System and method for resolving frequency and timing uncertainty in access transmissions in a spread spectrum communication system

• US 6,381,225 B1
• Filed: 08/27/1998
• Issued: 04/30/2002
• Est. Priority Date: 08/27/1998
• Status: Expired due to Term

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Tenny, Nathan E., Chockalingam, Ananthanarayanan, Miller, David S.
• Primary Examiner(s): VU, HUY DUY
• Application Number:
  US09/140,936
• Time in Patent Office: 1,342 Days
• Field of Search: 370/316, 370/319, 370/321, 370/324, 370/330, 370/343, 370/350, 370/436, 370/478, 370/480, 375/316, 375/322, 375/324, 375/325, 375/326, 455/11.1, 455/12.1, 455/13.1, 455/13.2, 455/13.3, 342/34, 342/131, 342/155, 342/157, 342/158
• US Class Current: 370/316
• CPC Class Codes: H04B 7/216 Code division or spread-spectrum multiple access [CDMA, SSMA]

Abstract

A method for reducing the number of required frequency and time hypotheses to acquire an access transmission in a satellite communication system. By reducing the number of required hypotheses, the amount of hardware required to acquire the access transmission is reduced. The method of the present invention reduces the number of required hypotheses by reducing the range of possible frequency and timing values of the access transmission. The range of possible frequency and timing values is reduced by determining the frequency and time uncertainties over individual satellite beams rather than over an entire satellite footprint.

Claims

1. A method for acquiring a signal transmitted by a user terminal to a satellite and relayed by the satellite to a gateway, comprising the steps of:

defining an arrival time and frequency search space for a communication beam associated with the satellite based on a predetermined beam coverage area of said communication beam;

searching said search space to resolve a timing and frequency uncertainty associated with the signal; and

demodulating a message portion of the signal based on a frequency increment and timing offset obtained as a result of resolving said timing and frequency uncertainty.

2. The method of claim 1, wherein said predetermined beam coverage area of said beam corresponds to an area defined by a range of azimuths and a range of elevations containing a nominal coverage region of said beam.

3. The method of claim 1, wherein said step of searching said search space comprises the steps of:

performing a coarse search of said search space to resolve said frequency uncertainty associated with the signal; and

performing a fine search to resolve said timing uncertainty associated with the signal.

4. The method of claim 1, wherein the signal comprises a preamble portion and said message portion.

5. The method of claim 4, wherein said preamble portion comprises null data.

6. The method of claim 4, wherein said preamble portion has a first stage and a second stage, said first stage having data modulated by a first signal, and said second stage having data modulated by a second signal.

7. The method of claim 6, wherein said first signal and said second signal are pseudonoise sequences.

8. The method of claim 1, wherein said search space is defined by a range of frequencies and a range of arrival times.

9. A method for recovering at a gateway information within a message portion of a signal transmitted by a user terminal and relayed by a satellite to the gateway, comprising the steps of:

assigning an access channel receiver within the gateway to a beam associated with the satellite;

assigning a search space to said access channel receiver, said search space corresponding to a frequency and timing uncertainty associated with said beam to which said access channel receiver is assigned;

searching said search space to acquire the signal; and

if the signal is acquired after searching said search space, demodulating the message portion of the signal to recover the information contained therein.

10. The method of claim 9, wherein said step of searching said search space comprises the steps of:

performing a coarse search of said search space to resolve a frequency uncertainty associated with the signal; and

performing a fine search to resolve a timing uncertainty associated with the signal.

11. The method of claim 10, wherein said signal has a preamble portion that is transmitted before the message portion is transmitted.

12. The method of claim 11, wherein said preamble portion comprises a first stage and a second stage.

13. The method of claim 12, wherein said step of performing a coarse search is performed while the gateway is receiving said first stage of said preamble portion.

14. The method of claim 9, wherein said search space is defined by a range of frequencies and a range of arrival times.

15. A system for recovering at a gateway information within a message portion of a signal transmitted by a user terminal to a satellite and relayed by the satellite to the gateway, comprising:

an access channel receiver within the gateway, said access channel receiver being assigned to a beam associated with the satellite;

a search space assigned to said access channel receiver, said search space corresponding to a frequency and timing uncertainty associated with said beam to which said access channel receiver is assigned;

means for searching said search space to acquire the signal; and

means for demodulating the message portion of the signal acquired after searching said search space to recover the information contained therein.

16. The system of claim 15, wherein said means for searching said search space comprises:

means for performing a coarse search of said search space to resolve a frequency uncertainty associated with the signal; and

means for performing a fine search to resolve a timing uncertainty associated with the signal.

17. The system of claim 16, wherein said signal has a preamble portion that is transmitted before the message portion is transmitted.

18. The system of claim 17, wherein said preamble portion comprises a first stage and a second stage.

19. The system of claim 15, wherein said search space is defined by a range of frequencies and a range of arrival times.