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Tracking time-varying correlated underwater acoustic channels in the signal subspace

Multipath arrivals in many underwater acoustic channels are often cross-correlated and as a result, the path cross-correlation matrix shows a smaller rank than the number of multipath delay taps. Channel tracking error, measured in terms of the signal prediction error, can be significantly reduced by tracking the signal components in the signal subspace as previously demonstrated for a slowly varying channel in which the signal basis vectors can be assumed to be time-invariant. For a rapidly time-varying channel, one needs to track the time variation of the signal basis vectors. A subspace tracker is used in this paper based on a coarse estimate of the channel impulse response (CIR). The channel amplitudes (components) are tracked using a recursive least squares method. Performance of the proposed algorithms is demonstrated with real sea data and compared with that obtained with conventional approaches. The results demonstrate the importance of tracking the channel basis vectors as well as the channel components.