Bit-error rate gradient descent Doppler estimation for underwater acoustic OFDM communication

Abstract With its robustness to multipath spread the Orthogonal frequency division multiplexing (OFDM) has drawn rapidly increasing interests as a high data rate underwater acoustic (UWA) communication solution, which is unfortunately subject to significant performance degradation caused by Doppler. Generally Doppler can be estimated by searching the best match between the received packet and the standard copy in time domain or frequency domain at signal-level. However, under dynamic or diffuse multipath in adverse UWA channel there may exist mismatch between the signal-level Doppler estimation and bit-level demodulation. To address this problem, a bit-error-rate (BER) based Doppler estimation strategy has previously been derived by searching the optimum BER among Doppler grids at the expense of high complexity caused by exhausting search. In this paper by adopting BER gradient descent strategy an iterative Doppler estimation algorithm is proposed to substantially reduce the BER search complexity. Moreover, to improve convergence rate as well as to avoid being trapped at local optimums, smoothed gradient and variable Doppler increment are adopted in the iterations. Finally the numerical simulation as well as sea trial result is provided to verify the effectiveness and superiority of the proposed BER gradient descent Doppler estimation algorithm.