Reconfigurable intelligent surface (RIS) is a promising candidate technology for the upcoming Sixth Generation (6G) communication system for its ability to manipulate the wireless communication environment by controlling the coefficients of reflection elements (REs). However, since the RIS usually consists of a large number of passive REs, the pilot overhead for channel estimation in the RIS-aided system is prohibitively high. In this paper, the channel estimation problem for a RIS-aided multi-user multiple-input-single-output (MISO) communication system with clustered users is investigated. First, to describe the correlated feature for RIS-user channels, a beam domain channel model is developed for RIS-user channels. Then, a pilot reuse strategy is put forward to reduce the pilot overhead and decompose the channel estimation problem into several subproblems. Finally, by leveraging the correlated nature of RIS-user channels, an eigenspace projection (EP) algorithm is proposed to solve each subproblem respectively. Simulation results show that the proposed EP channel estimation scheme can achieve accurate channel estimation with lower pilot overhead than existing schemes.

翻译：可重构智能表面（RIS）通过控制反射单元（RE）的系数来操控无线通信环境，因其在即将到来的第六代（6G）通信系统中的潜力而成为一项极具前景的候选技术。然而，由于RIS通常包含大量无源RE，RIS辅助系统中的信道估计导频开销过高。本文针对用户集群的RIS辅助多用户多输入单输出（MISO）通信系统中的信道估计问题展开研究。首先，为描述RIS-用户信道的相关性特征，建立了RIS-用户信道的波束域信道模型。随后，提出一种导频复用策略来降低导频开销，并将信道估计问题分解为若干子问题。最后，利用RIS-用户信道的相关特性，提出一种特征空间投影（EP）算法分别求解各子问题。仿真结果表明，与现有方案相比，所提EP信道估计方案能够在更低导频开销下实现精确的信道估计。