Symbol timing acquisition using early-late interpolation

• US 8,897,409 B2
• Filed: 04/19/2012
• Issued: 11/25/2014
• Est. Priority Date: 12/18/2008
• Status: Active Grant

• Current Assignee: ViaSat Incorporated
• Original Assignee: ViaSat Incorporated
• Inventors: Nimon, Matthew D., Thesling, William H., Becker, Donald W.
• Primary Examiner(s): PHU, PHUONG M
• Application Number:
  US13/451,104
• Publication Number:
  US 20120207198A1
• Time in Patent Office: 950 Days
• Field of Search: 375/355, 375/362, 375/365, 375/371, 375/149, 375/343
• US Class Current: 375/355
• CPC Class Codes: H04B 7/18513 Transmission in a satellite or space-based system
• CPC Class Codes: H04L 7/0029 interpolation of received data signal

Abstract

Symbol timing acquisition is described for a wireless broadband signal received at a user terminal from a gateway via a satellite. In-phase and quadrature channels of the wireless signal may each be sampled at a rate of one sample per symbol. The samples may be interpolated to generate an early interpolation and a late interpolation for each of the samples. A difference measurement is obtained between the early interpolation and the late interpolation for a set of the samples. A number of the difference measurements may be averaged, and symbol timing may be modified based on the average. This process may be continued on an iterative basis to acquire symbol timing.

Claims

1. A device for achieving symbol timing, the device comprising:

a sampling module configured to;

receive a signal comprising a series of symbols; and

sample the signal at a rate of one sample per symbol to generate a series of samples;

an interpolation module, communicatively coupled with the sampling module, and configured to interpolate each of the samples to generate an early interpolation and a late interpolation for each of the samples;

a difference calculation module, communicatively coupled with the interpolation module, and configured to calculate difference measurements between the early interpolation and the late interpolation for each of at least a first subset of the samples and a plurality of additional subsets of the samples, the plurality of additional subsets received after the first subset;

a filtering module, communicatively coupled with the difference calculation module, and configured to obtain filtered difference measurements based on performing a filtering function on the difference measurements for the at least the first subset and the plurality of additional subsets;

a symbol timing control module, communicatively coupled with the averaging module, and configured to iteratively modify symbol timing for the sampling module based at least in part on the filtered difference measurements; and

a threshold comparator module, communicatively coupled with the filtering module, and configured to

monitor the filtered difference measurements for the first subset and the plurality of additional subsets; and

determine when a symbol timing error value corresponding to the filtered difference measurements falls below a threshold to confirm that symbol timing is achieved.

2. The device of claim 1, wherein the sampling module is configured to sample the signal at a rate of one sample per symbol by:

identifying an in-phase channel and a quadrature channel in the received signal;

sampling the in-phase channel at a rate of one sample per symbol to generate a first subset of the series of samples comprising in-phase samples; and

sampling the quadrature channel at a rate of one sample per symbol to generate a second subset of the series of samples comprising quadrature samples.

3. The device of claim 1, wherein the interpolation module is configured to perform each early interpolation one-quarter symbol period early, and to perform each late interpolation one-quarter symbol period late.

4. The device of claim 1, wherein the symbol timing control module is further configured to decrease an amount of modification relative to the symbol timing error value when symbol timing is confirmed.

5. The device of claim 1, wherein the received signal comprises a wireless signal.

6. The device of claim 5, wherein the wireless signal is received via a satellite in a satellite communications system.

22. The device of claim 1, wherein the filtering function comprises averaging the difference measurements for the at least the first subset and the plurality of additional subsets.

7. A method comprising:

receiving a series of samples of a signal, each sample of the series sampled at a rate of one sample per symbol;

interpolating each of the samples using one or more processors to generate an early interpolation and a late interpolation for each of the samples;

obtaining difference measurements between the early interpolation and the late interpolation for at least a first subset of the samples and a plurality of additional subsets of the samples, the plurality of additional subsets received after the first subset;

filtering the difference measurements for the at least the first subset and the plurality of additional subsets to obtain filtered difference measurements;

iteratively modifying the symbol timing based at least in part on the filtered difference measurements;

monitoring the filtered difference measurements for the first subset and the plurality of additional subsets; and

determining when a symbol timing error value corresponding to the filtered difference measurements falls below a threshold to confirm that symbol timing is achieved.

8. The method of claim 7, further comprising:

receiving the signal; and

sampling an in-phase channel of a time segment of the received communications signal at a rate of one sample per symbol and sampling a quadrature channel of the time segment of the received communications signal at a rate of one sample per symbol, to generate the series of samples.

9. The method of claim 7, wherein receiving the series of samples comprises:

receiving a first subset of the series of samples comprising in-phase samples, the in-phase samples sampled at a rate of one sample per symbol; and

receiving a second subset of the series of samples comprising quadrature samples, the quadrature samples sampled at a rate of one sample per symbol.

10. The method of claim 7, wherein obtaining a difference measurement between the early interpolation and the late interpolation comprises:

subtracting a value comprising the early interpolation for an in-phase and quadrature pair from a value comprising the late interpolation for the in-phase and quadrature pair.

11. The method of claim 7, further comprising:

storing, at a first rate, a series of samples; and

outputting, at a second rate slower than the first rate, the stored series of samples, wherein the output series of samples comprises the received series of samples.

12. The method of claim 7, wherein each early interpolation is one-quarter symbol period early, and each late interpolation is one-quarter symbol period late.

13. The method of claim 7, further comprising:

obtaining a difference measurement between the early interpolation and the late interpolation for at least a second subset of the samples, the second subset received after the first subset;

filtering the difference measurements for the second subset; and

modifying the symbol timing based at least in part on the filtering for the second subset.

14. The method of claim 7, further comprising:

decreasing an amount of modification relative to the symbol timing error value when symbol timing is confirmed.

15. The method of claim 7, wherein the communications signal comprises a wireless communications signal.

16. The method of claim 7, wherein the one or more processors comprise one or more application specific integrated circuits, field programmable gate arrays, general purpose processors, or application-specific processors.

17. The method of claim 7, wherein the received series of samples comprises a subset of the sampled symbols of the signal.

23. The device of claim 7, wherein the filtering comprises averaging the difference measurements for the at least the first subset and the plurality of additional subsets.

18. A device for achieving symbol timing, the device comprising:

means for receiving a series of samples, each sample of the series sampled at a rate of one sample per symbol;

means for interpolating each of the samples to generate an early interpolation and a late interpolation for each of the samples;

means for obtaining difference measurements between the early interpolation and the late interpolation for at least a first subset of the samples and a plurality of additional subsets of the samples, the plurality of additional subsets received after the first subset;

means for filtering the difference measurements for the at least the first subset and the plurality of additional subsets to obtain filtered difference measurements;

means for iteratively modifying the symbol timing based at least in part on the filtered difference measurement;

means for monitoring the filtered difference measurements for the first subset and the plurality of additional subsets; and

means for determining when a symbol timing error value corresponding to the filtered difference measurements falls below a threshold to confirm that symbol timing is achieved.

19. The device of claim 18, further comprising:

means for receiving a communications signal; and

means for sampling a version of the communications signal to generate the series of samples.

24. The device of claim 18, wherein the means for filtering comprises means for averaging the difference measurements for the at least the first subset and the plurality of additional subsets.

20. A system for achieving symbol timing, the system comprising:

a source device configured to transmit a communication signal; and

a terminal device, in communication with the source device via a communication medium, and configured to;

receive the communication signal;

sample an in-phase channel associated with the received communication signal at a rate of one sample per symbol and sample a quadrature channel associated with the received communication signal to generate a series of symbols;

interpolate each of the samples to generate an early interpolation and a late interpolation for each of the samples;

obtain difference measurements between the early interpolation and the late interpolation for at least a first subset of the samples and a plurality of additional subsets of the samples, the plurality of additional subsets received after the first subset;

filter the difference measurements for the at least the first subset and the plurality of additional subsets to obtain filtered difference measurements;

iteratively modify the symbol timing based at least in part on the filtered difference measurement;

monitor the filtered difference measurements for the first subset and the plurality of additional subsets; and

determine when a symbol timing error value corresponding to the monitored filtered difference measurement falls below a threshold to confirm that symbol timing is achieved.

21. The system of claim 20, wherein:

the source device comprises a gateway;

the terminal device comprises user terminal; and

the communication signal comprises a wireless communication signal.

25. The device of claim 20, wherein the terminal device is configured to average the difference measurements for the at least the first subset and the plurality of additional subsets.