Optimizing Beamforming for Stacked RIS: Addressing Path Loss and Orientation Challenges in Multi-RIS 6G Networks

Reconfigurable Intelligent Surfaces (RIS) have emerged as a pivotal enabling technology with substantial potential for sixth-generation (6G) wireless networks. Due to their unique ability to manipulate wireless communication environments, RIS has garnered significant research interest in recent years. However, as multi-RIS cooperation, whether in parallel or cascaded configurations, is increasingly employed to improve signal guidance between transmitters and receivers, two primary challenges arise: mitigating path loss over wide area RIS deployments and addressing the impact of relative orientation angles between surfaces. To address these challenges, this letter introduces a novel approach that integrates multiple RIS units into a single structure, termed stacked RIS. This design aims to overcome the limitations associated with separately deployed RIS units. The sum rate is analyzed by optimizing the incorporation of active and combined passive beamforming for the base station and stacked RIS compared to multi-separated RIS with different configurations. Simulation results demonstrate the superior performance of stacked RIS over its multi-separated counterpart, highlighting its ability to reduce the required surface area while mitigating deployment constraints and minimizing the effects of relative orientation angles between surfaces.