Method and apparatus for mitigating the residual CFO effect on channel estimation for OFDM receivers

1. A carrier frequency offset (CFO) tracking loop module, comprising:

a numerically-controlled oscillator (NCO) that calculates an NCO phase output;

a CFO phase error block that generates a corrected NCO phase output by subtracting half of a residual CFO phase error from the NCO phase output;

a pilot processor that generates a phase error by averaging phases of pilot subcarriers;

a multiplier that corrects the pilot subcarrier's phase by multiplying complex values of the pilot subcarriers with a conjugated complex value of the corrected NCO phase output; and

a loop filter that filters the phase error from the pilot processor and updates the NCO with the filtered phase error.

2. The module of claim 1 , wherein a CFO phase error is estimated using a long sequence, the channel estimation sequence, and the pilot subcarriers, and

wherein the residual CFO phase error is estimated using a channel estimation sequence and before a frame synchronization is performed.

3. The module of claim 1 , further comprising an equalizer that performs channel estimation by removal of a CFO phase error before other estimation steps.

4. The module of claim 1 , further comprising an equalizer that de-rotates channel estimation coefficients based on the residual CFO phase error.

5. A carrier frequency offset (CFO) tracking loop module, comprising:

a numerically-controlled oscillator (NCO) that calculates a phase output;

a CFO phase error block that generates a corrected NCO phase output by subtracting half of a residual CFO phase error from the NCO phase output;

a pilot processor that generates a phase error by averaging phases over a plurality of pilot subcarriers;

a multiplier that corrects the phase of the pilot subcarriers by multiplying a plurality of complex values of the pilot subcarriers by a conjugated complex value of the corrected NCO phase output;

a loop filter that filters the phase error from the pilot processor and updates the NCO with the filtered phase error; and

an equalizer that de-rotates channel estimation coefficients based on a CFO phase error;

wherein the de-rotated channel estimation coefficients (X 2,k c ) are given by

X 2,k c ≦e −j·φ ·X 2,k ,

where φ is the CFO phase error in radians, j is the imaginary unit, e is the base of the natural logarithm, and ·X 2,k are the received channel estimation samples.

6. The module of claim 1 , further comprising a plurality of subcarriers;

wherein a first subcarrier having a higher Signal to Noise Ratio (SNR) than a second subcarrier is nominated to be a pilot subcarrier.

7. The module of claim 6 , wherein a transmit data of the first subcarrier is obtained at a receiver via a slicer operation.

8. The module of claim 1 , wherein the residual CFO phase error is estimated from two channel estimation symbols.

9. A method, comprising:

calculating a phase output;

estimating a residual carrier frequency offset (CFO) phase error using a channel estimation sequence;

performing a frame synchronization after estimating the residual CFO phase error;

generating a phase error by averaging a plurality of phases of a plurality of pilot subcarriers;

generating a corrected phase output by subtracting half of the residual CFO phase error from the phase output;

correcting the phase of the pilot subcarriers by multiplying a plurality of complex values of the pilot subcarriers by a conjugated complex value of the corrected phase output;

filtering the phase error from the pilot processor; and

updating a numerically-controlled oscillator (NCO) with the filtered phase error.

10. The method of claim 9 , wherein the phase error is estimated using a long sequence, the channel estimation sequence and the pilot sub carriers.

11. The method of claim 9 , further comprising performing channel estimation by removal of the phase error before other estimation steps such that the subtracting is not performed.

12. The method of claim 9 , further comprising de-rotating channel estimation coefficients based on the phase error such that the subtracting is not performed.

13. The method of claim 12 , wherein the de-rotated channel estimation coefficients (X 2,k c ) are given by

X 2,k c =e −j·φ ·X 2,k ,

where φ is the phase error in radians, j is the imaginary unit, e is the base of the natural logarithm, and ·X 2 k are the received channel estimation samples.

14. The method of claim 9 , further comprising nominating a first subcarrier having higher a Signal to Noise Ratio (SNR) than a second subcarrier to be a pilot subcarrier.

15. The method of claim 14 , further comprising slicing, at a receiver, the first subcarrier to generate a transmit data.

16. The method of claim 9 , wherein the residual CFO phase error is estimated from two channel estimation symbols.