Transmissive RIS: Modeling and Optimization for Multi-user Communications

Transmissive Reconfigurable Intelligent Surfaces (RISs) constitute a type of metasurfaces that can dynamically control how electromagnetic waves pass through them, facilitating, and even boosting, the performance of wireless communication applications in relevant scenarios. This chapter first presents a physically consistent model for the behavior of transmissive RISs, which is based on the scattering S-parameters. This model is then used for the joint active and passive beamforming optimization of transmissive-RIS-enabled multi-user Multiple-Input Single-Output (MISO) communication systems. In particular, a fully transmissive mode is considered for the studied RIS whose transmissive radiation pattern is optimized offline for a particular class of wireless channels describing the targeted wireless deployment environment. A dynamic, per-channel-realization optimization of the metasurface’s response-tunable elements jointly with the transmitter’s active precoder is also considered. The presented two-step overall design maintains the simplicity of conventional tunable phase configuration in the dynamic part, while introducing a sophisticated static part based on fundamental laws of physics, resulting in a non-local RIS structure. More importantly, the proposed design exhibits increased practicality compared to designs relying solely on fully-connected complex tunable phase profiles, which require increased computational overhead for their dynamic optimization on a per-channel-realization basis.