Experimental characterization of depth-dependent acoustic channels across the deep-ocean main thermocline

The deep-ocean main thermocline is typically located near the axis of the deep sound channel and exhibits a strong sound-speed gradient, with its vertical stratification structure exerting a significant impact on underwater acoustic communication channels. In this study, based on a vertical line array experiment conducted in the South China Sea, the depth-dependent characteristics of the underwater acoustic channel were systematically investigated through observations within the core region of the main thermocline. Contrary to the conventional assumption that channel characteristics vary monotonically with depth, experimental results reveal that the maximum multipath complexity and the strongest temporal decorrelation occur in the convergence region beneath the main thermocline, rather than at the bottom of the thermocline. Furthermore, the results demonstrate that the classical inverse relationship between coherence bandwidth and delay spread becomes invalid within the thermocline. Based on these physical-layer observations, we further translate the channel characteristics into communication system design guidelines and quantitatively analyze the receiver parameter requirements at different depths within the main thermocline. These findings provide a measurement-based foundation for environment-aware and adaptive underwater acoustic communication in resource-constrained deep-ocean systems.