On Sum-Rate Maximization for Cooperative STAR-RIS-Assisted Near-Field NOMA Network

Non-orthogonal multiple access (NOMA) and simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) are viewed as two enabling techniques for next-generation mobile networks. With the increase of STAR-RIS elements, near-field communication achieves higher performance than far-field communication in many scenarios. In this paper, we investigate the downlink transmission scheme of a cooperative STAR-RIS-assisted NOMA network with near-field communication. A sum-rate maximization problem is formulated by jointly designing the active beamformer at base station and the phase-shift coefficients at STAR-RIS. By invoking the semidefinite relaxation (SDR) and the successive convex approximation (SCA) methods, the formulated optimization problem is addressed by the proposed alternating optimization algorithm efficiently. The simulation results show that: 1) Compared with the far-field beamforming scheme, the proposed near-field beamformer can significantly improve the sum-rate of downlink transmission; 2) The proposed algorithm achieves a higher sum-rate than the existing schemes, i.e., the STAR-RIS units' alternating optimization and the block coordinate descent algorithm.