On the Optimality of NOMA in Two-User Downlink Multiple Antenna Channels

Non-orthogonal multiple access (NOMA) is proposed as downlink precoding scheme in cellular wireless networks. Applying multiple antennas at the base station allows for spatial precoding and spatial multiplexing. In particular, the capacity region for multiple antenna downlink channels is achieved by dirty paper precoding (DPC) and time sharing. It cannot be achieved by linear superposition coding and successive interference cancellation (SIC). In this work, we investigate the performance of NOMA in multiple antenna downlink channels. We show that NOMA cannot achieve the sum capacity except for parallel channels. Furthermore, we propose an efficient algorithm to compute the minimum power for DPC with time sharing under rate constraint. The results are compared with NOMA and zero-forcing (ZF) precoding. The numerical experiments show that NOMA requires significantly more transmit power than DPC. Moreover, the performance gap between ZF precoding and NOMA highly depends on the SNR region, number of antennas, and minimum rate demands.