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    • 专利摘要:
    The present invention relates to a transmission method of successive messages forming a frame for a telecommunications system with M sources (s1, ..., SM), possibly L relay and a destination, M ≥2, L ≥ 0 according to a diagram orthogonal multiple access access of the transmission channel between the nodes taken from among the M sources and the L relays with a maximum number of M + Tmax time slots per transmitted frame including M intervals allocated during a first phase to the successive transmission of the M sources , and Tmax intervals for one or more cooperative transmissions allocated during a second phase to one or more selected nodes according to a selection strategy. The link adaptation implemented by the destination is of a slow type and consists in maximizing a mean utility metric under constraint of an average individual BERE for each source, the utility metric being an average spectral efficiency conditioned to the selection strategy of the nodes intervening during the second phase.
    公开号:FR3078459A1
    申请号:FR1851593
    申请日:2018-02-23
    公开日:2019-08-30
    发明作者:Stefan Cerovic；Raphael Visoz
    申请人:Orange SA；
    IPC主号:
    专利说明:

    OMAMRC transmission method and system with slow link adaptation under constraint of a
    BLER
    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 transmit and can also in some cases relay messages from other sources.
    There are many relay 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 sensor networks.
    Such a sensor network is a multi-user network, consisting of several sources, several relays and a recipient using an orthogonal multiple access diagram of the transmission channel between the relays and the destination, noted OMAMRC (“Orthogonal Multiple-Access Multiple -Relay Charnel "according to English terminology).
    Prior art
    An OMAMRC network implementing a cooperation strategy known as IR-HARQ (Incremental Redundancy Hybrid-ARQ) based on selective relaying known as SDE (Selective Decoded and Forward) is known from [1].
    In this type of cooperation, the independent sources among themselves broadcast, in a first phase, their coded information sequences in the form of messages for the attention of a single recipient and in the presence of relays. Relay nodes can be either sources or relays. Relays considered to be of the “Half Duplex” HD type, (that is to say that a relay cannot receive and transmit simultaneously) receive messages from sources, decode them and generate a message only from messages from decoded sources. without error. The relays access the channel in a second phase orthogonally in time between them to transmit their message to the destination. Relay selectivity causes a relay to transmit a message only if it decodes at least one source without error. A source can behave during the second phase like a relay which has decoded without error at least one of the messages of the sources, the message of the source considered. Thus during the second phase, all the nodes of the system are considered as relay nodes which can access the channel orthogonally in time. The destination can choose during the second phase which node should transmit at a given time. It is also possible that the relay nodes follow an activation sequence known in advance by the destination and by the relay nodes during the second phase.
    Very limited flow control channels are allowed from the destination to the sources to allocate the flows to the sources. In addition, the sources and relays must periodically go back to the destination of the metrics representative of the SNR by means of the links they can observe.
    Furthermore, limited speed control channels are necessary to implement the strategy of selecting the transmission nodes.
    The retransmission of a node (which can be a relay or a source) contains one or more combined messages from the sources. It is requested by a control signal broadcast by the destination or follows an activation sequence known in advance and results in the emission of a redundancy by the selected node based on an incremental coding of one or more sources. . There may also be control channels for each node to inform the destination of the received and correctly decoded messages.
    Such a method is particularly suitable for a system deployed in an urban environment in which the transmission channel generally has a so-called Rayleigh fading profile. Indeed, such fainting disturbs the transmitted signal and results in a non-zero probability of false detection (so-called probability of cut or "outage" according to English terminology).
    Among the transmission techniques with relaying, there is known a slow link adaptation process for an OMAMRC system. Before any transmission, the destination implements a slow link adaptation, that is to say that it allocates bit rates to sources knowing the statistical distribution of all channels (CDI: Channel Distribution Information). In general, it is possible to go back to the CDI on the basis of knowledge of the average SNR of each link in the system.
    The flow rates determined during implementation take on discrete values. Each bit rate allocated R s to the source s corresponds to a coding and modulation scheme (MCS: Modulation and Coding Scheme) which belongs to the family of MCS defined by the system. In the following, the sum of these bit rates is called the sum bit rate and should not be confused with the average spectral efficiency.
    Source message transmissions are divided into frames during which link CSIs are assumed to be constant (slow fainting assumption). The rate allocation is assumed not to change for several hundred frames, it only changes with changes in CDI.
    The transmission of a frame takes place in two phases which may be preceded by an additional phase.
    During the first phase, the sources each transmit their respective messages in turn during time-slot intervals each dedicated to a source.
    A hybrid and automatic repeat request (HARQ) with incremental redundancy (IR) is used during the second phase as an effective a posteriori mechanism for rapid link adaptation according to which the source coding rates adapt to the known quality of the channel. During each time slot of the second phase, the destination schedules the transmitting node. The transmitting node implements Joint Network Channel Coding (JNCC) of the messages it has successfully decoded.
    The slow link adaptation process is implemented during the initial phase which occurs before the first phase and this once every several hundred frames (i.e. each time the channel / link quality statistics change).
    The slow link adaptation proposes to find the maximum sum flow under constraint of reaching a common average target BLER based on the probability of cut £ com at the end of a fixed number X> 1 of cooperative retransmissions. The method is thus based on the idea that with an appropriate choice of the target average common BLER, which is correlated with the average individual BLERs, it is possible to achieve good spectral efficiency.
    An algorithm based on an interference-free approach or "Genie Aided" is used to solve the problem of optimizing multidimensional flow allocation (rate). This approach is to independently determine each bit rate of a source assuming that all messages from other sources are known to the destination and the relay.
    Although the cooperation strategy is such that the sources and the relays cooperate to maximize the sum of the bit rates transmitted between the sources and the destination in a situation close to reality, that is to say where there is no of symmetry imposed between the links as it is the case for other previous techniques such as [1], a difficulty comes from the absence of a clear relation between the probability of common cut ("common outage" representative of the average common BLER ) and the individual shutdown probabilities (“individual outage” representative of the average individual BLER). This difficulty makes it impossible to demonstrate that this slow link adaptation process gives the best spectral efficiency. In addition, the "Genie Aided" hypothesis is not precise enough and often requires correction.
    Main features of the invention
    The subject of the present invention is a method of transmitting successive messages forming a frame for a telecommunications system with M sources, possibly L relay and a destination, M> 2, L> 0 according to an orthogonal multiple access diagram of the channel of transmission between the nodes taken from the M sources and the L relays with a maximum number of M + T max time intervals per transmitted frame of which M intervals allocated during a first phase to the successive transmission of the M sources, and T max intervals for a or several cooperative transmissions allocated during a second phase to one or more nodes selected according to a selection strategy. The process includes:
    an initial link adaptation phase with determination by the destination of an initial bit rate for each source on the basis of the knowledge by the destination of an average quality of each of the links in the system and with transmission by the destination to each source information on this initial bit rate, for each frame among several frames, successive transmissions of the messages of the M sources during the M intervals of the first phase with respectively modulation and coding schemes determined from the information on the initial debits.
    The method is such that the adaptation of the link implemented by the destination is of the slow type and consists in maximizing a metric of average utility under the constraint of an average individual BLER for each source, the metric of utility being a spectral efficiency. average conditioned to the strategy of selection of the nodes intervening during the second phase.
    The OMAMRC transmission system considered comprises at least two sources, each of these sources being able to operate at different times either as a source or as a relay node. The system may optionally further include relays. Node terminology covers both a relay and a source acting as a relay node.
    The links between the different nodes of the system are subject to slow fading and to white Gaussian 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 is not directly observable by the destination and would require an excessive exchange of information between the sources, the relays and the destination. To limit the cost of feedback overhead, only information on channel distribution / statistics (CDI: Channel Distribution Information) of all links, eg average quality (for example average SNR, average SNIR ) of all links, is assumed to be known by the destination for the purpose of determining the bit rates allocated to the sources.
    The sources which are independent of each other broadcast their coded information sequences during the first phase in the form of messages for the attention of a single recipient. Each source broadcasts its messages at an initial rate. The destination communicates to each source its initial flow rate through very limited flow control channels. The destination determines the initial debits from its knowledge of the statistics of all the links.
    Sources other than that which transmits and possibly the relays, of the "Half Duplex" type receive successive messages from the sources, decode them and, if selected, generate a message only from the messages of the sources decoded without error.
    The selected nodes then access the channel orthogonally in time with each other during the second phase to transmit their generated message to the destination.
    The destination can choose which node should transmit at a given time. It is also possible that the relay nodes follow an activation sequence known in advance by the destination and the relays. In the first case, the destination always chooses a node (source or relay) which could decode at least one message from a source without error.
    The destination communicates if necessary its strategy of selection of the nodes towards the sources and towards the relays via control channels with limited flow.
    The method is such that the link adaptation is of the slow type and, unlike certain known transmission techniques with the implementation of an IR-HARQ cooperation strategy based on SDF relaying, the method takes place in the context of a system with bit rates which can be asymmetrical between the sources and implements a strategy to maximize the average spectral efficiency within the system considered constrained to respect an individual quality of service (QoS) by source ie an average individual BLER by source.
    This system is such that the destination has no knowledge of the instantaneous quality of the links between the sources, between the sources and the relays, and between the relays, but only of the instantaneous quality of the links between the sources and the destination and between the relays and the destination.
    Thus, the cooperation strategy according to the invention is such that the sources and the relays cooperate to maximize the average spectral efficiency in a situation closer to reality i.e. where there is no imposed symmetry between the bit rates.
    The system considered is such that the sources can themselves be relays. A relay differs from a source because it does not have its own message to transmit i.e. it only retransmits messages from other nodes.
    Distinguishes three phases process, an initial phase, and for each frame to be transmitted, a l st phase and a 2 nd stage.
    During the initialization phase, the destination determines an initial bit rate for each source, taking into account the average quality (for example SNR) of each of the links in the system.
    The destination estimates the quality (for example SNR) of the direct links: source to destination and relay to destination according to known techniques based on the use of reference signals. The quality of the source-source, relay-relay and source-relay links is estimated by the sources and the relays by using, for example, these same reference signals. The sources and the 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 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 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 debits that 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 slots (time slots) using respectively modulation and coding schemes determined from the initial bit rates.
    During the 2 nd phase, the sources of messages are delivered cooperatively by the relay and / or sources. This phase lasts at most T max time slots.
    The utility metric which consists of a spectral efficiency is conditioned by the node selection strategy which intervenes during the second phase.
    According to one embodiment, the method further comprises a step of iterative calculation of the initial debits by the destination.
    The “Genie Aided” approach which consists in independently determining each bit rate of a source by supposing that all the messages of the other sources are known to the destination and the relays leads to initial bit rate values for each source which are not sufficient specific. The iterative calculation makes it possible to correct these initial values by taking into account the selection strategy which intervenes during the second phase which cannot by nature the "Genie Aided" approach alone.
    According to one embodiment, the cooperative transmission of a node during the second phase results in the emission of a redundancy based on an incremental coding at the sources.
    According to one embodiment, the strategy for selecting the nodes intervening during the second phase follows a sequence known in advance by all the nodes.
    According to one embodiment, the iterative step of calculating the initial bit rates takes into account a node selection strategy (strategy with random selection, strategy with cyclic selection, etc.).
    According to one embodiment, the strategy for selecting the nodes intervening during the second phase takes into account information coming from the nodes indicating their set of correctly decoded sources.
    According to one embodiment, the selection strategy of the nodes intervening during the second phase corresponds to each time interval to the selection of the node which has correctly decoded at least one source which the destination has not decoded correctly at the end of the previous time interval and which benefits from the best instantaneous quality among the instantaneous qualities of all the links between the nodes and the destination.
    According to one embodiment of the invention, the method for transmitting messages results from a software application divided into several specific software applications stored in the sources, in the destination and possibly in the relays. The destination can for example be the receiver of a base station. The execution of these specific software applications is suitable for implementing the transmission process.
    The invention also relates to a system comprising M sources, possibly L relays, and a destination, M> 1, L> 0, for implementing a transmission method according to a preceding object.
    The invention further relates to each of the specific software applications on one or more information carriers, 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 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 a network of the Internet type.
    The optional features presented above as part of the transmission process may possibly apply to the software application and to 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 nonlimiting examples, and of the appended drawings, among which:
    FIG. 1 is a diagram of an example of a system called OMAMRC (Orthogonal Multiple Access Multiple Relays Channel) according to the invention, FIG. 2 is a diagram of a transmission cycle of a frame which can be preceded by an initialization step according to the invention, FIG. 3 is a diagram of the OMAMRC system of FIG. 1 for which all the sources except the source sl are considered to be correctly decoded.
    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 channel uses is related to the available frequency band and the transmission time.
    In the “slow fading” case privileged in the description, the fading gains are constant during the M + T max time intervals where M + T max is the maximum number of time intervals to complete a transmission cycle.
    An embodiment of the invention is described in the context of an OMAMRC system illustrated in FIG. 1 and with the support of the diagram in FIG. 2 which illustrates a transmission cycle of a frame.
    This system includes M sources which belong to the set of sources = {s 1; L relays which belong to the relay set 3Î = [ry ..., r L } and a destination d. Each source of the game U communicates with the unique destination with the help of the other sources (user cooperation) and relays which cooperate
    By way of simplification of the description, the following assumptions are made subsequently on the OMAMRC system:
    - the sources, the relays are equipped with a single transmitting antenna;
    - the sources, the relays, and the destination are equipped with a single receiving antenna;
    - the sources, the relays, and the destination are perfectly synchronized;
    - the sources are statistically independent (there is no correlation between them);
    - all the nodes transmit with the same power;
    - use is made of a supposed CRC code included in the K s information bits of each source s to determine whether a message is correctly decoded or not;
    - the links between the different nodes suffer from additive noise and fading. The fading gains are fixed during the transmission of a frame carried out for a maximum duration M + T max time intervals, but can change independently from one frame to another. T max > 2 is a system parameter;
    - the instantaneous quality of the channel / direct link in reception (CSIR Channel State Information at Receiver) is available at the destination, at the sources and at the relays;
    - the returns are error free (no error on the control signals);
    - the duration of the time intervals is variable.
    Nodes include relays and sources that can behave as a relay when they are not sending their own message.
    The nodes, M sources and L relays, access the transmission channel according to an orthogonal multiple access scheme which allows them to listen without interference to the transmissions of the other nodes. The nodes operate in a half-duplex mode.
    The following notations are used:
    • x ak GC is the coded modulated symbol for the use of channel k transmitted by the node U £ £ $ R, • Va, b, k I e is the received signal at the node b GSU 3i U {d} {n } corresponding to a signal emitted by the node a GSU Ή.
    • Ya, b is I and report average signal to noise (SNR) that takes into account the channel attenuation effects (path-loss) and masking (shadowing) • h ab is the channel attenuation gain ( fading) which follows a symmetrical circular Gaussian distribution with zero mean and variance y a / 7 , the gains are independent of each other, • n a, b, k are samples of a Gaussian white noise (AWGN) distributed in a way identical and independent which follow a complex Gaussian distribution of circular symmetry with zero mean and unit variance.
    The signal received at node b G S U 3Î U {d} {a} corresponding to the signal transmitted by node a G S U 3Î can be written:
    Ya.bk ~ h a , b x a, k + n a, b, k (1)
    During the first phase of M time intervals, each source transmits its code words during N r uses of the channel, k G {1, During the second phase of T max time intervals, each selected node transmits information representative of the messages of the sources decoded without error by this node during / V 2 uses of the channel, k G {1, ..., N 2 }.
    By using reference signals (pilot symbols, SRS signals from 3GPP LTE, etc.), the destination can determine the gains (CSI Channel State Information) from direct links: h dir = that is, source links to destination and relay to destination and can therefore deduce the average SNR from these links.
    On the other hand, the gains from links between sources, links between relays and links between sources and relays are not known by the destination. Only sources and relays can estimate a metric of these links by using reference signals in a similar way to that used for direct links. Given that the channel statistics are assumed to be constant between two initialization phases, the transmission to the destination of the metrics by the sources and the relays can occur only at the same rate as the initialization phase. The channel statistics for each link is assumed to follow a centered circular complex Gaussian distribution and the statistics are independent between the links. It is therefore sufficient to consider only the average SNR as a measure of link statistics.
    The sources and relays therefore go back to the destination of the metrics representative of the average SNRs of the links they can observe.
    The destination thus knows the average SNR of each of the links.
    During an initial link adaptation phase which precedes the transmission of several frames, the destination goes up for each source a representative value (index, MCS, bit rate, etc.) of an initial bit rate. Each of the initial rates unambiguously determines an initial modulation and coding scheme (MCS) or vice versa each initial MCS determines an initial rate.
    These initial bit rates are determined by the destination so as to maximize an average spectral efficiency conditional on the strategy of selecting the nodes involved during the second phase and under the constraint of an average individual BLER for each source. The initial flow rates are raised via very limited flow control channels. The maximization is typically carried out under constraint of the average SNRs of the links of the system.
    Each source transmits its framed data to the destination with the help of the other sources and relays.
    A frame occupies time slots during the transmission of the M messages from the M sources respectively. The transmission of a frame (which defines a transmission cycle) is taught M + T max time intervals: M intervals for the st phase intervals T max for the 2nd phase.
    During the first phase, each source s E S = transmits after coding a
    K message u s comprising K s information bits u s EF 2 S , F 2 being the Galois body with two elements. The message u s includes a CRC type code which makes it possible to verify the integrity of the message u s . The message u s is coded according to the initial MCS. Since the initial MCS may be different between the sources, the lengths of the coded messages may be different between the sources. The coding uses an incremental redundant code. The code word obtained 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 incremental redundancy 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 produced for example by means of a finite family of punched linear codes with compatible yields or of no-performance codes modified to operate with finite lengths: raptor code (RC), punched turbo code of compatible performance ( RCPTC rate compatible punctured turbo code), convolutional code marked for compatible performance (RCPCC rate compatible punctured convolutional code), LDPC for compatible performance (RCLDPC rate compatible low density parity check code).
    During the first phase, the M sources successively transmit their message during the M intervals with respectively modulation and coding schemes determined from the initial bit rates. Each time interval includes N ± uses (channel uses) of the channel such that the time resource is shared equally between the sources.
    Each transmitted message corresponding to a correctly decoded message source is assimilated to the corresponding source by abuse of notation.
    When a source transmits, the other sources and the relays listen and try to decode the messages received at the end of each time-slot. The success of the decoding is decided using CRC.
    During the second phase, the selected node, source or relay, acts as a relay by cooperating with the sources to help the destination decode messages from all sources correctly. The selected node transmits ie it cooperates by transmitting the words or part of the words which it has correctly decoded. The second phase comprises at most T max time intervals (time slots) called rounds. Each round t E {1, ..., T max ] has a duration of N 2 uses of the channel.
    During this phase, the destination follows a certain strategy to decide which node transmits at each time interval (round). The destination informs the nodes using a limited feedback control channel to transmit a return message. This return message is based on its result of decoding the received frames. The destination thus controls the transmission of the nodes by using these return messages which makes it possible to improve the spectral efficiency and the reliability by increasing the probability of decoding of all the sources by the destination.
    If the decoding of all the sources is correct, the return is an ACK type message. In this case, a transmission cycle for a new frame begins with the erasing of the relay and destination memories and with the transmission by the sources of new messages.
    If the decoding of at least one source is wrong, the return message is typically a NACK. Each node a ESU 3Î transmits its set of sources correctly decoded at the end of the previous time interval (round) noted S a t _ i . By convention, we note £ bt Ç the set of messages (or sources) correctly decoded by the node b ESU 3Î U {d} at the end of the time interval t (round t), t E {0, .. ., T max }. The end of the time interval (round) t = 0 corresponds to the end of the first phase. The number of time slots used during the second phase t used = {1, ..., T max } depends on the success of decoding at the destination.
    The selected node transmits parities determined from messages in its set of correctly decoded sources using joint network coding and channel coding (Joint Network Channel Coding). This transmission occurs during a time interval of N 2 uses of the channel. The other nodes and the destination can improve their own decoding by exploiting the transmission of the selected node and update their set of correctly decoded sources accordingly.
    The initial transmission rate of a source s is R s = K s / N 1 in bits per complex dimension (bcu). The long-term rate R s of a source is defined as the number of bits transmitted relative to the total number of uses of the channel for a number of frames transmitted which tends towards infinity:
    R s = - (2) s M + aE (T) v 7
    T with EfT) = Σί = ι Χ tPr {T = t] the average number of retransmission time intervals used during the second phase and with a = N 2 / N 1 .
    Spectral efficiency can be defined as the sum of individual spectral efficiencies: ΣΜ _ ^ (1-PrfO ,, (3) = 1 with O sTmax the event that the source s is not decoded correctly by the destination at l 'after the time interval T max , hereinafter called individual outage event of the source s at the end of the time interval T max .
    In general, the individual cutoff event of the source s at the end of the time interval (round) t, O st (a t , S at , ti hdir> Pt-i) depends on the selected node a t GN = S {TR and the associated set of decoded sources S a t _ 1 . And this conditionally to the knowledge of the gains of the direct channels h dir and of Pt-γ. Pt-i is ' e j eu comprising all the nodes â L which have been selected at the time intervals (rounds) l G {1, ... t - 1} preceding the time interval (round) t as well as their associated decoding game and that I e had decoding of the destination $ d, tl ·
    The common outage event at the end of the time interval t, S t (a t , S at t _i | h dir , is defined as the event that at at least one source is not correctly decoded by the destination at the end of the time interval (round) t.
    The probability of the individual cutoff event of the source s at the end of the time interval (round) t for a candidate node a t can be expressed in the form:
    with E (.) the expectation operator and such that l {v} takes the value 1 if the event V is true and the value 0 otherwise.
    The probability of the common outage event can be defined in the same way. In the following, the dependence on the knowledge of h dir and of is omitted for the sake of simplification of the notations.
    The common cutoff event of a set of sources occurs when the vector of their bitrate is outside the corresponding MAC capacity region.
    For some source subsets B Ç S dt _ i with S dt _ i = 3 S dt _ 1 the set of sources not correctly decoded by the destination at the end of the time interval (round) t - 1, the common outage event can be expressed in the form:
    = UuçbFd. ^ CW (4) such that the sources which belong to L = S dt _ 1 B are considered as interference.
    B dS (îi) translates the non-respect of the MAC inequality associated with the sum bit rate of the sources contained in LL:
    • Fd, - {Σδβΐ / Rs>UsE'U h, d + Σ / = ι a Ιάι, άΛί ^ $} + a G t , dl [c UtS jj (5)
    OR
    C âhS = {{se Sa ^ n U] λ (¾ ^ n J = 0}], C at . S = {{sen U] λ n J = 0} j with Λ which represents the logical operator,
    I ab denotes mutual information between nodes a and b, â t , l = 1 to t - 1 a node already selected.
    The factor a makes it possible to normalize before addition the two terms associated respectively with the two phases for which the time intervals have respective durations N r and N 2 uses of the channel (channel use).
    The event of individual cutoff of the source s at the end of the time interval (round) t can be written:
    Os, t (, a t> $ a t , tl) ~ rijc <S £ t _i Ul / £ 7: se1 / ^ Σδεί / Rs> Σδ5ΐ / As, d + Σ / = 1 s j + ®G t , dl [c Ut s dd (6) where i = Jd.t-N and Dai, GZT and s, s have the same expression as for a (5).
    The destination implements according to the invention an adaptation of a slow type link. This adaptation consists in maximizing a metric of average utility after a number X <T max of retransmissions (cooperative transmissions) occurring during the second phase under the constraint of an average individual BLER for each source. The utility metric is an average spectral efficiency conditioned by the strategy of selection of the nodes intervening during this second phase.
    According to a first class of strategies, the selection of the nodes taken from the sources and the relays depends on the sets of the sources correctly decoded by the nodes. An example considered said preferred strategy is based on an IR-HARQ type selection which aims to maximize the spectral efficiency. According to this preferred strategy, at the time interval (round) t of the second phase the destination chooses the node with the best instantaneous quality of the link between itself and this node (for example the greatest mutual information between itself and this node) taken from among all the nodes which were able to correctly decode at least one source of the game, these nodes being said to be eligible. This strategy achieves a good compromise between computational complexity and performance but at the expense of a large number of control signals.
    According to a second class of strategies, the selection of the nodes taken from the sources and the relays does not depend on the sets of the sources correctly decoded by the nodes. According to this class, the selection is determined and known to all the nodes. An example considered is such that the selection sequence is cyclical and such that the selected node is selected only among the relays. In this example, each relay has at least a dedicated time interval (round) during the second phase to transmit. In order not to favor one relay over another, the sequence changes with each frame. In this example, only one return bit from the destination is sufficient to raise a common ACK / NACK message.
    During the first phase, each source is emitted with the initial rate R s .
    Let BLER S x (R s ) be the average probability of having the message from the source s not correctly decoded after X time intervals (rounds) of the second phase.
    In a point-to-point transmission, the individual throughput of the source is given by:
    (1 - BLER SiX (R s ))
    And to optimize this flow, the usual method consists in finding the optimal pair Çr s , BLERgxÇRg) ^.
    Such a usual method cannot be used for a system with M sources, possibly L relays, and a destination with an orthogonal multiple access diagram of the transmission channel since the BLER sX is dependent on all bit rates (R ± R M ). This comes from the fact that the decoding set of the selected node at the current time interval depends on all bit rates and that these influence the probability of incorrect decoding of the message from source s.
    In order not to overload the ratings we distinguish R s . the flow rate of the source Sj after optimization of / , which is a possible value of R s . in the whole set of possible bit rates ..., R nMCS }. n MCS is the number of different MCS. The method according to the invention is a solution to the following optimization problem:
    M
    under constraint that Pr {C s , x} <QoS s , Vs E <S
    In relation (7), X used is a random variable which represents the number of time intervals (rounds) used during the second phase X used <X- The distribution of X used depends on (R 1 ..., R M ) as well as Pr {0 s . x } which makes the optimization (7) multidimensional with cardinality n ^ cs or 3375 M-tuple (R ^ R M ) possible for a family of fifteen MCS and three sources. An exhaustive search quickly becomes impossible when the number of sources increases.
    The so-called “Genie Aided” approach consists in assuming during the initialization step that all the sources s except the source Sj for which the flow is to be initialized are considered to be correctly decoded, s ES s t = { s 1 , s 2 , All sources {s 1 , s 2 , ..., s i _ 1 , Sj +1 , other than Sj act as noted relays (r L + 1 , For source s, considered, the network is a multiple relay network denoted (1, L + M - 1.1) and no longer a multiple relay network and multiple users The corresponding system is illustrated by the diagram in FIG. 3 when s, = ^.
    According to the invention, this approach is completed with the taking into account of the quality of all the links which can help the transmission of the source s ,. This method gives a more precise solution, in particular in the case of a priori knowledge of the sequence of selection of the nodes intervening during the second step.
    Given the simplification of the network (1, L + M - 1.1), the search for the maximum speed
    R s . for source s, under the assumption "Genie Aided" can be written in the form:
    R s . = argmax i ----- r il - i R t > I s . d + Y
    Ri ^ Rl ..... Rn MCS ] + J JT —f
    Ri
    1 = 1
    J (8) such as Pr {O s . x } <QoS s .
    It is clear from equation (8) that the flow R s . under the “Genie Aided” hypothesis depends on the selected node â L at the time interval (round) l. To determine an upper bound on the flow R s . under the “Genie Aided” hypothesis for the source s i; it suffices to choose the optimal node selection strategy under the “Genie Aided” hypothesis for this same source s ,. This is described by algorithm 3 in appendix A.
    Furthermore, the optimization calculation (8) is given by algorithm 1 of appendix A. Each bitrate value of the set of possible bit rates {/, ..., Rn MCS } is considered one after the other according to a first loop on j. n MCS is the number of modulation and coding schemes. A second loop on cnt allows to average the individual BLER or Pr {t> sy } on Nb_MC channel draws according to the statistics given by the average SNR of all the links. Thus, inside the cnt loop all the channels are known resulting from a random draw. It then suffices to calculate equation (8) according to a Monte-Carlo approach where the integral is replaced by a sum:
    1 = 1 d P (PT) dH
    Nb MC cnt = l
    1 = 1 and where the variable out corresponds to:
    Nb_MC cnt = l
    1 = 1
    To determine an average approximation of the flow that the source s, can use under the “Genie Aided” hypothesis, a strategy of random selection of the node â ( (among all the relay nodes and for each time interval (round) Z) This is materialized during step 11 of algorithm 1 of ΓAnnex A and consists of a random selection of the node â x among all the possible relay nodes.
    To determine the best initial bit rates that all sources can use, an embodiment of the method according to the invention according to which the determination of the initial bit rates by the destination comprises an iterative calculation step can follow the sequence given in Annex A, algorithm 2 This algorithm takes as starting point the initial flows under the “Genie Aided” hypothesis and a particular selection strategy (for example random selection). The upper bound is based on the selection strategy given by algorithm 3 which can be used to minimize the number of computations (no flow higher than that given by the upper “Genie Aided” bound for a source s should be tested for this same source) or even as a starting point for algorithm 2.
    According to algorithm 2, all the flow rates of the sources are updated in a cyclic manner. The flow rate of a source Sj is a function of the flow rates of the sources having an index 1 ′ less than i, 1 ′ <i, updated in the same iteration and of the flow rates updated for the last time during the previous iteration for sources with an index i greater than i, i> i. The update at each iteration t of the flow rate of a source Si, i E {1, consists of the flow rate calculated in the previous iteration, R Si (t -
    1), to check whether the spectral efficiency increases or decreases by increasing the value of the flow rate R s . (T) to the just higher value (the flow rate values being quantified).
    If the spectral efficiency increases then the increase in the flow rate value is continued until the spectral efficiency decreases. The flow rate value retained R s . (T) is that just before the spectral efficiency decreases.
    If the spectral efficiency decreases when the flow rate R s . (T) is increased for the first time then the value of the flow rate is decreased until the spectral efficiency decreases. The flow rate value retained R s . (T) is that just before the spectral efficiency decreases.
    Any decrease or increase in flow is bounded by the upper bound as determined by algorithm 3.
    References :
    [1] A. Mohamad, R. Visoz and A. O. Berthet, “Cooperative Incrémental Redundancy Hybrid Automatic Repeat Request Strategies for Multi-Source Multi-Relay Wireless Networks,” IEEE Commun. Lett., Vol. 20, no. 9, pp. 1808-1811, Sept. 2016.
    Annex A
    Alg.l - Monte-Carlo simulation to determine the flows under the "Genie" hypothesis
    Helped ":
    1. re loop: select sequentially the possible candidate flow Rj which has not yet been considered in the game ..., R njVÎCS }. If all flows were considered going to end the era loop.
    2. Initialization of the counter out of Monte-Carlo realizations (of matrix of channels H) which lead to a cut-out: out = 0, of the counter X used of the number of time intervals (rounds) cumulated used during the second phase: X used = 0, of the set S d0 = S Sj according to the “Genie Aided” hypothesis.
    3. 2nd loop: sequentially selecting the counter cnt to the current embodiment of a Monte-Carlo simulation: 1 <cnt <Nb_MC Nb_MC with the maximum number of Monte Carlo embodiments, e.g. Nb_MC = 1000. If the counter has reached the maximum number cnt> Nb_MC going to end 2nd loop. 4. determine H cnt on the basis of P (H) the joint probability of the realizations of the channels of all the links h ab . 5. calculate 7 a , b (H cnt ) for all links 6. if Rj <I s . d then 7. • $ d, 0 = $ d, 0 U (Si), 8. continuous, (no change of the values of the out and X counters used ). 9. end of if 10. 3rd loop: for time chaqueintervalle (round), x = 1 to X 11. selection of the node â x by the destination by applying a selection strategy (for example Alg.3 for an upper bound or random selection) 12. calculate C 2 = I s . d + Zk = ialâ k , d l [ s . e§ J 13. if Rj <C 2 then 14. X used = x. (the number of time intervals (rounds) used in the current realization of the Monte-Carlo simulation) 15. break, (no change of counter value out) 16. end of if 17. if x = X then 18. out = out + 1 19. Xused X- 20. end of if 21. end of the 3 rd loop
    72- ^ used ^ used 3 ^ used
    23. end of the 2nd loop
    24. determine the average cutoff probability of the source Sj for the flow rate Rj: pout _ out s i , R j Nb_MC
    25. determine the average number of time intervals (rounds) used during the second phase:
    te 1 iy λ _ ^ used ^ used.Rj) ~ Nb _ MC
    26. end of the loop ere
    27. choose the maximum flow rate R s . that the source Sj can use:
    R Si f R J argmax <----------- R j E { R i ..... Rn MCs} + X used, Rj) (1-¾) such that P ° $. <QoS s ..
    Alg.2 - Iterative procedure for correcting "Genie Aided" debits:
    1. initialization of the iteration counter: t = 0,
    2. initialization of the flow rates of the sources using a so-called “Genie Aided” approach conditioned on a random selection of nodes,
    3. while (| P s . (T) - R If (t - 1) |> 0, Vt e {1, do
    4. increment the iteration counter: t = t + 1
    5. for each source s ,, ί E {1, ..., M} do
    6. considering the already determined flow rates of the sources:
    (fi S1 (t), .... Rs ^ tfRs ^ t - 1), ..., R SM (t - 1)), update the flow rate value R s . (t) of the source Sj such as BLER S. <QoS s . . REML. The value of R s . (T) is different from R s (t - 1) if BLER S. <QoS s . and the average spectral efficiency to increase. Rem 2: the calculation of the average individual B LE Rs as well as of the spectral efficiency follows a Monte-Carlo approach without “Genie Aided” hypothesis.
    7. end of pros
    8. end of as long as
    Alg.3 - Optimal selection strategy under the "Genie Aided" hypothesis:
    1. Loop: determine the decoding set for each candidate node a x E SUîl at the end of the time interval (round) x - 1.
    2. Initialization “Genie Aided”: S a X _ 1 =
    3. calculate Q = I s . Ux + Zk = i al âk , ax
    4. if Rj <C-l then
    5 · & α χ , χ- $ α χ , χ- U {Sj}.
    6. end of si
    7. end of the loop
    8. selection of the node â x by the destination: â x = argmax ae ^ u5i} pa x , dl { Si es ax _ ±
    权利要求:
    Claims (7)
    [1" id="c-fr-0001]
    1. Method (1) of transmitting successive messages forming a frame for a telecommunication system with M sources possibly L relay ..., r L ) and a destination (d), M> 2, L> 0 according to a scheme d orthogonal multiple access of the transmission channel between the nodes selected from among the M sources and the L relays with a maximum number of M + T max time intervals per transmitted frame including M intervals allocated during a first phase to the successive transmission of the M sources , and T max intervals for one or more cooperative transmissions allocated during a second phase to one or more nodes selected according to a selection strategy, characterized in that it comprises:
    an initial link adaptation phase with determination by the destination of an initial throughput for each source on the basis of the destination's knowledge of an average quality of each of the links in the system and with transmission by the destination to each source information on this initial bit rate, for each frame among several frames, successive transmissions of the messages of the M sources during the M intervals of the first phase with respectively modulation and coding schemes determined from the information on the initial rates, and characterized in that, the link adaptation implemented by the destination is of the slow type and consists in maximizing a metric of average utility under the constraint of an average individual BLER for each source, the metric of utility being an average spectral efficiency conditioned to the strategy of selection of the nodes intervening during the second phase.
    [2" id="c-fr-0002]
    2. Method (1) of transmission according to claim 1 according to which the method further comprises a step of iterative calculation of the initial debits by the destination.
    [3" id="c-fr-0003]
    3. The transmission method (1) according to claim 2, according to which the strategy for selecting the nodes intervening during the second phase follows a sequence known in advance by all the nodes.
    [4" id="c-fr-0004]
    4. Method (1) of transmission according to claim 2 according to which the iterative step of calculating the initial bit rates takes into account the node selection strategy.
    [5" id="c-fr-0005]
    5. Method (1) of transmission according to claim 2 according to which the strategy of selection of the nodes intervening during the second phase takes into account information coming from the nodes indicating their set of correctly decoded sources.
    [6" id="c-fr-0006]
    6. Method (1) of transmission according to claim 2, according to which the strategy of selection of the nodes intervening during the second phase corresponds, at each interval, to the selection of the node among the nodes which have correctly decoded at least one source than the destination has not decoded correctly at the end of the previous time interval known as eligible nodes which benefits from the best instantaneous quality among instantaneous qualities of all the links between these eligible nodes and the destination.
    [7" id="c-fr-0007]
    7. System comprising M sources (s x ..., s M ), L relay (r x ..., r L ~) and a destination (d), M> 2, L> 0, for an implementation of a transmission method according to one of claims 1 to 6.
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    US20210067284A1|2021-03-04|
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    FR1851593|2018-02-23|
    FR1851593A|FR3078459B1|2018-02-23|2018-02-23|OMAMRC TRANSMISSION METHOD AND SYSTEM WITH SLOW LINK ADJUSTMENT UNDER CONSTRAINT OF A BLER|FR1851593A| FR3078459B1|2018-02-23|2018-02-23|OMAMRC TRANSMISSION METHOD AND SYSTEM WITH SLOW LINK ADJUSTMENT UNDER CONSTRAINT OF A BLER|
    US16/970,154| US20210067284A1|2018-02-23|2019-02-11|OMAMRC transmission method and system with slow link adaptation under BLER constraint|
    PCT/FR2019/050294| WO2019162592A1|2018-02-23|2019-02-11|Omamrc transmission method and system with slow link adaptation under bler constraint|
    EP19710044.9A| EP3756295A1|2018-02-23|2019-02-11|Omamrc transmission method and system with slow link adaptation under bler constraint|
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