Frequency Offset Estimation

• US 20110261909A1
• Filed: 10/30/2009
• Published: 10/27/2011
• Est. Priority Date: 11/03/2008
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Fredrik Nordstrom, Niklas Andgart
• Inventors: Nordstrom, Fredrik, Andgart, Niklas
• Granted Patent: US 8,744,020 B2
• Time in Patent Office: Days
• US Class Current: 375/341
• CPC Class Codes: H04L 25/023 with extension to other symbols
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 5/0007 the frequencies being orthogonal, e.g. OFDM(A), DMT

Abstract

A frequency offset of a received signal comprising a number of subsequently received data symbols is estimated. A first estimate is determined from a calculated change in phase of the received signal between two received symbols having a first time distance between them. At least one further estimate is determined from a calculated change in phase of the received signal between two received symbols having a different time distance. A frequency periodicity is determined for each estimate from the distance between the two received symbols from which the estimate was determined. A set of integer values is determined for each estimate so that frequency values calculated for each estimate as the frequency periodicity multiplied by the integer value added to the estimate are at least approximately equal to each other, and a corrected estimate of the frequency offset is determined from the integer values.

Claims

1. A method of estimating a frequency offset of a received signal comprising a number of subsequently received data symbols, the method comprising the step of determining a first estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a first time distance between them, wherein the method further comprises the steps of:

determining at least one further estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a time distance between them, which is different from said first time distance;

determining for each of said first and further estimates a frequency periodicity from the distance between the two received symbols from which the estimate was determined;

determining a set of integer values comprising an integer value for each of said first and further estimates so that frequency values calculated for each estimate as the frequency periodicity multiplied by the integer value added to the estimate are at least approximately equal to each other; and

calculating from the determined set of integer values a corrected estimate of said frequency offset.

2. The method according to claim 1, wherein the step of determining a set of integer values for each estimate comprises the steps of:

calculating for each estimate and for each of a number of integer values a corresponding frequency value; and

comparing the frequency values calculated for different estimates and selecting the integer values for each estimate giving frequency values closest to each other.

3. The method according to claim 1, wherein the steps of determining a set of integer values for each estimate and calculating a corrected estimate of said frequency offset comprise the steps of:

calculating for each of a number of combinations of integer values an optimal frequency value as a frequency value that maximizes a probability function;

calculating a likelihood for each optimal frequency value; and

selecting the optimal frequency value having the largest likelihood as a corrected estimate of said frequency offset.

4. The method according to claim 1, wherein the steps of determining a set of integer values for each estimate and calculating a corrected estimate of said frequency offset comprise the steps of:

calculating at least one difference between said first and further estimates;

comparing the at least one difference to pre-calculated difference values stored in a table, which for each pre-calculated difference value indicates a corresponding set of integer values; and

reading in said table the set of integer values corresponding to the at least one calculated difference.

5. The method according to claim 4, wherein the step of calculating a corrected estimate of said frequency offset further comprises the step of:

calculating from the set of integer values read in said table a corrected estimate of said frequency offset for each of said first and further estimates.

6. The method according to claim 2, further comprising the step of:

selecting a frequency value corresponding to one of the integer values in the determined set of integer values as the corrected estimate of said frequency offset.

7. The method according to claim 2, further comprising the steps of:

calculating an average value of frequency values corresponding to the integer values in the determined set of integer values; and

selecting said average value as the corrected estimate of said frequency offset.

8. The method according to claim 2, further comprising the steps of:

calculating a weighted average value of frequency values corresponding to the integer values in the determined set of integer values; and

selecting said weighted average value as the corrected estimate of said frequency offset.

9. The method according to claim 2, further comprising the step of:

mixing frequency values corresponding to the integer values in the determined set of integer values with weights corresponding to respective signal-to noise ratios to obtain the corrected estimate of said frequency offset.

10. A receiver arranged to estimate a frequency offset of a received signal comprising a number of subsequently received data symbols, the receiver comprising a first frequency offset estimator configured to determine a first estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a first time distance between them,

wherein the receiver further comprises:

at least one further frequency offset estimator configured to determine at least one further estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a time distance between them, which is different from said first time distance;

means for determining for each of said first and further estimates a frequency periodicity from the distance between the two received symbols from which the estimate was determined;

means for determining a set of integer values comprising an integer value for each of said first and further estimates so that frequency values calculated for each estimate as the frequency periodicity multiplied by the integer value added to the estimate are at least approximately equal to each other; and

means for calculating, from the determined set of integer values, a corrected estimate of said frequency offset.

11. The receiver according to claim 10, wherein the receiver is configured to determine a set of integer values for each estimate by:

calculating for each estimate and for each of a number of integer values a corresponding frequency value; and

comparing the frequency values calculated for different estimates and selecting the integer values for each estimate giving frequency values closest to each other.

12. The receiver according to claim 10, wherein the receiver is configured to determine a set of integer values for each estimate and calculate a corrected estimate of said frequency offset by:

calculating for each of a number of combinations of integer values an optimal frequency value as a frequency value that maximizes a probability function;

calculating a likelihood for each optimal frequency value; and

selecting the optimal frequency value having the largest likelihood as a corrected estimate of said frequency offset.

13. The receiver according to claim 10, wherein the receiver is configured to determine a set of integer values for each estimate and calculate a corrected estimate of said frequency offset by:

calculating at least one difference between said first and further estimates;

comparing the at least one difference to pre-calculated difference values stored in a table, which for each pre-calculated difference value indicates a corresponding set of integer values; and

reading in said table the set of integer values corresponding to the at least one calculated difference.

14. The receiver according to claim 13, further configured to calculate a corrected estimate of said frequency offset by:

calculating from the set of integer values read in said table a corrected estimate of said frequency offset for each of said first and further estimates.

15. The receiver according to claim 11, further configured to:

select a frequency value corresponding to one of the integer values in the determined set of integer values as the corrected estimate of said frequency offset.

16. The receiver according to claim 11, further configured to:

calculate an average value of frequency values corresponding to the integer values in the determined set of integer values; and

select said average value as the corrected estimate of said frequency offset.

17. The receiver according to claim 11, further configured to:

calculate a weighted average value of frequency values corresponding to the integer values in the determined set of integer values; and

select said weighted average value as the corrected estimate of said frequency offset.

18. The receiver according to claim 11, further configured to:

mix frequency values corresponding to the integer values in the determined set of integer values with weights corresponding to respective signal-to-noise ratios to obtain the corrected estimate of said frequency offset.

19. A user equipment comprising a receiver according to claim 10.

20. (canceled)

21. A non-transitory computer readable medium having stored thereon program code means for performing a method when said program code means is run on a computer, wherein the method is a method of estimating a frequency offset of a received signal comprising a number of subsequently received data symbols, the method comprising the step of determining a first estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a first time distance between them, and wherein the method further comprises the steps of:

determining at least one further estimate of said frequency offset from a calculated change in phase of the received signal between two received symbols having a time distance between them, which is different from said first time distance;

determining for each of said first and further estimates a frequency periodicity from the distance between the two received symbols from which the estimate was determined;

determining a set of integer values comprising an integer value for each of said first and further estimates so that frequency values calculated for each estimate as the frequency periodicity multiplied by the integer value added to the estimate are at least approximately equal to each other; and

calculating from the determined set of integer values a corrected estimate of said frequency offset.