Carrier frequency offset estimation using precoded non-fixed pilot signals

1. A method of selecting a carrier frequency offset tracking pilot signal in an access point of a wireless network, the wireless network comprising the access point and a plurality of terminals, the method comprising:

receiving, at a plurality of antennas of the access point, signals from each of the plurality of terminals;

estimating, from the received signals, a spatial channel signature for each of the plurality of terminals;

selecting, for each terminal of the plurality of terminals, the carrier frequency offset tracking pilot signal from a plurality of pre-coded pilot signals, which minimises a cost function that depends on the spatial tracking signature for each of the plurality of terminals and the carrier frequency offset tracking pilot signal; and

transmitting an indication of the selected carrier frequency offset tracking pilot signals to the respective terminal.

2. A method according to claim 1 , wherein the carrier frequency offset tracking pilot signal is selected from a codebook and the indication of the selected carrier frequency offset pilot signal comprises a codebook index.

3. A method according to claim 1 , wherein the indication of the selected carrier frequency offset tracking pilot signals indicates weights for a plurality of subcarriers.

4. A method according to claim 1 , wherein the cost function is the sum over all of the terminals of the errors in the residual carrier frequency offset with a linear estimator.

5. A method according to claim 1 , wherein the cost function is the largest individual error in the carrier frequency offset over all of the terminals.

6. A method according to claim 1 , further comprising receiving a data packet signal including the carrier frequency offset tracking pilot signals from the terminals, and estimating a residual carrier frequency offset from the carrier frequency offset tracking pilot signals.

7. A method according to claim 6 , wherein the data packet signal including the carrier frequency offset tracking pilot signals further comprises a data signal, and the method further comprises:

decoding the data signal using: the spatial channel signatures, the residual carrier frequency offset, and an initial carrier freguencv offset estimated from the received signals.

8. A non-transitory computer readable medium carrying computer program instructions which when executed on a processor in an access point cause the access point to carry out a method of selecting a carrier freguencv offset tracking pilot signal comprising:

receiving, at a plurality of antennas of the access point, signals from each of the plurality of terminals;

estimating, from the received signals, a spatial channel signature for each of the plurality of terminals;

selecting, for each terminal of the plurality of terminals, the carrier freguencv offset tracking pilot signal from a plurality of pre-coded pilot signals, which minimises a cost function that depends on the spatial tracking signature for each of the plurality of terminals and the carrier freguencv offset tracking pilot signal; and

transmitting an indication of the selected carrier freguencv offset tracking pilot signals to the respective terminal.

9. An access point for selecting a carrier frequency offset tracking pilot signal in a wireless network, the wireless network comprising the access point and a plurality of terminals, the access point comprising:

a plurality of antennas configured to receive signals from each of the plurality of terminals;

a spatial channel estimator configured to estimate, from the received signals, a spatial channel signature for each of the plurality of terminals; and

a pilot selector configured to select, for each terminal of the plurality of terminals, the carrier frequency offset tracking pilot signal for that terminal from a plurality of pre-coded pilot signals, which minimises a cost function that depends on the spatial tracking signature for each of the plurality of terminals and the carrier frequency offset tracking pilot signal.

10. An access point according to claim 9 , further comprising storage for a codebook, the codebook comprising the plurality of pre-coded pilot signals.

11. An access point according to claim 9 , further comprising a residual carrier frequency estimator configured to estimate a residual carrier frequency offset from carrier frequency offset tracking pilot signals received by antennas of the plurality of antennas from the terminals.