Energy Efficient Secure MISO Non-Orthogonal Multiple Access Transmission

In this paper, we consider a multiple-input single-output (MISO) non-orthogonal multiple access (NOMA) system in the presence of an external single-antenna eavesdropper. We study secure downlink transmission strategies. The challenge is that the multiple antenna configurations make the successive interference cancellation (SIC) condition, which is necessary for applying NOMA schemes, much complex. We formulate the transmission power minimization problem under the quality of service (QoS), SIC and secrecy constraints. In particular, the transmission power minimization problems are formulated under both perfect channel state information (CSI) and the bounded CSI error models. These problems are non-convex, hence they are challenging to solve. Using semidefinite relaxation (SDR) and successive convex approximation (SCA), we propose an iterative algorithm for solving these problems. Our numerical results show that the proposed scheme succeeds in establishing secure communications and NOMA is capable of outperforming the conventional orthogonal multiple access schemes in terms of its power efficiency.