Reconfigurable intelligent surfaces for wireless communications: Overview of hardware designs, channel models, and estimation techniques

TLDR

The next‑generation 6G networks drive research into novel materials and RF front‑ends, with reconfigurable intelligent surfaces (RISs) emerging as planar, programmable structures capable of reshaping electromagnetic propagation for coverage, capacity, energy efficiency, secrecy, and reduced EM exposure. This paper surveys and classifies recent RIS hardware designs and the latest models of RIS unit elements and RIS‑enabled wireless propagation. We provide a detailed review of RIS channel‑estimation techniques and discuss their role in current and forthcoming wireless standards, outlining ongoing standardization efforts. RIS deployment can dynamically reconfigure any wireless environment, enabling coverage extension, capacity enhancement, spatiotemporal focusing, improved energy efficiency, stronger secrecy, and lower EM exposure.

Abstract

The demanding objectives for the future sixth generation (6G) of wireless communication networks have spurred recent research efforts on novel materials and radio-frequency front-end architectures for wireless connectivity, as well as revolutionary communication and computing paradigms. Among the pioneering candidate technologies for 6G belong the reconfigurable intelligent surfaces (RISs), which are artificial planar structures with integrated electronic circuits that can be programmed to manipulate the incoming electromagnetic field in a wide variety of functionalities. Incorporating RISs in wireless networks have been recently advocated as a revolutionary means to transform any wireless signal propagation environment to a dynamically programmable one, intended for various networking objectives, such as coverage extension and capacity boosting, spatiotemporal focusing with benefits in energy efficiency and secrecy, and low electromagnetic field exposure. Motivated by the recent increasing interests in the field of RISs and the consequent pioneering concept of the RIS-enabled smart wireless environments, in this paper, we overview and taxonomize the latest advances in RIS hardware architectures as well as the most recent developments in the modeling of RIS unit elements and RIS-empowered wireless signal propagation. We also present a thorough overview of the channel estimation approaches for RIS-empowered communications systems, which constitute a prerequisite step for the optimized incorporation of RISs in future wireless networks. Finally, we discuss the relevance of the RIS technology in the latest wireless communication standards, and highlight the current and future standardization activities for the RIS technology and the consequent RIS-empowered wireless networking approaches.