Time tracking loops for a communication system utilizing a cyclic prefix

• US 8,416,733 B2
• Filed: 06/28/2010
• Issued: 04/09/2013
• Est. Priority Date: 06/30/2009
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Srinivasan, Shivratna G., McCloud, Michael L., Gurski, Remi J., Kandukuri Narayana, Sunil Kumar
• Primary Examiner(s): Mui, Gary
• Application Number:
  US12/825,146
• Publication Number:
  US 20110149943A1
• Time in Patent Office: 1,016 Days
• Field of Search: 370208-210, 370/343, 370/359, 370/419, 370/463, 455/63.1
• US Class Current: 370/324
• CPC Class Codes: H04L 25/0212 of impulse response
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2665 Fine synchronisation, e.g. by positioning the FFT window

Abstract

Techniques for performing time tracking in a communication system utilizing a cyclic prefix are described. In an aspect, a receiver may detect for large timing errors based on early and late received samples obtained with early and late FFT windows, respectively. The receiver may derive first and second channel impulse response (CIR) estimates based on the early and late received samples, respectively, determine an early channel energy based on the first CIR estimate, determine a late channel energy based on the second CIR estimate, compute an update amount based on the early and late channel energies, and update the FFT window position based on the update amount. In another aspect, the receiver may perform time tracking with an inner time tracking loop (TTL) and an outer TTL. The receiver may update the FFT window position in coarse steps with the outer TTL and in fine steps with the inner TTL.

Claims

1. A method for communication, comprising:

deriving a first channel impulse response (CIR) estimate based on early received samples;

deriving a second CIR estimate based on late received samples;

determining a fast Fourier transform (FFT) window position based on the first and second CIR estimates; and

selecting received samples for processing based on the FFT window position.

2. The method of claim 1, further comprising:

determining an early FFT window and a late FFT window based on a current FFT window position;

selecting the early received samples based on the early FFT window; and

selecting the late received samples based on the late FFT window.

3. The method of claim 2, wherein the early FFT window is at least one cyclic prefix length earlier than the current FFT window position, and wherein the late FFT window is at least one cyclic prefix length later than the current FFT window position.

4. The method of claim 1, wherein the determining the FFT window position comprises:

updating the FFT window position with a time tracking loop(TTL); and

determining an amount to update the FFT window position based on the early and late received samples.

5. The method of claim 1, wherein the determining the FFT window position comprises:

determining an early channel energy based on the first CIR estimate;

determining a late channel energy based on the second CIR estimate; and

updating the FFT window position based on the early channel energy and the late channel energy.

6. The method of claim 5, wherein the updating the FFT window position comprises determining a difference between the early channel energy and the late channel energy, and determining an amount to update the FFT window position based on the difference between the early channel energy and the late channel energy.

7. The method of claim 5, wherein the FFT window position is updated to balance the early channel energy and the late channel energy and to minimize a difference between the early channel energy and the late channel energy.

8. The method of claim 5, wherein the determining the early channel energy comprises:

identifying a predetermined number of strongest channel taps in the first CIR estimate, and computing the early channel energy based on total energy of the identified channel taps.

9. The method of claim 5, wherein the determining the early channel energy comprises:

identifying channel taps in the first CIR estimate with energy exceeding a threshold, and computing the early channel energy based on total energy of the identified channel taps.

10. The method of claim 1, wherein the selecting the received samples for processing comprises selecting N received samples of an orthogonal frequency division multiplexing (OFDM) symbol comprising N+G samples based on the FFT window position, where N is a total number of subcarriers and G is a cyclic prefix length.

11. The method of claim 1, wherein the selecting the received samples for processing comprises selecting N received samples of a single-carrier frequency division multiplexing (SC-FDM) symbol comprising N+G samples based on the FFT window position, where N is a total number of subcarriers and G is a cyclic prefix length.

12. An apparatus for communication, comprising:

means for deriving a first channel impulse response (CIR) estimate based on early received samples;

means for deriving a second CIR estimate based on late received samples;

means for determining a fast Fourier transform (FFT) window position based on the first and second CIR estimates; and

means for selecting received samples for processing based on the FFT window position.

13. The apparatus of claim 12, further comprising:

means for determining an early FFT window and a late FFT window based on a current FFT window position;

means for selecting the early received samples based on the early FFT window; and

means for selecting the late received samples based on the late FFT window.

14. The apparatus of claim 12, wherein the means for determining the FFT window position comprises:

means for updating the FFT window position with a time tracking loop(TTL); and

means for determining an amount to update the FFT window position based on the early and late received samples.

15. The apparatus of claim 12, wherein the means for determining the FFT window position comprises:

means for determining an early channel energy based on the first CIR estimate;

means for determining a late channel energy based on the second CIR estimate; and

means for updating the FFT window position based on the early channel energy and the late channel energy.

16. An apparatus for communication, comprising:

at least one processor configured to;

derive a first channel impulse response (CIR) estimate based on early received samples;

derive a second CIR estimate based on late received samples;

determine a fast Fourier transform (FFT) window position based on the first and second CIR estimates; and

select received samples for processing based on the FFT window position.

17. The apparatus of claim 16, wherein the at least one processor is further configured to determine an early FFT window and a late FFT window based on a current FFT window position, to select the early received samples based on the early FFT window, and to select the late received samples based on the late FFT window.

18. The apparatus of claim 16, wherein the at least one processor is further configured to update the FFT window position with a time tracking loop (TTL), and to determine an amount to update the FFT window position based on the early and late received samples.

19. The apparatus of claim 16, wherein the at least one processor is further configured to determine an early channel energy based on the first CIR estimate, to determine a late channel energy based on the second CIR estimate, and to update the FFT window position based on the early channel energy and the late channel energy.

20. A computer program product, comprising:

a non-transitory computer-readable medium comprising;

code for causing at least one computer to derive a first channel impulse response (CIR) estimate based on early received samples;

code for causing at least one computer to derive a second CIR estimate based on late received samples;

code for causing at least one computer to determine a fast Fourier transform (FFT) window position based on the first and second CIR estimates; and

code for causing the at least one computer to select received samples for processing based on the FFT window position.