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Time-variant sea surface scattering and Doppler spreading due to source/receiver motion cause the signal fading stochastically in amplitude and phase fluctuation. Consequently, the performance of underwater acoustic communication systems are degraded, and high-speed digital communication is disrupted. In this study, a channel simulator for flat fading over time-variant sea surface scattering and Doppler spread in source/receiver motion is presented. Rayleigh and Rice fading models are adopted and their distributions are compared with the distribution measured in a water tank. The bit error rate of binary phase shift keying (BPSK) and binary frequency shift keying (BFSK) for flat fading is examined to evaluate the simulator performance.

In underwater acoustic communication, the transmitted signal is affected by multipath fading and background noise. Therefore, the intersymbol interference (ISI) of underwater acoustic communication systems depends on the effects of these parameters. To cope with ISI due to multipath fading, various equalizers and orthogonal frequency division multiplexing systems have been developed. Forward error correction coding is also adopted to reduce the error by background noise. Here, the performances of convolution code (CC) and Reed–Solomon (RS) code are evaluated in a multipath fading channel. The CC shows better performance in frequency nonselective fading but the RS code shows better performance in frequency selective fading.

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The results of an experiment conducted to understand the frequency dependence of acoustic propagation in very shallow water using a ship as an acoustic source are presented and the possible causes of the features appearing in the results are discussed. The result shows large propagation losses at a frequency of approximately 140 Hz. The results of numerical simulation including the shear property and layered structure of the bottom imply that the features might be caused by the shear property of the lower sediment that lies below the thin upper fluid sediment.