EP4173167A1

EP4173167A1 - Omamrc method and system with fdm transmission

Info

Publication number: EP4173167A1
Application number: EP21740143.9A
Authority: EP
Inventors: Raphaël Visoz; Ali AL KHANSA
Original assignee: Orange SA
Current assignee: Orange SA
Priority date: 2020-06-24
Filing date: 2021-06-18
Publication date: 2023-05-03
Also published as: CN115769510A; WO2021260308A1; US20230246704A1; FR3111495A1

Abstract

The present invention relates to a message method intended for an OMAMRC telecommunication system with M sources s_i iε{ 1,..., M], potentially L relays (r₁..., r _L ) and a destination. The transmission is of FDM type in a band divided into B mutually orthogonal sub-bands. The method comprises: a simultaneous transmission of the M sources in a time interval with allocation of at least one sub-band per source and at least one cooperative retransmission for a time interval of at least one relay node taken from among the M sources and the L relays which is selected using a selection strategy with allocation of at least one sub-band per node selected.

Description

DESCRIPTION

TITLE: OMAMRC process and system with FDM transmission

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, made up 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 the English terminology).

Prior art

An OMAMRC transmission system implementing a slow link adaptation is known from application WO 2019/162592 published on August 29, 2019. The OMAMRC telecommunication system describes has M sources, possibly L relays and a destination, M ≥ 2, L ≥ 0 and it uses an orthogonal time 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 successive transmissions from M sources and T _used ≤ T _max slots for one or more cooperative retransmissions 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. The 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. The relay differs from a source because it has no message to transmit of its own, ie it only retransmits messages from other nodes. The relay always performs a cooperative retransmission.

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 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 overhead of the return channel (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 slow, that is to say that before any transmission, the destination allocates initial rates to sources knowing the distribution of all 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 (slow fading assumption). The 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 throughput for each source taking into account the average quality (eg 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 refresh takes place on a long time scale, that is to say over a period of 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 messages from the sources are retransmitted in a cooperative manner 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 use is fixed and identical for each of the sources.

The independent sources broadcast during the first phase their coded information sequences in the form of messages to 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 (time slots) each dedicated to a source.

Sources other than the one transmitting and possibly the relays, of the "Half Duplex" type receive successive messages from the sources, decode them and, if they are selected, generate a message only from the messages from the sources decoded without error.

The selected nodes then access the channel orthogonal in time to each other during the second phase to retransmit their generated message to the destination.

The destination can choose which node should retransmit at any 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 from the discrete values taken in a finite set the number of flow rates corresponding to the different coding and modulation schemes (MCS, Modulation and Coding Scheme) available for transmission,

• T _used ≤ T _max presents the number of cooperative retransmissions used during the 2 ^nd phase, E, (T _used ) is the average of the number of cooperative 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: BLERi ≤

BLER _QoS I, ∀i ∈ {1, ..., M). The algorithm based on an interference-free or “Genie Aided” approach is used to solve the problem of optimization of multidimensional allocation of throughput (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. "Genie Aided". The utility metric which consists of spectral efficiency is conditioned by the node selection strategy that occurs during the second phase.

Main characteristics of the invention

The present invention relates to a method of transmitting messages intended for an OMAMRC telecommunication system with M sources s _t ie {1, ..., M), possibly L relay r ₁ ..., r _L and a destination, M ≥ 2, L ≥ 0, M ≤ B. The transmission is of the LDM type on a band divided into B orthogonal sub-bands between them. The method is such that it comprises: a simultaneous transmission of the M sources during a time interval with allocation of at least one sub-band per source and at least one cooperative retransmission during a time interval of at least one relay node taken from among the M sources and the L relays selected according to a selection strategy with allocation of at least one sub-band per selected node, to maximize a quality of service metric.

The allocation of sub-bands between the sources makes it possible to reduce the time necessary for transmitting data since the sources transmit simultaneously in one and the same first time slot (time slot). Such a process is therefore well suited for services requiring in terms of latency. The allocation of one or more sub-bands per source as well that the source selection strategy at subsequent intervals, are performed to maximize a quality of service metric, eg BLER, spectral efficiency. Maximizing the quality of service makes it possible to optimize the bit rate or to reduce the transmission power of the sources for the same bit rate.

The slot or slots following the first time slot are dedicated to retransmissions including at least one cooperative retransmission. A cooperative retransmission is either a transmission by a relay or a transmission by a source capable of helping the destination to decode at least one other source. We will call hereafter a non-cooperative retransmission, a retransmission by the source of its own message. A cooperative retransmission is a transmission by one node that contains information about at least one message from another node. The transmission of a relay is by nature a cooperative retransmission but also the transmission of a source (which is capable of cooperation) which includes in its transmission information relating to at least one message from another source. The cooperation of the relay nodes ensures an increase in the reliability of the transmissions.

According to one embodiment, the selection strategy is such that a relay node which decodes a set of sources at a time interval t can only cooperate at a time interval t + 1 for a single source of its set.

This embodiment makes it possible to obtain a direct expression of the individual cut-off event of a source, that is to say without the need to obtain the cut-off events of all the subgroups of the sources containing the source considered. The choice of the source, among the sources not yet decoded without error by the destination, with which the node cooperates can be random, the transmission from the node to the destination therefore includes the indication of the source with which it cooperates. Further, the common cutoff of a set of sources is obtained simply as the union of the individual cuts of the sources of the set.

According to one embodiment, the method is such that: the destination broadcasts to the relay nodes its set of correctly decoded sources among the sources received during a transmission interval, the relay nodes which have correctly decoded a source not correctly decoded by the destination informs the destination, the destination broadcasts to the relay nodes a vector α _t comprising the relay nodes selected for the subbands for cooperative or non-cooperative retransmission during the next transmission interval.

According to this protocol, the destination sends its set of correctly decoded sources back to the relay nodes after receiving data transmitted during a transmission interval. This feedback can take place via a control channel. According to one embodiment particularly simple, the destination goes up M bits which indicate for each of the M sources whether it is correctly decoded or not. If all the sources are correctly decoded by the destination ie its set of correctly decoded sources contains the M sources, a new frame is transmitted.

According to one embodiment, a relay node informs the destination by transmitting a single bit in a control channel.

According to this mode, the signaling of the relay nodes to the destination is minimal and therefore has the advantage of consuming very little channel resource. With this information, the destination can implement a selection strategy which, for example, consists in maximizing at a given time interval t the sum of the mutual information between the nodes that can help with their allocated subbands and the destination

According to one embodiment, a relay node informs the destination by transmitting its set of correctly decoded sources.

Compared to the previous mode, the signaling of the relay nodes to the destination according to the latter mode consumes more channel resources. But the information transmitted allows the destination to select relay nodes more efficiently to help it decode as many sources as possible.

According to one embodiment, the destination selects the relay nodes allowing it to correctly decode the most sources at the end of the cooperative or non-cooperative retransmission.

According to this mode, the destination returns to the relay nodes a vector in which are selected the relay nodes which maximize the number of sources correctly decoded by these relay nodes and not yet correctly decoded by the destination. The vector further includes the allocation of the subbands to the selected relay nodes.

According to one embodiment, the destination selects the relay nodes such that the sum of the mutual information between the nodes which can help with their allocated subbands and the destination is maximized.

In the case where several vectors a _t can lead to the same maximum number of newly decoded sources, the method chooses the vector which maximizes the sum of the mutual information between the nodes which can help with their associated subbands and the destination:

According to one embodiment, the method is with slow link adaptation and is such that rates allocated to the sources are determined to maximize a metric expressed as an average utility function under constraint of an average individual BLER for each source: a variable representing the initial flow allocated to the source i, i ∈ {1, ..., M)

K _j the number of data transmitted on n _{o, i} XF uses of the channel by source i,

• T _{used the} number of time slots used for cooperative / possibly non-cooperative retransmissions,

• E (T _used ) ^an average of the number of time slots used for cooperative / possibly non-cooperative retransmissions,

• BLER _j the block error rate for source i.

According to one embodiment, the method is with rapid link adaptation and is such that the bit rates allocated to the sources are determined to maximize a metric expressed in the form of an average utility function under the constraint of the individual cuts of the sources:

O _{i, t} the individual cut-off probability of source i at the cooperative / possibly non-cooperative retransmission interval t,

• T _used Ie number of time slots used for cooperative / possibly non-cooperative retransmissions, a variable representing the initial bit rate allocated to the source i, i ∈ {1,, M}.

A further subject of the invention is a system comprising M sources ..., s _M , L relay r ₁ ..., r _L and a destination d, 0, for implementing a transmission method according to invention.

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.

The invention further relates to 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 from 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 transmission cycle of a frame according to an exemplary implementation of the invention,

[Fig 3] FIG. 3 is a diagram of the protocol for the exchange of information between the destination and the nodes, sources and relays, according to one embodiment of the invention.

Description of particular embodiments Channel use is the smallest granularity in time-frequency resource defined by the system which allows the transmission of a modulated symbol. The number of times the channel is used is related to the available frequency band and the transmission time.

An OMAMRC system is illustrated by FIG. 1. Such a system comprises M sources which belong to the set of sources S = (1, ..., M), L relays which belong to the set of relays R = (M + 1,. .., M + L) and a destination d.

Each source in the game S communicates with the unique 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, relays, and 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 _i information bits of each source i to determine whether a message is correctly decoded or not, i ∈ S;

- 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 of 1 + T _max time intervals, but can change independently from one frame to another. T _max ≥ 1 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 in an orthogonal frequency multiple access pattern and operate in a full-duplex mode that allows them to listen to transmissions from other nodes without interference.

The channel strip is divided into B sub-bands the number of which is assumed to be greater than or equal to the number of sources: B ≥ M. Each sub-band associated with a time interval determines F uses of the channel (F element resource).

In the case of transmission with OFDM modulation, a sub-band can include, for example, as many sub-carriers as an OFDM symbol.

The number N of channel uses is assumed to be identical for each transmission interval: N = B X F.

A transmission cycle lasts 1 + T _used timeslots with T _used ≤ T _max and T _max the maximum number of timeslots. At each time interval, none, one or more sub-bands are allocated to a node according to a first partition.

During the first time interval (first phase) all the sources transmit, assuming that B ³ M, respectively on one or more sub-bands allocated to each source.

During the following so-called retransmission intervals (second phase), only the nodes selected from among the sources and the relays retransmit and their retransmission takes place on the sub-band (s) which are respectively allocated to them according to a partition determined for each current interval. Thus, the partitions can be different between all the transmission intervals including the first.

The selection of the nodes and the allocation of the sub-bands are implemented by a scheduler, typically hosted by the destination.

The following notations are used:

• if i ≤ M the selected node i is a source i denoted S _i , i ∈ {1, ..., M) otherwise i> M and the selected node is a relay i - M denoted r _iM , i ∈ {M + 1,, M + L},

• a _t ∈ {SU 9V) ^B is the vector of dimension B of the nodes selected for the transmission interval t, both during the first phase and during the second phase. The i ^th element a _ti of the vector a _t designates the i ^th sub-band and the selected node active during this time interval t in this sub-band i, i ∈ {1, ..., B}. The order in the vector corresponds to the order of the sub-bands.

• n _t ∈ {0, ..., B} ^{M + L} is the vector of dimension M + L of the number of sub-bands allocated for each node which varies between 0 (the node is inactive) and B (the node occupies all sub-bands), source or relay, for the transmission interval (time slot) t whether during the first phase or during the second phase. The i ^th element n _ti of the vector n _t designates the number of sub-bands allocated to node i at the transmission interval (time slot) t, i ∈ {1, ..., M + L}. The sum of the component elements the vector n _t is equal to B the number of sub-bands.

• h _ab is the attenuation gain of the channel (fading) between node a (source or relay) and node b (source, relay or destination) which follows a complex circular symmetric Gaussian distribution with zero mean and variance y _{a , b} , the gains are independent of each other,

• T _used ^is the minimum number of retransmission time intervals ie during the second phase which leads to zero faults for all the sources (the individual cut-off event of each of the sources is equal to zero): (1)

The individual cut-off event of the source s after the interval t (round t) retransmission depends on the vector _t of selecting nodes, the vector n _t allocation of sub-bands and the game _{has been} $ _i sources decoded at the end of the preceding interval, t-1. It is also conditional on the knowledge of the realizations of the channel of the direct links h _dir (of the gains of the channel) as well as of P _t -1P _t - i designates the set of selection vectors (therefore of the selected nodes) and of the allocation vectors with their associated set of decoded sources <¾ _{, ί} -i determined for the intervals (rounds) l preceding the interval t, l ∈ {1, ..., t - 1} and the set S _{d t -1} of sources decoded by the destination. It should be noted that a ₀ is the selection vector of the source nodes transmitting during the transmission phase, that n ₀ is the allocation vector of sub-bands allocated for each source during the transmission phase and that _< S _{d 0} is the set of sources decoded by the destination at the end of the first phase.

The common failure event F ^or ^I and sub set of sources S after time interval t (round t) is the event that at least one source of subset B is not decoded correctly by the destination at the end of this interval t. Subsequently, the dependencies of are omitted to simplify the notations. We denote I ^e j ^had sources not successfully decoded by the destination at the end of the time interval t (round t). From an analytical point of view, the event of common cut-off of a subset S of sources occurs ie is satisfied if the vector of the bit rates of these sources is not included in the corresponding MAC capacity region.

Thus, for a given subset of sources, for a vector _t nodes candidate selected and the _{corresponding sub-band allocation vector n t} , this event can be expressed in the form: (2) reflects the non-respect of the MAC inequality associated with the sum rate of the sources contained in 11: (3) with l the time interval index (round) of the second phase with the convention that l = 0 corresponds to the end of the first phase (transmission phase), l ∈ {1, .., T _used } _> s the index corresponding to the source node, s ∈ {1, ..., M), i the index corresponding to any node (source and relay), i ∈ {1, ..., M + L), rti i the number of sub-bands allocated to node i for the time interval l (round) l G {1, ..., T _{use (i} }, n _{0 s} the number of allocated sub-bands at the source s G {1, ..., M) by the destination for the first phase, · (4) , represents the set of interfering sources, C _{l: i} is equal to one if on the one hand the intersection between the set of sources correctly decoded by node i at the interval l - 1 and the set U is not empty and on the other hand the intersection between the set of sources correctly decoded by node i at the interval l - 1 and the set of interfering sources is empty,

A represents the logical "and",

[P] represents the brackets of Iverson i.e. which gives the value 1 if the event P is satisfied and the value 0 if not,

T _{; j} the mutual fading information block from node i to destination d for the rti i sub-bands allocated to node i at time interval l ∈ {1, .., T _used ): (5) where I _{ai /, d} ^is the mutual information between the node a which is allocated the sub-band / at the time interval (round) l E {1, ..., T _used } and the destination d. Mutual information depends on the power transmitted on the sub-band of the channel ie between node a _{; t} and the destination d with P _T the total power of this node. If node i is not selected at time interval l then the mutual information block is zero.

• _S the mutual fading information block from the source s to the destination d, for a ₀ andn ₀ given, at the time interval corresponding to the transmission phase (first phase),

• is the bit rate used during the first phase with K _s the number of useful information bits transmitted over n _{0 S} F channel uses.

Subsequently the cutout event for a given source s is defined in the form: which is by definition the intersection of all the common breaking events corresponding to a set of sources Έ including the source s. A source s is cut off if and only there is no set of sources Έ it including which can be associated with an error-free decoding, ie, e _ίΈ = 0. It comes:

This cut event indicates whether a source is decoded without error or if it is cut off (O _st = 1). This approach makes it possible to predict the result of implementing a parity check (CRC check) without going through the simulation of the entire transmission (modulation coding) and reception (detection / demodulation, decoding) chain. ). In this, it defines an abstraction of the physical layer. Certain adjustments obtained by simulation (called calibration within the framework of the abstractions of the physical layer) for a given coding scheme can be carried out by introducing weighting parameters of the mutual information and / or SNRs of the links. The two transmission phases of the transmission method can be preceded by an initial phase of determining an initial rate. This phase can occur once every several hundred frames (ie each time the quality statistics of the channel / link change) in the “slow fading” case, we speak of slow link adaptation. Or this phase can occur much more frequently and at most each cycle, we speak of rapid link adaptation. Whether the link adaptation is fast or slow, the bit rate of each source and the allocation of sub-bands are known before the start of transmission. By using reference signals (pilot symbols of the DMRS 3GPP LTE / NR type, reference signals of the SRS 3GPP LTE / NR type, etc.), the destination can determine the gains (CSI Channel State Information) of the direct links that is, source-to-destination and relay-to-destination links. The destination can therefore deduce average values for the direct links within the framework of a slow adaptation.

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 in the context of a slow adaptation, 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.

For rapid adaptation, optimizing spectral efficiency may rely on knowing all or some links. One possible solution, but very expensive in terms of control, is for the sources and the relays to feed back the coefficients of the links (quantified by sub-band) that they can estimate at the destination.

During the initial link adaptation phase which precedes the transmission of one or more frames, the destination transmits for each source s a representative value (index, MCS, bit rate, etc.) of an initial bit rate R _j . Each of the initial rates unambiguously determines an initial modulation and coding scheme (MCS) or conversely each initial MCS determines an initial rate. The rise of the initial flow rates R _[ is carried out via very limited flow control channels.

These initial rates are determined by the destination so as to maximize a quality of service metric, for example an average spectral efficiency.

In the case of a slow link adaptation, the quality of service metric is, according to one embodiment, an average spectral efficiency which is expressed in the form:

(6) with

• a variable representing the initial flow allocated to the source i

• K _i the number of data transmitted on n _oi XF uses of the channel by source i,

• T _used I ^e number of time slots used for cooperative or non-cooperative retransmissions, • E (T _used ) ^an average of the number of time slots used for the retransmissions whether they are cooperative or non-cooperative,

• BLER _j the average block error rate for source i.

In the case of a slow link adaptation, the bit rate and the allocation of sub-bands by source remain unchanged for several hundred transmissions of messages from the sources, which makes it possible to average the block error rate (BLER) of the source i on the channel statistics (CDI: Channel Distribution Information) known to the destination. A source i takes n _oi XF resource elements to transmit at the rate R _j the K _j data during the time interval of the first phase.

In the case of rapid link adaptation, the quality of service metric is, according to one embodiment, an average utility function under constraint of the individual cuts of the sources defined by transmitted message and the throughput and the allocation of sub. -bands can change from one message to the next:

(7) with

• O _{it is} the individual cut-off event of source i at the retransmission interval t which is equal to one in the event of a fault or zero in the event of success (correctly decoded source), O ^ ₀ is the individual cut-out event at the end of the transmission phase (first phase of a time interval),

• T _Used I ^e number of time slots used for cooperative or non-cooperative retransmissions (second phase can take the value 0 if

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

An embodiment of a transmission method according to the invention is described in support of the diagram of FIG. 2 which illustrates a transmission cycle of a frame in the context of an OMAMRC system with three sources, i = {1, 2, 3) two relays, i = {4,5), a destination and a transmission channel having a certain bandwidth. The band is split into B = 5 sub-bands and each sub-band associated with a time interval determines F = 5 uses of the channel, ie N = 25.

During the first phase of a time interval, each source i = {1, 2, 3) transmits its code words. According to the example, the number of sub-bands allocated to a source differs between the sources. Thus, the sub-bands fi, Î2 and fs are allocated to the source 1, the sub-band f ₃ is allocated to the source 2 and the sub-band f * is allocated to the source 3. The selection vector is therefore a ₀ = . The vector of allocation of sub-bands by node is thus During the second so-called retransmission phase and for the first time interval, only the sources 2, 3 and the relay 2 are selected and the sub-band fi is allocated to the source 3, the sub-bands f ₂ , Î ₃ and f * are allocated to relay 5 and the sub-band fs is allocated to source 2. The selection vector is therefore a ₁ = [s ₃ , r ₂ , r ₂ , r ₂ , s ₂ ] ^T = [3, 5,5,5,2] ^r . The sub-band allocation vector per node is therefore = [0,1,1,0,3] ^T.

During the second so-called retransmission phase and for the second time interval, only the source 1 and the relay 4 are selected and the sub-bands f ₁ , f ₂ and fs are allocated to the relay 4, the sub-bands f ₃ and f ₄ are allocated to source 1. The selection vector is therefore a ₂ =

= [4.4, l, l, 4] ^T. The sub-band allocation vector per node is therefore n ₂ = [2,0,0,3,0] ^T.

An embodiment of the protocol for the exchanges between the nodes and the destination is illustrated in FIG. 3.

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 respectively M sources. The transmission of a frame (which defines a transmission cycle) takes place during 1 + T _used time intervals: 1 interval for the 1 ^st phase of capacity n _oi uses the channel for each source i, T _used intervals for the 2 ^e capacity phase n _ti uses the channel for each source i.

During the first phase, each source s ∈ S transmits after coding a message u _s comprising K _s information bits u _s being the two-element Galois body. the message u _s includes a CRC type code which allows the integrity of 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 (RCPCC rate compatible punctured convolutional code), LDPC rate compatible (RCLDPC rate compatible low density parity check code). Thus, during the first phase, the M sources simultaneously transmit their message during the transmission interval on the sub-bands allocated in accordance with the vector a ₀ , with respectively modulation and coding schemes determined from the values of the initial rates .

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

Whether in the first phase or the second phase, when a node, especially a source transmits, the destination and the other nodes listen. Each full-duplex node can transmit and listen simultaneously to all the other nodes given that, to transmit, each node is allocated one or more sub-bands which are different between the nodes.

The destination, sources and relays attempt to decode messages received at the end of a time interval. Success of decoding at each node is decided using CRC. The destination and the nodes thus determine their set of correctly decoded sources.

During the second phase, at the time interval (round) t, the destination d transmits its set of correctly decoded sources at the end of the previous time interval S _dt _ ₁ using for example a control channel of feedback broadcast control channel. t = {1, ..., T _used ). This return can consist of a vector of M bits.

If the decoding of all the sources by the destination is correct S _dt _ ₁ = S. In this case, the current cycle is stopped, a new cycle can start. The 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. The number of timeslots (rounds) used during the second phase T _used = {1, ..., T _max } depends on the decoding success at the destination.

The nodes, sources and relays, compare the set S _dt _ ₁ to their set of correctly decoded sources.

If the set of a node includes at least one source not included in the set S _dt _ ₁ of the destination, the node informs the destination thereof by using for example a dedicated control channel of unicast type. The information transmitted by a node can consist of its set of correctly decoded sources or, as illustrated by FIG. 3, of a bit, for example set to one.

During this second phase, the destination follows a certain strategy to decide which node (s) selected to transmit at the time interval (round) t.

The destination informs the nodes of this selection by transmitting the vector a _t using for example the return broadcast control channel. Each node which receives the vector a _t can determine whether it is selected and on which sub-band (s) it should transmit.

During this second phase and for at least one retransmission interval among the T _used retransmission intervals, at least one selected node, source or relay, generates a cooperative retransmission. Outside of the at least one time slot, the retransmissions can be cooperative or non-cooperative.

The node selected for retransmission transmits u _at after multi-user encoding, the words or part of the words that it has correctly decoded. The selected node can transmit parities determined from messages from its correctly decoded source set using Joint Network Channel Coding. 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 destination thus controls the transmission of the nodes using a return channel. This improves spectral efficiency and reliability by increasing the likelihood that all sources will be decoded by the destination.

Selection strategy

According to a first strategy, the destination selects those which maximize the sum of the mutual information among the set Zt of the different allocations of sub-bands to the nodes which can help at the interval t. This strategy only requires knowledge of the nodes that can help, it is compatible with the way in which the nodes transmit information in the form of a bit.

The selection criterion can be expressed in the form: (8) with Z _t the set of _{possible vectors a t} which correspond to the selection of the nodes which can help the destination at the interval (round) t.

On the basis of the sets of correctly decoded sources received from the nodes and according to a second strategy, the destination selects the nodes which make it possible to obtain the most newly correctly decoded sources at the destination at the end of the current interval t ie which maximize the cardinality of the set of sources correctly decoded by the destination at the end of the current interval t.

According to this strategy, the method reviews all the possible values of the vector a _t and retains the one which leads to the greatest number of newly decoded sources. Thus, the method does not take into account the nodes which cannot help the non-sources. still decoded since it targets the greatest number of newly decoded sources, ie; only the nodes i which satisfy

This strategy further requires knowledge of the correctly decoded source set of all previously selected nodes. In the case where several vectors a _t can lead to the same maximum number of newly decoded sources, the method chooses the vector a _t which maximizes the sum of the mutual information. In fact, at a time interval t, this is the only element that can be maximized to maximize the right part of the individual cut-off events and of the common cut-off events. The presence of C _ti in the expression of the common cut event reflects the fact that the only nodes that can be selected are those that can help, ie having decoded at least one source not yet decoded by the destination. The selection criterion can then be expressed in the form: (9) with A _t the set of candidate nodes which maximize the clearance of the destination at the end of the interval (round) t.

It can be noted that for t = 0, the only candidate nodes for the first phase are the sources, their decoding set corresponds to itself and the relay nodes have an empty decoding set.

Claims

1. Method of transmitting framed messages intended for a telecommunication system with M sources ie {i, ..., M}, possibly L relay (r ₁ ..., r _L ) and a destination (d), M ≥ 2, L ≥ 0, M ≤ B with orthogonal multiple access to the transmission, called OMAMRC, characterized in that the transmission is of the so-called FDM frequency division multiplexing type on a band divided into B orthogonal sub-bands between them and the method is such that it comprises: simultaneous transmission of the M sources during a time slot with allocation of at least one sub-band per source and at least one cooperative retransmission of a time slot of at least one relay node taken from among the M sources and the L relays selected by the destination with allocation by the destination of at least one sub-band per selected node, knowing the sources correctly decoded by the nodes the destination selects the nodes which make it possible to obtain the most newly correctly decoded sources at the destination at the end of the retransmission cooperative.

2. Message transmission method according to claim 1, wherein the selection by the destination is such that a relay node which decodes a set of sources at a time interval t can only cooperate at a time interval t + 1 for only one source of his game.

3. Message transmission method according to claim 1, according to which: - the destination broadcasts to the relay nodes its set of correctly decoded sources among the sources received during a transmission interval, the relay nodes which have correctly decoded a source not correctly. decoded by the destination informs the destination, the destination broadcasts to the relay nodes a vector (a _t ) comprising the relay nodes selected for the subbands for cooperative or non-cooperative retransmission during the next transmission interval.

4. The transmission method according to claim 3, wherein a relay node informs the destination by transmitting its set of correctly decoded sources.

5. The transmission method according to claim 1, according to which the destination selects the relay nodes such that the sum of the mutual information between the nodes which can help with their allocated sub-bands and the destination is maximized.

6. Transmission method according to one of the preceding claims with an initial phase of determining initial rates and such that the initial rates allocated to the sources are determined to maximize a metric expressed in the form of an average utility function under constraint. of an average individual BLER for each source: _ayec

• a variable representing the initial flow allocated to the source i, i ∈ {1, ..., M)

• K _j the number of data transmitted on n _{oi X} F uses of the channel by source i,

• T _Used I ^e number of time slots used for cooperative or non-cooperative retransmissions,

• E (T _used ) an average of the number of time slots used for cooperative or non-cooperative retransmissions,

• BLER _j the block error rate for source i.

7. The transmission method according to one of claims 1-5 with an initial phase of determining initial rates for each frame and such that the initial rates allocated to the sources are determined to maximize a metric expressed in the form of a function d. '' average utility under constraint of individual power cuts: with

• O _{i, t} the individual cut-off probability of source i at the cooperative or non-cooperative retransmission interval t,

T ^e I _used many cooperatives broadcasts or uncooperative,

• a variable representing the initial flow allocated to the source i, i ∈ {1, ..., M) n _{0 j} a number of sub-bands allocated to node i for the time interval 0, i ∈ {1. M},

• F a number of uses of the channel.

8. System comprising M sources ..., s _M ), L relay (r ₁ ..., r _L ) and a destination (d), M ≥ 2, L ≥ 0, suitable for an implementation of a transmission method according to one of claims 1 to 9.