Non Orthogonal Multiple Access Using Reconfigurable Intelligent Surfaces

In this paper, we propose the use of Reconfigurable Intelligent Surfaces (RIS) for Non Orthogonal Multiple Access (NOMA). RIS is decomposed in N sets of reflectors serving N users. The ith RIS contains N-i reflectors dedicated to user i. When RIS is used, all reflections reach the ith user with the same phase. RIS has not been yet used in NOMA systems where the transmitter sends a combination of symbols dedicated to N user. The ith user U-i has to detect the symbols of remaining N-i users. It performs Successive Interference Cancelation and detects first the symbol of weakest user. Then, it removes the signal of weakest user and detect that of second weakest user. The process is continued until U-i detects its own symbol. We suggest to optimize power allocation coefficients to all users as well as the number of reflector dedicates to each user in order to maximize the total throughput using the alternating maximization algorithm. More power and a larger number of reflectors should be allocated to weak users. An example of RIS implementation is provided in Dai et al. (IEEE Access, 8:45913-45923, 2020).