Channel Estimation for RIS-Assisted Multiuser mmWave Systems With Direct Channels Based on a Novel Space Projection Approach

In this letter, we develop a novel two-stage joint direct and cascaded channel estimation strategy for reconfigurable intelligent surface (RIS)-assisted multi-user multiple-input single-output (MU-MISO) millimeter wave (mmWave) systems based on a novel space projection approach. Specifically, in Stage I, to obtain an equivalent signal for direct channel estimation, we carefully design the phase shifts of the RIS without switching off the RIS. In Stage II, the equivalent signal matrices for estimating the cascaded channels are obtained by employing the orthogonal complement space of the transmitted pilot sequence, which mitigates the component of the direct channels and thus completely prevents error propagation from the direct channels to the cascaded channels. Based on the proposed novel signal pre-processing method, the MU-MISO mmWave channels can be estimated by exploiting the sparsity and correlation. Comprehensive simulation results verify that the proposed method can improve the estimation accuracy and decrease pilot overhead.