Communication Models for Reconfigurable Intelligent Surfaces: From Surface Electromagnetics to Wireless Networks Optimization

TL;DR

This paper reviews and develops electromagnetic and optimization models for reconfigurable intelligent surfaces (RIS) in wireless networks, emphasizing surface impedance design and efficiency, with practical numerical insights.

Contribution

It provides a comprehensive tutorial on electromagnetically-consistent models for RIS surface impedance and compares local versus global design approaches.

Findings

Global designs can be approximated by locally passive RISs with zero resistance.

Surface impedance models enable efficient optimization of RIS reflection properties.

Numerical results demonstrate high power efficiency at large reflection angles.

Abstract

A reconfigurable intelligent surface (RIS) is a planar structure that is engineered to dynamically control the electromagnetic waves. In wireless communications, RISs have recently emerged as a promising technology for realizing programmable and reconfigurable wireless propagation environments through nearly passive signal transformations. With the aid of RISs, a wireless environment becomes part of the network design parameters that are subject to optimization. In this tutorial paper, we focus our attention on communication models for RISs. First, we review the communication models that are most often employed in wireless communications and networks for analyzing and optimizing RISs, and elaborate on their advantages and limitations. Then, we concentrate on models for RISs that are based on inhomogeneous sheets of surface impedance, and offer a step-by-step tutorial on formulating electromagnetically-consistent analytical models for optimizing the surface impedance. The differences between local and global designs are discussed and analytically formulated in terms of surface power efficiency and reradiated power flux through the Poynting vector. Finally, with the aid of numerical results, we discuss how approximate global designs can be realized by using locally passive RISs with zero electrical resistance (i.e., inhomogeneous reactance boundaries with no local power amplification), even for large angles of reflection and at high power efficiency.