Method for selecting at least one modulation and coding scheme, corresponding transmission method, selection device and user terminal

Abstract

The invention relates to a method for selecting at least one MCS scheme suitable for being used by a user terminal for transmitting data on at least one subcarrier frequency, implementing a calibration phase comprising, for the at least one subcarrier frequency: - for at least one collaborator satellite and for at least one candidate MCS scheme, obtaining a result of a comparison (E5) between a target value of at least one parameter representing a use of the at least one subcarrier frequency for the candidate MCS scheme expected by the collaborator satellite, and an actual value of the at least one parameter, which value is measured by the collaborator satellite, and selecting (E6-2) at least one MCS scheme suitable for being used by the user terminal for the transmission of data on the at least one subcarrier frequency, among the at least one candidate MCS scheme, taking into account the comparison result(s) obtained.

Description

[0001] DESCRIPTION

[0002] TITLE: Method for selecting at least one modulation and coding scheme, transmission method, selection device and corresponding user terminal.

[0003] Field of invention

[0004] The field of the invention is that of satellite communications. More specifically, the invention relates to a solution for improving the performance of a radio access network whose access nodes are embedded in telecommunications satellites.

[0005] The invention finds a very particular application when several satellites belonging or not to the same constellation of satellites collaborate with each other in order to improve the quality and performance of data transmission with a user terminal located in a cell of a communication network served by one of these satellites.

[0006] Prior art and its drawbacks

[0007] In recent years, the deployment of satellites, particularly in the form of satellite constellations consisting of several hundred to several thousand satellites in low orbit around the Earth, has been accelerating.

[0008] The objective of such deployment of satellite constellations is, among other things, to provide user terminals with high-speed access to a communications network when access to the latter via terrestrial radio access networks is not satisfactory.

[0009] A satellite constellation consists of a group of artificial satellites working together to provide a service, such as access to a communications network, by ensuring the most complete possible ground coverage of the Earth's surface in order to ensure continuity in the provision of this service. To this end, a satellite carries at least one access node of a radio access network interconnected to a core network. Furthermore, each satellite belonging to the same constellation circulates in orbits and trajectories chosen so that all the ground coverages of each of the satellites in the constellation complement each other.

[0010] A satellite's resources are not used continuously over time. Indeed, a satellite may be required to fly over desert areas, aquatic areas, forests, or agricultural areas, in which few, if any, user terminals are located. Thus, the radio communication resources of these satellites are free over many time slots.

[0011] In order to be able to exploit these radio communication resources, cooperation between several satellites of the same constellation or belonging to different constellations can be envisaged. Thus, a satellite belonging to a first constellation flying over an area in which there is no communication terminal to be served can share its radio communication resources with another satellite belonging to a second constellation located in a position in its orbit that does not allow it to optimally serve a given radio cell because its distance and / or its degree of inclination relative to the ground surface of this radio cell, and to the air zone attached to this surface, does not offer sufficient transmission conditions to guarantee an acceptable quality of service to the communication terminals located within this radio cell.

[0012] As satellites move relative to each other and to the user terminals with which they exchange data, this impacts the transmission of radio signals exchanged between these different entities. The effects generated by the movement of the satellites may, for example, consist of a phase shift of the exchanged radio signals, a modification of the amplitude of one or more of these radio signals, or a frequency shift, for example due to the Doppler effect, of the radio signals. This results in a reduction in the performance of the radio access network.

[0013] There is therefore a need for a technique to overcome all or part of the aforementioned drawbacks.

[0014] Statement of the invention

[0015] The invention proposes a new solution in the form of a method for selecting at least one modulation and coding scheme capable of being used by a user terminal for the transmission of data on at least one subcarrier frequency, said user terminal being located in a cell of a radio communication network served by at least one first satellite, called master satellite, said method implementing a calibration phase comprising, for said at least one subcarrier frequency: for at least one collaborating satellite of a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal and for at least one candidate modulation and coding scheme:

[0016] - obtaining a result of a comparison between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by said collaborating satellite, and an effective value of said at least one parameter measured by said collaborating satellite, a selection of at least one modulation and coding scheme capable of being used by said user terminal for the transmission of data on said at least one subcarrier frequency, from among said at least one candidate modulation and coding scheme, taking into account the comparison result(s) obtained.

[0017] Such a solution makes it possible to increase the performance of the radio access network by improving the decoding of a radio signal transmitted by the user terminal and received by at least one satellite participating in the collaboration. Such radio signals are, for example, radio signals multiplexed into orthogonal frequencies or OFDM signals (Orthogonal Frequency-Division Multiplexing in English). The principle of such multiplexing into orthogonal frequencies consists of distributing the data to be transmitted over a plurality of subcarrier frequencies. In order to transmit a large volume of data, the OFDM multiplexing technique is based on the use of subcarrier frequencies that are orthogonal to each other. Thus, even when certain subcarrier frequencies overlap, they do not interfere with each other due to their orthogonality.In an orthogonal frequency division multiplexed radio signal, or OFDM signal, the data intended to be transmitted on the different subcarrier frequencies are coded and modulated according to a modulation and coding scheme.

[0018] This solution has a definite advantage when another satellite collaborates with the master satellite to collect data transmitted by a user terminal located within a cell of the radio access network served by the master satellite. Indeed, the uplink radio signal, transmitted by the user terminal, is on the one hand received by the master satellite, and on the other hand by at least one so-called "collaborating" satellite. The radio signal received by the master satellite may be different from the radio signal received by a collaborating satellite, for example due to the distance between the user terminal and the satellites, or a degree of inclination. Taking into account the radio signal received directly by the master satellite, and the data collected by the collaborating satellite(s), therefore contributes to improving the decoding of the radio signal.

[0019] More particularly, the solution which is the subject of the invention consists of taking into account the effects that the movement of the user terminal relative to the master satellite may have on one or more parameters representing a use of at least one subcarrier frequency of the radio signal transmitted by the user terminal, such as a phase shift, a modification of the amplitude, or even a frequency shift for example due to the Doppler effect, in the choice of a suitable modulation and coding scheme to be used during the transmission of the radio signal by the user terminal.Indeed, the comparison between the target values ​​of the parameters representing a use of said at least one subcarrier frequency of the radio signal emitted by the user terminal, as expected by a collaborating satellite having received the radio signal, and the current values ​​actually measured by the collaborating satellite, makes it possible to identify to what extent one or more parameters representing a use of at least one subcarrier frequency of the radio signal emitted by the user terminal have been impacted by the movement or the context of the user terminal in relation to the satellites.It then becomes possible to choose a modulation and coding scheme which makes it possible to modulate and code said at least one subcarrier frequency of the radio signal, such that the parameters which represent a use of the subcarrier frequency of the radio signal, received by the collaborating satellite or by the master satellite, have current values ​​in accordance with the expected values. For example, said at least one parameter representing a use of said at least one subcarrier frequency belongs to the group comprising: at least one parameter representative of the transmission quality of said at least one subcarrier frequency (such as for example a spectral efficiency, or an error rate, or a retransmission factor, etc.); and the value of the phase and / or the amplitude of the constellation points carried by said at least one subcarrier frequency.As detailed below, these parameters (phase and amplitude) correspond for example to the polar coordinates of the points associated with the symbols received on each subcarrier frequency.

[0020] Advantageously, the solution developed by the inventors can rely on the use of existing modulation and coding tables and does not require any modification of the latter. The invention makes it possible to obtain reduced modulation and coding tables adapted to the collaboration between the master satellite and the collaborating satellite.

[0021] Similarly, such a method can be implemented in any terrestrial or RAN 5G equipment on board an existing satellite without requiring significant modifications to the latter, particularly hardware.

[0022] In particular, such a method can be implemented in the master satellite, in a geostationary satellite, or even in a satellite in high orbit or in low orbit.

[0023] It should be noted that the invention can also be implemented in other flying machines than satellites, in particular for RANs on board aircraft or drones.

[0024] According to a particular embodiment, the selection of at least one modulation and coding scheme selects at least one modulation and coding scheme making it possible to achieve the target value of said at least one parameter for a given minimum number of collaborating satellites.

[0025] Thus, if one or more collaborating satellites collaborate with the master satellite (i.e. at least two satellites participate in the collaboration - the master satellite and at least one collaborating satellite), the modulation and coding scheme is chosen so that the parameters which represent a use of the subcarrier frequency of the radio signal have current values ​​in accordance with the values ​​expected for the largest number of satellites.

[0026] In this way, the data carried by the radio signal generated by the user terminal using the modulation and coding scheme(s) selected at the end of the calibration phase can be decoded and reconstructed by most of the satellites participating in the collaboration (i.e. no or little phase shift or frequency offset for example). In another embodiment, said selection selects at least one modulation and coding scheme making it possible to achieve the target value of said at least one parameter for all of said subcarrier frequencies intended to be used to transmit said data.

[0027] Thus, according to this embodiment, it is sought to apply the same modulation and coding scheme to all the subcarrier frequencies of the radio signal intended to be transmitted by the user terminal. In this way, one (or more) same modulation and coding schemes for all the subcarriers can be selected and sent back to the user terminal at the end of the calibration phase, which simplifies the generation of the radio signal during a phase of transmission of useful data by the user terminal.

[0028] According to a particular embodiment, said method comprises sending a calibration message identifying said at least one selected modulation and coding scheme and / or at least one prohibited modulation and coding scheme.

[0029] Such a calibration message may be sent to said user terminal and / or said collaborating satellites, or even to said master satellite when said selection method is implemented by a geostationary satellite or a high-orbit satellite.

[0030] Information relating to the modulation and coding schemes authorized or prohibited for a given subcarrier frequency may in particular be used by the user terminal to generate a radio signal during a phase of transmission of useful data by the user terminal.

[0031] According to a particular embodiment, said at least one modulation and coding scheme is identified by an index in a modulation and coding table.

[0032] In this way, transmitting a simple index value allows the user terminal to determine the modulation type, modulation order and expected coding rate for example.

[0033] Furthermore, as mentioned previously, this makes it possible to rely on modulation and coding tables already used in existing networks.

[0034] In a particular embodiment, said method comprises, prior to said calibration phase, a determination of said group of collaborating satellites, implementing the sending of a collaboration request.

[0035] For example, said collaboration request carries at least one piece of information belonging to the group comprising: information relating to an envisaged duration of collaboration, a start time and / or an end time of collaboration, an identifier of one or more subcarrier frequencies to which the collaboration relates. In this way, it is sought to ensure that the collaboration between satellites is possible and effective in terms of improving the decoding of the data transmitted by the user terminal for example. According to a particular embodiment, the method implements the sending of at least one parameterization message comprising technical information belonging to the group comprising: at least one FSPi subcarrier frequency authorized to be used during said calibration phase, chosen for example from the subcarrier frequency(ies) to which the collaboration relates,at least one subcarrier frequency prohibited during said calibration phase, said at least one candidate modulation and coding scheme capable of being used by said user terminal to transmit data on said at least one subcarrier frequency FSPi, at least one modulation and coding scheme prohibited during said calibration phase, at least one symbol of a modulation associated with said at least one candidate modulation and coding scheme authorized to be transmitted during said calibration phase, at least one symbol of a modulation prohibited during said calibration phase.,

[0036] In particular, such a parameterization message may be transmitted by the master satellite, or by a geostationary or high-orbit satellite. Such a message may in particular be received by said user terminal and / or the satellite(s) participating in the collaboration.

[0037] The technical information received by the user terminal can be used by the latter during the calibration phase, so as to select a modulation and coding scheme capable of being used by said user terminal for the transmission of data during a useful data transmission phase.

[0038] According to a particular characteristic, the method implements the reception of at least one test message carrying data transmitted on at least one subcarrier frequency modulated according to one of said candidate modulation and coding schemes, taking into account said at least one authorized symbol, identified in said at least one parameterization message.

[0039] In particular, it is recalled that a radio signal to which a digital modulation such as quadrature amplitude modulation (QAM) or phase-shift keying (PSK) is applied can be represented by means of a set of points placed in a constellation diagram associated with coordinates. Each point is assigned a group of different bits called a symbol. The number of bits constituting a symbol is defined by the modulation order of the chosen modulation and coding scheme.

[0040] A constellation diagram is, for example, a two-dimensional diagram whose two axes, one representing the phase and the other the amplitude, delimit the complex plane at a sampling instant of the symbols of a given subcarrier frequency. A point in a constellation diagram represents a symbol at a given instant resulting from the modulation of a subcarrier frequency.

[0041] Once the radio signal has been received (by the master satellite, for example), a demodulator determines the positions of the points corresponding to the different subcarrier frequencies constituting the received signal. The subcarrier frequencies may have been disturbed during the transmission of the radio signal or by the equipment receiving the radio signal, so that a point corresponding to a given subcarrier, or received point, may be represented in the demodulator's constellation diagram by a point whose current coordinates are distinct from the expected coordinates for the symbol corresponding to this point.

[0042] In order to identify the symbol corresponding to the received point (also called the received symbol for the sake of simplification), the demodulator extracts the coordinates of the received point from the received signal and then searches for the closest point on the constellation diagram, for example the point on the diagram with the smallest Euclidean distance from the received point.

[0043] However, the demodulator may have selected a point in the constellation diagram that is different from the point chosen by the transmitter modulator. Indeed, in the case of a highly disturbed signal, it is not because it is closest to the current coordinate point that this point corresponds to the point used by the modulator. This is the case, for example, when the subcarrier frequency undergoes a phase, amplitude or frequency shift due to a very high disturbance of the signal caused by a building, or a displacement of the transmitter of the radio signal relative to the receiver of this same radio signal.

[0044] In the embodiment according to which the parameterization message carries at least one symbol of a modulation prohibited during the calibration phase, or equivalently points of the constellation diagram prohibited during the calibration phase, the grid used by the modulator contains prohibited points which increase the distance between the received points in order to allow the identification of a point very offset in the grid. Thus, the distance of the points in the constellation diagram being greater, the risks of selecting a point of expected coordinates which would not actually correspond to a transmitted symbol are reduced.

[0045] Similarly, the insertion in the parameterization message of technical information relating to at least one modulation and coding scheme prohibited during said calibration phase makes it possible to reduce the risks of selecting a point of expected coordinates of the constellation diagram of a demodulator which would not actually correspond to a symbol of a subcarrier frequency of the radio signal.

[0046] In another example of implementation of the solution which is the subject of the present invention, said selected modulation and coding scheme corresponds to a coding rate greater than or equal to a first threshold. By increasing the volume of redundant data transmitted, the reception quality is improved.

[0047] Generally, the selected modulation and coding scheme modifies one of the following parameters representing a use of said at least one subcarrier frequency of said radio signal:

[0048] - a phase of the radio signal,

[0049] - a radio signal frequency,

[0050] - an amplitude of the radio signal.

[0051] According to a particular embodiment, the method according to the invention comprises a determination of at least one phase, frequency and / or amplitude correction profile for said at least one modulation and coding scheme selected for said at least one subcarrier frequency.

[0052] In particular, said at least one correction profile can be sent in said calibration message or in a separate message, for example to said user terminal and / or said collaborating satellites, or even to said master satellite when the method is implemented by a geostationary satellite or by a satellite in high orbit.

[0053] It is thus possible to correct the phase and / or frequency and / or amplitude of the radio signal by applying an appropriate correction profile, either before transmission by the user terminal, or after reception by a satellite participating in the collaboration (including by the master satellite).

[0054] According to a particular characteristic, the selection method comprises the determination of said effective value of said at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme, from said at least one test message transmitted by the user terminal.

[0055] For example, several test messages may be transmitted by the user terminal using different candidate modulation schemes, so as to determine the current value of said at least one parameter on the signal actually received by the collaborating satellite.

[0056] In a particular embodiment, the selection method comprises determining a subgroup of collaborating satellites taking into account the similarities of the results of said comparison of said calibration phase obtained for at least two collaborating satellites.

[0057] This allows the number of satellites participating in the collaboration to be reduced, so that the satellites offering the best results can be kept in the collaboration.

[0058] In another embodiment, the invention relates to a corresponding selection device, comprising at least one processor configured to implement said calibration phase.

[0059] Such a device is particularly suitable for implementing the selection method described above. It is, for example, integrated into a master satellite, a geostationary satellite, a high-orbit satellite, a low-orbit satellite or even a group of low-orbit or high-orbit satellites. This device may of course include the various characteristics relating to the selection method according to the invention, which may be combined or taken in isolation. Thus, the characteristics and advantages of this device are the same as those of the method and are not detailed further.

[0060] In yet another embodiment, the invention relates to a method for transmitting data on at least one subcarrier frequency, implemented by a user terminal located in a cell of a radio communication network served by at least one first satellite, called the master satellite.

[0061] Such a method comprises: receiving, at the end of a calibration phase, at least one modulation and coding scheme selected from at least one candidate modulation and coding scheme, taking into account at least one comparison result between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by a collaborating satellite, and an actual value of said at least one parameter measured by said collaborating satellite, said collaborating satellite belonging to a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal, transmitting data on said at least one subcarrier frequency, modulated according to one of said at least one selected modulation and coding scheme.

[0062] The proposed solution thus allows a user terminal to transmit, during a useful data transmission phase, a radio signal constructed using a modulation and coding scheme adapted to the transmission conditions, taking into account in particular the position of the user terminal relative to a master satellite and at least one collaborating satellite.

[0063] In particular, the coding and modulation scheme can be selected by the master satellite (or by a geostationary satellite, or even a high-orbit satellite) during a calibration phase as described above.

[0064] Thus, in a particular embodiment, the method comprises beforehand: receiving at least one parameterization message comprising technical information belonging to the group comprising: at least one FSPi subcarrier frequency authorized to be used during said calibration phase (chosen for example from among the subcarrier frequency(ies) to which the collaboration relates), at least one subcarrier frequency prohibited during said calibration phase, at least one candidate modulation and coding scheme capable of being used by said user terminal to transmit data on said at least one FSPi subcarrier frequency, at least one prohibited modulation and coding scheme during said calibration phase, at least one symbol of a modulation associated with said at least one candidate modulation and coding scheme authorized to be transmitted during said calibration phase,at least one symbol of a modulation prohibited during said calibration phase, and for at least one authorized subcarrier frequency and for at least one candidate modulation and coding scheme: sending at least one test message carrying data transmitted on said at least one subcarrier frequency modulated according to said candidate modulation and coding scheme, taking into account said at least one authorized symbol, identified in said at least one parameterization message.,

[0065] According to a particular embodiment, the method comprises receiving at least one phase, frequency and / or amplitude correction profile for said at least one modulation and coding scheme selected for said at least one subcarrier frequency, and implements a correction of said at least one subcarrier frequency modulated according to said at least one selected modulation and coding scheme taking into account said correction profile.

[0066] As already indicated, it is thus possible to correct the phase and / or the frequency and / or the amplitude of the radio signal by applying an adequate correction profile to it, before transmission by the user terminal.

[0067] In another embodiment, the invention relates to a corresponding terminal, comprising at least one processor configured to: receive, at the end of a calibration phase, at least one modulation and coding scheme selected from at least one candidate modulation and coding scheme, taking into account at least one comparison result between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by a collaborating satellite, and an actual value of said at least one parameter measured by said collaborating satellite, said collaborating satellite belonging to a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal, transmit data on said at least one subcarrier frequency,modulated according to one of said at least one selected modulation and coding scheme.,

[0068] Such a user terminal is particularly suitable for implementing the transmission method described above. This terminal may of course include the various characteristics relating to the transmission method according to the invention, which may be combined or taken in isolation. Thus, the characteristics and advantages of this terminal are the same as those of the transmission method and are not detailed further. The invention finally relates to at least one computer program product comprising program code instructions for implementing the selection method and / or the transmission method as described above, when it is executed by a processor.

[0069] The invention also relates to a computer-readable recording medium on which is recorded a computer program comprising program code instructions for executing the steps of the selection method and / or the transmission method according to the invention as described above.

[0070] Such a recording medium may be any entity or device capable of storing the program. For example, the medium may include a storage medium, such as a ROM, for example a CD-ROM or a microelectronic circuit ROM, or a magnetic recording medium, for example a USB flash drive or a hard disk.

[0071] On the other hand, such a recording medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means, so that the computer program contained therein is remotely executable. The program according to the invention may in particular be downloaded over a network, for example the Internet.

[0072] Alternatively, the recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the selection method and / or the transmission method which are the subject of the aforementioned invention.

[0073] List of figures

[0074] Other aims, characteristics and advantages of the invention will appear more clearly on reading the following description, given as a simple illustrative, and non-limiting, example, in relation to the figures, among which:

[0075] [fig. 1]: this figure represents an example of a system in which the present invention can be implemented,

[0076] [fig. 2]: this figure represents a diagram of the exchanges occurring between the different elements of the system of figure 1 during the implementation of the method of selecting at least one modulation and coding scheme according to an embodiment of the invention,

[0077] [fig. 3]: this figure represents a constellation diagram of a 16QAM modulation,

[0078] [fig- 4]: This figure illustrates the amplitude and phase of each point in the constellation diagram of figure 3,

[0079] [fig- 5]: this figure represents a satellite capable of implementing all or part of the methods which are the subject of the invention,

[0080] [fig- 6]: this figure illustrates the simplified structure of a user terminal according to a particular embodiment of the invention. Detailed description of embodiments of the invention

[0081] The invention relates to a solution for improving the performance of a radio access network whose access nodes are embedded in telecommunications satellites. It applies more particularly when several satellites, whether or not belonging to the same constellation of satellites, collaborate with each other in order to improve the quality and performance of data transmission with a user terminal located in a cell of a communications network served by one of these satellites. More specifically, it is envisaged here that a plurality of satellites participate in the collection of radio signals transmitted by a user terminal, and that the data carried by the radio signals thus collected by these different satellites are aggregated by a so-called master satellite, which allows it to obtain better quality data, easier to demodulate and decode, and incidentally to improve the uplink performance of the access network.

[0082] The general principle of the invention is based on taking into account the effects that the movement of the user terminal, relative to the plurality of satellites participating in this collaboration, can have on parameters (frequency, phase and / or amplitude) representing a use of at least one subcarrier frequency of a radio signal transmitted by the user terminal. These effects can typically result in a transmission quality criterion not being achieved on this subcarrier frequency, a phase shift and / or a modification of the amplitude of a signal transmitted on this subcarrier frequency, or even a frequency shift (for example due to the Doppler effect) of this subcarrier frequency, and incidentally have an impact on the modulation (e.g. on its order) and the coding (e.g.on its performance) to be applied when transmitting data on this subcarrier frequency by the user terminal, on the satellites involved in the collaboration or even on possible corrections to be made (for example at the level of the user terminal or the collaborating satellites) to limit these effects.

[0083] More specifically, the invention proposes the implementation of a so-called calibration phase during which comparisons are made, for each satellite participating in the collaboration, and for different modulation and coding schemes (for example the different modulation and coding schemes or MCS (for "Modulation and Coding Scheme" in English) envisaged in a modulation and coding table implemented by the access network, known to those skilled in the art). These comparisons are made, for a given satellite and MCS scheme, between expected values ​​of the parameters representing a use of at least one subcarrier frequency of the radio signal transmitted by the user terminal using this MCS scheme and received by this satellite, and the current values ​​of these parameters.They make it possible to identify the extent to which these parameters have been impacted by the movement of the user terminal relative to the satellite in question for the modulation and coding scheme considered. This makes it possible, at the end of the calibration phase, to select one or more MCS schemes that can be used by the user terminal to modulate and code said at least one subcarrier frequency when it transmits a radio signal to the access network, and ensuring that the current values ​​(i.e. actually received) of the parameters in question are consistent with the expected values ​​despite the movement of the user terminal. In the present description, for the sake of simplification, the expression “coding and modulating a subcarrier frequency” is used to designate the processing which consists of coding and modulating the data which are then intended to be transmitted on this subcarrier frequency.

[0084] A system in which the present invention is implemented is now presented in relation to [fig. 1].

[0085] Such a system comprises a radio access network RAN ​​and a core network CORE interconnected with each other by means of a link established between at least one first satellite 4 and at least one GW equipment located on the ground or also on board a satellite.

[0086] In the example envisaged in Figure 1, the RAN access network comprises a plurality of satellite constellations const-1, const-2, const-3 placed in orbit around the Earth or any other set of stars (e.g. Earth and moon), at different altitudes. It will be understood that a RAN access network can comprise several dozen satellite constellations.

[0087] The first constellation of const-1 satellites includes three satellites 1-1, 1-2 and 1-3. Of course, such a constellation of const-1 satellites can include up to several thousand satellites.

[0088] The second satellite constellation, Const-2, consists of two satellites, 2-1 and 2-2. Of course, such a constellation, Const-2, can also consist of up to several thousand satellites.

[0089] Finally, the third satellite constellation const-3 also includes three satellites 3-1, 3-2 and 3-3. Of course, such a constellation of satellites const-3 can, just like the satellite constellations const-1 and const-2, include up to several thousand satellites.

[0090] The RAN radio access network also includes a fourth const-4 constellation comprising a single satellite 4. Of course, such a const-4 satellite constellation can also comprise up to several thousand satellites.

[0091] Each satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 here carries a gNB access node to the RAN radio access network and is likely to participate in a collaboration as described previously to improve the quality of uplink transmissions of user terminals of the RAN radio access network.

[0092] The RAN radio access network also comprises a plurality of cells, four of which, cell 1, cell 2, cell 3, cell 4, are represented in Figure 1. Each cell cell 1, cell 2, cell 3, cell 4, of the RAN radio access network is served by at least one satellite belonging to the RAN radio access network, for a given duration corresponding to the time of overflight of the cell by said satellite. In the remainder of this document, only cell cell 2 is considered for the purpose of simplification. Similarly, in the remainder of this document, it is considered that cell cell 2 is served by satellite 4.

[0093] A user terminal UE, such as a smartphone, a drone, a car or even an loT sensor for example, is located in cell 2 and has established a link with satellite 4 (or more specifically the gNB access node embedded in this satellite).

[0094] [Fig. 2] represents a diagram of the exchanges occurring between the different elements of the system illustrated in Figure 1 (user terminal UE and satellites of the radio access network RAN) during the implementation of the method for selecting at least one modulation and coding scheme (MCS scheme) which can be used to modulate and code at least one subcarrier frequency of a radio signal intended to be transmitted by the user terminal UE.

[0095] In the embodiment described herein, said at least one MCS scheme is selected from a set of MCS schemes listed in a modulation and coding table MCS-TAB used for uplink data transmissions on the RAN access network. Such a modulation and coding table is known per se. It is in the form of a plurality of combinations of modulations (which may in particular have different orders) and coding schemes (which may in particular have different rates), which can be used to modulate and code data transmitted in uplink by a user terminal. Each combination defines a distinct MCS scheme, uniquely identified in the MCS-TAB table by an index.Thus, selecting at least one MCS scheme in such a table amounts to selecting at least one index of the table, and knowledge of the table by the user terminal and by the satellites of the system of figure 1 makes it possible, from the index, to identify the corresponding modulation and coding scheme. The use of such a table therefore simplifies the exchanges within the system 1 to designate a given MCS scheme.

[0096] An example of an MCS-TAB table is illustrated in the following table, denoted [Table 1]:

[0097] In this example, 15 distinct MCS schemes are considered based on three quadrature amplitude modulations of different orders (2, 4 and 6), also known as QAM modulations (for "Quadrature Amplitude Modulation" in English), namely a 4-QAM modulation (equivalent to a QPSK modulation (for "Quadrature Phase Shift Keying" in English)), a 16-QAM modulation and a 64-QAM modulation. Different coding rates are also considered and associated with these three modulations.

[0098] Of course, this is only a non-limiting illustrative example of the invention. Alternatively, one can envisage other types of modulation (e.g. PSK, etc.), other orders, that the coding schemes use different coding techniques, or different puncturing patterns to achieve the same efficiency, etc. Furthermore, one can also envisage a different implementation of the invention which uses other formalisms to designate an MCS scheme than an index of a pre-established MCS table. It is assumed that the user terminal UE, located in the cell 2, wishing to establish a communication with another user terminal or to be provided with a service such as a content delivery service (not shown in the figures), attaches to the radio access network RAN ​​by establishing a radio link with the access node gNB of the radio access network RAN ​​serving the cell 2, in other words with the access node gNB on board the satellite 4.In the implementation example described below, satellite 4 serving cell 2 acts as the master satellite when inter-satellite collaboration is implemented for user terminal UE uplink transmissions.

[0099] More particularly, in a first step, the satellite 4 determines whether collaboration with other satellites of the RAN access network can be implemented to improve the reception of the data transmitted by the user terminal UE. To this end, the master satellite 4 broadcasts in a step E1 a collaboration request req-coll to the other satellites of the RAN access network. Such a collaboration request req-coll is intended to search for satellites likely to collaborate with it for the reception of data from the user terminal UE (i.e. in particular having resources available for this purpose) and comprises, in the embodiment described here:

[0100] • information relating to the duration of the planned collaboration,

[0101] • a start time (and possibly an end time) for the collaboration, and

[0102] • an identifier of one or more subcarrier frequencies FSPj to which the collaboration relates. It should be noted that the invention is not necessarily implemented on all the subcarrier frequencies of the radio signal transmitted by the user terminal UE but may only be applied to some of them.

[0103] No limitation is attached to the triggering event of the collaboration sought by the satellite 4. For example, the satellite 4 may trigger such collaboration after detecting that the power of the signals received from the user terminal UE is below a certain threshold or that any other quality of service parameter (e.g. error rate, etc.) does not satisfy a determined criterion. The master satellite 4 may typically carry out such a search for other satellites in order to obtain their collaboration to improve the collection of data transmitted by the user terminal UE while another satellite belonging to the constellation const-4, to which the master satellite 4 belongs, is in a position to be able to serve the cell cell-2 in which the user terminal UE is located.

[0104] Once the satellites likely to collaborate with the master satellite 4 for the uplink transmissions of the user terminal UE have been identified (for example, the satellites having responded favorably to the req-coll request, this response being able to reflect the extent of the accepted collaboration, and in particular to identify the FSPi subcarrier frequencies that the satellites in question can receive), a calibration phase is implemented with the user terminal UE and with these satellites.

[0105] This calibration phase aims to determine the modulation and coding schemes (MCS schemes) which can be applied in transmission by the user terminal UE on all or part of the subcarriers targeted by the collaboration in order to maximize the efficiency of the collaboration implemented between the satellites. It can also be used to identify, for all or part of the chosen MCS schemes, phase, frequency and / or amplitude correction profiles which can be applied in transmission, at the user terminal UE, or in reception, at the collaborating satellites and the master satellite 4, to further improve the efficiency of the selected MCS schemes. The calibration phase can also make it possible to restrict the number of satellites actually participating in the collaboration, by selecting a group of satellites for which this collaboration seems the most promising for the master satellite 4.

[0106] In the following description, for the sake of simplification, it is assumed that satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 have responded favorably to the request for collaboration from satellite 4.

[0107] In a first implementation example, the calibration phase is directed by the master satellite 4. Thus, during a step E2, the master satellite 4 sends to the user terminal UE at least one parameterization message MSG1 comprising technical information intended to be used by the user terminal UE during the calibration phase to communicate with the master satellite 4. This information comprises, in the embodiment described here, at least one identifier of a subcarrier frequency FSPi, at least one index of a modulation and coding table (or equivalently an MCS scheme) to be used to modulate and code said at least one subcarrier frequency FSPi, and a list LS identifying at least one symbol of said at least one modulation associated with said at least one index.The aforementioned technical information contained in the MSG1 message is also transmitted to the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 having accepted the collaboration. It should be noted that the invention applies in a context where a distinct MCS scheme per subcarrier frequency is applied as in a context where the same MCS scheme is applied to all or part of the subcarrier frequencies, the content of the MSG1 message being adapted according to the context.

[0108] As is known per se, a symbol of an M-ary modulation with M=2 Q, Q. denoting the order of the modulation, is obtained by grouping Q bits together. Each symbol of the modulation is represented by a point in the modulation constellation diagram (a two-dimensional diagram represented in an IQ coordinate system, the I axis denoting the in-phase component and the Q axis the quadrature component, and which represents the signals modulated by the modulation, in other words the points of the constellation). In the following, each point of the constellation is defined by its polar coordinates which include an amplitude and a phase.

[0109] In a first implementation variant, the list LS identifies a set of symbols of said at least one modulation (or equivalently points of the constellation diagram) authorized to be transmitted during the calibration phase on the subcarrier FSP1 by the terminal UE.

[0110] In a second implementation variant, the list LS identifies a set of symbols of said at least one modulation (or equivalently points of the constellation diagram) prohibited (forbidden) during the calibration phase (i.e. which must not be transmitted by the terminal UE during the calibration phase for the subcarrier FSP1).

[0111] It should be noted that the MSG1 message may comprise the identifiers of several FSPj subcarriers, several indices of the modulation and coding table (or an interval comprising these indices when they are consecutive, or designate the MCS-TAB table [Table 1] as a whole) or several modulation symbols in the LS list. The MSG1 message may then further include an indication of the order in which the user terminal UE must consider the indices and symbols of the LS list during the calibration phase in order to allow the satellites to demodulate and decode the data transmitted by the user terminal UE.

[0112] Such an MSG1 message is for example exchanged between the master satellite 4 and the user terminal UE when establishing a communication session, in particular during the exchange of information relating to operation, administration and maintenance or “OAM”.

[0113] As mentioned previously, the MSG1 message is also sent by the master satellite 4 to the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 having agreed to collaborate with it to collect the radio signals (and the data carried by these radio signals) transmitted by the user terminal UE. Then, a plurality of iterations are implemented so as to test each MCS scheme identified in the MSG1 message broadcast by the master satellite 4 and to take into account the symbols identified in the LS list. It should be noted that other MSG1 messages may be sent by the master satellite 4 to the user terminal UE and the satellites having agreed to collaborate during the calibration phase, these messages containing in the LS list, other symbols to be taken into account for one or more MCS schemes, as described in more detail later.Furthermore, in an alternative embodiment, separate MSG1 messages (one or more) may be broadcast by the master satellite 4 on the cell 2 for each MCS scheme of the MCS-TAB table [Table 1] to be tested, or for each MCS scheme corresponding to a separate modulation (typically, a message for a different constellation order in the illustrative example given for the MCS-TAB [Table 1]).

[0114] For a subcarrier frequency FSPi, each iteration, denoted iter, implemented on a given MCS scheme, denoted MCS(iter), identified by one of the indexes included in the message MSG1, comprises the steps E3 to E6 described below. For the sake of simplification, it is assumed here that all the subcarrier frequencies identified in the message MSG1 are coded and modulated with the same MCS(iter) scheme. However, in an alternative embodiment, as mentioned previously, a separate MCS scheme may be envisaged for all or part of the subcarrier frequencies targeted by the message MSG1. In this case, during the iter iteration, a combination of MCS schemes is applied to the different subcarrier frequencies envisaged and the iterations are implemented on the different possible combinations of MCS schemes on these different subcarrier frequencies.

[0115] In a step E3, the user terminal UE transmits, following receipt of the parameterization message MSG1, a test message MSG2 containing data carried by the subcarrier frequencies identified in the message MSG1, and coded and modulated using the MCS(iter) scheme.

[0116] The data included in the MSG2 message may be of different natures. It may be data of a known sequence (for example defined in an access network specification document) or application data of the UE terminal. The UE terminal, however, ensures that the modulated symbols conveying this data (after application of the MCS(iter) scheme) are in conformity with the symbols specified in the LS list transmitted in the MSG1 message. More particularly, if this LS list corresponds to a list of symbols of the modulation to be transmitted during the calibration phase, these symbols must be present in the symbols sent by the user terminal UE. Conversely, if the LS list includes a list of prohibited symbols, the symbols of the MSG2 message must not include any of the symbols identified in the LS list.

[0117] Each satellite among the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3, 4, receiving the message MSG2 determines, during a step E4, a current value of parameters representing the use of the subcarrier frequencies FSPi that it has captured in the radio signal carrying the message MSG2. In the embodiment described here, these parameters comprise: at least one parameter representative of the transmission quality of the subcarrier frequency, such as for example a spectral efficiency or an error rate or a retransmission factor, etc.; and the value of the phase and / or the amplitude of each constellation point carried by each subcarrier frequency FSPi. These parameters (phase and amplitude) correspond to the polar coordinates of the points associated with the symbols received on each subcarrier frequency.

[0118] It is possible to envisage that the satellites also determine other parameters such as, for example, the frequencies themselves of the subcarriers on which the points carried by the radio signals emitted by the user terminal UE are received by the satellite in question.

[0119] Then, during a step E5, in the embodiment described here, each satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 carries out the comparison between the current values ​​of the parameters (transmission quality parameter and polar coordinates of the points) determined during the step E4 with expected values ​​of these same parameters. These expected values ​​correspond here, for a quality parameter, to a target value of the latter, and for the polar coordinates, to the polar coordinates of the points of the constellation diagram of the modulation of the tested MCS scheme closest to the points defined by the polar coordinates determined during the step E4.The comparison is for example carried out by the satellite considered by calculating the difference between current values ​​and expected values ​​(for example a difference calculated for each quality parameter, and a difference calculated for each of the components of the polar coordinates, in other words one for the phase and one for the amplitude). No limitation is attached to the form of the calculated difference; it can in particular, depending on the parameter considered, take the form of a Euclidean distance or any other type of distance, a deviation (negative or positive), etc.

[0120] The results of the comparisons made by satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 (i.e. here the calculated deviations) are then transmitted by the latter during a step E6-1 to the master satellite 4.

[0121] In another embodiment, satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 transmit to master satellite 4 the current and expected values ​​of the parameters representing the use of the subcarrier frequencies that they captured during step E5 and it is master satellite 4 itself which carries out the comparisons between current values ​​and expected values ​​during step E6-1.

[0122] The master satellite 4 is then capable, during a step E6-2, of determining, using the results of the comparisons obtained, for each subcarrier frequency FSPi, one or more MCS schemes that can be used by the user terminal UE to modulate and code this subcarrier frequency. For example, the master satellite 4 determines one or more indexes Idxj of the modulation and coding table MCS-TAB [Table 1] that can be used to modulate and code a subcarrier frequency FSPj intended to be used for future communications with the user terminal UE.

[0123] For this purpose, several strategies can be implemented by the master satellite 4.

[0124] For example, according to a first strategy, the master satellite 4 selects, for a subcarrier frequency FSPi, the indexes Idxj from the modulation and coding table MCS-TAB [Table 1] which make it possible to achieve the target values ​​of the transmission quality parameter(s) considered at least for a given minimum number of satellites (for example for at least two satellites in order to be able to set up a collaboration).

[0125] According to a second strategy, which has a preferred application when a single MCS scheme is envisaged for all the subcarriers, the master satellite 4 selects the index(es) Idxj from the MCS-TAB table [Table 1] of modulation and coding which make it possible to guarantee the target values ​​of the transmission quality parameter(s) considered for all the subcarriers.

[0126] Of course, other strategies can be considered as alternatives.

[0127] The master satellite 4 can also, during step E6-2, use the comparison results obtained, and in particular the results of the comparisons made on the phase and amplitude parameters of the points received by each satellite, to group the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 into different groups with which it will collaborate or not to collect the data transmitted by the user terminal UE.

[0128] The master satellite 4 can also, during step E6-2, determine, from these comparison results, a phase, amplitude, and / or frequency correction profile (for example in the presence of a Doppler effect) to be applied for the MCS scheme considered for a subcarrier frequency to improve the decoding and demodulation of the signals received at the level of the master satellite 4 modulated and coded by means of the MCS schemes that it has identified as being able to be used by the user terminal UE.

[0129] For example, a correction profile is determined by the master satellite 4 so as to improve the demodulation and decoding of the data that it receives directly from the user terminal UE (from the comparison that it itself carries out on the data received directly from the user terminal UE) and it can consider collaborating with a group of satellites among the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-

[0130] 3 which obtained comparison results close to its own. Thus, for illustration purposes, the master satellite

[0131] 4 can select the group of satellites for which the phase distortion in reception measured by these satellites is identical or similar to the phase distortion measured in reception by the master satellite 4 from the signal that it receives directly from the user terminal UE. The master satellite 4 determines in this case a correction profile comprising the phase correction to be applied to compensate for the phase distortion in question. In a particular embodiment, the determined correction profile is sent to the user terminal UE so that it makes the phase, amplitude and / or frequency corrections corresponding to the transmission.

[0132] In another embodiment called "opportunistic", each collaborating satellite (satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 in the example envisaged here) or only some of them stores the results of its comparisons and then applies them when it receives application data from the user terminal UE to correct the position of the points in the constellation diagram corresponding to this received data. The way in which the master satellite 4 proceeds for the grouping of satellites and / or the determination of a correction profile is described in more detail later.

[0133] In a second implementation example, a geostationary satellite GEO (not shown in Figure 1), or a satellite in high orbit compared to satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3, directs the calibration phase. In the following, this second implementation example is described with reference to the geostationary satellite GEO but it can be implemented in an identical or similar manner by the satellite in high orbit.

[0134] Thus, in this second implementation example, it is the geostationary satellite GEO which is at the origin of the parameterization message MSG1 and which broadcasts it during a step E2' on the cell 2. The user terminal UE and the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 then proceed during steps E2' to E5', following the reception of this message MSG1, in a manner identical or similar to what has just been described for steps E2 to E5.

[0135] The results of the comparisons carried out in step E5' by satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 are transmitted by satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 during a step E6-1' to the geostationary satellite GEO.

[0136] In an alternative embodiment, the current and expected values ​​of the parameters representing the use of the subcarrier frequencies captured by the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 are transmitted by the latter to the geostationary satellite GEO during step E5' and it is this satellite, during step E6-1', which carries out the comparisons.

[0137] The geostationary satellite GEO is then capable, during a step E6-2', thanks to the results of the comparisons obtained, of determining, for each subcarrier frequency FSPj, one or more indices Idxi of the modulation and coding table which can be used to modulate and code a subcarrier frequency FSPj intended to be used for future communications with the user terminal UE. Step 6-2' implemented by the geostationary satellite GEO is similar or identical to step E6-2 of the calibration phase directed by the master satellite 4.

[0138] The geostationary satellite GEO transmits the information determined during step E6-2' to the master satellite 4 in a step E6-3'. Regardless of the implementation implemented for the calibration phase, at the end of this phase, the master satellite 4 has the indexes of the modulation and coding table that can be used to modulate and code a subcarrier frequency FSPi intended to be used for future communications of the user terminal UE with the master satellite 4. These indexes define a reduced modulation and coding table (compared to the initial MCS-TAB table) that can be used by the terminal UE during its uplink communications.

[0139] In another particular embodiment, these same indexes can be used for uplink transmissions from other user terminals located in cell-2 with master satellite 4.

[0140] In the embodiment described here, in a step E7, the master satellite 4 transmits, to the user terminal UE, a calibration message MSG3 comprising the index(es) Idxj identified during step E6-2 or E6-2' of the calibration phase which can be used by the user terminal UE to transmit data to the master satellite 4. The message MSG3 is also sent by the master satellite 4 to the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 which have agreed to collaborate with the master satellite 4, or only to those selected, where appropriate, by the master satellite 4 or by the geostationary satellite GEO or in high orbit for the collaboration.The MSG3 calibration message may further contain a correction profile to be applied by the user terminal UE (or by the satellites involved in the collaboration depending on the chosen embodiment), and determined where appropriate for each index Idxj identified in the MSG3 message, as described previously.

[0141] In an alternative embodiment, when the calibration phase was directed by the geostationary satellite GEO or by a high-orbit satellite, the MSG3 message can be sent by the latter instead of the master satellite 4 to the user terminal UE and / or to the satellites having accepted the collaboration or being selected for the collaboration.

[0142] The user terminal UE can then transmit application data to the master satellite 4, in a step E8, carried by subcarrier frequencies FSPi modulated and coded by means of an MCS scheme from the MCS-TAB table [Table 1] corresponding to one of the indices Idxj received in the message MSG3. The choice of the index from among the indices Idxj received in the message MSG3 is carried out dynamically, in a conventional manner known to those skilled in the art (for example, so as to comply with a given quality of service criterion, such as a block error rate lower than a certain threshold, typically 10%). As mentioned above, the invention applies in a context where all subcarrier frequencies are coded and modulated with the same MCS scheme (in which case, the MCS scheme in question must be allowed for all subcarrier frequencies) or different MCS schemes can be applied to different subcarrier frequencies.The data transmitted by the terminal UE during step E8 are received by the master satellite 4 and by the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 having agreed to collaborate with the master satellite 4. The data received by the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 are retransmitted to the master satellite 4 during a step E9 within the framework of the collaboration between satellites.

[0143] Thus, the master satellite 4 collects the data transmitted by the user terminal UE which reaches it directly and aggregates them during a step E10 with the data transmitted by the user terminal UE and collected by the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 which the latter have transmitted to it. This contributes to improving the performance of the radio access network for the uplink by taking advantage of the unused resources of certain satellites of the RAN access network.

[0144] In the embodiment described here, the calibration message MSG3 comprises the list of indexes of the modulation and coding schemes that can be used by the user terminal UE for its uplink transmissions to the master satellite 4. Alternatively, it may equivalently contain a list of indexes Idxj whose use for modulating and coding a subcarrier frequency FSPi is prohibited.

[0145] The methods for selecting the Idxi indices, for grouping the satellites for collaboration and for determining the correction profiles of the selected MCS schemes are now described in more particular, which can be implemented according to the implementation chosen in steps E4, E5, E6-1, E6-2 or E4', E5', E6-1', E6-2'.

[0146] In a particular implementation example, when a satellite among the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3, 4, receives a test message MSG2, transmitted by the user terminal UE during an iteration iter of the calibration phase using an MCS(iter) scheme (corresponding to an index provided in the parameterization message MSG1), a demodulator embedded in this satellite determines the positions, in the two-dimensional plane, of the points corresponding to the different subcarrier frequencies constituting the received signal (hereinafter referred to as “received points”).

[0147] In the following, we describe in detail the processing carried out for a single subcarrier frequency; all of these processing operations are applied in an identical manner to the other subcarrier frequencies targeted by the MSG1 message.

[0148] As mentioned above, in order to identify the modulation symbol corresponding to the received point (hereinafter referred to as the “received symbol”) on a subcarrier frequency FSPi, the demodulator selects a point in the constellation diagram of the modulation used to modulate this subcarrier frequency whose coordinates (called “expected” coordinates) are closest to the coordinates of the received point (called current coordinates), i.e. having the smallest Euclidean distance, with the current coordinates of the received point representing the received symbol. The demodulator then considers that the received symbol is the symbol corresponding to the point in the constellation diagram thus selected.

[0149] It should be noted that by doing so, the demodulator may have selected an incorrect point in the constellation diagram as representing the received symbol. Indeed, it is not because it is closest to the point of current coordinates representing the received symbol that this point of expected coordinates corresponds to the symbol actually transmitted by the UE terminal. This is for example the case when the subcarrier frequency undergoes an offset due to a movement of the user terminal transmitting the radio signal (e.g. user terminal UE) relative to the receiver of this same radio signal (e.g. master satellite 4 or any of the satellites collaborating with it).

[0150] As an example, consider the 16-QAM square-wave modulation whose constellation diagram is illustrated in Figure 3, each point of the constellation diagram being defined by polar coordinates including an amplitude and a phase, given in the table presented in Figure 4.

[0151] It is now assumed that the user terminal UE transmits, to the master satellite 4, a symbol 1110 of the 16-AQM modulation on a subcarrier frequency. This symbol corresponds in the constellation diagram of the 16-AQM modulation (in other words of the modulator of the UE terminal), to the point Z with polar coordinates (0.75V, 75°).

[0152] It is further assumed that the symbol 1110 thus transmitted by the user terminal UE and received by the satellite considered (among the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3, 4) is represented in the constellation diagram of the 16-AQM modulation, at the demodulator of the satellite in question, by a point X with current polar coordinates (0.12V, -50°). As illustrated in Figure 3, this point X, in the present example, is close to a point Y of the constellation diagram of the 16-AQM module with coordinates (0.25V, -45°) and corresponding to the symbol 1111. This point Y with coordinates (0.25V, -45°) being the point of the constellation diagram closest to the point X, the demodulator of the satellite chooses it as representing the symbol that it has received. In other words, the satellite demodulates the point X it received into the symbol 1111 corresponding to point Y in the constellation diagram.

[0153] However, in this case, the transmitted symbol 1110 normally corresponds to point Z of the constellation diagram. Since point X with current coordinates (0.12V, -50°) is very far from point Z with coordinates (0.75V, 75°) corresponding to the symbol actually transmitted by the UE terminal, it cannot be selected by the satellite demodulator. The satellite demodulator will therefore identify symbol 1111 as the demodulated symbol instead of symbol 1110, thus generating an error.

[0154] By restricting on the one hand during the calibration phase the MCS indexes of the MCS-TAB table [Table 1] which can be used by the UE terminal to communicate with the master satellite 4, and on the other hand, by selecting the most suitable satellites for effective collaboration with the master satellite 4, and / or by applying a correction profile at the level of the user terminal UE or of the satellites collaborating with each other, the invention aims to improve the reception of all or part of the satellites involved in the collaboration with the master satellite 4 and incidentally to limit these errors.

[0155] More particularly, as described previously, the Idxj indexes selected from among those examined during the iterations of step E6-2, E6-2' of the calibration phase, correspond to the MCS schemes of the MCS-TAB table which make it possible to achieve one or more transmission quality criteria according to a given strategy. The satellite directing the calibration phase, referred to hereinafter as SAT-CALIB (master satellite 4 or geostationary satellite GEO or even high-orbit satellite depending on the implementation variant selected) can also select, for a given modulation, the Idxi index(es) offering a coding rate greater than or equal to a given threshold SI. This makes it possible to increase the volume of redundant data transmitted in order to further increase the reception quality of the master satellite 4.More specifically, the SAT-CALIB satellite selects Idxj indexes for which a significant number of bits constituting a symbol are used to transmit redundant data coded according to an error correcting code or FEC (Forward Error Code in English).

[0156] Furthermore, in a particular embodiment, during step E6-2, E6-2' of an iteration iter of the calibration phase, a correction profile to be applied for the MCS(iter) scheme considered for at least one subcarrier frequency is determined by the SAT-CALIB satellite (master satellite 4 or geostationary satellite GEO or even high-orbit satellite depending on the implementation variant chosen). It should be noted that the determination of this correction profile may condition the selection or non-selection of the MCS(iter) scheme in question (typically depending on the consistency of the determined correction profile), and therefore be taken into account, in addition to the transmission quality parameters, to select the indexes Idxj.

[0157] To determine the correction profile of an MCS scheme for a subcarrier frequency FSPj, several iterations within the iter iter relating to the MCS(iter) scheme can be implemented. For illustration purposes, it is assumed in the following that the MCS(iter) scheme is based on the 16-QAM modulation whose constellation diagram is shown in Figure 3 and that the LS list included in the MSG1 message sent during step E1 comprises a list of symbols prohibited for this modulation (as well as for the other modulations corresponding to the indexes included in the MSG1 message). For example, the LS list comprises, for the 16-QAM modulation, the symbols corresponding to the points located on the circles of radius 0.25 and 1.In a first iteration iterl nested in the iteration iter, following the reception of the message MSG1, the user terminal UE then transmits on the subcarrier frequency FSPj, a message MSG2 conveying symbols of the MAQ-16 constellation which are distinct from the prohibited symbols identified in the list LS. In other words, only the symbols included on the circle of radius 0.75 can be transmitted by the user terminal UE during this first iteration iterl. Thus upon reception of the message MSG2, certain points of the constellation diagram of the demodulators of the satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3, 4 corresponding to the prohibited symbols cannot be selected by the latter during the step E4, E4' (i.e. as if they were absent from the constellation diagram). This is intended to facilitate the identification of the points of the constellation diagram actually corresponding to the symbols transmitted by the terminal UE.In the illustrative example considered previously of point Z corresponding to symbol 1110, the demodulator of a satellite receiving point X will demodulate point X into symbol 1110 of the constellation closest to point X received. The deviations calculated on the phase and amplitude during step E5, E5' or E6-1, E6-1' depending on the implementation envisaged, allow it to determine the phase and / or amplitude error between point X received and point Z corresponding to the demodulated symbol and to deduce therefrom the correction to be made where appropriate by the user terminal UE when it transmits point Z. The same procedure is applied for the other points of the constellation.

[0158] Different strategies can be considered by the SAT-CALIB satellite to determine the correction to be applied for a given point in the constellation.

[0159] For example, according to one strategy, the correction is determined solely from the current and expected values ​​determined by the master satellite 4 from the MSG3 message that it received directly from the UE terminal (or equivalently from the result of the comparison of these values). Thus, in the previous illustrative example, the correction to be applied to the phase of point Z is derived from the difference between the current value of the phase of point X received by the master satellite 4 and the expected value of this phase, i.e. the value of the phase of point Z; the correction to be applied to the amplitude of point Z is derived from the difference between the current value of the amplitude of point X received by the master satellite 4 and the expected value of this amplitude, i.e. the value of the amplitude of point Z.

[0160] According to another strategy, the correction is determined by taking into account the current and expected values ​​determined by the satellites having accepted the collaboration or selected for this collaboration, and for which the results of the comparisons between these values ​​are similar (within a determined limit) or identical to the results obtained by the master satellite 4. For example, the corrections result from the average of the corrections determined for each satellite.

[0161] According to yet another strategy, the corrections to be applied can take into account (i.e. anticipate), apart from the results of comparison between current values ​​and expected values, a movement of the master satellite 4 and / or of the satellites likely to collaborate with it.

[0162] According to yet another strategy, if the corrections determined for different points of the constellation of a modulation of an MCS scheme are not consistent with each other (for example when the bandwidth of the subcarrier frequency is small), the SAT-CALIB satellite may decide not to make any correction T1 or to exclude this MCS scheme from the list of MCS schemes authorized for the uplink transmissions of the user terminal UE.

[0163] Of course, other strategies can also be considered.

[0164] Then, at least a second iteration iter2 nested in the iteration iter can be implemented in a similar way to what has just been described for the first iteration, but using at the user terminal UE other symbols not to be transmitted (transmitted in another message MSG1 by the satellite SAT-CALIB to the user terminal UE). These symbols correspond for example to the points of the constellation diagram of the MAQ-16 modulation located on the circle of radius 0.75.

[0165] This process can be repeated until the UE terminal has tested all points in the 16-QAM constellation for the subcarrier frequency FSPj.

[0166] At the end of this process, the SAT-CALIB satellite has, for each point of the MAQ-16 modulation and for the FSPj subcarrier frequency, a correction profile to apply to improve the detection of these points in reception.

[0167] Furthermore, in a particular embodiment, the SAT-CALIB satellite can also use the comparison results carried out by the satellites involved in the collaboration (including its own comparisons) or by itself, to constitute groups of satellites and select a group of satellites with which the collaboration will actually be implemented. The satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 are for example grouped according to the corrections that the user terminal UE must make to the parameters (e.g. phase and / or amplitude) associated with the points of the constellation when transmitting the data and determined as described above.

[0168] As an illustration, the SAT-CALIB satellite can select, for example, the group of satellites whose phase distortion in reception measured by these satellites (i.e., the phase deviation calculated) is identical to the phase distortion (i.e., the phase deviation calculated) that the master satellite 4 itself measures in reception from the signal that it receives directly from the user terminal UE. Then it adds the phase correction to be applied to the list of indexes that it sends to the user terminal UE in the MSG3 message.

[0169] Thus, in a particular embodiment, different phase corrections can be applied to the list of indexes Idxj of the same coding and modulation table depending on the group of satellites considered. Indeed, the Doppler effect can induce a modification of the subcarrier frequency received by satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4. Thus, the subcarrier frequency actually received by a satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 which is approaching the user terminal UE while the latter has used the subcarrier frequency FSPj in transmission, can increase relative to the subcarrier frequency FSPj, whereas the subcarrier frequency actually received by a satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 which is moving away from the user terminal UE can decrease relative to the subcarrier frequency FSPi, due to a Doppler effect.Such a change in the subcarrier frequency disrupts the demodulators on board satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4.

[0170] Depending on the circumstances (for example its position and trajectory), the master satellite 4 may choose to collaborate with a given group of satellites, for example, the group of satellites that are moving away from the user terminal UE. For this, the SAT-CALIB satellite indicates, in the MSG3 message, the indexes selected for such a group of satellites, thus ensuring efficient collaboration of the master satellite 4 with these satellites because the impact of the Doppler effect during the demodulation of the subcarrier frequency is reduced.

[0171] The indexes of the MCS schemes authorized for the user terminal UE, and where applicable the correction profiles and the group of satellites selected for collaboration with the master satellite 4, are sent to the user terminal 4 and to the satellites concerned in the MSG3 message.

[0172] [Fig. 5] represents a satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 capable of implementing all or part of the methods which are the subject of the invention.

[0173] The satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 may comprise at least one hardware processor 10, a storage unit 11, at least one antenna 12, and a network interface 13 which are connected to each other through a bus 14. Of course, the constituent elements of the satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 may be connected by means of a connection other than a bus.

[0174] The processor 10 controls the operations of the satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4. The storage unit 11 stores at least one program for collecting, aggregating and / or processing data carried by at least one subcarrier frequency allocated to a terminal UE to be executed by the processor 10, and various data, such as parameters used for calculations performed by the processor 10, intermediate data of calculations performed by the processor 10, etc. The processor 10 may be formed by any known and suitable hardware or software, or by a combination of hardware and software. For example, the processor 10 may be formed by dedicated hardware such as a processing circuit, or by a programmable processing unit such as a central processing unit (Central Processing Unit) which executes a program stored in a memory thereof.

[0175] The storage unit 11 may be formed by any suitable means capable of storing the program, data in a computer-readable manner. Examples of the storage unit 11 include non-transitory computer-readable storage media such as semiconductor memory devices, and magnetic, optical, or magneto-optical recording media loaded into a read-write unit. The program causes the processor 10 to execute a method of managing the retransmission of missing data according to one embodiment of the invention.

[0176] The antennas 12 provide an interface between the satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 and the UE terminal or other satellites 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4.

[0177] A network interface 13 provides a connection between satellite 1-1 to 1-3, 2-1, 2-2, 3-1 to 3-3 and 4 and equipment 5.

[0178] [Fig. 6] finally illustrates the simplified structure of a user terminal UE capable of implementing all or part of the methods which are the subject of the invention.

[0179] A user terminal comprises a memory 61 consisting of a buffer memory, a processing unit 62, equipped for example with a processor P, and controlled by the computer program 73, implementing the transmission method according to the invention.

[0180] Upon initialization, the code instructions of the computer program 63 are for example loaded into a RAM memory before being executed by the processor of the processing unit 62. The processing unit 62 receives as input at least one modulation and coding scheme selected by a selection device (for example integrated into the master satellite, a geostationary satellite, or a high-orbit satellite). The processor of the processing unit 62 implements the steps of the transmission method described above, according to the instructions of the computer program 63, to generate a radio signal for the transmission of data on said at least one subcarrier frequency, modulated according to one of said at least one selected modulation and coding scheme.

Claims

CLAIMS 1. Method for selecting at least one modulation and coding scheme suitable for use by a user terminal for transmitting data on at least one subcarrier frequency, said user terminal being located in a cell of a radio communication network served by at least one first satellite, called master satellite, said method being implemented by said master satellite, a geostationary satellite, a high-orbit satellite or a low-orbit satellite, and implementing a calibration phase comprising, for said at least one subcarrier frequency: for at least one collaborating satellite of a group of collaborating satellites suitable for collaborating with said master satellite to collect data transmitted by said user terminal and for at least one candidate modulation and coding scheme: • obtaining a result of a comparison (E5) between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by said collaborating satellite, and an effective value of said at least one parameter measured by said collaborating satellite, a selection (E6-2) of at least one modulation and coding scheme capable of being used by said user terminal for the transmission of data on said at least one subcarrier frequency, from among said at least one candidate modulation and coding scheme, taking into account the comparison result(s) obtained.

2. Method according to claim 1, characterized in that said selection selects at least one modulation and coding scheme making it possible to achieve the target value of said at least one parameter for a given minimum number of collaborating satellites.

3. Method according to claim 1, characterized in that said selection selects at least one modulation and coding scheme making it possible to achieve the target value of said at least one parameter for all of said subcarrier frequencies intended to be used to transmit said data.

4. Method according to any one of claims 1 to 3, characterized in that it comprises sending a calibration message (MSG3) identifying said at least one selected modulation and coding scheme and / or at least one prohibited modulation and coding scheme.

5. Method according to any one of the preceding claims, characterized in that it implements the sending (E2) of at least one parameterization message (MSG1) comprising technical information belonging to the group comprising: at least one FSPi subcarrier frequency authorized to be used during said calibration phase, at least one subcarrier frequency prohibited during said calibration phase, said at least one candidate modulation and coding scheme capable of being used by said user terminal to transmit data on said at least one FSPi subcarrier frequency, at least one modulation and coding scheme prohibited during said calibration phase, at least one symbol of a modulation associated with said at least one candidate modulation and coding scheme authorized to be transmitted during said calibration phase, at least one symbol of a modulation prohibited during said calibration phase.

6. Method according to claim 5, characterized in that it implements the reception (E3) of at least one test message (MSG2) carrying data transmitted on at least one subcarrier frequency modulated according to one of said candidate modulation and coding schemes, taking into account said at least one authorized symbol, identified in said at least one parameterization message (MSG1).

7. Method according to any one of the preceding claims, characterized in that it comprises the determination (E4) of said effective value of said at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme, from said at least one test message (MSG2).

8. Method according to any one of the preceding claims, characterized in that it comprises the determination of a subgroup of collaborating satellites taking into account the results of said comparison of said calibration phase obtained for at least two collaborating satellites.

9. Method according to any one of the preceding claims, characterized in that it comprises a determination of at least one phase, frequency and / or amplitude correction profile for said at least one modulation and coding scheme selected for said at least one subcarrier frequency.

10. Method according to claim 9, characterized in that it comprises sending said at least one correction profile, in said calibration message (MSG3) or in a separate message.

11. Method according to any one of the preceding claims, characterized in that said at least one parameter representing a use of said at least one subcarrier frequency belongs to the group comprising: at least one parameter representative of the transmission quality of said at least one subcarrier frequency; and the value of the phase and / or the amplitude of the constellation points carried by said at least one subcarrier frequency.

12. Device for selecting at least one modulation and coding scheme capable of being used by a user terminal for the transmission of data on at least one subcarrier frequency, said user terminal being located in a cell of a radio communication network served by at least one first satellite, called master satellite, said device comprising at least one processor configured to implement a calibration phase comprising, for said at least one subcarrier frequency: for at least one collaborating satellite of a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal and for at least one candidate modulation and coding scheme: • obtaining a result of a comparison (E5) between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by said collaborating satellite, and an effective value of said at least one parameter measured by said collaborating satellite, selecting (E6-2) at least one modulation and coding scheme capable of being used by said user terminal for the transmission of data on said at least one subcarrier frequency, from among said at least one candidate modulation and coding scheme, taking into account the comparison result(s) obtained; said selection device belonging to a group comprising: said master satellite, a geostationary satellite, a high-orbit satellite, a low-orbit satellite.

13. Method for transmitting data on at least one subcarrier frequency, implemented by a user terminal located in a cell of a radio communication network served by at least one first satellite, called master satellite, characterized in that it comprises: receiving, at the end of a calibration phase, at least one modulation and coding scheme selected from at least one candidate modulation and coding scheme, taking into account at least one comparison result between a target value of at least one parameter representing a use of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by a collaborating satellite, and an actual value of said at least one parameter measured by said collaborating satellite,said collaborating satellite belonging to a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal, transmitting data on said at least one subcarrier frequency, modulated according to one of said at least one selected modulation and coding scheme.

14. Method according to claim 13, characterized in that said method comprises beforehand: the reception of at least one parameterization message (MSG1) comprising technical information belonging to the group comprising: • at least one FSPi subcarrier frequency authorized to be used during said calibration phase, at least one subcarrier frequency prohibited during said calibration phase, • at least one candidate modulation and coding scheme capable of being used by said user terminal to transmit data on said at least one FSPi subcarrier frequency, • at least one modulation and coding scheme prohibited during said calibration phase, • at least one symbol of a modulation associated with said at least one candidate modulation and coding scheme authorized to be transmitted during said calibration phase, • at least one symbol of a modulation prohibited during said calibration phase, and for at least one authorized subcarrier frequency and for at least one candidate modulation and coding scheme: • sending (E2) at least one test message (MSG2) carrying data transmitted on said at least one subcarrier frequency modulated according to said candidate modulation and coding scheme, taking into account said at least one authorized symbol, identified in said at least one parameterization message (MSG1).

15. Method according to any one of claims 13 and 14, characterized in that it comprises the reception of at least one phase, frequency and / or amplitude correction profile for said at least one modulation and coding scheme selected for said at least one subcarrier frequency, and in that it implements a correction of said at least one subcarrier frequency modulated according to said at least one selected modulation and coding scheme taking into account said correction profile.

16. User terminal capable of transmitting data on at least one subcarrier frequency, when it is located in a cell of a radio communication network served by at least one first satellite, called master satellite, said terminal comprising at least one processor configured to: receive, at the end of a calibration phase, at least one modulation and coding scheme selected from at least one candidate modulation and coding scheme, taking into account at least one comparison result between a target value of at least one parameter representing a usage of said at least one subcarrier frequency for said candidate modulation and coding scheme expected by a collaborating satellite, and an effective value of said at least one parameter measured by said collaborating satellite, said collaborating satellite belonging to a group of collaborating satellites capable of collaborating with said master satellite to collect data transmitted by said user terminal, transmit data on said at least one subcarrier frequency, modulated according to one of said at least one selected modulation and coding scheme.

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