Study of Multi-state Phase Gradient Surface with Single Beam Anomalous Reflection for Passive RIS Applications

Reflective Reconfigurable Intelligent Surfaces (RISs) are artifically engineered surfaces that can redirect an incident wave towards any arbitrary direction. As a result, RISs are envisioned to play a significant role in 5G and 6G communications. However, for practical implementation, active RISs may consume significant power besides their complex circuitry, especially when deployed on a large scale. To address this challenge, passive surfaces with nearly continuous phase gradients are being explored as a more efficient alternative. These surfaces consist of periodically arranged arrays of unit cells with linearly varying phase profiles, which can be implemented using two or more unit cells per period. However, using fewer unit cells per period can result in specular reflection besides anomalous reflection. This study demonstrates Phase Gradient Surface (PGS) designs having 18×18-unit cells of two-state to five-state, which effectively redirect incoming waves towards an anomalous direction, with suppressed specular reflection by upto 12.4 dB. They can also adaptively reconfigure the anomalous beam for smaller range of incident angles. Comparative reflection patterns for two- to five-state PGSs are presented, demonstrating that the five-state design achieves the greatest suppression of specular and other undesired reflections, even for incident angle of 20º.