  Large beam training overhead has been considered as one of main issues in the
channel estimation for reconfigurable intelligent surface (RIS)-aided systems.
In this paper, we propose an atomic norm minimization (ANM)-based low-overhead
channel estimation for RIS-aided multiple-input-multiple-output (MIMO) systems.
When the number of beam training is reduced, some multipath signals may not be
received during beam training, and this causes channel estimation failure. To
solve this issue, the width of beams created by RIS is widened to capture all
multipath signals. Pilot signals received during beam training are compiled
into one matrix to define the atomic norm of the channel for RIS-aided MIMO
systems. Simulation results show that the proposed algorithm outperforms other
channel estimation algorithms.
