Reception and retransmission methods, destination device, and relay node

Abstract

The invention relates to a reception method implemented by a device d having received messages transmitted successively by M≥2 source devices and comprising first redundancy versions of information messages from the source devices, which method comprises, following an inability of the device d to decode messages received and during at least one time interval of a retransmission phase: - selecting a set Sopt of at least two undecoded source devices and a set Bopt of relay nodes that know the information messages from these source devices; - sending retransmission instructions to these relay nodes so that they simultaneously transmit, to the device d, the same second redundancy version of each information message from the source devices of the set Sopt ; and - receiving the second redundancy versions and using same to decode the information messages from the undecoded source devices of the set Sopt. Figure for the abstract:

Description

Description Title of the invention: Reception and retransmission methods, destination device and relay node Previous technique

[0001] The invention falls within the general field of telecommunications.

[0002] It relates more specifically 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 competing for 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 of, for example, 5G or 6G communication networks, as defined by the 3GPR 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 (UEs). However, the invention can also 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 s1, s2, ..., sM, and one destination device d, and relies on cooperative relaying of messages sent by the source devices s1, s2, ..., sM, which is done 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 time division multiplexing (TDM).

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

[0006] More specifically, in the context of a 5G network, for example as defined by the 3GPR standard, an information message from a source device is typically encoded with a coding efficiency R, before being mapped to the symbols of a constellation (q bits per symbol) and then transmitted on N1 resource elements, or REs (for "Resource Element"). For an OFDMA (Orthogonal Frequency Division Multiple Access) multiplexing technique such as that used in a 5G network, an RE corresponds to an OFDM symbol in the time domain and a subcarrier in the frequency domain. The size L of the coded information message is therefore equal to L = N1.Rq

[0007] In the case of the HARQ R-type I protocol, the information message associated with a source device is first encoded with a low efficiency R0 < R (typically 1 / 3 or 1 / 5), resulting in a coded source message size Lc = DR0 strictly greater than L. This encoded information message is then stored in a circular buffer. At each (re-)transmission, the coded bits to be sent are read from fixed positions in the buffer. For example, during the first transmission, the source device reads the N1.q coded bits from position POS0; then during the first retransmission, N2.q coded bits are read from position POS2; during the second retransmission, N2.q coded bits are read from position POS3; during the third retransmission, N2.q coded bits are read from position POS1, and so on. The coded bits sent with each (re-)transmission are called "redundancy versions".

[0008] In an OMAMRC system using the HARQ-type IR 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.In this transmission phase, the number N1 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 the information messages that the destination device d has not been able to decode are cooperatively retransmitted, for a number Tused of time intervals, by relay nodes selected by the destination device d. The selection of relay nodes is carried out 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 have succeeded in decoding 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 links or feedback channels, often limited in scope. This exchange of information aims to allow the destination device d, on the one hand, to identify the nodes that have knowledge of the information messages it was unable to decode and that can be used as relay nodes for these information messages. undecoded data, and on the other hand, to inform the selected relay node(s) for each time interval to retransmit the same information message associated with a source device not decoded by the destination device. With each retransmission of an information message associated with a source, a new redundancy version resulting from the encoding of the information message in question is used by the selected relay node(s), 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 retransmit (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 allows for 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 allow 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 (MRT) in reception.This is achieved through the application, during 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 redundancy version resulting from the encoding of the information message associated with this source device, a precoding vector to be applied to this redundancy 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 signal-to-noise ratio (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) it could benefit from thanks to the plurality of relay nodes, and in particular its spatial multiplexing capabilities. Description of the invention

[0013] The invention notably overcomes this drawback by proposing a method of reception by a multi-antenna destination device that has received messages transmitted successively by M>2 source devices. Each message transmitted by a source device includes a first redundant version resulting from the encoding of an information message associated with the source device. This method comprises, following the inability of the destination device to decode messages received from several of the source devices (referred to as undecoded), for at least a time interval of a retransmission phase: a selection step, according to a determined performance criterion, of a set of SP pt comprising at least two undecoded source devices and a set B opt of relay nodes with a cardinality greater than or equal to the cardinality of the SP assembly pt, 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 set S*'; a step of sending retransmission instructions to the relay nodes of the set B opt so that they simultaneously transmit to the destination device the same second versions of redundancy resulting from the encoding of the information messages associated with the undecoded source devices of the set S*'; and a stage of receiving the second versions of redundancy transmitted by the relaying nodes.

[0014] Correspondingly, the invention also relates to a multi-antenna receiving device capable of receiving messages transmitted successively by M>2 source devices, each message transmitted by a source device comprising a first redundant version resulting from the encoding of an information message associated with the source device, said receiving device comprising modules activated following an inability of the receiving 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 set S 31 * comprising at least two undecoded source devices and a set B opt of relay nodes with a cardinality greater than or equal to the cardinality of the set S 31", 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 SP set pt ; a sending module, configured to send retransmission instructions to the relay nodes of set B opt so that they simultaneously transmit to the destination device the same second versions of redundancy resulting from the encoding of the information messages associated with the undecoded source devices of the set S*'; and a receiving module, configured to receive second versions of redundancy transmitted by the relaying nodes.

[0015] The second redundancy versions received are typically used by the destination device to decode messages received from undecoded source devices in the set S*'.

[0016] 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 verified, new redundancy versions are transmitted during each time interval. Such a criterion is, for example, reaching a maximum number Tmax of iterations or the absence, after a retransmission, of source devices not decoded by the destination device.

[0017] The invention also relates to a method for retransmitting, via a relay node, a communication system comprising a multi-antenna destination device and M to 2 source devices that have successively transmitted M messages. Each message transmitted by a source device includes a first redundancy version resulting from the encoding of an information message associated with that source device. The relay node may be a source device or an intermediate device of the communication system. The method comprises: a step of informing the destination device by the relay node of messages transmitted by several of the source devices; and a step of receiving, by the destination device, a retransmission instruction, during a retransmission phase interval, of second redundancy versions resulting from the encoding of the information messages associated with a set of SPs. ptof source devices known to the relay node but not decoded by the destination device; and a step of execution of said retransmission instruction during said time interval.

[0018] Correspondingly, the invention also relates to a relay node of a communication system comprising a multi-antenna destination device and M>2 source devices that have successively 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 relay node comprising: an information module, configured to inform the destination device of its knowledge of messages transmitted by several of said source devices; a reception 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 the information messages associated with a set SP ptof 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.

[0019] 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 relay nodes according to the invention, said relay node being able to be said source device or an intermediate device distinct from the source devices.

[0020] 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.

[0021] Thus, according to the invention, when the destination device has failed to decode the information messages from the source devices, it selects based on a performance criterion determined for each time interval of the phase of retransmission, a plurality S ppt of undecoded source devices, and a plurality B opt of relay nodes, which 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 managed to decode them correctly. The relay nodes of set B opt are therefore able to simultaneously retransmit the same redundant versions of the information messages associated with all source devices in the SP set ptA spatial multiplexing of the second redundancy versions is thus performed by the relay nodes, during the considered time interval of the retransmission phase, on o spatial layers, where o corresponds to the number of undecoded source devices in the SP set pt 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, by: 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 set B opt given the number of undecoded source devices in the set S*'; and the presence of multiple receiving antennas at the destination device.

[0022] The selection of SP sets pt and B opt is advantageously done according to a predetermined performance criterion, for example the destination device selects sets S*' and B opt which optimize this performance criterion. Such a performance criterion is, for example, the mutual information associated with the simultaneous transmission by the relay nodes of set B opt selected second versions of redundancy, and the destination device selects the SP sets pt and B opt which 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's performance on the propagation channels between the relay nodes and the destination device. This ensures that the selected sets S' 1 * and B optoptimize the overall performance of the system at each time interval of the retransmission phase.

[0023] Of course, other performance criteria can be considered, such as a bit error rate in reception. Furthermore, during the selection process, one can aim to optimize such a performance criterion or, alternatively, ensure that this performance criterion reaches at least a given threshold value.

[0024] It should be noted that, unlike a conventional M1 MO system consisting of a 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 assembly B opt is suitable for the S' set ptand that these two sets are likely to change between two time intervals. It follows that 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 within 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 configuration. possible at each time interval of the retransmission phase.

[0025] Thanks to these features, the invention offers the possibility of fully exploiting the capabilities of the distributed M1 MO system consisting of the destination device with multiple receiving antennas, the source devices, and the relay nodes (source devices and / or intermediate devices) participating in the relaying of information messages sent by the source devices but not decoded by the destination device, including 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.

[0026] 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 redundant version resulting from the encoding of this information message associated with the source device.

[0027] The invention therefore makes it very advantageous to improve the overall spectral efficiency of the communication system.

[0028] 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.

[0029] This embodiment offers a simple and efficient way to select the relay nodes at each time interval of the retransmission phase.

[0030] S 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 a number |B| of relay nodes having knowledge of the information messages associated with each source device in this set such that |B| >Q0 cannot be found, the search can be stopped for a number of undecoded source devices greater than Q0.

[0031] It is also possible to limit the value of Qmax to a number strictly less than the number of receiving antennas on 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 within the SP set. pt By deliberately limiting this number, we limit the complexity of the selection step.

[0032] According to the invention, the retransmission instructions sent by the device The retransmission of destination messages to selected relay nodes triggers the retransmission of messages from source devices that were not decoded by the destination device, according to 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.

[0033] In a particular embodiment, the retransmission instructions sent by the destination device to the relay nodes of assembly B opt include precoding vectors to be applied by the relay nodes of set B opt to the second versions of redundancy resulting from the encoding of information messages associated with the undecoded source devices of the SP set pt .

[0034] 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.

[0035] 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 coherently combine the received redundancy versions, and thus facilitate the elimination of interference at the destination device.

[0036] In one particular embodiment, the retransmission instructions include the SP set ptselected, 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.

[0037] This embodiment is particularly well-suited for a limited return channel between the destination device and the relay nodes, shared by all relay nodes (e.g., a broadcast type 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 the relay nodes to be informed of the precoding to be applied, enabling the destination device to utilize the retransmitted redundancy versions. This precoding 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 the following to the relay nodes of set B: opt an MRT type transmission technique.

[0038] In a particular embodiment, the precoding vectors are determined on the basis of a power transmitted by each relay node equally distributed among the undecoded source devices of the SP set pt .

[0039] This embodiment makes it possible to limit the amount of information sent back to the relay nodes of assembly B opt with retransmission instructions. Typically, when the assumption of equal power distribution across the spatial layers (i.e., the same PO transmission power 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 for determine the precoding matrix to be applied by the relay nodes of set B optIt is not necessary for the receiving device to include in the retransmission instructions the powers applied at each spatial layer. This is sufficient for the relay nodes of set B opt to have knowledge of the PO value. This knowledge can result from the transmission of the PO value (or equivalent information) by the destination device to the relay nodes with retransmission instructions, or 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.

[0040] In another embodiment, the precoding vectors are determined on the basis of a power transmitted by each relay node distributed among the undecoded source devices of the set S 31* so as to maximize mutual information associated with simultaneous transmission through the relay nodes of set B opt selected from said second versions of redundancy.

[0041] This embodiment is more optimal than the one based on an assumption of equal power distribution, but also requires a greater amount of information to be fed back to the relay nodes of set B. opt , since the destination device must inform them of the powers allocated to each spatial layer.

[0042] In either of these embodiments, the retransmission instructions may therefore also include an indication of the distribution of the power transmitted by each relay node between the undecoded source devices of the SP set pt .

[0043] In a particular embodiment, the reception process and / or the retransmission process is / are implemented by a computer.

[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 destination device conforming to the invention and comprising instructions adapted to the implementation of a reception process as described above.

[0045] 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 method as described above.

[0046] Each of these programs can use any programming language, and be in the form of source code, object code, or code somewhere between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0047] The invention also relates to an information carrier or a recording medium readable by a computer, and comprising instructions for a computer program as mentioned above.

[0048] 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 drive, or a flash memory.

[0049] On the other hand, the information or recording medium can be a transmissible medium such as an electrical or optical signal, which can be carried via an electrical or optical cable, by radio link, by wireless optical link or by other means.

[0050] The program according to the invention can in particular be downloaded onto an Internet-type network.

[0051] 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.

[0052] It can also be envisaged, in other embodiments, that the receiving process, the destination device, the retransmission process, the relaying node and the communication system according to the invention have in combination all or part of the aforementioned characteristics. Brief description of the drawings

[0053] Other features and advantages of the present invention will become apparent from the description given below, with reference to the accompanying drawings which illustrate an example of an embodiment without being limiting in any way. In the figures: [Fig. 1] Figure 1, already described, represents a state-of-the-art cooperative communication system; [Fig. 2] Figure 2 represents a communication system according to the invention, in a particular embodiment; [Fig. 3] Figure 3 represents the hardware architecture of the destination device and the relaying nodes of the communication system of Figure 2, in a particular embodiment; [Fig. 4] Figure 4 represents the functional modules of the destination device (Fig. 4A) and the relaying nodes (Fig. 4B) of the communication system of Figure 2, in a particular embodiment; [Fig. 5] Figure 5 represents the main stages of a reception process and a retransmission process as implemented respectively by the destination device and by the relay nodes of the communication system in Figure 2; and [Fig. 6] Figure 6 represents an incremental coding implemented in the context of the invention. Description of the invention

[0054] Figure 2 represents, in its environment, a communication system 1 conforming to the invention, in a particular embodiment.

[0055] The communication system 1 comprises: a plurality of source devices Si, Sa, ..., 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 transmitting antenna; and a multi-antenna destination device D, according to the invention and intended to receive the information messages sent by the source devices Si, Sa, ..., SM. The destination device D is equipped here with a number NR of receiving antennas, NR denoting an integer greater than or equal to 2.

[0056] According to 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 be correctly decoded by the destination device D. These relay nodes can be selected from among the source devices Si, &, SM (which are then said to be cooperative) and / or, according to the configuration of the communication system 1, from among other devices called intermediate (or relay) RM+I, RM+L 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, &, SM to the destination device D (unlike the source devices, they do not have information messages of their own to transmit to the destination device D).In the following, for the sake of simplification, we index by j, je JV= { 1 , ...,M+ L} the devices of the communication system 1 (then designated by "device j", for je J\T) with the exception of the destination device D: thus the indexes j= 1. respectively designate the source devices Si, Sa, ..., SM and the indices j = M+ 1 ,...,M+ L respectively designate the intermediate devices RM+ I , ..., RM+L.

[0057] Note 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 only selected from the source devices Si , &, ..., SM.

[0058] No limitations are attached to the nature of the various devices belonging to the communication system 1. For example, the source devices Si, Sa, ..., SM can be user equipment (UEs) such as terminals, IoT 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 for illustrative purposes 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.

[0059] It should be noted that the notion of "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" in English) defined in particular in the specifications of the 3GPP standard. An antenna port is an abstract notion 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)" v17.7.0 (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.

[0060] In the embodiment described here, the communication system 1 uses an orthogonal multiple access scheme of the OMAMRC type, based on an incremental redundancy retransmission protocol of the "I R-type HARQ" type, as previously mentioned. The (re-)transmissions between the devices of the communication system 1 are organized according to three phases: an initialization phase cpO, during which the destination device D determines the modulation and coding scheme to be used by each source device Si, Sa, ..., SM, depending on the quality of the transmission channel separating them. of this source device. The destination device D informs the source devices 51, &, Siw modulation and coding schemes chosen via a return link; a power line communication (PLC) transmission phase, during which the M source devices Si, 52, SM successively transmit their respective information messages, coded using the modulation and coding schemes determined by the destination device D during the initialization phase. During this transmission phase, the number N1 of resource elements (and incidentally of uses of the transmission channel) is fixed and identical for all source devices Si SM; and a retransmission phase cp2, which lasts at most Tmax time intervals, and which relies at each time interval, according to the invention, on a plurality of relaying nodes which simultaneously retransmit a plurality of information messages (more precisely redundancy versions resulting from the encoding of these information messages) associated with a plurality of source devices not decoded by the destination device D.

[0061] 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 represented in Figure 3. This computer 2 includes in particular a processor PROC, a random access memory MEM, a read-only memory ROM, a non-volatile memory NVM, and COM means of communication.

[0062] 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.

[0063] This PROG-D program includes instructions defining the main steps of a reception 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 computer 2 mentioned previously. These functional modules include, in particular, in the embodiment described here, as illustrated in Figure 4A, modules activated following the inability of the destination device D to (correctly) decode messages received from several so-called undecoded source devices for at least one time interval of the retransmission phase cp2. These modules include: - a selection module 3, configured to select, according to a given performance criterion, a set S*' comprising at least two source devices not decoded by the destination device D, and a set Bopt of relay nodes adapted to this set S*' having a cardinal | BP pt | greater than or equal to the cardinal | SR pt | of set S*'. In the embodiment described here, the selection module 3 is configured to select sets S*' and B opt which 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 sets S*' and B opt which allow the performance criterion in question to reach a determined threshold value (for example, such that the performance criterion is greater than the threshold value in question); - a sending module 4, configured to send retransmission instructions to the relay nodes of set B optso 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 set SPP* ; and - a receiving module 5, configured to receive redundant versions transmitted by the relay nodes, these redundant versions being intended to be used by the destination device D to decode the messages received from the undecoded source devices of the SP set pt .

[0064] The functions of modules 3 to 5 are described further later with reference to the steps of the acceptance process according to the invention and to Figure 5.

[0065] As mentioned previously, in the embodiment described here, the source devices Si, S2, ..., SM and the intermediate devices Riv i, RM+2, ..., Rwi+i 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.

[0066] 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 the source and intermediate devices) that 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 Figure 4B: - an information module 6, configured to inform the destination device D of its awareness, if applicable, of messages emitted by several of the source devices Si SM. Knowledge of an information message associated with a source device, as mentioned previously, may come from the fact that the relay node is said source device in question, or that it correctly decoded the message received from the source device during the cpl transmission phase or at the end of a previous time interval of the cp2 retransmission phase; - a receiving module 7 configured to receive, if necessary from the destination device D, a retransmission instruction, during a given time interval of the retransmission phase cp2, of second redundancy versions resulting from the encoding of the information messages associated with a set SP pt of source devices known to the relay node but not decoded by the destination device D; and - a transmission module 8 configured to execute this retransmission instruction during the given time interval.

[0067] 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 Figure 5 described now.

[0068] Figure 5 represents the main stages 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 cp2.

[0069] 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, &, ..., SM are assumed to be statistically independent (no correlation between them).

[0070] As mentioned previously, during the cpO initialization phase that precedes the Data frame transmission by source devices Sm, ..., SM: the destination device D determines, for each source device Sm, m = 1, ..., Jv, the modulation and coding scheme (MCS) that it must use to transmit data (step E10). The way in which the destination device D accomplishes 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 = 1, ..., M, via a limited-rate control channel, information unambiguously identifying the MCS scheme assigned to it (step E20). This information is, for example, in the context of a 5G network, an MCS scheme index pointing in a specific table to a MOD modulation, a coding efficiency R mto obtain at the output of a determined COD coding scheme (for example an LDPC code), and a spectral efficiency value to use to transmit its useful data.

[0071] No limitations are attached to the modulations or coding types that can be envisaged in the context of the invention. For example, one can consider using phase shift keying (PSK) or quadrature amplitude modulation (QAM) of various orders, LDPC (Low Rarity Check Code) type coding schemes, turbo codes or convolutional codes, systematic or non-systematic, or any other modulation and / or coding scheme.

[0072] Following this cpO initialization phase, the cp1 transmission phase starts.

[0073] Each source device Sm, m= 1 , ...,M, attaches to the useful data (information message in the sense 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.

[0074] When using an I-type R-HARQ protocol based on incremental coding, the information message including the CRC is encoded by the source device S with a very low-efficiency master code (e.g., 1 / 3 or 1 / 5), typically lower than the efficiency of the MCSm scheme specified during the initialization phase. The number of bits K m The information message from the source device Sm depends on the coding efficiency and MOD modulation of the MCSm scheme indicated during the cpO initialization phase. More specifically, K m = R m . Nl. q m where q mdenotes the number of bits per symbol carried by the modulation. The coded 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 Figure 6. In the example shown in Figure 6, a systematic master code with a 1 / 3 efficiency is considered. As is 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.

[0075] The coded bits stored in the circular buffer are organized into a number of so-called redundancy versions, denoted RVOm, RV1 metc., starting at predetermined positions POSOm, POS1m, ..., on the buffer, and whose dimensions are fixed according to the number of resource elements available for each (re)transmission. Thus, the first version of RV0 redundancy m is of dimension N1,q m , then the subsequent redundancy versions are of dimension N2.q m . The first version of RVOm redundancy is also chosen so as to be self-decodable. For example, it includes systematic bits as illustrated in Figure 6. Note the configuration of the POSOm, POS1 positions. m POS2 m The POS3m configuration illustrated in Figure 6 is that proposed by the 3GPP standard and is only an illustrative example, not a limiting one. Other configurations can be considered.

[0076] During the cpl transmission phase, the first redundancy version RVOm is transmitted by the source device Sm, after modulating the coded bits contained in this redundancy version RVOm with the MOD modulation corresponding to the MCSm scheme determined by the destination device D (step E30). The other redundancy versions are transmitted subsequently if necessary during the cp2 retransmission phase, as described later.

[0077] The source devices Si, Sa, ..., SM transmit their respective first redundancy versions RV0i, RVO2, ..., RVO to the destination device D, in turn during M time intervals of the cpl transmission phase. It is noted that when a source device Sm transmits its first redundancy version RVOm during a time interval, the other devices of the communication system 1 (i.e. the destination device, the other source devices 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).

[0078] At the end of the power line communication (PLC) transmission phase (or each time interval of the PLC transmission phase), 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, "correctly decoding a message received from a device" is sometimes referred to as "decoding the device" in question.

[0079] The destination device D identifies the source devices whose messages it was able to correctly decode (referred to here as "decoded devices") using the CRCs attached to the information messages of the source devices, in a way known per se. We denote S D 0 the so-called decoding assembly of the destination device D comprising the source devices correctly decoded by the destination device D at the end of the transmission phase, and by S D 0 the complementary set of the decoding set S D0 which includes the source devices that the destination device D was not able to decode correctly.

[0080] The other devices j, i, J\T of communication system 1 proceed identically from the messages they received during the transmission phase (step E50). S is denoted j 0 , I J\T the decoding set of a device j of the communication system 1 , and by S j 0 , i J\T, the set of source devices that were not correctly decoded by device j. Note that if device j is a source device of communication system 1, the decoding set S j 0 includes at a minimum device j.

[0081] SS D 0If the signal is not empty (i.e., there is at least one source device not decoded by the destination device D at the end of the powerline transmission phase), the destination device D informs the other devices, i.e., TV, of the communication system 1, for example by sending a non-acknowledgement (NACK) message in a control or feedback channel (step E60). Otherwise, the destination device D sends an acknowledgment (ACK) message in the control or feedback channel, and a new powerline transmission phase (possibly preceded by a new cpO initialization phase if propagation conditions require it) can be implemented. by the source devices Si, &, SM.

[0082] Note that acknowledgment (ACK) and non-acknowledgment (NACK) messages are not necessarily explicit. For example, the destination device may send the set of source devices S to the devices in communication system 1. D 0 that it was unable to decode during the transmission phase, and if this set is not empty, this is interpreted by the devices of communication system 1 as a non-acknowledgment message. Conversely, if the set S D 0 The transmitted message is empty; the devices of communication system 1 interpret it as an acknowledgment message.

[0083] The sending of the non-acknowledgment message by the destination device D triggers the retransmission phase cp2. The cp2 retransmission phase is designed to allow the destination device D to correctly decode the information messages from the source devices in its set S. D 0and lasts for a maximum number Tmax of time intervals (in other words, a maximum number Tmax of retransmissions is allowed). Here, we assume that the set S D 0 includes at least two source devices not disconnected by the destination device D.

[0084] The steps described below are repeated for at least T US ed>l time interval(s) of the retransmission phase cp2, 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, at the end of the time interval t of the retransmission phase, a set S D t empty. Of course, other stopping criteria can be considered.

[0085] In the following, each time interval of the retransmission phase cp2 is indexed by t, and we denote by S D t and 5) tthe respective decoding sets of the destination device D and each device j of the communication system 1, with j → J\T, at the end of the time interval t of the retransmission phase cp2, and and their complementary sets (i.e. including the source devices not correctly decoded).

[0086] During a time interval t of the retransmission phase cp2, l <t<T US ed, a device j of the communication system 1 can help the destination device D to decode a source device Sm of the set and therefore serve as a relay node for this source device if it knows the information message associated with the source device Sm, either because this device is itself the source device Sm or because it correctly decoded its information message during a time interval prior to the retransmission phase cp2 or at the end of the transmission phase. As mentioned previously, if device j is a source device of the communication system 1, the decoding set 5) t includes at a minimum device j.

[0087] 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 ^D,ti (E80).

[0088] To this end, different approaches can be considered.

[0089] For example, upon receipt of the non-acknowledgement message NACK from the destination device D, the devices j, i, JV, 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 channel or feedback), of their respective decoding sets I'm JV.

[0090] According to another example, the destination device D informs the devices ] , i J\T of the communication system 1 of its decoding set S D or its set of undecoded source devices S D t-, and the devices ] , I JV, 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 information messages not decoded by the destination device D by identifying the known message(s).

[0091] According to the invention, when the assembly S D t-r comprising at least two source devices, the destination device D can select a set B*' (index t omitted for simplicity) of several relay nodes from among the devices j, i JV, of the communication system 1 to simultaneously retransmit during the transmission interval t the redundancy versions associated with the same set SPP' (index t also omitted) comprising at least two source devices from the set t _!. Set Bopt groups together devices of the communication system 1 which all know, within the meaning of the invention, the information messages associated with each of the source devices of the set S 31 His cardinal | B op, | is at least greater than the cardinal | SP p, | of the S set 313 *.

[0092] The sets S*' and B opt are selected by the destination device D, via its selection module 3, at the end of each time interval t-1 in order to optimize a specific performance criterion (step E90). For example, the destination device D selects sets that optimize mutual information. The method of this selection is described in more detail later, with reference to Annexes 1 and 2. The relay nodes of set B are hereafter denoted by bi, ..., b|Bopti. opt and by S mi , ... , S mvthe undecoded source devices of set S 3pt .

[0093] 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, subject to change from one time interval to another during the retransmission phase.

[0094] Once the SP sets pt and B opt selected, the destination device D determines the complex precoding matrix denoted V = [vt v2... v n including (online) the precoding vectors to be applied at each relay node of the entire B° system pt on the retransmitted messages from the source devices of the SP set pt The matrix V has dimensions |B op, | xo where o=|S pp, | denotes the number of source devices in the set S*' (step E100).

[0095] 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. As is known, the precoding matrix V consists of the best orthogonal eigenvectors of the HR matrix. P t|x|Bo P t| (ranked in descending order of their corresponding eigenvalues) where H denotes the matrix of the equivalent propagation channel between the relay nodes of set B optand 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 estimate of the matrix H is assumed here to be known by the destination device D, using techniques known to those skilled in the art. The eigenvector v t is intended to be applied to precode the redundancy version associated with the source device S m; of set S 31 *.

[0096] Of course, other criteria can be considered.

[0097] The destination device D then sends, via its sending module 4 and the control or feedback channel, retransmission instructions to the relay nodes bi, ..., b|Bo P t| of set B optfor 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 mi , ... , S mv of set S*' (step E1 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 set S ppt , the precoding matrix V (which includes the precoding vectors to be applied by the relay nodes) and a vector b of dimension | B op,| 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 v J t to the message associated with the l-th source device designated in the set S*'.

[0098] Alternatively, the destination device D can be considered to send each selected relay node its specific retransmission instruction. In this variant, the retransmission instruction comprises the set S 31", the precoding vector to be applied by the relay node in question, as well as an indication of the redundancy versions to be transmitted for the source devices of the SP set pt .

[0099] The destination device D can also provide, in the retransmission instructions, an indication of the distribution between the source devices S mi , ... , S mv of the SP set pt of the power transmitted by each relay node bi ,..,b| Bo ti , such that this distribution was taken into account when determining the precoding matrix V, and that the relay nodes bi ,..,b| Bo P t| of set B opt must apply during retransmission.

[0100] For example, if the precoding matrix V is determined on the basis of a power transmitted by each relay node of set B opt equally distributed across the source devices Smi , ... , S mv of set S 3pt The destination device D can send, in the retransmission instructions, an indication of the PO power to be applied to each source device S mi , ... , S mv of set S ppt .

[0101] Alternatively, the value of PO can be calculated by the relay nodes of set B opt 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: where P denotes the transmission power of a relay node bj.

[0102] S the assumption of an equal distribution of power between the source devices S mi , ... , S mvis not applied, then in this case the destination device D transmits, in the retransmission instructions, the vector p = [Pi, ...,P„] of powers to be applied to each source device S mi , ... , S mv of the SP set pt .

[0103] Upon receiving the retransmission instructions from the receiving device D, each relay node bi ,..,b|Bopt| of the set B opt simultaneously retransmits to the destination device D, during the time interval t, redundant versions of the information messages associated with the source devices of set S*', to which are applied the coefficients of the vector of the precoding matrix associated with it (step E120). Thus, during step E120, the relay nodes bi, ..., b|Bopt| of set B opt execute, via their respective retransmission modules 8, the retransmission instructions that have been transmitted to them.

[0104] More specifically, during the time interval t, all relay nodes bi, ..., b|Bopti of set B opt 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 from the source devices S mi , ... , S mvof the set S*' 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 up-to-date the redundancy version to be retransmitted for a source device 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. This is denoted RVt mi the redundancy version transmitted at time interval t for the source device S m; with, in the particular embodiment described here based on Figure 6 and 4 distinct redundancy versions, t mi = t m ' l mod 4 where t m 'l denotes the number of retransmissions of the information message associated with the source device S m; (including that of the time interval t) in the form of redundancy versions.

[0105] Each relay node bj, j= 1 , ..., | B op, | , thus simultaneously transmits, during the time interval t, the linear combination of the o redundancy versions RVt m RVtm - respectively associated with the o source devices S mi , ... , S mv of the set S^' weighted by the coefficients of the precoding matrix V, i.e.: where Vj i, 1= 1 , ..., o, denote coefficients of the precoding matrix V (and more precisely of the precoding vector allocated to the relay node bj), and denotes the k-th (fc e {0, ... ,N2- 1}) coded modulated symbol of the t m; -first redundancy version resulting from the encoding of the information message associated with the source device Sm; with = P t The RVt redundancy version mi transmitted at interval t is ob- held by the relay node from a circular buffer such as the one shown in Figure 6, generated from its knowledge of the information message associated with the source device S m; The redundancy version 7?yt m; transmitted is obtained by reading the coded bits from the POSc position into the buffer. 1; Thus, for the time interval indexed by t=1, the redundancy version RVl m; is read from the POSl position m; in the circular buffers corresponding to each information message associated with the source device s mi , for 1= 1 ,..., ü. Subsequently, the redundancy versions are transmitted, in the embodiment described here, in the following order: RV2 mi RV3 mi RV0 mi RVl mi RV2 mi RV3 mi, etc.

[0106] During the time interval t and step E120, all relay nodes bi, ..., b| Bopti of set B opt therefore simultaneously transmit a linear combination of the same redundancy versions associated with the source devices S mi , ... , S mv of the SPP* set, the linear combination differs from one relay node to another depending on the precoding vectors applied. During this time interval t, all relay nodes therefore use the same number of resource elements (the same number N2 of channel uses).

[0107] 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 set B optThe destination device D attempts to decode the messages received during the powerline transmission phase from the source devices of the SP set. pt using the new redundancy versions transmitted by the re-layage nodes (step E130). The way in which the destination device D does this is known in itself and falls under techniques classically used within the framework of an I R-HARQ protocol, not described here.

[0108] 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 S D t and the complementary set S D t at the end of the time interval t.

[0109] S the decoding set S D tis empty (all source devices & , ...,SM are correctly decoded), the destination device D sends an acknowledgment message ACK in the control or feedback channel, and the retransmission phase cp2 is completed (step E140). A new transmission phase cpl (possibly preceded by a new initialization phase cpO if propagation conditions require it) can be implemented for new information messages from the source devices SI , ...,SM.

[0110] SS D t is not empty (that is, there still exists at least one source device not decoded by the destination device D at the end of the time interval t), the device [Destination D informs the other devices], I J\T, of communication system 1, by sending a non-acknowledgement (NACK) message 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.

[0111] We will now describe how the selection of sets B opt and S*' is performed by the selection module 3 of the destination device D during the selection step E90, for each transmission interval t, in a particular embodiment. As mentioned previously, the assemblies B opt and S 31 * may differ from one time interval to another; however, for the sake of simplification, the index t is omitted when these sets are designated.

[0112] In the embodiment described here, to select the sets B opt and S ppt used for a time interval t, the destination device D calculates the set of parts of the set of source devices not decoded by the destination device D at the interval t-1, S D t-1 by restricting ourselves to subsets with 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 part of a set A, denoted Part(A), we mean the set of all possible non-empty subsets made up of elements of A. The resulting set is denoted Part(S). D t-1 '). The subsets of Part S D t-1 ') of cardinality Q e {1, ... , Qmax are denoted PartQ^Sp f^) and are such that Part(S D t-1 ') = \J^^ NR,Qmax ^ Part Q (S D t-1 ).

[0113] Then, for each subset of source devices S Part Q (S D t _ , )' , Q {1, .... Qmax], the destination device D determines the set B of devices] e JV, of the communication system 1 that have decoded at least the source devices of S with the constraint |S| < |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. No subset is eligible for a given value Q o Therefore, the destination device D does not test the values ​​of Q > Q o .

[0114] 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 redundant versions of the information messages associated with all source devices of S. The calculation of the mutual information is explained in more detail later.

[0115] The destination device D selects SP as the set pt , the subset S and the associated set B that maximizes this mutual information.

[0116] For example, = {1,4,5} and Qmax= NR=3, the selection module 3 obtains: = {{1}, {4}, {5}, {1,4}, {1,5}, {4, 5}, {1,4, 5} For each subset S e Part(S D It determines the set of relay nodes ® that have decoded the source devices contained in S, i.e., I J\T such that S Su-!. 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 such that |S| <Qmax= NR, par exemple pour NR=2 : Part S D ^1) = {{1}, {4}, {5}, {1,4}, {1,5}, {4,5}}

[0117] 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 |S| > |B|.

[0118] Annex 1 presents an example of pseudocode that can be used by the selection module 3 during the selection step E90.

[0119] The time complexity of such a selection algorithm can be measured in the number of iterations required to obtain the sets B opt and S 31 * The inventors demonstrated that in the worst-case scenario, the number of iterations required is:

[0120] In other words, if we consider, for illustrative purposes, different values ​​of NR (which is the maximum value that Qmax can take) and of |R D ,ti| : o NR = 2: 3 iterations. o NR = 4: 3 iterations. - for |S D;t -i | = 3 and: o NR = 2: 6 iterations. o NR = 4: 7 iterations. - for |S D;t -i | = 7 and: o NR = 2: 28 iterations. o NR = 4: 98 iterations. o NR = 8: 127 iterations.

[0121] It is clear that temporal complexity increases with the number of transmitted spatial layers, that is, the number of source devices not decoded in S 3ptOne way to control this increase is to set a maximum number Qmax < NR of source devices for which redundant versions are retransmitted simultaneously. For example, for |S D 1 = 7, NR = 8 and Qmax = 2, 28 iterations are executed against 127 iterations in the case where Qmax = NR.

[0122] We will now describe how 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.

[0123] It has been described previously how the precoding matrix V to be applied by the set B is obtained. It can easily be demonstrated that the signal-to-noise ratio (SNR) of the lth channel seen by the redundancy version associated with a source device S mi of the set S is given by where A ( denotes the eigenvalue associated with the l ème best eigenvector of the equivalent channel matrix HR'H between the relay nodes of set B and the receiving antennas of the destination device D, as introduced previously. Thus, the equivalent mutual information I s contributed by the parallel channels is written as follows: where I denotes mutual information and o 2 the variance of the additive Gaussian noise (known from the destination device D). Assuming that the parallel channels have With Gaussian inputs, we obtain:

[0124] 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.

[0125] Different hypotheses can be considered by the destination device D concerning the power transmitted on each spatial layer (Le. allocated to the transmission of a redundancy version associated with each source device of the sub-assembly S).

[0126] In one variant, the destination device D can allocate this power Pi to the source device S mi so as to maximize the equivalent mutual information I s This amounts to maximizing the equivalent mutual information I s , under the following constraints: where Vj 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.

[0127] According to a second variant, the destination device D can consider that the power is equally distributed over all spatial layers, i.e.:

[0128] 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.

[0129] Annex 2 presents an example of pseudocode that can be used by the selection module 3 of the destination device D to evaluate and maximize the equivalent mutual information I s during the E90 selection step, when an assumption of equal power distribution across all source devices is considered.

Claims

1. Claims

1. A method for receiving messages 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 the encoding 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, referred to as undecoded, and for at least a time interval of a retransmission phase: o a step (E90) of selecting, according to a determined performance criterion, a set SP pt comprising at least two undecoded source devices and a set B opt of relay nodes with a cardinality greater than or equal to the cardinality of the SP assembly pt, a said relay node being a source device or an intermediate device knowing each of the information messages associated with the source devices of set S*'; o a step (Et 10) of sending retransmission instructions to the relay nodes of set B opt 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 set S*'; o a step (E120) of receiving the second versions of redundancy transmitted by the relaying nodes.

2. A receiving method according to claim 1, wherein said retransmission instructions comprise precoding vectors to be applied by the relay nodes of assembly B opt to the second redundancy versions resulting from the encoding of information messages associated with the undecoded source devices of set S31 *.

3. A receiving method according to claim 2 wherein said retransmission instructions comprise the selected set S*', 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 set S*'.

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 assembly S pptso as to maximize mutual information associated with simultaneous transmission through the relay nodes of set B opt selected from said second versions of redundancy.

6. A receiving method according to claim 4 or 5, wherein said retransmission instructions further comprise an indication of a distribution of the power transmitted by each relaying node between the undecoded source devices of the set S*'.

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 relaying nodes of assembly B opt selected from said second versions of redundancy.

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 receiving are repeated for at least one other time interval of the retransmission phase until a predefined stopping criterion is verified, 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 successively 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: o a step of informing the destination device of knowledge by the relay node of messages emitted by several of the source devices;o a step (E1 10) of receiving 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 the relaying node but not decoded by the destination device; and o a step (E120) of executing said retransmission instruction during said time interval.;

11. 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, said undecoded, and for at least one time interval of a retransmission phase, said modules comprising: o a selection module (3) configured to select, according to a determined performance criterion, a set S*' comprising at least two undecoded source devices, and a set B opt of relay nodes with a cardinality greater than or equal to the cardinality of the set S 3pt, a said relay node being a source device or an intermediate device knowing each of the information messages associated with the source devices of set S*'; o a sending module (4), configured to send retransmission instructions to the relay nodes of set B opt 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 SP set pt ; and o a receiving module (5), configured to receive second redundancy versions transmitted by the relay nodes.

13. Relay node in a communication system comprising a multi-antenna destination device (D) and M>2 source devices (S1, ..., SM) having successively 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 relay node comprising: o an information module (6), configured to inform the destination device of its knowledge of messages emitted by several of said source devices;o 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 S*' of source devices known to said relaying node but not decoded by the destination device; and o a retransmission module (8) configured to execute said retransmission instruction during said time interval.;

14. Communication system (1) comprising a destination device (D) according to claim 12, a plurality of source devices (SI, .., SM) and a plurality of relaying nodes according to claim 13, a said relaying node being able to be a said source device (SI ,..,SM) OR an intermediate device (RM+I ,.. ,RM+L) distinct from said source devices.

15. Communication system (1) according to claim 14 using an orthogonal multiple access multiple-relay channel scheme of the OMAMRC type, between source devices, relay nodes and destination device.

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