Two-Timescale Channel Estimation for Reconfigurable Intelligent Surface Aided Wireless Communications

TL;DR

This paper introduces a two-timescale channel estimation method for RIS-aided wireless systems, significantly reducing pilot overhead by exploiting the quasi-static nature of the BS-RIS channel and the mobility of the RIS-UE channel.

Contribution

It proposes a novel two-timescale framework and a dual-link pilot scheme with a coordinate descent algorithm for efficient channel estimation in RIS systems.

Findings

Achieves accurate channel estimation with reduced pilot overhead.

Effectively exploits the different timescales of BS-RIS and RIS-UE channels.

Simulation confirms improved efficiency and accuracy.

Abstract

Channel estimation is challenging for the reconfigurable intelligent surface (RIS)-aided wireless communications. Since the number of coefficients of the cascaded channel among the base station (BS), the RIS and the user equipments (UEs) is the product of the number of BS antennas, the number of RIS elements, and the number of UEs, the pilot overhead can be prohibitively high. In this paper, we propose a two-timescale channel estimation framework to exploit the property that the BS-RIS channel is high-dimensional but quasi-static, while the RIS-UE channel is mobile but low-dimensional. Specifically, to estimate the quasi-static BS-RIS channel, we propose a dual-link pilot transmission scheme, where the BS transmits downlink pilots and receives uplink pilots reflected by the RIS. Then, we propose a coordinate descent-based algorithm to recover the BS-RIS channel. Since the quasi-static BS-RIS channel is estimated less frequently than the mobile channel be, the average pilot overhead can be reduced from a long-term perspective. Although the mobile RIS-UE channel has to be frequently estimated in a small timescale, the associated pilot overhead is low thanks to its low dimension. Simulation results show that the proposed two-timescale channel estimation framework can achieve accurate channel estimation with low pilot overhead.