  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.
