Channel Estimation and Pilot Allocation for Practical RIS-Aided Wideband OFDMA Systems.

Channel state information (CSI) acquisition is a crucial but challenging issue in reconfigurable intelligent surface (RIS)-aided systems due to the passive property of RIS. In this paper, we investigate channel estimation in practical RIS-assisted multiuser orthogonal frequency division multiplexing access (OFDMA) systems. Different from prior works which assume that the RIS has an ideal reflection model (i.e., each reflecting element has constant amplitude, variable phase shift, and the same response for signals at different subcarriers), in this work the channel estimation is investigated with a practical RIS reflection model by considering the amplitude-phase-frequency relationship of the reflected signals. Aiming at enhancing the accuracy and efficiency of the channel estimation, a novel channel estimation method with pilot subcarrier allocation is developed. Simulation results demonstrate the necessity of considering practical RIS reflection model for channel estimation and the effectiveness of our proposed channel estimation method and pilot subcarrier allocation scheme.