Estimating frequency offset at a subscriber station receiver

1. A method of estimating frequency offset at a subscriber station, comprising:

the subscriber station receiving a composite signal, the composite signal comprising multi-carrier signals transmitted from a plurality of base stations;

the subscriber station selecting a subset of the plurality of base stations;

the subscriber station selecting at least one pair of multi-carrier symbols of the composite signal, the at least one pair of multi-carrier symbols each comprising a plurality of pilot sub-carriers;

the subscriber station selecting a subset of the plurality of pilot sub-carriers of the at least one pair of multi-carrier symbols based on transmit pilot symbols of the pilot sub-carriers of the multi-carrier signals transmitted by the subset of the plurality of base stations; and

estimating the frequency offset based on a conjugate product between received symbols across at least one pair of multi-carrier symbols of each of the pilot sub-carriers of the selected subset of the plurality of pilot sub-carriers.

2. The method of claim 1 , wherein selecting a subset of the plurality of pilot sub-carriers of the at least one pair of multi-carrier symbols based on pilot symbols of the pilot sub-carriers of the multi-carrier signals transmitted by the subset of the plurality of base stations comprises:

identifying pilot sub-carriers that have transmit pilot symbols that do not change from one multi-carrier symbol to the other multi-carrier symbol of the pair of multi-carrier symbols of the multi-carrier signals for any base station of the subset of the plurality of base stations.

3. The method of claim 1 , wherein selecting a subset of the plurality of pilot sub-carriers of the at least one pair of multi-carrier symbols based on pilot symbols of the pilot sub-carriers of the multi-carrier signals transmitted by the subset of the plurality of base stations comprises:

identifying pilot sub-carriers that have transmit pilot symbols that change in polarity from one multi-carrier symbol to the other multi-carrier symbol of a pair of multi-carrier symbols of the multi-carrier signals for all of the subset of the plurality of base stations.

4. The method of claim 1 , wherein selecting a subset of the plurality of pilot sub-carriers of the at least one pair of multi-carrier symbols based on pilot symbols of the pilot sub-carriers of the multi-carrier signals transmitted by the subset of the plurality of base stations comprises:

identifying pilot sub-carriers that have transmit pilot symbols that do not change from one multi-carrier symbol to the other multi-carrier symbol of a pair of multi-carrier symbols of the multi-carrier signals for any of the subset of the plurality of base stations, and

identifying pilot sub-carriers that have transmit pilot symbols that change in polarity from one multi-carrier symbol to the other multi-carrier symbol of a pair of multi-carrier symbols of the multi-carrier signals for all of the subset of the plurality of base stations.

5. The method of claim 4 , further comprising:

summing conjugate products between received symbols on pilot sub-carriers that have transmit pilot symbols that do not change from one multi-carrier symbol to the other multi-carrier symbol of the pair of multi-carrier symbols

summing conjugate products between received symbols on pilot sub-carriers that have transmit pilot symbols that do change in polarity from one multi-carrier symbol to the other multi-carrier symbol of a pair of multi-carrier symbols of the multi-carrier signals; and

estimating the frequency offset based on subtracting the summed conjugate products of received symbols on pilot sub-carriers that have transmit pilot symbols that do change in polarity from the summed conjugate products of received symbols on pilot sub-carriers that have transmit pilot symbols that do not change.

6. The method of claim 1 , wherein the subscriber station selecting a subset of the plurality of base stations comprises:

the subscriber station determining a received signal strength of each of the multi-carrier signals transmitted by each of the plurality of base stations;

the subscriber station selecting a subset of the plurality of base stations that have a received signal strength greater than a threshold level.

7. The method of claim 6 , further comprising the subscriber station reducing the subset of the plurality of base stations to N base stations having N greatest received signal strength.

8. The method of claim 6 , further comprising the subscriber station reducing the subset of the plurality of base stations to two base stations having two greatest received signal strengths.

9. The method of claim 1 , wherein the subscriber station selecting a subset of the plurality of base stations comprises:

selecting different subsets of base stations for each of different groups of sub-carriers, wherein the selections are based on a received signal strength for each of the groups of sub-carriers.

10. The method of claim 1 , wherein the pilot symbols of the multi-carrier signals transmitted by the plurality of base stations are BPSK modulated.

11. The method of claim 1 , wherein the subscriber station selecting at least one pair of multi-carrier symbols of the composite signal, comprises the subscriber station selecting multi-carrier symbols where the pilot sub-carriers indices are the same.

12. The method of claim 1 , wherein the subscriber station selecting at least one pair of multi-carrier symbols of the composite signal, comprises the subscriber station selecting multi-carrier symbols that are not adjacent.

13. The method of claim 1 , wherein the subscriber station selecting at least one pair of multi-carrier symbols of the composite signal, comprises the subscriber station selecting multi-carrier symbols with a separation in time that is inversely proportional to a Doppler spread.

14. The method of claim 1 , further comprising selecting multiple pairs of multi-carrier symbols of the composite signal.

15. The method of claim 14 , wherein the multiple pairs are within a common frame.

16. The method of claim 14 , wherein the multiple pairs are from different frames.

17. The method of claim 1 , further comprising using the frequency offset estimate as an error signal in a frequency tracking loop.

18. A subscriber station, comprising:

means for receiving a composite signal, the composite signal comprising multi-carrier signals transmitted from a plurality of base stations;

means for selecting a subset of the plurality of base stations;

means for selecting at least one pair of multi-carrier symbols of the composite signal, the at least one pair of multi-carrier symbols each comprising a plurality of pilot sub-carriers;

means for selecting a subset of the plurality of pilot sub-carriers of the at least one pair of multi-carrier symbols based on transmit pilot symbols of the pilot sub-carriers of the multi-carrier signals transmitted by the subset of the plurality of base stations; and

means for estimating the frequency offset based on a conjugate product between received symbols across at least one pair of multi-carrier symbols of each of the pilot sub-carriers of the selected subset of the plurality of pilot sub-carriers.