Design of a Power Amplifying-RIS for Free-Space Optical Communication Systems

The steering dynamics of reconfigurable intelligent surfaces (RISs) have hoisted them to the front row of technologies that can be exploited to solve skip-zones in wireless communication systems. They can enable a programmable wireless environment, turning it into a partially deterministic space that plays an active role in determining how wireless signals propagate. However, RIS-based communication systems' practical implementation may face challenges such as higher signal attenuation. Furthermore, the transmitted signal may face a double-fading effect over the two portions of the channel. This article tackles this double-fading problem in near-terrestrial free-space optical (nT-FSO) communication systems using a RIS module based on liquid crystal (LC) on silicon (LCoS). A doped LC layer can directly amplify a light when subjected to an external field. Leveraging on this capacity of a doped LC, we mitigate the double-attenuation faced by the transmitted signal. We first revisit the nT-FSO power loss scenario, then discuss the direct-light amplification, and consider the system performance. Results show that at 51° of the incoming light incidence angle, the proposed LCoS design has minimal RIS depth, implying less LC material. The performance results show that the number of bits per unit bandwidth is upper-bounded and grows with the ratio of the sub-links' distances. Finally, we present and discuss open issues to enable new research opportunities toward the use of RIS and amplifying-RIS in nT-FSO systems.