Near-Field LoS Channel Estimation for RIS-Aided MU-MIMO Systems

We study the channel estimation problem for a reconfigurable intelligent surface (RIS)-assisted millimeter-wave (mmWave) multi-user multiple-input multiple-output (MU-MIMO) system. In particular, it is assumed that the channel between the RIS and a base station (BS) exhibits a near-field line-of-sight (LoS) channel as a dominant signal path in mm Wave communication system. Due to the high-rankness and non-sparsity of the RIS-BS channel, the existing methods, constructed based on far-field or near-field non-LoS channel, cannot be applied to this system. We for the first time develop an efficient channel estimation method with the idea of a piece-wise low-rank approximation. Via simulations, we demonstrate the effectiveness of our channel estimation method.