Path-Specific Beamforming and Nonuniform Doppler Compensation for Underwater Acoustic Communications

This article introduces methods for path-specific beamforming and adaptive compensation of nonuniform Doppler drifts in single-carrier broadband underwater acoustic communications. We focus on scenarios where different propagation paths experience varying Doppler drifts, which can significantly degrade conventional receivers. The proposed method employs broadband beamforming to separate the multipath components. Each path is then individually time-aligned and Doppler-synchronized before being processed by a multichannel fractionally spaced decision-feedback equalizer (DFE). By time-aligning the signals before equalization, the DFE filters need only cover the dispersion of each isolated path rather than the entire multipath spread, which leads to an overall reduction in the equalizer size. To account for time compression and dilation, we incorporate a delay-locked loop alongside a phase-locked loop to perform adaptive online resampling or delay tracking, thus eliminating the need for front-end resampling even in high-Doppler scenarios. Simulation and experimental results demonstrate that the proposed approach operates successfully under conditions that cause conventional receivers to fail. In cases where the conventional receiver performs well, our proposed algorithm achieves comparable performance with significantly fewer training symbols, yielding excellent performance in terms of data detection mean squared error and bit error rate.