Reconfigurable Intelligent Surfaces: Channel Characterization and Modeling

Reconfigurable intelligent surfaces (RISs) are 2-D metasurfaces, which can intelligently manipulate electromagnetic waves by low-cost near passive reflecting elements. RIS is viewed as a potential key technology for the sixth-generation (6G) wireless communication systems mainly due to its advantages in tuning wireless signals, thus smartly controlling propagation environments. In this article, we aim at addressing channel characterization and modeling issues of RIS-assisted wireless communication systems. First, the concept, principle, and potential applications of RIS are given. An overview of RIS-based channel measurements and experiments is presented by classifying frequency bands, scenarios, system configurations, RIS constructions, experiment purposes, and channel observations. Then, RIS-based channel characteristics are studied, including reflection and transmission, the Doppler effect and multipath fading mitigation, channel reciprocity, channel hardening, rank improvement, far field, near field, and so on. RIS-based channel modeling works are investigated, including large-scale path loss models and small-scale multipath fading models. Finally, future research directions related to RIS-assisted channels are also discussed.

I. Introduction

Although the fifth-generation (5G) wireless communication systems are being deployed worldwide, the performance metrics of 5G are not fully satisfied, and the three application scenarios, i.e., enhanced mobile broadband (eMBB), massive machine-type communications (mMTCs), and ultrareliable and low latency communication (URLLC), are not well supported. Hence, 5G will not meet all requirements of the future in 2030 and beyond [1]. Research on the sixth-generation (6G) wireless communication key technologies has been launched, with global coverage, all spectra, full applications, and strong network security as the vision and paradigm shifts [1], [2].