Frequency offset estimation in a CDMA system

1. A method of estimating a frequency offset, the method comprising:

receiving a wireless signal timed according to a first frequency;

generating a local signal timed according to a second frequency;

performing a first synchronization search, the first synchronization search comprising obtaining a set of correlation results indicative of a correlation between the wireless signal and the local signal at a first timing offset of the wireless signal relative to the local signal;

performing at least one subsequent synchronization search, the at least one subsequent synchronization search comprising obtaining a subsequent set of correlation results indicative of a correlation between the wireless signal and the local signal at least one subsequent timing offset different to said first timing offset of the wireless signal relative to the local signal, whereby the first synchronization search and the at least one subsequent synchronization search constitute a plurality of synchronization searches;

finding a series of results, with a result from each of a plurality of said synchronization searches, for which the difference in timing offset between results from adjacent searches in the series is within a maximum specified value; and

determining from said series a frequency offset between the first and second frequencies.

2. The method of claim 1 , comprising:

for each synchronization search, ordering the respective correlation results in order of a corresponding received signal energy.

3. The method of claim 2 , wherein said step of finding of said series is performed using only a top portion of the correlation results having the greatest received signal energy from each synchronization search.

4. The method of claim 1 , comprising using said estimation to correct for the frequency offset.

5. The method of claim 1 , wherein the step of determining the frequency offset comprises determining the frequency offset from a difference in timing offsets between at least two of the correlation results from different searches in said series.

6. The method of claim 1 , wherein the step of determining the frequency offset comprises determining a phase from sums of real and imaginary parts of the correlation results in said series.

7. The method of claim 1 , comprising wherein the step of determining the frequency offset is conditional on a magnitude of a sum of real and imaginary parts of the correlation results in said series being above a predetermined threshold.

8. The method of claim 1 , wherein the step of receiving the wireless signal comprises receiving a synchronization channel on which is broadcast a synchronization code, and the step of generating the local signal comprises generating a local version of the synchronization code.

9. The method of claim 8 , wherein the synchronization channel is a primary synchronization channel of a W-CDMA cellular communications system.

10. A wireless receiver, comprising:

reception means arranged to receive a wireless signal timed according to a first frequency;

generation means arranged to generate a local signal timed according to a second frequency;

correlation means arranged to perform a plurality of synchronization searches, by performing a first synchronization search, the first synchronization search comprising obtaining a set of correlation results indicative of a correlation between the wireless signal and the local signal at a first timing offset of the wireless signal relative to the local signal;

performing at least one subsequent synchronization search, the at least one subsequent synchronization search comprising obtaining a subsequent set of correlation results indicative of a correlation between the wireless signal and the local signal at least one subsequent timing offset different to said first timing offset of the wireless signal relative to the local signal, whereby the first synchronization search and the at least one subsequent synchronization search constitute a plurality of synchronization searches; and

frequency offset estimation means arranged to find a series of results, with a result from each of a plurality of said synchronization searches, for which the difference in timing offset between results from adjacent searches in the series is within a maximum specified value, and to estimate from said series a frequency offset between the first and second frequencies.

11. The receiver of claim 10 , wherein the frequency offset estimation means is arranged, for each synchronization search, to order the respective correlation results in order of a corresponding received signal energy.

12. The receiver of claim 11 , wherein the frequency offset estimation means is arranged to find said series using only a top portion of the correlation results having the greatest received signal energy from each synchronization search.

13. The receiver of claim 10 , wherein the frequency offset estimation means is arranged to determine the frequency offset from a difference in timing offsets between at least two of the correlation results from different searches in said series.

14. The receiver of claim 10 , wherein the frequency offset estimation means is arranged to determine the frequency offset from a phase determined from sums of real and imaginary parts of the correlation results in said series.

15. The receiver of claim 10 , wherein the frequency offset estimation means is arranged to determining the frequency offset on the condition that a magnitude of a sum of real and imaginary parts of the correlation results in said series is above a predetermined threshold.

16. The receiver of claim 10 , comprising automatic frequency correction means for using said estimation to correct for the frequency offset.

17. The receiver of claim 10 , wherein the reception means is arranged to receive a synchronization channel in said wireless signal on which is broadcast a synchronization code, and the local signal comprises a local version of the synchronization code.

18. The receiver of claim 17 , wherein the synchronization channel is a primary synchronization channel of a W-CDMA cellular communications system.

19. A computer program product for estimating frequency offset between a received wireless signal timed according to a first frequency and a generated local signal timed according to a second frequency, the program comprising code stored on a non-transitory computer readable medium which when executed by a processor is configured to perform the steps of:

performing a first synchronization search, the first synchronization search comprising obtaining a set of correlation results indicative of a correlation between the wireless signal and the local signal at a first timing offset of the wireless signal relative to the local signal; and

performing at least one subsequent synchronization search, the at least one subsequent synchronization search comprising obtaining a subsequent set of correlation results indicative of a correlation between the wireless signal and the local signal at least one subsequent timing offset different to said first timing offset of the wireless signal relative to the local signal, whereby the first synchronization search and the at least one subsequent synchronization search constitute a plurality of synchronization searches;

finding a series of results, with a result from each of a plurality of said synchronization searches, for which the difference in timing offset between results from adjacent searches in the series is within a maximum specified value; and

determining the frequency offset from said series.

20. A wireless cellular communication system comprising:

a base station arranged to transmit a wireless signal timed according to a first frequency, and

a mobile terminal comprising:

reception means arranged to receive said wireless signal;

generation means arranged to generate a local signal timed according to a second frequency;

correlation means arranged to perform a plurality of synchronization searches, by:

performing a first synchronization search, the first synchronization search comprising obtaining a set of correlation results indicative of a correlation between the wireless signal and the local signal at a first timing offset of the wireless signal relative to the local signal; and

performing at least one subsequent synchronization search, the at least one subsequent synchronization search comprising obtaining a subsequent set of correlation results indicative of a correlation between the wireless signal and the local signal at least one subsequent timing offset different to said first timing offset of the wireless signal relative to the local signal, whereby the first synchronization search and the at least one subsequent synchronization search constitute a plurality of synchronization searches; and

frequency offset estimation means arranged to find a series of results, with a result from each of a plurality of said synchronization searches, for which the difference in timing offset between results from adjacent searches in the series is within a maximum specified value, and to estimate from said series a frequency offset between the first and second frequencies.