The present invention concerns a method for establishing radiofrequency links via a telecommunication satellite having several spots, designated multispot satellite, between at least one terrestrial station (designated as gateway) and a service area composed of a plurality of elementary covering zones, designated as cells, each cell comprising a plurality of terrestrial terminals. This type of satellite allows the use of several antenna spots on board the satellite to cover contiguous geographical zones or cells, instead of a single broad spot.
Such multispot satellites allow several radiofrequency links to be established occupying the same frequency band on different spots.
In the case of a broadband satellite telecommunication system, the satellite is used in a bidirectional manner, i.e. at the same time for:                relaying data emitted by a terrestrial station towards a plurality of terrestrial terminals: this first link of the point-multipoint type constitutes the forward link;        relaying towards the terrestrial station the data emitted by the terrestrial terminals this second link, of the multipoint-point type, constitutes the return link.        
It will be noted that a radiodiffusion service by satellite can be considered as equivalent to the forward link of a bidirectional system such as described above.
An example of forward link in a multispot configuration is illustrated in FIG. 1.
Signals are sent towards a multispot satellite 3 on an uplink LM by a terrestrial station 2 such as a gateway connected to an internet backbone 5. These signals are then processed at the level of the satellite 3 then retransmitted on a downlink LD in the form of a plurality of spots forming elementary covering zones or cells C1 to C8 in which terrestrial terminals 6 are situated. Each cell C1 to C8 is associated with a spot SP1 to SP8. It will be noted that in the case of configuration 1, the eight cells C1 to C8 associated respectively to the eight spots SP1 to SP8 form a group of cells served by the same terrestrial station 2. The return link of the terrestrial terminals 6 towards the terrestrial station 2 functions in an identical manner with an inverse direction of communication.
The coordination of the frequencies between operators is carried out within the framework of a regulation decreed by the International Union of Telecommunications (IUT): thus, by way of example, the Ka band for Region 1 (Europe, Africa, Middle East) is defined in Table 1 below:
TABLE 1Forward linkUplink (of the terrestrial station)27.5 GHz to 29.5 GHzDownlink (towards the terrestrial19.7 GHz to 20.2 GHzterminals)Return linkUplink (of the terrestrial terminals)29.5 GHz to 30.0 GHzDownlink (towards the terrestrial17.7 GHz to 19.7 GHzstation)
Other bands such as band Ku can likewise be used.
Given that the gain of an antenna is inversely proportional to the opening of the spot, it is necessary to use multispot antennae to cover an extensive zone with a homogeneous and high gain. The greater number of spots, the smaller the opening of each spot will be. Thus, the gain on each spot and hence the gain on the service area to be covered will be increased. As we have mentioned above, a service area to be covered is formed by a plurality of contiguous cells (elementary covering zones), a spot being associated with each cell. A homogeneous multispot covering zone SA is represented in FIG. 2a), each cell being represented by a hexagon FH such that the covering zone is composed of a plurality of hexagons FH in which φcell is the external dimension of the cell expressed by the angle of the satellite associated with the covering. However, the antenna spot associated with each cell is not capable of producing a hexagonal shape, a good approximation consisting in considering a plurality of circular spots FC such as represented in FIG. 2 b). The association of a spot with a cell is carried out taking into account the best performances of the satellite for said spot, in particular in terms of EIRP (Effective Isotropic Radiated Power) and of merit factor G/T (ratio gain over noise temperature): a cell is determined as the part of the service area associated with the spot offering the highest gain on this zone from all the spots of the satellite.
Configuration 1, as represented in FIG. 1, uses a technique designated frequency re-use: this technique allows the same range of frequencies to be used several times in the same satellite system so as to increase the total capacity of the system without increasing the attributed band pass.
Frequency re-use schemes are known, designated as color schemes, making a color correspond to each of the spots of the satellite. These color schemes are used to describe the attribution of a plurality of frequency bands to the spots of the satellite with a view to radiofrequency transmissions to be realized in each of these spots. In these schemes, each color corresponds to one of these frequency bands.
In addition, these multispot satellites allow polarised transmissions to be emitted (and received): the polarisation can be linear (in this case the two directions of polarisation are respectively horizontal and vertical) or circular (in this case the two directions of polarisation are respectively circular left or circular right). It will be noted that in the example of FIG. 1, the uplink leaving the station 2 uses two polarisations with four channels for each polarisation, respectively Ch1 to Ch4 for the first polarisation and Ch5 to Ch8 for the second polarisation: the use of two polarisations allows the total number of terrestrial stations to be reduced. The eight channels Ch1 to Ch8, after processing by the payload of the satellite 3 will form the eight spots SP1 to SP8 (one channel being associated with one spot in this example).
According to a scheme with four colors (red, yellow, blue, green) with a frequency spectrum of 500 MHz for each polarisation, the transmissions being polarised in one of the two polarisation directions: circular right or circular left, each color is associated with a band of 250 MHz and a polarisation direction.
In the whole of the following description, we will take the following convention:                the color red is represented by hatched lines toward the right;        the color yellow is represented by dense points;        the color blue is represented by hatched lines toward the left;        the color green is represented by dispersed points.        
A color is thus associated with each spot of the satellite (and hence a cell) so that the spots of a same “color” are non-adjacent: the contiguous cells therefore correspond to different colors.
FIGS. 3a) and 3b) take up again the example of FIGS. 2a) and 2b) with a scheme having four colors. FIG. 3a) illustrates a homogeneous multispot covering zone, each cell being represented by a hexagon associated with a color so that the contiguous cells therefore correspond to different colors. FIG. 3b) represents the circular spots associated with each cell (the color of which is identical to that of the associated cell).
An example of a scheme with four colors for the coverage of Europe is represented in FIG. 4. In this case, 80 cells are necessary to cover Europe. This scheme allows there to be a European coverage towards and from terminals using a spectrum of 500 MHz but with the re-use of the frequencies. The coverage for the terrestrial stations is less constraining and can be provided by a sub-assembly of spots or a separate coverage.
This type of scheme is equally applicable in uplink and in downlink. At the satellite level, the creation of a spot is made from a horn radiating towards a reflector. A reflector can be associated with a color so that a coverage with four colors is ensured by four reflectors.
FIG. 5 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 designate respectively the right and left circular directions of polarisation.
The PMVA plan corresponding to the uplink on the forward link (of the terrestrial station to the satellite) has 2 GHz available frequency spectrum so that 16 channels of 250 MHz band pass are generated by a terrestrial station (8 channels for each polarisation). These 16 channels, after processing by the payload of the satellite will form 16 spots. In this example, 16 spots (and hence 16 cells) are generated by a terrestrial station.
It will be noted that the scheme with four colors, for the forward link, associates one of the following four colors with each spot of a pattern of four adjacent spots:                a first color red corresponding to a first band of 250 MHz (lower part of the available spectrum of 500 MHz) and to the circular right polarisation direction;        a second color blue corresponding to the same first band of 250 MHz and to the circular left polarisation direction;        a third color yellow corresponding to a second band of 250 MHz (upper part of the available spectrum of 500 MHz) and to the circular right polarisation direction;        a fourth color green corresponding to the same second band of 250 MHz and to the circular left polarisation direction.        
On the return link, the polarisations are inverted so that the colors red and yellow have a circular left polarisation and the colors blue and green have a circular right polarisation.
However, such a configuration is liable to involve certain difficulties.