Unified RIS-Assisted Spatial Modulation: A Constellation Redesigning Approach for Performance Enhancement

This paper explores the limitations of a conventional reconfigurable intelligent surface-assisted spatial modulation (RIS-SM) system by analyzing the signal-spatial constellation diagram. These limitations include closely packed signalspatial points and a lack of flexibility in adjusting their positions, negatively affecting system performance. To address these challenges, this work proposes a unified RIS-SM (U-RIS-SM) system that utilizes optimal RIS reflector phases and a modified modulation scheme with a tuning parameter. The proposed U-RIS- SM system facilitates the constellation redesigning of signalspatial points by adjusting the tuning parameter. Additionally, the optimal tuning parameter value is determined to achieve the best possible performance of U-RIS-SM. Closed-form expressions for the symbol error rate (SER), average SER (ASER), and average mutual information (MI) of the proposed U-RIS-SM system under the Nakagami-m fading channel are derived and validated through Monte-Carlo simulations. Moreover, since the proposed U-RIS-SM system can be transformed into existing RIS-SM and RIS-assisted space shift keying (RIS-SSK) systems by varying the tuning parameters and RIS reflector phases, the performance metrics for existing RIS-SM/SSK systems are derived from expressions for U-RIS-SM. The U-RIS-SM system is compared with existing RIS-SSK and RIS-SM systems across various parameters, demonstrating its superior performance. Furthermore, to assess its effectiveness under practical conditions, its performance is examined using a low-complexity two-stage maximum likelihood detector, as well as under RIS phase errors and discrete RIS phase shifts.