Discrete Reflecting Beamforming Optimization for RIS Enhanced Space Shift Keying MIMO Systems

This paper presents a novel reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) multiple-input multiple-output (MIMO) system, where the optimization of discrete phase shifts in the RIS is explored to minimize the average bit error probability (ABEP). While previous studies have focused on RIS configurations that assume alignment of all RIS elements to a single antenna, this work addresses the challenge of phase interference between multiple antennas in the receive array, which complicates the calculation of composite channel distributions. We formulate an optimization problem that maximizes the minimum Euclidean distance among constellation points while adhering to discrete RIS phase shifts. The problem is solved using successive convex approximation (SCA) and penalty-alternating optimization techniques, yielding a suboptimal solution. Our simulation results show that the proposed RIS-SSK-MIMO system outperforms existing benchmarks, offering significant improvements in ABEP performance. Additionally, we analyze the impact of RIS phase quantization and antenna configurations on system reliability, demonstrating the advantages of RIS in enhancing signal quality and spectral efficiency.