  In this paper, we investigate physical layer security (PLS) for full-duplex
(FD) multi-user systems. To simultaneously protect uplink (UL) and downlink
(DL) transmissions and ensure efficient use of time-frequency resources, we
consider a base station (BS) that operates in FD mode and enables to emit the
artificial noise (AN). Conventional fixed-position antennas (FPAs) at the BS
struggle to fully exploit spatial degrees of freedom (DoFs). Therefore, we
propose a new paradigm for secure FD multi-user systems, where multiple
transmit and receive movable antennas (MAs) are deployed at the BS to serve UL
and DL users and effectively counter the cooperative interception by multiple
eavesdroppers (Eves). Specifically, the MA positions, the transmit, receive,
and AN beamformers at the BS, and the UL powers are jointly optimized to
maximize the sum of secrecy rates (SSR). To solve the challenging non-convex
optimization problem with highly coupled variables, we propose an alternating
optimization (AO) algorithm. This algorithm decomposes the original problem
into three sub-problems, which are iteratively solved by the proposed
multi-velocity particle swarm optimization (MVPSO) and successive convex
approximation (SCA). Simulation results demonstrate that the proposed scheme
for MA-aided secure FD multi-user systems can significantly enhance security
performance compared to conventional FPA systems.
