Channel estimation for reconfigurable intelligent surface aided multiuser millimeter-wave/THz systems

It is assumed that reconfigurable intelligent surface (RIS) is a key technology to enable the potential of mmWave communications. The passivity of the RIS makes channel estimation difficult because the channel can only be measured at the transceiver and not at the RIS. In this paper, we propose a novel separate channel estimator via exploiting the cascaded sparsity in the continuously valued angular domain of the cascaded channel for the RIS-enabled millimeter-wave/Tera-Hz systems, i.e., the two-stage estimation method where the cascaded channel is separated into the base station (BS)-RIS and the RIS-user (UE) ones. Specifically, we first reveal the cascaded sparsity, i.e., the sparsity exists in the hybrid angular domains of BS-RIS and the RIS-UEs separated channels, to construct the specific sparsity structure for RIS enabled multi-user systems. Then, we formulate the channel estimation problem using atomic norm minimization (ANM) to enhance the proposed sparsity structure in the continuous angular domains, where a low-complexity channel estimator via Alternating Direction Method of Multipliers (ADMM) is proposed. Simulation findings demonstrate that the proposed channel estimator outperforms the current state-of-the-arts in terms of performance.