A study of multiplicity and diversity in MIMO underwater acoustic communications

Multiple-input/multiple-output (MIMO) communications can significantly increase the data rate using transmitter/receiver diversity in theory. However, experimental demonstrations of the increase of data rate in MIMO underwater acoustic communications so far were limited to a (small) fraction of what is theoretically possible, beyond that the bit error rate (BER) becomes unacceptably high due to severe inter-symbol and co-channel interference. This raises the question whether the underwater acoustic channel can support high multiplicity (increase in data rate) and at the same time provide enough diversity to minimize the BER. We demonstrate in this paper, based on at sea data, that the multiplicity of an 8×8 MIMO system is indeed 8, using a direct-sequence code-division-multiple-access (DS-CDMA) communication scheme. Data from all eight sources are all received with minimum BER using code orthogonality to remove the co-channel interferences. This implies that a base station with eight sources/receivers can communicate to eight different users simultaneously using CDMA signaling.