The study of spatial diversity and combination technology for phase-coherent underwater acoustic communication

The UWA channel is characterized as a time-dispersive rapidly fading channel, which in addition exhibits Doppler instabilities. To eliminate inter-symbol interference caused by multipath propagation, spatial diversity equalization is the main technical means. This paper mainly uses the method of complex weighted coefficient to achieve high SNR signal and maximize array gain. This paper uses the passive-phase-conjugation (PPC) algorithm to calculate signal-to-noise ratio(SNR) and signal-to-interference ratio(SIR) with impulse response function and communication signal. This paper firstly calculates the signal-to-noise-plus-interference ratio (SNIR) and uses the Sigmoid function to optimize the weighted coefficients of each channel. Secondly, the optimized weighted coefficients need to be normalized. It implements space and time joint equalization. Thirdly, by introducing the second order phase locked loop, it can track signal phase change caused by UWA channel. Lastly, the paper adopts fractional-decision feedback diversity equalizer(FDFDE). It achieves diversity equalization by using different channel weighted coefficients. The simulation and lake trial data processing results show that, the optimized diversity receiving equalization algorithm can improve communication system's ability in tracking the change of underwater acoustic channel, offset the impact of multipath and noise, and improve the performance of communication system. The performance of the communication receiving system is better than that of the combination with equal gain. At the same time, the bit error rate (BER) reduces for 1.7%.