Three-Phase Channel Estimation for RIS-Aided MIMO mmWave Systems with Direct Channels

In this paper, a three-phase joint direct and cascaded channel estimation strategy is proposed for reconfigurable intelligent surface (RIS)-aided multi-user (MU) multiple-input multipleoutput (MIMO) millimeter wave (mmWave) systems with the existence of the direct channels. The base station (BS), the users and the RIS are equipped with uniform planar array (UPA). The effectiveness of the devised three-phase strategy is contingent upon the meticulous design of the pilot signal sequence and the RIS phase shift vectors. Specifically, in Phase I, by reversing the RIS phase shift vectors, we remove the cascaded channel components to estimate the direct channels. In Phase II, we employ the orthogonal subspace projection to obtain equivalent signal matrices for the estimation of angles of departure (AoDs) of the user-RIS channel. In Phase III, we combine the signals of time slots with the same pilots and project the obtained measurement matrix to the orthogonal complement space of the component consisting of the portion of the direct channel, which removes the direct components and thus prevents error propagation from the direct channels to the cascaded channels. Then, we estimate the angles of arrival (AoAs) of the RIS-BS channel and the remaining parameters of the cascaded channels. Simulation results show that the proposed method outperforms existing methods.