Joint Relay Selection and Global Energy-Efficient Power Allocation in NOMA Networks

In this paper, the problem of joint relay selection and global energy-efficient power allocation in energy-harvesting cooperative non-orthogonal multiple-access (NOMA) networks is studied. In particular, a base-station communicates with multiple users via a selected energy-harvesting relay, such that network global energy-efficiency (GEE) is maximized, subject to quality-of-service (QoS) constraints. To that end, the problem of joint-relay selection and global energy-efficient power allocation (J-RS-GEE-PA) is formulated, which happens to be non-convex (i.e. computationally-expensive). In turn, the J-RS-GEE-PA problem is decoupled into two sub-problems: (1) relay selection, and (2) GEE power allocation. Different relay selection schemes are explored, and then a computationally-efficient algorithm is devised to optimally solve the GEE power allocation problem for the selected relay. In addition, a low-complexity solution procedure is proposed to solve the J-RS-GEE-PA problem, so as to achieve the global optimal solution. Simulation results are presented to evaluate the resulting network GEE for the different relay selection schemes, and the proposed solution procedure. More importantly, the proposed solution procedure is shown to yield the optimal network GEE when compared to the J-RS-GEE-PA, and superiority to the other relay selection schemes, while maintaining QoS.