EP2120281A1 - Multi-beam antenna system for multi-spot coverage and satellite comprising such a system - Google Patents

Abstract

The system has an antenna including a control unit coupled with a power supply unit such that sub-sets of radiating elements in a radiating element array of the antenna are successively powered to form beams illuminating one of sub-spots (A1-A3) in each of spots of a coverage obtained according to an initial reutilization scheme. The obtained coverage is equivalent to a reutilization scheme of reutilization factor equal to a product of an initial reutilization factor and number of sub-spots in each spot.

Description

GENERAL TECHNICAL FIELD

The invention relates to the field of satellite communications and more particularly relates to a multibeam antennal system for the coverage of a given geographical area decomposed into several ground spots (in English, "spots").

STATE OF THE ART

Multi-beam antennal systems for multispots coverage of a given geographical area are used in satellite communications.

The main objective of this technology is to reduce the cost of bit transmission by making the best use of the frequency band allocated to a given application.

Current techniques use multi-beam antennal systems operating in frequency diversity and polarization diversity.

These techniques are similar to those used for so-called "cellular" terrestrial communications networks.

Within these networks, the distribution of the signals is such that two adjacent cells have no signals having the same characteristics, that is to say signals with the same frequency and the same polarization.

In contrast, the identical signals are reused in non-adjacent cells to increase the capacity of the system.

The noted reuse factor k characterizes the number of different cells, in terms of signal characteristics, over the entire coverage.

The figure 1 presents an example of a theoretical cellular network with a fourfold reuse scheme. The arrangement of the cells maximizes the distance D between identical cells, materialized on the figure 1 by the same filling pattern.

In the usual way for satellite transmissions, the allocated frequency band is divided into two sub-frequency bands F ₁ and F ₂ , and two orthogonal linear (horizontal H and vertical V) or circular (right PCG or left PCD) polarizations are used.

• F1 + H (ou PCG) ;
• F1 + V (ou PCD) ;
• F2 + H (ou PCG) ;
• F2 + V (ou PCD).

Thus the four cells of a fourfold reuse scheme will be defined as follows:

• F1 + H (or PCG);
• F1 + V (or PCD);
• F2 + H (or PCG);
• F2 + V (or PCD).

In the context of multi-beam applications, the main parameter is the signal to interference ratio (in English, " Carrier to Interference Ratio ", (C / I)).

For a given cell, signals using the same frequency and polarization in another cell contribute to the noise and degrade the link.

The C / I can be improved by increasing the reuse factor, which has the direct consequence of increasing the distance D between two cells using the same signal (same frequency, same polarization) for a given reflector antenna geometry.

However, given the development constraints related to the types of antennas used for satellite telecommunications (single offset reflector antennas, double reflectors, etc.), it is difficult to envisage different reuses of four, as this increases the number of antennas to be arranged on the satellite for passive antennas of reflector type fed by a focal network (in English, " Focal Array Fed Reflector ", (FAFR)) or increases the complexity of these antennas (active, power supply circuit digital, etc.).

PRESENTATION OF THE INVENTION

The invention makes it possible, from a reference configuration covering a given geographical area by means of a reuse scheme by k, to obtain a coverage equivalent to a multiple reuse scheme of the reuse factor k of the configuration of reference.

Thus, according to a first aspect, the invention relates to a multi-beam antennal system for covering a given geographical area decomposed into a plurality of spots, comprising at least one antenna comprising: an array of radiating elements; network supply means such that in operation the radiating elements form a plurality of beams each illuminating a spot, the coverage of the area being performed according to an initial reuse factor reuse scheme.

The system of the invention is characterized in that each antenna further comprises control means coupled to the supply means such that, in operation, subsets of radiating elements are successively supplied to form a plurality of beams illuminating a beam. portion of each spot of the coverage obtained according to the initial reuse scheme, the resulting coverage being equivalent to a reuse factor reuse scheme equal to the product of the initial reuse factor by the number of portions of each spot.

The reuse factor K of the coverage obtained by the antenna system of the invention can therefore be written K = k.n, where n is an integer.

The multiplication is obtained by dividing in n sub-spots each spot of the reference configuration. Thus each sub-spot has a reduced area of a factor n relative to that of the reference spots.

With identical reflector antenna geometry, this characteristic makes it possible to improve the minimum directivity on the considered sub-spot with respect to the reference configuration.

In order to ensure the coverage of all the sub-spots without increasing the number of reflectors, the invention uses the technique of beam jumping between sub-spots of the same reference spot.

• les moyens d'alimentation et de commande sont adaptés pour modifier l'amplitude et la phase de la loi d'alimentation des éléments rayonnants par sous-ensemble selon la portion du spot de la couverture à éclairer ;
• chaque antenne comprend en outre un réflecteur disposé en regard du réseau d'éléments rayonnants ;
• le réflecteur est de forme parabolique, le réseau étant disposé dans le plan focal du réflecteur ;
• les moyens de commande sont adaptés pour commander à un instant donné l'alimentation d'un nombre d'éléments rayonnants inférieur au nombre total d'éléments rayonnants du sous-ensemble du réseau correspondant à un spot du schéma de réutilisation initial ;
• les moyens d'alimentation permettent de discriminer : les polarisations orthogonales, ou les sous bandes de fréquence ou les deux simultanément ;
• chaque sous-ensemble du réseau comprend douze éléments rayonnants ;
• les moyens de commande sont adaptés pour commander l'alimentation de sept éléments rayonnants parmi douze ;
• les moyens de commandes sont constitués d'une matrice de commutation ;
• le nombre d'antennes est égal au facteur de réutilisation initial ;
• le nombre d'antennes est égal à trois ou quatre.

The system of the invention may furthermore comprise the following characteristics:

• the supply and control means are adapted to change the amplitude and the phase of the supply law of the radiating elements by subassembly according to the portion of the spot of the cover to be illuminated;
• each antenna further comprises a reflector arranged facing the array of radiating elements;
• the reflector is of parabolic shape, the array being disposed in the focal plane of the reflector;
• the control means are adapted to control at a given instant the supply of a number of radiating elements less than the total number of radiating elements of the subset of the network corresponding to a spot of the initial reuse scheme;
• the supply means discriminate: orthogonal polarizations, or frequency sub-bands or both simultaneously;
• each subset of the network comprises twelve radiating elements;
• the control means are adapted to control the supply of seven of twelve radiating elements;
• the control means consist of a switching matrix;
• the number of antennas is equal to the initial reuse factor;
• the number of antennas is equal to three or four.

And, according to a second aspect, the invention relates to a satellite comprising at least one antenna system according to the first aspect of the invention.

PRESENTATION OF FIGURES

• les figures 2a et 2b illustrent respectivement la couverture de l'Europe selon un motif de réutilisation par quatre et par un spot de chaque motif ;
• la figure 3 illustre une antenne réflecteur alimentée par un réseau focal (FAFR) ;
• la figure 4 illustre la couverture produite par trois sous-spots de chaque motif dans le cas d'une réutilisation K=12 (n=3) couvrant une aire équivalente à la couverture de référence selon un motif de réutilisation par quatre ;
• les figures 5a et 5b illustrent respectivement les réseaux focaux de la configuration de référence et de la configuration de l'invention dans le cas d'une réutilisation équivalente à K=12 ;
• les figures 6a, 6b et 6c illustrent le principe du saut de faisceau entre sous-spots au niveau du réseau focal ;
• la figure 7 illustre un satellite de télécommunication assurant une couverture multifaisceaux avec quatre antennes FAFR.

Other features and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and should be read with reference to the appended figures in which, in addition to the figure 1 already discussed:

• the Figures 2a and 2b respectively illustrate the coverage of Europe according to a pattern of reuse by four and a spot of each pattern;
• the figure 3 illustrates a reflective antenna fed by a focal grating (FAFR);
• the figure 4 illustrates the coverage produced by three sub-spots of each pattern in the case of a re-use K = 12 (n = 3) covering an area equivalent to the reference coverage with a reuse pattern of four;
• the Figures 5a and 5b respectively illustrate the focal networks of the reference configuration and the configuration of the invention in the case of a reuse equivalent to K = 12;
• the Figures 6a, 6b and 6c illustrate the principle of beam jumping between sub-spots at the focal network;
• the figure 7 illustrates a telecommunication satellite providing multibeam coverage with four FAFR antennas.

DETAILED DESCRIPTION OF THE INVENTION

In what follows will be used for radiating element, the source term and vice versa.

The proposed technique is based on a 3 or 4 reuse scheme in which each cell is subdivided into sub-spots, illuminated successively by the beam-jumping technique.

At equal geometry, this reduction in the size of the spots actually used at a given instant naturally ensures that better performance in minimum directivity and consequently in roll-off and C / I.

Finally, the proposed system can be seen as an antenna-level implementation of both spatial division multiplexing ( Spatial Division Multiple Access , (SDMA)) and time division ( Time Division Multiple Access ) techniques. (TDMA)), frequently used in terrestrial systems to improve their capacity.

Basic configuration for reuse by k = 4 (reference configuration)

We have shown on the figure 2a a cover of Europe with forty spots. The cover shown in this figure is obtained with a reuse pattern of four, k = 4, the pattern is composed of four spots A, B, C, D.

As known per se, each of these four spots corresponds to the association of a sub-frequency band and a polarization.

We have shown on the figure 2b , the coverage achieved by the spots A of the figure 2a .

Each subset of spots in the cover is obtained by means of a reflector antenna, ie four identical antennas in the case of an FAFR antenna solution where a source of the focal grating produces a spot.

As known per se, the number of necessary reflector antennas could be reduced by using several sources of the focal grating to produce a spot.

However, it should be noted that in this case, the complexity of the focal network supply circuit is significantly increased, each elementary source having to discriminate orthogonal polarizations or frequency sub-bands or both at once.

We have shown on the figure 3 a reflector antenna fed by a focal grating (or FAFR).

The reflector 33 is typically of a diameter of 2 m for an operating frequency of 20 GHz, to allow if necessary the arrangement of two reflectors on the same side face of the satellite.

Each antenna used to obtain the cover comprises a reflector 33 disposed facing the network 30 of radiating elements.

In the case of the reference configuration, each of the spots on the geographical area to be covered is obtained by the radiation of a source of the focal network on the reflector.

We illustrated on the figure 5a a network of sources. Such a network is in particular arranged in the focal plane of the parabolic reflector, possibly formed to improve the overall performance of the antenna.

Each source of the focal network is typically a corrugated horn whose maximum diameter is geometrically imposed by the minimum distance between two spots of the cover illustrated on the diagram. figure 2b .

Each source emits different signals, but the polarization and frequency subband used is the same for all the sources of the focal network.

For complete coverage as illustrated on the figure 2a , four antennas are used each producing one of the four subsets of spots A, B, C or D

• la fréquence de fonctionnement considérée est de 20 GHz ;
• l'antenne réflecteur est caractérisée par un diamètre de 1,7 m et une focale de 3m ;
• chaque spot au sol présente un diamètre de 0,65° ;
• les spots sont répartis à quelques exceptions près selon un maillage triangulaire également appelé maillage hexagonal défini par un pas de 0,56°.

Subsequently, in order to establish certain performances, we consider the following hypotheses:

• the operating frequency considered is 20 GHz;
• the reflector antenna is characterized by a diameter of 1.7 m and a focal length of 3m;
• each spot on the ground has a diameter of 0.65 °;
• the spots are distributed with a few exceptions according to a triangular mesh also called hexagonal mesh defined by a step of 0.56 °.

Note that the angular distances mentioned correspond to a representation in spherical coordinates (r, θ, φ) centered at the apex of the paraboloid defining the shape of the reflector.

The pitch of the mesh is calculated in order to ensure a cross-matching between spots at a directivity level typically 4 dB below the maximum of directivity, thus allowing coverage without a significant dip in directivity.

où p est le pas du maillage et d le diamètre d'un spot.The relationship between spot size and mesh pitch in the triangular case is as follows $$p\le \frac{\sqrt{3}}{2}d,$$

where p is the pitch of the mesh and the diameter of a spot.

It is noted that the spots made are circular in view of the electromagnetic properties of the antennas used but the service area is hexagonal. The hexagon has been retained for cellular networks because it is the polygon closest to the circle for paving the plane.

pour une zone de service de $\frac{3\sqrt{3}{d}^2}{8}\mathrm{.}$

Le rapport entre la zone de service et la zone analysée est donc de $\frac{\mathrm{3}\sqrt{\mathrm{3}}}{\mathrm{2}\mathrm{\pi }}$

soit environ 83%.For a spot of diameter d, the analyzed zone therefore has a $\frac{{\mathrm{\pi }d}^{\mathrm{2}}}{\mathrm{4}},$

for a service area of $\frac{3\sqrt{3}{d}^2}{8}\mathrm{.}$

The ratio between the service area and the analyzed area is therefore $\frac{\mathrm{3}\sqrt{\mathrm{3}}}{\mathrm{2}\mathrm{\pi }}$

about 83%.

This beam overlap nevertheless offers a certain flexibility since the users located in these zones can be allocated indifferently to one or the other of the spots thus relieving the spots with a high density of users.

It is noted that it is possible to obtain similar results with a direct radiation (or DRA) array antenna, that is to say without a reflector.

Advanced configuration for reuse by K = k.n = 12

The antennal system that will be described here produces an area coverage equivalent to the coverage of the reference configuration as illustrated on the Figures 2a and 2b .

The advantage of the system is that to obtain a reuse by twelve the number of antennas implemented in the antennal system is equal to the initial reuse factor (k = 4).

We therefore place ourselves here in the case of a re-use K = 12, ie a division of the reference spots into n = 3 sub-spots.

The diameter of the sub-spots used is related to the diameter of the reference spots according to the following formula $$d_{K=\mathit{kn}}=\frac{d_k}{\sqrt{\mathrm{not}}}\mathrm{.}$$

Each sub-spot therefore has a diameter of 0.38 ° according to the preceding hypotheses.

The shape of the reference spot, corresponding to an equivalent reuse by four, is therefore substantially modified since it is the combination of three sub-spots of a cover with reuse by twelve as illustrated on the figure 4 , but it is important to note that the covered service area is the same (Europe in this case).

The superposition of four covers A, B, C, D, similar to those of the figure 4 but each produced by the antenna system allowing a factor of reuse by twelve, are necessary to form a coverage equivalent to that of the figure 2a .

Each spot A, B, C, D - said reference spot - is therefore divided into three sub spots (n = 3).

We have shown on the figure 4 the reference spot A divided into three sub-spots A1, A2, A3. The other reference spots B, C and D are not shown but are divided in the same way.

To obtain the coverage in a twelve-fold reuse pattern, each reference focal plane source is replaced by an aggregate of twelve smaller sources as shown respectively on the figure 5b .

The sources used are typically walking horns whose diameter of the opening is of the order of the wavelength.

The maximum diameter for a source is imposed geometrically by the distance between two sub-spots of the same reference spot.

In addition, control means 32 are coupled to the power supply means 31 of each sub-network of twelve sources in order to produce the beam jump.

More precisely, successively, seven radiating elements (out of twelve) of each sub-array are fed so as to form the beams making it possible to illuminate successively the sub-spots A1, A2 and A3, according to the notations of the figure 4 .

The implementation of a grouping of seven sources makes it possible to obtain iso-level curves while maintaining a supply circuit that is simple.

We have shown on Figures 6a, 6b and 6c the radiating elements successively fed to obtain the cover shown on the figure 4 . The radiating elements fed at a given moment are blackened in these figures.

More precisely, let δt be the period of the beam jump and T _{0 be} the instant at which all the spots A ₁ are illuminated simultaneously. Thus, all the spots A ₂ and all spots A ₃ will be illuminated respectively at T ₀ + δt and at T ₀ + 2.δt.

It is important to note that the focal network does not show any overlap between sub-networks of twelve radiating elements as illustrated on the figure 5b , which considerably simplifies the design of the supply circuit 31 as well as control means 32.

Thus, according to the embodiment described, the control means 32 are limited to a switching matrix, whose technology is to adapt to the constraints of size and power levels sought.

In addition, the power supply circuit is simplified in that the amplitudes and phases are the same for all the radiating elements of a sub-array of twelve sources.

However, it is possible to consider optimization in either amplitude or phase, or both simultaneously as known to further improve the overall performance of the antenna.

However, it should be noted that the power supply and control means are adapted to modify the amplitude and the phase of the source supply law by subassembly according to the portion of the spot of the cover to be illuminated.

Depending on the embodiments, it will be possible to envisage placing the switching matrix before, after or even integrated in the supply circuit 31.

The beam jump pattern is similarly reproduced for spots B, C and D.

As in the reference configuration, it is possible to reduce the number of antennas to produce the total coverage if the elementary sources are able to discriminate the orthogonal polarizations or the sub-frequency bands or both simultaneously.

Reducing the number of antennas therefore requires increased complexity in the supply circuit of the radiating elements.

This complexity seems to be lower in the case of an association of orthogonal polarizations, which thus makes it possible to reduce to two the number of antennas necessary to achieve the total coverage.

performances

The table below summarizes the performance of the two configurations presented above. Reuse factor C / I (dB) Directivity (dBi) Roll-off (dB) Low Average Low Max Max average k = 4 13.87 19.29 45.79 49.96 4.10 4.07 K = kn = 12 17.62 22.38 46.70 48.43 1.38 1.30

The antennal system with a reuse factor of K = 12 allows a C / I improvement of almost 4dB, the edge directivity is increased by 1dB and the roll-off increases by 4.1 dB at 1.38 dB.

applications

The antennary system described is notably used on a telecommunications satellite as illustrated on the figure 7 for the transmission of multimedia contents.

Claims (12)

Multi-beam antennal system for covering a given geographical area decomposed into a plurality of spots (A, B, C), comprising at least one antenna (3) comprising: ○ a network (30) of radiating elements (1-12); ○ power supply means (31) of the network such that in operation the radiating elements form a plurality of beams each illuminating a spot, the coverage of the zone being performed according to an initial reuse factor reuse scheme (k) ;
the antennal system being characterized in that each antenna further comprises Control means (32) coupled to the supply means (31) such that in operation the subsets of radiating elements (1-12) are fed successively to form a plurality of beams illuminating a portion of each spot the coverage obtained according to the initial reuse scheme, the coverage thus obtained being equivalent to a reuse factor reuse scheme (K) equal to the product of the initial reuse factor (k) by the number (n) of portions (A1 -A3) of each spot. System according to claim 1, characterized in that the supply means (31) and the control means (32) are adapted to modify the amplitude and the phase of the supply law of the radiating elements by subassembly according to the portion from the spot of the cover to be illuminated. System according to claim 1, characterized in that each antenna further comprises a reflector (33) arranged facing the network (30) of radiating elements (1-12). System according to the preceding claim, characterized in that the reflector is of parabolic shape, the array being disposed in the focal plane of the reflector (33). System according to claim 1, characterized in that the control means (32) are adapted to control at a given instant the supply of a number of radiating elements less than the total number of radiating elements of the subset of the network. corresponding to a spot of the initial reuse scheme. System according to the preceding claim, characterized in that the supply means (31) make it possible to discriminate: the orthogonal polarizations, or the frequency sub-bands or both simultaneously. System according to one of the preceding claims, characterized in that each subset of the network (30) comprises twelve radiating elements. System according to one of claims 5 to 7, characterized in that the control means (32) are adapted to control the supply of seven of twelve radiating elements. System according to the preceding claim, characterized in that the control means consist of a switching matrix. System according to one of the preceding claims, characterized in that the number of antennas is equal to the initial reuse factor. System according to one of the preceding claims, characterized in that the number of antennas is equal to three or four. Satellite (60) comprising a multi-beam antenna system for multispots coverage according to one of the preceding claims.

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