Hybrid Time-Frequency Domain Turbo Equalization for Single Carrier MIMO Underwater Acoustic Communication

Abstract

Shallow seawater acoustic channel has serious multipath effect. We proposed hybrid time-frequency domain Turbo equalization (HTFDTE) and bidirectional time-frequency domain Turbo equalization (Bi-HTFDTE) for multiple-input multiple-output (MIMO) single carrier (SC) underwater acoustic system with PN code to improve the quality of underwater acoustic communication. At the receiving end, the channel is estimated using the known PN code. The frequency domain equalization with minimum mean square error (MMSE) criterion and the time domain Turbo iterative processing are combined together to further improve performance. To eliminate error propagation effect, the Bi-HTFDTE includes two parallel HTFDTE structures, which requires roughly twice as many computational operations per iteration as the HTFDTE but achieving a lower bit error rate (BER). From the simulation results, we can find that the BER performance of HTFDTE is better than frequency domain Turbo equalization (FDTE). At the order of 10−4, the HTFDTE with three iterations gains about 0.5dB and 1dB over the HTFDTE with first iteration and the FDTE with first iteration for QPSK modulation, respectively. The Bi-HTFDTE with three iterations gains about 0.15dB over the HTFDTE with three iterations.