Inter-RIS Beam Focusing Codebook Design in Cooperative Distributed RIS Systems

This paper explores distributed Reconfigurable Intelligent Surfaces (RISs) by introducing a cooperative dimension that enhances adaptability and performance. It focuses on strategically deploying multiple RISs to improve Line-of-Sight (LoS) connectivity with the Base Station (BS) and among RISs, thereby aiding users in areas with weak BS coverage and enhancing spatial multiplexing gain. Each RIS can act as a main RIS (mRIS) to directly support users or as an intermediate RIS (iRIS) to reflect signals to another mRIS. This dual functionality allows for flexible responses to changing conditions. We implement an inter-RIS signal focusing design for phase shifts, creating a tailored codebook for precise control over signal direction. This design considers the interplay of incidence and reflection angles to maximize reflected signal power, based on the RIS response function and the physical properties of the RIS elements.