  This paper considers a movable antenna (MA)-aided secure multiple-input
multiple-output (MIMO) communication system consisting of a base station (BS),
a legitimate information receiver (IR) and an eavesdropper (Eve), where the BS
is equipped with MAs to enhance the system's physical layer security (PLS).
Specifically, we aim to maximize the secrecy rate (SR) by jointly optimizing
the transmit precoding (TPC) matrix, the artificial noise (AN) covariance
matrix and the MAs' positions under the constraints of the maximum transmit
power and the minimum distance between MAs. To solve this non-convex problem
with highly coupled optimization variables, the block coordinate descent (BCD)
method is applied to alternately update the variables. Specifically, we first
reformulate the SR into a tractable form by utilizing the minimum mean square
error (MMSE) method, and derive the optimal TPC matrix and the AN covariance
matrix with fixed MAs' positions by applying the Lagrangian multiplier method
in semi-closed forms. Then, the majorization-minimization (MM) algorithm is
employed to iteratively optimize each MA's position while keeping others fixed.
Finally, simulation results are provided to demonstrate the effectiveness of
the proposed algorithms and the significant advantages of the MA-aided system
over conventional fixed position antenna (FPA)-based system in enhancing
system's security.
