Communication system for bidirectional message transmission between ground stations, using a communication satellite.

p0001The invention relates to a communication system according to the preamble of claim 1.

p0002Such news or satellite systems for mobile applications for communication between stations or earth stations via a communication satellite are subject to a number of requirements to the ground stations and the satellite-side systems that can not be satisfactorily resolved by conventional means.

p0003A significant limitation is the size limit of the antennas of the base stations that are as integrated as planar antennas in roofs of cars or to be housed in portable subscriber units in the size of a briefcase. The demand for cost-effective production, as well as health aspects, also make a limitation of the emitted by the subscriber unit RF power, ie RF power at max. few watts required.

p0004On the satellite side, pay attention to a high RF / DC conversion efficiency, ie to obtain a low DC power high performance radio to achieve the highest possible and therefore economical channel capacity, at least with power-limited satellite. In addition where a large coverage area on the Earth from communications satellites to capture and illuminate, so a flexible allocation of subscriber channels should spot beams to be possible at the transmitting end then inevitable multibeam antenna system for economic reasons.

p0005In conventional communications satellites "transparent" transponders are usually called. Used, which do not alter the signals except for a use frequency conversion and thus on the uplink and downlink, the same access method. Recent proposals access for transmission of high data rates and the reduction of ground station antennas often back on multi-beam antennas. They also utilize mostly transparent transponder with the same access method on the uplink and downlink, wherein each time division multiple access (TDMA) and frequency division multiple access (FDMA) or per carrier frequency, a single channel (SCPC) may be used; see. Lenormand, R. et al, "A 30/20 GHz MULTIBEAM ARRAY ANTENNA SUITABLE FOR FUTURE HIGH RATE TDMA EUROPEAN SATELLITE SYSTEMS", IEEE AP-S, June 1987 pp 179-183, or Rose, HA, " FREQUENCY ADDRESSABLE BEAMS FOR SATELLITE COMMUNICATIONS "Telecom 1987 Geneva. . A deviating solution, the so-called time division multiplex with the uplink and downlink beams bouncing (hopping beams) used is also known; see. Campanella, J. et al, "A USERS PERSPECTIVE OF THE ACTS HOPPING BEAM TDMA SYSTEM", Proc. AIAA March 1990, pp 484-489.

p0006However, such built-up systems can not optimally meet the requirements described above satellite systems for mobile applications.

p0007In DE-OS 32 00 249, which corresponds to US-PS 44 25 639, the advantage of a combination of time division multiplexed signals of higher bit rate in the downlink by frequency-interleaved channels is already exposed on the uplink, that is a combination of time-division multiplex and frequency division multiplexing. Here, more solid, only reconfigurable in transmission capacity spot beams for uplink and downlink are used. Number and size of the satellite spot beams here are for uplink and downlink equal, said each spot beam assigned frequency bandwidth for uplink and downlink is also the same size.

p0008Such an arrangement requires, however, the bottom side antennas with high gain and the appropriate action face to keep the satellite-side effort to DC power and ground into a viable framework. However, this condition can not be met by earth stations for satellite-mobile communications. The demand for small earth station antennas can be realized only by a high effective radiated power of the satellite. Due to the limited transmission power of the satellite spot beams must therefore be relatively tightly bundled, so that corresponding to the respective spot beams floor areas on Earth are also relatively small. For large areas cover accordingly a plurality of such spot beams are required, and thus a corresponding number of beamforming networks and a corresponding number of carrier frequencies. The operation of such a multi-beam transmitter antenna with frequency division multiplexing signals but also with several simultaneously radiated in different spot beams time division multiplexed signals leads even with limited flexibility in the allocation of channels to spot beams to a high performance and mass budget for beamforming networks and switching matrices. In addition, a multi-carrier system to avoid intermodulation distortion requires the use of linear power amplifiers that minimize the mentioned RF / DC efficiency further.

p0009The invention addresses the problem of specifying an appropriate combination of access methods, optimally fulfills the requirements described and the disadvantages of time-division multiplexing or frequency division multiple access method can be largely avoided.

p0010This object is achieved according to the invention by the features stated in the characterizing part of patent claim 1.

p0011Thus, the invention is a combination of access methods, which called on the downlink in time division using an electronically controllable satellite-side transmitting antenna with a deflectable spot beam, a. Reached "flying spot" small beamwidth, many small ground stations in a large coverage area consecutively, while on the uplink frequency division multiplexing is used to separate the subscriber channels.

p0012From out of satellite ground stations are in rapid chronological order succession with down signals in the form of time-limited signal packets, so-called. Operated "bursts". The deflectable beam includes a point substantially lower than the beam width and the beam used on the uplink. The sum of the bandwidths of all in an area covered by the deflectable spot beam subarea, the so-called. "Virtual spot", located and operated simultaneously subscriber stations is less than the bandwidth of the signal packet, with the same subscriber stations are supplied with down signals on the uplink. The number of applicable for downlink virtual spots is considerably larger than the number of valid for uplink and covered by the or the satellite receiving antennas subregions. for example, only one satellite receive antenna may be used for the uplink.

p0013A time-division multiplex system having a single RF carrier allows the use of power amplifiers of high efficiency, has only one beam forming network and no switching matrix. The time interleaving the channels or channel groups and their staggered alignment with the respective desired reception area on the earth can be made of high-speed digital circuits with low mass and benefits paid in connection with an active electronically controlled array antenna. Each time slot within the time interleaving of the downlink signals may be flexibly assigned to any virtual spot on the earth.

p0014In this case, when n time slots, and m virtual spots in the practical case in which n is greater than m, both a uniform distribution of the channel capacity of all virtual spots in the coverage area as well as a concentration of the channel capacity on only one or a few virtual spots possible. If you want to extend the switching times of the electronically controlled antenna, so with reduced demands on the flexibility and the allocation of festverschalteten channel groups to be longer time slot is possible.

p0015The use of the time division principle for the uplink would require a high radiated RF pulse line with many, including several thousand directly communicating with the satellite and the satellite "accessing" participants because of the unfavorable duty on the part of the ground stations and also pose a synchronization problem. As a receiving antenna on the satellite would, on the transmission side, an electronically controlled antenna are required. When using a frequency division (or SCPC) access method on the uplink, however, can reduce the RF peak output drastically because of the votes of the ground station in the assigned frequency band carrier or CW signal at equal energy contributions. Thus no discrete power transistors more transmission amplifiers are required for the base stations in addition to microwave monolithic integrated circuits, which leads to a considerable cost reduction of the user equipments. In addition, can be used on Earth illuminating antenna instead of a complex electronically controlled receiving antenna on the side of the satellite simple, the entire coverage area. Synchronization problems can not occur in this case. The advantage of a simple side satellite receiving antenna is reduced by the necessary with the frequency division multiplexing using a multi-carrier demodulator though; as the main advantage of the access method in the frequency-division multiplexing on the uplink, however, is the lower cost subscriber equipment to look at.

p0016Depending on the system design, from an organizational perspective and the currently available technologies is also a frequency division multiple access method with a, in several large spots disassembled coverage area possible in the uplink. In this case, each spot is associated with a frequency sub-band of the uplink.

p0017For the customer premises equipment of the ground stations resulting from the illustrated optimal combination of access methods and new requirements:

p0018In many cases, for example to the angular separation of satellite systems that operate in the same frequency band, and also for higher-quality transmission of voice signals will be required on the part of the ground station due to the required higher antenna gain with an electronic or mechanical antenna tracking. In addition, the tracking of the antenna will be necessary if the user equipment is a vehicle, aircraft or ship mounted on a movable platform, eg. The criterion for the tracking can serve the received power, with the "flying spot" an additional radiated from the satellite beacon frequency for tracking, especially for acquisition is recommended because of the transfer process.

p0019The transmitting part of the ground station emits a narrow-band signal in a predetermined frequency bandwidth is allocated by the satellite whose frequency position. The transmitting device of the subscriber device can be adjusted possible frequencies for the uplink via satellite to all the operation of the satellite system.

p0020The receiving part of the ground station, however, receives a very broadband signal in the form of said short signal packet or burst, wherein a burst according to a frame length follows the next packet. A burst may contain in addition to the specific for the currently receiving subscriber downlink signal and information for other participants who are in the same virtual spot. In addition, depending on traffic further signal packets can be received during a frame length, containing only information for foreign participants. Task for the receiving part of the subscriber device is accordingly not only the actual receipt of the identification and temporal filtering of the own signal component.

p0021The other modules of the user equipment or base stations, such as demodulator, modulator, terminal equipment and tracking device based on known principles.

p0022A satellite system according to the invention can be used both with satellites in geostationary orbits as well as with satellites in elliptical hochinklinierten quasi-stationary orbits.

p0023Further embodiments of the invention are apparent from the subclaims. The invention is explained in more detail in an embodiment shown in the drawing. The drawings show in:<dl id="dl0001"><dt>Fig. 1</dt><dd>an illustration of a cover area on the earth's surface, which is divided into multiple virtual sites, which are supplied from a communication satellite according to the invention in each case by a deflectable spot beam (flying spot);</dd><dt>FIG. 2</dt><dd>schematically the representation of the time-division multiplex access method on the downlink with an assignment of the individual channels or channel groups to the virtual spots shown in FIG. 1;</dd><dt>Fig. 3</dt><dd>the coverage area shown in Figure 1 with the possible arrangement of a plurality of spots on the uplink. </dd><dt>Fig. 4</dt><dd>a schematic block circuit diagram for the communications satellite, wherein only necessary for the different access method on the uplink and downlink functional blocks are shown;</dd><dt>Fig. 5 and Fig. 6</dt><dd>schematic block diagrams of a ground station, storing only those necessary for the different access methods function blocks are shown.</dd></dl>

p0024In Fig. 1 at 1 is a coverage area on the Earth's surface shown in which a plurality of ground stations are located that can exchange messages via a satellite. This coverage area 1 is from a r plurality of partial areas S_j, So-called. Virtual Spots composed of S₁ to S_m are numbered. These subregions S_j are the points of incidence of a pivotal point beam or flying spot FS, cf. Fig. 4. On the downlink between the satellite and the ground station is carried out the transmission in a time division method as shown in FIG. 2, wherein for each channel or each channel group C_i a time slot is provided within one frame length. Within each time slot the spot beam shines in a virtual spot S_jBut this classification is adapted to the respective requirements. In the uplink the frequency division multiplex method is used in which a spot beam is transmitted toward the satellite by each base station 3 and for each point a separate frequency beam is provided.

p0025On the part of the satellite receiving antenna is either a present, covering the whole coverage area. 1 Or, this coverage area for the uplink as shown in FIG. 3 in several subregions, four sub-regions T1 to T4, divided in this case, each sub-region is associated with a portion of the frequency band. then satellite side either each a receiving antenna is provided for the sub-areas or only one receiving antenna having a plurality of independently acting apart beamformers.

p0026In FIG. 4 is represented by 2 a transmitter / receiver unit of the communications satellite. A receiving antenna 4 with its antenna network 5 receives the light emitted by the earth stations 3 spot beams and performs this allocation according to the channel 6 to a Vielträgerdemodulator which demodulates and regenerates all channels. A subsequent switch matrix 8 acts as on-board switching device and determined together with a multiplexer 10, with which they can be integrated, the temporal arrangement of the channels in the serial downward operation. The signals are then passed to a modulator 11 and a transmitting device 12, the signal via a deflection device 13 and an antenna network 14 is fed to a transmitting antenna 15th The functional blocks 13 to 15 will be typically performed as an electronically controlled active array antenna. With the help of the deflector 13, the transmitting antenna 15 produces the spot beam or flying spot FS, which according to the above time frame to the individual sub-areas in accordance with the virtual spots S_j is directed.

p0027In addition, still by means of a beacon transmitter 16, a beacon signal is generated which is used for acquisition and alignment of the antennas of the base stations third The deflection device 13 and the beacon transmitter 16 are controlled by a control unit. 9

p0028In Fig. 5 and 6, a transmitting / receiving device 20 is a base station 3 is shown. This has a combined transmitting / receiving antenna 21, and separate transmit / receive antennas 31,32, which may be mechanically or electronically 22 precisely aligned with the communications satellite by means of a tracking device. The antenna 21 is connected to a diplexer 23rd On the transmitter side, an input unit 24, a modulator 25 and a transmitter 26 is provided, which transmits the signals via the diplexer 23 to the working in the transmission mode antenna 21 or directly to the antenna 31st The signals received by the antenna 21 are a receiver supplied via the diplexer 23 or antenna 32 directly 27, which is connected to a demodulator 29th This is a device 28 for synchronization, downstream identification and temporal filtering of the own signal component. In this the assigned for this ground station message is filtered out of the broadband signal packet. This message is finally fed to an output unit 30th

p0029The transmitting device emits toward the satellite from a narrow usable frequency band, wherein the frequency layer is separated from the satellite assigned to each subscriber within the overall operational frequency band. The modulator is used here for the digital modulation of the subscriber signal.

p0030The receiving device is suitable for the reception of broadband signals in the form of said short signal packets, intended for the respective ground station signal portions are identified and filtered out in time from the signal packet. Signal inputs and signal output occur in a conventional manner, as is the power supply, of course, interface units for connecting to other devices, such as fax, etc., may be provided.

p0031Further, central antenna systems can be provided for groups of ground stations, which are in turn transponder to the satellites.