Multichannel combining and equalization for underwater acoustic MIMO channels

In order to achieve high data rate digital communications, multiple-input/multiple-output (MIMO) techniques have attracted growing interests in the underwater acoustic communication studies. In this paper, multichannel combining and decision feedback equalization (MCC/DFE) has been proposed for underwater acoustic MIMO channels. In order to overcome the difficulties introduced by the fast fluctuating channel, Doppler tracking and frequent channel estimation are performed. Then time reversal combining followed by a single channel DFE is used to demodulate individual symbol sequences transmitted by the multiple element source. To improve the performance, successive interference cancellation is also incorporated into the receiver structure. Using data from the Makai experiment conducted around Kauai Island, HI, 2005, we have shown that the achievable data rate can be increased up to 4 times using the same bandwidth as single source systems. For example, 32 kilobits/s could have been achieved by simultaneous transmission of four 4 kilosymbols/s 4-phase shift keying (QPSK) symbol sequences when both the source and the receiver were drifting at a 2 km range in the ocean.