Fairness secure transmission for mmWave NOMA system with internal eavesdroppers

In this paper, we investigate the secure transmission problem for the millimeter wave (mmWave) nonorthogonal multiple access (NOMA) system, where users are divided into multiple clusters. Each cluster is comprised of an entrusted far user and an untrusted near user (potential eavesdropper). In order to prevent near users from eavesdropping on the signal of far users and in consideration of the fairness of each cluster, we propose minimal secrecy rate maximization problem by optimizing the beamforming vector and powers allocated to users while the power constrain and users rate threshold requirement are imposed. Due to the its difficulty to directly solve the proposed problem, we first employ the zero-forcing and maximum ratio transmission to design the beamforming vector, and then adopt successive convex approximation to iteratively optimize the power allocation. Numerical results are showed to evaluate the security performance of proposed approach to mmWave NOMA system.