Multi-beam TDMA satellite mobile communications system

• US 6,314,269 B1
• Filed: 08/11/1997
• Issued: 11/06/2001
• Est. Priority Date: 07/22/1994
• Status: Expired due to Fees

• Current Assignee: Inmarsat Global Limited (Inmarsat PLC)
• Original Assignee: International Mobile Satellite Organization
• Inventors: Hart, Nicholas, Bjornstrom, Gunnar
• Primary Examiner(s): Eisenzopf, Reinhard
• Assistant Examiner(s): KINCAID, LESTER G
• Application Number:
  US08/750,915
• Time in Patent Office: 1,548 Days
• Field of Search: 455/12.1, 455/13.1, 455/13.2, 455/427, 455/69, 455/428, 455/429, 455/430
• US Class Current: 455/12.1
• CPC Class Codes: H04B 7/18534 for enhancing link reliablility, e.g. satellites diversity
• CPC Class Codes: H04B 7/18539 Arrangements for managing radio, resources, i.e. for establishing or releasing a connection
• CPC Class Codes: H04B 7/1855 using a telephonic control signal, e.g. propagation delay variation, Doppler frequency variation, power variation, beam identification
• CPC Class Codes: H04B 7/2041 Spot beam multiple access

Abstract

An earth station receives a return signal via more than one satellite link from a mobile terminal using TDMA. The earth station selects one or more of the satellite links for transmitting a forward signal on the basis of the quality of signal received via each link. The earth section allocates frequency channels to the mobile terminals according to their location on the surface of the earth, so that the propagation time to and from those mobile terminals which share the same frequency channel is approximately the same. The satellite includes an antenna which generates an array of beams which are individually pointed to fixed regions of the earth, until the elevation of the satellite relative to a fixed region falls below a minimum value, in which case the corresponding beam is redirected to a new area, while the other beams remain pointed at the corresponding fixed areas. In this way, beam-to-beam handover is reduced, while maintaining the boresight of the antenna pointing at the nadir.

Claims

1. A method of satellite communication between a first earth station and a second earth station comprising:

receiving at the second earth station information transmitted from the first earth station within one or more time-division multiplexed time slots and relayed via a plurality of beams generated by one or more satellites;

said method being characterized by the steps of;

selecting one or more of said satellite beams according to a property of the information received via said satellite beams; and

transmitting further information from said second earth station to the first earth station such that the further information is relayed via said selected one or more satellite beams.

2. A method as claimed in claim 1, further comprising:

calculating variations in the delay in the transmission link between the first earth station and the second earth via said selected one or more satellite beams, and transmitting the further information from the second earth station to the one or more satellites which generates the or each selected satellite beam with a timing determined so as to compensate for said variations.

3. A method as claimed in claim 2, wherein the further information is transmitted from the second earth station to the first earth station via two or more said selected beams, and the timings of the transmissions are determined so that the information is received substantially simultaneously at the first earth station via the selected beams.

4. A method as claimed in any preceding claim, further comprising measuring the Doppler shift from a predetermined frequency of the frequency of a signal containing the information received at the second earth station, and selecting the frequency of a signal containing the further information so as to compensate for the measured Doppler shift.

5. A method as claimed in any preceding claim, further comprising determining the position of the first earth station;

wherein the further information is transmitted from the second earth station with a frequency determined according to the derived position of the first earth station.

6. A method as claimed in claim 5, further comprising transmitting to the first earth station a control signal for controlling the transmission and reception frequencies of the first earth station, the control signal being generated according to the derived position of the first earth station.

7. A method as claimed in claim 1 or 2, wherein the second earth station receives the information more than once in different time slots via a corresponding number of said satellite beams.

8. A method as claimed in claim 7, wherein the second earth station receives the information transmitted by the first earth station within a first time slot in a first frequency channel (f₁′

) of a second satellite beam, and

receives the information which is transmitted again by the first earth station during a second time slot in a second frequency channel of a second satellite beam.

9. A method as claimed in claim 8, further comprising:

comparing respective properties of the received information transmitted by

the first earth station during the first and second time slots and transmitting the further information at a selected frequency corresponding to one of the first and second satellite beams according to the result of the comparison.

10. A method as claimed by any preceding claim, wherein if a property of the received information fails to satisfy a predetermined criterion, the second earth station transmits the further information at a lower rate, and if the property of the received information satisfies said predetermined criterion, the second earth station transmits the further information at a higher rate.

11. A method as claimed in claim 10, wherein the information is transmitted by the steps of:

dividing the further information into first and second portions;

transmitting the first portion to the first earth station within a third time slot; and

transmitting the second portion to the first earth station within a fourth time slot.

12. A method as claimed in any preceding claim, wherein said property relates to the quality of previous information previously received by the first earth station from the second earth station.

13. A method as claimed in any one of claims 1 to 11, wherein the property relates to the quality of said information received by the second earth station.

14. A method of satellite communication using TDMA channels, comprising:

transmitting the same information from a first earth station within each of a plurality of time slots to a second earth station, at respective, different frequencies so that the information is received in respective, different beams generated by one or more satellites.

15. Apparatus for use in a satellite communications earth station using TDMA channels, comprising:

a receiver arranged to receive information relayed by one or more satellites from a remote earth station within one or more time slots, the information being relayed via a plurality of beams generated by said one or more satellites;

beam selecting means for selecting one or more of said satellite beams according to a property of the information received therein, and a transmitter arranged to transmit further information to the remote earth station such that the information is relayed to the remote earth station via the selected one or more satellite beams.

16. Apparatus as claimed in claim 15, further comprising:

means for calculating variations in the transmission delay to the remote earth station via the selected one or more satellite beams, and control means for controlling the timing of the transmitter to compensate for the variations.

17. Apparatus as claimed in claim 16, wherein the beam selecting means is arranged to select two or more of said satellite beams, and the control means is arranged to control the timing of the transmitter so that the transmitted information is received substantially simultaneously at the remote earth station via each of the selected beams.

18. Apparatus as claimed in any one of claims 15 to 17, further comprising means for measuring the Doppler shift in a received signal containing the information, and means for adjusting the frequency of the transmitter to compensate for the measured Doppler shift.

19. Apparatus as claimed in any one of claims 15 to 18, further comprising means for deriving the position of the remote earth station;

and frequency selecting means for selecting the frequency of the transmitter according to the derived position of the remote earth station.

20. Apparatus as claimed in claim 19, including means for generating a control signal for controlling the transmission and reception frequency of the remote earth station according to the derived position thereof the transmitter being arranged to transmit the control signal to the remote earth station.

21. Apparatus as claimed in claim 15 or 16, wherein the receiver is arranged to receive the information more than once sequentially via a corresponding number of said beams.

22. Apparatus as claimed in claim 21, wherein the receiver is arranged to receive the information transmitted by the remote earth station within a first time slot in a first frequency channel of a first satellite beam and to receive the information which is repeated by the remote earth station during a second time slot (t₇) in a second frequency channel of a second satellite beam.

23. Apparatus as claimed in claim 21 or 22, further comprising:

comparing means for comparing a property of the information transmitted by the remote earth station during the first and second time slots;

wherein

the transmitter is arranged to transmit a signal in a frequency channel corresponding to one of the first and second beams selected by the comparing means.

24. Apparatus as claimed in any one of claims 15 to 23, wherein the transmitter is arranged to transmit at a lower rate if the received information fails to satisfy a predetermined criterion and to transmit at higher rate if the predetermined criterion is satisfied.

25. Apparatus as claimed in claim 24, wherein the transmitter is arranged to divide the information to be transmitted into first and second portions, to transmit the first portion to the remote earth station within a third time slot and to transmit a second portion to the remote earth station within a fourth time slot.

26. Apparatus as claimed in any one of the claims 15 to 25, wherein said property relates to the quality of previous information previously received by the remote earth station from the satellite communications earth station.

27. Apparatus as claimed in any one of claims 15 to 25, wherein the property relates to the quality of the received information.

28. A user station for satellite communications, comprising a transmitter arranged to transmit the same information within each of a plurality of time slots to a base station at respective different frequencies so as to be receivable in respective different beams generated by one or more satellites.