Reconfigurable Intelligent Surfaces Assisted SM-Cooperative NOMA for THz Communications

In terahertz (THz) communication, the integration of reconfigurable intelligent surface (RIS), cooperative non-orthogonal multiple access (CNOMA), and multiple-input multiple-output (MIMO) techniques holds promise for next-gen wireless networks, enhancing spectral efficiency and overall system access. However, traditional MIMO-NOMA faces power consumption and implementation challenges. In this paper, we propose a novel RIS-assisted spatial modulation (SM)-CNOMA for THz communication, termed RSM-CNOMA. In RSM-CNOMA, the base station manages communication for two distinct users by transmitting their information through the RIS using SM. Additionally, a user in close proximity to the RIS serves as a relay, forwarding information to a user located at a greater distance through an index modulation (IM) scheme. We have derived the bit error rate (BER) and sum-rate of users in the RSMCNOMA system. The RSM-CNOMA system exhibited better BER and sum-rate performance compared to the conventional RIS-CNOMA system, as observed in the results. Furthermore, we conducted a comprehensive comparative assessment of achievable sum-rate performance across various systems, including RISassisted SM-NOMA (RSM-NOMA), RIS-CNOMA, and RISsupported orthogonal multiple access (OMA).