Multipath of sound propagation in shallow water and underwater spread spectrum communication

Abstract

Due to the influence of the sea bottom and the sound speed profile, the propagation of sound in shallow water appears to be a complicated multipath. The time spread and the amplitude fluctuation of different arriving paths cause strong fading and intersymbol interference (ISI). At the receiving point the multipath signal can be expressed as r(t)=A0s(t−τ0)+∑i=1N-1Ais(t−τi)+n(t), where the first term is arriving signal directly, the second term is arriving signals by different paths, the third term is noise. When the difference of time delays is over the duration of code chip, a strong ISI occurs. As we know, the pseudo-random signal has high resolution in both time and frequency domains. It is often used in spread spectrum communication. In this paper a diversity technique with spread spectrum is used for realizing coherent accumulation of the arriving paths. The results of the lake experiment and the sea experiment are shown in this paper. The efficiency of coherent accumulation is obvious.