Active-RIS enabled secure NOMA networks: Joint transmit/reflect beamforming optimization

Abstract

The integration of Non-Orthogonal Multiple Access (NOMA) and Reconfigurable Intelligent Surface (RIS) provides a powerful method to improve the coverage and capacity of wireless communication systems. However, protecting the communication security of RIS assisted networks is particularly important in situations where eavesdroppers are able to actively interfere with the communication. Passive RIS cannot effectively cope with the dual fading problem in signal transmission. To address this, we propose an active RIS assisted NOMA scheme. Active RIS can enhance communication security by adjusting the reflective characteristics of the signal. Additionally, we introduce artificial noise to reduce the information accessible to eavesdroppers. Specifically, by jointly optimizing the transmit beamforming at the base station, the artificial noise signal and the reflection coefficients of the active RIS, the security of the legitimate users and the maximization of the rate are achieved under the conditions of satisfying the user rate requirements, the power constraints of the base station and the active RIS, and the Successive Interference Cancellation (SIC). To solve this nonconvex optimization problem, this paper decomposes it into two subproblems and converts it into a convex problem using Successive Convex Approximation (SCA) and first-order Taylor’s method, and proposes an alternative optimization algorithm for iterative solution. Empirical data indicates that the scheme in question yields a minimum enhancement of 50% when active RIS and artificial noise are utilized, as opposed to a passive RIS scheme, thus offering a robust and safeguarded approach for the development of future wireless communication systems.