  In this letter, secure beamforming in a multiple intelligent reflecting
surfaces (IRSs)-aided millimeter-wave (mmWave) system is investigated. In this
system, the secrecy rate is maximized by controlling the on-off status of each
IRS as well as optimizing the phase shift matrix of the IRSs. This problem is
posed as a joint optimization problem of transmit beamforming and IRS control,
whose goal is to maximize the secrecy rate under the total transmission power
and unit-modulus constraints. The problem is difficult to solve optimally due
to the nonconvexity of constraint conditions and coupled variables. To deal
with this problem, we propose an alternating optimization (AO)-based algorithm
based on successive convex approximation (SCA) and manifold optimization (MO)
technologies. Numerical simulations show that the proposed AO-based algorithm
can effectively improve the secrecy rate and outperforms the traditional single
IRS-aided scheme.
