Power allocation scheme and spectral efficiency analysis for downlink non-orthogonal multiple access systems

In this study, the authors investigate the power allocation (PA) problem and spectral efficiency (SE) analysis for the single antenna downlink non-orthogonal multiple access (NOMA) system under the sum rate maximising criteria with minimum rate constraints (SRMC-MRC). For the PA problem, they propose the duality scheme for SRMC-MRC which is considered as the optimal solution for the PA problem on one orthogonal subband in the single antenna NOMA system. They propose the specific user rate maximising criteria with minimum rate constraints (SURMC-MRC) scheme to decrease the computational complexity of SRMC-MRC. The PA problem on one subband under SURMC-MRC is proved to be equivalent to SRMC-MRC. Numerical results show that both the SE and fairness performance for SURMC-MRC can strictly approach to those of the duality scheme in the whole signal-to-noise ratio (SNR) region. They prove that NOMA under SRMC-MRC can obtain SE performance advantage in the high SNR region but accompany with some disadvantages in the low SNR region over the orthogonal multiple access system. This conclusion is verified through numerical results under different parameter conditions.