Multipath DOA based MIMO beamforming receiver scheme for high-rate underwater acoustic communications

Underwater acoustic (UWA) channels exhibit a sparse multipath structure, with each path having its corresponding direction of arrival (DOA). Conventional MIMO receivers process received signals hydrophone by hydrophone, disregarding the angle domain information. This paper presents a MIMO beamforming receiver for UWA communications. The proposed method estimates DOAs of multipath signals firstly, then forms directional signals with high signal-to-noise ratio by beamforming. Subsequently, it demodulates MIMO signals for these directional signals, leading to performance improvement. The wideband and high -frequency characteristics of UWA communication signals result in the receiving array element spacing being significantly larger than the signal wavelength. The proposed method adopts frequency -difference (FD) DOA estimation, solving the issue of bearing ambiguity arising from the large element spacing. Moreover, non -overlapping pilots of MIMO-OFDM signals are employed by the proposed method to estimate multipath DOA, resulting in increased resolution for multipath DOA from different transducers. Simulation results demonstrate the outstanding performance of the proposed beamforming receiver under low signal-to-noise ratio (SNR) and multi -interference conditions. The stability of the proposed beamforming receiver is additionally validated in lake trials employing UWA communication signals in the frequency bands of 25 - 31 kHz and 20.5 - 53.5 kHz. The receiver utilizes an 8 -element receiving array with a 0.25 m element spacing.