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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.

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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.

The inherent attenuation of a homogeneous viscous medium limits radiation propagation, thereby restricting the use of many high-frequency acoustic devices to only short-range applications. Here, we design and experimentally demonstrate an acoustic metamaterial localization cavity which is used for sound pressure level (SPL) gain using double coiled up space like structures thereby increasing the range of detection. This unique behavior occurs within a subwavelength cavity that is 1/10th of the wavelength of the incident acoustic wave, which provides up to a 13 dB SPL gain. We show that the amplification results from the Fabry-Perot resonance of the cavity, which has a simultaneously high effective refractive index and effective impedance. We also experimentally verify the SPL amplification in an underwater environment at higher frequencies using a sample with an identical unit cell size. The versatile scalability of the design shows promising applications in many areas, especially in acoustic imaging and underwater communication.