Flexible Intelligent Metasurface in Aerial BD-RIS-Assisted Terahertz Communications System With RSMA

Abstract

This letter proposes a novel communication framework that synergistically integrates flexible intelligent metasurfaces (FIMs), beyond-diagonal reconfigurable intelligent surfaces (BD-RIS), and rate-splitting multiple access (RSMA) to enhance terahertz (THz) aerial-assisted communications. Despite the promise of ultra-wide bandwidth, THz communications are hindered by severe propagation loss and blockage. FIMs, composed of low-cost and morphable radiating elements, enable fine-grained wavefront control to counteract these propagation challenges. To further improve signal coverage and spatial diversity, an aerial active BD-RIS (AABD-RIS) is deployed to provide richer reflection patterns, while RSMA enhances interference management via adaptive message splitting. We formulate a joint optimization problem over BS active beamforming, FIM surface morphing, RSMA rate allocation, and AABD-RIS trajectory and configuration to maximize the system achievable rate. To solve it, we propose a meta soft actor-critic (Meta-SAC) algorithm leveraging metagradient adaptation for rapid convergence in dynamic environments. Simulation results demonstrate the superiority of the proposed scheme over conventional systems, achieving significant gains in data rates and connectivity, particularly under obstructed propagation conditions.