On the Practical Aspects of Joint Passive Phase Conjugation and Equalization in Underwater Communication Systems

Underwater acoustic communication is suffered from the channel effects which results in time spreading of the transmitted signal. In underwater environment multiple replicas of the transmitted signal are received at the receiver through different paths, which causes significant Inter-Symbol Interference (ISI). Decision Feedback Equalizers (DFE) was utilized to overcome this type of interference in digital communications so many years ago. However, because of the complexity of adaptive equalizers Passive Phase Conjugation (PPC) was widely exploited in underwater communications in the recent years. Because of the poor performance of the PPC method, adaptive equalizers are jointed to improve the PPC performance. In this paper, experimental results conducted in Shiraz Electronic Industrials’ pool are reported. Three different approaches are compared in terms of Symbol Error Rate (SER) versus the number of used channels at the pool: 1) the PPC method; 2) DFE equalizer for only a single channel; 3) the PPC method combined with adaptive DFE. The experimental results showed that the third approach outperforms the others while a simpler receiver is obtained.