Mobile Underwater Acoustic Communication with Constantly Changing Doppler Effect

Abstract

Recently an underwater acoustic communication (UWA) experiment with a mobile source was conducted in downward-refracting shallow water environment. During this experiment, the source moved towards and then away from the receiving array repeatedly, Doppler effect kept changing in this process due to the time-varying radial velocity between the source and receiving array. In this paper, a three-stage data processing method is proposed in order to effectively decode the experimental data with severe Doppler interference. In the first stage, fractional Fourier transform (FrFT) is used to estimate the delay-Doppler function of the underwater channel based on the linear chirp signal at the front of the received data sequence. Large Doppler effect are coarsely removed according to the Doppler estimation result by resampling and delay estimation is used to determine the number of channel taps. Then the resampled data are demodulated and downsampled to the baseband. In the second stage, data are divided into small blocks and block-based passive time reversal (PTR) is adopted to suppress the mutipath in a block by block manner. Fine Doppler is also compensated in each block by maximum likelihood estimation method. In the third stage, decision feedback equalizer (DFE) is used to remove the residual inter symbol interference (ISI) and Doppler effect. Data proceesing results show that this proposed three-stage equalization method can realize effective trakcing and compensation for time-varying channels. The output signal-to-noise ratio (OSNR) after equalization is greatly improved and the bit error rate (BER) is significantly reduced.