Bidirectional DFE Turbo Equalization for MIMO Underwater Acoustic Communications

A turbo equalization scheme using bidirectional decision feedback equalizer (BiDFE) is proposed for multiple-input-multiple-output (MIMO) underwater acoustic communications (UAC). Turbo equalization in MIMO communications needs to combat both inter-symbol interference (ISI) due to frequency-selective fading and co-channel interference (CCI) among data streams transmitted by different antennas. The proposed BiDFE turbo equalization (BiDFE-TEQ) receiver structure includes a time reverse mirror, BiDFE and soft-input-soft-output (SISO) mapper/demapper. The BiDFE combines the output of the normal mode and time-reversal mode DFEs and is known to provide improved performance, the inherent diversity between the soft outputs of the two DFEs is described, we propose to select the optimal combining weight coefficient by minimizing the variance of the two DFEs soft outputs, thereby achieving optimal diversity performance in time-varying channels. From the point of view of reducing the length of filters and the number of filters, we embed the time reverse mirror (TRM) technology into the BiDFE-TEQ scheme, in a N × M MIMO system, the residual ISI length is greatly shortened by using the focusing effect of TRM, only N filters are needed instead of N × M filters, so the TEQ computational complexity is greatly reduced. Sea trial data verification results show that BiDFE-TEQ has better performance and can effectively improve the BER performance of MIMO underwater acoustic communications.