Joint Beamforming and Power-Splitting Control in Downlink Cooperative SWIPT NOMA Systems

This paper investigates the application of simultaneous wireless information and power transfer (SWIPT) to cooperative non-orthogonal multiple access (NOMA). A new cooperative multiple-input single-output (MISO) SWIPT NOMA protocol is proposed, where a user with a strong channel condition acts as an energy-harvesting (EH) relay by adopting power splitting (PS) scheme to help a user with a poor channel condition. By jointly optimizing the PS ratio and the beamforming vectors, we aim at maximizing the data rate of the "strong user" while satisfying the QoS requirement of the "weak user". To resolve the formulated nonconvex problem, the semidefinite relaxation (SDR) technique is applied to reformulate the original problem, by proving the rank-one optimality. And then an iterative algorithm based on successive convex approximation (SCA) is proposed for complexity reduction, which can at least attain its stationary point efficiently. In view of the potential application scenarios, e.g., Internet of Things (IoT), the single-input single-output (SISO) case is also studied. The formulated problem is proved to be strictly unimodal with respect to the PS ratio. Hence, a golden section search (GSS) based algorithm with closed-form solution at each step is proposed to find the unique global optimal solution. It is worth pointing out that the SCA method can also converge to the optimal solution in SISO cases. In the numerical simulation, the proposed algorithm is numerically shown to converge within a few iterations, and the SWIPT-aided NOMA protocol outperforms the existing transmission protocols.