Reconfigurable Intelligent Surface Empowered Downlink Non-Orthogonal Multiple Access

Power-domain non-orthogonal multiple access (NOMA) has become a promising technology to exploit the new dimension of power domain enhance spectral efficiency wireless networks. However, most existing NOMA schemes rely on strong assumption that users’ channel gains are quite different, which may be invalid in practice. To unleash potential power-domain NOMA, we propose reconfigurable intelligent surface (RIS)-empowered scheme introduce desirable gain differences among users by adjusting phase shifts at RIS. Our goal is minimize total transmit jointly optimizing beamforming vectors base station, phase-shift matrix RIS, and user ordering. address challenge due highly coupled optimization variables, present an alternating framework decompose non-convex bi-quadratically constrained quadratic problem under specific ordering into two rank-one matrices problems via lifting. accurately detect feasibility constraints improve performance avoiding early stopping procedure, equivalently represent constraint as difference between nuclear norm norm. A difference-of-convex (DC) algorithm further developed solve resulting DC programs successive convex relaxation, followed establishing convergence proposed DC-based method. We efficient with closed-form expressions, considering both conditions target data rates. Simulation results validate ability RIS enlarging channel-gain when original similar superiority method reducing power.