Efficient Channel Probing and Phase Shift Control for mmWave Reconfigurable Intelligent Surface-Aided Communications

Recently, a reconfigurable intelligent surface (RIS) that controls the reflection characteristics of incident signals has received a great deal of attention. To make the most of the RIS-aided systems, an acquisition of RIS reflected channel information at the base station (BS) is crucial. However, this task is by no means easy due to the pilot overhead induced by the large number of reflecting elements. In this paper, we propose an efficient channel estimation and phase shift control technique reducing the pilot overhead of the RIS-aided mmWave systems. Key idea of the proposed scheme is to decompose the RIS reflected channel into three major components, i.e., static BS-RIS angles, quasi-static RIS-UE angles, and time-varying BS-RIS-UE path gains, and then estimate them in different time scales. By estimating the BS-RIS and RIS-UE angles occasionally and estimating only the path gains frequently, the proposed scheme achieves a significant reduction on the pilot overhead. Further, by optimizing the phase shifts using the channel components with relatively long coherence time, we can improve the channel estimation accuracy. From the performance analysis and numerical evaluations, we demonstrate that the proposed scheme achieves more than 60% pilot overhead reduction over the conventional techniques.