  This paper considers a MIMO secure wireless communication system aided by the
physical layer security technique of sending artificial noise (AN). To further
enhance the system security performance, the advanced intelligent reflecting
surface (IRS) is invoked in the AN-aided communication system, where the base
station (BS), legitimate information receiver (IR) and eavesdropper (Eve) are
equipped with multiple antennas. With the aim for maximizing the secrecy rate
(SR), the transmit precoding (TPC) matrix at the BS, covariance matrix of AN
and phase shifts at the IRS are jointly optimized subject to constrains of
transmit power limit and unit modulus of IRS phase shifts. Then, the secrecy
rate maximization (SRM) problem is formulated, which is a non-convex problem
with multiple coupled variables. To tackle it, we propose to utilize the block
coordinate descent (BCD) algorithm to alternately update the TPC matrix, AN
covariance matrix, and phase shifts while keeping SR non-decreasing.
Specifically, the optimal TPC matrix and AN covariance matrix are derived by
Lagrangian multiplier method, and the optimal phase shifts are obtained by
Majorization-Minimization (MM) algorithm. Since all variables can be calculated
in closed form, the proposed algorithm is very efficient. We also extend the
SRM problem to the more general multiple-IRs scenario and propose a BCD
algorithm to solve it. Finally, simulation results validate the effectiveness
of system security enhancement via an IRS.
