Joint NOMA Power Factor and STAR-RIS Amplitude Allocation for Reliable Communication

In this letter, we demonstrate that the symbol error rate (SER) performance of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted downlink non-orthogonal multiple access (DL-NOMA) communication system can be improved by jointly setting the NOMA power allocation and the STAR-RIS amplitude parameters. The power allocation parameter mainly determines the relative symbol distances in the NOMA superimposed signal constellation, while the STAR-RIS amplitude parameter can dictate the relative strength of the users' effective channel. By scaling both parameters together, the overall detection error can be reduced. Using a simplified two-user model with QAM modulation, the proposed schemes are explained, and simulations confirm the improved SER performance versus baseline schemes.