METHOD AND SYSTEM FOR INSERT A TERMINAL INTO A SATELLITE NETWORK

Description

[0001] The invention relates to a method and a system allowing a terminal to connect to a satellite communications network and to move within this network without knowing a priori the configuration of the satellite spots, the latter being able to vary during a mission.

[0002] It applies in Satcom systems where the satellite is configured to form N orthogonal spots on the return path (terminal to hub) and an aggregation of these spots on the forward path (hub to terminals) and where the terminals are mobile.

[0003] One of the existing problems in the Satcom field is to allow a terminal to enter and move within a Satcom network without a priori knowledge of the satellite's spot configuration, as they can be reconfigured during the mission.

[0004] Various solutions are described in the prior art. A first solution is known as the "iDirect solution." The terminal that wishes to enter and move will detect adjacent spots by implementing two reception chains. This first solution, by implementing two reception chains, increases costs by a ratio of two.

[0005] Another method is described in patent application FR 3052012. The "uplink handover" method requires the terminal to have a priori knowledge of the geometric configuration of each spot, the adjacency relationships between the spots and the configuration of the bands used to receive in each of the spots. This results in very significant and more complex initial planning.

[0006] GB 2303764 describes a method for determining a spot beam from a plurality of spot beams projected by a satellite in which a radio frequency transceiver is located, all beams of the plurality of spot beams being capable of receiving a common signaling path.

[0007] In patent JP 3653722 an IR router is connected to each VSAT.

[0008] The solutions of the prior art do not allow the problem of "entry" of a terminal into a satellite network mentioned above to be solved in a flexible and inexpensive manner. In the known techniques of the prior art, the complexity of mobility is shifted either into the upstream planning or into the multiplication of the reception chain. If the spots become smaller and smaller, and therefore more and more numerous in the same area, these solutions are not flexible, nor easily modifiable, when an operator changes the configuration, or the number and width of the bands in the different spots. The expression "SATVLAN" will be used subsequently to designate the band and bandwidth parameters used in a spot and possibly the power parameters.

[0009] The object of the present invention is to provide a method and a system for solving the problem of locating terminals in different spots in order to allow the entry, the hooking of a terminal in a satellite network, without transferring the complexity to the initial planning or to the multiplication of reception chains.

[0010] The invention relates to a method for configuring the entry of at least one mobile terminal A into a satellite network comprising a satellite transmitting on several spots, a hub comprising a set of modem hubs and at least two frequency hopping laws, the method comprising at least the following steps: During a first access, synchronize terminal A on a first frequency hopping law LS 1 and identify the frame and the slots using a second frequency hopping law LS 2, then synchronize the terminal on this second frequency hopping law LS 2 by listening to said slots, Terminal A uses one or more slots of a return channel configured according to the second frequency hopping law to transmit an identification signal, said identification signal is listened to by all the modem hubs configured to receive on the slots according to this second frequency hopping law, Terminal A listens on the common channel defined by all the modem hubs which communicate on the slots identified according to this same second frequency law, to be notified by the elected modem hub, the network allocation parameters,the election of the hub being carried out in the following manner: • Each modem hub measures the power level of the signal(s) emitted by one or more terminals of the satellite network, • Each modem hub communicates to the other modem hubs the power values ​​of the measured signals, and selects the maximum power value and the modem hub which received the corresponding signal, The modem hub elected by the other modem hubs notifies the terminal of the radio resource and the frequency hopping law to be used.

[0011] Terminal A, which wishes to attach itself to the satellite network, identifies, for example, the slots for which all the modem hubs are listening on the same frequency, in order to constitute an access and measurement channel, and the terminal uses said common channel to synchronize itself on the first frequency law.

[0012] The method can also use GPS-type geolocation information to synchronize.

[0013] Modem hubs can exchange signals received via a common router.

[0014] The modem hubs can also exchange the signals received via an external network R configured to communicate with the routers of each modem hub.

[0015] A message or signal can be encoded with Code Divisional Multiple Access (CDMA) modulation and the hub decodes the signal using the set of CDMA codes assigned to the network and estimates the signal strength based on the correlation rate between the received signal and the CDMA code.

[0016] The invention also relates to a system for the entry into a satellite network of one or more mobile terminals A comprising a satellite, several modem hubs installed in one or more hub stations characterized in that each modem hub is configured to execute the power measurement steps, the election of a hub by executing the steps of the method according to the invention and said elected modem hub transmits to the terminal a configuration for entry into the network and its movement within said network.

[0017] The modem hubs can be arranged in the same hub station, said station includes a router allowing the exchange of signals received by each of the modem hubs and a common clock.

[0018] The modem hubs are, for example, arranged in the same hub station, said station comprising a router allowing the exchange of signals received by each of the modem hubs and an external means of synchronization.

[0019] The modem hubs can be arranged in separate hub stations, said hub stations being connected via a suitable R network.

[0020] Other characteristics and advantages of the present invention will appear more clearly on reading the description of exemplary embodiments attached to the figures which represent: Figure 1 , an example of architecture of the system according to the invention Figure 2 , a second example of architecture of the system according to the invention, and Figure 3 , an example of a frame.

[0021] There figure 1 represents an example of architecture of the system according to the invention comprising a satellite 1, several modem hubs 2 1 , 2 2 , installed in a hub station 3 or hub, said modems being designated as modem hubs, several terminal stations A, B, C, or terminals, only station A being represented in the figure.

[0022] A 2 i modem hub (with i=1, 2 on the figure 1 ) comprises a processor 21i configured to execute the steps of the method according to the invention, a memory 22i making it possible to record the signals received at the modem hub for a duration of a time slot or slot. The modem hub 2 i comprises a coding / decoding module 23i for the content of the signals.

[0023] The hub station or hub 3 comprises a radiofrequency RF transmission chain comprising a set 31 consisting of antennas, amplifiers, transposition modules, known to those skilled in the art. It also comprises a router 32 allowing the exchange of signals and messages between the modem hubs, and a clock 33 common to all the modems. The clock 33 has the particular function of synchronizing the modem hubs on the frame and slot boundaries. An example of a frame structure is given in figure 3 . In the case where all the spots are not managed by the same modem hub, the modem hubs are synchronized by an external means 34 so that the different spots - whose receptions are managed by modems of different hubs - all listen according to at least two frequency hopping laws F 1 , F 2 , at the indicated frequency hopping steps. The external synchronization means is, for example, a GPS (Global Positioning System) type geolocation system. It is assumed that the satellite and its spots have a broadband listening capacity so that the satellite does not need to know the frequency hopping law(s) applied and known to the modems alone.

[0024] According to one embodiment, the satellite is said to be regenerative, that is to say that it demodulates the received signal to correct the errors and remodulates it afterwards. Such a satellite will need to know the two frequency laws and to synchronize itself on these two laws. It must also know the time interleaving law, that is to say, the identification of the slots for which the first law applies and those for which the second law applies.

[0025] The process is implemented at the terminal level and at the hub or hub station level.

[0026] On the example given in the figure 1 , the message requesting entry into the satellite network and transmitted by terminal A "I am terminal A" is heard by the two spots SP 1 , SP 2 , of the satellite. The amplitude of the signal reaching the two spots is identical, but due to the differences in orientation of these two spots, the antenna gain at the satellite is different. As a result, the same source signal is transformed by passing through the spots SP 1 and SP 2 , into two signals SA 1 and SA 2 of different amplitudes at the input of the transposition 10, amplification 11 and retransmission 12 stages of satellite 1 to the hub. It is assumed that these stages carry out identical processing for all the spots. This difference in amplitude is found in the modulated signals SM 1 , SM 2 and transposed to the processing modem hubs 2 1 , 2 2 of hub 3.

[0027] Both the modems of the hub 3 and the terminals share common data necessary for the implementation of the invention and are preconfigured in a consistent manner, for example in the following manner: The duration of a frame, The number of frames in a cycle, at the end of which the frame number is reset, The number of slots / frames, and steps per slot, A first frequency hopping law F 1 , used in transmission and reception, and specific to each modem hub, A second frequency hopping law F 2 , used in reception only and common to all modem hubs, An identification of one or more slots per frame for which the second frequency hopping law applies, the first frequency hopping law applying to all other slots.

[0028] The slots for which the second frequency hopping law applies are not necessarily of the same duration as the other slots.

[0029] A terminal identifier (Id(terminal)) is unique in the network. The terminal identifier will be used to associate a signal received at the satellite and at a modem hub with its transmitting terminal.

[0030] When a terminal A wishes to attach to the network, it must first be able to identify the slots in which all the modem hubs will be listening on the same frequency. All of these access slots then form the access and measurement channel common to all the spots.

[0031] According to one embodiment, the terminal does not depend on external synchronization to synchronize on the frequency laws (GPS type) and uses the channel common to all the spots to synchronize on the frequency hopping law used (first frequency law) to carry the traffic transmitted by all the modem hubs. The terminal knows the frequency hopping law used, for example LS 1 = {F 1 , F 2 , F 4 , F 5}; LS 2 = {F 1 , F 3 , F 2 , F 5 .}. To synchronize and knowing the frequency hopping law used, the terminal waits for the arrival of one of the known frequencies of the first frequency hopping law. The terminal does not initially know where it is in the sequence of frequencies, it knows the frequency hopping law of the global spot. Knowing the frequency law used, the terminal waits for the advent of one of the known frequencies and makes a hypothesis on the following frequencies which will be used by the modem hubs.It makes the assumption that it must be for example on the frequency F 1 of the first sequence and checks if it finds the frequency F 2 and the following predefined ones. If this assumption is false, the terminal, the terminal waits for another occurrence of this frequency or another predefined frequency and emits a new hypothesis. It repeats this internal procedure until the hypothesis is verified, that is to say, the terminal has recognized the frequency hopping law and its position. This step allows the terminal to synchronize on a first frequency law before detecting the slots using the second frequency law to transmit.

[0032] Once this synchronization is acquired, the terminal is able to detect the slots using the second frequency hopping law F 2 . These slots (using the second frequency law) are positioned in a predefined manner by the modem hubs and their position is also known by terminal configuration (at application startup). These slots S(F 2 ) and this second frequency hopping law F 2 form the access and measurement channel in the upstream direction (terminal == > hub) and a broadcast channel in the downstream direction (hub == > terminal).

[0033] According to a procedure identical to that for synchronization on the first frequency hopping law, the terminal synchronizes on the second frequency hopping law. To the frequency hopping law and to the slots of the broadcast channel on which the terminal synchronizes in reception, corresponds a single frequency hopping law F u and a set of slots S(F u ), identical or not to the slots and to the frequency hopping law used by the modem hubs in reception and which the terminal can use in reception. It is this second frequency hopping law that the terminal must use in transmission on the predetermined slots which are the counterpart on the return channel of the slots of the broadcast channel used by the modem hubs on the outbound channel.

[0034] At the end of these steps, a common channel usable by the terminal in transmission has been defined.

[0035] During a first access to the satellite network, the terminal A synchronizes itself on the frame in reception, then determines the synchronization peak of the frame usable in transmission of configuration parameters and the synchronization peak of the frame in reception, then determines the slots usable for random access according to the configuration parameters of the frame, the frame number and a random number. It proceeds in this / these slots to a transmission towards the satellite according to the second frequency hopping law, of a signal containing its identifier. This signal is, for example, depending on the embodiment, either a CDMA type signal with a unique code per terminal used to carry out the spread spectrum function, or a short signal modulated according to the message containing the identifier.

[0036] The terminal then listens on the common downstream channel to be notified by the modem hub which will have determined, to process the terminal, an initial allocation in the form of a radio resource and a frequency hopping law. This procedure on the hub side is described below.

[0037] If we refer to the figure 1 , terminal A transmits a satellite network access request message “I am terminal A” to the satellite. The satellite amplifies using its amplifier 10, transposes 11 and routes the message using the retransmission module 12. The signal is then transmitted via the RF transmission chain from the hub station to the first hub modem 2 1 , and to the second hub modem 2 2 . The first hub modem 2 1 transmits the SM 1 message that it received to the second hub modem 2 2 with an RSSI of -30dB for example, the second hub modem transmits the SM 2 message that it received with an RSSI of -50 dB to the first hub modem. The transmission is carried out via the router of the hub station 3 in the configuration of the figure 1 . The modem hubs each carry out a power measurement on reception of the received signal, P(SM 1 ), P(SM 2 ) and communicate the results to each other. The modem hub that has received the highest power signal will be designated "elected" to attach the terminal. The chosen modem hub notifies the corresponding terminal of the configuration parameters, for example the resource and the frequency hopping law that the latter must use. Each modem hub is configured to decide on which modem hub the terminal will connect and enter the satellite network. The signal power measurements are made per cycle (time interval for the same frame) in order to allow the terminal to change modem hub once it has entered the satellite network.

[0038] After attachment to the satellite network and during traffic, the terminal receives, via dedicated signaling, the frequency at which it must signal itself on the access and measurement channel so that the modem hub network can carry out a reception power measurement (received signal power level) used to decide on a modem hub change procedure. The measurement of the signal power emitted by the terminal to request its entry into the "I am terminal" network is carried out according to a periodicity determined by the network, depending for example on the traffic.

[0039] Once the terminal is synchronized, the process will search for the modem hub to which the terminal will connect to enter the satellite network. The procedure at each of the modem hubs follows, for example, the following steps: Determination of the frame number N(T) contained in the message received by a modem hub from the hub's internal clock, Determination of the slot / step number N(P) for a modem hub from the hub's internal clock, If slot P is a reception slot for which the second frequency hopping law applies, the modem hub records all the signals emitted by the different terminals using the same slot for reception, which it receives during the duration of this slot. These signals are stored in the modem hub's memory.

[0040] Regardless of the satellite network configuration, the modem hubs will exchange the power measurements they make on the received signals, which will allow them to identify the highest power signal, the associated terminal and whether they should consider themselves as the connection modem hub for the terminal.

[0041] According to a first embodiment, the modem hub will select the highest power signal that has been stored. The selection is made by comparing the powers of the different signals and retaining the highest power signal. The modem hubs exchange the signal power measurements that they have made upon receiving the signals transmitted by different terminals transmitting on the same slot.

[0042] Then the modem hub that detected the higher power signal decodes the message contained in the higher power S MP signal and containing the identifier of the terminal transmitting this signal.

[0043] If the modem hub that detected the higher power signal manages to decode it correctly, the modem hub reconstructs the signal as it would arrive if there were no interference from other messages. To do this, knowing the content of the initial message and the modulation used, the modem hub is able to reconstruct the initial signal. On the other hand, the modem hub estimates the power level P(S MP ) received from this signal (RSSI received signal strength indicator). The modem hub subtracts the theoretical initial signal Sp the from the received and measured signal S MP . It deduces the secondary signals that correspond to the lower power messages and then identifies these so-called secondary messages

[0044] The modem hub estimates the amplitude of secondary signals, i.e., signals whose power level is lower than the maximum power value.

[0045] There figure 2 represents an architectural variant with two modem hubs processing two spots of the satellite SP 1 , SP 2 . In this case, compared to the architecture of the figure 1 , each modem hub 2 1 , 2 2 comprises its own RF chain, 31 1 , 31 2 , and a router 32 1 , 32 2 , and the system comprises a network R (known to those skilled in the art) allowing the exchange of messages from one modem hub to another. The implementation of the method is identical to that described previously for the case of the architecture described in figure 1 .

[0046] In the case of a system using CDMA code division multiple access coding, the modem hub decodes all messages using CDMA demodulation. Each CDMA code used for demodulation corresponds to a unique identifier "Ident". The modem hub therefore attempts to decode an "ident" message using all the CDMA codes allocated in the network.

[0047] For each correctly decoded modem, the modem hub estimates the signal strength based on the correlation rate between the received signal and the CDMA code.

[0048] For all terminals whose "Ident" message does not correspond to a terminal supported by the modem hub, the latter constructs a message M{(ldent,RSSI)} consisting of a list of pairs containing the terminal "Ident" identifier and the RSSI under which the modem hub receives this ident signal. This message is broadcast periodically to all other modem hubs in the network, i.e. both to the modem hubs belonging to the same station and to the modem hubs of other Satcom hubs. When a modem hub receives this message, it identifies in the list all the terminals for which it handles communications. For each terminal in the list, the modem hub sorts this list according to the RSSI level as perceived by it and by the modem hubs processing neighboring spots.If the terminal is not seen by a neighboring hub, it is indicated with an RSSI = 0. The modem hub maintains a histogram of the difference between the RSSI it measures and the RSSI estimated by neighboring modem hubs. This difference histogram is used to determine the average transmission frequency of the terminal's Ident signal.

[0049] There figure 3 illustrates an example of a frame configuration comprising 14 slots of 10ms. The Modem hub 2 1 listens on three frequencies F 1 , F 2 , F 3 , The Modem hub 2 2 listens on the same frequencies but for different slots, the two modem hubs transmit on the three frequencies. Sc represents the slots of each frame for which the frequency used for reception is common. This frequency changes with each frame according to a frequency hopping law known to all the modems and all the terminals.

[0050] The invention offers a simple solution, as terminals do not need to know the configuration of the SATVLANs in the different spots. It offers security and availability, as the configuration of the SATVLANs usable on the return path is not broadcast, which eliminates the risk of interception of this configuration and its use.

Claims

1. A method for configuring the input of at least one mobile terminal A in a satellite network comprising a satellite (1) which transmits over several spots, the transmissions being organised in the form of frames and slots which are cut up into levels which are associated with times and frequencies of transmission, a set of modem hubs (2i) being installed in one or more hub stations (3), each modem hub being configured to exchange signals corresponding to one or more spots with the satellite and to carry out power measurements, the modem hubs being connected with each other, the modem hubs being associated with at least three frequency hopping laws among which one frequency hopping law which is referred to as the first frequency hopping law LS1 which is specific to each modem hub and which gives the frequency associated with the traffic slot levels, and one frequency hopping law which is referred to as the second frequency hopping law LS2 which gives the frequency associated with the slot levels of a channel which is common to all of the modem hubs, the method comprising at least the following steps: • during a first access, synchronising the terminal A to the first frequency hopping law LS1, identifying the frame and the slots which use the second frequency hopping law LS2, then synchronising the terminal to this second frequency hopping law LS2 whilst listening to the slots, • the terminal A uses one or more slots of the common channel to transmit an identification signal, said identification signal is heard by all the modem hubs which are configured to receive on the slots according to this second frequency hopping law, • the terminal A begins listening on said common channel to be notified of the radio resource and the frequency hopping law to be used by a modem hub which is determined to be the connection modem hub for the terminal, the determination of the connection modem hub being carried out in the following manner: • each modem hub measures the power level of the identification signal transmitted by the terminal A, • each modem hub communicates to the other modem hubs the measured power value of the identification signal, ∘ the modem hub having the maximum power value is considered to be the connection modem hub for the terminal A.

2. The method according to claim 1, characterised in that the terminal A which wishes to become attached to the satellite network identifies the slots for which all the modem hubs are listening at the same frequency in order to constitute an access and measurement channel, and the terminal uses said common channel to synchronise to the first frequency law.

3. The method according to claim 1, characterised in that it further uses a geolocation information item of the GPS type in order to be synchronised.

4. The method according to any one of claims 1 to 3, characterised in that the modem hubs exchange the signals received via a router (12) which is common to all.

5. The method according to any one of claims 1 to 3, characterised in that the modem hubs exchange the signals received via an external network R which is configured to interact with the routers of each modem hub.

6. The method according to claim 1, characterised in that a message or signal is encoded with the CDMA modulation and in that hub decodes the signal using all of the CDMA codes attributed to the network and estimates the power of the signal on the basis of the rate of correlation between the signal received and the CDMA code.

7. A system for input into a satellite network of at least one mobile terminal A comprising a satellite (1) which transmits over several spots, the transmissions being organised in the form of frames and slots which are cut up into levels which are associated with times and frequencies of transmission, several modem hubs (2i) being installed in one or more hub stations (3), each modem hub being configured to exchange signals corresponding to one or more spots with the satellite and to carry out power measurements, the modem hubs being connected with each other, the modem hubs being associated with at least three frequency hopping laws among which one frequency hopping law which is referred to as the first frequency hopping law LS1 which is specific to each modem hub and which gives the frequency associated with the traffic slot levels, and one frequency hopping law which is referred to as the second frequency hopping law LS2 which gives the frequency associated with the slot levels of a channel which is common to all of the modem hubs, the system being characterised in that the equipment items of the system are configured to implement the steps of the method according to any one of claims 1 to 6.

8. The system according to claim 7, characterised in that the modem hubs are arranged in the same hub station (3), the station comprises a router (12) which enables the exchange of the signals received by each of the modem hubs and a common clock (33).

9. The system according to claim 7, characterised in that the modem hubs are arranged in the same hub station (3), said station comprises a router (12) which enables the exchange of the signals received by each of the modem hubs and an external synchronisation means (34).

10. The system according to claim 7, characterised in that the modem hubs are arranged in different hub stations (31, 32), said hub stations being connected via a suitable network R.