Sum Secrecy Rate Maximization in Movable Antenna-Assisted NOMA Secure Transmission Systems

Abstract

In this paper, we investigate the downlink non-orthogonal multiple access (NOMA) secure transmission systems, where a base station (BS) uses superposition coding to transmit information to users in the presence of multiple colluding eavesdroppers. To improve the information security, we propose a movable antenna (MA) and active beamforming (AB) assisted NOMA (MAABaN) secure transmission scheme, in which the BS is equipped with multiple MAs to enhance the physical layer security (PLS). To this end, we formulate a sum secrecy rate maximization optimization problem by jointly optimizing the AB vectors and the MA positions. To solve the complex non-convex problem, we employ the alternating optimization method to transform the original problem into two subproblems. Regarding the subproblem of MA positions, we analyze the coupling relationship between the position variables, and optimize one set of position variables while keeping others fixed. Then, we propose a low-complexity iterative algorithm for the MA positions based on multi-start gradient ascent (MSGA), and construct a joint optimization framework of MA and AB. Numerical results demonstrate that the proposed MAABaN scheme can effectively enhance PLS performance, and the proposed MSGA-based algorithm can also be applied to MA-assisted NOMA secure transmission systems.