MIMO OFDM over underwater acoustic channels

MIMO OFDM communication is considered for spatial multiplexing of independent data streams over band-limited, frequency-selective underwater acoustic channels. Long acoustic multipath, however, limits the applicability of MIMO channel estimation methods that require inversion of a matrix whose size is proportional to both the number of transmit elements and the multipath spread. To overcome this problem, an adaptive algorithm is used that does not require matrix inversion and operates in a decision-directed manner, thus reducing both the computational complexity and the overhead. The algorithm makes use of a phase synchronization method to compensate for the non-uniform Doppler shifting in a wideband acoustic system, and exploits the remaining temporal coherence. System performance is successfully demonstrated using real data transmitted over 1 km in shallow water, with a varying number of carriers (128-1024), transmitters (1-3), and modulation levels (4 and 8 PSK) in the 8-18 kHz band.