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The performance of transmission schemes is heavily influenced by the wirelesschannel, which is typically considered an uncontrollable factor. However, theintroduction of reconfigurable intelligent surfaces (RISs) to wirelesscommunications enables the customization of a preferred channel for adoptedtransmissions by reshaping electromagnetic waves. In this study, we proposemulti-timescale channel customization for RIS-assisted multiple-inputmultiple-output systems to facilitate transmission design. Specifically, wecustomize a high-rank channel for spatial multiplexing (SM) transmission and ahighly correlated rank-1 channel for beamforming (BF) transmission by designingthe phase shifters of the RIS with statistical channel state information in theangle-coherent time to improve spectral efficiency (SE). We derive closed-formexpressions for the approximation and upper bound of the ergodic SE and comparethem to investigate the relative SE performance of SM and BF transmissions. Interms of reliability enhancement, we customize a fast-changing channel in thesymbol timescale to achieve more diversity gain for SM and BF transmissions.Extensive numerical results demonstrate that flexible customization of channelcharacteristics for a specific transmission scheme can achieve a tradeoffbetween SE and bit error ratio performance.

Multi-timescale Channel Customization for Transmission Design in RIS-assisted MIMO Systems