RIS-Aided Channel Estimation for Multi-User mmWave Systems Under Practical Hybrid Architecture With Direct Path

This paper develops a two-stage channel estimation protocol for a reconfigurable intelligent surface (RIS)-aided multi-user (MU) millimeter wave (mmWave) system, in which the base station (BS) is equipped with a practical hybrid RF architecture and the direct channel between the BS and the user equipment (UE) always exists. In Stage I, the direct and cascaded channels of a typical UE are estimated. This stage consists of two sub-stages: Sub-stage I estimates the common angles of arrival (AoAs), while Sub-stage II estimates the direct and cascaded channels. In Stage II, there are two sub-stages to estimate the direct channels and the cascaded channels of the remaining UEs, which exploits the information obtained in Stage I to reduce the pilot overhead and the complexity. Simulation results demonstrate that the proposed method has lower pilot overhead and higher accuracy than the existing benchmark approaches.