Abstract:
The present invention relates to a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a telecommunication satellite with several spot beams, known as a multispot satellite. The network includes a multispot satellite including a payload for the reception, processing and retransmitting of telecommunication signals received by said satellite, a service area comprised of a plurality of basic coverage areas, known as cells, each cell including a plurality of ground terminals and a plurality N GWactive  of active gateways interconnected by a ground network, N GWactive  being an integer, said satellite relaying signals sent by said N GWactive  active gateways to said cells. Furthermore, the network includes N GW  gateways, N GW  being an integer strictly greater than the number N GWactive  of active gateways, said N GW  gateways being interconnected by said ground network and including said N GWactive  active gateways such that N GW −N GWactive  gateways are not active.

Description:
This claims priority to French Patent Application FR 08/51885, filed Mar. 21, 2008, the entire disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a multispot telecommunication satellite. This type of satellite enables the use of several spot beams from antennas on board the satellite to cover contiguous geographic areas or cells, instead of a single large spot beam. 
     Such multispot satellites enable several radiofrequency links occupying the same frequency band on different spot beams to be established. 
     In the case of a high bandwidth broadband satellite telecommunication system, the satellite is used bidirectionally, that is to both:
         relay data sent by a gateway (connected to the ground network) to a plurality of ground terminals: this first point to multipoint type link constitutes the forward link;   relay to the gateway data sent by the ground terminals: this second multipoint to point type link constitutes the return link.       

     It will be noted that a satellite broadcasting service may be considered to be equivalent to the forward link of a bidirectional system as described above. 
     An example of a forward link in a multispot telecommunication network is illustrated in  FIG. 1 . 
     Signals are sent to a multispot satellite  3  over an uplink LM by a gateway  2  (also called a central station) such as a ground communication gateway connected to an Internet backbone  5 . The gateway controls the network through a network management system that allows the operator to monitor and control all the components in the network. The signals sent by the gateway are then processed at the level of satellite  3  that amplifies the signals, transposes the signals at a generally lower frequency and then retransmits the signals from the satellite antenna or antennas on a downlink LD in the form of a plurality of spot beams or spots forming basic coverage areas or cells C 1  to C 8  in which ground terminals  6  are situated. Each cell C 1  to C 8  is associated with a spot beam SP 1  to SP 8 . It will be noted that, in the case of configuration  1 , the eight cells C 1  to C 8  associated respectively with eight spot beams SP 1  to SP 8  form a group of cells served by the same gateway  2 . In practice, network  1  is formed by a plurality of gateways that are interconnected via a ground network (an Internet network, for example). The return link of ground terminals  6  to gateway  2  operates identically with a reverse direction of communication. 
     Coordination of frequencies between operators is done in the context of regulation issued by the International Telecommunication Union (ITU): thus, by way of example, the band Ka for region 1 (Europe, Africa, Middle East) is defined in table 1 below: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Forward link 
                 Uplink (from the gateway) 
                 27.5 GHz to 29.5 GHz 
               
               
                   
                 Downlink (to the ground 
                 19.7 GHz to 20.2 GHz 
               
               
                   
                 terminals) 
               
               
                 Return link 
                 Uplink (from the ground 
                 29.5 GHz to 30.0 GHz 
               
               
                   
                 terminals) 
               
               
                   
                 Downlink (to the gateway) 
                 17.7 GHz to 19.7 GHz 
               
               
                   
               
             
          
         
       
     
     It is observed that the spectrums from band Ka in uplink are adjacent (i.e., the intervals [27.5; 29.5] and [29.5; 30.0] do not present any discontinuity). The same is true for spectrums from band Ka in downlink (i.e., the intervals [17.7; 19.7] and [19.7; 20.2] do not present any discontinuity). 
     Given that the gain from an antenna is inversely proportional to the opening of the spot beam, using multispot antennas to cover an extended area with a homogeneous and elevated gain is necessary. The larger the number of spot beams, the smaller the opening of each spot beam will be. Thus, the gain on each spot beam and so the gain on the service area to cover will be increased. As we mentioned above, a service area to cover is formed by a plurality of contiguous cells (basic coverage areas), one spot beam being associated with each cell. A homogeneous multispot coverage area SA is represented in  FIG. 2   a ), each cell being represented by a hexagon FH such that the coverage area is comprised of a plurality of hexagons FH in which θ cell  is the outer size of the cell expressed by the angle of the satellite associated with the coverage. However, as the antenna spot beam associated with each cell is not capable of producing a hexagonal form, a good approximation consists of considering a plurality of circular spot beams FC such as represented in  FIG. 2   b ). The association of a spot beam with a cell is done by considering the best performance of the satellite for said spot beam, particularly in terms of EIRP (Equivalent Isotropically Radiated Power) and G/T figure of merit (gain to noise temperature ratio): a cell is determined to be the part of the service area associated with the spot beam that offers the highest gain on this area from among all the satellite spot beams. 
     Configuration  1  such as represented in  FIG. 1  uses a technique known as the frequency reuse technique: this technique enables the use of the same frequency range several times in the same satellite system in order to increase the total capacity of the system without increasing the allocated bandwidth. 
     Frequency reuse schemes, known as color schemes, making one color correspond to each of the satellite spot beams, are known. These color schemes are used to describe the allocation of a plurality of frequency bands to the satellite spot beams in view of radiofrequency transmissions to carry out in each of these spot beams. In these schemes, each color corresponds to one of these frequency bands. 
     In addition, these multispot satellites enable the sending (and receiving) of polarized transmissions: the polarization may be linear (in this case the two polarization directions are horizontal and vertical, respectively) or circular (in this case the two polarization directions are left circular or right circular, respectively). It will be noted that in the example from  FIG. 1 , the uplink leaving the gateway  2  uses two polarizations with four channels for each polarization, respectively Ch 1  to Ch 4  for the first polarization and Ch 5  to Ch 8  for the second polarization: the use of two polarizations allows the total number of gateways to be reduced. The eight channels Ch 1  to Ch 8 , after processing by the payload of the satellite  3 , will form the eight spot beams SP 1  to SP 8  (one channel being associated with one spot beam in this example). 
     According to a four-color scheme (red, yellow, blue, green) with a frequency spectrum of 500 MHz for each polarization, the transmissions being polarized in one of two right circular or left circular polarization directions, each color is associated with a 250 MHz band and a polarization direction. 
     In the rest of the description, we will take the following convention: 
     the color red is represented by lines hatched to the right; 
     the color yellow is represented by dense dots; 
     the color blue is represented by lines hatched to the left; 
     the color green is represented by dispersed dots. 
     A color is thus associated with each satellite spot beam (and thus a cell) such that the spot beams with the same “color” are non-adjacent: contiguous cells thus correspond to different colors. 
     An example of a four-color scheme for covering Europe is represented in  FIG. 3 . In this case, 80 cells are necessary to cover Europe. 
     This type of scheme is applicable equally well in uplink and in downlink. At the satellite level, a spot beam is created from a feedhorn radiating towards a reflector. A reflector may be associated with a color such that four-color coverage is ensured by four reflectors. In other words, the generation of 16 spot beams from each gateway may be done by using four antennas (one per color) each having a reflector, four feedhorns being associated with each reflector. 
       FIG. 4  illustrates a frequency plan broken down into an uplink frequency plan PMVA on the forward link, a downlink frequency plan PDVA on the forward link, an uplink frequency plan PMVR on the return link and a downlink frequency plan PDVR on the return link. The notations RHC and LHC respectively designate the right and left circular directions of polarization. 
     The PMVA plan corresponding to the forward uplink (from the gateway to the satellite) disposes 2 GHz (from 27.5 to 29.5 GHz) of available frequency spectrum such that 16 channels of 250 MHz of bandwidth are generated by a gateway (8 channels for each polarization). These 16 channels, after processing by the satellite payload, will form 16 spot beams. The assumption made here consists of considering that the entire 2 GHz spectrum is used: however, it will be noted that it is also possible, particularly for operational reasons, to use only one part of the spectrum and to generate fewer channels. In the example above, 16 spot beams (and thus  16  cells) are generated from two signals multiplexing the 8 channels (a signal multiplexed by polarization) generated by a gateway. Each multiplexed signal corresponding to a polarization is then processed at the satellite transponder level so as to provide 8 spot beams; each of these eight spot beams is associated with a frequency interval among the two frequency intervals [19.7; 19.95] and [19.95; 20.2] and with an RHC or LHC polarization as represented on the downlink frequency plan PDVA. 
     The PDVR plan corresponding to the return downlink (from the satellite to the gateway) disposes 2 GHz (from 17.7 to 19.7 GHz) of available frequency spectrum such that 16 spot beams of 250 MHz of bandwidth (associated with a frequency interval from among the two frequency intervals [29.5; 29.75] and [29.75; 30.0] and with an RHC or LHC polarization such as represented on the downlink frequency plan PMVR) issued from cells are multiplexed at the satellite level into two signals (corresponding to each polarization) to be returned to the gateway (8 channels for each polarization). We are still assuming that the entire 2 GHz spectrum is used. Thus, in the case of Europe with a spectrum of 2 GHz used, there is a number N c  of cells equal to 80 and a number of active gateways N GWactive  equal to 5 (or the number 80 of cells divided by the number 16 of spot beams). It will be noted that It may be that part of the band is not usable, for example the part going from 17.7 to 18.45 GHz in return link and the part going from 27.5 to 28.25 GHz in forward link: in this case, the number of channels Ns per polarization is equal to 5: therefore, the number of cells always being equal to 80 for Europe, the number of active gateways N GWactive  becomes equal to 5. In any case, the number of gateways N GWactive  is always less than the number N c  of coverage area cells. 
     For the forward link, each spot beam is associated with one of the four following colors: 
     a red color corresponding to a first band of 250 MHz (lower part of the available spectrum of 500 MHz) and to the right circular polarization direction; 
     a yellow color corresponding to the same first band of 250 MHz and to the left circular polarization direction; 
     a blue color corresponding to a second band of 250 MHz (upper part of the available spectrum of 500 MHz) and to the right circular polarization direction; 
     a green color corresponding to the same second band of 250 MHz and to the left circular polarization direction; The four adjacent spot beams with the same pattern are each associated with a different color. 
     On the return link, the polarizations are reversed so that the red and yellow colors have a left circular polarization and the blue and green colors have a right circular polarization. The ground terminals send and receive according to an opposite polarization such that one may easily separate the uplink signals from the downlink signals: such a configuration enables less costly terminals to be used. 
     The satellite payload designates the part that allows it to fulfill the mission for which it was designed, that is for a telecommunication satellite  3  such as that represented in  FIG. 1 , to ensure the reception, processing (frequency conversion, filtering, amplification) and resending of telecommunication signals from gateway  2 . The payload essentially includes satellite antennas and transponders (and not the equipment for control, propulsion or electrical power equipment which belong to the platform of the satellite). 
       FIG. 5  represents in a known manner a function block diagram of a payload  10  architecture in forward link (from gateways to cells including ground terminals) with multispot sending over the downlink. 
     After reception and selection of polarization, 2N GWactive  multiplexed signals (in the example cited above, N GWactive  signals from 8 channels for each of two polarizations) received from N GWactive  gateways (or communication gateway) are each amplified by a 12 LNA low noise amplifier. Each signal is then separated into N c  uplink channels by a signal dividing device (demultiplexer)  13 . The N c  uplink channels are then translated in frequency by a frequency converter circuit  14  generally formed by a local oscillator and filtered by a receiver filter  15  (of the band-pass filter type) so as to form N c  channels in agreement with the downlink frequency plan on the forward link (PDVA). The N c  translated frequency channels are amplified through a HPA (High Power Amplifier) high power amplifier  16  generally formed by a CAMP (Channel AMPlifier) channel amplifier  17  and a TWTA (Traveling Wave Tube Amplifier) traveling wave tube amplifier  18  forming N c  downlink spot beam signals. Each of the N c  signals is then filtered through a transmit band-pass filter  19  then sent over a feed  20  such as a feedhorn radiating to a reflector for forming a spot beam. According to this functional configuration, the payload  10  comprises: 
     2N GWactive  LNA low noise amplifiers  12 ; 
     2N GWactive  signal dividing devices  13 ; 
     N c  frequency converter circuits  14 ; 
     N c  receiver filters  15 ; 
     N c  HPA power amplifiers  16 ; 
     N c  transmit band-pass filters  19 . 
       FIG. 6  represents in a known manner a function block diagram of a payload  100  architecture in return link (from cells including ground terminals to gateways) with multispot sending over the uplink. 
     N c  signals received from N c  cells comprising user terminals are each amplified by an LNA (Low Noise Amplifier) low noise amplifier  112 . Each signal is then transposed in frequency by a frequency converter circuit  114  generally formed by a local oscillator and filtered by a receiver filter  115  (of the band-pass filter type) so as to form N c  channels in agreement with the downlink frequency plan on the return link (PDVR). The channels intended for the same gateway (for the same polarization) are then regrouped to form a signal multiplexed by a multiplexer  113  (at N c  inputs and 2N GWactive  outputs): the structure of this multiplexed signal is identical to that of a signal sent by a gateway to the satellite on the forward uplink. Thus there are 2N GWactive  signals in output from the multiplexer  113 . Each of the 2N GWactive  signals is amplified through a HPA power amplifier  116  generally formed by a CAMP channel amplifier  117  and a TWTA traveling wave tube amplifier  118  forming 2N GWactive  downlink signals in return link. Each of the 2N GWactive  return downlink signals is then filtered through a transmit band-pass filter  119  then sent over a radiating device  120  such as a feedhorn radiating to a reflector to form 2N GWactive  signals to N GWactive  gateways. According to this functional configuration, the payload  100  comprising: 
     N c  LNA low noise amplifiers  112 ; 
     N c  frequency converter circuits  114 ; 
     N c  receiver filters  115 ; 
     A multiplexer device  113  with N c  inputs and 2N GWactive  outputs; 
     2N GWactive  HPA power amplifiers  116 ; 
     2N GWactive  transmit band-pass filters  119 . 
     It will be noted that channel amplifiers  17  and/or  117  are generally gain control amplifiers that allow the power level of input signals of traveling wave tubes  18  and/or  118  to be adjusted. Tubes  18  and/or  118  may be replaced by SSPA (Solid State Power Amplifier) solid state power amplifiers. It is also possible to use more sophisticated architectures comprising MPA (Multiport Amplifier) type devices offering more flexibility. 
     However, payloads  10  and  100  such as presented above may pose several difficulties, particularly in the case of TWTA tube breakdowns. 
     A known solution to this problem consists of using redundant tubes. Such a configuration is illustrated in  FIG. 7 .  FIG. 7  schematically represents the part  200  of a return link payload situated between the multiplexer and the transmit filter and including CAMPs and redundant TWTA tubes. 
     As explained above, each of the 2N GWactive  signals is amplified through an HPA high power amplifier  216  generally formed by a CAMP channel amplifier  217  and a TWTA traveling wave tube amplifier  218  forming 2N GWactive  return downlink signals. The difference with  FIG. 6  resides in the fact that the payload  200  includes N TWTA  HPA power amplifiers  216  formed by N TWTA  CAMP channel amplifiers  217  and N TWTA  TWTA traveling wave tube amplifiers, N TWTA  being strictly greater than 2N GWactive . 
     The payload  200  comprises first selection means  201  receiving in input the 2N GWactive  signals to amplify and selecting 2N GWactive  power amplifiers  217  from among the N TWTA  present that will perform the amplification. In case of breakdown of a power amplifier (tube failure, for example), it is then possible to use another amplifier by switching the signal on this amplifier. In addition, payload  200  comprises second selection means  202  receiving as inputs the outputs of N TWTA  power amplifiers to produce in output 2N GWactive  amplified return downlink signals that will then be filtered through an output band-pass filter then sent over a feedhorn radiating towards a reflector to form 2N GWactive  signals to N GWactive  gateways. Such a configuration allows N TWTA -2N GWactive  amplifier failures to be supported. It will be noted that the same type of configuration may be provided in forward link with a redundancy of LNA low noise amplifiers. 
     However, such a configuration may also present certain difficulties. 
     SUMMARY OF THE INVENTION 
     In fact, it may also be that a gateway fails; in such a case, a known solution consists of introducing a redundancy in the components used in the gateway so as to be able to switch to a redundant component in case of failure of a component from one of the gateways. Of course, such redundancy leads to much higher costs. 
     In addition, it is also possible that a gateway is temporarily unavailable or unusable (for example, in case of adverse weather making data traffic difficult or in case of maintenance of the gateway) without it being failure. 
     It is an object of the present invention to provide a telecommunication network to establish radiofrequency links between gateways and ground terminals via a multispot telecommunication satellite, said network allowing the aforementioned problems to be eliminated. 
     For this purpose, the invention proposes a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a telecommunication satellite with several spot beams, known as a multispot satellite, said network comprising: 
     a multispot satellite including a payload for receiving, processing and resending the telecommunication signals received by said satellite, 
     a service area comprised of a plurality of basic coverage areas, called cells, each cell comprising a plurality of ground terminals, 
     a plurality N GWactive  of active gateways interconnected by a ground network, N GWactive  being an integer, said satellite relaying the signals sent by said N GWactive  active gateways to said cells, said network being characterized in that the network comprises N GW  gateways, N GW  being an integer strictly greater than the number N GWactive  of active gateways, said N GW  gateways being interconnected by said ground network and including said N GWactive  active gateways such that N GW −N GWactive  gateways are not active. 
     “Gateway” is understood to refer to any central station such as a ground communication gateway connected to an Internet backbone. The gateway sends signals over a forward link that are then processed at the satellite level which amplifies the signals, transposes the signals at a different frequency (generally lower), then retransmits the signals from the satellite antenna or antennas over a downlink in the form of a plurality of spot beams or spots forming basic coverage areas or cells in which the ground terminals are situated. 
     Active gateway is understood to refer to a gateway that effectively participates in the broadcasting and reception of signals to and from ground user terminal cells. Conversely, a non active gateway does not participate in data traffic; on the other hand, all of the active and inactive gateways are interconnected via a ground network such as an Internet type network. 
     Thanks to the invention, at least one additional inactivated gateway (N GW −N GWactive ≧2) is used in addition to the total number of active gateways N GWactive  necessary to cover the service area. The gateways N GW  are located in different locations and the means to select the payload on board the satellite enable switching from one failing or temporarily unavailable gateway to another gateway from among the N GW −N GWactive  gateways provided for this purpose. Such a configuration is particularly interesting not only in case of failure of a gateway but also and especially operationally in case of the temporary unavailability of a gateway, for example in case of adverse weather (rain leading to an alteration in radiofrequency signals) or in case of scheduled maintenance. 
     Advantageously, said network is a bidirectional network such that said satellite relays: 
     signals sent by said N GWactive  active gateways to said cells, this first link forming the forward link, 
     signals sent by said cells to said N GWactive  active gateways, this second link forming the return link. 
     The present invention also provides a return payload of a multispot telecommunication satellite used in a network according to the invention, said payload comprising means to reroute a multiplexed signal from a plurality of signals sent by said cells, said signal initially being intended for one of said N GWactive  active gateways that had become unavailable, to one of said N GWactive  gateways including one newly activated gateway selected from among the N GW −N GWactive  initially inactivated gateways. 
     The payload according to the invention may also present one or more of the characteristics below, considered individually or according to all technically possible combinations: 
     In a particularly advantageous manner, the payload comprises means to amplify said multiplexed and rerouted signal, the rerouting operation being done before the amplification operation. 
     Advantageously, the payload comprises amplification means including N TWTA  amplification paths able to amplify N TWTA  multiplexed signals, each path having an input and an output, N TWTA  being an integer such that 2N GWactive  is strictly less than N TWTA , 2N GWactive  paths forming nominal amplification paths and N TWTA −2N GWactive  paths forming backup amplification paths. 
     Advantageously, said amplification means are formed: 
     either by N TWTA  amplification units comprising N TWTA  traveling wave tube amplifiers or N TWTA  solid state power amplifiers, each amplification unit being able to amplify a multiplexed signal; 
     or by a MPA multiport amplification device comprising N TWTA  inputs and N TWTA  outputs and able to amplify N TWTA  multiplexed signals. 
     According to a first embodiment, the payload comprises: 
     first means to selectively connect 2N GWactive  outputs selected from among N TWTA  outputs to 2N GWactive  inputs able to receive 2N GWactive  multiplexed signals, the 2N GWactive  signals including N GWactive  signals polarized according to a first polarization and N GWactive  signals polarized according to a second polarization opposite from said first polarization, said N TWTA  outputs being connected to said N TWTA  inputs of said N TWTA  amplification paths; 
     second means to selectively connect 2N GWactive  inputs selected from among N TWTA  inputs to 2N GWactive  outputs, said N TWTA  inputs being connected to said N TWTA  outputs of said N TWTA  amplification paths; 
     third means to selectively connect 2N GWactive  inputs to 2N GWactive  outputs selected from among 2N GW  outputs, said 2N GWactive  inputs being connected to said 2N GWactive  outputs of said second selective connection means. 
     According to a second particularly advantageous embodiment, 2N GW  is less than or equal to N TWTA  and said payload comprises: 
     first means to selectively connect 2N GWactive  outputs selected from among N TWTA  outputs to 2N GWactive  inputs able to receive 2N GWactive  multiplexed signals, the 2N GWactive  signals including N GWactive  signals intended for a first polarization and N GWactive  signals intended for a second polarization opposite from said first polarization, said N TWTA  outputs being connected to said N TWTA  inputs of said N TWTA  amplification paths; 
     second means to selectively connect 2N GW  inputs selected from among N TWTA  inputs to 2N GW  outputs, said N TWTA  inputs being connected to said N TWTA  outputs of said N TWTA  amplification paths. 
     In a particularly advantageous manner, the payload comprises: 
     amplification means including N 2  amplification paths able to amplify N 2  multiplexed signals, each path having an input and an output, N 1  paths forming nominal amplification paths and N 2 -N 1  paths forming backup amplification paths, N 2  and N 1  being integers such that the integer N 1  is strictly less than N 2 , the integer N 1  either being equal to 2N GWactive  or equal to N GWactive ; 
     first means to selectively connect N 1  outputs selected from among N 2  outputs to N 1  inputs able to receive N 1  multiplexed signals, said N 2  outputs being connected to said N 2  inputs of said N 2  amplification paths; 
     second means to selectively connect N 3  inputs selected from among N 2  inputs to N 3  outputs, the number N 3  being either equal to N GW  or equal to 2N GW , N 3  being less than or equal to N 2 , said N 2  inputs being connected to said N 2  outputs of said N 2  amplification paths. 
     Advantageously, said selective connection means are formed by a plurality of R type switches. 
     The present invention also provides_a method for reconfiguring a network in case of unavailability of one of said N GWactive  active gateways, said method utilizing a payload according to the invention and comprising the following steps: 
     activation of a gateway selected from among the N GW −N GWactive  inactivated gateways, said selected gateway receiving at least one signal from an output of said second means, said output being connected via said second means to an output of one of said N TWTA  or N 2  amplification paths, known as a reconfiguration path, said network thus comprising a group N GWactive  of gateways including one newly activated gateway selected from among the N GW −N GWactive  inactivated gateways; 
     control of said first means to connect the input of said reconfiguration path to an input able to receive multiplexed signals intended for one of the gateways from among the group of N GWactive  gateways including one newly activated gateway. 
     The present invention also provides_a satellite comprising a return payload according to the invention and a forward payload comprising: 
     a plurality of low noise amplifiers able to amplify the multiplexed signals sent by said active gateways; 
     means to reroute the multiplexed signals sent by the gateway ensuring the traffic from said gateway that had become unavailable to the low noise amplifier intended to amplify the multiplexed signals sent by said gateway that had become unavailable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will emerge more clearly from the description given below, for indicative and in no way limiting purposes, with reference to the attached figures, among which: 
         FIG. 1  is a simplified schematic representation of a multispot configuration; 
         FIG. 2   a ) represents an example of a coverage area comprised of a plurality of adjacent hexagons; 
         FIG. 2   b ) represents an approximation of the coverage area from  FIG. 2   a ) comprised of a plurality of circular spot beams; 
         FIG. 3  illustrates a four-color scheme for the coverage of Europe; 
         FIG. 4  illustrates a frequency plan in band Ka; 
         FIG. 5  is a functional block diagram of a reference payload architecture in forward link according to the prior art; 
         FIG. 6  is a functional block diagram of a reference payload architecture in return link according to the prior art; 
         FIG. 7  schematically represents the part of a return payload situated between the multiplexer and the output filter and including CAMPs and redundant TWTA tubes according to the prior art; 
         FIG. 8  schematically represents a return payload according to a first embodiment of the invention; 
         FIG. 9  schematically represents a return payload according to a second embodiment of the invention; 
         FIGS. 10 ,  11  and  12  represent a payload according to the second embodiment of the invention in three different configurations. 
         FIGS. 13   a ,  13   b ,  13   c  and  13   d  represent an R type switch in its four possible positions. 
     
    
    
     In all figures, common elements bear the same reference numbers. 
     DETAILED DESCRIPTION 
     The invention relates to a telecommunication network for establishing radiofrequency links between gateways and ground terminals via a multispot telecommunication satellite. An example of such a network is represented in  FIG. 1 . The network according to the invention comprises a plurality N GWactive  of active gateways (i.e., which participate in the data traffic within the network) interconnected by a ground network such as an Internet network. 
     Subsequently we will place ourselves in the assumption of a bidirectional network of the type that the satellite relays: 
     signals sent by said N GWactive  active gateways to said cells, this first link forming the forward link, 
     signals sent by said cells to said N GWactive  active gateways, this second link forming the return link. 
     In addition, the network comprises N GW  gateways, N GW  being an integer strictly greater than the number N GWactive  of active gateways (for example, N GW =N GWactive +1). The N GW  gateways include N GWactive  active gateways and are interconnected via the ground network mentioned above. Among these N GW  gateways, N GW −N GWactive  gateways are not active (i.e., they do not participate in the data traffic within the network). 
     As we will see in further detail subsequently with reference to  FIGS. 8 to 14 , thanks to the invention, at least one additional inactive gateway is used in addition to the total number of active gateways N GWactive  necessary for the service area coverage. The N GW  gateways are located in different locations. The presence of at least one additional gateway allows switching from a failing or temporarily unavailable gateway to this gateway. Of course, such a configuration implies that all of the gateways are connected to the same telecommunication network to be interchangeable; this configuration is particularly interesting not only in case of failure of a gateway but also and especially operationally in case of the temporary unavailability of a gateway, for example in case of adverse weather (rain leading to an alteration in radiofrequency signals) or in case of scheduled maintenance. 
       FIG. 8  schematically represents the part of a payload  300  in return link allowing establishment of a network according to the invention. This payload part  300  is situated between the multiplexer and the transmit filter. In a known manner, not represented, the payload  300  receives N c  signals received from a plurality N c  of cells comprising user terminals; these N c  signals are each amplified by an LNA low noise amplifier. Each signal is then translated in frequency by a frequency converter circuit generally formed by a local oscillator and filtered by a receiver filter (of the band-pass filter type) so as to form N c  channels in agreement with the downlink frequency plan on the return link. The channels intended for the same gateway (for the same polarization) are then regrouped to form a signal multiplexed by a multiplexer (at N c  inputs and 2N GWactive  outputs): the structure of this multiplexed signal is identical to that of a signal sent by a gateway to the satellite on the forward uplink. Thus there are 2N GWactive  output signals from the multiplexer: the 2N GWactive  signals include N GWactive  signals intended for a first polarization and N GWactive  signals intended for a second polarization opposite from the first polarization (it may be a polarization with a right or left circular direction or a linear polarization with a horizontal and vertical direction). Each of the 2N GWactive  signals is amplified through an HPA high power amplifier  316  generally formed by a CAMP channel amplifier  317  and a TWTA traveling wave tube amplifier  318  forming 2N GWactive  return downlink signals. The payload  300  includes N TWTA  HPA power amplifiers  316  (subsequently also known as N TWTA  amplification paths) formed by N TWTA  CAMP channel amplifiers  317  and N TWTA  TWTA traveling wave tube amplifiers  318 , N TWTA  being strictly greater than 2N GWactive . The payload  300  thus comprises 2N GWactive  nominal amplifiers intended to amplify the 2N GWactive  signals and N TWTA -2N GWactive  redundant amplifiers. 
     The payload  300  also comprises first means  301  (at 2N GWactive  inputs and N TWTA  outputs) to selectively connect 2N GWactive  outputs selected from among N TWTA  outputs to 2N GWactive  inputs. The 2N GWactive  inputs receive 2N GWactive  multiplexed signals. For example, the first means  301  are formed by switches such as R type switches. The N TWTA  outputs from first means  301  are connected to the N TWTA  inputs from N TWTA  amplification paths  316 . In case of failure of one of the 2N GWactive  power amplifiers  316  (failure of a tube, for example) receiving the 2N GWactive  multiplexed signals, it is then possible to use another amplifier by rerouting the signal over another amplifier from among the N TWTA -2N GWactive  redundant amplifiers. 
     In addition, payload  300  comprises second means  302  (at N TWTA  inputs and 2N GWactive  outputs) to selectively connect 2N GWactive  inputs selected from among N TWTA  inputs to 2N GWactive  outputs, the N TWTA  inputs being connected to the N TWTA  outputs of the N TWTA  amplification paths  316 . The second means  302  are also, for example, made in the form of R type switches. The second means  302  receive as inputs the outputs from the N TWTA  power amplifiers  316  to produce in output 2N GWactive  amplified return downlink signals. These 2N GWactive  are intended for N GWactive  active gateways. 
     In case of failure of one of the 2N GWactive  power amplifiers  316 , we have seen that another amplifier was used by rerouting the signal over this other amplifier via the first means  301 . Second means  302  allow this amplification path to be connected to the output of said second means  302  corresponding to the gateway initially served by the failing amplification path. 
     The payload  300  also comprises third means  303  (at 2N GWactive  inputs and 2N GW  outputs) to selectively connect 2N GWactive  inputs to 2N GWactive  outputs selected from among 2N GW  outputs. The 2N GWactive  inputs from the third means  303  are connected to the 2N GWactive  outputs from the second selective connection means  302 . Thus, in case of unavailability of an initially active gateway, it is possible to reroute the signal initially intended for this gateway to one of the N GW −N GWactive  inactive gateways that had become active. Incidentally, it should be noted that in the example presented here, two signals will be rerouted to two outputs of the third means  303  (since the network uses two polarizations, each gateway receives two signals corresponding to the two polarizations). 
     Of course, it should be noted that the payload  300  also comprises NGW output filters and 2N GW  feedhorns in order to be able to potentially send to each of the N GW  available gateways. 
     However, payload  300  presents a disadvantage inasmuch as the switching performed by the third means  303  is done over a signal at high power, switching in power not being desirable. Purely as an illustration, a signal in input from the amplification path has a power on the order of a milliwatt while the amplified signal has a power on the order of a hundred watts. One way to eliminate this disadvantage consists, for example, of using the mute function of the CAN channel amplifier  317  of amplification path  316  over which the switching operation will take place: the CAN  317  amplifier gain is then sufficiently reduced such that the tube amplifier  318  that follows it has practically no signal to amplify, the switching operation is thus done at reduced power. 
       FIG. 9  illustrates a more elegant solution allowing the switching in power problem mentioned above to be eliminated. 
       FIG. 9  schematically represents the part of a payload  400  in return link allowing establishment of a network according to the invention. As in the case of  FIG. 3 , this part of the payload  400  is situated between the multiplexer and the output filter. In a known manner, not represented, the payload  400  receives NC signals received from a plurality NC of cells comprising user terminals; these NC signals are each amplified by an LNA low noise amplifier. Each signal is then translated in frequency by a frequency converter circuit generally formed by a local oscillator and filtered by a receiver filter (of the band-pass filter type) so as to form NC channels in agreement with the downlink frequency plan on the return link. The channels intended for the same gateway (for the same polarization) are then regrouped to form a signal multiplexed by a multiplexer (at N c  inputs and 2N GWactive  outputs): the structure of this multiplexed signal is identical to that of a signal sent by a gateway to the satellite on the forward uplink. Thus there are 2N GWactive  output signals from the multiplexer: the 2N GWactive  signals include N GWactive  signals intended for a first polarization and N GWactive  signals intended for a second polarization opposite from the first polarization (it may be a polarization with a right or left circular direction or a linear polarization with a horizontal and vertical direction). Each of the 2N GWactive  signals is amplified through an HPA high power amplifier  417  generally formed by a CAMP channel amplifier  417  and a TWTA traveling wave tube amplifier  418  forming 2N GWactive  return downlink signals. The payload  400  includes N TWTA  HPA high power amplifiers  416  formed by N TWTA  CAMP channel amplifiers  417  and N TWTA  TWTA traveling wave tube amplifiers, N TWTA  being strictly greater than 2N GWactive . The payload  400  thus comprises 2N GWactive  nominal amplifiers intended to amplify the 2N GWactive  signals and N TWTA -2N GWactive  redundant amplifiers. 
     The payload  400  also comprises first means  401  (at 2N GWactive  inputs and N TWTA  outputs) to selectively connect 2N GWactive  outputs selected from among N TWTA  outputs to 2N GWactive  inputs. The 2N GWactive  inputs receive 2N GWactive  multiplexed signals. For example, the first means  401  are formed by switches such as R type switches. The N TWTA  outputs from first means  401  are connected to the N TWTA  inputs from N TWTA  amplification paths  416 . In case of failure of one of the 2N GWactive  power amplifiers  416  (failure of a tube for example) receiving the 2N GWactive  multiplexed signals, it is then possible to use another amplifier by rerouting the signal over another amplifier from among the N TWTA −2N GWactive  redundant amplifiers. 
     The payload  400  also comprises second means  402  (at N TWTA  inputs and 2N GW  outputs) to selectively connect 2N GW  inputs selected from among N TWTA  inputs to 2N GW  outputs. The N TWTA  inputs of the second means  402  are connected to the N TWTA  outputs of the N TWTA  amplification paths  416 . The second means  402  are, for example, made in the form of R type switches. 
     In case of failure of one of the 2N GWactive  power amplifiers  416  (failure of a tube, for example) receiving 2N GWactive  multiplexed signals, it is possible to use another amplifier by rerouting the signal via first means  401  over another amplification path from among the N TWTA −2N GWactive  redundant amplifiers. Second means  402  allow this amplification path to be connected to the output of the second means  402  corresponding to the gateway initially served by the failing amplification path. 
     In case of unavailability of one of the N GWactive  active gateways, the network reconfiguration method is as follows: 
     the method starts by activating a gateway selected from among the N GW −N GWactive  initially inactivated gateways. The selected gateway is able to receive a signal issued from an output from the second means  402 , this output being connected via the second means  402  to an output from one of the N TWTA  amplification paths that we are calling the reconfiguration path. The network thus comprises a group of N GWactive  active gateways including a newly activated gateway selected from among the N GW −N GWactive  inactivated gateways; 
     the first means  401  are controlled to connect the input of the reconfiguration path to an input able to receive multiplexed signals intended for one of the gateways from among the group of N GWactive  gateways including one newly activated gateway. 
     It will be noted that two solutions are possible: either the reconfiguration path is powered after activation of the selected gateway or the potential reconfiguration path or paths is or are always powered (on standby) to ensure that the path or paths is or are ready to be used. In the first case, a warming period connected to the powering of the reconfiguration path is introduced. In the second case, this period is eliminated (on the other hand, maintaining a certain powering is required, which induces higher consumption). 
     Thus, it is entirely possible to arrange for the reconfiguration path to be put on standby when a gateway maintenance operation is scheduled: thus the period induced by warming the amplification path is avoided. 
     It will be noted that reconfiguration depends on the type of selective connection means used: for example, one may directly connect the input of the reconfiguration path to the input able to receive multiplexed signals initially intended for the unavailable gateway. One may also connect the input of the reconfiguration path to an input different from the input able to receive multiplexed signals initially intended for the unavailable gateway. In this case, another switching will have to take place (see for example  FIGS. 10 to 13 ). 
     This reconfiguration is very rapid since, even in the case where the reconfiguration path is not on standby, powering of the reconfiguration path and switching of the first means  401  only takes several minutes (typically 4 minutes). 
     Thanks to the payload  400  according to the invention, it is thus possible to reroute the signals initially intended for an unavailable gateway to a substitution gateway without performing a switching in power (the switching of second means  402  is done upstream from amplification paths  416 , so at low power). 
     Traffic is thus redirected without reducing the gain of the CAMP amplifiers  417  (or even completely shutting down the amplification paths  416 ). 
     According to this embodiment, a distinction is no longer made between the nominal amplifiers and the redundant amplifiers, a redundant amplifier may become a nominal amplifier to satisfy a replacement gateway. 
     The operating condition of a network using a payload such as payload  400  is that number 2N GW  is less than or equal to the total number N TWTA  of amplifiers. Here it should be noted that the network according to the invention allows a number N GW  of gateways to be used that is not doubled with relation to the number of active gateways. 
     The problematics linked on the one hand to the reconfiguration linked to the loss of an amplifier and on the other hand to the reconfiguration linked to the unavailability of a gateway are two decorrelated problematics. 
     It will be noted that, in the embodiment presented here, in case of unavailability of a gateway, there are two reconfiguration paths, each receiving the multiplexed signals according to a polarization initially intended for the unavailable gateway (the network in fact uses two polarizations, each gateway receiving two signals corresponding to the two polarizations). 
     As for the payload  300  of  FIG. 8 , it will also be noted that the payload  400  of course also comprises N GW  output filters and 2N GW  feedhorns in order to be able to potentially send to each of the N GW  available gateways. 
       FIGS. 10 to 12  illustrate a simplified example of a payload  500  according to the second embodiment of the invention (the case of payload  400  such as represented in  FIG. 9 ) in three different configurations. For simplification, we will not introduce (contrary to payload  400 ) the requirement linked to the two amplification systems (one system per polarization) in the case of  FIGS. 10 to 13 , the principle remains the same depending on whether one operates with one amplification system or two amplification systems. 
     Before describing  FIGS. 10 to 12 , we recall with reference to  FIGS. 13A to 13D  the operation of an R type waveguide switch  503 .  FIGS. 13A to 13D  represent the four possible configurations of an R type switch. R type switches are adapted to turn by 45° steps to connect any given port to any of the three other ports, only the case of the connection between opposite ports involves insulation of each of the other ports; these switches have four distinct connection configurations. This switch has four ports P 1 , P 2 , P 3  and P 4  (clockwise numbering) and one switching means adapted to: 
     connect two opposite ports by insulating the others ( FIGS. 13A and 13D ); 
     connect two adjacent ports as well as the other ports ( FIGS. 13A and 13D ). 
     Payload  500  comprises first means  501  comprising N 1  (N 1 =4 in our example) inputs E (E 1  to E N1  with N 1 =4) and N 2  (N 2 =6 in our example) outputs S (S 1  to S N2  with N 2 =6). The N 1  inputs are able to receive N 1  multiplexed signals; in comparison with the example from  FIG. 9 , N 1  is equal to 2N GWactive . However, if a payload is used with two amplification systems (one system per polarization), N 1  is equal to N GWactive  for each amplification system. The invention thus applies to both a payload with 2N GWactive  inputs and a payload organized into two amplification systems each having N GWactive  inputs. First means  501  comprise a series of N 1  R type switches  503  ( 503   1  to  503   N1  with N 1 =4): with reference to  FIGS. 13A to 13C , the N 1  inputs from first means  501  correspond to ports P 1  of switches  503 . In addition, each port P 4  of a switch is connected to port P 2  of the next switch. The N 2  outputs correspond to ports P 3  of switches  503  as well as to port P 2  of switch  503   1  situated at a first end of the series of switches  503  and to port P 4  of switch  503   4  situated at a second end of the series of switches  503 . N 2  is an integer strictly greater than N 1 ; in comparison with the example of  FIG. 9 , N 2  is equal to N TWTA . If a payload is used with two amplification systems (one system per polarization), N 2  is equal to N TWTA /2 for each amplification system. 
     Payload  500  includes N 2  power amplifiers  516  or amplification paths ( 516   1  to  516   N2  with N 2 =6) formed by N 2  CAMP channel amplifiers  517  ( 517 , to  517   N2  with N 2 =6) and N 2  TWTA traveling wave tube amplifiers  518  ( 518   1  to  518   N2  with N 2 =6). The N 2  outputs are connected to the N 2  inputs from N 2  amplification paths  516 . Thus there are N 2 −N 1  backup amplifiers  516  (amplifiers  516   1  and  516   6  in our example). In the configuration from  FIG. 10 , first means  501  are such that inputs E 1  to E 4  are connected to outputs S 2  to S 5 . 
     Payload  500  in addition comprises second means  502  comprising N 2  inputs E′ (E′ 1  to E′ N2  with N 2 =6) and N 3  (N 3 =6 in our example) outputs S′ (S′ 1  to S′ N2  with N 2 =6). The N 2  inputs are able to receive N 2  amplified multiplexed signals. Number N 3  is less than or equal to the total number N 2  of amplifiers (in our example N 3 =N 2 ). 
     The N 2  inputs E′ 1  to E′ N2  from second means  502  are connected to N 2  outputs from N 2  amplification paths  516   1  and  516   N2 . 
     The N 3  outputs S′ 1  to S′ N3  from second means  502  are connected to N 2  inputs from N 2  amplification paths E′ 1  and E′ N2 . 
     Second means  502  comprise a series of N 3  R type switches  503 ′ ( 503 ′ 1  to  503 ′ N3  with N 3 =6): with reference to  FIGS. 13A to 13C , N 1  inputs from second means  502  correspond to ports P 1  of switches  503 ′. In addition, each port P 4  of a switch is connected to port P 2  of the following switch. The N 3  outputs correspond to ports P 3  of switches  503 ′. The N 3  outputs correspond to the output ports allowing signals to be sent to gateways. In comparison with the example of  FIG. 9 , N 3  is equal to 2N GW . However, if a payload is used with two amplification systems (one system per polarization), N 3  is equal to N GW . In  FIG. 10 , R type switches are such that their port P 1  is electrically connected to their port P 3 . 
     In the example, inactive gateways are gateways connected to outputs S′ 1  and S′ 6 . 
     The case of unavailability (maintenance, adverse weather, etc.) of one of the N GWactive  active gateways (in our example the gateway connected to output S′ 3 ) is illustrated by the passage from  FIG. 10  to  FIG. 11 . 
     The method starts by activating a gateway selected from among the N GW −N GWactive  initially inactivated gateways. In our example, this is the gateway connected to the output S′ 1 . This output S′ 1  is connected via input E′ 1  from the second means  502  to the output of the amplification path  516   1  that we are calling the reconfiguration path. This reconfiguration path  516   1  is powered (as we mentioned before, this step is not necessary if the reconfiguration path is already on standby). First means  501  are then controlled to connect the input from the reconfiguration path to the input E 1  (the first R switch is turned so that its port P 1  is connected to its port P 2  and its port P 3  is connected to its port P 4 ) able to receive multiplexed signals initially intended for the gateway receiving signals from output S′ 2 . First means  501  are also controlled to connect the input from the amplification path  516   2  to input E 2  (the second R switch is turned so that its port P 1  is connected to its port P 2  and its port P 3  is connected to its port P 4 ) able to receive multiplexed signals initially intended for the gateway that had become unavailable. Here it may be seen that reconfiguration of the network requires two switchings at the level of the first means  501  (however, more sophisticated switching means allow the input from the reconfiguration path to be directly connected to the input E 2  able to receive multiplexed signals initially intended for the gateway that had become unavailable so as to eliminate this “domino” effect). It is also observed that this reconfiguration does not require any action on the second means  502  and that switchings are performed over low power signals. In addition, it will be noted that N 2 −N 1  backup amplifiers  516  have changed: these are amplifiers  516   1  and  516   6  in the case of  FIG. 10 ; these are amplifiers  516   3  and  516   6  in our example. 
     The case of a failure of one of the N 2  power amplifiers  516  (failure of a tube, for example) is illustrated by the passage from  FIG. 10  to  FIG. 12 . In the case of  FIG. 12 , tube  518   1  is failing and tube  518   2  becomes the backup tube. In this case, all of the R switches from the first means  501  are turned so that their port P 1  is connected to their port P 4  and that their port P 2  is connected to their port P 3 . Furthermore, all of the R switches from the second means  502  are turned so that their port P 1  is connected to their port P 2  and that their port P 3  is connected to their port P 4 . In this case, the amplification path  518   2  becomes the backup amplification path. Outputs S′ 2  to S′ 5  from the second means  502  sending signals to gateways remain unchanged. Thus it is observed that the failure of a tube (or an amplification path) does not lead to reconfiguration of the active gateways. 
     The invention was more particularly described in the case of a return payload. However, it may also be desirable to apply the same principle to a forward payload at the level of the low noise amplifiers (such as the LNA amplifiers  112  represented in  FIG. 6 ). In this case, the forward payload comprises: 
     a plurality of low noise amplifiers able to amplify the multiplexed signals sent by the active gateways; 
     means to reroute the multiplexed signals sent by the gateway ensuring the traffic from the gateway that had become inactive to the low noise amplifier intended to amplify the multiplexed signals sent by the gateway that had become unavailable. 
     Of course, the invention is not limited to the embodiment that has just been described. 
     Thus, the invention was more particularly described in the case of an amplifier formed by a CAMP followed by a TWTA. However, it will be noted that the invention also applies to the case of an SSPA amplifier or MPA type equipment.