Engineering of Intelligent Reflecting Surfaces: Reflection Locality and Angular Stability

Reconfigurable intelligent surfaces (RISs) are electromagnetically passive controllable structures, deflecting incident wave beams in tunable directions. A usual way to design RIS using metasurfaces (MSs) is based on the approximation in which the reflective properties of a uniform MS are attributed to unit cells of the nonuniform one. We call this approximation the reflection locality (RL) and analyze its applicability. We prove that RL holds for a wide range of incidence and deflection angles if and only if uniform MSs based on which the nonuniform one is generated possess so-called angular stability (AS). AS of an infinite uniform MS (that we call generic MS) means that its reflection coefficient is independent of the incidence angle for both TE and TM polarizations.