Low-Complexity MMSE Iterative Equalization for Multiband OFDM Systems in Underwater Acoustic Channels

Underwater acoustic communication systems use a bandwidth that is comparable to the center frequency of the signal and qualify as broadband in a relative sense. And underwater acoustic channels are time-varying and multipath channels, giving rise to serious inter-symbol interference (ISI), inter-carrier interference (ICI) and time-domain telescopic changes in the underwater acoustic OFDM system. In this paper, we use a multiband OFDM system by dividing the available frequency band of the underwater acoustic signal into multiple sub-bands, and the channel in each sub-band can be regarded as the time-varying channel of Doppler shift. Then minimum mean square error (MMSE) iterative equalization based on soft information is adapted to counteract the ICI. At the same time, we exploit the banded structure of the frequency-domain channel matrix, as well as a receiver time-domain window that enforce this banded structure. This method leads to a considerable reduction in the equalization complexity while get a better bit error rate (BER) performance.