Transmission method and omamrc system with a selection strategy during retransmissions taking into account the throughput of the sources and of a single control exchange

Abstract

The present invention relates to a method for transmitting a frame carrying messages for an OMAMRC telecommunication system having N nodes, including M sources si i ∈ {1,..., M} and a destination, where N ≥ M ≥ 2. The transmission comprises a maximum number of M + Tmax time intervals per transmitted frame, distributed between a first phase and a second phase. The selection of the sources to be assisted during the second phase takes into account the estimated numbers of retransmission intervals sufficient for the destination to decode the sources not yet correctly decoded and a sum of throughputs assigned to the sources.

Description

[0001] DESCRIPTION

[0002] TITLE: OMAMRC transmission method and system with a selection strategy during retransmissions taking into account the flow rate of the sources and a single control exchange Field of the invention

[0003] The present invention relates to the field of digital communications. Within this field, the invention relates more particularly to the transmission of coded data between at least two sources and a destination with relaying by at least one node which may be a relay or a source.

[0004] It is understood that a relay does not have a message to transmit. A relay is a node dedicated to relaying messages from sources while a source has its own message to transmit and can also in certain cases relay messages from other sources i.e. the source is said to be cooperative in this case.

[0005] There are many relaying techniques known by their Anglo-Saxon names: "amplify and forward", "decode and forward", "compress-and-forward", "non-orthogonal amplify and forward", "dynamic decode and forward", etc.

[0006] The invention applies in particular, but not exclusively, to the transmission of data via mobile networks, for example for real-time applications, or via, for example, sensor networks.

[0007] Such a sensor network is a multi-user network, consisting of several sources, several relays and a recipient using an orthogonal multiple access scheme in time of the transmission channel between the relays and the sources, noted OMAMRC ("Orthogonal Multiple-Access Multiple-Relay Channel" according to English terminology).

[0008] Prior art

[0009] The considered OMAMRC telecommunication system illustrated in Figure 1 has N nodes and one destination with an implementation of a time orthogonal multiple access scheme of the transmission channel that applies between the N nodes. The N nodes include M sources and L relays. The maximum number of time slots per transmitted frame is M + T max with M intervals allocated during a first phase to the successive transmission of the M sources and T U sed T max intervals for one or more cooperative transmissions allocated during a second phase to one or more nodes selected by the destination according to a selection strategy.

[0010] Such an OMAMRC transmission system implementing a selection strategy in the second phase is known from the article by S. Cerovic, R. Visoz, and L. Madier entitled "Efficient Cooperative HARQ for Multi-Source Multi-Relay Wireless Networks.", 14th International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob). IEEE, 2018. The OMAMRC transmission system described is such that each of the sources can operate at different times either exclusively as a source or as a relay node. The node terminology covers both a relay and a source acting as a relay node or as a source. A relay is distinguished from a source because it has no message to transmit of its own, i.e. it only retransmits messages from other nodes.

[0011] The links between the different nodes of the system are subject to slow fading and Gaussian white noise. Knowledge of all the links in the system (CSI: Channel State Information) by the destination is not available. Indeed, the links between sources, between relays, between relays and sources are not directly observable by the destination and their knowledge by the destination would require too much information exchange between the sources, the relays and the destination. To limit the cost of the feedback overhead, represented by dotted lines in Figure 1, only information on the channel distribution / statistics (CDI: Channel Distribution Information) of all the links, e.g. average quality (e.g. average SNR, average SINR) of all the links, is assumed to be known by the destination in order to determine the rates allocated to the sources.

[0012] Link adaptation is said to be slow type, that is to say that before any transmission, the destination allocates initial rates to the sources knowing the distribution of all the channels (CDI: Channel Distribution Information). In general, it is possible to go back to the CDI distribution on the basis of knowledge of the average SNR or SINR of each link in the system.

[0013] Message transmissions from sources are formatted into frames during which the CSI of the links are assumed to be constant (slow fading assumption). The rate allocation is assumed not to change for several hundred frames, it changes only with CDI changes.

[0014] The process distinguishes three phases: an initial phase and, for each frame to be transmitted, a 1st phase and a 2nd phase. The transmission of a frame takes place in two phases which are possibly preceded by an additional phase called the initial phase.

[0015] During the initialization phase, the destination determines an initial rate Ri for each source S[ by taking into account the average quality (for example SNR) of each of the links in the system. The destination estimates the quality (for example SNR) of the direct links: source to destination and relay to destination according to known techniques based on the exploitation of reference signals. The quality of the source-source, relay-relay and source-relay links is estimated by the sources and the relays by exploiting for example the reference signals. The sources and the relays transmit to the destination the average qualities of the links. This transmission occurs before the initialization phase. Since only the average value of the quality of a link is taken into account, its refresh occurs on a long time scale, i.e. over a time which makes it possible to average the rapid variations (fast fading) of the channel.This time is of the order of the time required to travel several tens of wavelengths of the transmitted signal frequency for a given speed of a node in the system. The initialization phase occurs, for example, every 200 to 1000 frames. The destination sends the initial rates it has determined back to the sources via a return path. The initial rates remain constant between two occurrences of the initialization phase.

[0016] In the first phase, the M sources successively transmit their message during the M time slots using modulation and coding schemes determined from the initial bit rates. During this phase, the number of channel uses (channel use, i.e. resource element according to 3GPP terminology) is fixed and identical for each of the sources.

[0017] During the first phase, independent sources broadcast their coded information sequences in the form of messages to a single recipient. Each source broadcasts its messages at its initial rate. The destination communicates its initial rate to each source via very limited rate control channels. Thus, during the first phase, the sources each transmit their respective messages in turn during time slots, each dedicated to a source.

[0018] Sources other than the one transmitting and possibly relays, of the “Half Duplex” type, receive successive messages from the sources and decode them.

[0019] In the second phase, the destination selects for the current interval t a single node taken from the sources and the relays to cooperate. This node randomly selects the source it helps from among the one it has correctly decoded and that the destination has not yet correctly decoded by transmitting a redundancy of the message from this source.

[0020] This phase lasts a maximum of T max time slots. During this phase, the number N2 of channel uses is fixed and identical for each of the selected nodes (sources and relays).

[0021] This article teaches control signals which consist, for the destination to broadcast M bits which indicate its set of correctly decoded sources at the interval t — 1, for the nodes which have correctly decoded a source that the destination has not yet correctly decoded to transmit a signal on a dedicated unicast channel and for the others to remain silent and finally for the destination to broadcast the result of its selection according to the retained selection strategy. Although these exchanges limit the signaling overhead while allowing a maximization of the average spectral efficiency (utility metric) within the considered system under the constraint of respecting an individual quality of service (QoS) per source, it may be desirable to limit the signaling overhead even more.

[0022] The present invention meets this objective.

[0023] Main features of the invention

[0024] The present invention relates to a method for transmitting a frame carrying messages intended for an OMAMRC telecommunications system with M sources, possibly L relay and a destination, 0, nodes operating in half-duplex mode, according to an orthogonal multiple access scheme of the transmission channel between the N nodes with a maximum number of M + T max time intervals per transmitted frame distributed between a l ere phase and a 2 nde phase, 1 ≤ T max , the message from a source having been coded before transmission using an incremental redundancy type coding which generates several redundancies, the 1 ere phase includes M intervals allocated respectively to successive transmissions of the M sources and the 2 nde phase includes at least one retransmission interval for a transmission from nodes having correctly decoded the same source s isuch that these nodes simultaneously transmit during the same retransmission interval the same redundancy of the message from the same source not yet correctly decoded by the destination, called the source to be helped. The method according to the invention is such that it comprises: a single transmission by the nodes of at least their set of sources correctly decoded and not yet correctly decoded by the destination, these transmissions allowing the destination to determine, for each of these sources, a quality of an equivalent channel based on a quality of the channels between the nodes having correctly decoded this source and the destination, an estimation of a sufficient number of retransmission intervals (x i (0)) for the destination to decode a source (Si) not yet correctly decoded and correctly decoded by at least one node on the basis of the quality of an equivalent channel for this source between this at least one node and the destination and a rate (R i) attributed to this source (S i ), a selection by the destination of the sources to help taking into account the numbers (x i (0)) estimates of retransmission intervals sufficient for the destination to decode the sources not yet correctly decoded and a sum of rates (R i ) attributed to the sources.

[0025] Thus, the number of control exchanges between the source and the nodes is reduced to a single exchange during which the nodes communicate to the destination their set of correctly decoded sources. If the destination has previously communicated its own set of correctly decoded sources, the transmission by the nodes may contain only their set of correctly decoded sources minus those already correctly decoded by the destination. The transmission of the nodes allows the destination to evaluate the quality of the node-destination channels to estimate per source a sufficient number of retransmission intervals for the destination to correctly decode this source. Knowing these sufficient numbers of intervals, the destination can then successively select the sources to be helped either randomly or in an ordered manner among those whose sufficient number of intervals is less than the time remaining before reaching T max. The scheduling can be done by successively selecting the sources according to the increasing number of sufficient intervals. The invention has the advantage of not wasting transmission if no source can be helped in the remaining time. The invention further relates to a system comprising M sources ... , s M , possibly L relays and a destination d, M ≥ 2, L ≥ 0, for an implementation of a transmission method according to the invention.

[0026] According to one embodiment of the invention, the single transmission by the nodes of at least their set of correctly decoded sources and not yet correctly decoded by the destination is carried out at the start of the 2nd phase.

[0027] According to one embodiment of the invention, the method further comprises a comparison between a sum of estimated numbers of retransmission intervals sufficient to help the destination to decode sources not yet correctly decoded and a number of time intervals remaining during the 2nd phase to help the destination to correctly decode one or more sources.

[0028] In this mode, the method adds at least two estimated numbers of sufficient retransmission intervals and compares the result with the remaining time. If the result of the addition is less than the remaining time, all sources involved in the addition can be helped during the 2nd phase.

[0029] According to one embodiment of the invention, the comparison is updated after the correct decoding of a source by the destination.

[0030] Since correct decoding by the destination of a source may occur before the end of the estimated number of sufficient retransmission intervals (cooperative transmissions), this mode allows another source to benefit from the unused time.

[0031] According to one embodiment of the invention, the single transmission by the nodes of at least their set of correctly decoded sources not yet correctly decoded by the destination is part of a control exchange during which the source transmits its set of correctly decoded sources. According to one embodiment of the invention, the method is such that during the exchange, a node sends only its set of correctly decoded sources not yet correctly decoded by the destination.

[0032] According to one embodiment of the invention, the method is such that during the exchange, a node sends its set of correctly decoded sources.

[0033] According to one embodiment of the invention, the method further comprises a comparison between the estimated numbers of sufficient retransmission intervals so that the selection takes into account an ordering of these estimated numbers of sufficient retransmission intervals. According to this mode, the estimated numbers of sufficient retransmission intervals are classified according to their value. And, preferably, the method first selects the source for which the estimated number of sufficient retransmission intervals is the smallest. The same source is helped for the duration corresponding to this estimated number or for a shorter duration if its correct decoding by the destination occurs before the end of the estimated number. The method thus successively considers the sources remaining to be correctly decoded.The process is stopped when all sources are correctly decoded by the destination, when no source that has not yet been correctly decoded can be helped, or when the maximum time is reached.

[0034] According to one embodiment of the invention, the method further comprises a determination of a set of sources that can be helped taking into account the estimated numbers of sufficient retransmission intervals and a time remaining before the end of the 2nd phase and such that the selection of the sources to be helped is done randomly from among the set of sources that can be helped.

[0035] According to one embodiment of the invention, the method is such that if no source can be helped in the time remaining before the end of the 2nd phase then the transmission of the frame is interrupted before the use of the maximum number of sufficient retransmission intervals. The invention further relates to each of the specific software applications on one or more information media, said applications comprising program instructions adapted to the implementation of the transmission method when these applications are executed by processors.

[0036] The invention further relates to configured memories comprising instruction codes corresponding respectively to each of the specific applications.

[0037] The memory can be incorporated into any entity or device capable of storing the program. The memory can be of the ROM type, for example a CD ROM or a microelectronic circuit ROM, or of the magnetic type, for example a USB key or a hard disk. Furthermore, each specific application according to the invention can be downloaded from a server accessible on an Internet-type network.

[0038] The optional features presented above in the transmission method may optionally be applied to the software application and memory mentioned above.

[0039] List of figures

[0040] Other characteristics and advantages of the invention will appear more clearly on reading the following description of embodiments, given as simple illustrative and non-limiting examples, and the appended drawings, among which:

[0041] [Fig 1] Figure 1 is a diagram of an example of a so-called Cooperative OMAMRC (Orthogonal Multiple Access Multiple Relays Channel) system described with regard to the prior art, [Fig 2] Figure 2 is a diagram of a transmission cycle of a frame according to an example of implementation of the invention,

[0042] [Fig 3] Figure 3 is a diagram of the protocol for decoding control exchanges between the destination and the nodes, sources and relays, according to one embodiment of the invention.

[0043] Description of particular embodiments

[0044] A channel use is the smallest granularity in time-frequency resource defined by the system that allows the transmission of a modulated symbol. The number of channel uses is related to the available frequency band and the transmission duration. An OMAMRC system is illustrated in Figure 1 already described.

[0045] An OMAMRC system according to the invention comprises M sources which belong to the set of sources, possibly L relays which belong to the set of relays and a destination d. By convention, it is considered that and in other words, we can confuse a source and its index, and a relay and its index (shifted by the value M of the number of sources). Each source in set 5 communicates with the single destination with the help of other sources (user cooperation) and cooperating relays.

[0046] Transmission cycle of a frame according to the invention

[0047] A transmission cycle of a frame according to an exemplary implementation of the invention is illustrated in Figure 2.

[0048] The method according to the invention distinguishes two phases for each frame to be transmitted, one ere phase and a 2 ndephase. The transmission of a frame is possibly preceded by an additional phase called initial during which the flow rates are allocated.

[0049] The M sources access the transmission channel according to a time orthogonal multiple access scheme during the 1 ere phase. During the 2 nde phase, access to the transmission channel of the N nodes which include the M sources and possibly the L relays is considered orthogonal because at each retransmission interval the active nodes transmit in parallel the same redundancy of the same message from the same source i.

[0050] The N nodes operate in a half-duplex mode that allows them to listen to the transmissions of other nodes without interference. The sources can act as a relay when they are not transmitting only their own message.

[0051] The CSI of the links is assumed to be constant (slow fading assumption) during the transmission of a frame. From time to time, the destination allocates rates to sources knowing the distribution of all channels (CDI: Channel Distribution Information). The rate allocation is assumed not to change for several hundred frames, it only changes with CDI changes.

[0052] Each of the allocated bit rates unambiguously determines a modulation and coding scheme (MCS) and conversely each MCS determines a bit rate. The allocated bit rates are transmitted from the destination to the sources via very limited bit rate control channels (shown in dotted lines in Figure 1).

[0053] For the sake of simplification of the description, the following assumptions are subsequently made about the OMAMRC system:

[0054] - the sources, relays and destination are equipped with a single transmit antenna (or transmit antenna port);

[0055] - sources, relays and destination are equipped with a single receiving antenna (or receiving antenna port);

[0056] - sources, relays and destination are perfectly synchronized;

[0057] - the sources are statistically independent (there is no correlation between them);

[0058] - all nodes transmit with the same power;

[0059] - use is made of a CRC code supposedly included in the K i information bits from each source i to determine whether the message associated with the information bits is correctly decoded or not,

[0060] - links between different nodes suffer from additive noise and fading. Fading gains are fixed during the transmission of a frame carried out for a maximum duration of M + T max time intervals, but can change independently from frame to frame. T max ≥ 1 is a system parameter;

[0061] - the instantaneous quality of a direct channel / link in reception (CSIR Channel State Information at Receiver) is available at the destination, sources and relays;

[0062] - returns are error-free (no errors on control signals / channels).

[0063] The following notations are used: • is a discrete variable representing the source throughput ^^ provided by a link adaptation process implemented before the transmission of frames, • is the number of retransmission intervals used during the 2 nde phase, it corresponds to the number of transmissions during this phase, • 1 is the ratio of the number of channel uses available at each time interval (slots) of the 2 nde phase and the number of channel uses available at each time interval (slots) of the 1 ère phase, • is the set of sources not correctly decoded by the destination at the end of the retransmission interval • is the set of sources correctly decoded by the node at the end of the retransmission interval • is the outage indicator which takes the value one when an individual outage event occurs and the value zero in other cases. represents the outage event source defect ie the source is not decoded correctly after sending a frame given that the maximum number of retransmission intervals was reached without this source ^^ being correctly decoded. The individual fault event O ^^, ^^ depends on each retransmission interval (slot) of mutual information of nodes that have correctly decoded the source , • represents the mutual information between the source and the destination ^^, • represents the mutual information between all the nodes helping the source ^^ and the destination at the retransmission interval ^^ (it is considered an equivalent channel formed by the different channels between these nodes and the destination). By convention, is equal to Transmission of a frame according to the invention During the first phase of the method, the sources successively transmit their message after coding comprising bits of information being the two-element Galois field. The message ^^ includes a CRC-type code that allows the integrity to be checked of the message The message is encoded according to the MCS determined by the allocated bit rate. Since the MCSs may be different between sources, the lengths of the encoded messages may be different between sources. The encoding uses an incremental redundancy type code. The resulting codeword is segmented into successive redundancies. The incremental redundancy code may be of a systematic type, the information bits are then included in the first redundancy. Whether or not the incremental redundancy code is of a systematic type, it is such that the first redundancy can be decoded independently of the other redundancies.The incremental redundancy type code can be implemented, for example, by means of a finite family of rate-compatible punctured linear codes or rate-free codes modified to operate with finite lengths: raptor code (RC), rate-compatible punctured turbo code (RCPTC), rate-compatible punctured convolutional code (RCPCC), rate-compatible punctured convolutional code (RCLDPC), rate-compatible low-density parity check code (LDPC).

[0064] Transmission by a source typically includes one or more reference signals. The destination estimates the channel and therefore its quality between each of the sources and the destination in a known manner, for example by exploiting the reference signal(s) received.

[0065] Whether during the first phase or the second phase, when a node transmits, especially a source, the destination and other nodes listen.

[0066] The destination, sources, and relays attempt to decode the received redundancies at the end of a time interval. The success of decoding at each node is decided using the CRC. The destination and nodes thus determine their set of successfully decoded sources at each interval.

[0067] The 2 nde transmission phase of the process includes t = {1, ... , T used ] retransmission intervals with the convention that t = 0 corresponds to the last transmission interval of the first phase. The term retransmission associated with an interval is used in connection with the 2 nde phase to clearly indicate that any transmission during this phase of a n ieme redundancy of the message from a source i occurs when this source has already transmitted the lere redundancy of this same message during the l ere phase.

[0068] Unlike the prior art, there is no exchange of decoding control between the destination and the nodes at each retransmission interval: the destination does not systematically send back at each retransmission interval its set of correctly decoded sources or any indication of correct decoding or not, the nodes do not systematically transmit at each retransmission interval their set of correctly decoded sources or any indication of their correct decoding or not. The exchange of decoding control for an update of the destination's knowledge of the sets of sources correctly decoded by the nodes occurs only when t = 1, that is to say at the start of the second phase. This exchange can occur in an equivalent manner at the end of the first phase.During this exchange, the nodes transmit to the destination their set of correctly decoded sources or at least their set of correctly decoded sources not yet correctly decoded by the destination. The transmission by a node typically includes one or more reference signals. The destination estimates in a known manner the channel and therefore its quality between each of the nodes and the destination by exploiting, for example, the reference signal(s) received at this interval t = 1.

[0069] Thus, the nodes transmit to the destination their set of correctly decoded sources or at least their set of correctly decoded sources not yet correctly decoded by the destination only once, i.e. during the last transmission interval, t = 0 or equivalently during the 1 er retransmission interval, t = 1.

[0070] On the other hand, the destination selects at each retransmission interval a source called source to be helped by using a broadcast control channel from the destination to the nodes. The nodes having correctly decoded this source then transmit the same redundancy of the message from this source during this interval using a data channel. Helping a source means helping the destination to decode this source by transmitting by the nodes having correctly decoded this source a redundancy of the message from this source during the 2 nde phase.

[0071] Selection according to the invention

[0072] The selection by the destination of a source to be assisted at each retransmission interval is explained below with the support of Figure 3 which illustrates the single exchange of decoding control between the destination and the nodes according to one embodiment. According to this mode, the destination sends back to the nodes its set of correctly decoded sources and the nodes transmit their set of correctly decoded sources. According to another mode, the destination sends back to the nodes its set of correctly decoded sources and the nodes transmit their set of correctly decoded sources and not yet correctly decoded by the destination. According to another mode, the destination sends back to the nodes a signal indicating a lack of correct decoding, NACK and the nodes transmit their set of correctly decoded sources.

[0073] During the 2 ndephase, the destination schedules in turn the sources not yet correctly decoded for a given number of successive transmissions in order to maximize the received sum throughput. A transmission, during a retransmission interval, to assist a source i corresponds to the transmission of the same redundancy version of its message by all the nodes having correctly decoded this source. Transmissions to assist a source i begin at the retransmission interval t s i and end when the source is decoded.

[0074] The individual fault event of source i, for which the retransmission intervals start at t s i , at the end of the retransmission interval t — 1, O i t-1 , can be expressed in the form: (1)

[0075] This expression reflects the fact that source i is not decoded correctly at retransmission interval t — 1 if the source rate is greater than the sum of the transmission capacities. This transmission capacity includes the channel capacity between this source i and the destination that occurs during the l ere phase and a weighted sum by a of the capacities of the equivalent channels which occur during the second phase from t s i up to the retransmission interval t — 1. At the retransmission interval l ≥ t s i of the second phase, an equivalent channel is considered for source i. The equivalent channel considered at a retransmission interval l groups the channels between each of the nodes helping source i during this interval and the destination. The capacity of the channel between this source i and the destination is deduced from the quality of the channel i.e. the mutual information I i dbetween source i E {1, ... , M] and destination d. The capacity of the equivalent channel considered when source i is helped during interval l is evaluated by the mutual information between the set of nodes that help source i at retransmission interval l and the destination. This capacity depends on time l since a node can benefit from transmissions to help a source i during the 2 nde phase and correctly decode this source i from a retransmission interval of the 2 nde phase when he had not decoded it at the end of the l ere phase.

[0076] X(t) is defined as the number of retransmission intervals elapsed up to (but not including) the current interval during the second phase since the last decoding control exchange between the destination and the nodes.

[0077] X m(t) defines the value of X(t) that triggers a new exchange of decoded source sets. For example, triggers an exchange of decoded source sets for

[0078] By convention, X m (l) = 0 triggers an exchange of decoded source sets at the start of the 2 nde transmission phase.

[0079] It is assumed that a source i is aided over one or more consecutive retransmission intervals starting with retransmission interval t s i E {1, ... , T max}. At each retransmission interval (time slot) t ≥ t s i , and for the selected source i not yet correctly decoded by the destination, the variable is defined according to the invention. This variable is defined as the maximum number (i.e. sufficient number) of retransmission intervals for the destination to decode this source i (starting from and counting the retransmission interval t s ), that is, source i is decoded at the latest at the end of the retransmission interval This variable %;(t) is estimated by the destination based on its knowledge of Let the set determined by X m (t) retransmission intervals starting with a exchange of sets of decoded sources. Let be the set of intervals of retransmission starting with an exchange of decoded source sets coinciding with the transmissions to assist source i, i.e.,

[0080] In the event that an exchange of decoded source sets took place before t s i , that is, before the start of the selection of source i to help during the 2 ndephase, the destination knows H is to emphasize that an exchange of sets of sources decoded at start of the 2 nde transmission phase allows the destination to know for all sources. Indeed, transmissions preceding t s i being linked to another source they do not impact the number of nodes having decoded source i nor For all

[0081] Otherwise, the destination is only known to estimate For the sake of simplicity of notation, the source index i of t s i is omitted when it is obvious.

[0082] For retransmission intervals t s ≤ t ≤ t0the invention considers: (2) And the number of sufficient retransmission intervals estimated by the destination from the retransmission interval t is: (4) where [q] represents the upper integer function (ceiling function) which takes the integer value just greater than or equal to q, eg, [2.3] = 3.

[0083] For t = t0, an exchange of decoded source sets takes place, which allows the destination to evaluate as follows: with (6)

[0084] More generally, it is possible to construct recursively (t) like: ) or, equivalently: (8) with always: Noticed : Between two decoding control exchanges the number of retransmission intervals sufficient to decode source i is decremented at each transmission helping source i For , it comes: Finally, in the event that no exchange of decoded source sets is made either before or after he comes: (11) In the case where a single exchange of decoded source sets is carried out at the beginning of the 2 nde transmission phase then: (12) is the number of retransmission intervals to assist source i from the retransmission interval included sufficient for error-free decoding of source i which is estimated by the destination based on its knowledge of The destination uses as an approximation of a previous value (known closest) with consequently 1}. The number of retransmission intervals required in practice requires the knowledge of . It is given by the smallest value of ^^ such as : , (13) The estimate is x t the smallest value of such that: (14)

[0085] As it comes that In case it comes:

[0086] (15)

[0087] Indeed, the smallest positive integer such that:

[0088] (16) East :

[0089] (17)

[0090] Thus, it is certain that the event of default (outage) does not occur at The source i is systematically decoded correctly by the destination at if it is helped times from the interval t included.

[0091] The generalization of this demonstration in the case of several decoding exchanges is immediate.

[0092] It should be noted that the evaluation of the number of retransmission intervals sufficient for a source i is the same regardless of t s i its retransmission interval chosen for the first transmission intended to help this source during the 2 nde phase. At a given retransmission interval t, depends only on the number of transmissions n1= t — ts i helping the source i during the 2 nde phase and before t. Thus, the notation denotes the necessary number of transmissions helping source i knowing the number of transmissions ni having helped source i (already carried out) during the 2 nde phase. Subsequently, we denote x t the counter of the number of transmissions remaining to help source i, this counter being initialized to Xj(0) and being decremented each time source i is helped without decoding exchange. This counter is updated at each decoding control exchange at the retransmission interval l = tj j = 0, ... , M — 1 which allows Jt to be known d (L).

[0093] If the estimate of x t is such that there has been no previous transmission to help source i or such that Xi(0) relies solely on knowledge of direct links and that for all sources the following inequality is satisfied: (18) then all sources can be decoded without exchanging sets of decoded sources. Since all sources can be decoded correctly by the destination in the remaining time, the method chooses, for example, successively the sources to be helped and randomly. The method according to the invention is particularly interesting when since it allows the destination to correctly decode an optimal number of sources by optimizing spectral efficiency while very strongly limiting the signaling overhead by performing the selection of the source to be helped according to a certain strategy.

[0094] Furthermore, according to the invention the exchange of control of source sets correctly decoded between the destination and the nodes occurs only once and at the beginning of the 2 nde phase.

[0095] At t = 1 ie at the beginning of the 2 ndephase, the method comprises an exchange of decoding control between the destination and the nodes. The method determines the sufficient number of retransmission intervals for the destination to decode the source i not yet decoded at the end of the l ere phase knowing its allocated flow rate:

[0096] (19) with :

[0097] (20) or, again with

[0098] (21) with rtj the number of transmissions already made to help source i. Following the exchange of decoding control between the destination and the nodes at t = 1, the destination knows Without decoding control exchange at t=l, the destination is based on i.e. on knowledge of the direct link between source i and destination.

[0099] The estimation of the channel between source i and destination is performed, for example, on the basis of the reference signals emitted by source i when it transmits during the first phase. Since the channels are assumed to be invariant during a frame, this value is independent of the transmission or retransmission interval. This knowledge of the channel quality between source i and destination allows the destination to estimate mutual information I i d representative of this quality and therefore of the capacity of the channel.

[0100] In the second phase, the estimation of the channel between the node and the destination is carried out, for example, on the basis of a reference signal emitted by node j during the control exchange during which it transmits its set or a subset of this set of correctly decoded sources. This knowledge of the quality of the channel between node j and the destination at the interval t = 1 allows the destination to estimate mutual information Ji,d(1) representative of the quality of the equivalent channel between all the nodes having decoded source i and the destination.

[0101] For t > 1 the process considers that each transmission leads to mutual information so that decreases depending on the number of transmissions made to help source i.

[0102] The destination selects, according to the invention, for each retransmission interval t the source i to be helped.

[0103] At each retransmission interval t and before selection, the remaining number of retransmission intervals

[0104] The selection is determined to maximize spectral efficiency. Maximizing spectral efficiency can be expressed as determining the subset A taken from the set of possible subsets A of sources not yet decoded correctly by the destination at the interval preceding the current interval t leading to the largest sum of the rates of the sources and such that the sources of this subset A can be decoded in the remaining time, T av ie such that the remaining time is greater than or equal to the sum of the number of retransmission intervals sufficient to decode each of the sources of this subset A: (22) is called the power play of

[0105] The method then successively considers each source i of this subset

[0106] For each source i considered of the destination transmits to the nodes the indication of the selection of source i at the retransmission interval t.

[0107] The nodes that have correctly decoded this source i transmit the same redundancy during this interval t to help the decoding of the source i by the destination.

[0108] The destination repeats the transmission of the indication of the selection of the same source i until the destination correctly decodes this source. The number of retransmission intervals elapsed before correct decoding is at most equal to x t (0).

[0109] According to a particularly efficient embodiment, if the destination correctly decodes the source i before the number Xi(0) of retransmission intervals has elapsed, it recalculates the best subset if and only if the set from which source i is removed does not does not contain all sources not decoded by the destination otherwise the destination goes to another source in the subset from which source i was previously removed. This case can occur when the set of sources correctly decoded by a node changes to include source i during Xi(0) retransmission intervals. Thus, this node becomes active during the transmission at interval l of a redundancy for source i which results in an increase in mutual information If source i is decoded after exactly xi(0) retransmission intervals then the destination switches to another source in the subset from which the source has been previously removed.

[0110] Implementation example

[0111] The following description of an embodiment of the invention is illustrated with an implementation by an OMARC system with M = 4 sources, S = {1,2, 3, 4}, L = 3 relays, R = {5,6,7}, and one destination. The parameter T max is set to 8.

[0112] At the end of the l ere phase, ie, t = 0, the sets of sources correctly decoded by the nodes are as follows:

[0113] In other words, sources 1, 2, 3, 4 and relay 7 have not yet decoded anything correctly at the end of the l ere phase but since a source knows its own message its game contains at least this message. Relay 5 correctly decoded sources 2 and 3 and relay 6 correctly decoded sources 1, 2 and 3 at the end of the l ere phase. Destination d has not yet decoded anything correctly and at the end of the l ere phase.

[0114] During the 2 ndephase, the determination by the destination of the set of sources A to help takes place for example according to the algorithm in Appendix A.

[0115] During the 2 nde phase, the selection of source i to help among the set at each retransmission interval, for example, the algorithm in Appendix B can be used since

[0116] Sequence of the Algorithm in Appendix B:

[0117] Step 2. The destination determines the estimate of the number of intervals needed for the destination to decode a source i not yet decoded on the basis of knowledge of mutual information of the source i destination link and a rate assigned to this source i. The destination therefore calculates x i for everything

[0118] For this example, the variables x ihave the following values ​​knowing the direct source-destination links: x1= 6, x2= 2, x3= 3, x4= 12

[0119] R flow rates t have the following values:

[0120] R1= 1, R2= 2, R3= 3, R4= 4

[0121] Step 3. The sum of the x t exceeds the remaining time: 6 + 2 + 3 + 12 = 23 > T av = T max = 8 Step 4. A single decoding control exchange takes place between the destination and the nodes. During this exchange, the nodes transmit their set of correctly decoded sources or only their set of correctly decoded sources that have not yet been correctly decoded by the destination.

[0122] Step 5. For a source i not yet decoded by the destination, the destination updates the estimate of x i using the quality of the equivalent channel considered as the aggregation of the channels between the nodes having correctly decoded this source and the destination.

[0123] According to the example, knowing the node-destination links, the variables x i become:

[0124] Although x2 has decreased, the sum of the xi: 6 + 1 + 3 + 12 = 22 remains greater than the remaining time T av T max •

[0125] Step 7. All sources A to help at the end of the 1 ere phase is determined, according to the algorithm in Appendix A, by the destination knowing the x ( and the flow rates allocated to the sources.

[0126] According to the example, the possible choices for  that satisfy are: {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}. The set that leads to the maximum sum of the flow rates and which is determined according to the algorithm in Appendix A is

[0127] Step 8. The process decodes the frame as long as t < T max and that there remains a source i to be decoded in the set  ie  #= Φ .

[0128] According to the example  = {2, 3}.

[0129] Step 9. At retransmission interval t, the destination selects source i with the smallest Xi.

[0130] According to the example, the destination first selects source 2.

[0131] Step 10. The process repeats steps 11-20 until the source i is successfully decoded by the destination.

[0132] According to the example, the destination repeats for source 2 until it has successfully decoded it and then repeats for source 3.

[0133] Step 11. At each retransmission interval t the destination sends back the number of the source i to be helped by the nodes.

[0134] According to the example, the destination first retransmits number 2 until it is correctly decoded, then number 3 until it is correctly decoded. Step 12. The method increments the value of the current retransmission interval, t «- t + 1, decrements the remaining time, Tav ← T av — 1 and decrements the value of x t since I was helped once by the knots.

[0135] According to the example, for source i = 2, at the first pass, x i = 0 since x2= 1 and the current interval becomes t = 2 , and T av =7 For source i = 3, on the first pass, x i = 2 since x3= 3 and the current interval becomes t = 3 and T av =6.

[0136] For source i = 3, on the second pass, x i = 1 and the current interval becomes t = 4 and

[0137] T = 5

[0138] Step 13. If the destination has correctly decoded the source i at the current interval then proceed to steps 14-18.

[0139] According to the example, source i = 2 is correctly decoded in a single retransmission interval since x2= 1. S d l = {2}.

[0140] For source i = 3, the process loops back to step 11 after a retransmission interval.

[0141] S d 2 = {2}- After a second retransmission interval, the source i = 3 is correctly decoded. S d 3 = {2,3}.

[0142] Step 14. The source i correctly decoded by the destination is removed from the set Â.

[0143] According to the example, at the beginning of the 2 e retransmission interval,

[0144] At the end of the 2 e retransmission interval, A is unchanged: A = {3}.

[0145] At the end of the 3 e retransmission interval,

[0146] Step 15. If the x ( intervals were not consumed and that the set A did not contain all the sources not decoded correctly by the destination (when determining A the sum of the x (exceeded the remaining time) then the process proceeds to steps 16-17.

[0147] According to the example, for source i = 2, x2= 1 — 1 = 0 so the condition xi > 0 is not fulfilled although the condition is fulfilled, the process goes to step 19, ie the method repeats steps 10-18 for source i = 3.

[0148] For source i = 3 it is correctly decoded according to the example after two retransmission intervals x3= 3 — 2 = 1. As x t > 0 and then the method updates the set  according to step 16.

[0149] Step 16. The method determines set A according to the algorithm in Appendix A with the updated set of sources not decoded correctly by the destination.

[0150] According to the example, after the correct decoding of the source i = 3 the algorithm of Appendix A is called with to update A.

[0151] According to the algorithm in Appendix A since and that x1= ​​6 > 5 and that x4= 12 > 5 then

[0152] Step 20. The process loops back to step 8 with  updated.

[0153] According to the example, ie no other source can be decoded in the remaining time. The transmission of the frame is interrupted, there is a decoding fault (outage event) of sources 1 and 4. The method moves on to the transmission of the next frame. According to another embodiment, the selection of a source in step 9 can be done randomly among the sources of the set Â.

[0154] Appendix A

[0155] Determination of the set sources that can be helped:

[0156] [Table 1]

Claims

CLAIMS 1. Method for transmitting a frame carrying messages intended for an OMAMRC telecommunications system with N nodes including M sources and a destination (d), N ≥ M ≥ 2, the nodes operating in half-duplex mode, according to an orthogonal multiple access scheme of the transmission channel between the N nodes with a maximum number of M + T max time intervals per transmitted frame distributed between a l ere phase and a 2 nde phase, 1 ≤ T max , the message from a source having been coded before transmission using an incremental redundancy type coding which generates several redundancies, the l ere phase includes M intervals allocated respectively to successive transmissions from the M sources and the 2 nde phase includes at least one retransmission interval for a transmission from nodes having correctly decoded the same source s isuch that these nodes transmit simultaneously during the same retransmission interval the same redundancy of the message from the same source not yet correctly decoded by the destination, called the source to be helped, the method is such that it comprises: a single transmission by the nodes of at least their set of sources correctly decoded and not yet correctly decoded by the destination, these transmissions allowing the destination to determine, for each of these sources, a quality of an equivalent channel based on a quality of the channels between the nodes having correctly decoded this source and the destination, an estimation of a sufficient number of retransmission intervals (X i (0)) for the destination to decode a source (s i ) not yet correctly decoded and correctly decoded by at least one node based on the quality of an equivalent channel for this source between this at least one node and the destination and a rate (R i) attributed to this source (s i ). a selection by the destination of the sources to be helped taking into account the estimated numbers (0) of retransmission intervals sufficient for the destination to decode the sources not yet correctly decoded and a sum of rates (R i ) attributed to the sources.

2. Transmission method according to claim 1, such that the single transmission by the nodes of at least their set of correctly decoded sources and not yet correctly decoded by the destination is carried out at the start of the 2 nde phase.

3. Transmission method according to one of claims 1 and 2, further comprising a comparison between a sum of estimated numbers of retransmission intervals sufficient to help the destination to decode sources not yet correctly decoded and a number of time slots remaining during the 2 ndephase to help the destination correctly decode one or more sources.

4. Transmission method according to the preceding claim such that the comparison is updated after the correct decoding of a source by the destination.

5. Transmission method according to one of claims 1-4 such that the single transmission by the nodes of at least their set of correctly decoded sources and not yet correctly decoded by the destination is part of a control exchange during which the source transmits its set of correctly decoded sources.

6. Transmission method according to claim 5 such that during the exchange, a node sends only its set of sources correctly decoded and not yet correctly decoded by the destination.

7. Transmission method according to claim 5 such that during the exchange, a node sends its set of correctly decoded sources.

8. Transmission method according to one of claims 1-7, further comprising a comparison between the estimated numbers of retransmission intervals sufficient for the selection to take into account a sufficient ordering of these estimated numbers of retransmission intervals.

9. Transmission method according to one of claims 1-7, further comprising a determination of a set of sources which can be helped taking into account the numbers (x i (0)) estimates of sufficient retransmission intervals and time remaining before the end of the 2 nde phase and such that the selection of sources to be helped is done randomly among all the sources that can be helped.

10. The method of claim 9 such that, if no source can be helped in the time remaining before the end of the 2 ndephase then the transmission of the frame is interrupted before the use of the maximum number of sufficient retransmission intervals (T used < Tmax)- 11. System comprising N nodes including M sources S i i∈{l, ... , M] and a destination (d), N ≥ M > 2, for an implementation of a transmission method according to one of claims 1 to 10.

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