Method and apparatus for synchronizing to a multi-beam satellite TDMA communication system

• US 5,463,400 A
• Filed: 06/30/1994
• Issued: 10/31/1995
• Est. Priority Date: 06/30/1994
• Status: Expired due to Fees

• Current Assignee: Torsal Technology Group Limited LLC (Intellectual Ventures LLC)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Tayloe, Daniel R.
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US08/269,273
• Time in Patent Office: 488 Days
• Field of Search: 342/352, 342/354, 455/33.1
• US Class Current: 342/352
• CPC Class Codes: H04B 7/18513 Transmission in a satellite or space-based system
• CPC Class Codes: H04B 7/2125 Synchronisation
• CPC Class Codes: H04W 56/0005 synchronizing of arrival of multiple uplinks
• CPC Class Codes: H04W 56/0035 detecting errors in frequency or phase

Abstract

In a satellite based cellular communication system where the satellites move with respect to the surface of the Earth and have antenna patterns covering thousands of miles, subscriber units must initially access the communication system at a proper frequency and during a proper time slot. Subscriber units compensate for the propagation delay and frequency-shift associated with the distance to a satellite and the velocity of the satellite. The subscriber unit determines the antenna beam of the satellite within which it is located and receives the antenna beam'"'"'s center coordinates. The location of the satellite is calculated and the propagation distance to the satellite is estimated. Based on the distance to the satellite and the satellite'"'"'s velocity, the subscriber unit transmits an acquisition request at the proper time and frequency so that it is received within the satellite'"'"'s timing window. The subscriber unit also adjusts is transmission frequency to compensate for the frequency shift associated with satellite'"'"'s motion.

Claims

1. A method for accessing a communication system by a subscriber unit, said communication system comprising at least one communication station that projects a plurality of antenna beams on the surface of the Earth and is moving with respect to the surface of the Earth, said method comprising the steps of:

(a) determining, by said subscriber unit using said one communication station, a first location of said subscriber unit;

(b) determining one of said antenna beams in which said subscriber unit is located;

(c) receiving from said one communication station, a beam center identification indicating a center of a ground projection of said one antenna beam;

(d) determining a second location of said one communication station using said beam center identification;

(e) calculating a propagation time to said one communication station based on the difference between said first and second locations; and

(f) transmitting an acquisition request to said one communication station at a time based on said propagation time, wherein said acquisition request arrives at said one communication station within a timing window.

2. A method as claimed in claim 1 further comprising the step of:

(g) calculating a timing advance to synchronize said subscriber unit to said one communication station, said timing advance being based on said propagation time and being approximately twice said propagation time.

3. A method as claimed in claim 1 further comprising the steps of:

(g) estimating a velocity of said one communication station with respect to said subscriber unit;

(h) calculating a frequency shift associated with said velocity; and

(i) adjusting a transmission frequency of said acquisition request based on said frequency shift.

4. A method as claimed in claim 1 wherein step (b) comprises the steps of:

determining said one of said antenna beams by measuring a power level of each of said antenna beams; and

choosing said one of said antenna beams having a greatest power level.

5. A method as claimed in claim 1 wherein step (c) comprises the step of receiving said beam center identification indicating a ground projection center of said one communication station.

6. A method as claimed in claim 1 wherein step (a) comprises the step of determining said first location within a predetermined range, said predetermined range based on said timing window.

7. A method as claimed in claim 6 wherein step (a) further comprises the step of determining said first location within said predetermined range, said predetermined range being within a 160 mile radial accuracy from a center ground projection of said one communication station.

8. A method as claim in claim 1 wherein said step (c) comprises the step of receiving said beam center identification on a ring alert channel, and wherein said beam center identification includes coordinates of said center of said one antenna beam.

9. A method as claimed in claim 1 wherein step (b) comprises the step of determining said one of said plurality of antenna beams in which said subscriber unit is located, said one antenna beam being an outer beam of said plurality and having a substantially elliptical ground projection.

10. A method as claimed in claim 9 wherein said one communication station has a receive timing window for receiving said acquisition request, and wherein step (b) comprises the step of determining said one antenna beam, and wherein a difference between said propagation time associated with a near end of said one antenna beam and said propagation time associated with a far end of said one antenna beam exceeds said receive timing window.

11. A method as claimed in claim 10 wherein said timing window is between three and four milliseconds, and said difference is between ten and twenty-one milliseconds.

12. A method as claimed in claim 1 wherein step (a) comprises the steps of:

(a1) receiving at said subscriber unit, a signal comprising information unique to said one of said antenna beams;

(a2) processing a beam geometry based on said information to determine a region where said subscriber unit is located, said beam geometry being stored in said subscriber unit; and

(a3) comparing said region with a stored value of a last reported region.

13. A subscriber unit that synchronizes to a timing window of a communication station, said communication station projecting a plurality of antenna beams on the surface of the Earth and moving with respect to the surface of the Earth, said subscriber unit comprising:

an receiver for receiving a beam center identification from said communication station;

a processor for determining a first location of said subscriber unit within a predetermined range and determining one of said plurality of antenna beams within which said subscriber unit is located;

a memory for storing antenna pattern information for said communication station; and

a transmitter for transmitting an acquisition request to said communication station,

said processor determining a second location of said communication station using said beam center identification and calculating a propagation time to said communication station based on a difference between the first and second locations, said transmitter transmitting said acquisition request based on said propagation time.

14. A subscriber unit as claimed in claim 13 wherein said processor includes means for calculating a timing advance which is approximately twice said propagation time, and wherein said transmitter includes means for transmitting said acquisition request at a time ahead by said timing advance, said acquisition request being received at said communication station within a predetermined timing window.

15. A subscriber unit as claimed in claim 13 wherein said beam center identification indicates a center location of one antenna beam.

16. A subscriber unit as claimed in claim 13 wherein said processor includes means for determining said first location by measuring power levels of said antenna beams of said communication station.

17. A subscriber unit as claimed in claim 13 wherein at least some of said antenna beams are projected at an angle with respect to the surface of the Earth producing substantially elliptical ground projection regions, said ground projection regions having a timing differential from a near end to a far end exceeding an acquisition timing window of said communication station.

18. A subscriber unit as claimed in claim 17 wherein said timing differential is between ten and twenty-one milliseconds, and said timing window is between three and four milliseconds.

19. A method of operating a subscriber unit in a communication system comprising at least one low-Earth orbiting satellite that projects a plurality of moving antenna beams onto the surface of the Earth, at least some of said antenna beams extending a radial distance from an overhead point of said satellite, said method comprising the steps of:

(a) determining a location of said subscriber unit within a predetermined range, said predetermined range based on an acquisition timing window of said satellite;

(b) determining a propagation distance from said subscriber unit to said satellite using said location and beam center information received from said satellite; and

(c) transmitting an acquisition request to said satellite at a time advanced by approximately twice a propagation time based on said propagation distance.

20. A method as claimed in claim 19 wherein said radial distance is large enough to cause a differential propagation time across an antenna beam within which said subscriber unit is located to exceed said acquisition timing window of said satellite, and wherein step (a) comprises the step of determining said location within said predetermined range, said predetermined range having said differential propagation time less than said timing window.