  We consider the channel estimation problem in point-to-point reconfigurable
intelligent surface (RIS)-aided millimeter-wave (mmWave) MIMO systems. By
exploiting the low-rank nature of mmWave channels in the angular domains, we
propose a non-iterative Two-stage RIS-aided Channel Estimation (TRICE)
framework, where every stage is formulated as a multidimensional
direction-of-arrival (DOA) estimation problem. As a result, our TRICE framework
is very general in the sense that any efficient multidimensional DOA estimation
solution can be readily used in every stage to estimate the associated channel
parameters. Numerical results show that the TRICE framework has a lower
training overhead and a lower computational complexity, as compared to
benchmark solutions.
