Satellite communications system using multiple earth stations

• US 7,305,211 B2
• Filed: 07/20/2004
• Issued: 12/04/2007
• Est. Priority Date: 07/26/2000
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Dent, Paul W.
• Primary Examiner(s): Nguyen, Lee
• Application Number:
  US10/895,210
• Publication Number:
  US 20040259497A1
• Time in Patent Office: 1,232 Days
• Field of Search: 455/12.1, 455/13.3, 455/427, 455/430, 370/316, 342/368, 342/373
• US Class Current: 455/12.1
• CPC Class Codes: H04B 7/18515 Transmission equipment in satellites or space-based relays

Abstract

A satellite communications system uses multiple ground stations and one or more satellites for communicating between mobile subscribers and a land-based communications network, such as the PSTN or the Internet. Multiple ground stations geographically dispersed minimize toll charges incurred routing calls from a mobile subscriber through the land network by reducing the need for long-distance calling. Further, because each ground station communicates with a given satellite using the same frequency spectrum, the subscriber capacity of the system increases and/or bandwidth requirements for the communications link between ground stations and satellites may be reduced. The present system uses ground-based beamforming techniques enabling each satellite to transmit signals in multiple transmission beams, each beam supporting one or more mobile subscribers. Each beam may reuse the same frequency spectrum, thereby increasing the number of subscribers supported by each satellite. Multiple ground stations cooperatively relay signals through a given satellite in a manner complementary with ground-based beamforming.

Claims

1. A communications satellite system having at least one satellite for providing communications between at least one of a plurality of mobile terminals and one of at least two ground stations, said at least one satellite comprising:

a plurality of feeder link antennas that receive feeder link signals from said at least two ground stations;

a separate demultiplexer connected to each feeder link antenna that demultiplexes each feeder link signal to obtain power amplifier drive signals;

a plurality of combiners that combine power amplifier drive signals corresponding to the same power amplifier but received from different ones of said ground stations to obtain combined drive signals;

power amplifiers that amplify corresponding combined drive signals to obtain transmit signals; and

a multi-element transmitting antenna coupled to said power amplifiers that transmits said transmit signals so as to form directive transmission beams in multiple directions for communicating information to said plurality of mobile terminals.

2. The system of claim 1 wherein said multi-element transmitting antenna is a Direct Radiating Array with each radiating element coupled one-for-one to a corresponding one of said power amplifiers.

3. The system of claim 1 wherein said multi-element transmitting antenna comprises multiple antenna feeds and a reflector, said multiple antenna feeds being coupled to said power amplifiers using at least one Butler matrix.

4. The system of claim 3 wherein said multiple antenna feeds are located out of the focal plane of said reflector.

5. The system of claim 1 wherein said plurality of combiners comprise:

a plurality of up-converters that upconvert each one of said demultiplexed power amplifier drive signals to a transmit frequency band which is the same for all power amplifier drive signals originating from the same ground station and different for power amplifier drive signals originating from different ground stations; and

a plurality of signal adders that add upconverted signals that correspond to drive signals for the same power amplifier to obtain said combined drive signals.

6. A communications satellite system having at least one satellite for providing communications between at least one of a plurality of mobile terminals and one of at least two ground stations, said at least one satellite comprising:

a feeder link antenna for receiving feeder link signals from said at least two ground stations;

a downconverter for downconverting said received feeder link signals to the complex baseband comprising an Inphase (I) signal and a Quadrature (Q) signal;

quadrature demultiplexer for demultiplexing said I and Q signals to obtain demultiplexed signals corresponding to a synchronization channel and a plurality of power amplifier drive signal channels;

a synchronization processor for processing the demultiplexed signal corresponding to said synchronization channel to determine timing errors for signals received from each of said ground stations; and

a feeder link transmitter for transmitting said timing errors to said ground stations such that each ground station may correct its respective timing error by advancing or retarding its transmit timing.

7. The communications system of claim 6 further comprising:

up-converters for upconverting each of said power amplifier drive signals to a frequency band for transmitting to said mobile communications terminals to obtain upconverted drive signals;

power amplifiers for amplifying a corresponding upconverted drive signal to obtain a transmit signal; and

a multi-element antenna, coupled to said power amplifiers, for transmitting said transmit signals such that multiple directive transmission beams are formed for communicating signals to said mobile communications terminals.

8. The system of claim 7 wherein said multi-element antenna is a Direct Radiating Array having a plurality of radiating elements coupled one-for-one with corresponding ones of said power amplifiers.

9. The system of claim 7 wherein said multi-element antenna comprises multiple antenna feeds and a reflector, said multiple antenna feeds being coupled to said power amplifiers using at least one Butler matrix.

10. The system of claim 9 wherein said multiple antenna feeds are located out of the focal plane of said reflector.

11. The system of claim 7 wherein channel frequency components of signals radiated in the same one of said multiple directive beams originating from one of said ground stations are different from channel frequency components of signals radiated at the same time in the same or overlapping beams that originated from a different one of said ground stations.

12. A method for communicating between a plurality of mobile terminals and a plurality of ground stations using at least one orbiting satellite, comprising:

allocating different channel frequencies to ones of said mobile terminals located in such proximity to prevent discrimination between signals transmitted from said mobile terminals and reallocating the same channel frequencies to other ones of said mobile terminals separated by a sufficient distance to allow discrimination between signals transmitted from said mobile terminals;

dividing said plurality of mobile terminals into a plurality of groups and allocating each group to be served by a corresponding ground station;

using beamforming coefficients to form, at each ground station and in dependence on the information to be communicated to the plurality of groups of mobile terminals allocated to be served by the ground station, a set of antenna array drive signals for transmission to said at least one orbiting satellite;

quadrature multiplexing at each of said ground stations the set of antenna drive signals formed at the ground station to form a quadrature multiplexed signal;

upconverting said quadrature multiplexed signals to a feeder link frequency common to all ground stations to obtain a feeder link signal at each ground station;

transmitting said feeder link signals from each ground station to said at least one orbiting satellite;

receiving at said satellite the overlapping sum of said feeder link signals from said second plurality of ground stations;

quadrature demultiplexing said received sum of feeder link signals to obtain reconstructed antenna array drive signals; and

using said reconstructed antenna array drive signals to form corresponding amplified transmit signals for a multi-element transmit antenna on said at least one satellite such that said transmit antenna radiates signals to each of said plurality of mobile terminals in a corresponding directive beam, each signal having originated from the ground station allocated to serve the corresponding mobile terminal and each corresponding beam direction being under the control of the allocated ground station by choice of said beamforming coefficients.

13. The method of claim 12, further comprising synchronizing said quadrature multiplexing at each ground station to said quadrature demultiplexing on board said at least one orbiting satellite.

14. The method of claim 13 wherein said synchronizing said quadrature multiplexing at each ground station to said quadrature demultiplexing on board said at least one orbiting satellite is achieved by using a timing error feedback signal received from said at least one orbiting satellite.

15. A method of transmitting signals from a ground station to a satellite for transmission using a multi-beam antenna, comprising:

assembling a sample of each signal to be transmitted using different ones of said multiple antenna beams into a vector;

performing an orthogonal transformation of the vector to obtain a transformed vector;

quadrature modulating the multiplex signal to a feeder link frequency to obtain a feeder link signal and transmitting said feeder link signal to the satellite;

receiving, downconverting and quadrature demultiplexing said feeder link signal at the satellite to obtain power amplifier drive signals;

quadrature modulating the power amplifier drive signals to the frequency for transmission from the satellite and driving a set of power amplifiers with the corresponding quadrature modulated signals; and

coupling the amplified signals from said power amplifiers to the feeds of a multiple-feed reflector antenna using Butler matrices such that an inverse orthogonal transform is performed by the coupling, thereby achieving increased insensitivity to mis-synchronization between said quadrature multiplexing and said quadrature demultiplexing.