Frequency Domain Turbo Equalization with Iterative Channel Estimation for Single Carrier MIMO Underwater Acoustic Communications

In this paper, we propose a low-complexity single- carrier frequency-domain iterative receiver for triply-selective underwater acoustic (UWA) fading channels, which combines frequency domain decision feedback equalization (FD-DFE) with iterative channel estimation. Due to long multipath channels, frequency domain turbo equalization has to use large block size to achieve low computational complexity and high data efficiency, but UWA channel coherence time is often much shorter than block length. We utilize pilots or soft decision symbols obtained at the previous iteration to re-estimate channel at each turbo iteration, thus achieving satisfactory performance. Although having slightly inferior bit error performance to time-domain turbo equalizers, the proposed FDE scheme reduces the complexity by three orders of magnitude, making it affordable for real-time implementation. The proposed iterative receiver has been verified through both numerical simulations and undersea experiment data collected in the Surface Processes and Acoustic Communications Experiment 2008 (SPACE08).