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A pilot-aided Doppler estimator for underwater acoustic channels

The paper presents a practical Doppler estimator to be used in underwater acoustic (UWA) channels. The estimation is performed using OFDM pilot signals. The OFDM pilot signals are passed through a Doppler channel that adds the above effects to the signal. The receiver then performs Doppler estimation on the signal. The estimator finds an estimate for the Doppler scaling factor (also called the mach number) using the centroid of the energy spectral density (ESD) of the received signal. A suitable domain (“window of estimation”) for each pilot is chosen to avoid pilot ambiguity resulting from large Doppler shifts. This leads to a necessary condition for the estimator's accuracy that must be satisfied by the designer. After estimating the mach number, Doppler compensation is performed in a two-step process. First the received signal is resampled using the mach number estimate to undo the signal dilation/compression. The second step is to compensate for the residual Doppler shift that persists by back-shifting the frequency spectrum of the pilots. Due to the high accuracy of the estimator, the pilot positions are recovered exactly. A Simulink model that uses the proposed estimator and compensator is presented, and the results show how the estimator performs in a typical underwater channel.