  Reconfigurable intelligent surface (RIS) is an important candidate technology
for 6G. We provide an analysis of RIS-assisted MIMO communication in sparse
channel typically found in the mmW or THz range. By exploring the sparse
property, we maximize the capacity in the singular space of the channel and
developed efficient algorithms for SU-MIMO or DL MU-MIMO. We also proved it is
more difficult to support high rank transmission in the RIS reflection channel
than in the traditional MIMO channel.
