  Frequency diverse array (FDA) is a promising antenna technology to achieve
physical layer security by varying the frequency of each antenna at the
transmitter. However, when the channels of the legitimate user and eavesdropper
are highly correlated, FDA is limited by the frequency constraint and cannot
provide satisfactory security performance. In this paper, we propose a novel
movable FDA (MFDA) antenna technology where the positions of antennas can be
dynamically adjusted in a given finite region. Specifically, we aim to maximize
the secrecy capacity by jointly optimizing the antenna beamforming vector,
antenna frequency vector and antenna position vector. To solve this non-convex
optimization problem with coupled variables, we develop a two-stage alternating
optimization (AO) algorithm based on block successive upper-bound minimization
(BSUM) method. Moreover, to evaluate the security performance provided by MFDA,
we introduce two benchmark schemes, i.e., phased array (PA) and FDA. Simulation
results demonstrate that MFDA can significantly enhance security performance
compared to PA and FDA. In particular, when the frequency constraint is strict,
MFDA can further increase the secrecy capacity by adjusting the positions of
antennas instead of the frequencies.
