Adaptive linear turbo equalization of large delay spread time-varying channel responses

In this paper, we investigate applying linear turbo equalization techniques to underwater acoustic communications. First, we elaborate on two popular linear turbo equalizers (TEQ), a channel estimate-based minimum mean square error TEQ (CE-based MMSE-TEQ) and a direct-adaptive TEQ (DA-TEQ). We compare the behavior of both TEQ approaches in the presence of channel estimation errors and adaptation filter adjustment errors. By analyzing extrinsic information transfer (EXIT) charts, we confirm that the performance gap between these two TEQs is small after convergence. Next, we introduce an underwater receiver architecture based on the LMS DA-TEQ technique that dramatically improves the performance of the conventional decision-feedback equalizer at a feasible complexity. This receiver architecture is demonstrated using data collected from the SPACE 08 experiment. The experimental results demonstrate that the LMS DA-TEQ yields more than an order of magnitude performance gain over the conventional equalizer.