On Optimal Power Allocation for Downlink NOMA Transmissions under PHY QoS Constraints

Non-orthogonal multiple access (NOMA) is one of the fundamental techniques that have been actively investigated for the 5th generation (5G) of wireless communication systems, and will be adopted on a wider scale in the coming versions of beyond-5G networks. It presents the benefit of supporting a higher number of users; as compared to the conventional orthogonal multiple access (OMA) schemes, thanks to the successive interference cancellation (SIC) process. However, resources allocation optimization remains one of the main open research challenges in this area. In this paper, we are interested in the power allocation aspect in a downlink NOMA system. We first propose an efficient optimal power allocation strategy maximizing the system's average sum rate, under physical layer (PHY) quality of service (QoS) conditions. Then, we derive the necessary system conditions to guarantee optimal SIC-stable NOMA schemes. This provides an insight on the operational regions where the use of NOMA is of interest. Various numerical results are presented to illustrate the analysis.