New RIS-Assisted SSK Schemes With Passive Beamforming Using Channel-Dependent RIS Partitioning

This letter presents new phase configuration schemes for systems employing reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) signaling with passive beamforming (RIS-SSK-PB). Unlike most RIS-assisted SSK schemes, which necessitate adjusting the RIS phase configuration at the symbol rate, the RIS-SSK-PB scheme permits phase adjustments at a slower rate, provided it is not slower than the inverse of the channel coherence time. This approach significantly reduces system overhead and mitigates hardware constraints. The main novelty in the proposed schemes lies in the idea of optimized channel-dependent RIS partitioning. Two RIS-partitioning schemes are first proposed and formulated as mixed-integer linear programming (MILP) problems. Suboptimal algorithms based on linear programming relaxation are subsequently presented to achieve much lower complexity with negligible performance degradation. Extensive numerical simulations demonstrated the superior performance of the proposed schemes.