Iterative multi-channel FH-MFSK reception in underwater acoustic communication

Abstract

With the challenges of long multipath, large Doppler spreads, high ambient noise and rapid phase fluctuation in mobile shallow underwater acoustic (UWA) channels, design of robust UWA communication system should be carefully considered. Aiming at this topic, a convolutionally encoded noncoherent frequency-hopped M-ary frequency-shift-keyed (FH-MFSK) modulation scheme with a novel iterative multi-channel reception is proposed. To obtain the reception suitable for noncoherent MFSK, the ideas of iterative multi-channel demodulation and decoding are combined and extended. A modified multi-channel soft input soft output (SISO) demodulator based on maximum a posterior (MAP) criterion is derived, and substantial gains of several decibels in power efficiency are achieved. Extra gain can be obtained by utilizing time diversity with long multi-path delay. Simulation shows that a noncoherent receiver equipped with 5 arrays obtains around 5.5 dB gain over single noniterative receiver. A shallow water field testing at Songhua Lake with 500 m distance verifies the robustness and usability of the proposed receiver.