Spatial diversity in multichannel processing for underwater acoustic communications

This paper discusses spatial diversity for underwater communications and presents a passive-phase conjugation (PPC) based multichannel equalizer. Multichannel processing involves taking advantage of multiple receivers distributed in space to take advantage of spatial diversity. The multichannel decision feedback equalizer (DFE) uses multiple nonlinear adaptive filters to remove intersymbol interference (ISI), and its complexity increases with time spread plus the number of receiving channels. Passive time reversal realized by the passive-phase conjugation (PPC) approach refocuses time delayed arrivals at the receiving array, where spatial diversity is obtained by the multichannel combining. The property of refocusing can be used to mitigate ISI for underwater communications, and only one single channel equalizer is required to remove residual ISI. PPC achieves pulse compression for time delayed arrivals, where the number of taps is significantly reduced for a PPC based equalizer. Based on temporal diversity obtained by pulse compression, the proposed structure improves the performance of time reversal communications by adaptive diversity combining. Three receiver structures are evaluated by processing real data collected in an experiment, which was conducted in a range-dependent acoustic channel over a range of 4 km. Results have demonstrated superior performance of the proposed receiver structure.