Exploitation of Spatial Coherence for Reducing the Complexity of Acoustic OFDM Systems

This paper addresses the question as to how to exploit spatial coherence between receiving elements in multichannel multicarrier acoustic communication systems in order to reduce the signal processing complexity without compromising the performance. To answer this question, an adaptive pre-combining method is proposed. Without requiring any a priori knowledge about the spatial distribution of received signals, the method exploits spatial coherence between receive channels by linearly combining them into fewer output channels so as to reduce the number of subsequent channel estimators. The algorithm learns the spatial coherence pattern recursively over the carriers, thus effectively achieving broadband beamforming. The reduced-complexity pre-combining method relies on differentially coherent detection which keeps the receiver complexity at a minimum and requires a very low pilot overhead. Using the experimental data transmitted over a 3-7 km shallow water channel in the 10.5-15.5 kHz acoustic band, we study the system performance in terms of data detection mean squared error (MSE) and show that the receiver equipped with the proposed reduced-complexity pre-combining scheme requires three times fewer channel estimators while achieving the same MSE performance as the full-complexity receiver.