Cross power spectral density based beamforming for underwater acoustic communications

Abstract

In underwater acoustic (UWA) communications, beamforming is often used to improve the detection performance of a receiver. For beamforming, there have been methods presented in time domain, e.g. fractional delay (FD) method, and in frequency domain, e.g. time-frequency-time with cross spectral density matrix (TFT-CSDM) method. The former brings accurate direction of arrival (DOA) estimation but with high complexity and is vulnerable to noise; while the latter brings less accuracy but with lower complexity. In this paper, we propose and investigate a time-frequency-time with cross power spectral density (TFT-CPSD) beamforming method for a vertical linear array (VLA) of hydrophones. The proposed method is compared with the FD and the TFT-CSDM methods in a receiver designed for guard-free orthogonal frequency-division multiplexing (OFDM) with superimposed data and pilot signals. The comparison is based on data obtained in sea trials at distances 30–50 km in the northwest Pacific Ocean. The results demonstrate that the proposed TFT-CPSD method possesses higher accuracy than the TFT-CSDM method, and lower complexity than the FD method. Besides, the OFDM receiver with the TFT-CPSD beamforming outperforms a receiver with the TFT-CSDM beamforming and the FD beamforming at signal to noise ratio (SNR) −14 to 14 dB. The proposed beamforming technique possesses the merits of energy conservative and energy leakage reduction, which can also be applied to single-carrier transmission.