Transmit Beamforming for Underwater Acoustic OFDM Systems

This article addresses the problem of transmit beamforming for underwater acoustic communication systems within the framework of multicarrier signaling based on orthogonal frequency division multiplexing (OFDM). The system consists of a transmitter equipped with a uniform linear array and a single receiver. Transmit beamforming requires the transmitter to have complete knowledge of the channel to the receiver; however, this assumption is often not justified in acoustic channels with long feedback delays. To counteract this problem, we propose a technique that targets only those features of the channel that can withstand the feedback delay. One such feature is the angle of arrival of the principal propagation path, which does not experience rapid variations caused by surface scattering, and is thus, varying sufficiently slowly that it can tolerate long feedback delays. OFDM provides an ideal platform for implementing broadband beamforming, and we study the system performance in terms of the data detection mean squared error (MSE) and bit error rate (BER), using synthetic data transmitted over a 1 km shallow water channel in the 10-15 kHz acoustic band. Specifically, we show that beamforming in the principal path's direction achieves excellent MSE performance, with only a few dB degradation with respect to optimal beamforming. We present results for different receive-side detection methods, namely, differentially coherent detection and coherent detection. In addition, we propose an angle tracking algorithm to reduce the complexity in mobile systems, and we demonstrate the system performance using an over-the-air acoustic communications testbed.