Multiple RIS-Assisted Cooperative NOMA with User Selection

This paper proposes a novel transmission scheme that leverages the synergistic benefits of multiple reconfigurable intelligent surfaces (RISs) and cooperative non-orthogonal multi-ple access (NOMA) to improve both spectral and energy efficiency in 5G and beyond wireless systems. The proposed scheme involves selecting one of cell-center users to an access point (AP), which then relays the data to a cell-edge user without a direct connection to the AP with assistance of multiple distributed RISs. In this respect, we propose a cooperative NOMA scheme and develop a user selection strategy for two RIS exploitation scenarios: (i) the RIS selection scheme where the transmission between the selected cell-center and cell-edge users is performed via a selected RIS, and (ii) the distributed RIS scheme where all RISs assist in the end-to-end communications. The system outage performance is statistically characterized and its closed-form expressions are derived. Additionally, simulations are carried out to validate the analytical expressions and compare the performance of these two schemes to a single RIS-assisted network under different numbers of RISs, reflecting elements, and cell-center users. The results show that combining multiple RIS in cooperative NOMA can yield high spectral efficiency gains and improved outage performance, even with a low number of RIS elements.