I. Introduction
Reconfigurable intelligent surface (RIS) is a two-dimensional surface composed of programmable electromagnetic materials, which can adjust the phase of incident electromagnetic waves by manual programming [1], [2]. With these characteristics, RIS can construct intelligent radio communication environments to eliminate adverse channel factors, such as blocking and fading, and improve the performance of mobile communication systems. Consequently, it has recently emerged as a promising technology to tackle the coverage enhancement problem in mobile communication systems [3].