Anchor-Aided Channel Estimation for RIS-Assisted Multiuser Broadband Communications

Due to the passive nature of reconfigurable intelligent surface (RIS) and a large number of users, it is very challenging to estimate the cascaded base station (BS)-RIS-user channel directly, especially for multiuser broadband communication systems. In this letter, we propose a novel two-phase method to estimate multiuser broadband cascaded channels with the assistance of two anchor nodes. Specifically, in the first phase, the cascaded channel from the two anchors to the BS and that between two anchors are estimated to acquire the BS-RIS channel state information (CSI) with sign ambiguities. It is accomplished based on cyclic-prefixed single-carrier transmission and phase configuration at the RIS, which creates multiple cyclic delayed versions of transmitted signals for isolating all broadband channel taps. In the second phase, the RIS-user channel and subsequently the cascaded BS-RIS-user channel are estimated in the frequency domain using the knowledge of the partial BS-RIS CSI. Simulation results verify the superiority of the proposed scheme over the state-of-the-art methods in terms of estimation overhead, normalized mean-squared error and bit error rate.