On the Capacity of Reconfigurable Intelligence Surface: the Sparse Channel Case

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.