Two-Stage Channel Estimation for RIS-Aided Multiuser mmWave Systems With Reduced Error Propagation and Pilot Overhead

In this paper, we propose a novel two-stage uplink channel estimation strategy with reduced pilot overhead and error propagation for reconfigurable intelligent surface (RIS)-aided multi-user (MU) millimeter wave (mmWave) multiple-antenna systems. The base station (BS) and the RIS are equipped with a uniform linear array (ULA) and a uniform planar array (UPA), respectively. Specifically, in Stage I, by carefully designing the RIS phase shift matrix and introducing a matching matrix, all users jointly transmit pilot signals for the estimation of the correlation factors between different paths of the common RIS-base station (BS) channel, which facilitates a significant MU diversity gain. Then, because of the inherent scaling and angle ambiguities of the mmWave cascaded channel, an ambiguous common RIS-BS channel composed of the estimated correlation factors is constructed. In Stage II, given the constructed ambiguous RIS-BS channel, each user independently sends a few pilots for estimating their own ambiguous user-RIS channel so as to obtain the entire cascaded channel. Simulation results demonstrate that the channel estimation accuracy of the proposed algorithm is improved as the number of users increases, and existing channel estimation algorithms are outperformed in terms of both pilot overhead and estimation accuracy.