EP4150808A1 - Omamrc transmission method and system with variation in the number of uses of the channel - Google Patents

Abstract

The present invention relates to a transmission method with slow link adaptation intended for an OMAMRC telecommunication system with M sources (s1...,sM), optionally L relays and a destination, M ≥ 2, L ≥ 0. A number of uses of the channel is allocated to each source for transmission and this number is determined by the destination during slow link adaptation for each source based on statistical knowledge of all of the channels.

Description

DESCRIPTION

TITLE: OMAMRC method and system of transmission with variation of the number of uses of the channel

Field of the Invention 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 two nodes which can be relays or sources.

It is understood that a relay has no message to transmit. A relay is a node dedicated to relaying messages from sources while a source has its own message to be transmitted and can also in certain cases relay messages from other sources i.e. the source is said to be cooperative in this case.

There are many relaying techniques known by their Anglo-Saxon name: "amplify and forward", "decoded and forward", "compress-and-forward", "non-orthogonal amplify and forward", "dynamic decoded and forward" , etc.

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 networks of sensors.

Such a network of sensors is a multi-user network, consisting of several sources, several relays and a recipient using a time-orthogonal multiple access scheme of the transmission channel between the relays and the destination, denoted OMAMRC (“Orthogonal Multiple- Access Multiple-Relay Charnel ”according to Anglo-Saxon terminology).

Prior art

An OMAMRC transmission system implementing slow link adaptation is known from application WO 2019/162592 published August 29, 2019. The content of this application is included by reference.

An OMAMRC telecommunication system has M sources, possibly L relays and a destination, M ≥ 2, L ≥ 0 and a time-orthogonal multiple access scheme of the transmission channel which applies between the nodes taken from among the M sources and the L relays. The maximum number of time slots per transmitted frame is M + T _max with M slots allocated during a first phase to the successive transmission of the M sources and T _used ≤ T _max slots 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.

The OMAMRC transmission system considered comprises at least two sources, each of these sources being able to operate at different times either as a source or as a relay node. The system may optionally further include relays. Node terminology covers both a relay and a source acting as a relay node or as a source. The system considered is such that the sources can themselves be relays. A relay differs from a source because it has no message to transmit which is specific to it, ie it only retransmits messages from other nodes.

The links between the different nodes of the system are subject to slow fading and Gaussian white noise. Knowledge of all system links (CSI: Channel State Information) by the destination is not available. Indeed, the links between the sources, between the relays, between the relays and the sources are not directly observable by the destination and their knowledge by the destination would require an excessive exchange of information between the sources, the relays and the destination. . To limit the cost of the return channel overhead (feedback overhead), only information on the distribution / statistics of the channels (CDI: Channel Distribution Information) of all the links, eg average quality (for example average SNR, average SINR ) of all links, is assumed to be known by the destination in order to determine the bit rates allocated to the sources.

The link adaptation is of the 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 trace the CDI distribution back on the basis of knowing the average SNR or SINR for each link in the system.

The transmissions of the messages from the sources are divided into frames during which the CSIs of the links are assumed to be constant (assumption of slow fading). Rate allocation is assumed not to change for several hundred frames, it only changes with CDI changes.

Distinguishes three phases process, an initial phase, and for each frame to be transmitted, a 1 ^st stage and a 2 ^nd stage. The transmission of a frame takes place in two phases which are optionally preceded by an additional so-called initial phase.

During the initialization phase, the destination determines an initial rate for each source 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 use of reference signals. The quality of the source - source, relay - relay and source - relay links is estimated by the sources and the relays by using, for example, the reference signals. The sources and the relays transmit to the destination the average qualities of the links. This transmission takes place before the initialization phase. Only the average value of the quality of a link being taken into account, its refreshing takes place on a long time scale, that is to say over a time which allows 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 frequency of the signal transmitted for a given speed. The initialization phase occurs for example every 200 to 1000 frames. The destination goes back to the sources via a return path the initial rates that it determined. The initial flow rates remain constant between two occurrences of the initialization phase.

During the first phase, the M sources successively transmit their message during the M time slots using respectively modulation and coding schemes determined from the initial bit rates. During this phase, the number N ₁ of uses of the channel (channel use, ie resource element according to 3GPP terminology) is fixed and identical for each of the sources.

During the 2 ^nd phase, the source messages are transmitted cooperatively by the relays and / or by the sources. This phase lasts at most T _max time slots. During this phase, the number N ₂ of channel uses is fixed and identical for each of the sources.

During the first phase, the sources which are independent from one another broadcast their coded information sequences in the form of messages for the attention of a single recipient. Each source broadcasts its messages at the 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 message in turn during time slots each dedicated to a source.

The sources other than the one which transmits and possibly the relays, of the “Half Duplex” type receive the successive messages from the sources, decode them and, if they are selected, generate a message solely from the messages from the sources decoded without error. The selected nodes then access the channel orthogonally in time to each other during the second phase to transmit their generated message to the destination.

The destination can choose which node should transmit at a given time.

The method implements a strategy to maximize the average spectral efficiency (utility metric) within the system considered under constraint to respect an individual quality of service (QoS) per source ie an average individual BLER per source:

Or

R _i = K _i / N ₁ represents the initial bit rate of source i with K _i the number of information bits of the message from source i ∈ {1, ..., M). R _i is a variable which takes discrete values taken from a set with n _MCS Ie number of flow rates corresponding to the different diagrams Modulation and Coding Scheme (MCS) available for transmission.

• T _used ≤ T- _ma x shows the number of retransmissions used during the 2 ^nd phase, is the average of the number of retransmissions used during the second phase

• a = N ₂ / N ₁ is the ratio between the number of uses of the channel during the second phase and the number of uses of the channel during the first phase

• BLERi represents the block error rate for source i. BLERi denotes the function with multiple variables BLERi (R ₁ , ..., R _M ) which depends on the current value taken by the flow variables R ₁ , ..., R _M.

The QoS constraint on the individual BLER given by source is written: BLER _i ≤ BLER _{QoS, i} , ∀i∈ {1,, M}. The algorithm based on an interference-free or “Genre Aided” approach is used to solve the multidimensional allocation optimization problem (rate). This approach consists in independently determining each initial rate of a source by assuming that all the messages of the other sources are known by the destination and the relays then in iteratively determining the rates by initializing their value with the values determined according to the approach. "Kind Aided". The utility metric which consists of spectral efficiency is conditioned by the node selection strategy that occurs during the second phase. Main features of the invention

The present invention relates to a transmission method with slow link adaptation for an OMAMRC telecommunication system according to which the number of uses of the channel is determined by the destination for each source ie by time interval, on the basis of a knowledge of the statistical distribution of all channels (Channel Distribution Information according to English terminology).

By introducing a variable number of uses of the channel, with slow adaptation controlled by the destination, unlike the known method for which this number is fixed, the present invention substantially improves the performance of the method.

To simplify the method, only the number of uses of the channel for the first phase is variable, the number N ₂ of uses of the channel for the second phase is fixed.

The number N _{1, i} of uses of the channel by a source for the first phase can thus be adapted taking into account the distribution of the CDI channels.

The distribution of CDI channels can be based, for example, on knowing an average SNR (or SINR, or equivalent indicator) of each of the channels, subject to considering that all the links are independent of each other and that the rapid fading ( fast fading) follows a distribution Gaussian ie a so-called Rayleigh fading. From the knowledge of the average SNRs, the destination aims to optimize the ratio = N ₂ / N _{1, i} together with the throughput transmission by source i during the first phase i.

During the second phase, the destination selects at each time interval t∈ {1, ... T _used ) the node among the sources and the relays which will transmit using the N ₂ uses of the channel. The slow link adaptation implemented by the destination tends to maximize the average utility metric modified by the introduction of the ratio α _i = N ₂ / N _{1, i} . This metric is thus a function with multiple variables which depends on the current value taken by the flow variables R ₁ ..., R _M and by the variables of the ratio α ₁ , ..., α _M between the number N ₂ d 'uses of the channel during the second phase and the number N _{1, i} of uses of the channel during the first phase:

According to one embodiment, the uses of the channel consisting of time and / or frequency resources, a variation in the number of uses of the channel consists of a variation in the number of resources allocated to each source which takes account of the average qualities of the channels.

According to one embodiment, the mean utility metric takes into account M ratios α _i which are considered to be of discrete values and belonging to a finite set of possible values.

According to one embodiment, the values of a ratio of uses of the channel between the two phases determined for each source belong to a finite set of discrete values, preferably {1, 0.5, 2). The method maximizes the average utility metric with an initialization of the ratio for each source to the closest discrete value to an average of the ratio values denoted α _GA and implementing a “Genie Aided” approach. The "Genie Aided" approach of independently determining each bit rate from a source, assuming all messages from other sources are known to the destination and assuming a relationship for each source, leads to initial flow values for each source which are not sufficiently precise.

According to one embodiment, an iterative calculation of the initial throughputs by the destination makes it possible to correct these initial throughput values by taking into account the selection strategy which occurs during the second phase, which the "Genie Aided" approach cannot by nature. »Only which assumes that the active node for each time interval of the second phase is chosen randomly among the nodes. This mode is advantageous in that it makes it possible to use a “Genie Aided” approach for the initialization of the iterative algorithm. According to one embodiment, the cooperative transmission of a node during the second phase results in the transmission of a redundancy based on incremental coding to the sources.

According to one embodiment, the strategy for selecting the nodes occurring during the second phase follows a sequence known in advance by all the nodes.

According to one embodiment, the iterative calculation of the initial flow rates takes into account a node selection strategy (strategy with random selection, strategy with cyclic selection, etc.). According to one embodiment, the strategy for selecting the nodes intervening during the second phase takes into account information coming from the nodes indicating their set of correctly decoded sources.

According to one embodiment, the strategy for selecting the nodes intervening during the second phase corresponds to each time interval to the selection of the node which has correctly decoded at least one source that the destination has not decoded correctly at the end of the previous time interval and which benefits from the best instantaneous quality among the instantaneous qualities of all the links between the nodes and the destination.

According to one embodiment of the invention, the message transmission method results from a software application divided into several specific software applications stored in the sources, in the destination and possibly in the relays. The destination can be for example the receiver of a base station. The execution of these specific software applications is suitable for implementing the transmission method.

A further subject of the invention is a system comprising M sources, possibly L relays, and a destination, M> 1, L ≥ 0, for implementing a transmission method according to a preceding object.

A further subject of the invention is 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.

A further subject of the invention is configured memories comprising instruction codes corresponding respectively to each of the specific applications.

The memory can be incorporated into any entity or device capable of storing the program. The memory may be of the ROM type, for example a CD ROM or a microelectronic circuit ROM, or else of the magnetic type, for example a USB key or a hard disk.

On the other hand, each specific application according to the invention can be downloaded from a server accessible on an Internet type network. The optional characteristics presented above in the context of the transmission method can optionally be applied to the software application and to the memory mentioned above.

List of Figures

Other characteristics and advantages of the invention will emerge more clearly on reading the following description of embodiments, given by way of simple illustrative and non-limiting examples, and the appended drawings, among which:

[Fig 1] FIG. 1 is a diagram of an example of a so-called Cooperative OMAMRC (Orthogonal Multiple Access Multiple Relays Channel) system according to the invention,

[Fig 2] FIG. 2 is a diagram of a frame transmission cycle which can be preceded by an initialization step according to the invention,

[Fig 3] FIG. 3 is a diagram of the OMAMRC system of FIG. 1 for which all the sources except the source s-, are considered to be correctly decoded.

Description of particular embodiments

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 uses of the channel is linked to the available frequency band and the transmission time.

In the “slow fading” case favored in the description, the fading gains are constant during the M + T _max time intervals where M + T _max is the maximum number of time slots to accomplish a transmission cycle.

An embodiment of the invention is described in the context of an OMAMRC system illustrated by FIG. 1 and in support of the diagram of FIG. 2 which illustrates a transmission cycle of a frame.

This system includes M sources which belong to the set of sources where, for convention to simplify the notations, s _i = i∀i∈ (1, ..., M), L relays which belong to the set of relays and a destination d.

Every source in the game communicates with the single destination with the help of other sources (user cooperation) and cooperating relays.

To simplify the description, the following assumptions are made subsequently on the OMAMRC system:

- the sources, the relays are equipped with a single transmitting antenna;

- the sources, the relays, and the destination are equipped with a single receiving antenna;

- the sources, the relays, and the destination are perfectly synchronized;

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

- all the nodes transmit with the same power;

- use is made of a CRC code assumed to be included in the K _s information bits of each source s to determine whether a message is correctly decoded or not;

- the links between the different nodes suffer from additive noise and fading. The fading gains are fixed during the transmission of a frame carried out for a maximum duration M + T _max time intervals, but can change independently from one frame to another. T _max ≥ 2 is a system parameter;

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

- the returns are error free (no error on the control signals).

Nodes include relays and sources that can behave like a relay when they are not sending their own message.

The nodes, M sources and L relays, access the transmission channel according to an orthogonal time multiple access scheme which allows them to listen without interference to the transmissions of the other nodes. The nodes operate in a half-duplex mode.

The following notations are used: is the modulated symbol encoded for the use of the k channel transmitted by the node

• is the signal received at the node corresponding to a signal transmitted by the node

• γ _{α, b} is the mean signal-to-noise ratio (SNR) which takes into account the effects of attenuation of the channel (path-loss) and of masking (shadowing), · h _{α, b} is the attenuation gain of the channel (fading) which follows a complex circular Gaussian distribution symmetric with zero mean and of variance γ _{α, b} , the gains are independent between them,

• ⁿ _{a, b, k} are samples of an identically and independently distributed Gaussian white noise (AWGN) which follow a complex Gaussian distribution of circular symmetry with zero mean and unit variance. R _s is a variable representing the initial flow of the source s which can take its values in the finite set Similarly, α _s is a variable representing the ratio N ₂ / N _{1, s} which can take its values in a finite set

The signal received at the node corresponding to the signal sent by the node can be written: (1)

During the first phase of M time intervals, each source transmits its code words for N _{1, s} uses of the channel, k ∈ {1, ..., N _{1, s} }, the number N _{1, s} of uses of the channel depending on the source s. During the second phase of T _used , T _used ≤ T _max , time intervals, each selected node transmits information representative of the messages of the sources decoded without error by this node during N ₂ uses of the channel, k ∈ {1, ... , / V ₂ ), the number N ₂ of uses of the channel being by simplification for the identical process between the sources By exploiting reference signals (pilot symbols, SRS signals from 3GPP LTE, etc.), the destination can determine the gains (CSI Channel State Information) of the direct links: i.e. source to destination and relay links to destination and can therefore deduce the average SNRs of these links.

On the other hand, the gains of links between sources, links between relays and links between sources and relays are not known to the destination. Only sources and relays can estimate a metric of these links by exploiting reference signals in a manner similar to that used for direct links. Given that the statistics of the channels are assumed to be constant between two initialization phases, the transmission to the destination of the metrics by the sources and the relays may only take place at the same rate as the initialization phase. The channel statistic of each link is assumed to follow a circular-centered complex Gaussian distribution and the statistics are independent between the links. It is therefore sufficient to consider only the average SNR as a measure of the statistics of a link.

The sources and the relays therefore go back to the destination of the metrics representative of the average SNRs of the links that they can observe. The destination thus knows the average SNR of each of the links.

During an initial link adaptation phase which precedes the transmission of several frames, the destination transmits for each source s a representative value (index, MCS, bit rate, etc.) of an initial bit rate and a value

Each of the initial rates unambiguously determines an initial modulation and coding scheme (MCS) or conversely each initial MCS determines an initial rate.

The feedback of initial flows and reports is carried out via control channels at very limited flow.

These initial flows are determined by the destination so as to maximize an average utility metric eg an average spectral efficiency, conditioned on the selection strategy of the nodes intervening during the second phase and under the constraint of an average individual BLER for each source, this metric being modified by the introduction of the sound report expression is given by equation (21).

This metric (21) is thus a function with multiple variables which depends on the current value taken by the flow variables R _1, ..., R _M and by the variables of the ratio α ₁ , ..., α _M between the number N ₂ · of uses of the channel during the second phase and the number N _{1, i} of uses of the channel during the first phase:

According to one embodiment, the mean utility metric takes into account M ratios α _i which are considered to be of discrete values and belonging to a finite set of possible values.

Each source transmits its framed data to the destination with the help of other sources and relays.

A frame occupies time slots during the transmission of the M messages from the M sources respectively. The transmission of a frame (which defines a transmission cycle) takes place during M + T _used time intervals: M intervals for the 1 ^st phase of respective capacities N _{1, i} uses of the channel for each source i, T _used intervals for the 2 ^nd phase. During the first phase, each source transmits after coding a message u _s comprising K _s information bits being the two-element Galois body. The message u _s includes a CRC type code which allows the integrity of the message u _s to be checked. The message u _s is coded according to the initial MCS. Since the initial MCSs may be different between sources, the lengths of the encoded messages may be different between the sources. The encoding uses an incrementally redundant code. The resulting code word is segmented into redundancy blocks. The incremental redundancy code can be of the systematic type, the information bits are then included in the first block. Whether or not the incrementally redundant code is of the systematic type, it is such that the first block can be decoded independently of the other blocks. The incremental redundancy code can be achieved, for example, by means of a finite family of punched linear codes with compatible yields or of non-yielding codes modified to operate with finite lengths: raptor code (RC), punched turbo code of compatible yield ( RCPTC rate compatible punctured turbo code), rate compatible punctured convolutional code), rate compatible punctured convolutional code, LDPC rate compatible (RCLDPC rate compatible low density parity check code).

Thus, during the first phase, the M sources successively transmit their message during the M intervals with respectively modulation and coding schemes determined from the values of the initial bit rates.

Each message transmitted corresponding to a source s∈ S, a correctly decoded message is assimilated to the corresponding source by abuse of notation.

When one source is transmitting, other sources and relays listen and attempt to decode messages received at the end of each time interval. Success of decoding is decided using CRC. In the second phase, the selected node, source or relay, acts as a relay by cooperating with the sources to help the destination correctly decode messages from all sources. The selected node transmits ie it cooperates by transmitting the words or part of the words that it has correctly decoded. The second phase comprises at most T _max time intervals (time slots) called rounds. Each round t ∈ {1, ..., T _max ] has a capacity of N ₂ uses of the channel.

During this phase, the destination follows a certain strategy to decide which node to transmit at each time interval (round). The destination informs the nodes using a limited feedback control channel to transmit a feedback message. This return message is based on its result of decoding received messages. The destination thus controls the transmission of the nodes using these return messages which improves spectral efficiency and reliability by increasing the probability of decoding of all sources by the destination.

If the decoding of all the sources is correct, the return is an AC K type message. In this case, a cycle of transmission of a new frame begins with the erasure of the memories of the relays and of the destination and with the transmission by the sources of new messages.

If the decoding of at least one source is wrong, the return message is typically a NACK. Each node has ∈ transmits its set of correctly decoded sources at the end of the previous noted time interval (round) By convention, we denote] _c set of messages (or sources) correctly decoded by the node at the end of the interval t of time (round t), t ∈ {0, ..., T _max }. The end of the time interval (round) t = 0 corresponds to the end of the first phase. The number of time slots used during the second phase T _used = {1, ..., T _max ) depends on the decoding success at the destination.

The selected node transmits parities determined from the messages of its correctly decoded source set using joint network coding and joint channel coding. This transmission takes place during a time interval of N ₂ uses of the channel. The other nodes and the destination can improve their own decoding by exploiting the transmission of the selected node and update their correctly decoded source set accordingly.

The initial transmission rate of a source s is R _s = K _s / N _{1, s} in bits per complex dimension (bcu). The long-term bit rate R _s of a source is defined as the number of bits transmitted compared to the total number of uses of the channel for a number of frames transmitted which tends to infinity: with the average number of retransmission time slots (rounds) used during the second phase and with α _i = N ₂ / N _{1, i} .

Spectral efficiency can be defined as the sum of individual spectral efficiencies: with the event that the source s is not decoded correctly by the destination at the end of the time interval (round) T _max , hereinafter called the individual outage event of the source s at the end of the time interval (round) T _max . In general, the individual cut-off event from source to the outcome of the time interval (round) depends on the selected node and the game associated with decoded sources and this conditionally on knowledge of the gains direct channels and of is the game including all the nodes that have been selected at the time intervals (rounds) l ∈ {1, ... t - 1) preceding the time interval (round) t as well as their associated decoding set and the destination decoding set

The common outage event at the end of the time interval (round) t, is defined as being the event that at least one source is not decoded correctly by the destination at the end of the time interval (round) t. The probability of the individual outage event of the source s at the end of the time interval (round) t for a candidate node a _t can be expressed as: with the expectation operator and such that takes the value 1 if the event V is true and the value 0 otherwise.

The probability of the common cutoff event can be defined in the same way. In the following, the dependence on the knowledge of is omitted for the sake of simplification ratings.

The common cut-off event of a set of sources occurs when the vector of their rate is outside the corresponding MAC capacity region.

For certain sub-sets of sources, the set of non-sources decoded correctly by the destination at the end of the time interval (round) t - 1, the common break event can be expressed in the form: such as sources that belong to are considered interference. reflects the non-respect of the MAC inequality associated with the sum rate of the sources contained in :

I _{α, b} the mutual information between nodes a and b and an already selected node. The factor 1 / a _s makes it possible to standardize before addition the two terms associated respectively with the two phases for which the time intervals have respective channel use times N _{1, s} and N ₂ .

The individual cut-off event of the source s at the end of the time interval (round) t can be written: have the same expression as for (5).

According to the invention, the destination implements a slow type link adaptation. This adaptation consists in maximizing the average utility metric (21), including the ratio α _i = N ₂ / N _{1, i} , after a number T _used ≤ T _max of retransmissions (cooperative transmissions) occurring during the second phase under constraint of an average individual BLER for each source. The utility metric is an average spectral efficiency conditioned on the selection strategy of the nodes intervening during this second phase.

According to a first class of strategies, the selection of the nodes taken from among the sources and the relays depends on the sets of the sources correctly decoded by the nodes. An example considered, called a preferred strategy, is based on a selection of the IR-HARQ type which aims to maximize the spectral efficiency. According to this preferred strategy, at the time interval (round) t of the second phase the destination chooses the node with the best instantaneous quality of the link between itself and this node (for example the greatest mutual information between itself and this node) taken among all the nodes which were able to correctly decode at least one source of the game, these nodes being said to be eligible. This strategy makes it possible to achieve a good compromise between computational complexity and performance, but to the detriment of a large number of control signals.

According to a second class of strategies, the selection of the nodes taken from among the sources and the relays does not depend on the sets of the sources correctly decoded by the nodes. According to this class the selection is determined and known to all nodes. One example considered is such that the selection sequence is cyclical and such that the selected node is selected only from among the relays. According to this example, each relay benefits from at least one dedicated time interval (round) during the second phase to transmit. In order not to favor one relay over another, the sequence changes with each frame. According to this example, only one return bit from the destination is sufficient to send up a common ACK / NACK message.

During the first phase, each source is transmitted with the initial rate R _s .

Let be the average probability of having the message from the source s not decoded correctly after T _used time intervals (rounds) of the second phase. In point-to-point transmission, the individual throughput of the source is given by:

And to optimize this throughput, the usual method is to find the optimal pair

Such a usual method cannot be used for a system with M sources, possibly L relays, and a destination with a multiple access scheme orthogonal to the transmission channel since the is dependent on all the bit rates (R ₁ ..., R _M ) and all reports α ₁ , ..., α _M. This is because the decoding set of the node selected at the current time interval (round) depends on all the rates and all the ratios and these influence the probability of incorrect decoding of the message from the source s. In order not to overload the ratings, we distinguish the flow rate of the source s after optimization, which is a possible value of R _s in the set of possible flow rates · n _MCS is a number of different MCS. Likewise we distinguish the ratio of the source s after optimization, which is a possible value of α _s in the set of possible relations The method according to the invention is a solution to the following optimization problem: under constraint that

In relation (7), T _used is a random variable which represents the number of time intervals (rounds) used during the second phase T _used ≤ T _max . The distribution of T _used depends on (R ₁ ..., R _M ), on (a, ..., a _M ) and on ^what makes the optimization (7) multidimensional cardinality i.e. 91125 (2M) -tuple ..., R _M , α ₁ , ..., α _M ) possible for 3 sources, a family of fifteen MCS and 3 possible values of a. An exhaustive search very quickly becomes impossible when the number of sources increases.

According to one embodiment, the method according to the invention follows a so-called “Genie Aided (GA)” approach which consists in making the assumption during the initialization step that all the sources s except the source s _{i, of} which we wants to initialize the bitrate are considered correctly decoded, All sources

{s ₁ , s ₂ , ..., s _i _ ₁ , s _{i + 1} , other than act as relays denoted {r _{L + 1} , ..., r _{L + M-1} }.

For the source s _t considered, the network is a network with multiple relays denoted (1, L + M - 1,1) and no longer a network with multiple relays and multiple users. The corresponding system is illustrated by the diagram of FIG. 3 when = s · ,.

The search for the optimal pair for the source s under the “Genie Aided (GA)” hypothesis, there is an additional difficulty compared to the state of the art where N _{1, i} = N ₁ ∀ i. Indeed, equation (2) shows that the choice of the flow rate of the source s depends even under the assumption GA on the ratios Thus, by making another hypothesis known as GAI, the same value a _GA of the ratio α _i is fixed for all the sources as follows:

Finally, the search for the optimal pair for the source s under the assumptions GA and GAI can be written in the form of a one-dimensional optimization conditioned on : where is the bit rate of the source s after optimization, R _s is the bit rate variable and where P (H) is the joint probability of the realizations of the channels of all the links of the network. It is clear that the rate R _s depends on the selected nodes â _; and therefore the selection strategy applied by the destination.

A third hypothesis GA2, consisting in considering an equiprobable random selection strategy of the active node at time l = 1, -, T _max can be adopted, where T _max is the maximum number of retransmission. The approximation according to (8) with the assumptions GA, GAI, GA2 can be calculated by implementing a Monte Carlo process. An implementation mode is detailed by algorithm 1 (in Annex) for the source s, its principle is as follows. First, the reports are all set to Then, each bitrate value of the set of possible bitrates is considered one after the other according to a first loop on i. n _MCS is the number of modulation and coding schemes. A second loop on cnt makes it possible to average the individual BLER or on Nb_MC draws of channels according to the statistics given by the average SNRs of all links. Thus, inside the cnt loop all the channels are known resulting from a random draw.

The calculation of equation (8) is then carried out according to a Monte-Carlo process where the integral is replaced by a sum: the out variable of algorithm 1 corresponding to:

The conjunction of the assumptions GA, GAI and GA2 makes it possible to obtain a first approximation according to (8) of the initial flows and the ratios α _i of the sources which can constitute the initial starting point (iteration 0) of an iterative optimization algorithm known under the name of best response dynamics or “Best Response Dynamics”, an implementation of which corresponds to algorithm 2 (in Appendix).

Algorithm 2 is based on a Monte Carlo simulation calculation of the spectral efficiency using the cutoff events given by equation (6). for everything should be evaluated in the Monte Carlo loop based on the destination active node selection strategy and the P (H) statistic similar to Algorithm 1.

Calculation of spectral efficiency can follow the sequence given by algorithm 3.

The method calculates the BLER _i of the source i and the expectation of the number of retransmissions According to this mode, the method considers that the individual cut-off probability is a good approximation of individual BLER BLER _i of source i. The individual outage probability is the average of the individual outage event given by equation (6) on PH link statistics) of the MAMRC system. For a given rate of channel profiles of the MAMRC system and a set of ratios {R ₁ , ..., R _M , α _i , ..., α _M `conditioned on a selection strategy given by the destination, the process relies on the evaluation of the cutoff events of equations (5) and (6) to determine the decoding set at the destination node and the relay nodes. All the sources which are not in the decoding set of the destination after T _max retransmissions are declared out of service, i.e. their error counter is increased by 1.

Agorithm 3 has a main part and two subroutines.

The first subroutine is the check_outage (t, I _list , activated_nodes_list, which takes into account the current time interval t of retransmission and the subsets for which the common outage must be calculated for . This function expresses the event of equation (4) having as inputs the list of mutual information between the I _list nodes and the selected nodes as well as their set of decoded sources without error from l = 1 to t. Thus all the sub-games of the sub-game should be searched to get a result of a decoding set for the subset. These events are reviewed in the first loop of this function. This function also contains two other loops, one to calculate the sum rate of the subset current as well as the mutual information included, and another loop to calculate the additional mutual information included according to the nodes activated in the retransmission phase.

The second subroutine is the get_decoding_set (t, I _list , activated_nodes_list) function which determines the decoding set at a certain time interval t by taking into account the previous parameters I _list and activated_nodes_list set determined. The principle of this function is to find the sub games cardinality Card _max up that leads to a common cutoff event zero. Indeed,

Whatever the source added to B, the associated common cutout event is equal to 1 by definition,

It is not possible to have multiple subsets with the same Card _max cardinalities associated with a common cutoff event equal to zero. If it were possible, a subset of greater cardinality associated with a common cutoff event equal to zero would be obtained by performing the union of these subsets.

The main part has four loops. As for algorithm 1, there is the Monte-Carlo loop such that each of its iterations leads to an individual cut event as well as an expected duration of retransmission for the second phase. The functions defined previously are used to check the cutout event.

If there is no outage then the process proceeds to a new iteration and the counters of the individual outage events as well as the expected retransmission time counter do not change.

If there is an outage then the process starts a 2 ^nd loop, taking into account the retransmission phase and a certain allocation strategy whereby a certain node is selected to retransmit and is added to the set of activated nodes. Once an outage event is reached during the retransmission phase time interval, the expected retransmission duration is updated. Finally, if the number T _max of retransmission time slots is reached, the method checks whether there is an outage or not and updates the individual outage event counter of each user accordingly. After the end of the Monte Carlo loop, the individual BLERs and the expected length of retransmission time slots are calculated using an average. The spectral efficiency is then calculated according to equation (3).

Annex

Algorithm 1 - Monte-Carlo approach to determine the flow rates under the “Genie Aided” hypothesis of the source s:

1.calculation of

2. 1 ^st bung out of 1 from 1 to n _MCS

3.

4. Initialization of the out counter of Monte-Carlo realizations (of H-channel matrix) which lead to a cutoff: out - 0, of the number counter cumulative time intervals (rounds) used during the second phase: according to the “Genie Aided” hypothesis.

5. 2 ^nd loop on cnt beyond> b_MC (example Nb_MC> 1000)

6. determine H _cnt on the basis of P (H) the joint probability of the realizations of the channels of all the links h _{α, b} . η calculate for all links

8. so

9.

10.continue or return to point 5, (no change in the values of the counters or

11. end of if

12. 3 * loop on t from 1 to Tmax

13. selection of the node by the destination by applying a strategy of selection (for example random selection according to hypothesis GA2)

14. calculate

15. if then

16. (the number of time intervals (rounds) used in the current realization of the Monte-Carlo simulation)

17. break the output of the ^3rd loop to move item 23 (no change in the value of out counter)

18. end of if

19. if t ~ T, _mx then

20. out = * out + 1 26. determine the average outage probability of the source s for the flow:

27. determine the average number of time intervals (rounds) used during the second phase:

28-

29. choose the maximum flow rate R _s that the source can use: such as .

Algorithm 2 - “Best response dynamics algorithm” (BRD) for each individual BLER under QoS constraint

1.t ← 0 (counter of the number of iterations)

2. Initialize the reports and flow sets:

3. The flow rates and the reports have been initialized under the assumptions

6.t ← t + 1

7.for i ← 1 to M do (For each source choose the best knowing 9. Such as (which satisfies the constraint)

10. End of for

11. End of While

Algorithm 3 Monte-Carlo Simulations to determine the spectral efficiency and BLER events used in the BDR algorithm:

1. initialization of E _total; : cumulative number of rounds of the 2nd phase on NB_MC draws of channel initialization of the table cumulates the number of events of channel prints random draw of H on the basis of P (H) the joint probability of the realizations of the channels of all the links h _{α, b}

4.compute for all the links for the place between nodes a and b.) their associated set of decoded sources during retransmissions)

6.2 ^nd loop on t from 1 to T _max

7.

8.if out ≈ 0 then

9.

10. 11. break or weed from plug 2 to go to point 2: 12. Otherwise

13.

14.

15. 16. dn selection by the destination by applying a selection strategy;

17. addition of and I 'associated decoded sources to the list

18. then

Claims

1. Transmission method, implemented by an OMAMRC telecommunication system with M sources possibly L relays (r ₁ ..., r _L ), and a destination (d), M ≥ 2, L ≥ 0, comprising an initial phase and, for each frame to be transmitted, a first phase of successive transmission of the M sources during M time intervals of N _{1, i} uses of the channel per source s _i i∈ {1, ..., M] and a second phase reserved for one or more cooperative transmissions of N ₂ uses of the channel per time interval allocated to one or more nodes taken from among the M sources and the L relays, a use of the channel consisting of a time-frequency resource, with adaptation of link controlled by the destination of the type allocating initial rates on the basis of a knowledge of a statistical distribution of the channels of the system, characterized in that the one or more nodes taken from among the M sources and the L relays are selected according to a selection strategy and according to which the sources are informed by the destination of the values of a ratio of uses of the channel between the two phases determined for each source, the number N _{1, i} of uses ns of the channel allocated to each source to transmit during the first phase of transmission is variable between the sources and determined by the destination during link adaptation by determining a maximum of an average utility function for values of the ratios d 'uses of the channel taken from a finite set of discrete values and constrained by an average individual BLER for each source.

2. The transmission method according to claim 1, according to which the values of a ratio of uses of the channel between the two phases determined for each source belong to the finite set of discrete values {1, 0.5, 2}.

3. Transmission method according to one of claims 1 and 2, according to which the maximization of an average utility function under the constraint of an average individual BLER for each source is expressed in the form: under constraint that with

• R _i a variable representing the initial bit rate allocated to the source i, i∈ {1, ..., M}

• T _used the number of cooperative transmissions used during the second phase, • E (T _used ) an average of the number of cooperative transmissions used during the second phase,

• α _i = N ₂ / N _{1, i} a variable representing the ratio between the number of uses of the channel during the second phase and the number of uses of the channel during the first phase for the source i, i∈ {1, ..., M), the probability of the event of individual cut-off of the source s at the end of T _used to be less than a given quality of service QoS _{s value} .

4. Transmission method according to one of claims 1 to 3, the method is such that the maximization of the average utility metric comprises an initialization of the ratio for each source to the discrete value closest to an average of the values. of the report and independently determining each rate of a source assuming that all messages from the other sources are known to the destination.

5. The transmission method according to claim 4 in that the initial values are modified by means of an iterative calculation of the initial rates by the destination which takes into account a strategy for selecting the nodes during the second phase.

6. System comprising M relay sources (r ₁ , ..., r _L ) and one destination (d), M ≥ 2, L ≥ 0, suitable for implementing a transmission method according to one of claims 1 to 5.