Direct Channel Estimation between MIMO Transmitter and Reconfigurable Intelligent Surface

Reconfigurable Intelligent Surfaces (RIS) have emerged as a key component in wireless communication and sensing systems, offering the potential to significantly enhance network performance. However, due to the passive nature of RIS, most existing studies focus on the cascaded channels from the transmitter (TX) through RIS to the users, leaving a gap in our understanding of the channel between the TX and RIS. This paper addresses this lacuna by proposing a novel channel estimation protocol specifically designed for the TX-RIS link. This new protocol provides a foundation for a more complete understanding and optimization of RIS-aided wireless communication and sensing systems. Our methodology deploys a least squares (LS) estimator, for which we utilize a matrix eigenvalue decomposition method. We prove that our proposed two-step approach is optimal under appropriate pilot and reflective coefficient design. Simulation results verified the viability of the proposed method.