Reception and retransmission processes, destination device and relay node

Abstract

Reception and retransmission processes, destination device and relay node. The reception process is implemented by a device d having received messages emitted successively by M≥2 source devices comprising first redundancy versions of information messages from the source devices, and comprises, following an inability of device d to decode received messages and for at least a time interval of a retransmission phase: the selection of a set Sopt of at least two undecoded source devices and a set Bopt of relay nodes knowing the information messages from these source devices; the sending of retransmission instructions to these relay nodes so that they transmit simultaneously to device d the same second redundancy version of each information message from the source devices of Sopt;and the reception of the second redundancy versions and their use to decode the information messages from the undecoded source devices of Sopt. Figure for the abstract: Fig. 5;

Description

Title of the invention: Reception and retransmission methods, destination device and relay node. Prior art.

[0001] The invention belongs to the general field of telecommunications.

[0002] It relates more particularly to a cooperative communication technique involving a plurality of communicating devices connected to a communication network. Such a technique aims to make the communicating devices cooperate with each other to transmit their respective messages to a destination device rather than operating in competition with regard to the resources of the transmission channel as is the case in traditional communication networks.

[0003] The invention has a preferred, but not limiting, application in the context, for example, of 5G or 6G communication networks, as defined by the 3GPP standard. In this context, the destination device is, for example, a network base station equipped with several receiving antennas, and the communicating devices cooperating with each other are user equipment or UEs (for "User Equipment"). The invention can, however, be applied in other contexts.

[0004] Figure 1 illustrates an example of a SYS system implementing such a cooperative communication technique. The SYS system is an OMAMRC (Orthogonal Multiple Access Multiple Relay Channel) type system. It comprises M>2 source devices si, s2, ..., sM, and a destination device d, and relies on cooperative relaying of messages sent by the source devices si, s2, ..., sM, which is carried out via so-called relay nodes that can be source devices and / or dedicated intermediate nodes rl, r2, ..., rL, L>0, which know the messages to be relayed. Transmissions within this system use an orthogonal multiple access channel scheme, such as, for example, time division multiplexing (TDM).

[0005] The document by S. Cerovié et al. entitled “Efficient Cooperative HARQ for Multi-Source Multi-Relay Wireless Networks”, August 2018, 14th International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob), pages 61-68, proposes an OMAMRC system using a cooperative HARQ (Hybrid Automatic Repeat reQuest) protocol based on single-user incremental or IR (Incremental Redundancy) coding using LDPC (Low Parity Check Codes) or turbo codes such as those used in 3GPP standards. This “IR-type HARQ” protocol relies on an incremental retransmission of an information message by a source device (called "source message") not correctly decoded by the destination device, rather than an identical retransmission of that source message.

[0006] More specifically, in the context of a 5G network, for example, as defined by the 3GPP standard, an information message from a source device is conventionally encoded with a coding efficiency R, before being mapped to the symbols of a constellation (q bits per symbol) and then transmitted on NI resource elements or REs (for "Resource Element"). For an OFDMA (for "Orthogonal Frequency Division Multiple Access") multiplexing technique such as that used in a 5G network, an RE resource element corresponds to an OFDM symbol in the time domain and a subcarrier in the frequency domain. The size L of the encoded information message is therefore equal to L = Nl.Rq

[0007] In the case of the IR-type HARQ protocol, the information message associated with a source device is first encoded with a yield R0 <R bas (typiquement 1 / 3 ou 1 / 5), conduisant à une taille de message source codé Lc=L / R0 strictement supérieure à L. Ce message d’information codé est ensuite mémorisé dans un buffer circulaire. A chaque (re-)transmission, les bits codés à envoyer sont lus à partir de positions fixes dans le buffer. Par exemple, lors de la première transmission, le dispositif source lit les Nl.q bits codés à partir d’une position POS0 ; puis lors d’une première retransmission, N2.q bits codés sont lus à partir d’une position POS2 ; lors d’une deuxième retransmission, N2.q bits codés sont lus à partir d’une position POS3 ; lors d’une troisième retransmission, N2.q bits codés sont lus à partir d’une position POS1, etc. Les bits codés envoyés à chaque (re-)transmission sont appelés « versions de redondance ».

[0008] In an OMAMRC system using the IR-type HARQ protocol, (re-)transmissions are organized according to three phases: - an initialization phase, during which the destination device d determines the modulation and coding scheme to be used by each source device, depending on the quality of the transmission channel separating it from each source device; - a transmission phase, during which the M source devices Si, ..., sM successively transmit their respective coded information messages using the modulation and coding schemes determined by the destination device d during the initialization phase. During this transmission phase, the number NI of resource elements (and incidentally of uses of the transmission channel) is fixed and identical for all source devices; and - a retransmission phase, during which information messages that the destination device d failed to decode are cooperatively retransmitted, over a number of time intervals, by relay nodes selected by the destination device d. The selection of relay nodes is performed at each time interval from among the source devices and / or intermediate nodes that know the information messages that could not be decoded by the destination device d, either because they are their own information messages or because they themselves successfully decoded them. At each time interval, an information message associated with a source device not decoded by the destination device d is retransmitted by one or more relay nodes. The maximum number of possible retransmissions is Tmax; in other words, the retransmission phase lasts at most Tmax time intervals.

[0009] To implement the retransmission phase, an exchange of information must take place before the start of each retransmission between the destination device d and the other nodes of the system (source devices and / or intermediate nodes) via feedback links or channels, which are often limited. This exchange of information aims to allow the destination device d, firstly, to identify the nodes that have knowledge of the information messages that it has not been able to decode and that can be used as relay nodes for these undecoded information messages, and secondly, to inform the relay node(s) selected for each time interval to retransmit the same information message associated with a source device not decoded by the destination device.Each time an information message associated with a source is retransmitted, a new redundancy version resulting from the encoding of the information message in question is used by the selected relay node(s), with the different selected nodes, if any, transmitting the same redundancy version.

[0010] Document WO2023 / 242295 proposes an OMAMRC system based on the principles described above. In this system, the destination device selects a source information message to be retransmitted (or equivalently, a source device whose information message is to be retransmitted) in order to maximize the overall signal-to-noise ratio (SNR) received on the receiving NR antennas of the destination device. Thus, when several nodes know the information message in question and are able to simultaneously retransmit the same redundant version resulting from the encoding of this information message during the retransmission phase, such a system makes it possible to benefit from spatial diversity in transmission.

[0011] The choice of the source device to be retransmitted therefore depends on the number of relay nodes capable of retransmitting a redundant version of the information message associated with that source device, as well as the quality of their transmission links with the destination device. Once the source device is selected, the destination device informs the system nodes, and the nodes that have correctly decoded the information message associated with that source device retransmit the same redundant version resulting from the encoding of that information message. To enable the destination device to coherently combine the redundant versions received on the receiving NR antennas of the destination device, the relay nodes implement a transmission technique that maximizes the signal-to-noise ratio in reception, or MRT (for "Maximum Transmission Ratio").This is achieved through the application in transmission of a precoding vector, each coefficient of which is applied by one of the relay nodes involved in the retransmission phase. Thus, at each time interval of the retransmission phase, the destination device informs the OMAMRC system nodes of the selected source device; it also indicates to the relay nodes involved in the retransmission of a redundant version resulting from the encoding of the information message associated with this source device, a precoding vector to be applied to this redundant version.

[0012] If precoding is included in the transmission channel, such a system, by its configuration, resembles a single-input multiple-output (SIMO) system. Although such a system maximizes the overall SNR in reception by exploiting the presence of multiple antennas at the destination device and the spatial diversity offered when a plurality of relay nodes retransmit the same redundancy version associated with a source message, it does not fully take advantage of the degree of freedom MIMO could offer thanks to the plurality of relay nodes, and in particular its spatial multiplexing capabilities. Description of the invention

[0013] The invention makes it possible in particular to overcome this drawback by proposing a method of reception by a multi-antenna destination device having received messages emitted successively by M>2 source devices, each message emitted by a source device comprising a first redundancy version resulting from a coding of an information message associated with the source device, said method comprising, following an inability of the destination device to decode said messages received from several of said source devices said not decoded, for at least a time interval of a retransmission phase: - a selection step according to a determined performance criterion, of a Sopt set comprising at least two undecoded source devices and a Bopt set of relaying nodes with a cardinality greater than or equal to the cardinality of the Sopt set, a said relaying node being a source device or an intermediate device knowing each of the information messages associated with the source devices of the Sopt set; - a step of sending retransmission instructions to the relay nodes of the Bopt set so that they simultaneously transmit to the destination device the same second redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set; and - a stage of receiving the second versions of redundancy transmitted by the relaying nodes, said second versions of redundancy received being used by the destination device to decode the messages received from the undecoded source devices of the Sopt set.

[0014] Correspondingly, the invention also relates to a multi-antenna destination device, capable of receiving messages transmitted successively by M>2 source devices, each message transmitted by a source device comprising a first redundancy version resulting from an encoding of an information message associated with the source device, said destination device comprising modules activated following an inability of the destination device to decode said messages received from several of said source devices, said undecoded, and for at least a time interval of a retransmission phase, said modules comprising: - a selection module, configured to select according to a determined performance criterion, a Sopt set comprising at least two undecoded source devices and a Bopt set of relaying nodes with a cardinality greater than or equal to the cardinality of the Sopt set, a said relaying node being a source device or an intermediate device knowing each of the information messages associated with the source devices of the Sopt set; - a sending module, configured to send retransmission instructions to the relay nodes of the Bopt set so that they simultaneously transmit to the destination device the same second redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set; and - a receiving module, configured to receive second versions of redundancy transmitted by the relay nodes, said second versions redundancy received being used by the destination device to decode messages received from undecoded source devices of the Sopt set.

[0015] It should be noted that the selection, instruction sending, and reception steps can be repeated for at least one other time interval of the retransmission phase, i.e., in practice, until a predefined stopping criterion is met, with new redundancy versions being transmitted during each time interval. Such a criterion is, for example, reaching a maximum number Tmax of iterations or the absence, at the end of a retransmission, of source devices not decoded by the destination device.

[0016] The invention also relates to a method of retransmitting, via a relay node, a communication system comprising a multi-antenna destination device and M>2 source devices having simultaneously transmitted M messages, each message transmitted by a source device comprising a first redundancy version resulting from the encoding of an information message associated with that source device, said relay node being able to be said source device or an intermediate device of the communication system, said method comprising: - a step of informing the destination device of a knowledge via the relay node of messages emitted by several of the source devices; - a reception step, configured to receive from the destination device a retransmission instruction, during a time interval of a retransmission phase, of second redundancy versions resulting from the encoding of information messages associated with a set of source devices known to the relay node but not decoded by the destination device; and - a step of executing said retransmission instruction during said time interval.

[0017] Correspondingly, the invention also relates to a relay node of a communication system comprising a multi-antenna destination device and M>2 source devices having simultaneously transmitted M messages, each message transmitted by a source device comprising a first redundancy version resulting from an encoding of an information message associated with that source device, said relay node being able to be said source device or an intermediate device of the communication system, said relay node comprising: - an information module, configured to inform the destination device of its knowledge of messages emitted by several of said source devices; - a receiving module, configured to receive from the destination device a retransmission instruction, during a time interval of a retransmission phase, of second redundancy versions resulting from the encoding of information messages associated with a set of source devices known to said relaying node but not decoded by the destination device; and - a retransmission module configured to execute said retransmission instruction during said time interval.

[0018] The invention also relates to a communication system comprising a destination device according to the invention, a plurality of source devices and a plurality of relaying nodes according to the invention, said relaying node being able to be said source device or an intermediate device distinct from the source devices.

[0019] The invention has a preferred but not limiting application in the context of a communication system using an orthogonal multiple access multiple-relay channel scheme of the OMAMRC type, between source devices, relaying nodes and the destination device.

[0020] Thus, according to the invention, when the destination device fails to decode the information messages from the source devices, it selects, based on a performance criterion determined for each time interval of the retransmission phase, a plurality Sopt of undecoded source devices and a plurality Bopt of relay nodes that possess knowledge of these information messages either because they are their own information messages (when the relay nodes are source devices) or because they have successfully decoded them. The relay nodes in the Bopt set are therefore able to simultaneously retransmit the same redundant versions of the information messages associated with all the source devices in the Sopt set.A spatial multiplexing of the second redundancy versions is thus performed by the relay nodes, during the considered time interval of the retransmission phase, on U spatial layers, where U corresponds to the number of undecoded source devices in the Sopt set. The elimination of interference generated by this spatial multiplexing is advantageously permitted, provided the propagation channels between the relay nodes and the destination device are known, thanks to: . - the simultaneous retransmission by the relay nodes of the same redundancy versions associated with several source devices not decoded by the destination device; - a sufficient number of relay nodes selected from the Bopt set, taking into account the number of undecoded source devices in the Sopt set; and - the presence of multiple receiving antennas at the destination device.

[0021] The selection of the SOPT and BOPT sets is advantageously based on a predetermined performance criterion. For example, the destination device selects the SOPT and BOPT sets that optimize this performance criterion. Such a performance criterion is, for example, the mutual information associated with the simultaneous transmission by the relay nodes of the selected BOPT set of the second redundancy versions. The destination device selects the SOPT and BOPT sets that maximize this mutual information. The use of such a performance criterion, and more specifically the optimization of mutual information, makes it possible to average the system performance on the propagation channels between the relay nodes and the destination device. This ensures that the selected SOPT and BOPT sets optimize the overall system performance at each time interval of the retransmission phase.

[0022] Of course, other performance criteria can be considered, such as a receive bit error rate. Furthermore, during the selection process, one can seek to optimize such a performance criterion or, alternatively, ensure that this performance criterion reaches at least a given threshold value.

[0023] It should be noted that, unlike a conventional MIMO system consisting of one device with multiple transmitting antennas and another device with multiple receiving antennas, the configuration of the communication system according to the invention is likely to change at each time interval of the retransmission phase, since the BOPT set is adapted to the SOPT set, and these two sets are likely to change between two time intervals. As a result, the number of spatial layers considered during the retransmission phase (and therefore, incidentally, the number of relay nodes) can vary from one time interval to another during the retransmission phase: transmission over a fixed number of spatial layers is not necessarily the most optimal strategy at every instant.Taking such a performance criterion into account makes it possible to identify the best possible configuration at each time interval of the retransmission phase.

[0024] Thanks to these characteristics, the invention offers the possibility of fully exploiting the capabilities of the distributed MIMO system consisting of the destination device having several receiving antennas, the source devices, and the relaying nodes (source devices and / or intermediate devices) participating in the relaying Information messages sent by source devices are not decoded by the destination device, even when the relay nodes have only one (or use only one) transmitting antenna. The invention allows for the selection, during the retransmission phase, of a relay node configuration that optimizes system performance at each time interval of the retransmission phase, and incidentally, makes the best use of the spatial dimension of the system thus constituted when appropriate, given the propagation conditions.

[0025] It should be noted that when we refer here, for the sake of simplification, to the retransmission by a relay node of an information message associated with a source device, it is in fact the transmission by this relay node of a redundancy version resulting from the encoding of this information message associated with the source device.

[0026] The invention therefore makes it very advantageous to improve the overall spectral efficiency of the communication system.

[0027] In a particular embodiment of the reception process, the selection step includes an evaluation of said performance criterion determined for each possible set S comprising Q undecoded source devices with 2 <Q<Qmax pour lequel il existe au moins un ensemble B de nœuds de relayage connaissant chacun des messages d’information associés aux dispositifs sources non décodés dudit ensemble S, Qmax désignant un nombre déterminé choisi inférieur ou égal à un nombre d’antennes de réception du dispositif de destination.

[0028] This embodiment provides a simple and efficient way to select the relay nodes at each time interval of the retransmission phase.

[0029] If Qmax is taken to be equal to the number of receiving antennas of the destination device, this embodiment approximates an exhaustive or near-exhaustive search of all possible combinations of source devices not decoded by the destination device and all possible combinations of relay nodes suitable for these combinations of source devices. However, some combinations can be quickly excluded, such as, for example, combinations of undecoded source devices for which it is not possible to identify a set of relay nodes with a cardinality greater than or equal to the cardinality of the set of undecoded source devices under consideration.Furthermore, if for a given combination of Q0 undecoded source devices, a combination including an IBI number of relay nodes with knowledge of the information messages associated with each source device in this set such that IBI>Q0 is not found, the search can be stopped for a number of undecoded source devices greater than Q0.

[0030] It is also possible to limit the value of Qmax to a number strictly less than the number of receiving antennas of the destination device. Indeed, it can easily be shown that the complexity of the selection step increases with the number of source devices associated with redundancy versions to be retransmitted simultaneously, considered in the Sopt set. By deliberately limiting this number, the complexity of the selection step is limited.

[0031] According to the invention, the retransmission instructions sent by the destination device to the selected relay nodes trigger the retransmission of messages from source devices not decoded by the destination device, based on the information conveyed by these retransmission instructions. These retransmission instructions can, for example, be broadcast on a broadcast channel, sent in a channel dedicated to each of the selected relay nodes, etc.

[0032] In a particular embodiment, the retransmission instructions sent by the destination device to the relay nodes of the Bopt set include precoding vectors to be applied by the relay nodes of the Bopt set to the second redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set.

[0033] Correspondingly, in a particular mode of the retransmission process, the execution step includes the retransmission, during said time interval of the retransmission phase, of said second versions of redundancy by applying to them a precoding vector transmitted in said retransmission instruction.

[0034] Such precoding vectors make it possible in particular to implement an MRT type transmission technique at the relay nodes to allow the destination device to combine the received redundancy versions coherently, and thus facilitate the elimination of interference at the destination device.

[0035] In a particular embodiment, the retransmission instructions include the selected Sopt set, a precoding matrix comprising said precoding vectors and an indication for each relay node of the precoding vector to be applied by that relay node.

[0036] This embodiment is particularly well-suited for a limited return channel between the destination device and the relay nodes, shared by all the relay nodes (e.g., used as a broadcast channel). It allows the destination device to inform the selected relay nodes of the retransmission phase for a specific time interval while limiting the amount of information transmitted in the return channel. The information transmitted in this embodiment allows, in effect, The relay nodes are informed of the precoding to be applied to allow the destination device to utilize the retransmitted redundancy versions, which has led to an optimization (e.g., a maximization) of the performance criterion considered during the selection step. For example, the precoding matrix leads to the application of an MRT-type transmission technique at the relay nodes of the BOPT system.

[0037] In a particular embodiment, the precoding vectors are determined on the basis of a power transmitted by each relaying node equally distributed between the undecoded source devices of the Sopt set.

[0038] This embodiment makes it possible to limit the amount of information sent back to the relay nodes of the Bopt system with the retransmission instructions. Typically, when the assumption of equal power distribution across the U spatial layers (i.e., the same transmission power PO is applied to each layer, in other words, to each source device for which a redundant version is retransmitted) is considered at the destination device level during the selection step and to determine the precoding matrix to be applied by the relay nodes of the Bopt system, it is not necessary for the destination device to include in the retransmission instructions the powers applied at the level of each spatial layer. It is sufficient for the relay nodes of the Bopt system to have knowledge of the value of PO.This knowledge can result from the transmission of the PO value (or equivalent information) by the destination device to the relay nodes along with retransmission instructions, or it can be obtained by the relay nodes by any other means. For example, the relay nodes can calculate the PO value themselves from knowledge of the precoding matrix.

[0039] In another embodiment, the precoding vectors are determined on the basis of a power transmitted by each relaying node distributed among the undecoded source devices of the Sopt set so as to maximize mutual information associated with the simultaneous transmission by the relaying nodes of the selected Bopt set of said second redundancy versions.

[0040] This embodiment is more optimal than that based on an assumption of equal power distribution, but also requires a greater amount of information to be sent back to the relay nodes of the Bopt system, since the destination device must inform them of the powers allocated to each spatial layer.

[0041] In either of these embodiments, the retransmission instructions may therefore further include an indication of a power distribution transmitted by each relay node between the undecoded source devices of the Sopt set.

[0042] In a particular embodiment, the reception process and / or the retransmission process is / are implemented by a computer.

[0043] The invention also relates to a computer program on a recording medium, this program being capable of being implemented in a computer or more generally in a destination device according to the invention and comprising instructions adapted to the implementation of a reception process as described above.

[0044] The invention also relates to a computer program on a recording medium, this program being capable of being implemented in a computer or more generally in a relay node according to the invention and comprising instructions adapted to the implementation of a retransmission process as described above.

[0045] Each of these programs can use any programming language, and be in the form of source code, object code, or code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0046] The invention also relates to an information medium or a recording medium readable by a computer, and comprising instructions for a computer program as mentioned above.

[0047] The information or recording medium can be any entity or device capable of storing programs. For example, the medium may include a storage means, such as a ROM, for example a CD-ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a hard disk drive, or a flash memory.

[0048] On the other hand, the information or recording medium can be a transmissible medium such as an electrical or optical signal, which can be transmitted via an electrical or optical cable, by radio link, by wireless optical link or by other means.

[0049] The program according to the invention can in particular be downloaded onto an Internet-type network.

[0050] Alternatively, the information or recording medium may be an integrated circuit in which a program is incorporated, the circuit being adapted to execute or to be used in the execution of the receiving or retransmitting process according to the invention.

[0051] In other embodiments, it is also possible to consider that the receiving process, the destination device, the retransmission process, the node of relaying and the communication system according to the invention present in combination all or part of the aforementioned characteristics. Brief description of the drawings

[0052] Other features and advantages of the present invention will become apparent from the description below, with reference to the accompanying drawings, which illustrate an example of an embodiment without being limiting in any way. In the figures:

[0053] [Fig-1] [Fig.1], already described, represents a cooperative communication system of the state of the art;

[0054] [Fig.2] [Fig.2] represents a communication system according to the invention, in a particular embodiment; [Fig.3] [Fig.3] represents the hardware architecture of the destination device and the relaying nodes of the communication system of [Fig.2], in a particular embodiment;

[0055] [Fig.4] [Fig.4] represents the functional modules of the destination device (fig. 4A) and the relaying nodes (fig. 4B) of the communication system of [Fig.2], in a particular embodiment;

[0056] [Fig. 5] [Fig. 5] represents the main stages of a reception process and a retransmission process as implemented respectively by the destination device and by the relaying nodes of the communication system of [Fig. 2]; and

[0057] [Fig.6] [Fig.6] represents an incremental coding implemented in the context of the invention. Description of the invention

[0058] Fig. 2 represents, in its environment, a communication system 1 according to the invention, in a particular embodiment.

[0059] The communication system 1 comprises: - a plurality of source devices Sb, S2, ..., SM, M, denoting an integer greater than or equal to 2, each source device being capable of sending at least one information message. It is assumed here that each source device comprises a single transmission antenna; and - a multi-antenna destination device D, according to the invention and intended to receive information messages sent by the source devices Si, S2, ...., SM. The destination device D is here equipped with a number NR of receiving antennas, NR denoting an integer greater than or equal to 2.

[0060] In accordance with the invention, the communication system 1 implements a cooperative communication technique based on relay nodes configured to retransmit messages from source devices that could not to be correctly decoded by the destination device D. These relaying nodes can be selected from the source devices Si, S2, .SM (which are then said to be cooperative) and / or according to the configuration of the communication system 1, from other devices called intermediate (or relays) Rm+i, • RM+r of the communication system 1, L designating an integer greater than or equal to 1. These intermediate devices Rm+i, RM+L are equipped with a single transmission antenna and are dedicated solely to relaying messages from the source devices Si, S2, ..., SM to the destination device D (unlike the source devices, they do not have their own information messages to transmit to the destination device D). In the following, for the sake of simplicity, we will index the devices of communication system 1 (referred to as "device") by j, je N={ 1,.. .,M+L] for je N) with the exception of the destination device D: thus the indices j=l,...,M respectively denote the source devices Sb S2, ..., SM and the indices j=M+l,...,M+L respectively denote the intermediate devices Rm+i, • • - , Rm+l- .

[0061] It is noted that the communication system 1 may not include intermediate devices Rm+i, ..., Rm+l solely dedicated to relaying, in which case the relaying nodes are selected only from the source devices Si, S2, ..., SM-

[0062] No limitations are attached to the nature of the various devices belonging to the communication system 1. For example, the source devices Si, S2, ..., SM can be user equipment or UEs (for "User Equipment") such as terminals, IoT (Internet of Things) devices, etc., the destination device D can be a base station of a 5G or 6G network, and the intermediate devices (pure relays) can be other user equipment connected to the network. This example is, however, given only by way of illustration and is not limiting in itself; the invention can be applied in other contexts, such as within a proprietary network, an ad hoc network, etc.

[0063] It should be noted that the term "antenna" here covers indifferently a physical antenna, a radiating element of such a physical antenna, or a logical antenna such as an antenna port (or AP for "Antenna Port") as defined in particular in the specifications of the 3GPP standard. An antenna port is an abstract concept defined in paragraph 4.4.1 of the 3GPP document TS 38.211 entitled "Technical Specification Group Radio Access Network; NR; Physical channels and modulation (Release 17)" v 17.7.0 (2024-03), according to which the channel on which a symbol of an antenna port is transmitted can be deduced from the channel on which another symbol of the same antenna port is transmitted.

[0064] In the embodiment described herein, the communication system 1 uses an orthogonal multiple access scheme of the OMAMRC type based on a Incremental redundancy retransmission protocol of the "IR-type HARQ" type as previously mentioned. (Re)transmissions between devices of communication system 1 are organized according to three phases: - an initialization phase q>0, during which the destination device D determines the modulation and coding scheme to be used by each source device Si, S2, ..., SM, depending on the quality of the transmission channel separating it from that source device. The destination device D informs the source devices Si, S2, ..., SM of the chosen modulation and coding schemes via a return link; - a transmission phase <pl, au cours de laquelle les M dispositifs sources Si, S2, ..., SM transmettent successivement leurs messages d’information respectifs, codés en utilisant les schémas de modulation et de codage déterminés par le dispositif de destination D lors de la phase d’initialisation. Pendant cette phase de transmission, le nombre NI d’éléments de ressource (et incidemment d’utilisations du canal de transmission) est fixe et identique pour tous les dispositifs sources Si, S2, ..., SM; et - a retransmission phase q>2, lasting at most Tmax time intervals, and which relies, at each time interval, according to the invention, on a plurality of relay nodes that simultaneously retransmit a plurality of information messages (more precisely, redundant versions resulting from the encoding of these information messages) associated with a plurality of source devices not decoded by the destination device D.

[0065] In the embodiment described here, the devices (source devices, destination devices, and, where applicable, intermediate devices) of the communication system 1 have the hardware architecture of a computer 2 as shown in [Fig. 3]. This computer 2 includes, in particular, a processor PROC, random access memory MEM, read-only memory ROM, non-volatile memory NVM, and communication means COM.

[0066] The non-volatile NVM memory of the destination device D constitutes a recording medium according to the invention, readable by the PROC processor and on which a PROG-D program according to the invention is recorded.

[0067] This PROG-D program includes instructions defining the main steps of a receiving process according to the invention, and more specifically defines the functional modules of the destination device D that rely on and / or control all or part of the PROC, MEM, ROM, NVM, and COM elements of the computer 2 mentioned above. These functional modules include, in particular, in the embodiment described here, as illustrated in Fig. 4A, modules activated following a inability of the destination device D to decode (correctly) messages received from several so-called undecoded source devices for at least a time interval of the retransmission phase q>2, these modules comprising:

[0068] - a selection module 3, configured to select according to (i.e., based on) a defined performance criterion, a Sopt set comprising at least two source devices not decoded by the destination device D, and a Bopt set of relay nodes adapted to this Sopt set having an IBoptl cardinality greater than or equal to the ISoptl cardinality of the Sopt set. In the embodiment described here, the selection module 3 is configured to select the Sopt and Bopt sets that optimize the performance criterion in question. However, in an alternative embodiment, it is possible to consider that the selection module 3 is configured to select Sopt and Bopt sets that allow the performance criterion in question to reach a defined threshold value (for example, such that the performance criterion is greater than the threshold value in question);

[0069] - a sending module 4, configured to send retransmission instructions to the relay nodes of the Bopt set so that they simultaneously transmit to the destination device D the same second redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set; and

[0070] - a receiving module 5, configured to receive redundant versions transmitted by the relay nodes, these redundancy versions are intended to be used by the destination device D to decode messages received from the undecoded source devices of the Sopt set.

[0071] The functions of modules 3 to 5 are described further later with reference to the steps of the reception process according to the invention and to [Fig.5].

[0072] As mentioned previously, in the embodiment described herein, the source devices Si, S2, ..., SM and the intermediate devices Rm+i, Rm+2, • Rm+l of the communication system 1 are all capable of being selected as relay nodes by the destination device D during the retransmission phase. Each of these devices is therefore capable of being a relay node according to the invention, and includes in its non-volatile memory NVM, readable by the PROC processor, a program PROG-R according to the invention.

[0073] This PROG-R program includes instructions defining the main steps of a retransmission process according to the invention implemented by a relay node, and more specifically the functional modules of such a relay node (and therefore of the source devices and intermediate devices) which rely on and / or control all or part of the PROC, MEM, ROM, NVM, and COM elements of computer 2 mentioned previously. These functional modules include, in particular, in the embodiment described here, as illustrated in Fig. 4B:

[0074] - an information module 6, configured to inform the destination device D of its knowledge where applicable of messages emitted by several of the source devices Si, S2, ..., SM. The knowledge of an information message associated with a source device, as mentioned previously, may come from the fact that the relaying node is said source device in question, or that it correctly decoded the message received from the source device during the transmission phase q> 1 or at the end of a previous time interval of the retransmission phase q>2;

[0075] - a receiving module 7 configured to receive, where applicable, from the device destination D a retransmission instruction, during a given time interval of the retransmission phase q>2, of second redundancy versions resulting from the encoding of information messages associated with a set Sopt of source devices known to the relay node but not decoded by the destination device D; and

[0076] - a transmission module 8 configured to execute this instruction retransmission during the given time interval.

[0077] The functions of modules 6 to 8 are described further later with reference to the steps of the retransmission process according to the invention and to [Fig.5] described now.

[0078] Fig. 5 represents the main steps of the reception and retransmission processes as implemented respectively by the destination device D and by the relaying nodes selected by this destination device D during the retransmission phase q>2.

[0079] The different devices (source devices, destination device and intermediate devices) of the communication system 1 are assumed to be synchronized, and the source devices Si, S2, ..., SM are assumed to be statistically independent (no correlation between them).

[0080] As mentioned previously, during the initialization phase q>0 which precedes the transmission of data frames by the source devices Sb S2, ..., SM, the destination device D determines, for each source device Sm, m=l,... .M, the modulation and coding scheme (or MCS) that it must use to transmit data (step E10). The way in which the destination device D does this involves link adaptation techniques, known to those skilled in the art and not described here. The destination device D sends to each source device Sm, m=l,... .,M, via a limited-rate control channel, information unambiguously identifying the MCSm scheme that has been assigned to it (step E20). This information is, for example, in the context of a 5G network, an MCS scheme index pointing in a determined table to a MOD modulation, an Rm coding efficiency to be obtained at the output of a determined COD coding scheme (e.g., an LDPC code), and a spectral efficiency value to be used to transmit its useful data.

[0081] No limitations are attached to the modulations or types of coding that can be envisaged in the context of the invention. For example, it is possible to consider using phase shift keying (PSK) or quadrature amplitude modulation (QAM) of various orders, LDPC (Low Parity Check Code) type coding schemes, turbo codes or convolutional codes, systematic or non-systematic, or any other modulation and / or coding scheme.

[0082] Following this initialization phase q>0, the transmission phase q>1 starts.

[0083] Each source device Sm, m=l,...,M, attaches to the useful data (information message within the meaning of the invention) that it wishes to transmit to the destination device D a CRC (for "Cyclic Redundancy Check" in English) for error detection, then codes the useful data completed by the CRC using the COD coding scheme.

[0084] When using an IR-HARQ type protocol based on incremental coding, the information message including the CRC is encoded by the source device Sm with a very low-efficiency master code (for example, 1 / 3 or 1 / 5), typically lower than the efficiency of the MCSm scheme indicated during the initialization phase. The number of bits Km in the information message of the source device Sm depends on the coding efficiency and the MOD of the MCSm scheme indicated during the initialization phase q>0. More specifically, Km = RmN Lqm, where denotes the number of bits per symbol carried by the modulation. The encoded information message resulting from this encoding (or codeword) is then stored by the source device Sm, for example in its non-volatile memory NVM, in a circular buffer as illustrated in [Fig. 6]. In the example shown in [Fig. 6], a systematic master code with an efficiency of 1 / 3 is used.As is well known, the coded bits obtained at the output of a systematic code consist of the information bits (called "systematic" bits) supplied as input to the code (possibly punched) and redundancy bits.

[0085] The coded bits stored in the circular buffer are organized into a number of so-called redundancy versions, denoted RV0m, RVlm, etc., starting at specific positions POSOm, POSlm, ..., on the buffer, and whose dimensions are fixed according to the number of resource elements available for each (Re)transmission. Thus, the first RVOm redundancy version has dimensions of N1.qm, and subsequent redundancy versions have dimensions of N2.qm. The first RVOm redundancy version is also chosen to be self-decodable. For example, it includes systematic bits as illustrated in [Fig. 6]. Note that the configuration of the POS0m, POS1m, POS2m, and POS3m positions illustrated in [Fig. 6] is that proposed by the 3GPP standard and is only an illustrative example, not a limiting one. Other configurations can be considered.

[0086] During the transmission phase <pl, la première version de redondance RV0m est transmise par le dispositif source Sm, après avoir modulé les bits codés contenus dans cette version de redondance RV0m avec la modulation MOD correspondant au schéma MCSm déterminé par le dispositif de destination D (étape E30). Les autres versions de redondance sont transmises ultérieurement si nécessaire pendant la phase de retransmission q> 2, as described later.

[0087] The source devices Si, S2, ..., SM transmit their respective first redundancy versions RVOi, RV02, ..., RV0M to the destination device D, in turn during M time intervals of the transmission phase q> 1. It is noted that when a source device Sm transmits its first redundancy version RV0m during a time interval, the other devices of the communication system 1 (i.e. the destination device, the other source devices Sj, j=l,..., M and j^m, and where applicable the intermediate devices (Rm+i, RM+2, • • - , Rm+l) listen to the transmission channel (without transmitting).

[0088] At the end of the transmission phase q> 1 (or each time interval of the transmission phase q> 1), the destination device D attempts to decode the messages received from the source devices Si, S2, ..., SM (step E40). For simplicity, in the following description, "decoding (correctly) a message received from a device" is sometimes referred to as "decoding the device" in question.

[0089] The destination device D identifies the source devices whose messages it was able to correctly decode (referred to herein as "decoded devices") using the CRCs attached to the information messages of the source devices, in a manner known per se. S^q denotes the decoding set of the destination device D, comprising the source devices correctly decoded by the destination device D at the end of the transmission phase, and S^q denotes the complementary set of the decoding set S^q, comprising the source devices that the destination device D was not able to correctly decode.

[0090] The other devices j, i N of the communication system 1 proceed identically from the messages they received during the transmission phase (step E50). We denote by Syo, je N the decoding set of a device j of the communication system 1, and by Sj^, j GN, the set of source devices that have not been correctly decoded by the device j. We note that if the device j is a source device of the communication system 1, the decoding set Sjq includes at a minimum the device j.

[0091] If SpQ is not empty (i.e., if there is at least one source device not decoded by the destination device D at the end of the transmission phase q>1), the destination device D informs the other devices j, i, N of the communication system 1, for example by sending a non-acknowledgment message NACK in a control channel or a feedback channel (step E60). Otherwise, the destination device D sends an acknowledgment message ACK in the control or feedback channel, and a new transmission phase q>1 (possibly preceded by a new initialization phase q>0 if propagation conditions require it) can be implemented by the source devices Si, S2, ..., SM.

[0092] It should be noted that the acknowledgment (ACK) and non-acknowledgment (NACK) messages are not necessarily explicit. For example, the destination device may send to the devices of communication system 1 the set of source devices Spg that it was unable to decode during the transmission phase, and if this set is not empty, it is interpreted by the devices of communication system 1 as a non-acknowledgment message. Conversely, if the transmitted set S^o is empty, the devices of communication system 1 interpret it as an acknowledgment message.

[0093] Sending the non-acknowledgment message by the destination device D triggers the retransmission phase q>2. The retransmission phase q>2 is intended to allow the destination device D to correctly decode the information messages from the source devices in its set SD0 and lasts for a maximum number Tmax of time intervals (in other words, a maximum number Tmax of retransmissions is permitted). It is assumed here that the set Sp0 includes at least two source devices not decoded by the destination device D.

[0094] The steps described below are repeated for at least Tused>l time interval(s) of the retransmission phase q>2, until a predetermined stopping criterion is met (test step E70). In the embodiment described here, this stopping criterion is reaching the maximum number Tmax of retransmissions (i.e., t = Tmax) or obtaining an empty set at the end of the time interval t of the retransmission phase. Of course, other stopping criteria can be considered.

[0095] In the following, each time interval of the retransmission phase q>2 is indexed by t, and Spt and Sj denote the respective decoding sets of the destination device D and of each device j of the communication system 1, with j ∈ N, at the end of the time interval t of the retransmission phase q>2, and Sji and Sji their complementary sets (i.e. including the source devices not correctly decoded).

[0096] During a time interval t of the retransmission phase q>2, l <t<Tused, un dispositif j du système de communication 1 peut aider le dispositif de destination D à décoder un dispositif source Sm de l’ensemble et donc servir de nœud de relayage pour ce dispositif source s’il connait le message d’information associé au dispositif source Sm, soit parce que ce dispositif est lui-même le dispositif source Sm soit parce qu’il a correctement décodé son message d’information lors d’un intervalle de temps précédent de la phase de retransmission q> 2 or at the end of the transmission phase. As mentioned previously, if device j is a source device of communication system 1, the decoding set Sjj includes at least device j.

[0097] Following the sending of the non-acknowledgment message from the destination device D, the destination device D is informed of the devices of the communication system 1 knowing the information messages associated with the source devices of the set SD^ (E80).

[0098] To this end, different ways of proceeding can be envisaged.

[0099] For example, upon receipt of the non-acknowledgement message NACK from the destination device D, the devices j, je N, of the communication system 1 inform the destination device D, via their respective information modules 6 (for example by means of a message sent to the destination device D in the control or feedback channel), of their respective decoding sets S je N.

[0100] According to another example, the destination device D informs the devices j, je N of the communication system 1 of its decoding set or of its set of undecoded source devices and the devices j, je N, of the communication system 1 inform the destination device D, via a message sent back by their respective information modules 6 (for example in the control or feedback channel), when they know one or more undecoded information messages by the destination device D by identifying the known message(s).

[0101] According to the invention, when the assembly comprises at least two source devices, the destination device D can select a set Bopt (index t omitted for simplicity) of several relaying nodes from among the devices j, i, N, of communication system 1 to simultaneously retransmit, during the transmission interval t, the redundancy versions associated with the same Sopt set (index t also omitted) comprising at least two source devices of the Spt-i set. The Bopt set groups devices of communication system 1 that all know, in the sense of the invention, the information messages associated with each of the source devices of the Sopt set. Its cardinality IBoptl is at least greater than the cardinality ISoptl of the Sopt set.

[0102] The Sopt and Bopt sets are selected by the destination device D, via its selection module 3, at the end of each time interval t-1 so as to optimize a given performance criterion (step E90), for example, the destination device D selects sets optimizing mutual information. The way in which this selection is carried out is described in more detail later, with reference to Annexes 1 and 2. Hereafter, the relay nodes of the Bopt set are designated by bi,..,b|Bopti and the undecoded source devices of the Sopt set by Sm p .. ■.

[0103] It should be noted that, in the embodiment described here, if during the selection step E90, it is not possible for the destination device D to identify several devices of the communication system 1 all having knowledge of the same plurality of source devices not decoded by the destination device D, or if there remains only one source device not decoded by the destination device D, or if the optimization of the performance criterion leads to such a configuration, the destination device D may decide to retransmit during the time interval t only one information message associated with a single undecoded source device, relying on one or more relaying nodes, as in the prior art.In other words, in the embodiment described here, the destination device D attempts, as far as possible, to prioritize the implementation of spatial multiplexing (retransmission of the same redundancy versions associated with a plurality of distinct source devices via a plurality of relay nodes); however, if conditions are unfavorable or lead, for example, to a less optimal performance criterion value than without implementing spatial multiplexing, the destination device D triggers the retransmission of the redundancy version associated with only one undecoded source device during the time interval considered. This situation is, of course, likely to change from one time interval to another during the retransmission phase.

[0104] Once the Sopt and Bopt sets are selected, the destination device D determines the complex precoding matrix denoted y_y^yJ comprising (inline) the precoding vectors to be applied at each relay node of the Bopt set on the retransmitted messages from the source devices of the Sopt set. The matrix V has dimensions IBoptl x U where ü=|Soptl denotes the number of source devices in the Sopt set (step E100).

[0105] The precoding matrix V is determined, for example, by the destination device D so as to allow the selected relay nodes to apply an MRT transmission technique. In a manner known per se, the precoding matrix V consists of the U best orthogonal eigenvectors of the matrix JJ^RJJ e (ranked in descending order of eigenvalues). (corresponding) where H denotes the matrix of the equivalent propagation channel between the relay nodes of the Bopt assembly and the NR receiving antennas of the destination device D, and -R is the covariance matrix of noise plus interference (this can be approximated by an identity matrix to reduce complexity). An estimation of the matrix Hest, assumed here to be known by the destination device D, is performed using techniques known to those skilled in the art. The eigenvector vtest is intended to be applied to precode the redundancy version associated with the source device Sm;dc the Sopt assembly.

[0106] Of course, other criteria can be considered.

[0107] The destination device D then sends, by means of its sending module 4 and via the control or feedback channel, retransmission instructions to the relay nodes bi,..,b|Bopti of the Bopt set for the time interval t so that they simultaneously transmit, during this time interval t, the same redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices S»ip ..., of the Sopt set (step El 10). In the embodiment described here, the retransmission instructions are broadcast to all devices of the communication system 1.In the embodiment described here, the retransmission instructions include the Sopt set, the precoding matrix V (which includes the precoding vectors to be applied by the relay nodes), and a vector b of dimension IBoptl establishing the correspondence between the selected relay nodes and the precoding vectors transmitted in the precoding matrix to be applied by each relay node. Typically, the device of the communication system 1 whose index is designated by the j-th component bj of the vector b (in other words, the relay node bj with the notations introduced previously) applies the j-th row of the precoding matrix V, and more specifically the coefficient to the message associated with the 1-th source device designated in the Sopt set.

[0108] Alternatively, it can be envisaged that the destination device D sends the instruction to each selected relay node individually

[0109]

[0110] [YES]

[0112]

[0113]

[0114]

[0115] retransmission concerning it. In this variant, the retransmission instruction includes the Sopt set, the precoding vector to be applied by the relaying node in question, as well as an indication of the redundancy versions to be transmitted for the source devices of the Sopt set. The destination device D can also provide, in the retransmission instructions, an indication of the distribution between the source devices SOTi, ..., S,,,, of the Sopt set of the power transmitted by each relay node bi,..,b|Bopt|, such that this distribution was taken into account when determining the precoding matrix V, and that the relay nodes bi,..,b|Bopt| of the Bopt set must apply during retransmission. For example, if the precoding matrix V is determined on the basis of a power transmitted by each relay node of the Bopt set equally distributed over the U source devices Sot,, .. •, Sot,, of the Sopt set, the destination device D can send in the retransmission instructions an indication of the power PO to be applied to each source device Sm,, .... Sm, of the Sopt set. Alternatively, the value of PO can be calculated by the relay nodes of the Bopt set from the knowledge of the precoding matrix V (for example when this precoding matrix is ​​broadcast by the destination device D on the control channel to all devices of the communication system 1) and the following expression: p — p--— where P denotes the transmission power of a relay node bj. If the assumption of equal power distribution between the source devices Sot,, ..., Sm, is not applied, then in this case the destination device D transmits, in the retransmission instructions, the vector P - [ / % ...,^] of the powers to be applied to each source device Sot,, .. •, Sot,, of the set Sopt. Upon receiving retransmission instructions from the receiving device D, each relay node bi,..,b|Bopt| of the Bopt set simultaneously retransmits to the destination device D, during the time interval t, redundant versions of the information messages associated with the source devices of the Sopt set, to which the coefficients of the vector of the precoding matrix associated with it are applied (step E120). Thus, during step E120, the relay nodes bi,..,b|Bopt| of the Bopt set execute, via their respective retransmission modules 8, the retransmission instructions that have been transmitted to them.

[0116] More specifically, during the time interval t, all bi-relay nodes The Bopti devices of the Bopt set simultaneously transmit to the destination device D (via their respective retransmission modules 8) the same redundancy versions resulting from the encoding of the information messages of the source devices Sw, ..., Sm of the Sopt set, to which they apply the precoding vectors of the precoding matrix V that were communicated to them in the retransmission instructions. It should be noted that when the retransmission instructions are broadcast by the destination device D to all devices of the communication system 1, each device is able to keep the redundancy version to be retransmitted for a source device up to date at each time interval of the retransmission phase, and thus to identify the redundancy version to be retransmitted for the time interval t for which it has been selected, if applicable.RVtm denotes the redundancy version transmitted at time interval 1 for the source device Sm, with, in the particular embodiment described here based on Figure 6 and 4 distinct redundancy versions, tm, = tml mod 4 where t'^ denotes the number of retransmissions of the information message associated with the source device S™, (including that of the time interval t) in the form of redundancy versions.

[0117] Each relay node bj, j=l,..., IBoptl, thus transmits simultaneously, during of the time interval t, the linear combination of the U redundancy versions RVtmY RVtm? .. •, RVtm associated respectively with the U source devices SOT|, ..., S,„.

[0118]

[0119] of the set Sopt weighted by the coefficients of the precoding matrix V, i.e.: where vj}, 1=1,..., ü, denote coefficients of the precoding matrix V (and more precisely of the precoding vector allocated to the relay node bj), and (RVtm,) k denotes the k-th (ke {0, ..., N2- 1} ) coded modulated symbol of the 6th redundancy version resulting from the encoding of the information message associated with the source device SM / with £ [ | ( RVtm ) 121 = P» The redundancy version RVtm, transmitted at interval f, is obtained by the relay node from a circular buffer such as that represented in Figure 6, generated from its knowledge of the information message associated with the source device Sn(;). The transmitted redundancy version RVtm is obtained by reading the coded bits from the POS position in the buffer. Thus, for the time interval indexed by t=1, the redundancy version RVU is read from the POS position in the circular buffers corresponding to each information message associated with the source device Sm', for 1=1,..., U. Subsequently, the redundancy versions are transmitted, in the embodiment described here, in the following order: RV2m„ RV3m„ RV0mp RVlmi, RV2m„ RV3m„ etc.

[0120] During the time interval t and step E120, all the relay nodes bi,..,b iBopti of the Bopt set therefore simultaneously transmit a linear combination of the same redundancy versions associated with the source devices, Sm. of the Sopt set, the linear combination differing from one relay node to another depending on the precoding vectors applied. During this time interval t, all the relay nodes therefore use the same number of resource elements (i.e., the same number N2 of channel uses).

[0121] Upon receipt by the receiving device D (via its receiving module 5) of the redundancy versions transmitted during the time interval t by the relay nodes bi,..,b|Bopti of the Bopt set, the destination device D attempts to decode the messages received during the transmission phase q> 1 from the source devices of the Sopt set using the new redundancy versions transmitted by the relay nodes (step E130). The method by which the destination device D performs this function is known per se and involves techniques classically used within the framework of an IR-HARQ protocol, not described here.

[0122] The destination device D identifies the new source devices whose messages it has been able to correctly decode using the CRCs attached to the information messages of these source devices, as described previously, and updates its decoding set and the complementary set at the end of the time interval t.

[0123] If the decoding set SDj is empty (all source devices Si,...,SM are correctly decoded), the destination device D sends an acknowledgment message ACK in the control or feedback channel, and the retransmission phase q>2 is completed (step E140). A new transmission phase q>1 (possibly preceded by a new initialization phase q>0 if propagation conditions require it) can be implemented for new information messages from the source devices Si,...,SM.

[0124] If Spt is not empty (i.e., if at least one source device not decoded by the destination device D still exists at the end of the time interval t), the destination device D informs the other devices j, i, N of the communication system 1 by sending a non-acknowledgment message NACK in the feedback control channel in the same way as was done previously in step E60 (step E140). If the maximum number of retransmissions is not reached (test step E70), steps E80 to E140 are repeated for a new time interval t+1.

[0125] We will now describe how the selection of the Bopt and Sopt sets is carried out by the selection module 3 of the destination device D during the step of E90 selection, for each transmission interval t, in a particular embodiment. As mentioned previously, the Bopt and Sopt sets may differ from one time interval to another; however, for the sake of simplicity, the index t is omitted when these sets are designated.

[0126] In the embodiment described here, to select the Bopt and Sopt sets used for a time interval t, the destination device D calculates the set of subsets of the set of source devices not decoded by the destination device D at the interval t-1, restricting itself to subsets of cardinality less than or equal to a given value Qmax (for example, Qmax is taken to be equal to the number of receiving antennas of the destination device D) and which are different from the empty subset. By subset of a set A, denoted Part^A^, we mean the set of all possible non-empty subsets consisting of elements of A. The resulting set is denoted Part. The subsets of Part (SD^) of cardinality ge(1, ..., Qmax) are denoted PartQ (SDt_) and are such that Part^) = ^^Part^S^)'

[0127] Then, for each subset of source devices S ePart^S^ ), Q e {1, ..., Qmax}'le d'-'l'™1'!dc destination D determines the set B of devices] e N, of the communication system 1 that have decoded at least the source devices of S with the constraint ISI < \B\ for S to be eligible. In the following, the devices of set B being potential relay nodes, they are called "relay nodes" for the sake of simplicity. If no subset is eligible for a given value Qq then the destination device D does not test the values ​​of Q > Q^

[0128] For each eligible subset S, the destination device D evaluates a specific performance criterion. For example, in the embodiment described here, this evaluated performance criterion is the equivalent mutual information associated with activating the relay nodes of set B to simultaneously retransmit the redundancy versions of the information messages associated with all source devices of S. The calculation of the mutual information is explained in more detail later.

[0129] The destination device D selects as set Sopt, the subset S and the associated set B which maximizes this mutual information.

[0130] For illustrative purposes, if 5 = {1, 4, 5} and Qmax = NR = 3, the selection module 3 obtains:

[0131] Part^Su;^ = {{if {4), {5), {1, 4}, {1, 5}, {4, 5}, {1, 4, 5}

[0132] For each subset S e Part (S), it determines the set of relay nodes B that have decoded the source devices contained in S, i.e., j ≤ N such that SCS ≤ 4. Then, the selection module 3 calculates the mutual information associated with the simultaneous transmission of a linear combination of the redundancy versions associated with the source devices of S by these relay nodes B. Note that if Qmax = NR < 3, the destination device is limited to subsets S ≤ QSjj such that ISI <Qmax=NR, par exemple pour NR=2 :

[0133] = {{1}.{4), (5). {1.4}, (1,5). {4.5}!

[0134] Furthermore, if S = {1,4, 5} and B = [ 1,4}, the destination device D does not consider the set S as eligible for retransmission because ISI > IB!.

[0135] Annex 1 presents an example of pseudo-code that can be used by the selection module 3 during the selection step E90.

[0136] The time complexity of such a selection algorithm can be measured in the number of iterations required to obtain the Bopt and Sopt sets. The inventors have demonstrated that, in the worst case, the number of iterations required is:

[0137] 3 k ^(|SD.w[7c)!

[0138] which reduces to, when Qmax > :

[0139] 2^-1

[0140] In other words, if we consider, for illustrative purposes, different values ​​of NR (which is the maximum value that can be taken by Qmax) and of |:

[0141] - for | = 2 and : • NR = 2:3 iterations. NR = 4: 3 iterations. - for = 3 and: NR = 2: 6 iterations. NR = 4: 7 iterations. -for =7 and: • NR - 2; 28 iterations. • NR = 4; 98 iterations. • NR = 8; 127 iterations.

[0142] It is clear that the temporal complexity increases with the number of transmitted spatial layers, i.e., the number of source devices not decoded in Sopt. One way to control this increase is to set a maximum number Qmax <NR de dispositifs sources pour lesquels des versions de redondance are retransmitted simultaneously. For example, for = 7, NR=8 and Qmax=2, 28 iterations are executed compared to 127 iterations in the case where Qmax=NR.

[0143] We will now describe how the mutual information is calculated in the embodiment described here, for a subset S and a corresponding set B, and then optimized during the selection step E90.

[0144] It has been described previously how the precoding matrix V to be applied by the assembly B is obtained. It can easily be shown that the signal-to-noise ratio (SNR) of the 1st channel seen by the redundancy version associated with a source device Sm, of the assembly S, is given by VA, where \ denotes the eigenvalue associated with the leme O2 best eigenvector of the equivalent channel matrix between nodes of relaying assembly B and the receiving antennas of the destination device D, as introduced previously. Thus, the equivalent mutual information I§ contributed by the parallel channels is written as follows:

[0145] T _yv o2 /

[0146] where I denotes the mutual information and o2 the variance of the additive Gaussian noise (known from the destination device D). Assuming that the parallel channels have Gaussian inputs, we obtain: 101471 Is = ^,108,(1 + ^)

[0148] This expression is used by the selection module 3 of the destination device D to evaluate the equivalent mutual information for a given subset S and corresponding set B.

[0149] Different hypotheses can be considered by the destination device D concerning the power P / transmitted on each spatial layer (i.e. allocated to the transmission of a redundancy version associated with each source device of the subset S).

[0150] In a first variant, the destination device D can allocate this power Pi to the source device Smi so as to maximize the equivalent mutual information 1$. This amounts to maximizing the equivalent mutual information 1$, subject to the following constraints: 101511 E) = |M 2 P z <P. j=l, .... IBI

[0152] where vjJ denotes the coefficients of the precoding matrix V applied to the redundancy versions of the source devices of S and P denotes the maximum power that can be transmitted by a relay node. This constrained maximization can be achieved by the destination device D by applying Lagrange multipliers in a manner known per se.

[0153]

[0154]

[0155]

[0156]

[0157] According to a second variant, the destination device D can consider that the power is equally distributed across all spatial layers, i.e.: Pi — P o = p » Z = 1, ..., v maxL, As mentioned previously, the destination device D informs in the retransmission instructions the power allocation strategy chosen for it to be applied by the relay nodes of set B. However, it can be considered that if the destination device D does not indicate in the retransmission instructions the power allocation strategy chosen, an equal distribution of power is considered by default by the relay nodes, and they calculate the value PO from the precoding matrix V using the expression above. Annex 2 presents an example of pseudo-code that can be used by the selection module 3 of the destination device D to evaluate and maximize the equivalent mutual information I5 during the selection step E90, when an assumption of equal distribution of power over all source devices is considered. Appendix 1 [Tables 1]

[0158] Algorithm 1: Selection Inputs; set N comprising all potential relay nodes of the communication system I, sets and je jV Outputs: set of source devices S0^ for i 'time interval t of the retransmission phase and set of corresponding relaying nodes Bf* Initialization; Max = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 for il 11 Q ® {!> ■" ' ÇîïlGX} do =? bouoie su tous les ensembles possibles ùe dispositifs source?; aon-dècodés parD 6 F?lsc £ varîabEe bcoiéetitse tadiquani if ​​at least one subset is eùgible for tme value of Q butinée for ail S in Part^ ) of * for each possible 3rd disp^sicif& sources not deco3éî- ‘B 0 Reforms: Reforms forÜII J ifi 2^ of the tone? kz and if S c Ss t_1 then 7* Yes <?■ FlQïîtâ / Ci .ivSiÏÜjT / -ÉZ càd.^S .S | BBU y} ? 0« / ssiEgre æ3HS X'exsixéfé ü end if end for if [S[ < |^| tken # ie 5JGÆDKÏ •arj :£? 7^0^£735:^ SG«Î. NAIL rfoii then JE^ and <- True ~ ?sjLre»i£fe j$' srf Compute the equivalent mutual information ls (e.g. Algorithm 2) if h Max then MaX <— 1; and wsss up to date !ü of few equivalent ri^sr^sdeiï& S #.tntse àjow des en^m&leseéîectiems: peur îa B end if end if end for if s = Fa ine then | Break end if End for! Appendix 2 [Tables 2] ® not descus-eîssembSeéiisible ue P^rt^ :_f petus a given QO value Algorithm 2: Calculating Mutual Information Inputs: set S and set B of corresponding relay nodes, power P transmitted per relay node, noise variance a2 Outputs: equivalent mutual information Ig and precoding matrix V Imtialization: Is= 0.. Max = 0 1 2 3 4 5 6 H <- Channel between the relay nodes in B and the .¾ receiving antennas V, S EVD,,(H!H) £ EVDyt.j furnîtta precoding matrix t'r and tes eigenvalues ​​S in decreasing order of their values 8 9 10 11 12 13 14 for all j in {1,..., jSj} do tmp *— 0 for ail l in (1,..., i'} do | tmp +- tmp + end for if tmp > Max then | Max *- tmp end if # for all relay nodes of B: # Initialization of the sum of precoding coefficients on all source devices for a relay node j ? for all devices that are sources of S £ ÈskcttG^ of the maximum on all the relaying sisters; ù [1,, |£?]) ead for 15 16 Po P / Max for ail Z ia {lf v} do P SNHi = ¢72 end faith* £ èqiiirépartiticaaj de la. puissance # Calculation of 1: equivalent mutual information l $ for F set S

Claims

1.

2. Demands A method of reception by a multi-antenna destination device (D) having received messages transmitted successively by M>2 source devices (Si,...,SM), each message transmitted by a source device (Sm) comprising a first redundancy version (RVOm) resulting from a coding of an information message associated with the source device, said method comprising, following an inability of the destination device to decode said messages received from several of said source devices, said undecoded, and for at least a time interval of a retransmission phase: • a selection step (E90), according to a determined performance criterion, of a Sopt set comprising at least two undecoded source devices and a Bopt set of relaying nodes with a cardinality greater than or equal to the cardinality of the Sopt set, a said relaying node being a source device or an intermediate device knowing each of the information messages associated with the source devices of the Sopt set; • a step (El 10) of sending retransmission instructions to the relay nodes of the Bopt set so that they simultaneously transmit to the destination device the same second versions of redundancy (RVtm) resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set; • a step (El20) of receiving the second redundancy versions transmitted by the relay nodes, said second redundancy versions received being used by the destination device to decode the messages received from the undecoded source devices of the Sopt set. A receiving method according to claim 1 in which said retransmission instructions include precoding vectors to be applied by the relay nodes of the Bopt assembly to the second redundancy versions resulting from the coding of the information messages associated with the undecoded source devices of the Sopt assembly.

3. A receiving method according to claim 2 wherein said retransmission instructions comprise the selected Sopt set, a precoding matrix comprising said precoding vectors and an indication for each relay node of the precoding vector to be applied by that relay node.

4. A receiving method according to claim 2 or 3 wherein the precoding vectors are determined on the basis of a power transmitted by each relaying node equally distributed between the undecoded source devices of the Sopt assembly.

5. A receiving method according to claim 2 or 3 wherein the precoding vectors are determined on the basis of a power transmitted by each relay node distributed among the undecoded source devices of the Sopt set so as to maximize mutual information associated with the simultaneous transmission by the relay nodes of the selected Bopt set of said second redundancy versions.

6. A receiving method according to claim 4 or 5 wherein said retransmission instructions further include an indication of a distribution of the power transmitted by each relaying node between the undecoded source devices of the Sopt assembly.

7. A receiving method according to any one of claims 1 to 6 wherein said determined performance criterion is mutual information associated with the simultaneous transmission by the relay nodes of the selected Bopt set of said second redundancy versions.

8. A receiving method according to any one of claims 1 to 7, wherein the selection step comprises an evaluation of said performance criterion determined for each possible set S comprising Q undecoded source devices with 2 <Q<Qmax pour lequel il existe au moins un ensemble B de nœuds de relayage connaissant chacun des messages d’information associés aux dispositifs sources non décodés dudit ensemble S, Qmax désignant un nombre déterminé choisi inférieur ou égal à un nombre d’antennes de réception du dispositif de destination.

9. A receiving method according to any one of claims 1 to 8, wherein the steps of selection, sending instructions, and receptions are repeated for at least one other time interval of the retransmission phase until a predefined stopping criterion is verified, with new redundancy versions being transmitted at each time interval.

10. Method of retransmitting by a relay node of a communication system comprising a multi-antenna destination device and M>2 source devices having simultaneously emitted M messages, each message emitted by a source device comprising a first redundancy version resulting from an encoding of an information message associated with that source device, said relay node being able to be said source device or an intermediate device of the communication system, said method comprising: • a step of informing the destination device of knowledge by the relay node of messages emitted by several of the source devices;• a receiving step (El 10), configured to receive from the destination device a retransmission instruction, during a time interval of one retransmission phase, of second redundancy versions resulting from the encoding of information messages associated with a set Sopt of source devices known to the relaying node but not decoded by the destination device; and • a step (El20) of executing said retransmission instruction during said time interval.

11. A retransmission method according to claim 10 wherein the execution step comprises retransmitting, during said retransmission phase time interval, said second redundancy versions by applying to them a precoding vector transmitted in said retransmission instruction.

12. A multi-antenna destination device (D) capable of receiving messages transmitted successively by M>2 source devices, each message transmitted by a source device comprising a first redundancy version resulting from an encoding of an information message associated with the source device, said destination device comprising modules activated following an inability of the destination device to decode said messages received from several of said source devices, referred to as undecoded, and for at least one time interval of a retransmission phase, said modules comprising: • a selection module (3) configured to select, according to a determined performance criterion, a Sopt set comprising at least two undecoded source devices, and a Bopt set of relay nodes with a cardinality greater than or equal to the cardinality of the Sopt set, a said relay node being a source device or an intermediate device knowing each of the information messages associated with the source devices of the Sopt set; • a sending module (4), configured to send retransmission instructions to the relay nodes of the Bopt set so that they simultaneously transmit to the destination device the same second redundancy versions resulting from the encoding of the information messages associated with the undecoded source devices of the Sopt set; and • a receiving module (5), configured to receive second versions of redundancy transmitted by the relaying nodes, said second versions of redundancy received being used by the destination device to decode messages received from the undecoded source devices of the Sopt set.

13. A relay node in a communication system comprising a multi-antenna destination device (D) and M>2 source devices (SI,...,SM) having simultaneously transmitted M messages, each message transmitted by a source device comprising a first redundancy version resulting from an encoding of an information message associated with that source device, said relay node being able to be said source device or an intermediate device of the communication system, said relay node comprising: an information module (6), configured to inform the destination device of its knowledge of messages emitted by several of said source devices;

14.

15. • a receiving module (7), configured to receive from the destination device a retransmission instruction, during a time interval of a retransmission phase, of second redundancy versions resulting from the encoding of information messages associated with a set Sopt of source devices known to said relaying node but not decoded by the destination device; and • a retransmission module (8) configured to execute said retransmission instruction during said time interval. Communication system (1) comprising a destination device (D) according to claim 12, a plurality of source devices (Si,...,SM) and a plurality of relay nodes according to claim 13, said relay node being able to be said source device (Si,...,SM) or an intermediate device (Rm+i,...,R M+L) distinct from said source devices. Communication system (1) according to claim 14 using an orthogonal multiple access multiple-relay channel scheme of type OMAMRC, Orthogonal Multiple Access Multiple-Relay Channel, between source devices, relaying nodes and destination device.

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