Method for estimating frequency offset at a subscriber station receiver in a multi-carrier system
View Patent ↗A method and system of estimating frequency offset of a subscriber station is disclosed. The method includes the subscriber station receiving a composite signal, wherein the composite signal includes multi-carrier signals transmitted from a plurality of base stations. The subscriber station selects a subset of the plurality of base stations. The subscriber station selects at least one pair of multi-carrier symbols of the composite signal, wherein each of the at least one pair of multi-carrier symbols includes a plurality of pilot sub-carriers. The subscriber station selects at least one group of the plurality of pilot sub-carriers within 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. The frequency offset is estimated based on a conjugate product between a linear combination of received symbols of a first multi-carrier symbol of a pair of multi-carrier symbols with a linear combination of received symbols of a second multi-carrier symbol of the pair of multi-carrier symbols on each of the pilot sub-carriers of the at least one group of the plurality of pilot sub-carriers.
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 comprising a first multi-carrier signal including a plurality of first multi-carrier pilot sub-carriers, and a second multi-carrier signal including a plurality of second multi-carrier pilot sub-carriers;
the subscriber station selecting at least one group of the plurality of first multi-carrier pilot sub-carriers within 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;
estimating the frequency offset based on a complex conjugate product between a linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a first multi-carrier symbol of the pair of multi-carrier symbols and a linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a second multi-carrier symbol of the pair of multi-carrier symbols.
2. The method of claim 1 , wherein the subscriber station selecting at least one group of the plurality of pilot sub-carriers within the at least one pair of multi-carrier symbols based on transmit pilot symbols of the pilot sub-carriers of the multi-carrier signals comprises:
Identifying at least one group of pilot sub-carriers of the pair of multi-carrier symbols by identifying a linear combination of transmit pilot symbol vectors across the group of pilot sub-carriers of a first of the pair of multi-carrier symbols that is substantially equal to a linear combination of transmit pilot symbol vectors across the group of pilot sub-carriers of a second of the pair of multi-carrier symbols.
3. The method of claim 2 , wherein weights used to obtain the linear combination of transmit pilot symbol vectors across the group of pilot sub-carriers of the first of the pair of multi-carrier symbols are substantially equal to weights used to obtain linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a first multi-carrier symbol of the pair of multi-carrier symbols, and weights used to obtain the linear combination of transmit pilot symbol vectors across the group of pilot sub-carriers of a second of the pair of multi-carrier symbols are substantially equal to weights used to obtain the linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a second multi-carrier symbol of the pair of multi-carrier symbol.
4. The method of claim 1 , wherein selecting at least one group of the plurality of pilot sub-carriers is dependent on a coherence bandwidth.
5. The method of claim 1 , wherein a range of sub-carrier indices included within the at least one group is dependent upon a flatness of a pass-band of a transmission channel.
6. The method of claim 1 , further comprising:
summing conjugate products obtained from multiple groups of the pairs of multi-carrier symbols.
7. The method of claim 1 , further comprising selecting multiple pairs of multi-carrier symbols of the composite signal.
8. The method of claim 6 , wherein the multiple pairs are within a common frame.
9. The method of claim 6 , wherein the multiple pairs are from different frames.
10. 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.
11. The method of claim 9 , further comprising the subscriber station reducing the subset of the plurality of base stations to N base stations having N greatest received signal strength.
12. The method of claim 9 , further comprising the subscriber station reducing the subset of the plurality of base stations to two base stations having two greatest received signal strengths.
13. 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.
14. The method of claim 1 , wherein the pilot symbols of the multi-carrier signals transmitted by the plurality of base stations are BPSK modulated.
15. 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 are the same.
16. 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.
17. 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 the Doppler spread.
18. The method of claim 1 , further comprising selecting multiple pairs of multi-carrier symbols of the composite signal.
19. The method of claim 17 , wherein the multiple pairs are within a common frame.
20. The method of claim 17 , wherein the multiple pairs are from different frames.
21. The method of claim 1 , further comprising using the frequency offset estimate as an error signal in a frequency tracking loop.
22. The method of claim 1 , wherein weights selected to determine the linear combinations of transmit pilot symbol vectors across the group of pilot sub-carriers comprise complex numbers.
23. The method of claim 2 , wherein for a desired base station, the linear combination of transmit pilot symbols across the group of pilot sub-carriers of a first of the pair of multi-carrier symbols is substantially equal to a linear combination of transmit pilot symbols across the group of pilot sub-carriers of a second of the pair of multi-carrier symbols, and is non-zero, and for at least one non-desired base station the linear combination of transmit pilot symbols across the group of pilot sub-carriers of a first of the pair of multi-carrier symbols is substantially equal to a linear combination of transmit pilot symbols across the group of pilot sub-carriers of a second of the pair of multi-carrier symbols, and is substantially equal to zero.
24. A subscriber station that estimates frequency offset, 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 comprising a first multi-carrier signal including a plurality of first multi-carrier pilot sub-carriers, and a second multi-carrier signal including a plurality of second multi-carrier pilot sub-carriers;
means for selecting at least one group of the plurality of first multi-carrier pilot sub-carriers within 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;
means for estimating the frequency offset based on a complex conjugate product between a linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a first multi-carrier symbol of the pair of multi-carrier symbols and a linear combination of received pilot symbols of the at least one group of the plurality of sub-carriers of a second multi-carrier symbol of the pair of multi-carrier symbols.