WO2017101973A1

WO2017101973A1 - Information provision and processing for receiver driven precoding

Info

Publication number: WO2017101973A1
Application number: PCT/EP2015/079646
Authority: WO
Inventors: Luca Rose; Marco MASO; Apostolos Destounis
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2015-12-14
Filing date: 2015-12-14
Publication date: 2017-06-22
Also published as: CN108292937A; CN108292937B

Abstract

The invention proposes a receiver comprising R₀ receiving antenna(s) for receiving data over a downlink from T₀≥2 transmitting antennas of a transmitter, a connection unit for a connection with a first list of potentially cooperating receivers that respectively comprise R_PCDL(i) receiving antenna(s) for receiving data from T_PCDL(i)≥2 transmitting antennas of a transmitter, i being an index of the first list, and a computing unit for selecting a second list of cooperating receivers by recursively selecting the potentially cooperating receiver at index i=c, adding it to the second list if formula (I) and incrementing c, j being an index of the second list, R_CDL(j) the number of receiving antenna(s) of the cooperating receiver at index j and T_CDL(j) the number of transmitting antennas of its transmitter. The computing unit computes a precoder for the downlink based on the second list.

Description

INFORMATION PROVISION AND PROCESSING FOR RECEIVER DRIVEN

PRECODING TECHNICAL FIELD

The present invention generally relates to the field of wireless communication, and specifically relates to a wireless communication supporting precoding functionality. BACKGROUND

In autonomous networks two types of solutions are known for reducing or avoiding the interference level arising from the lack the cooperation. A first type of solutions, which may be labeled resource allocation (RA) solutions aim at designing algorithms that orthogonalize the communications by smartly allocated resource radio blocks, so that if one transmitter transmits data over a particular radio resource (e.g., frequency, time slot, code) the other transmitters do not transmit data on the same radio resource. For instance TDMA and CSMA follow this approach. A drawback of these approaches is that they are not efficient for two reasons. First, they waste radio resource, either in time and/or frequency domain. Second, they require some level of synchronization between the transmitters. A second type of solution assumes the existence of a noiseless high throughput backhaul connecting the transmitters and/or among all the receivers. Through this solution the network can employ a theoretically optimal precoding and decoding scheme that maximizes the overall throughput. However, the practical limits of such a system are evident, since the existence of a noiseless high throughput backhaul among SCs or WiFi access points is very unlikely.

In terms of precoding, two strategies can be distinguished. The first strategy is based on a codebook-based precoding. In this approach the precoder, as well as if necessary the corresponding decoder at the receiver, is chosen by each transmitter among a finite number of pre-computed non-optimized precoding matrices, wherein these matrices are known at both the transmitter and the receiver. In practice when a receiver receives the pilots/training symbols from a transmitter, it estimates the channel matrix and computes the index corresponding to the best precoder based on some metric. This first strategy however does not fully exploit the potential of using multiple antennas. The second strategy requires the receiver to estimate the channel and then feed it back as channel state information (CSI) to the transmitter. Afterwards, the transmitter uses this CSI to design a specific precoder. Instances of possible precoders are the maximum ratio combining (MRC) precoding, in which W^^RC = H , the zero-forcing (ZF) precoder in which W^^F = H_n(H H_n) ^{1 an}d the minimum mean square error (MMSE) transmitter in which W_J^^rMSE = H_n(H^ H_n + σ²/)^-1 , where W_n is the precoder employed by transmitter n, and H_n is the overall CSI feedback from all the receivers served by the transmitter n . This strategy is efficient for reducing the negative effects of intra-cell interference, but does not account for the negative effects of the inter-cell interference.

In view of the above, many wireless communication networks are based on two paradigms: multi-antenna devices, also known as multiple-input and multiple-output (MIMO) method, and full frequency reuse. Instances of these paradigms can be found in WiFi or LTE wireless communication networks. MIMO has the potential of largely improving the performance of a wireless communication network by mitigating the negative effect of interference and fading, whereas full frequency reuse maximizes spectral occupation at the price of an increased interference level. Centralized networks offer many ways of achieving this goal. In this context, mutually interfering base stations (BSs) or transmitters that dispose of perfect CSI with respect to the channels towards the non-served receivers or users can manage the multi-user interferences by means of precoding/beamforming. In practice, by exploiting the peculiar structure of MIMO precoding, base stations or transmitters can mitigate the multi-user interferences by employing ZF precoders. Even more efficiently, the base stations can employ MMSE transmit filtering to maximize the signal to interference plus noise ratio (SINR) at all receivers. However, there is no existing solution for wireless communication networks, in which neither cooperation nor coordination is possible between the transmitters. Examples of such wireless communication networks are e.g., WiFi, small-cells (SCs), femto-cells (FCs).

Fig. 1 shows an example of a non-centralized wireless communication system 100 with MIMO devices. The system 100 comprises three base stations in the form of three multi antenna transmitters 101, 102, 103 as well as receivers 111, 112, 113. The transmitters 101, 102, 103 are for example WiFi access points and the receivers 111, 112, 113 are for example handheld devices like tablets, laptops or smartphones.

A first receiver 111 is served by a first transmitter 101 over a first downlink 121. Similarly, a second receiver 112 is served by a second transmitter 102 over a second downlink 124 and a third receiver 113 is served by a third transmitter 103 over a third downlink 128. Further on, even if the first receiver 111 is not served by the second and third transmitters 102, 103, there is a respective channel 126, 129 between respectively the second and third transmitters 102, 103 and the first receiver 111. Likewise, the second receiver 112 is not served by the first and third transmitters 101, 103, and the third receiver 113 is not served by the first and second transmitters 101, 102. However, a respective channel 123, 127 exists from the first and third transmitters 101, 103 to the second receiver 112, and a respective channel 122, 125 exists from the first and second transmitters 101, 102 to the third receiver 113. While the first receiver 11 1 is served by the first transmitter 101 via the downlink 121, the channels 126, 129 received by the first receiver 111 from the further second and third transmitters 102, 103 cause interferences at the first receiver 1 11. In Fig. 1, these channels 126, 129 are therefore identified as being interfering channels. Interference strongly limits the performance of autonomous multi-user/multi-cell networks such as WiFi and small-cell networks. On the one hand, known multi antenna transmitters have the capability of zeroing or mitigating the negative effect of interference on the performance of the network, but at the cost of centralization. On the other hand, interference is problematic in non-centralized wireless communication networks, even more if the transmitters do not cooperate.

SUMMARY

Having recognized the above-mentioned disadvantages and problems, the present invention aims at improving the state of the art. In particular, the object of the present invention is to provide an improved wireless communication with reduced interferences.

The present invention particularly intends to improve the wireless communication in a network that is not centralized and in which cooperation between the transmitters is not envisaged. The invention intends to mitigate the interferences caused at the receivers.

The above-mentioned object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the respective dependent claims.

A first aspect of the present invention provides a receiver for being served by a multi antenna transmitter. The receiver comprises a number Ro of at least one receiving antenna adapted to receive data over a downlink from a number T₀ of transmitting antennas of the transmitter, wherein T₀ > 2. The receiver comprises a connection unit adapted to establish a connection with a first list of potentially cooperating receivers over respective communication links. A potentially cooperating receiver is served by a respective multi antenna transmitter and comprises a number R_PCDL© of at least one receiving antenna adapted to receive data over a respective downlink from a number T_PCDL(i₎ of transmitting antennas of the respective transmitter, wherein T_PCDL(i₎ > 2 and i is an index of the first list. The receiver comprises a computing unit adapted to select a second list of cooperating receivers from the first list by means of an initialization phase and a recursive phase. The initialization phase comprises initializing the second list as an empty list, and initializing a counter c. The recursive phase comprising selecting the potentially cooperating receiver located at index i=c, adding to the second list the selected potentially cooperating receiver if

^Ro ⁺ ) , and incrementing the counter

c, wherein j is an index of the second list, R_CDLQ is ^me number of receiving antenna(s) of the cooperating receiver at index j, and T_CDL(j) is the number of transmitting antennas of the transmitter serving the cooperating receiver at index j . The computing unit is adapted to compute a precoder for the downlink from the transmitter to the receiver depending on channel state information with respect to the downlink and with respect to respective channels from the transmitter to each cooperating receiver of the second list. In other words, the inequality comprises the term ∑^RCDLU) ^mat is ^{me sum} °f ^me j

receiving antenna(s) of all cooperating receivers of the second list, and the term { _{c7J£( /)}|y} that is a set comprising, for each cooperating receiver of the second list, the number of transmitting antennas of the transmitter serving the cooperating receiver. Thereby, the invention may compensate for the absence of cooperation between transmitters, particularly in the context of MIMO non-cooperative networks, and may reduce interference and increase the SINR at the receivers. The receiver may advantageously exploit the knowledge about the interfering channel from the transmitter to the cooperating receiver in order to compute the precoder that may be used by the transmitter. The receiver may for example comprise a feedback unit adapted to transmit to the transmitter either parameters of the computed precoder or information derived from the computed precoder. Also, the invention advantageously provides the second list of cooperating receivers based on which the precoder for the downlink may be computed. Particularly, the precoder for the downlink from the transmitter to the receiver may depend on the channels from the transmitter to each cooperating receiver.

In a first implementation form of the receiver according to the first aspect, the computing unit is adapted to order the first list according to the number R_PCDL© of receiving antenna(s) of each potentially cooperating receiver of the first list, so that the computing unit is adapted to select the second list from the ordered first list.

Thereby, the first list can accordingly be ordered prior to the selection of the second list. Correspondingly, the selection of the selection list can be optimized according to desired criteria. In a second implementation form of the receiver according to the first aspect, the computing unit is adapted to order the first list in decreasing order of the number RpcDL(i) of receiving antenna(s) of each potentially cooperating receiver of the first list.

Thereby, it can be ensured that the highest possible number of receiving antennas is selected, which in turn allows for maximizing the throughput.

In a third implementation form of the receiver according to the first aspect, the computing unit is adapted to order the first list in increasing order of the number RpcDL(i) of receiving antenna(s) of each potentially cooperating receiver of the first list.

Thereby, this ordering arrangement has the advantage that the inter-cell interference can be reduced for a maximum number of cooperating receivers. Accordingly, an increased number of cooperating receivers can benefit from an interference reduction. This also correspondingly increases the fairness among the receivers. In a fourth implementation form of the receiver according to the first aspect, the computing unit is adapted to order the potentially cooperating receivers in the first list in decreasing order of the respective number T_PCDL(i₎ of transmitting antennas of the transmitter serving said potentially cooperating receivers.

Thereby, this ensures that the potential cooperating receivers, which are served by the transmitters having the highest number of transmitting antennas, are selected first during the recursive phase. In view of the inequality that shall be satisfied for adding a potentially cooperating receiver to the second list, the theoretical maximum number of receiver antennas is given by mm(T₀ ,

. Ordering the first list in decreasing order of T_PCDL(i₎ thus in turn ensures that this theoretical maximum number is maximized.

In a fifth implementation form of the receiver according to the first aspect, the recursive phase is carried out until all potentially cooperating receiver of the first list are selected.

Thereby, it is guaranteed that all potentially cooperating receivers have been checked. This is particularly advantageous for example in case the first list is randomly ordered, i.e. in case the first list is not ordered according to a specific criterion like the number RPCDL(i) of receiving antenna(s) of the potentially cooperating receivers

Alternatively, in the fifth implementation form of the receiver according to the first aspect, the recursive phase is carried out until R₀ +∑^CDL ) ^{= mm}( o > ^CDLU) }) ·

Thereby, by carrying out the recursive phase until the equation is verified, it is possible to stop the recursive phase earlier without having to select each potentially cooperating receiver of the first list. In a sixth implementation form of the receiver according to the first aspect, the receiver comprises a broadcast unit adapted to broadcast a request for cooperation to the potentially cooperating receivers. The receiver comprises a reception unit adapted to receive a respective response message from the potentially cooperating receivers in response to the request for cooperation. The first list corresponds to the potentially cooperating receivers from which the reception unit has received a response message.

Thereby, this configuration is advantageous in that the receiver can easily establish a connection with the potentially cooperating receivers.

In a seventh implementation form of the receiver according to the first aspect, the response message received from a given potentially cooperating receiver comprises information about the number R_PCDL® of its receiving antenna(s) and the number TpcDL(i) of transmitting antennas of the transmitter serving it.

Thereby, the receiver can easily gather the information required for selecting the second list and carrying out the recursive phase.

In an eight implementation form of the receiver according to the first aspect, the request for cooperation broadcasted by the receiver comprises information about the number T₀ of transmitting antennas of the transmitter and/or about the number R₀ of receiving antenna(s).

Thereby, this is advantageous in that a potentially cooperating receiver can actively decide not to transmit a response message after having received a request for cooperation. For example, if its number of receiving antennas is higher than the number T₀ comprised in the request for cooperation, it is not possible for the potentially cooperating receiver to be added to the second list during the recursive phase. By not transmitting the response message, the recursive phase can be simplified. Likewise, a potentially cooperating receiver receiving a request for cooperation can compare the number of transmitting antennas of the transmitter serving it to the number Ro of receiving antenna(s). If Ro is above said number of transmitting antennas, is not possible for the potentially cooperating receiver to be added to the second list during the recursive phase. The recursive phase can thus be simplified by not transmitting the response message.

In a ninth implementation form of the receiver according to the first aspect, the request for cooperation broadcasted by the receiver comprises information about a position of the receiver.

Thereby, the potential cooperating receiver receiving the request for cooperation can then compare the position of the receiver with its own position and decide to transmit the response message or not depending on the distance between the two positions. If said distance is sufficiently large, for example higher than a threshold, it can be assumed that the interference between the receiver and the potentially cooperating receiver is low enough and that there is no need for the potentially cooperating receiver to be part of the second list. In this case, the potentially cooperating receiver can decide not to transmit the response message such that the selection of the second list can be simplified and accelerated.

In a tenth implementation form of the receiver according to the first aspect, the response message received from a given potentially cooperating receiver comprises information about a position and/or a speed and/or a battery level of the given potentially cooperating receiver.

Thereby, the receiver can correspondingly optimize the first list depending on the received information. In an eleventh further implementation form of the receiver according to the first aspect, the computing unit is adapted to order the first list according to the information about the position and/or the speed and/or the battery level comprised in the response messages received from the potentially cooperating receivers.

Thereby, for example if the distance between the position of the receiver and the position of the potentially cooperating receiver increases, the potentially cooperating receiver can be moved towards the end of the first list, since the potential interference between the receiver and the potentially cooperating receiver then would rather decrease. Similarly, if the speed of the potentially cooperating receiver increases, the latter can be moved towards the end of the first list, since also in this case the potential interference between the receiver and the potentially cooperating receiver would rather decrease. Also, if the battery level decreases, the corresponding potential cooperating receiver could be move towards the beginning of the first list in order to avoid interferences with said potential cooperating receiver the latter: it can then be avoided that a potentially cooperating receiver with low battery level has to re-transmit data

In a twelfth implementation form of the receiver according to the first aspect, the computing unit is adapted to remove from first list a potentially cooperating receiver depending on the position and/or the speed and/or the battery level of the latter. Thereby, the selection of the second list of cooperating receivers from the first list and particularly the recursive phase can be simplified and accelerated.

A second aspect of the present invention provides a method for a receiver to be served by a multi antenna transmitter. Said receiver comprises a number Ro of at least one receiving antenna adapted to receive data over a downlink from a number T₀ of transmitting antennas of the transmitter, wherein T₀ > 2. The receiver establishes a connection with a first list of potentially cooperating receivers over respective communication links. A potentially cooperating receiver is served by a respective multi antenna transmitter and comprises a number RpcDL(i) of at least one receiving antenna adapted to receive data over a respective downlink from a number T_PCDL(i) of transmitting antennas of the respective transmitter, wherein T_PCDL(i) > 2 and i is an index of the first list. The receiver selects a second list of cooperating receivers from the first list by means of an initialization phase and a recursive phase. The initialization phase comprises initializing the second list as an empty list, and initializing a counter c. The recursive phase comprises selecting the potentially cooperating receiver located at index i=c, adding to the second list the selected potentially cooperating receiver if

^RO ⁺∑^RCDLU) ^{+ R}pcDL(i) ^{≤ m}(^To cDL(j) \j ^TpcDL(i) ) , and incrementing the counter j

c, j is an index of the second list, R_CDLQ₎ is the number of receiving antenna(s) of the cooperating receiver at index j, and T_CDL(j) is the number of transmitting antennas of the transmitter serving the cooperating receiver at index j. The receiver computes a precoder for the downlink from the transmitter to the receiver depending on channel state information with respect to the downlink and with respect to respective channels from the transmitter to each cooperating receiver of the second list. Further implementation forms of the method according to the second aspect of the invention correspond to the different implementation forms of the receiver according to the first aspect. In other words, further features or implementations of the method according to the fourth aspect of the invention can perform the functionality of the receiver according to the first aspect of the invention and its different implementation forms.

A third aspect of the present invention provides a computer program having a program code for performing the method according to the second aspect of the present invention when the computer program runs on a computing device.

It has to be noted that the receiver proposed by the invention may advantageously comprise a transmission unit adapted to transmit the second list to each cooperating receiver belonging to said second list. This is advantageous in that each cooperating receiver may then also compute a precoder for the respective downlink from its respective transmitter, i.e. from the respective transmitter serving said cooperating receiver, to said cooperating receiver. Particularly, each cooperating receiver may comprise a computing unit adapted to compute said precoder depending on channel state information with respect to the respective downlink from said respective transmitter to said cooperating receiver, with respect to a channel from said respective transmitter to the receiver that has computed the second list, and with respect to channels from said respective transmitter to the remaining cooperating receivers of the second list. Accordingly, the interferences occurring at the receiver and at the remaining cooperating receivers may be reduced. Further, it has to be noted that the receiver proposed by the invention may advantageously comprise a reception unit adapted to receive a request for cooperation broadcasted by another receiver, and a transmission unit adapted to transmit a response message to said other receiver. All above-mentioned aspects regarding the request for cooperation and the response message may also apply to this case. Accordingly, said other receiver may comprise a similar computing unit for selecting the second list, wherein the receiver may be part of such second list and may comprise a reception unit adapted to receive the second list.

An idea of the invention is to provide a receiver driven precoding, RDP, for reducing interferences and increasing the SINR at receivers as well as for compensating for the absence of cooperation between transmitters. An idea of the invention is further on to select the cooperating receivers so as to ensure that the highest possible number of cooperating receivers is selected. This may be of importance for the performance of the overall precoding system comprising the receiver, the transmitter, the cooperating receivers and the respective transmitters serving the cooperating receivers, since the amount of interference that is suppressed may be increased.

The rationale behind the invention is that the "cost" of a cooperating receiver added to the second list is given by the amount of degrees of freedom it costs to the overall system and by the amount of transmitting antennas its transmitter is equipped with. The first aspect may be taken care of by counting the total amount of degrees of freedom and reducing it on the go when adding new cooperating receivers to the second list. Conversely, the second aspect may be accounted for by ordering the potentially cooperating receivers in the first list based on the number of antennas their transmitter has. The information provision and processing protocol proposed by the present invention is advantageous to frame the ideal scenario for RDP to deliver its best performance.

Furthermore, the present invention targets RDP systems in which a group of MIMO transmitters communicate with their intended receivers without any required cooperation or coordination between the transmitters. The receivers implementing the invention establish a communication channel in which some side information about the CSI are exchanged, allowing each receiver to build a feedback that, once received by the respective serving transmitter, can influence the precoder implemented in the serving transmitter. This precoder is adapted to improve the achievable rate for all the cooperating receiving devices.

This happens in a complete blind or transparent way for the transmitters, making the invention particularly attractive in many scenarios. For instance, in one of its embodiments this invention could be adopted in a multi- vendor/operator systems in which a vendor/operator cannot control the behavior of transmitters manufactured by another vendor or owned by another operator. In another embodiment this invention could be adopted in systems in which already the deployed infrastructure/standard does not allow to perform any mutually beneficial precoding/beamforming at the transmitter side.

This invention proposes a solution to the problem of interferences particularly in a network that is not centralized and in which cooperation between the transmitters is not required. The invention particularly proposes to transfer the load of computing an efficient method to avoid interference, i.e. to transfer the load of computing an efficient precoder, from the transmitter to the receiver. In other words, the invention proposes to frame a receiver-centric decision process. This invention has the advantage of increasing the overall network throughput while being transparent for the transmitters, without requiring expensive network/transceiver modifications in order to achieve the same goal. In other words, by means of the present invention each transmitter contributes to the overall performance increase while adopting the legacy precoding schemes, without however the necessity for an explicit or implicit cooperation or coordination between the transmitters.

An idea of the invention is to exploit knowledge about the interfering channels of cooperating receivers in order to feedback a precoder that mitigate the negative effects for the cooperating receivers. This mitigation is advantageous in that is occurs in a complete blind way for the transmitters. Since all receivers of a network may apply the strategy of the invention, all the receivers that cooperate through the invention may experience a significant performance gain, in turn increasing the overall network throughput. This invention yields a receiver-centric scenario, in which state-of-the-art network limitations are overcome without further costs for the operators or manufacturers.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be full formed by external entities not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF DRAWINGS

The above aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

Fig. 1 shows a non-centralized wireless communication network according to the state of the art, Fig. 2 shows a wireless communication network according to an embodiment of the present invention,

Fig. 3 shows a wireless communication network according to a further embodiment of the present invention,

Fig. 4 shows a wireless communication network according to a further embodiment of the present invention,

Fig. 5 shows a method according to an embodiment of the present invention,

Fig. 6 shows a method according to a further embodiment of the present invention,

Fig. 7 shows an algorithm for selecting the second list of cooperating receivers according to an embodiment of the present invention,

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 2 shows a wireless communication network 200 according to an embodiment of the present invention. The embodiment of Fig. 2 is a specific embodiment of a general wireless communication network according to the present invention and being composed of N autonomous transmitters indexed as n G {1, ... , N}, with transmitter n being connected to K_n receivers. Here, the feature autonomous refers to the fact that the transmitters do not exchange, or particularly cannot exchange, any information between them, and therefore behave as if no other transmitter is present in the same radio resource block.

An embodiment of such a network is depicted in Fig. 2 and encompasses among the others WiFi networks and SC networks. Each transmitter in the network is equipped with M_n > 1 antennas, while the receivers are equipped with N_r≥ 1 antennas. The MIMO downlink channel between a transmitter n and a receiver r is indicated by the matrix h^_r. It is assumed that the matrix h^ _r is estimated at each receiver through downlink pilot/training sequences, e.g. as done for frequency-division duplexing (FDD) communications. By stacking the channel matrices for all the intended receivers of transmitter n, i.e. for all the receivers that are served by transmitter n, the channel matrix according to the following equation (1) is obtained:

Thus the signal received by receiver r can be expressed according to the following equation (2):

wherein:

- x_n represents the transmit symbols vector of transmitter n, and

- w_r the additive noise vector.

In the equation (2), it is possible to identify three terms

- a first term h_n x_n representing the useful signal, - a second term∑_i≠r Ji„ representing the intra-cell interference, and

- a third term∑^≠n∑i ^t\^xe representing the inter-cell interference.

In this context, the downlinks operate in the same frequency band. As a consequence, mutual interference arises and the performance of the communications degrades. Even if the transmitters do not cooperate, MIMO techniques can be exploited to mitigate the intra-cell interference. In particular, it is assumed that each transmitter employs a precoder of its choice based on the CSI feedback from its receivers. That is, each transmitter implements the following general precoding strategy:

W_n = f_n{H_n) (3) wherein:

- W_n is the precoder employed by transmitter n,

- f_n is a certain invertible precoding function, for example any of the standard linear precoding strategies such as maximum ratio combining (MRC), ZF or MMSE transmission that are invertible, and

- H_n is the CSI feedback by all the receivers connected to transmitter n, i.e. H_n is the overall CSI feedback by all the receivers that are served by transmitter n.

According to known techniques, the second term of the above equation (2) representing the intra-cell interference can be minimized, or even more efficiently, the SINR for all the receivers can be maximized. Coming back to Fig. 2, the wireless communication network 200 comprises two transmitters 201, 202. The transmitters 201, 202 are multi antenna transmitters and each transmitter particularly comprises three antennas. The wireless communication network 200 comprises a first set of receivers 21 1, 212, 213 served by a first transmitter 201, and respective downlinks 221, 222, 223 from the first transmitter 201 to the first set of receivers 21 1, 212, 213. The wireless communication network 200 also comprises a second set of receivers 214, 215 served by a second transmitter 202, and respective downlinks 234, 235 from the second transmitter 202 to the second set of receivers 214, 215.

Further on, even if the first set of receivers 211, 212, 213 is not served by the second transmitters 202, there is a respective channel 231, 232, 233 between the second transmitter 202 and the first set of receivers 211, 212, 213. Similarly, even if the second set of receivers 214, 215 is not served by the first transmitter 201, there is a respective channel 224, 225 between the first transmitter 201 and the second set of receivers 214, 215.

The wireless communication network 200 of Fig. 2 in fact comprises two autonomous networks that respectively comprise the first transmitter 201 and the second transmitter 202. The transmission of the first transmitter 201 causes interferences at the receivers 214, 215 served by the second transmitter 202 and vice versa. The channel 224, 225 correspondingly cause inter-cell interference at the receivers 214, 215, while the channel 231, 232, 233 cause inter-cell interference at the receivers 211, 212, 213.

The receivers 211, 212, 213, 214, 215 of the embodiment of Fig. 2 respectively comprise one antenna. A receiver according to the present invention may alternatively comprise more than one antenna and may consequently be a multi antenna receiver. The transmitters 201, 202 are for example base stations or WiFi access points. The receivers 211, 212, 213, 214, 215 are for example handheld devices like tablets, laptops or smartphones. Fig. 3 shows a wireless communication network 300 according to a further embodiment of the present invention.

According to an embodiment of the present invention, Fig. 3 shows a receiver 311 for being served by a multi antenna transmitter 301. The receiver 311 comprises a number Ro of at least one receiving antenna 311a adapted to receive data over a downlink 321 from a number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301, wherein T₀ > 2. The receiver 311 comprises a connection unit adapted to establish a connection with a first list of potentially cooperating receivers 312 over respective communication links 331, 332. The first list may be referred to as PCDL, and a potentially cooperating receiver 312 of the first list PCDL may be referred to as PCDL(i), i being an index of the first list PCDL.

A potentially cooperating receiver 312, PCDL(i) is served by a respective multi antenna transmitter 302 and comprises a number RpcDL(i) of at least one receiving antenna 312a adapted to receive data over a respective downlink 323 from a number TpcDL(i) of transmitting antennas 302a, 302b, 302c of the respective transmitter 302, wherein T_PCDL(i) > 2.

The receiver 311 comprises a computing unit adapted to select a second list of cooperating receivers from the first list PCDL. The second list may be referred to as CDL, and a cooperating receiver of the second list CDL may be referred to as CDL(j), j being an index of the second list CDL. The computing unit is adapted to select the second list by means of

- an initialization phase comprising initializing the second list CDL as an empty list, and initializing a counter c, and

- a recursive phase comprising selecting the potentially cooperating receiver 312, PCDL(i) located at index i=c, adding to the second list CDL the selected potentially cooperating receiver 312, PCDL(i) if

and incrementing the counter c. The value R_CDLO) is ^me number of receiving antenna(s) of the cooperating receiver CDL(j) at index j, and T_CDL(j) is the number of transmitting antennas 302a, 302b, 302c of the transmitter 302 serving the cooperating receiver CDL(j) at index j.

The computing unit is adapted to compute a precoder for the downlink 321 from the transmitter 301 to the receiver 311 depending on channel state information with respect to the downlink 321 and with respect to respective channels 322 from the transmitter 301 to each cooperating receiver CDL(j) of the second list CDL.

Particularly, the receiver 311 may comprise an estimation unit adapted to estimate said channel state information with respect to the downlink 321. The connection unit may be adapted to receive from each potentially cooperating receiver 312 said channel state information with respect to the respective channels 322 from the transmitter 301 to each cooperating receiver of the second list. The receiver 311 may comprise a feedback unit adapted to transmit to the transmitter 301 information characterizing the computed precoder or information derived from the computed precoder.

Correspondingly, while the receiver 311 is served by the transmitter 301 over the downlink 321, the potentially cooperating receiver 312 is not served by the transmitter 301. The channel 322 that is defined between the transmitter 301 and the potentially cooperating receiver 312 represents an interfering channel for the reception at the potentially cooperating receiver 312.

Therefore, the receiver 311 is adapted to exploit the knowledge about the interfering channel 322 related to a cooperating receiver from the second list in order to compute a precoder that mitigates negative effects for the this cooperating receiver, i.e. that reduces inter-cell interferences at this cooperating receiver.

According to an embodiment of the present invention, the receiver 311 comprises an estimation unit adapted to estimate channel state information with respect to a channel 324 from the further transmitter 302 to the receiver 311. The receiver 311 comprises a connection unit adapted to establish a connection with a cooperating receiver 312 over the respective communication link 331, 332. The receiver 311 is not served by the transmitter 302, and the cooperating receiver 312 is served by the transmitter 302 over the downlink 323. The connection unit is adapted to transmit to the cooperating receiver 312 the estimated channel state information.

Correspondingly, not the receiver 311 but the cooperating receiver 312 is served by the further transmitter 302 over the downlink 323. The channel 324 that is defined between the further transmitter 301 and the receiver 312 represents an interfering channel for the reception at the receiver 312.

Therefore, the receiver 311 is adapted to estimate the channel state information of this interfering channel 324, and to transmit the estimated channel state information to the cooperating receiver 312. In turn, it is then possible for the cooperating receiver 312 to exploit the knowledge about the interfering channel 324 in order to feed back to the further transmitter 302 a precoder that mitigates negative effects for the receiver 31 1, i.e. that reduces inter-cell interferences at the receiver 311.

Similarly to the embodiment Fig. 2, the transmitters 301, 302 are multi antenna transmitters and each transmitter particularly comprises three antennas 301a, 301b, 301c and 302a, 302b, 302c respectively. The receivers 311, 312 of Fig. 2 respectively comprise one antenna. A receiver according to the present invention may alternatively comprise more than one antenna and may consequently be a multi antenna receiver. The transmitters 301, 302 of Fig. 3, and more generally the transmitters according to the present invention, are for example base stations or WiFi access points. The receivers 311, 312 of Fig. 3, and more generally the receivers according to the present invention, are for example handheld devices like tablets, laptops or smartphones.

In Fig. 3, h₂ represents the channel matrix of the unidirectional communication link 331 from the receiver 311 to the potentially cooperating receiver 312, while hi_t2 represents the channel matrix of the unidirectional communication link 332 from the potentially cooperating receiver 312 to the receiver 311. The receiver 311 and the potentially cooperating receiver 312 are equipped with some capabilities of exchanging data among them. The exchange of data may be carried out wirelessly at least when both the receiver 311 and the potentially cooperating receiver 312 are in proximity. This may be accomplished, for instance, by WiFi Direct, Bluetooth or any device-to-device (D2D) radio access technology (RAT).

Fig. 4 shows a wireless communication network 400 according to a further embodiment of the present invention. The wireless communication network 400 corresponds to the wireless communication network 300 of Fig. 3. The wireless communication network 400 comprises a receiver 411 being served by a transmitter 401 over a downlink 421 , which corresponds to the receiver 311 being served by the transmitter 301 over the downlink 321. The wireless communication network 400 comprises a potentially cooperating receiver 412 being served by a respective transmitter 402 over a respective downlink 423, which corresponds to the potentially cooperating receiver 312 being served by the respective transmitter 302 over the respective downlink 323. Further on, the wireless communication network 400 comprises a channel 422 from the transmitter 401 to the potentially cooperating receiver 412 and a channel 424 from the further transmitter 402 to the receiver 411, which respectively correspond to the channel 322 from the transmitter 301 to the potentially cooperating receiver 312 and to the channel 324 from the further transmitter 302 to the receiver 311.

The difference between the embodiments of Fig. 3 and Fig. 4 consists in the data exchange between the receiver 411 and the potentially cooperating receiver 412. The data exchange of channel state information is performed via a further network 430. A data exchange from the receiver 411 to the potentially cooperating receiver 412 is carried out by means of a unidirectional communication link 432 to the further network 430, by means of the further network 430 and by means of a unidirectional communication link 433 from the further network 430 to the potentially cooperating receiver 412. A data exchange from the potentially cooperating receiver 412 to the receiver 411 is carried out by means of a unidirectional communication link 434 to the further network 430, by means of the further network 430 and by means of a unidirectional communication link 431 from the further network 430 to the receiver 41 1. The further network 430 may be for example based on cloud computing, or on a relay or repeater.

Fig. 5 shows a method 500 according to an embodiment of the present invention for a receiver 31 1 to be served by a multi antenna transmitter 301 , said receiver 31 1 comprising a number Ro of at least one receiving antenna 31 1a adapted to receive data over a downlink 321 from a number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301, wherein T₀ > 2.

According to the method 500, the receiver 31 1 establishes 501 a connection with a first list PCDL of potentially cooperating receivers 312, PCDL(i) over respective communication links 331, 332. A potentially cooperating receiver 312, PCDL(i) is served by a respective multi antenna transmitter 302 and comprises a number RPCDL® of at least one receiving antenna 312a adapted to receive data over a respective downlink 323 from a number T_PCDL(i₎ of transmitting antennas 302a, 302b, 302c of the respective transmitter 302, wherein T_PCDL(i₎ > 2 and i is an index of the first list PCDL. According to the method 500, the receiver 31 1 selects 502 a second list CDL of cooperating receivers CDL(j) from the first list PCDL by means of

- an initialization phase 503 comprising initializing 504 the second list CDL as an empty list, and initializing 505 a counter c, and

- a recursive phase 506 comprising selecting 507 the potentially cooperating receiver 312, PCDL(i) located at index i=c, adding 508 to the second list CDL the

312, PCDL(i) if

(4) and incrementing 509 the counter c,

wherein j is an index of the second list CDL, RCDLQ₎ is the number of receiving antenna(s) of the cooperating receiver CDL(j) at index j, and T_CDL(j) is the number of transmitting antennas 302a, 302b, 302c of the transmitter 302 serving the cooperating receiver CDL(j) at index j.

According to the method 500, the receiver 311 computes 510 a precoder for the downlink 321 from the transmitter 301 to the receiver 31 1 depending on channel state information with respect to the downlink 321 and with respect to respective channels 322 from the transmitter 301 to each cooperating receiver CDL(j) of the second list CDL. Fig. 6 shows a method 600 according to a further embodiment of the present invention.

According to this embodiment, the step of the receiver 311 establishing 501 a connection with a first list PCDL of potentially cooperating receivers 312 comprises a step of broadcasting request for cooperation 601, a step of listening 602 for answer to discovery and, if any device answers 603, a step of creating 606 the first list PCDL.

The purpose of this establishing 501 step is that the receiver 311 establishes a first connection or handshake with the devices that implement the invention and are located within the range of their neighbors. This step is carried out for example by means of a discovery and answer to discovery. The receiver 311 broadcasts 401 a request for cooperation through an appropriate RAT, like Bluetooth or Wi-Fi as mentioned with respect to Fig. 3. Each potentially cooperating receiver 312 implementing the invention answers via a response message. A connection is established 501 between the receiver 311 and the responding potentially cooperating receivers 312. This phase of establishing 501 connections comprises creating 606 the first list PCDL of potentially cooperating devices.

Advantageously, the response message sent by the potentially cooperating receiver 312 and received by the receiver 311 comprises information about number R_PCDL© of receiving antenna(s) 312a of the potentially cooperating receiver 312 as well as information about the number T_PCDL(i) of transmitting antennas 302a, 302b, 302c of the transmitter 302 serving said potentially cooperating receivers 312.

The step of creating 606 the first list PCDL may further comprise a sub-step of ordering the first list PCDL. For example, the first list PCDL may be ordered based on the number RpcDL(i) of receiving antenna(s) 312a of the potentially cooperating receiver 312, in decreasing order.

In one embodiment of the invention, the potentially cooperating receiver 312 already involved in cooperation do not answer. In other words, if a potentially cooperating receiver 312 receives a request for cooperation from the receiver 311 a first time and later receives a request for cooperation from the same receiver 311, then preferably no response message is sent by the potentially cooperating receiver 312. In one embodiment of the invention, the request for cooperation broadcasted by the receiver 311 comprises information about the number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301 and/or about the number R₀ of receiving antenna(s) 311a. Thereby, the potentially cooperating receiver 312 may advantageously decide not to transmit a response message. For example, if its number of receiving antennas is higher than the number T₀ comprised in the request for cooperation, it is not possible for the potentially cooperating receiver to be added to the second list during the recursive phase. By not transmitting the response message, the recursive phase can be simplified. In one embodiment of this invention, the request for cooperation may include information on both the number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301 and the identity of the transmitter 301. This advantageous in that the potentially cooperating receiver 312 then knows the identity of the transmitter 301 and may easily identify pilots received from the transmitter 301 when estimating channel state information with respect to the channel 322 from the transmitter 301 to the potentially cooperating receiver 312. In one embodiment of this invention, the request for cooperation may include information on the number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301, on the identity of the transmitter 301 and on the number Ro of receiving antenna(s) 311a.

In the embodiment of Fig. 6, the step of the receiver 31 1 selecting 502 a second list CDL of cooperating receivers CDL(j) from the first list PCDL corresponds to the step of identifying 607 the set of cooperating devices CDL(j), i.e. identifying 607 the second list CDL of cooperating receivers CDL(j).

In one embodiment of the invention, the selection 502, 607 of the second list of cooperating receivers is carried out according to the algorithm shown in Fig. 7. In an initialization phase, the second list CDL is initialized as an empty list. Also, a variable K is a counter that counts a total amount of receiving antennas, and is initialized to the number Ro of receiving antenna(s) 311a of the receiver 311. A variable i represents an index of the first list PCDL and is initialized to a value corresponding to a counter c=l. The variable i is used to scroll the first list PCDL. A variable M represents the minimum number of antennas among the transmitters 301, 302, and is initialized to the number T₀ of transmitting antennas 301a, 301b, 301c of the transmitter 301.

In a recursive phase of the algorithm, the potentially cooperating receiver 312, PCDL(i) located at index i is selected 507. The variable K counting the total amount of receiving antennas is incremented by the number RpcDL₍i₎ of receiving antenna(s) 312a of this selected potentially cooperating receiver 312, PCDL(i). The variable M, which represents the minimum number of antennas among the transmitter 301 of the receiver 311 and the transmitters 302 of the cooperating receivers 312 of the second list, is updated by considering the number T_PCDL(i₎ of transmitting antennas 302a, 302b, 302c of the transmitter 302 serving the potentially cooperating receiver 312, PCDL(i) identified by the index i.

The selected potentially cooperating receiver 312, PCDL(i) is added to the second list if K≤ M, i.e. if above-mentioned equation (4) is verified. Otherwise, the selected potentially cooperating receiver 312, PCDL(i) is not added to the second list and the variables K and M are updated. Each time the recursive phase is carried out, the index i is incremented, i.e. the counter c is incremented. The second list CDL of cooperating devices is identified by the receiver 31 1 that established the connection with the potentially cooperating devices.

Advantageously, the receiver 31 1 may order the first list or even remove some potentially cooperating receivers from the first list. This ordering or removing may be based on different criteria. In one embodiment, this may be the number of antennas of the potentially cooperating receivers. This phase is advantageous, for instance, to keep the number of cooperating receiver antennas smaller than the number of transmitting antennas and e.g. to exclude devices with very high mobility. The minimum output for this phase is the second list of cooperating receivers in which the total amount of antennas of the receiver 31 1 and of the cooperating receivers 312 is smaller than the minimum number of antennas of the transmitters involved.

In one embodiment of the invention, a parameter for ordering the first list or removing a potentially cooperating receiver from the first list may be the position, for example the GPS-position, of the potentially cooperating receiver. Indeed, if a potentially cooperating receiver is sufficiently far from the receiver 31 1, the level of interference will be sufficiently low between the two devices so that the corresponding potentially cooperating receiver can be removed from the first list or reordered at the end of the first list. A threshold approach can be used in that if the distance between receiver 31 1 and potentially cooperating receiver 312 is above the threshold, said removing or reordering may be carried out. In a further embodiment of the invention, a parameter for ordering the first list or removing a potentially cooperating receiver from the first list may be the speed of the latter or its speed with respect to the receiver 31 1. A potentially cooperating receiver that is moving e.g. with a speed higher than a given threshold, may have a coherence time too short to really exploit the benefits of the invention, such that a removing or reordering may be carried out.

In a further embodiment of the invention, a parameter for ordering the first list or removing a potentially cooperating receiver from the first list may be the battery level or the potentially cooperating receiver. A potentially cooperating receiver may be more in need than other of using the invention since its battery is low and cannot afford several retransmissions: the potentially cooperating receiver having low battery level may correspondingly be moved at the beginning of the first list. Alternatively, in the case in which it will come out that the invention is not energy efficient, this parameter can be taken into account inversely, i.e. the potentially cooperating receiver having low level battery is removed from the first list.

Each of these parameters may be communicated during the set up of the collaboration, at the latest during the creation 606 of the first list PCDL, and shall be included in the exchange message procedure.

These parameters can also be combined according to the following equation: maximize^ f ({X}, {S}, {B}) ^

where {X},{S}, {B} represent, respectively, the set of all the positions, the set of all speed values, and the set of all battery levels of the potentially cooperating devices 312. In this respect the equation (4) shall also be verified when maximizing equation (5). A possible implementation of the function f is a weighted sum according to the following equation:

/({ }, {SI {B}) = _Wl{X} + w₂{S} + w₃{B} (6)

The method 600 comprises a further step of bargain on precoding strategy 608, i.e. a further step of cooperation level bargaining and establishment. The receiver 311 and the cooperating receivers CDL(j) selected in step 607 bargain or negotiate what kind of precoding strategy should be adopted based on the amount of information available. The minimum output of this step is the precoding strategy to be adopted, like for example ZF or MRC. An optional output of this step is a level of selfishness and/or a transmit SNR level. The level of selfishness may be defined as a parameter a defining the computed precoder as a combination of a maximum ratio combining precoder and a zero-forcing precoder.

The method 600 comprises a further step of information gathering 609 consisting in estimating the state of direct and interfering channels. The necessary information, e.g. information regarding the interfering channels, is estimated based on the pilot/training signals sent by the interfering transmitters, i.e. sent by the transmitter 301. Accordingly, the receiver 31 1 may estimate channel state information, CSI, with respect to the downlink 321, and the cooperating receiver 312, CDL(j) may estimate channel state information with respect to the interfering channel 322 from the transmitter 301 to the cooperating receiver 312, CDL(j). In one embodiment of the invention, the CSI with respect to non-sparse interference channels is estimated by means of a linear estimator. In one embodiment of the invention, the CSI with respect to non-sparse interference channels is estimated by means of a non-linear estimator. In one embodiment of the invention, the CSI with respect to sparse interference channels is estimated by compressed sensing. The method 600 comprises a further step of information exchange 610. The receiver 311 and the cooperating receivers 312 selected in step 607 exchange the information needed for computing the precoder, and particularly the information regarding the interfering channels. Particularly, the cooperating receiver 312 may transmit to the receiver 31 1 its estimation of the channel state information with respect to the interfering channel 322 from the transmitter 301 to the cooperating receiver 312. This transmission is carried out over the communication link 332. In one embodiment, the necessary information is exchanged through RAT, like for example Bluetooth or WiFi Direct, or through a further network 430 for example based on cloud computing or on a relay or repeater.

In one embodiment of the invention, the transmitted CSI is a quantized version of the estimated CSI at the corresponding cooperating receiver. In one embodiment of the invention, the transmitted CSI is the actual analog realization of the estimated CSI at the corresponding cooperating receiver. In one embodiment of the invention, the transmitted CSI is a compressed version of the estimated CSI at the corresponding cooperating receiver.

The method 600 comprises a further step of precoder computation 611. This step may be designed according to several criteria.

In one embodiment of the invention, the criteria could be determined in order to reach a Pareto optimal solution, as described in the followings. In a first sub-step, the receiver 311 labeled "r" builds a extended channel matrix H_r by stacking together its own channel matrix and the channel matrices of the cooperating receivers 312 according to the following equation:

H_r— [ _{n r}, h_n i, ... , h_n j] (7) wherein

- h_{n r} represents the channel matrix of the receiver 311, and - h_{n l}, ... , h_{n i} represent the channel matrices of the i cooperating receivers 312 labeled from "1" to "f.

The dimension of H_r is M_n X K, wherein:

- K is the total amount of antennas of the receiver 31 1 and of the cooperating receivers 312, and

- M_n is the number of antennas of transmitter n. a second sub-step, a ZF precoder is evaluated according to the following equation:

wherein

- IN is the identity matrix of dimension N_r, and

- 0_M__Nr is a matrix of dimension ( — N_r) X N_r composed only of zeros.

In a third sub-step, an MRC precoder is evaluated according to the following equation:

In a fourth sub-step, the wished precoder is computed according to the following equation:

P = apMRc + (1 - a) PZF (10) wherein a is the level of selfishness.

The value a = 1 would basically impose MRC precoding to the transmitter 301 and may be referred to as a selfish choice, while the value a = 0 would impose ZF precoding and may be referred to as an altruistic choice. The method 600 comprises a further step of feedback signal transmission 605 i.e. a step of feedback precoder/CSI. In this step, the receiver 31 1 finally creates a feedback signal and transmits it to the transmitter 301 serving the receiver 31 1. Based on the precoder P computed in step 61 1, the receiver 31 1 for example computes and transmits to the transmitter as feedback the vector: f- P). (1 1) The precoding function / is assumed to be invertible. In this regard, it is noted that standard and popular precoding functions such as MRC, ZF and MMSE are invertible. In particular, MMSE is a widely used close-to-the-optimum linear solution.

In case the receiver does not receive any response message after having broadcasted the request for cooperation, the receiver estimates 604 its own downlink 321. The feedback signal transmission 605 then only depends on the CSI of the downlink 321 since no cooperating receiver 312 is cooperating with the receiver

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

1. Receiver (311) for being served by a multi antenna transmitter (301), comprising:

- a number Ro of at least one receiving antenna (31 1a) adapted to receive data over a downlink (321) from a number T₀ of transmitting antennas (301a, 301b, 301c) of the transmitter (301), wherein T₀ > 2,

- a connection unit adapted to establish (406) a connection with a first list (PCDL) of potentially cooperating receivers (312, PCDL(i)) over respective communication links (331, 332),

wherein a potentially cooperating receiver (312, PCDL(i)) is served by a respective multi antenna transmitter (302) and comprises a number R_PCDL® of at least one receiving antenna (312a) adapted to receive data over a respective downlink (323) from a number T_PCDL(i₎ of transmitting antennas (302a, 302b, 302c) of the respective transmitter (302), wherein T_PCDL(i) > 2 and i is an index of the first list (PCDL), - a computing unit adapted to select (407) a second list (CDL) of cooperating receivers (CDL(j)) from the first list (PCDL) by means of

- an initialization phase comprising initializing the second list (CDL) as an empty list, and initializing a counter c, and

- a recursive phase comprising selecting the potentially cooperating receiver (312, PCDL(i)) located at index i=c, adding to the second list (CDL) the selected potentially cooperating receiver (312, PCDL(i)) if

^Ro ⁺ , and incrementing the counter c,

wherein j is an index of the second list (CDL), R_CDLQ₎ is the number of receiving antenna(s) of the cooperating receiver (CDL(j)) at index j, and T_CDL(j) is the number of transmitting antennas (302a, 302b, 302c) of the transmitter (302) serving the cooperating receiver (CDL(j)) at index j,

wherein the computing unit is adapted to compute a precoder (pZF, pMRC, P) for the downlink (321) from the transmitter (301) to the receiver (311) depending on channel state information with respect to the downlink (321) and with respect to respective channels (322) from the transmitter (301) to each cooperating receiver (CDL(j)) of the second list (CDL).

2. Receiver (311) according to claim 1, wherein the computing unit is adapted to order the first list (PCDL) according to the number RpcDL(i) of receiving antenna(s) (312a) of each potentially cooperating receiver (312, PCDL(i)) of the first list (PCDL), so that the computing unit is adapted to select (407) the second list (CDL) from the ordered first list (PCDL).

3. Receiver (311) according to claim 2,

wherein the computing unit is adapted to order the first list (PCDL) in decreasing order of the number R_PCDL© of receiving antenna(s) (312a) of each potentially cooperating receiver (312, PCDL(i)) of the first list (PCDL).

4. Receiver (311) according to claim 2,

wherein the computing unit is adapted to order the first list (PCDL) in increasing order of the number R_PCDL© of receiving antenna(s) (312a) of each potentially cooperating receiver (312, PCDL(i)) of the first list (PCDL).

5. Receiver (311) according to claim 2,

wherein the computing unit is adapted to order the potentially cooperating receivers (312, PCDL(i)) in the first list (PCDL) in decreasing order of the respective number TpcDL(i) of transmitting antennas (302a, 302b, 302c) of the transmitter (302) serving said potentially cooperating receivers (312, PCDL(i)).

6. Receiver (311) according to any of the preceding claims,

wherein the recursive phase is carried out until all potentially cooperating receiver (312, PCDL(i)) of the first list (PCDL) are selected, or

7. Receiver (311) according to any of the preceding claims,

comprising:

- a broadcast unit adapted to broadcast a request for cooperation to the potentially cooperating receivers (312, PCDL(i)),

- a reception unit adapted to receive a respective response message from the potentially cooperating receivers (312, PCDL(i)) in response to the request for cooperation,

wherein the first list (PCDL) corresponds to the potentially cooperating receivers (312, PCDL(i)) from which the reception unit has received a response message.

8. Receiver (311) according to claim 7,

wherein the response message received from a given potentially cooperating receiver (312, PCDL(i)) comprises information about the number R_PCDL© of its receiving antenna(s) (312a) and the number T_PCDL(i₎ of transmitting antennas (302a, 302b, 302c) of the transmitter (302) serving it.

9. Receiver (311) according to any of the claims 7 to 8,

wherein the request for cooperation broadcasted by the receiver (311) comprises information about the number T₀ of transmitting antennas (301a, 301b, 301c) of the transmitter (301) and/or about the number R₀ of receiving antenna(s) (311a).

10. Receiver (311) according to any of the claims 7 to 9,

wherein the request for cooperation broadcasted by the receiver (311) comprises information about a position of the receiver (311).

11. Receiver (311) according to any of the claims 7 to 10,

wherein the response message received from a given potentially cooperating receiver (312, PCDL(i)) comprises information about a position and/or a speed and/or a battery level of the given potentially cooperating receiver (312, PCDL(i)).

12. Receiver (311) according to claim 11,

wherein the computing unit is adapted to order the first list (PCDL) according to the information about the position and/or the speed and/or the battery level comprised in the response messages received from the potentially cooperating receivers (312, PCDL(i)).

13. Receiver (311) according to claim 11 or 12,

wherein the computing unit is adapted to remove from first list (PCDL) a potentially cooperating receiver (312, PCDL(i)) depending on the position and/or the speed and/or the battery level of the latter.

14. Method (500) for a receiver (311) to be served by a multi antenna transmitter (301), said receiver (311) comprising a number R₀ of at least one receiving antenna (311a) adapted to receive data over a downlink (321) from a number T₀ of transmitting antennas (301a, 301b, 301c) of the transmitter (301), wherein T₀ > 2,

wherein

- the receiver (311) establishes (501) a connection with a first list (PCDL) of potentially cooperating receivers (312, PCDL(i)) over respective communication links (331, 332),

wherein a potentially cooperating receiver (312, PCDL(i)) is served by a respective multi antenna transmitter (302) and comprises a number R_PCDL® of at least one receiving antenna (312a) adapted to receive data over a respective downlink (323) from a number T_PCDL(i₎ of transmitting antennas (302a, 302b, 302c) of the respective transmitter (302), wherein T_PCDL(i₎ > 2 and i is an index of the first list (PCDL),

- the receiver (311) selects (502) a second list (CDL) of cooperating receivers (CDL(j)) from the first list (PCDL) by means of

- an initialization phase (503) comprising initializing (504) the second list (CDL) as an empty list, and initializing (505) a counter c, and

- a recursive phase (506) comprising selecting (507) the potentially cooperating receiver (312, PCDL(i)) located at index i=c, adding (508) to the second list (CDL) the selected potentially cooperating receiver (312, PCDL(i)) if + ) , and incrementing (509)

the counter c,

- the receiver (311) computes (510) a precoder (pZF, pMRC, P) for the downlink (321) from the transmitter (301) to the receiver (311) depending on channel state information with respect to the downlink (321) and with respect to respective channels (322) from the transmitter (301) to each cooperating receiver (CDL(j)) of the second list (CDL).

15. Computer program having a program code for performing the method according to claim 14, when the computer program runs on a computing device.