RIS-Assisted NOMA With Interference-Cancellation and Interference-Alignment Scheme

Reconfigurable intelligent surface (RIS) is a cost-effective method to enhance the end-to-end wireless channel. Meanwhile, Non-Orthogonal Multiple Access (NOMA) improves the overall spectral efficiency, connectivity and latency by allowing different users to utilize the same resource blocks. Combining these two technologies has been the main focus of recent research. In this letter, we propose a RIS-assisted NOMA with an interference-cancellation scheme, wherein a group of phase shift elements is assigned to mitigate interference to other users not being supported. We also propose an interference-alignment scheme when RIS is supporting all the NOMA users. In this particular scheme, we align the RIS-amplified signal of each user to the interference for better reception. We analyze the symbol error rate (SER) performance of these schemes in a downlink
${M}$
-QAM NOMA system. We adopt a simple model where we derive approximate SER expressions and bounds, and simulate them to validate the results.