Simulation of an Underwater Acoustic Communication Channel Characterized by Wind-Generated Surface Waves and Bubbles

Sea-surface scattering by wind-generated waves and bubbles is regarded to be the main nonplatform related cause of the time variability of shallow acoustic communication channels. Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modeling of these dynamic sea-surface effects. For frequencies in the range of 1-4 kHz, there is an important effect of bubbles on sea-surface reflection loss due to refraction, which can be modeled with a modified sound-speed profile (SSP) accounting for the bubble void fraction in the surface layer. The bubble cloud then acts as an acoustic lens, enhancing the rough-surface scattering by the resulting upward refraction. It is shown here that, for frequencies in the considered range of 4-8 kHz, bubble extinction, including both the effects of bubble scattering and absorption, provides a significant additional contribution to the surface loss. Model-based channel simulations are performed by applying a ray tracer, together with a toolbox for generation of rough sea-surface evolutions. This practical simulation framework is demonstrated to provide realistic results for both stationary and mobile communication nodes by capturing specific features observed in experiments, such as time variability, fading reverberation tails, and wind-speed dependence of the Doppler power spectrum.