Reconfigurable Intelligent Surfaces: Three Myths and Two Critical Questions

The search for physical layer technologies that can play a key role in beyond 5G systems has started. One option is reconfigurable intelligent surfaces (RISs), which can collect wireless signals from a transmitter and passively beamform them toward the receiver. The technology has exciting prospects and is quickly gaining traction in the communication community, but in the current hype we have witnessed how several myths and overstatements are spreading in the literature. In this article, we take a neutral look at the RIS technology. We first review the fundamentals and then explain specific features that can be easily misinterpreted. In particular, we debunk three myths: 1) current network technology can only control the transmitter and receiver, not the environment in between; 2) a better asymptotic array gain is achieved than with conventional beamforming; 3) the path loss is the same as with anomalous mirrors. To inspire further research, we conclude by identifying two critical questions that must be answered for RIS to become a successful technology: 1) What is a convincing use case for RIS?; 2) How can we estimate channels and control an RIS in real time?

Introduction

The electromagnetic waves that carry information in wireless communications interact with objects and surfaces on their way from the transmitter to the receiver. Although the superposition of many propagation paths gives rise to random-like fading phenomena, every propagation path has a constant behavior. However, there are engineered materials whose interactions with electromagnetic waves are not constant but reconfigurable. These materials are not naturally occurring but can be manufactured and deployed to shape the propagation environment. The prospects of including such reconfigurable intelligent surfaces (RISs) as a part of beyond 5G network architectures are attracting much attention [1], [2]. RISs have also been called software-controlled metasurfaces [3] and intelligent reflecting surfaces [4].