Secure Beamforming Assisted by Active RIS for mmWave-NOMA Network With Dual-Identity User

Millimeter-wave non-orthogonal multiple access (mmWave-NOMA) technology combines the wideband characteristics of millimeter waves with the power domain superposition multiplexing mechanism of NOMA, significantly enhancing the communication efficiency of the network. However, this characteristic also poses greater challenges to secure transmission, especially in networks with dual-identity nodes. These nodes not only act as legitimate users but also can eavesdrop on the information of secure users. Such behavior will further threaten the security of information transmission in this system. To address this issue, the active reconfigurable intelligent surface (RIS) provides a promising solution by suppressing eavesdropping channels and enhancing the quality of legitimate links. This paper focuses on a challenging security communication scenario in mmWave-NOMA networks. In the network, there is a dual-identity user who not only acts as a legitimate user but also attempts to eavesdrop on the information of the secure user. To address the above issue, this paper proposes an efficient security transmission scheme. Specifically, a successive interference cancellation order is designed for all users. Then, the sum secrecy rate is maximized by jointly optimizing the transmit beamforming, power allocation coefficients, and the reflecting coefficients matrix. To address this non-convex problem, we divide it into three subproblems. By applying the successive convex approximation (SCA) and the semi-definite relaxation (SDR) method to transform them into convex ones. Simulation results confirm the effectiveness of the proposed scheme and the potential of the active RIS in enhancing the security of mmWave-NOMA networks.