Acoustic RIS for Massive Spatial Multiplexing: Unleashing Degrees of Freedom and Capacity in Underwater Communications

TL;DR

This paper introduces acoustic Reconfigurable Intelligent Surfaces (aRIS) to significantly improve underwater acoustic communication capacity by actively creating virtual paths, enhancing spatial degrees of freedom, and deploying adaptive beamforming with autonomous underwater vehicles.

Contribution

It proposes a novel aRIS architecture and the Light-Point concept to maximize spatial DoFs, along with an adaptive beam control mechanism for underwater environments.

Findings

Channel capacity increased by up to 265% in shallow-sea scenarios.

Deployment of aRIS at the Light-Point optimizes DoFs.

Active beamforming with UUVs enhances communication robustness.

Abstract

Underwater acoustic (UWA) communications are essential for high-speed marine data transmission but remain severely constrained by limited bandwidth, significant propagation loss, and sparse multipath structures. Conventional underwater acoustic multiple-input multiple-output (MIMO) systems primarily utilize spatial diversity but suffer from limited array resolution, causing angular ambiguity and insufficient spatial degrees of freedom (DoFs). This paper addresses these limitations through acoustic Reconfigurable Intelligent Surfaces (aRIS) to actively generate orthogonally distinguishable virtual paths, significantly enhancing spatial DoFs and channel capacity. An ocean-specific DoF-channel coupling model is established, explicitly deriving conditions for spatial rank enhancement. Subsequently, the optimal geometric locus, termed the Light-Point, is analytically identified, where deploying a single aRIS maximizes DoFs by introducing two and three additional resolvable paths in deep-sea and shallow-sea environments, respectively. Furthermore, an active simultaneous transmitting and reflecting (ASTAR) aRIS architecture with independent beam control and adaptive beam-tracking mechanism integrating unmanned underwater vehicles (UUVs) and acoustic intensity gradient sensing is proposed. Extensive simulations validate the proposed joint aRIS deployment and beamforming framework, demonstrating substantial UWA channel capacity improvements-up to 265% and 170% in shallow-sea and deep-sea scenarios, respectively.