  Physical layer security (PLS) technology based on the fixed-position antenna
(FPA) has {attracted widespread attention}. Due to the fixed feature of the
antennas, current FPA-based PLS schemes cannot fully utilize the spatial degree
of freedom, and thus a weaken secure gain in the desired/undesired direction
may exist. Different from the concept of FPA, mobile antenna (MA) is a novel
technology that {reconfigures} the wireless channels and enhances the
corresponding capacity through the flexible movement of antennas on a minor
scale. MA-empowered PLS enjoys huge potential and deserves further
investigation. In this paper, we, for the first time, investigate the secrecy
performance of MA-enabled PLS system where a MA-based Alice transmits the
confidential information to multiple single-antenna Bobs, in the presence of
the single-antenna eavesdropper (Eve) {in the absence} of perfect channel state
information (CSI). For the purpose of the secrecy rate maximization of the
worst Bob, we jointly design the transmit beamforming and antenna positions at
the Alice, subject to the minimum moving distance of the antenna, uncertainty
CSI of Eve, and maximum transmit power. Furthermore, the projected gradient
ascent (PGA), alternating optimization (AO), and simulated annealing (SA) {are}
adopted to solve the non-convex characteristics of the problem of the secrecy
rate maximization. Simulation results demonstrate the effectiveness and
correctness of the proposed method. In particular, MA-enabled PLS scheme can
significantly enhance the secrecy rate compared to the conventional FPA-based
ones for different settings of key system parameters.
