Underwater acoustic channels are wideband in nature due to the fact that the signal bandwidth is not negligible with respect to the center frequency. OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. In this paper, we propose a two-step approach to mitigating the frequency-dependent Doppler drifts in zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem; and (2) high-resolution uniform compensation on the residual Doppler. Based on block-by-block processing, our receiver does not rely on channel dependence across OFDM blocks, and is thus desirable for fast-varying UWA channels. We test our receiver with data from a shallow water experiment at Buzzards Bay, Massachusetts. Our receiver achieves excellent performance even when the transmitter and the receivers have a relative speed up to 10 knots, where the Doppler drifts are several times larger than the OFDM subcarrier spacing.
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