A Low-Complexity Estimation Scheme for Separated Reflecting Channels of RIS-Assisted MIMO Systems towards Extended Coverage

Reconfigurable intelligent surface (RIS) technologies can effectively extend the coverage of MIMO systems with improved signal-to-interference-and-noise ratio (SINR). However, the channel estimation of RIS-assisted MIMO systems is very challenging, though crucially important. First, the passive reflective nature of RIS makes it hard to obtain channel state information (CSI) of the transmitter-to-RIS and RIS-to-receiver links, respectively. Second, limited by the number of transmit antennas, reflective elements typically need to be divided into groups, and RIS needs to turn on/off element groups successively to obtain the full CSI, causing extra overhead and difficulty in control. To tackle these problems, we first propose a Two-fold-SVD based Separated Reflecting Channel Estimation (2-SVD-SRCE) scheme, which based on pilot blocks can effectively extract transmitter-to-RIS and RIS-to-receiver links' CSI, respectively. Then, we propose a novel pilot reconstruction design at the transmitter and develop a Pilot Reconstruction based Separated Reflecting Channel Estimation (PR-SRCE) scheme, which can overcome the limitation that the number of reflective elements in each group cannot exceed transmit antennas. Analyses and simulations show that our proposed scheme can significantly simplify the RIS control procedures, while achieving good performances in both complexity and channel estimating accuracy.