Passive time reversal acoustics for horizontal shallow water acoustic communication to mitigate the effect of intersymbol interference

Abstract

Open ocean shallow water channels are characterized to be time varying, dispersive and multipath environment causes severe Inter Symbol Interference (ISI). Passive time reversal has ability to mitigate the effect of ISI with less expensive hardware. This paper presents the experimental results of a passive time reversal experiment conducted in ashallow water environment in Bay of Bengal during 2018. One Acoustic projector with 4 element receiver array was used for this experiment for 3 km and 5 km ranges at 1000 and 2000 bps with the center frequency of 11 kHz. The performance of passive time reversal communication is evaluated for two different approaches such as with passive time reversal technique and equalization combined with passive time reversal. Decision Feedback Equalizer (DFE) with Recursive Least Square (RLS) algorithm is used to remove the residual intersymbol interference (ISI). RLS algorithm in a DFE is used for calculating the tap-coefficient vector in order to minimize the squared equalization error due to input noise and due to channel dynamics in shallow water environment. The BER values of 2 × 10−2 to 1 × 10−1for 3 km range and 2 × 10−1 to 5 × 10−1 for 5 km range are obtained with time reversal and DFE combination as shown in Fig. 12. Time reversal cascaded with RLS DFE equalizer has also reduced scattering in the I/Q plan. Thus the algorithm demonstrates that the performance of shallow water communication improves by combining passive time reversal with an adaptive equalization.