Satellite Communications System And Method For Transmitting A Bit Stream Therewith

Abstract

The present invention relates to a satellite communications system comprising a satellite with a spatial digital modulator for transmitting an incoming bit stream in multiple beams, wherein the spatial digital modulator configured to generate transmit symbols by dividing the incoming bit stream into symbols of a symbol alphabet and to allocate each transmit symbol to a specific beam. The invention further relates to a method for transmitting a bit stream by such a satellite communications system.

Claims

1. A satellite communications system comprising a satellite with a spatial digital modulator for modulating an incoming bit stream to transmit in multiple beams wherein the spatial digital modulator is configured to generate transmit symbols by dividing the incoming bit stream into symbols of a symbol alphabet and to allocate each transmit symbol to a specific beam.

2. A satellite communications system according to claim 1, wherein the spatial digital modulator is configured to allocate each transmit symbol to a specific beam based on fixed or dynamic mapping rules.

3. A satellite communications system according to claim 2, wherein the spatial digital modulator is configured to use an amplitude-phase-shift-keying or space-shift-keying modulation scheme for encoding the one or more transmit symbols that are allocated to different beams.

4. A satellite communications system according to claim 1, wherein the satellite is configured to add artificial noise to the transmit symbols.

5. A satellite communications system according to claim 1, wherein a receiving station is provided, and the spatial digital modulator is configured to select the beams required for transmission of the symbol alphabet such that they have the highest off-axis gain at the location of the receiving station.

6. A satellite communications system according to claim 1, wherein a receiving station is provided, and the spatial digital modulator is configured to select the beams required for transmission of the symbol alphabet such that the average load is balanced between each transmit section.

7. A satellite to communication system according to claim 5, wherein the satellite is configured to adjust the signal power in the required beams such that the Euclidian distances between the transmit symbols received at the receiving station is optimized while minimizing the service area produced by the beam pattern on ground.

8. A satellite communication system according to claim 1, wherein the spatial modulator is placed in the digital satellite payload.

9. A satellite communication system according to claim 1, wherein the spatial modulator is placed at the transmitter ground station.

10. A satellite communication system according to claim 1, wherein the service area is controlled by the code rate of the channel coding of the transmitted bit stream.

11. A satellite communication system according to claim 1, wherein the spatial modulator is combined with MIMO communication to increase eavesdropping resistance.

12. A satellite communication system according to claim 5, wherein the satellite is configured to adjust the signal power in the required beams such that the Euclidian distances between the transmit symbols received at the receiving station is optimized while minimizing the service area produced by the beam pattern on ground.

Description

[0022] Further advantageous features and applications of the invention can be found in the dependent claims as well as in the following description of the drawings illustrating the invention. In the drawings like reference signs designate the same or similar elements throughout the several figures of which:

[0023] FIG. 1 shows a schematic representation of an embodiment of a satellite communications system according to the invention, based on a digital regenerative payload.

[0024] FIG. 2 shows a schematic representation of an embodiment of a satellite communications system according to the invention, based on a transparent (analog or digital) payload and the spatial modulator at the transmitting ground station.

[0025] FIG. 3 shows an exemplary positioning of a receiving station inside a beam pattern produced by the several antennas of the satellite communications system of the invention,

[0026] FIG. 4 shows exemplary spatial modulations achieved with the present invention, using 2 and 3 beams.

[0027] FIG. 5 shows a constellation diagram achieved with the present invention at the wanted (left hand side) and at the eavesdropper's position (right hand side).

[0028] FIG. 1 shows an embodiment of a satellite communications system 1 of the present invention with a satellite 2, a transmitting station 3 and a receiving station 4 in form of a ground station positioned on the Earth's surface. The satellite comprises any kind of receiving antenna 5, a typical analog-to-digital receive section 8, a spatial digital modulator 6 and a typical digital-to-analog transmit section 10, as well as one transmitting antenna 12. To form multiple beams on the Earth's surface different technical realizations of the transmitting antenna 12 can be implemented such as single-feed-per-beam, multiple-feeds-per-beam or a direct radiating array.

[0029] The spatial digital modulator 6 modulates an incoming bit stream by dividing the bit stream into symbols of a linear symbol alphabet that are to be transmitted to the ground station, i.e. ground station 4 (so-called transmit symbols). Each symbol contains a specific number of bits that is given by the overall modulation order m.sub.SM of the spatial digital modulator 6. The spatial digital modulator 6 allocates the generated transmit symbols to beams that shall be radiated to the ground station 4. I.e., a beam index is assigned by the spatial digital modulator 6 to each transmit signal. In the embodiment, the beams are downlink beams.

[0030] The modulated transmit symbols are each modulated by the transmit section 10 to an analog carrier signal, each symbol representing a particular state of/change in the carrier signal. The transmit section typically consists of multiple channels with a digital-to-analog converter, up-converter, amplifier and filter. In each channel the signal for one beam is generated. The modulated carrier signals are then transmitted via the beam-forming antenna 12 to generate the beams at the ground of the satellite communications system 1 whose contours, in particular their points of boresight PoB are not co-located, as can be seen in FIG. 3.

[0031] Spatial modulation is also possible with a transparent payload 7 which can be digital or analog. This is shown in FIG. 2. In this case the spatial modulated signals are generated by the spatial digital modulator 6 at the transmitting ground station 3. The transparent payload 7 consists of multiple channels (transponders). Each of them receives, converts and amplifies the signal for one beam.

[0032] The transmitted equivalent isotropic radiated power (EIRP) of each downlink beam vary with the location of the receiving station within the footprint (the beam contour). The wanted receiving station 4 is thus preferentially positioned within the beam contour of the main beam 18.1 and the beams required for transmission of the transmit symbols are preferably selected in such manner from the maximum number of beams, in particular from the remaining beams 18.2-18.7, that the selected beams have the highest off-axis gain at the location of the receiving station 4.

[0033] According to the present invention the number of beams required for transmission of the symbol alphabet, i.e., for each distinct symbol of the symbol alphabet, depends on the intended overall modulation order m.sub.SM of the satellite communications system 1 as well as the intended modulation order in each beam.

[0034] The transmit symbols can be either encoded by a combination of space-shift keying and amplitude-phase-shift keying, wherein each transmit symbol is allocated to a specific beam (assigned a specific beam index) based on predefined fixed mapping rules.

[0035] As can be seen from FIG. 4 (left-hand side), for an exemplary overall modulation order m.sub.SM of 2, i.e., two bits per symbol, the symbols 00 and 01 are allocated to the main beam 18.1 with beam index 1, whereas the symbols 10 and 11 are allocated to the second and the third beam 18.2 and 18.3, respectively. The symbols 00 and 01 in the main beam are modulated by binary-phase-shift keying (so-called APM symbols of binary-phase-shift keying) and the symbols 10 and 11 are each modulated by space-shift keying. The phases and amplitudes of each symbol are chosen to provide maximum Euclidean distance at the receiver site.

[0036] In another example, the spatial digital modulator 6 of the satellite 2 of the satellite communications system 1 employs a spatial modulation scheme based on amplitude-phase modulation only, whose modulation order is the overall modulation order m.sub.SM for encoding the transmit symbols that shall be transmitted via the main beam 18.1 and one adjacent beam, e.g. beam 18.2. For a modulation order of m.sub.SM of 3, i.e., three bits per symbol, the symbols 000, 001, 010, 011 are allocated to the main beam 18.1, whereas the symbols 100, 101, 110, 111 are allocated to the second beam, respectively (see FIG. 4, right-hand side). In case of an intended overall modulation order m.sub.SM of 3, for example quadrature-phase-shift keying (QPSK) may be used to modulate the main beam and the adjacent beam.

[0037] For each signal path from the spatial digital modulator 6 on board of the satellite 2 via the respectively activated antenna per transmit symbol a phase shift is caused by different path lengths, attenuation/damping, manufacturing technology tolerances or hardware imperfections. These phase shifts of each signal path for each transmit symbol and the beam gain at the receiver ground station location 4 must be considered in the spatial modulator to optimize the Euclidian distances in the complex plane at the receiving ground station 4. An exemplary constellation diagram for an overall modulation order m.sub.SM of 3 using two QPSK modulated signals in two beams is shown in FIG. 5 (left hand side). The constellation points on the outside represent the symbols transmitted with the beam 18.1 (beam 1), the constellation points on the inside represent the symbols transmitted with the beam 18.2 (beam 2). FIG. 5 (right hand side) shows the best case for eavesdropping resistance at the eavesdroppers' locations. The amplitudes of beam 18.1 and beam 18.2 are equal, so the eavesdropper is not able to detect which beam has transmitted the amplitude-phase-modulated symbol.

LIST OF REFERENCE SIGNS

[0038] 1 satellite communications system [0039] 2 satellite [0040] 3 transmit station [0041] 4 receiving station [0042] 5 receiving antenna [0043] 6 spatial digital modulator [0044] 7 transparent payload [0045] 8 typical receive section [0046] 10 typical transmit section [0047] 12 transmitting antenna [0048] 18.1-18.7 beams [0049] PoB point of boresight