  In this paper, a novel movable antenna (MA) empowered secure transmission
scheme is designed for cell-free symbiotic radio (SR) systems in the presence
of an eavesdropper (Eve). Specifically, multiple distributed access points
(APs) equipped with MAs collaboratively transmit confidential information to
the primary user (PU), in the meanwhile the backscatter device (BD) transmits
its own information to the secondary user (SU) by reflecting incident signals
from the APs. The MAs deployed at the APs can adjust their positions flexibly
to improve channel conditions between the APs and the PU/SU/BD and suppress the
eavesdropping from the Eve on confidential information at the PU. Under this
setup, we maximize the secrecy rate of primary transmission through jointly
optimizing the APs' transmission beamforming vectors and the positions of the
MAs, while adhering to the quality of service constraints at the SU. To address
the challenges caused by the non-convexity and find a near-optimal solution, an
alternating optimization (AO) framework is proposed, utilizing the successive
convex approximation method, the semi-definite relaxation technology and a
genetic algorithm modified particle swarm optimization (GA-PSO) algorithm.
Numerical results demonstrate the secrecy rate enhancement provided by
utilizing the MAs and show the impact of the GA-PSO algorithm for improving the
solving accuracy.
