Method of non-uniform doppler compensation for wideband orthogonal frequency division multiplexed signals
Described is a method for compensation of an OFDM signal propagating through Doppler-distorted, time-varying multipath channels. The method is based on low-complexity post-FFT signal processing. Minimum mean square error combining of signals is performed for signals received at spatially-distributed receiver elements using adaptive channel estimation and phase tracking. Doppler shifts are modeled as a consequence of motion-induced time compression and dilation. The Doppler rate is assumed constant over one OFDM block but can vary between OFDM blocks. Thus a non-constant Doppler shift is accommodated by the method. Non-uniform Doppler compensation across subchannels is based on adaptive estimation and prediction of the Doppler rate. A single adaptively estimated parameter is used to track the phases of all the carriers and channel estimates are updated on a block by block basis.
1. A method for Doppler compensation of an orthogonal frequency division multiplexed (OFDM) signal, the method comprising:
determining a predicted phase for each subchannel in a plurality of subchannels for a current OFDM block in response to a phase estimate and a Doppler rate for a previous OFDM block;
determining an angular offset for each of the subchannels over the duration of the previous OFDM block;
generating an estimate of the Doppler rate for the current OFDM block;
determining a final phase estimate for each of the subchannels in response to the estimate of the Doppler rate for the current OFDM block;
generating a data symbol estimate for each of the subchannels in response to a respective one of the final phase estimates; and
deciding a data symbol value for each of the subchannels based on a respective one of the data symbol estimates.
2. The method of claim 1 wherein determining the angular offset for each of the subchannels comprises:
determining a preliminary estimate of a data symbol for the subchannel from a current signal vector and the predicted phase for the subchannel; and
generating a tentative data symbol decision in response to the preliminary estimate of the data symbol for the subchannel.
3. The method of claim 2 wherein the preliminary estimate of the data symbol for the subchannel is determined from the current signal vector, the predicted phase for the subchannel and an estimate of a channel vector for the previous OFDM block.
4. The method of claim 1 wherein generating the estimate of the Doppler rate for the current OFDM block comprises generating a mean Doppler rate for at least two subchannels.
5. The method of claim 4 wherein the mean Doppler rate is determined for at least one pilot channel.
6. The method of claim 4 wherein the mean Doppler rate is first determined for at least one pilot channel and subsequently determined for a plurality of subchannels.
7. The method of claim 2 wherein the current signal vector is determined from a spatially-distributed sampling of the OFDM signal.
8. The method of claim 1 wherein the OFDM signal is an acoustic signal.
9. The method of claim 1 wherein the OFDM signal is a phase-shift keying modulated signal.
10. A receiver for detection of an orthogonal frequency division multiplexed (OFDM) signal, comprising:
an array of spatially-distributed receiver elements;
a plurality of Fast Fourier Transform (FFT) demodulators each in communication with a respective one of the spatially-distributed receiver elements;
an adaptive phase tracker in communication with the FFT demodulators and configured to track the phases of a plurality of subchannels of the OFDM signal for non-uniform Doppler compensation of the subchannels based on a phase estimate for each of the subchannels, each phase estimate being determined from a predicted phase for each subchannel for a current OFDM block in response to a phase estimate and a Doppler rate for a previous OFDM block, an angular offset for each of the subchannels over the duration of the previous OFDM block and an estimate of the Doppler rate for the current OFDM block; and
a minimum mean square error (MMSE) combiner in communication with the adaptive phase tracker to perform adaptive channel estimation.
11. The receiver of claim 10 wherein the OFDM signal is an acoustic signal.
12. The receiver of claim 11 wherein the receiver elements are acoustic receiver elements.