Secure Energy Efficiency Transmission for mmWave-NOMA System

In this article, we investigate the secure energy efficiency (SEE) problem for the millimeter wave (mmWave) nonorthogonal multiplex access (NOMA) systems in the presence of multiple legitimate receivers (LRs) and an eavesdropper. LRs are first grouped multiple clusters according to their channel correlations and gains, and the LRs of each cluster are served by one beam with the NOMA technique. Two sparse radio frequency chain antenna structures are adopted. Based on this, we formulate an SEE maximization problem by optimizing the hybrid analog/digital precoding, while the minimum secure rate requirements of LRs and the transmit power constraints of the base station are imposed. Due to its difficulty to directly solve the formulated problem, we first design the analog precoding based on the discrete phase and digital precoding according to the zero forcing technique, and then, the original problem is transformed into a power allocation problem. By applying the first-order Taylor approximation and successive convex approximation, the transformed problem becomes a convex one, and an iterative algorithm is proposed to solve it. Finally, numerical results are conducted to evaluate the SEE performance of the proposed algorithm in the mmWave-NOMA system.