Time-variant Adaptive Passive Time Reversal Equaliser and a Perspective for Environmental Focusing Method

High digital data throughput in Underwater Acoustic Communications (UAComm) is a challenging subject, specially in shallow water where the channel is a wave-guide causing multipath propagation and where Doppler effect usually occurs due to relative source-receiver motion jointly to ocean dynamics. The source and receiver sensors can be used for telemetry in point-to-point underwater communications or as nodes of an underwater acoustic network within the scope of oceanic research observatory or offshore activities. However, channel tracking is required for reliable digital underwater communications between the sensors, which is a hard task due to the complicated propagation of acoustic waves in the ocean. Equalisation is often required to perform a compensation method aiming to overcome the inter-symbol interference (ISI) caused by multipath propagation. The motivation of this work is to propose a compensation method deploying the adaptive passive time-reversal (ApTR) equaliser, aiming to perform ISI mitigation jointly to Doppler compensation in time-variant channels. The benefit given by ApTR processing would be the performance improvement in underwater communications between an active sensor and a vertical line array of receiver sensors, relying in well-succeed time-variant channel impulse response estimation. Furthermore, this position paper discusses the perspective of use an environmental focusing method for channel estimation within the ApTR equaliser, based on the idea that a set of oceanic acoustic physical parameters – which are generally estimated in low-frequency matched field processing problems like geoacoustic assessment, ocean tomography and source localization – could be conveniently used for channel compensation in high frequency underwater communications using a carefully chosen search space of replicas. The results are two fold: in one hand the equalisation shall improve the UAComm system, and in the other hand, the best match of channel parameters consequently yields a refined local environmental assessment.