Adaptive Turbo Equalization for Differential OFDM Systems in Underwater Acoustic Communications

In this correspondence paper, an adaptive turbo equalizer for differential OFDM systems in underwater acoustic communications is proposed. This scheme first reconstructs the demodulation symbols using the soft information fed back from the channel decoder, and then eliminates the residual intercarrier interference (ICI) induced by estimated symbols using an adaptive turbo equalizer. Moreover, a pilot-based differential encoding method is presented to mitigate the impact of the feedback error, thereby improving the convergence of turbo equalization. Theoretical analysis and simulation results demonstrate that, the proposed equalizer outperforms the linear minimum mean-squared error (MMSE) estimation-based turbo equalizer for coherent systems in medium-to-high signal-to-noise ratio (SNR) region, while enjoys significantly reduced computational complexity, and its performance is more robust against the change of Doppler frequency.