-c in an MU-MIMO mode, it needs to transmit its data to each STA 12a-c such that this transmission does not interfere with the transmission intended for the other STAs. In other words, the AP 11 a attempts to keep the multi-user interference (MUI) to a minimum (optimally zero). Therefore, the precoder used by the multi-antenna AP 11 a for a respective STA 12a-c needs to lie in the null space of the channels of all the other STAs. In the example shown in figure 2, the AP 11 a has six Tx antennas transmitting to the three STAs 12a-c each with 2 Rx antennas (and, thus, 2 spatial streams per STA 12a-c).

The precoder used by the AP 11a for the first STA 12a takes the form P1 = N _2,3 ▪ A ₁ , wherein and H ₂ , H ₃ denotes the respective channel matrix between the AP 11a and the second STA 12b and between the AP 11a and the third STA 12c (V ₂ * and V ₃ denote a respective component of the SVD of the channel matrix H ₂ and H ₃ , respectively). By choosing the precoder in this way, Pi lies in the null space of the channels of the second and third STA 12b, 12c (and precoders). A ₁ is an additional precoding term which can, for example, be computed based on the singular vectors of H1▪ N _{2, 3} .The precoder Pi can be determined by the AP 11a using, for example, the Pseudo-Inverse.

Figure 3 shows a variant of the MU-MIMO example of figure 2, where there are more Rx antennas than streams (per STA). Thus, in the example shown in figure 2, each STA 12a-c has three Rx antennas. Also, for this example the AP 11a may compute the precoder as P ₁ = N _2,3 ▪ A ₁ , wherein

However, since for this example the channel matrices H ₂ and H ₃ are each of size 3 x 6 the AP 11a would need at least eight Tx antennas (to transmit 2 streams, otherwise no degrees- of- freedom).

An alternative approach would be to separate the ‘used’ components (in the example of figure 2, the first two components) from the ‘unused’ components (in the example of figure 2, the third component) of a SVD of the first channel matrix in the following way:

In this case the AP 11a may determine the precoder as P1 = N _2,3 ▪ A ₁ , wherein

Since for the example of figure 2 are each of size 2 x 6, the AP 11 a requires at least six Tx antennas, which is the case in the example shown in figure 2.

For estimating the MUI for the example of figure 2 the received signal at the first STA 12a can be determined in the following way:

Thus, as will be appreciated, the MUI from all STAs 12a-c is modulating Consequently, at high SNR, degrees of freedom (DoF) of the receiver have to be "sacrificed" with the rank of the MUI covariance matrix (e.g. through MVDR). In other words, for the example of figure 2 it is necessary to give up a single DoF in order to fully mitigate the MUI from all STAs 12a-c. Figure 4 shows a further variant of the examples shown in figures 2 and 3. In the example shown in figure 4 the wireless network 10 comprises two APs 11a, 11b having eight Tx antennas and four ST As 12a-d having three Rx antennas each. As already described above, in coordinated beamforming (C-BF) each AP 11a, 11b transmits (in MU-MIMO style) to its STAs, while placing a null on the OBSS STAs. Applying the known approach described above in the context of the example of figure 3 (for MU-MIMO with more Rx antennas than streams) completely fails, when there are more Rx antennas than streams. For example, if there are two cooperating APs 11 a, 11 b, the MUI is modulated over 2 vectors, so MVDR (with a single DoF) will fail. The corresponding "collapse" of the performance can be taken from the graphs shown in figure 5.

Thus, as will be appreciated from figure 5, using the existing feedback, C-BF will fail when the number of Rx antennas at the STA side is larger than the number of streams allocated to it. As will be described in more detail in the following, embodiments disclosed herein may address this issue based on the idea for each STA to compute the precoder it feedbacks to an OBSS AP based on the precoder it feedbacks to its own BSS AP (such that the precoders used by the OBSS AP are within the null space of the precoders used by the BSS AP).

An embodiment of the multi-antenna non-AP station 120a will be described in more detail in the following under reference to figure 6. The multi-antenna non-AP station 120a is configured to communicate with the multi-antenna AP 110a within its BSS. As illustrated in figure 6, the multi-antenna non-AP station 120a comprises a communication interface 123a with a plurality of antennas 124a1-3 configured to receive one or more first pilot signals from the first BSS multi-antenna AP 110a and one or more second pilot signals from the second OBSS multi-antenna AP 110b.

Moreover, the multi-antenna non-AP station 120a comprises a processing circuitry 121a configured to estimate a first channel matrix H _1,1 representing the channel between the multi- antenna non-AP station 120a and the first multi-antenna AP 110a based on the one or more first pilot signals and a second channel matrix H _1,2 representing the channel between the multi-antenna non-AP station 120a and the second multi-antenna AP 110b based on the one or more second pilot signals. The processing circuitry 121a is further configured to determine, based on the first channel matrix H _1,1 , an interference rejection combiner matrix W- _L for suppressing a multi-user interference caused by the first BSS multi-antenna AP 110a at the multi-antenna non-AP station 120a. In addition, as will be described in more detail below, the processing circuitry 121a is configured to determine, based on the second channel matrix H _1,2 and the interference rejection combiner W ₁ feedback information for the second OBSS multi-antenna AP 110b which will be used to suppress multi-user interference caused by the second BSS multi-antenna AP 110b.

The communication interface 123a of the multi-antenna non-AP station 120a is further configured to send first feedback information based on the first channel matrix H 1,1 to the first BSS multi-antenna AP 110a and second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 to the second OBSS multi- antenna AP 110b. Based on the second feedback information the second OBSS multi- antenna AP 110b may use an improved beamformer causing less (optimally zero) multi-user interference at the multi-antenna non-AP station 120a.

In the example shown in figure 6, the first multi-antenna AP 110a and the second multi- antenna AP 110b are transmitting to a total of four STAs 120a-d, including the multi-antenna non-AP station 120a according to an embodiment. As illustrated in figure 6, H _{k, m} denotes the matrix representing the channel between the k- th STA 120a-d and the m-th AP 110a, 110b.

In an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a may be configured to determine the first feedback information for the first BSS multi-antenna AP 110a in a conventional manner, such as by decomposing the first channel matrix H _1,1 = Then, the communication interface 123a may provide the first feedback information based on and to the first AP (this can be fed-back immediately after the receipt of the one or more first pilot signals or after both the one or more first pilot signals and the one or more second pilot signals have been received by the multi-antenna non-AP station 120a).

As already described above, in order to mitigate the MUI of the multi-antenna AP 110a the multi-antenna non-AP station 120a uses an interference rejection combiner W ₁ . In an embodiment, the interference rejection combiner W ₁ is configured to project onto the null- space basis of which is The interference from the second OBSS multi-antenna AP 110b to the multi-antenna non-AP station 120a can be expressed as: wherein P ₃ and P ₄ denote the precoding matrices used by the second OBSS multi-antenna AP 110b for the 3 ^rd and 4 ^th STAs 120c-d, respectively, and s _i denotes the signal for the i-th STA 120a-d. In an embodiment, this becomes:

Therefore, if P ₃ and P ₄ are in the null space of the composite matrix W ₁ ▪ H _1,2, in particular then there is zero MUI at the multi-antenna non-AP station 120a.

As already described above, the processing circuitry 121a of the multi-antenna non-AP station 120a may compute the first feedback information for the first BSS multi-antenna AP 110a in a conventional manner based on the one or more first pilot signals. Thus, in an embodiment, the multi-antenna non-AP station 120a may determine a SVD of the first channel matrix, i.e. and prepare the first feedback information on the basis of In an embodiment, the first feedback information may comprise D _1,1 and V _1,1 or at least portions thereof. For instance, in an embodiment, the first feedback information may comprise K columns of the matrixV _1,1 . This feedback information can be sent immediately in response to the one or more first pilot signals or at a later stage (e.g. in response to receiving the one or more second pilot signals as well).

In an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a may be configured to determine the second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 in the following way.

In an initial stage, which will be described in more detail in the context of figures 8 to 10 below, the communication interface 123a of the multi-antenna non-AP station 120a receives a NDP frame comprising the one or more second pilot signals from the second OBSS AP 110b. In a further stage, as already described above, the processing circuitry 121a of the multi-antenna non-AP station 120a is configured to compute based on the one or more first pilot signals the first channel matrix H _1,1 and the interference rejection combiner matrix W ₁ and to estimate based on the one or more second pilot signals the second channel matrix H _1,2 .

In an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a is configured to generate the second feedback information based on a basis (i.e. a set of basis vectors), in particular an orthonormal basis of the matrix product of the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 , i.e. a basis of W ₁ ▪ H _1,2 (which is herein denoted as B = W ₁ ▪ H _1,2 ). For instance, the processing circuitry 121a of the multi- antenna non-AP station 120a may be configured to generate the orthonormal basis of B = W ₁ ▪ H _1,2 based on a QR decomposition of B* (i.e. the complex conjugate of the matrix B = W ₁ ▪ H _1,2 ) in the form Q ▪ R, a Gram-Schmidt factorization, a Cholesky factorization or a singular value decomposition (SVD). In an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a is configured to generate the second feedback information based on one or more orthonormal vectors of the basis of the row space of the matrix B = W ₁ ▪ H _1,2 .

Thus, in an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a is configured to compute the QR decomposition in the form B* = Q ▪ R, in particular Q ▪ R and to feedback K column vectors of the orthogonal matrix Q of the QR decomposition back to the OBSS AP 110b. Herein the K column vectors of the orthogonal matrix Q of the QR decomposition are denoted as Based on the second feedback information provided by the multi-antenna non-AP station 120a, the second OBSS multi- antenna AP 110b computes the precoders to use as usual, such that P ₃ and P ₄ are in the null space of The precoders (e.g. P ₃ and P ₄ ) may be computed using, for example, the pseudo-inverse.

In the example shown in figure 6, the communication interface 123a of the multi-antenna non-AP station 120a comprises, by way of example, three antennas 124a1-3. Thus, in principle, the multi-antenna non-AP station 120a may communicate with the multi-antenna BSS non-AP station 120a using up to three spatial streams (SS). In the exemplary embodiment shown in figure 6, the communication interface 123a is configured to communicate with the first BSS multi-antenna AP 120 using, by way of example, only two spatial communication streams. In such a scenario, for determining the interference rejection combiner matrix W ₁ the processing circuitry 121a of the multi-antenna non-AP station 120a is configured to decompose a singular value decomposition H of the first channel matrix H _1,1 into a first component due the used spatial communication streams and a second component due to the unused spatial communication streams, i.e. and to determine the interference rejection combiner matrix W ₁ as the matrix wherein denotes the complex conjugate of the matrix As illustrated in figure 6, the multi-antenna non-AP station 120a may further comprise a memory 125a configured to store the interference rejection combiner matrix Storing the interference rejection combiner matrix in the memory 125a of the multi-antenna non-AP station 120a allows re-using the interference rejection combiner matrix for efficiently generating the feedback information for a further OBSS multi-antenna AP.

In an embodiment, the processing circuitry 121a of the multi-antenna non-AP station 120a may be further configured to determine a signal strength of the one or more second pilot signals from the second OBSS multi-antenna AP 110b, wherein the communication interface 123a is further configured to send the second feedback information to the second OBSS multi-antenna AP 110b depending on the signal strength of the one or more second pilot signals. For instance, the communication interface 123a may be configured to send the second feedback information to the second OBSS multi-antenna AP 110b, only if the signal strength of the one or more second pilot signals is larger than a minimum signal threshold value.

Figure 7 shows the improved MUI mitigation performance due to the second feedback information provided by the multi-antenna non-AP station 120a according to an embodiment. More specifically, figure 7 shows the PER as a function of the SNR for different configurations of the wireless communication network 100 shown in figure 6. For the results shown in figure 7 the following configuration of the wireless communication network 100 of figure 6 has been used: two collaborating APs 110a, 110b, each with eight Tx antennas; four STAs 120a-d, two spatial streams per STA, MCS 7 (64QAM rate 5/6), 40 MHz bandwidth. As can be taken from figure 7, the multi-antenna non-AP station 120a manages to not only operate when the number of Rx antennas is larger than the number of spatial streams, but improves the performance compared with the baseline scenario (number of Rx antennas identical to number of streams) since it yields zero MUI. Using the conventional feedback leads to a poor performance (including a high error floor).

Figure 8 shows a sequence diagram illustrating a sequential sounding procedure for C-BF implemented by the wireless communication network 100 of figure 6, including the multi- antenna non-AP station 120a according to an embodiment. As illustrated in figure 8, each AP 110a, 110b transmits a sounding sequence 801 a, b, 805a, b comprising a NDPA, NDP and BFRP (more specifically a BFRP trigger frame), wherein the respective NDP comprises the one or more first pilot signals or the one or more second pilot signals. In response thereto, the respective STA 120a, 120b provides by means of a beamforming report 803a, b, 807a, b the first feedback information and the second feedback information. As will be appreciated, in figure 8 (where for the sake of clarity the details of TXOP sharing have been ignored) the order of the transmitting APs 110a, 110b is chosen such that it allows the STA(s) 120a, 120b to compute and apply the respective interference rejection matrix. In the exemplary sequential sounding procedure shown in figure 8 each transmission of an NDPA/NDP is followed by a BFR 803a, b, 807a, b from the STA(s) 120a, 120b. In a further embodiment, the STA(s) 120a, 120b may be configured to provide the feedback to all APs 110a, 110b substantially simultaneously, and not as a response to each separate NDPA/NDP (as illustrated in figure 8). Thus, in an embodiment, the processing circuitry 121a of the multi- antenna non-AP station 120a is configured to determine the first feedback information based on the first channel matrix H _1,1 , in response to receiving the one or more first pilot signals, and to determine the second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 in response to receiving the one or more first pilot signals and the one or more second pilot signals.

Figure 9 shows a sequence diagram illustrating a variant of the sounding procedure illustrated in figure 8. According to the variant shown in figure 9, the NDPA and NDP may be skipped, when an AP 110a, 110b is sounding an OBSS STA 120a, 120b. More specifically, the transmissions 805a, b shown in figure 9 include a BFRP, but no NDPA or NDP. Thus, in an embodiment, the communication interface 123a is configured to send the first feedback information to the first multi-antenna AP 110a, in response to receiving a first null data packet, NDP, including the one or more first pilot signals, and a first beamforming report poll, BFRP, from the first multi-antenna AP 110a, and to send the second feedback information to the second multi-antenna AP 110b, in response to receiving a second BFRP from the second multi-antenna AP 110b, wherein the second feedback information is based on the one or more first pilot signals of the first NDP and the one or more second pilot signals of a second NDP, wherein the second NDP was transmitted at an earlier stage by the second multi- antenna AP 110b to the further multi-antenna non-AP station 120b and also received by the multi-antenna non-AP station 120a.

Figure 10 shows a sequence diagram illustrating a further variant of the sounding procedure illustrated in figure 8, namely a joint sounding protocol for C-BF. According to the joint sounding protocol illustrated in figure 10, all APs 110a, 110b transmit the sounding sequence 801a, 805a together, i.e. substantially simultaneously. As illustrated in figure 10, the NDPA sequence transmitted to an intended receiver (i.e. the STA 120a or the STA 120b) may request the first feedback information (referred to as "regular feedback" in figure 10), while the NDPA sequence transmitted to a non-intended receiver (i.e. the STA 120a or the STA 120b) may request the second feedback information (referred to as "advanced feedback" in figure 10).

In an embodiment, the NDPA received by the multi-antenna non-AP station 120a according to an embodiment may comprise an indication instructing the multi-antenna non-AP station 120a to store the interference rejection matrix (based on the feedback computed for the first beamformer).

In an embodiment, the NDPA received by the multi-antenna non-AP station 120a according to an embodiment may further comprise an indication for the non-intended receiver, i.e. station to compute the second feedback information based on the interference rejection matrix. This indication may also be supported by a capability indication so that a STA (or an AP) that supports this capability can be directed to feedback the first feedback information (i.e. the "regular feedback") or the second feedback information (i.e. the "advanced feedback").

Figure 11 is a flow diagram illustrating a method 1100 of operating the multi-antenna wireless non-AP station 120a according to an embodiment.

The method 1100 comprises a step 1101 of receiving by the plurality of antennas 124a1-3 of the multi-antenna non-AP station 120a one or more first pilot signals from the first BSS multi- antenna AP 110a and one or more second pilot signals from the second OBSS multi-antenna AP 110b. Moreover, the method 1100 comprises a step 1103 of estimating a first channel matrix H _1,1 representing the channel between the multi-antenna non-AP station 120a and the first BSS multi-antenna AP 110a based on the one or more first pilot signals and a second channel matrix H _1,2 representing the channel between the multi-antenna non-AP station 120a and the second OBSS multi-antenna AP 110b based on the one or more second pilot signals.

Moreover, the method 1100 comprises a step 1105 of determining, based on the first channel matrix an interference rejection combiner matrix W ₁ for suppressing a multi- user interference caused by the first BSS multi-antenna AP 110a. The method 1100 further comprises a step 1107 of sending first feedback information based on the first channel matrix H _1,1 to the first BSS multi-antenna AP 110a and second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 to the second OBSS multi-antenna AP 110b. In an embodiment, the method 1100 further comprises the steps of determining the first feedback information based on the first channel matrix H _1,1 in response to receiving the one or more first pilot signals and determining second feedback information based on the interference rejection combiner matrix W ₁ and the second channel matrix H _1,2 in response to receiving the one or more first pilot signals and the one or more second pilot signals.

In an embodiment, the first feedback information is sent by the multi-antenna non-AP station 120a, in response to receiving a first null data packet, NDP, including the one or more first pilot signals, and a first beamforming report poll, BFRP, from the first BSS multi-antenna AP 110a, and the second feedback information is sent, in response to receiving a second BFRP from the second OBSS multi-antenna AP 110b and by making use of a second NDP, including the one or more second pilot signals, provided at an earlier stage by the second OBSS multi-antenna AP 110b to a further multi-antenna non-AP station, such as the further multi-antenna non-AP station 120b shown in figure 6.

In an embodiment, the one or more first pilot signals and the one or more second pilot signals are received by the multi-antenna non-AP station 120a substantially simultaneously from the first BSS multi-antenna AP 110a and the second OBSS multi-antenna AP 110b and the first feedback information and the second feedback information are sent by the multi- antenna non-AP station 120a substantially simultaneously to the first BSS multi-antenna AP 110a and the second OBSS multi-antenna AP 110b.

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).

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.

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.

In addition, functional units in the embodiments of the invention 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.