Doppler tracking and symbol synchronization method for mobile M-ary spread spectrum underwater acoustic communications

Doppler effect leads to the distortion of the wide-band underwater acoustic (UWA) communication signal. It brings frequency offset to the signal carrier. Simultaneously, the UWA signal length is compressed or broadened. For the UWA communication with a large relative velocity, the influence of Doppler effect is more prominent. This paper presents a method of Doppler tracking and symbol synchronization for movable M-ary spread spectrum (MSS) communication system, which provides a solution for mobile spread spectrum underwater acoustic communication with variable Doppler and large environmental noise. Using a large time bandwidth product of the spread spectrum code carrying information, the Doppler estimation is realized by searching the frequency of M spread spectrum codes within a certain range, and the next symbol is synchronously corrected in the time domain according to the estimation results. This method does not need the re-sampling operation for Doppler compensation. Simulation results show that the proposed method can accurately estimate and track the rapidly changing Doppler, and is suitable for mobile spread spectrum underwater acoustic communication. Compared with the conventional M-ary spread spectrum method, it is more robust.