DOA tracking in time-varying underwater acoustic communication channels

Underwater acoustic (UWA) communication channels are characterized by spreading of received signals in directions of arrival (DOAs). The DOAs often vary rapidly within small angular intervals, which are usually produced for the most part by moving ocean surface/internal waves and platforms. In this paper, the time-varying UWA communication channels are investigated. Based on the investigation, a beamforming technique that tracks DOAs is proposed, and used for UWA communications with guard-free orthogonal frequency-division multiplexing (OFDM) signal transmission. This beamforming is compared with a beamforming without DOA tracking, and the results show that the receiver with this beamforming outperforms that without DOA tracking. The comparison is based on data from a 14-element non-uniform vertical linear array in a simulation at a distance of 80 km, and in two sea trials at distances of 30 km and 105 km.