WO2024002899A1

WO2024002899A1 - Transmission method and omamrc system with a selection strategy during retransmissions taking into account the throughput of the sources and of a single control exchange

Info

Publication number: WO2024002899A1
Application number: PCT/EP2023/067145
Authority: WO
Inventors: Ali AL KHANSA; Raphaël Visoz
Original assignee: Orange
Priority date: 2022-06-28
Filing date: 2023-06-23
Publication date: 2024-01-04
Also published as: FR3136915A1

Abstract

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

Description

DESCRIPTION

TITLE: Transmission method and OMAMRC system with a selection strategy during retransmissions taking into account the flow rate of the sources and a single exchange of control 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 one node which can be a relay or a source.

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

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

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

Such a sensor network is a multi-user network, made up of several sources, several relays and a recipient using an orthogonal multiple access scheme in time of the transmission channel between the relays and the sources, denoted OMAMRC ("Orthogonal Multiple- Access Multiple-Relay Channel” according to Anglo-Saxon terminology).

Prior art

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

Such an OMAMRC transmission system implementing a selection strategy during the second phase is known from the article by S. Cerovic, R. Visoz, and L. Madier entitled "Efficient Cooperative HARQ for Multi-Source Multi-Relay Wireless Networks.", 14th International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob). IEEE, 2018. The OMAMRC transmission system described is such that each of the sources can operate at different times either exclusively as a source or as a relay node. The node terminology covers both a relay and a source acting as a relay node or as a source. A relay is distinguished from a source because it does not have its own message to transmit, ie it only retransmits messages from other nodes.

The links between the different nodes of the system are subject to slow fading and white Gaussian 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 too much exchange of information between the sources, the relays and the destination . To limit the cost of feedback channel overhead, shown in dotted lines in Figure 1, only information on the channel distribution/statistics (CDI: Channel Distribution Information) of all links, e.g. average quality (for example average SNR, average SINR) of all links, is assumed to be known by the destination in order to determine the flow rates allocated to the sources.

Link adaptation is said to be slow, meaning that before any transmission, the destination allocates initial flow rates to sources knowing the distribution of all channels (CDI: Channel Distribution Information). In general, it is possible to trace the CDI distribution based on knowledge of the average SNR or SINR of each link in the system.

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

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

During the initialization phase, the destination determines an initial rate Ri for each source S[ taking into account the average quality (for example SNR) of each of the links in the system. The destination estimates the quality (for example SNR) of the direct links: source to destination and relay to destination according to known techniques based on the exploitation of reference signals. The quality of the source - source, relay - relay and source - relay links is estimated by the sources and the relays by using, for example, the reference signals. Sources and relays transmit to the destination the average qualities of the links. This transmission occurs 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 makes it possible to average out the rapid variations (fast fading) of the channel. This time is of the order of the time necessary to travel several tens of wavelengths of the frequency of the transmitted signal for a given speed of a node of the system. The initialization phase occurs for example every 200 to 1000 frames. The destination goes back to the sources via a return path the initial flow rates it has 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 intervals (time-slots) respectively using modulation and coding schemes determined from the initial bit rates. During this phase, the number of channel uses (channel use i.e. resource element according to 3GPP terminology) is fixed and identical for each source.

During the first phase, the independent sources broadcast their coded information sequences in the form of messages to a single recipient. Each source broadcasts its messages with its initial rate. The destination communicates to each source its initial rate via very limited rate control channels. Thus, during the first phase, the sources each in turn transmit their respective message during “time-slot” intervals each dedicated to a source.

Sources other than the one which transmits and possibly the relays, of the “Half Duplex” type, receive successive messages from the sources and decode them.

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

This phase lasts at most T _max time intervals (time-slots). During this phase, the number N ₂ of channel uses is fixed and identical for each of the nodes (sources and relays) selected.

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

The present invention meets this objective.

Main characteristics of the invention

The subject of the present invention is a method of transmitting a frame carrying messages intended for an OMAMRC telecommunications system with M sources,

possibly L relay and a destination,

0, the nodes operating in

half-duplex mode, according to an orthogonal multiple access scheme of the transmission channel between the N nodes with a maximum number of M + T _max time intervals per transmitted frame distributed between a ^1st phase and a ^2nd phase, 1 ≤ T _max , the message from a source having been coded before transmission according to incremental redundancy type coding which generates several redundancies, the ^1st phase comprises M intervals allocated respectively to the successive transmissions of the M sources and the ^2nd phase comprises at least a retransmission interval for a transmission of nodes having correctly decoded the same source s _i such that these nodes transmit simultaneously during the same retransmission interval the same redundancy of the message from the same source not yet correctly decoded by the destination, called source to help. The method according to the invention is such that it comprises: a single transmission by the nodes of at least their set of sources correctly decoded and not yet correctly decoded by the destination, these transmissions allowing the destination to determine, for each of these sources, a quality of an equivalent channel based on a quality of the channels between the nodes having correctly decoded this source and the destination, an estimate of a sufficient number of retransmission intervals (x _i (0)) so that the destination decodes a source (Si) not yet correctly decoded and correctly decoded by at least one node on the basis of the quality of an equivalent channel for this source between this at least one node and the destination and a bit rate (R _i ) assigned to this source (S _i ), a selection by the destination of the sources to be helped taking into account the estimated numbers (x _i (0)) of retransmission intervals sufficient for the destination to decode the sources not yet correctly decoded and a sum of flow rates (R _i ) attributed to the sources.

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

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

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

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

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

As the correct decoding by the destination of a source can occur before the end of the estimated number of sufficient retransmission intervals (cooperative transmissions), this mode allows another source to help to benefit from the unused time.

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

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

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

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

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

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

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 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 as part of the transmission process may possibly apply to the software application and the memory mentioned above.

List of Figures

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

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

Description of particular embodiments

A 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 uses of the channel is linked to the available frequency band and the transmission duration. An OMAMRC system is illustrated by Figure 1 already described.

An OMAMRC system according to the invention comprises M sources which belong to the set of sources, possibly L relays which belong to the set of relays

and a destination d. By convention, it is considered that and

in other words, we can confuse a source and its index, and a relay

and its index (shifted from the value M of the number of sources). Each source of game 5 communicates with the single destination with the help of other sources (user cooperation) and relays which cooperate.

Transmission cycle of a frame according to the invention

A transmission cycle of a frame according to an example of implementation of the invention is illustrated in Figure 2.

The method according to the invention distinguishes two phases for each frame to be transmitted, a ^1st phase and a ^2nd phase. The transmission of a frame is possibly preceded by an additional so-called initial phase during which the flow rates are allocated.

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

The N nodes operate in a half-duplex mode which allows them to listen to transmissions from other nodes without interference. Sources can behave like a relay when they do not only send their own message.

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

Each of the allocated rates determines in an unambiguous manner a modulation and coding scheme (MCS, Modulation and Coding Scheme) and conversely each MCS determines a rate. The allocated flow rates are reported from the destination to the sources via very limited flow control channels (shown in dotted lines in Figure 1).

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

- the sources, relays and destination are equipped with a single transmitting antenna (or transmitting antenna port);

- the sources, relays and destination are equipped with a single reception antenna (or reception antenna port);

- sources, relays and destination are perfectly synchronized;

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

- all nodes transmit with the same power;

- use is made of a supposed CRC code included in the K _i information bits of each source i to determine whether the message associated with the information bits 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 of M + T _max time intervals, but can change independently from one frame to another. T _max ≥ 1 is a system parameter;

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

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

The following notations are used: •

is a discrete variable representing the source rate ^^ provided by a link adaptation process implemented before the transmission of the frames, • is the number of retransmission intervals used during the ^2nd phase,

it corresponds to the number of transmissions during this phase, •

₁ is the ratio between the number of channel uses available at each time interval (slots) of the ^2nd phase and the number of channel uses available at each time interval (slots) of the ^1st phase, • is the set of sources not correctly decoded by the destination at the end of the retransmission interval

• is the set of sources correctly decoded by the node

at the end of the retransmission interval

• is the fault indicator (outage) which takes the value one when an individual fault event occurs and the value zero in other cases. represents the event of

source defect

ie the source is not decoded correctly after sending a frame given that the maximum number of retransmission intervals

was reached without this source ^^ being correctly decoded. The individual fault event O _{^^, ^^} depends on each retransmission interval

(slot) of the mutual information of the nodes having correctly decoded the source

, • represents the mutual information between the source

and the destination ^^, • represents the mutual information between all the nodes helping the source ^^ and the destination at the retransmission interval ^^ (it is considered an equivalent channel formed by the different channels between these nodes and the destination). By convention,

is equal to

Transmission of a frame according to the invention During the first phase of the process, the sources

successively transmit their message after coding

comprising

information bits

being the two-element Galois body. The message ^^ includes a CRC type code which allows the integrity to be checked

of the message

The message

is coded according to the MCS determined by the allocated bit rate. Given that MCSs may be different between sources, the lengths of encoded messages may be different between sources. The encoding uses incremental redundancy type code. The codeword obtained is segmented into successive redundancies. The incremental redundancy code can be of systematic type, the information bits are then included in the first redundancy. Whether or not the incremental redundancy code is of systematic type, it is such that the first redundancy can be decoded independently of the other redundancies. The incremental redundancy type code can be produced for example at means of a finite family of punctured linear codes with compatible efficiency or codes without efficiency modified to operate with finite lengths: raptor code (RC), punctured turbo code with compatible efficiency (RCPTC rate compatible punctured turbo code), punctured convolutional code compatible yield (RCPCC rate compatible punctured convolutional code), LDPC compatible yield (RCLDPC rate compatible low density parity check code).

Transmission by a source conventionally comprises one or more reference signals. The destination estimates the channel and therefore its quality between each of the sources and the destination in a known manner by using, for example, the reference signal(s) received.

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

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

The ^2nd transmission phase of the method includes t = {1, ..., T _used ] retransmission intervals with the convention that t = 0 corresponds to the last transmission interval of the first phase. The term retransmission associated with an interval is used in connection with the ^2nd phase to clearly indicate that any transmission during this phase of an ^nth redundancy of the message from a source i occurs while this source has already transmitted the ^1st redundancy of this same message during the ^first phase.

Unlike the prior art, at each retransmission interval there is no exchange of decoding control between the destination and the nodes: the destination does not systematically send back its set of correctly decoded sources at each retransmission interval nor indication on correct decoding or not, the nodes do not systematically transmit at each retransmission interval their set of correctly decoded sources nor indication on their correct decoding or not. The exchange of decoding control for an update of the knowledge by the destination of the sets of sources correctly decoded by the nodes occurs only when t = 1, that is to say at the start of the second phase. This exchange can take place in an equivalent manner at the end of the first phase. During this exchange the nodes transmit to the destination their set of correctly decoded sources or at least their set of sources correctly decoded and not yet correctly decoded by the destination. Transmission by a node conventionally comprises one or more reference signals. The destination estimates the channel and therefore its quality between each of the nodes and the destination by exploiting for example the reference signal(s) received at this interval t = 1.

Thus, the nodes transmit to the destination their set of sources correctly decoded or at least their set of sources correctly decoded and not yet correctly decoded by the destination once only, ie during the last transmission interval, t = 0 or equivalently during of ^the 1st retransmission interval, t = 1.

On the other hand, the destination selects at each retransmission interval a so-called source to help using a broadcast control channel from the destination to the nodes. The nodes that have correctly decoded this source then transmit the same redundancy of the message from this source during this interval using a data channel. Helping a source means helping the destination to decode this source by transmitting, through the nodes that have correctly decoded this source, a redundancy of the message from this source during the ^2nd phase.

Selection according to the invention

The selection by the destination of a source to help at each retransmission interval is explained below in support of Figure 3 which illustrates the unique exchange of decoding control between the destination and the nodes according to one embodiment . According to this mode, the destination sends its set of correctly decoded sources to the nodes and the nodes transmit their set of correctly decoded sources. According to another mode, the destination sends its set of correctly decoded sources to the nodes and the nodes transmit their set of sources correctly decoded and not yet correctly decoded by the destination. According to another mode, the destination sends a signal to the nodes indicating an absence of correct decoding, NACK and the nodes transmit their set of correctly decoded sources.

During the ^2nd phase, the destination orders in turn the sources not yet correctly decoded for a given number of successive transmissions in order to maximize the sum rate received. A transmission, during a retransmission interval, to help a source i corresponds to the transmission of the same redundant version of its message by all the nodes having correctly decoded this source. Transmissions to support a source i begin at retransmission interval t _si and end when the source is decoded.

The individual fault event of source i, for which the retransmission intervals start at t _si , at the end of the retransmission interval t — 1, O _{i t-1} , can be expressed in the form: ( 1)

This expression reflects the fact that the source i is not decoded correctly at the retransmission interval t — 1 if the bit rate of the source is greater than the sum of the capacities of transmission. This transmission capacity includes the capacity of the channel between this source i and the destination which intervenes during the ^first phase and a sum weighted by a of the capacities of the equivalent channels which intervene during the second phase from t _si until retransmission interval t — 1. At the retransmission interval l ≥ t _if of the second phase, an equivalent channel is considered for source i. The equivalent channel considered at a retransmission interval l groups the channels between each of the nodes helping the source i during this interval and the destination. The capacity of the channel between this source i and the destination is deduced from the quality of the channel ie the mutual information I _id between the source i E {1, ... , M] and the destination d. The capacity of the equivalent channel considered when the source i is helped during the interval l is evaluated by the mutual information between the set of nodes which help the source i at the retransmission interval l and the destination. This capacity depends on time l since a node can benefit from transmissions to help a source i during the ^2nd phase and correctly decode this source i from a retransmission interval of the ^2nd phase while it does not had not been decoded at the end of the ^first phase.

X (t) is defined as the number of retransmission intervals sold to the current interval (not included) during the second phase from the last exchange of decoding control between the destination and the nodes.

X ^m (t) defines the value of X(t) which triggers a new exchange of sets of decoded sources. For example triggers an exchange of sets of decoded sources for

By convention, X ^m (l) = 0 triggers an exchange of sets of decoded sources at the start of the ^2nd transmission phase.

It is assumed that a source i is aided over one or more consecutive retransmission intervals starting with retransmission interval t _if E {1, ... , T _max }. At each retransmission interval (time slot) t ≥ t _if , and for the selected source i not yet decoded correctly by the destination, the variable

is defined according to

the invention. This variable

is defined as being the maximum number (ie sufficient number) of retransmission intervals for the destination to decode this source i (from and counting the retransmission interval t _s ), that is to say, the source i is decoded at the latest at the end of the retransmission interval

This variable %;(t) is estimated by the destination based on its knowledge of

Either the whole

determined by X ^m (t) retransmission intervals starting with a

exchange of sets of decoded sources. Let the set of intervals be

retransmission starting with an exchange of decoded source sets coinciding with transmissions to assist source i, i.e.,

In the case where an exchange of sets of decoded sources took place before t _si , that is to say before the start of the selection of the source i to help during the ^2nd phase, the destination knows H must be underlined that an exchange of sets of decoded sources at

start of the ^2nd transmission phase allows the destination to know

for all sources. Indeed, the transmissions preceding t _if being linked to another source they do not impact the number of nodes having decoded the source i nor For all

Otherwise, the destination only knows to estimate

for For the sake of simplicity of notations, the source index i of t _si is omitted

when it is obvious.

For the retransmission intervals t _s ≤ t ≤ t ₀ the invention considers: (2)

And

the sufficient number of retransmission intervals estimated by the destination from the retransmission interval t is:

(4) where [q] represents the upper integer function (ceiling function) which takes the integer value just greater than or equal to q, eg, [2.3] = 3.

For t = t ₀ , an exchange of sets of decoded sources takes place, which allows the destination to evaluate as follows:

with

(6)

More generally, for it is possible to construct

recursively (t) like: )

or, equivalently: (8)

with always:

Noticed :

Between two decoding control exchanges the number of retransmission intervals sufficient to decode source i is decremented at each transmission helping source i For , it comes:

Finally, in the case where no exchange of decoded source sets is carried out, whether before or after

he comes: (11)

In the case where a single exchange of sets of decoded sources is carried out at the start of the ^2nd transmission phase then: (12)

is the number of retransmission intervals to assist source i from the retransmission interval

included sufficient for error-free decoding of source i which is estimated by the destination based on its knowledge of

The destination uses as an approximation of

a previous value (closest known) with consequently

1}. The number of retransmission intervals required in practice requires the

knowledge of . It is given by the smallest value of ^^

such as :

_, (13) The estimate is x

t the smallest value of such that:

(14)

As

it just comes

In case it comes:

(15)

Indeed, the smallest positive integer such that:

(16)

East :

(17)

Thus, it is certain that the fault event (outage) does not occur at

Source i is systematically decoded correctly by the destination

if it is helped times from the interval t included.

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

It should be noted that the evaluation of the number of retransmission intervals sufficient for a source i is the same whatever t _if its retransmission interval chosen for the first transmission intended to help this source during the ^2nd phase. At a given retransmission interval t, depends only on the number of transmissions n ₁ = t — t _if helping source i during

the ^2nd phase and before t. Thus, the notation denotes the necessary number of transmissions helping source i knowing the number of transmissions neither having helped source i (already carried out) during the ^2nd phase. Subsequently, we denote x _t the counter of the number of transmissions remaining to help source i, this counter being initialized at Xj(0) and being decremented each time source i is helped without decoding exchange. This counter is updated at each exchange of decoding control at the retransmission interval l = tj j = 0, ..., M — 1 which makes it possible to know Jt _d (l).

If the estimate of x _t is such that there has been no previous transmission to help source i or such that Xi(0) relies solely on knowledge of direct links and that for all sources

the following inequality is satisfied: (18)

then all sources can be decoded without exchanging sets of decoded sources. All the sources can be decoded correctly by the destination in the remaining time, the method chooses for example the sources to help successively and randomly. The process according to the invention is particularly interesting when

since it allows the destination to correctly decode an optimal number of sources by optimizing spectral efficiency while very strongly limiting the signaling overhead by selecting the source to help according to a certain strategy.

Furthermore, according to the invention the exchange of control of source sets

correctly decoded between the destination and the nodes occurs only once and at the start of the ^2nd phase.

At t = 1 ie at the start of the ^2nd phase, the method includes an exchange of decoding control between the destination and the nodes. The method determines the sufficient number of retransmission intervals for the destination to decode the source i not yet decoded at the end of the ^first phase knowing its allocated bit rate:

(19)

with :

(20)

or, again with

(21)

with rtj the number of transmissions already carried out to help source i. Following the exchange of

decoding control between destination and nodes at t = 1, destination knows

Without exchanging decoding control at t=l, the destination is based on

ie on the knowledge of the direct link between the source i and the destination.

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

During the second phase, the estimation of the channel between the node and the destination

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

For t > 1 the process considers that each transmission leads to mutual information

so that

decreases depending on the number of transmissions made to help source i.

The destination selects, according to the invention, for each retransmission interval t the source i to help.

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

The selection is determined to maximize spectral efficiency. The maximization of spectral efficiency can be expressed in the form of determining the subset A taken from the set of possible subsets A of sources not yet decoded

correctly by the destination at the interval preceding the current interval t leading to the greatest sum of the flow rates of the sources and such that the sources of this subset A can be decoded in the remaining time, T _av ie such that the time remaining is greater than or equal to the sum of the number of retransmission intervals sufficient to decode each of the sources of this subset A: (22)

is called the power play of

The method then successively considers each source i of this subset

For each source i considered of

the destination transmits to the nodes the indication of the selection of source i at the retransmission interval t.

The nodes having correctly decoded this source i transmit the same redundancy during this interval t to help the decoding of source i by the destination.

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

According to a particularly efficient embodiment, if the destination correctly decodes source i before the number Xi(0) of retransmission intervals has elapsed, it recalculates the best subset

if and only if the set from which source i is removed does not

does not contain all the sources not decoded by deetination otherwise the destination passes to another source in the subset

from which the source was previously removed. This case can appear when the set of sources correctly decoded by a node changes to include source i during Xi(0) retransmission intervals. Thus, this node becomes active during the transmission at interval l of redundancy for source i which leads to an increase in mutual information If source i is decoded after exactly xi(0) retransmission intervals then the destination passes to another source of the subset

from which the source was previously removed.

Implementation example

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

At the end of the ^first phase, ie, t = 0, the sets of sources correctly decoded by the nodes are as follows:

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

During the ^2nd phase, the determination by the destination of all sources A

to help takes place for example according to the algorithm in Appendix A.

During the ^2nd phase, the selection of the source i to help among the set

at each retransmission interval takes place for example according to the algorithm in Appendix B which can be used since

Process of the Algorithm in Appendix B:

Step 2. The destination determines the estimated number of intervals needed

so that the destination decodes a source i not yet decoded on the basis of knowledge of mutual information of the source i destination link and a rate allocated to this source i. The destination therefore calculates x _i for all

For this example, the variables x _i have the following values knowing the direct source-destination links: x ₁ = 6, x ₂ = 2, x ₃ = 3, x ₄ = 12

The flow rates R _t have the following values:

_R1 = 1, _R2 = 2, _R3 = 3, _R4 = 4

Step 3. The sum of x _t exceeds the remaining time: 6 + 2 + 3 + 12 = 23 > T _av = T _max = 8 Step 4. A single exchange of decoding control takes place between the destination and the nodes. During this exchange the nodes transmit their set of sources correctly decoded or only their set of sources correctly decoded but not yet correctly decoded by the destination.

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

According to the example, knowing the node-destination links, the variables x _i become:

Although x ₂ has decreased, the sum of xi: 6 + 1 + 3 + 12 = 22 remains greater than the remaining time T _av T _max •

Step 7. The set of sources A to be helped at the end of the ^1st phase is determined, according to the algorithm in appendix A, by the destination knowing the x ₍ and the flow rates allocated to the sources.

According to the example, the possible choices for Â that satisfy are: {1}, {2}, {3},

{1, 2}, {1, 3}, {2, 3}. The set which leads to the maximum sum of the flow rates and which is determined according to the algorithm in Appendix A is

Step 8. The method decodes the frame as long as t < T _max and there remains a source i to decode in the set Â ie Â #= Φ.

According to the example Â = {2, 3}.

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

According to the example, the destination first selects source 2.

Step 10. The process repeats steps 11-20 until source i is correctly decoded by the destination.

According to the example, the destination repeats for source 2 until it has correctly decoded it then repeats for source 3.

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

According to the example, the destination first returns the number 2 as long as this is not correctly decoded then the number 3 as long as this is not correctly decoded. Step 12. The method increments the value of the current retransmission interval, t «- t + 1, decrements the remaining time, T _av ← T _av — 1 and decrements the value of x _t since i was helped once by the nodes.

According to the example, for the source i = 2, on the first pass, x _i = 0 since x ₂ = 1 and the current interval becomes t = 2, and T _av =7 For the source i = 3, on the first pass, x _i = 2 since x ₃ = 3 and the current interval becomes t = 3 and T _av =6.

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

T = 5

Step 13. If the destination has correctly decoded source i at the current interval then follow steps 14-18.

According to the example, the source i = 2 is correctly decoded in a single retransmission interval since x ₂ = 1. S _dl = {2}.

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

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

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

According to the example, at the start of the ^2nd retransmission interval,

At the end of the ^2nd retransmission interval, A is unchanged: A = {3}.

At the end of the ^3rd retransmission interval,

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

According to the example, for the source i = 2, x ₂ = 1 — 1 = 0 therefore the condition xi > 0 is not met although the condition is met, the process goes to step

19, ie the process repeats steps 10-18 for source i = 3.

For the source i = 3 it is correctly decoded according to the example after two retransmission intervals x ₃ = 3 — 2 = 1. As x _t > 0 and

then the method updates the set Â according to step 16.

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

According to the example, after the correct decoding of the source i = 3, the algorithm in Appendix A is used to update A.

According to the algorithm in appendix A since

and that x ₁ = 6 > 5 and that x ₄ = 12 > 5 then

Step 20. The process loops back to step 8 with Â updated.

According to the example,

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

Annex A

Determination of the whole

sources that can help:

[Table 1]

Claims

1. Method for transmitting a frame carrying messages intended for an OMAMRC telecommunications system with N nodes including M sources and one

destination (d), N ≥ M ≥ 2, the nodes operating in half-duplex mode, according to an orthogonal multiple access scheme of the transmission channel between the N nodes with a maximum number of M + T _max time slots per frame transmitted distributed between a ^1st phase and a ^2nd phase, 1 ≤ T _max , the message from a source having been coded before transmission according to incremental redundancy type coding which generates several redundancies, the ^1st phase includes M allocated intervals respectively to successive transmissions of the M sources and the 2 ^nd phase includes at least one retransmission interval for a transmission of nodes having correctly decoded the same source if _i such that these nodes transmit simultaneously during the same retransmission interval the same redundancy of the message d the same source not yet correctly decoded by the destination, called the source to be helped, the method is such that it comprises: a single transmission by the nodes of at least their set of sources correctly decoded and not yet correctly decoded by the destination , these transmissions allowing the destination to determine, for each of these sources, a quality of an equivalent channel based on a quality of the channels between the nodes having correctly decoded this source and the destination, an estimate of a number of intervals sufficient retransmission (X _i (0)) for the destination to decode a source (s _i ) not yet correctly decoded and correctly decoded by at least one node on the basis of the quality of an equivalent channel for this source between this at minus a node and the destination and a flow rate (R _i ) allocated to this source (s _i ). a selection by the destination of the sources to be helped taking into account the estimated numbers (0)) of retransmission intervals sufficient for the destination to decode the sources not yet correctly decoded and a sum of bit rates (R _i ) allocated to the sources .

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

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

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

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

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

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

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

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

10. Method according to claim 9 such that, if no source can be helped in the time remaining before the end of the ^2nd phase then the transmission of the frame is interrupted before the use of the maximum number of sufficient retransmission intervals (T _used < Tmax)-

11. System comprising N nodes including M sources S _i i∈{l, ... , M] and a destination (d), N ≥ M > 2, for an implementation of a transmission method according to one of claims 1 to 10.