Theory, Analysis, and Design of Metasurfaces for Smart Radio Environments

The term ``metasurface'' (MTS) denotes an artificial surface constituted by a distribution of electrically small elements that collectively exhibit equivalent homogeneous boundary conditions (BCs) to an interacting electromagnetic field. MTSs are becoming increasingly popular due to the technological simplification that they offer with respect to volumetric metamaterials. In this article, we review the basic theory behind microwave MTSs seen as reconfigurable intelligent surfaces (RISs), oriented to the future visionary challenge of a smart radio environment. To this end, two different typologies of MTS are reviewed: surface-wave-based MTSs and nonspecular reflective MTSs. Both types can be effectively characterized using simplified problems that locally match the homogenized, modulated BCs. A different use of these problems allows for an accurate design of radiated and scattered fields. An accurate ray representation is also suggested, which allows for an effective description of the scattered field also in the Fresnel region and for the insertion of the MTS description in ray-tracing tools for network planning. Several examples of practical implementation are shown, and the challenges in applying electronic reconfigurability are discussed.

Automated summary

The term "metasurface" refers to an artificial surface composed of electrically small elements that collectively behave like a homogeneous boundary to an interacting electromagnetic field. Metasurfaces, or MTSs, are gaining popularity due to their technological simplicity compared to volumetric metamaterials. This article provides a review of the basic theory behind microwave MTSs, specifically as reconfigurable intelligent surfaces (RISs), and their potential for creating a smart radio environment. The article explores two different types of MTSs: surface-wave-based MTSs and nonspecular reflective MTSs. Simplified problems are used to effectively characterize the behavior of these MTSs and accurately design the radiated and scattered fields. The article also suggests using a ray representation to describe the scattered field in the Fresnel region and incorporate MTS descriptions into ray-tracing tools for network planning. Practical implementation examples are shown, and the challenges of electronic reconfigurability are discussed.