Energy Efficiency and Spectral-Efficiency Tradeoff in Downlink NOMA Systems

Non-orthogonal multiple access (NOMA) has been investigated recently as a candidate radio access technology for the fifth generation (5G) mobile communication due to its high spectral efficiency (SE). Green radio (GR) which focuses on energy efficiency (EE) is becoming an inevitable trend. In this paper, we study the tradeoff between EE and SE in downlink NOMA systems. The tradeoff problem is formulated as an optimization problem in which the basic idea is to maximize EE under a satisfying minimum SE requirement. It is proved that the system EE is a strictly quasi-concave function of transmission power. By formulating the optimization problem as a bilayer optimization problem, the optimal solution to the optimization problem is obtained, including optimal power allocation coefficients among multiplexed users and optimal transmission power value. Simulation results are provided to further confirm our theoretical findings and the effectiveness of our proposed power allocation scheme for achieving a flexible tradeoff between EE and SE.