Energy-Efficient and Fair Power Allocation Approach for NOMA in Ultra-Dense Heterogeneous Networks

Non-orthogonal multiple access (NOMA) is considered as a promising candidate for fifth-generation mobile communication systems due to its high spectral efficiency (SE). Many earlier works focused on the power allocation for NOMA to achieve the sum rate maximization. However, the energy efficiency (EE) optimization problem in NOMA systems has not been well studied. In this paper, the EE of a two-tier downlink ultra-dense heterogeneous network (HetNet) with NOMA is studied. The goal is to maximize the EE subject to the constraint of allocated powers for users. To achieve this goal, an optimization problem is formulated which aims to find out the optimal number of users. In order to solve the optimization problem, a fair power allocation approach is proposed. Simulation results show that NOMA has a superior EE performance in comparison with orthogonal frequency division multiple access (OFDMA). Moreover, the fairness of the proposed fair power allocation (FPA) approach exceeds the conventional fractional transmit power allocation (FTPA) approach. At the same time, the optimal number of users can be found out by simulations.