OFDM pilot tone tracking for wireless LAN
First Claim
1. A pilot phase error metric for an orthogonal frequency division multiplexed (OFDM) receiver comprising:
 a discrete Fourier transform portion for receiving an incoming signal corresponding to an OFDM waveform, wherein the discrete Fourier transform portion outputs complex signal measurements corresponding to each of a plurality of pilots of a preamble portion of the OFDM waveform and complex signal measurements corresponding to each of a plurality of pilots of a subsequent OFDM symbol of the OFDM waveform, wherein the discrete Fourier transform portion is separate from a fast Fourier transform operation of the OFDM receiver;
a maximum likelihood phase error/weighting processor coupled to the discrete Fourier transform portion for processing the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol in comparison to pilot reference points; and
a phase error estimator coupled to the maximum likelihood phase error/weighting processor for estimating a phase error of the subsequent OFDM symbol relative to a phase corresponding to the preamble portion based on the processed complex signal measurements and the pilot reference points.
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Abstract
A pilot phase tracking loop for an OFDM receiver including a phase rotator receiving an incoming signal, a fast Fourier transform coupled to a phase rotator output, and a pilot phase error metric including a discrete Fourier transform portion coupled to the phase rotator output. The pilot phase error metric determines a phase error estimate associated with a received OFDM symbol, e.g., a data symbol, from the phase rotator output. A loop filter is coupled to the pilot phase error metric output and an oscillator is coupled to the loop filter output. The oscillator output is coupled to the phase rotator to adjust the phase of subsequent OFDM symbols of the incoming signal. Phase noise introduced by a radio portion of the OFDM receiver and OFDM transmitter is reduced by the baseband portion of the OFDM receiver improving OFDM signal tracking under poor SNR conditions.
132 Citations
29 Claims

1. A pilot phase error metric for an orthogonal frequency division multiplexed (OFDM) receiver comprising:

a discrete Fourier transform portion for receiving an incoming signal corresponding to an OFDM waveform, wherein the discrete Fourier transform portion outputs complex signal measurements corresponding to each of a plurality of pilots of a preamble portion of the OFDM waveform and complex signal measurements corresponding to each of a plurality of pilots of a subsequent OFDM symbol of the OFDM waveform, wherein the discrete Fourier transform portion is separate from a fast Fourier transform operation of the OFDM receiver;
a maximum likelihood phase error/weighting processor coupled to the discrete Fourier transform portion for processing the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol in comparison to pilot reference points; and
a phase error estimator coupled to the maximum likelihood phase error/weighting processor for estimating a phase error of the subsequent OFDM symbol relative to a phase corresponding to the preamble portion based on the processed complex signal measurements and the pilot reference points.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)


12. A method of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform;
processing, in a parallel path to the determining step, the OFDM preamble waveform with a fast Fourier transform;
determining a phase error estimate of a subsequent OFDM symbol relative to the pilot reference points; and
processing, in the parallel path to the determining step, the subsequent OFDM symbol with the fast Fourier transform;
wherein the determining the phase error estimate step is completed prior to the completion of the processing the subsequent OFDM symbol with the fast Fourier transform in the parallel path.  View Dependent Claims (13, 14, 15)


16. A method of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver comprising:

determining, in a separate processing path parallel to a fast Fourier transform processing path of the OFDM receiver, pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform;
estimating, in the separate processing path, a phase error of a subsequent OFDM symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol and the pilot reference points, the estimating the phase error for the purpose of phase tracking; and
rotating a phase of OFDM symbols occurring after the subsequent OFDM symbol according to the phase error.  View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29)
where {circumflex over (θ
)}_{m }is an aggregate phase error for the subsequent OFDM symbol having a time index m, wherein u_{k }and v_{k }represent inphase (I) and quadrature (Q) values, respectively, for the pilot reference points for n pilots of the OFDM preamble waveform, and where I_{k,m }and Q_{k,m }represent the complex signal measurements corresponding to the k^{th }pilot of the m^{th }subsequent OFDM symbol. 
1 Specification