Editorial Introduction to the Issue on Advanced Signal Processing for Reconfigurable Intelligent Surface-Aided 6G Networks

The papers in this special issue aim to report on the latest advances in signal processing for reconfigurable intelligent surface (RIS)-aided 6G networks with focus on theoretical development, algorithmic design, and future applications.RIS or intelligent reflecting surface (IRS) technology has been widely regarded as one of the most promising techniques to deal with the blockage issue in millimeter-wave (mmWave) communications. An RIS is a planar surface consisting of an array of nearlypassive reflecting elements, each of which can independently induce an controllable phase shift on the incident signal. By installing RIS on walls or ceilings, a virtual line-of-sight (LoS) link between a mmWave base station (BS) and the users can be established so as to bypass any blockage between them. In addition, RIS is a promising technology for applications in conventional sub-6 GHz communications. Specifically, by judiciously adjusting the phase shifts of the reflecting elements, the reflected signals can be constructively superimposed (even with the direct path if it is available) to enhance the desired signal power or can be cancelled out to mitigate the impact of co-channel interference or signal leakage to eavesdroppers. Since the elements of an RIS reflect the incoming signals without any signal processing operations that require radio-frequency (RF) chains, an RIS has a lower implementation cost than conventional active receivers and transmitters. An RIS can be fabricated with light weight and small-thickness layers, and thus can be readily integrated into the environment.