Covering Underwater Shadow Zones using Acoustic Reconfigurable Intelligent Surfaces

To enhance our exploration and understanding of Earth's oceans, seamless communication coverage across the vast three-dimensional underwater space has long been sought. Unlike terrestrial networks that utilize radio signals, underwater acoustic communications face a significant challenge: nodes located in underwater shadow zones cannot connect to the broader network, even when they are within the Line of Sight (LoS). These shadow zones can extend for tens of kilometers, preventing acoustic signals from propagating and resulting in disconnection of nodes within these areas. Existing solutions primarily focus on deploying nodes outside of shadow zones; however, these approaches fail to guarantee seamless coverage in dynamic ocean environments. This paper aims to address the shadow zone issue fundamentally by leveraging acoustic Reconfigurable Intelligent Surfaces (aRIS) to actively manage the underwater communication channel. Initially, the shadow zones are modeled analytically, and based on this model, optimal aRIS deployment strategies are developed to cover shadow zones in both deep and shallow seas. The initial aRIS design presented in this paper is refined to account for practical engineering constraints, enabling implementation on real-world hardware, with performance validated through pool tests. The shadow zone coverage performance, assessed through Bellhop-based simulations, indicates that without aRIS deployment, coverage is limited to less than 20%, regardless of the increased radiated energy from the initial coverage source. The ocean environment leads to persistent shadow zones, providing coverage only in bending regions. In contrast, optimal aRIS deployment achieves nearly 100% energy coverage across all areas, including previously inaccessible shadow zones, by introducing energy into these regions and utilizing energy-rich areas to enhance coverage in shadow zones.