Optimal Power Allocation in Downlink NOMA

Power-domain non-orthogonal multiple access (NOMA) has arisen as a promising multiple access technique for the next-generation wireless networks. In this work, we address the problem of finding globally optimal power allocation strategies for the downlink of a generic single-cell NOMA system including multiple NOMA clusters each operating in an isolated resource block. Each cluster includes a set of users in which the well-known superposition coding (SC) combined with successive interference cancellation (SIC) technique (called SC-SIC) is applied among them. Interestingly, we prove that in both the sum-rate and energy efficiency maximization problems, network-NOMA can be equivalently transformed to a virtual network-OMA system, where the effective channel gain of these virtual OMA users are obtained in closed-form. Then, the latter problems are solved by using very fast water-filling and Dinkelbach algorithms, respectively. The equivalent transformation of NOMA to the virtual OMA system brings new insights, which are discussed throughout the paper. Extensive numerical results are provided to show the performance gap between fully SC-SIC, NOMA, and OMA in terms of system outage probability, BS's power consumption, users sum-rate, and system energy efficiency.