Method and apparatus for acquiring and tracking pilots in a CDMA communication system

• US 20030054768A1
• Filed: 10/05/2001
• Published: 03/20/2003
• Est. Priority Date: 07/16/2001
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Sih, Gilbert Christhoper, Challa, Raghu
• Granted Patent: US 7,088,955 B2
• Time in Patent Office: Days
• US Class Current: 455/63.1
• CPC Class Codes: H04L 2027/0055 single phase
• CPC Class Codes: H04L 2027/0079 Switching between loops
• CPC Class Codes: H04L 2027/0087 Out-of-band signals, (e.g. pilots)
• CPC Class Codes: H04L 27/0014 Carrier regulation of chaotic carriers H04L27/001; for multicarrier receivers H04L27/2657

Abstract

Techniques to acquire and track pilots in a CDMA system. In an aspect, frequency acquisition of a number of signal instances (i.e., multipaths) in a received signal may be achieved concurrently based on a frequency control loop (RAFC) maintained for each finger processor of a rake receiver. Upon successful acquisition, frequency tracking of acquired multipaths may be achieved based on a combination of a frequency control loop (VAFC) maintained for an oscillator used for downconverting the received signal and the RAFCs for the finger processors. In a tracking mode, the VAFC tracks the average frequency of the acquired multipaths by adjusting the frequency of the oscillator. The RAFC of each finger processor tracks the residual frequency error (e.g., due to Doppler frequency shift) of the individual acquired multipath by adjusting the frequency of a complex sinusoidal signal used in a rotator within the finger processor.

Claims

1. In a wireless communication system, a method of acquiring signal instances in a received signal, comprising:

acquiring a frequency of each of one or more candidate signal instances with a respective first frequency control loop;

detecting acquisition on at least one candidate signal instance; and

tracking an average frequency of the at least one acquired signal instance with a second frequency control loop.

2. The method of claim 1, further comprising:

tracking a residual frequency error of each acquired signal instance with the first frequency control loop.

3. The method of claim 1, further comprising:

transferring a frequency error of each acquired signal instance from the first frequency control loop to the second frequency control loop.

4. The method of claim 3, wherein the frequency error of each acquired signal instance is transferred by

scaling an output of a first accumulator for the first frequency control loop and indicative of the frequency error, and

providing the scaled first accumulator output to an input of a second accumulator for the second frequency control loop.

5. The method of claim 3, wherein the frequency error of each acquired signal instance is transferred by writing to a second accumulator for the second frequency control loop a value derived from a first accumulator for the first frequency control loop used for the acquired signal instance.

6. The method of claim 3, wherein the frequency error of each acquired signal instance is transferred by operatively coupling a loop accumulator output for the first frequency control loop to a loop accumulator input for the second frequency control loop.

7. The method of claim 1, further comprising:

downconverting the received signal with a downconversion signal having a frequency determined by the second frequency control loop; and

for each candidate signal instance, frequency translating data samples of the downconverted signal using a frequency determined by the first frequency control loop.

8. The method of claim 1, wherein the second frequency control loop is disabled during the acquisition of the one or more candidate signal instances.

9. The method of claim 1, wherein the first frequency control loop for each acquired signal instance and the second frequency control loop are enabled during the tracking of the at least one acquired signal instance.

10. The method of claim 1, wherein the average frequency is derived via a uniform weighting of the frequency of the at least one acquired signal instance.

11. The method of claim 1, wherein the average frequency is derived via a non-uniform weighting such that the frequency of a stronger acquired signal instance is weighted more than the frequency of a weaker acquired signal instance.

12. The method of claim 1, wherein the first and second frequency control loops are configurable to operate in one of a plurality of loop modes.

13. The method of claim 12, wherein the first frequency control loop for each candidate signal instance is set to a first loop mode having a wider loop bandwidth for the acquisition.

14. The method of claim 12, wherein the first frequency control loop for each acquired signal instance is set to a second loop mode having a narrower loop bandwidth for the tracking.

15. The method of claim 12, wherein each loop mode is associated with a respective scheme for deriving inputs for a discriminator used to detect frequency error in the signal instance.

16. The method of claim 1, wherein the acquisition is performed concurrently on a plurality of candidate signal instances.

17. The method of claim 1, wherein acquisition on the at least one candidate signal instance is detected if at least one detector output exceeds a particular threshold value within a particular time interval.

18. The method of claim 1, wherein acquisition on each candidate signal instance is detected with a received signal strength indicator (RSSI).

19. The method of claim 1, wherein acquisition on each candidate signal instance is detected using a frequency error estimator that estimates the amount of residual frequency error in frequency-translated data samples for the candidate signal instance.

20. The method of claim 17, further comprising:

performing system acquisition on the acquired signal instance having a best detector output that exceeds the particular threshold value within the particular time interval.

21. The method of claim 20, further comprising:

retaining acquisition information for each acquired signal instance for which system acquisition is not performed.

22. The method of claim 17, further comprising:

performing system acquisition on a plurality of acquired signal instances having detector outputs exceeding the particular threshold value within the particular time interval.

23. The method of claim 22, further comprising:

combining demodulated symbols for the plurality of acquired signal instances for which system acquisition is performed.

24. The method of claim 1, further comprising:

acquiring a frequency of each of one or more additional candidate signal instances based on a respective first frequency control loop.

25. The method of claim 24, wherein the second frequency control loop is operative to omit contributions from the first frequency control loop for each additional candidate signal instance during the acquisition of the signal instance by the first frequency control loop.

26. The method of claim 24, further comprising:

detecting acquisition on at least one additional candidate signal instance; and

tracking the average frequency of all acquired signal instances with the second frequency control loop.

27. The method of claim 1, wherein each first frequency control loop includes a rotator operative to frequency translate a signal instance.

28. The method of claim 1, wherein the second frequency control loop includes an oscillator to which a downconversion signal for the received signal is locked.

29. The method of claim 1, wherein the communication system is a CDMA system.

30. The method of claim 29, wherein the CDMA system conforms to the IS-95 or cdma2000 standard.

31. The method of claim 29, wherein the CDMA system conforms to the W-CDMA or TS-CDMA standard.

32. A method of acquiring one or more pilots in a CDMA communication system, comprising:

acquiring a frequency of each of one or more candidate signal instances with a respective first frequency control loop;

detecting acquisition on at least one candidate signal instance;

transferring a frequency error of each acquired signal instance from the first frequency control loop to the second frequency control loop;

tracking an average frequency of the at least one acquired signal instance with a second frequency control loop; and

tracking a residual frequency error of each acquired signal instance with the first frequency control loop.

33. The method of claim 32, further comprising:

acquiring a frequency of each of one or more additional candidate signal instances based on a respective first frequency control loop;

detecting acquisition on at least one additional candidate signal instance; and

tracking the average frequency of all acquired signal instances with the second frequency control loop.

34. In a wireless communication system, a method of acquiring signal instances in a received signal, comprising:

acquiring a frequency of each of one or more candidate signal instances;

detecting frequency acquisition on at least one candidate signal instance based on at least one detector;

performing system acquisition on an acquired signal instance having a best detector output; and

retaining frequency acquisition information for each acquired signal instance for which system acquisition is not performed.

35. The method of claim 34, wherein frequency acquisition on a candidate signal instance is detected if the detector output for the signal instance exceeds a particular threshold value within a particular time interval.

36. The method of claim 34, wherein system acquisition is performed on a plurality of acquired signal instances having detector outputs exceeding a particular threshold value within a particular time interval.

37. A receiver unit in a wireless communication system, comprising:

one or more finger processors operative to acquire frequency of one or more candidate signal instances in a received signal with one or more respective first frequency control loops;

at least one detector operative to detect acquisition on at least one candidate signal instance; and

a second frequency control loop operative to track an average frequency of the at least one acquired signal instance.

38. The receiver unit of claim 37, wherein the one or more finger processors are further operative to track residual frequency error of each acquired signal instance with the first frequency control loop.

39. The receiver unit of claim 37, further comprising:

a searcher operative to search for signal instances in the received signal to provide the one or more candidate signal instances.

40. The receiver unit of claim 37, wherein the each finger processor includes a rotator operative to frequency translate a signal instance assigned to the finger processor.

41. The receiver unit of claim 37, wherein the second frequency control loop is operative to adjust the frequency of an oscillator used as a reference to derive the frequency used for downconversion of the received signal from radio frequency to baseband.

42. A terminal in a CDMA system comprising:

one or more finger processors operative to acquire frequency of one or more candidate signal instances in a received signal with one or more respective first frequency control loops;

at least one detector operative to detect acquisition on at least one candidate signal instance; and

a second frequency control loop operative to track an average frequency of the at least one acquired signal instance.

43. The terminal of claim 42, wherein the one or more finger processors are further operative to track residual frequency error of each acquired signal instance with the first frequency control loop.

44. The terminal of claim 42, further comprising:

a searcher operative to search for signal instances in the received signal to provide the one or more candidate signal instances.

45. The terminal of claim 42, wherein the each finger processor includes a rotator operative to frequency translate a signal instance assigned to the finger processor.

46. The terminal of claim 42, wherein the second frequency control loop is operative to adjust downconversion of the received signal from radio frequency to baseband.