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

2025-12-18

AI Summary

Underwater acoustic communication suffers from narrow bandwidth, high path loss, and sparse multipath propagation; conventional MIMO systems face severe angular ambiguity and insufficient spatial degrees of freedom (DoF) due to low array resolution. To address this, we propose acoustic Reconfigurable Intelligent Surfaces (aRIS), introducing the first DoF-channel coupling model to analytically determine optimal “spotlight” deployment. We design the first Active Simultaneous Transmit-and-Reflect (ASTAR) aRIS architecture and a UUV-coordinated acoustic intensity gradient tracking method. Through geometric optimization of aRIS placement, joint ASTAR beam design, and adaptive beamforming, we actively synthesize orthogonal virtual multipath components, thereby significantly enhancing channel rank. Experiments demonstrate 170% and 265% capacity gains in shallow-water and deep-water channels, respectively; a single aRIS introduces 2–3 additional resolvable virtual paths, effectively overcoming limitations imposed by array resolution and angular ambiguity.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Multiagent Systems: Agent Communication
• Intelligent Robots: Multi-Robot Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Search and Retrieval-Augmented AI: Personalized, context-aware and across-device search
• Responsible Web: Machine-in-the-loop, human agency and autonomy

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.

Problem

• Enhancing underwater acoustic communication spatial degrees of freedom
• Addressing limited bandwidth and sparse multipath in underwater MIMO systems
• Increasing channel capacity using acoustic reconfigurable intelligent surfaces

Innovation

• Acoustic RIS generates orthogonal virtual paths
• Light-Point deployment maximizes spatial degrees of freedom
• ASTAR architecture enables independent beam control and tracking