Effective Doppler compensation for underwater acoustic communications under relative acceleration

The performance of coherent acoustic communication system is adversely affected by Doppler shift resulting from relative motion of the transmitter and receiver. Existing methods of Doppler compensation^[1-5] are, in our opinion, deficient in effectiveness under relative acceleration. We now present a method believed to be effective for Doppler compensation for underwater acoustic communications. In the full paper, we explain in detail our effective method; in this abstract, we just add some pertinent remarks to listing the two topics of explanation: (1) Doppler shift in broadband communication and (2) the method of Doppler compensation under relative acceleration; in topic 2, with the help of Fig.1 in the full paper, we point out that LFM (linear frequency modulation) chirp pilot is fed to a group of matched filters (MFs) corresponding to different Doppler shift values and that the frequency of the MF with maximum value is chosen as the output of estimated Doppler shift frequency; also in topic 2, with the help of Fig.2, we explain how to calculate the interpolating factor of each symbol using Doppler shift; still in topic 2, with the help of Fig.3, which shows the structure of the receiver, we explain how to use a DFE (decision feedback equalizer) with phase-locked loop (PLL) to suppress intersymbol interference (ISI) and residual Doppler errors. We performed Monte Carlo simulations on medium-range BPSK system and short-range QPSK system respectively. Simulation results are given in Figs.5 and 6 for BPSK system and in Figs.7 and 8 for QPSK system. Fig.7 shows that, when the chirp signal is longer than ISI, the unusual phenomenon of the occurrence of more than one peak in the output of a MF was observed. Figs.6 and 8 show preliminarily that, when Doppler shift difference of a data frame is higher than 0.02%, the conventional method, which uses the same interpolating factor on the whole frame of symbols, causes high bit error rate, while we can get higher receiver reliability by using the method proposed in this paper.