On the Physical Layer Security of Millimeter Wave NOMA Networks

For the capability of providing multi-giga BPS (bits per second) rates, millimeter wave (mmWave) communication is one of the key enabling technologies for the new and future generations of mobile communications, i.e., the fifth generation (5G) and beyond. Meanwhile, non-orthogonal multiple access (NOMA) can significantly increase the spectral efficiency by simultaneously serving multiple users in the same channel. Thus, mmWave NOMA networks have recently attracted considerable research attention. Meanwhile, a large number of confidential messages exchanged within highly interconnected systems has posed tremendous challenges on secure wireless communications, and thus in this article, we investigate the physical layer security of mmWave NOMA networks. Considering the limited scattering characteristics of mmWave channels and imperfect successive interference cancellation at receivers, we develop an analytic framework for the secrecy outage probability (SOP) for mmWave NOMA networks, in which legitimate users and eavesdroppers are randomly distributed. Based on the directional transmission property of mmWave signals, we propose a minimal angle-difference user pairing scheme to reduce the SOP of users. Considering the spatial correlation between the selected user pair and eavesdroppers, we develop two maximum ratio transmission (MRT) beamforming schemes to further enhance the secrecy performance of mmWave NOMA networks. Closed-form SOPs for the paired users with different eavesdropper detection capacities are derived. Numerical results show the effectiveness of our analysis and that there exists an optimal radius of network coverage ranges and transmit power to minimize the SOP of the user pair.