  The performance of transmission schemes is heavily influenced by the wireless
channel, which is typically considered an uncontrollable factor. However, the
introduction of reconfigurable intelligent surfaces (RISs) to wireless
communications enables the customization of a preferred channel for adopted
transmissions by reshaping electromagnetic waves. In this study, we propose
multi-timescale channel customization for RIS-assisted multiple-input
multiple-output systems to facilitate transmission design. Specifically, we
customize a high-rank channel for spatial multiplexing (SM) transmission and a
highly correlated rank-1 channel for beamforming (BF) transmission by designing
the phase shifters of the RIS with statistical channel state information in the
angle-coherent time to improve spectral efficiency (SE). We derive closed-form
expressions for the approximation and upper bound of the ergodic SE and compare
them to investigate the relative SE performance of SM and BF transmissions. In
terms of reliability enhancement, we customize a fast-changing channel in the
symbol timescale to achieve more diversity gain for SM and BF transmissions.
Extensive numerical results demonstrate that flexible customization of channel
characteristics for a specific transmission scheme can achieve a tradeoff
between SE and bit error ratio performance.
