  In this paper, we propose to utilize intelligent reflecting surfaces (IRSs)
for enhancing the physical layer security of wireless communications systems.
In particular, an IRS-assisted secure wireless system is considered, where a
multi-antenna transmitter communicates with a single-antenna receiver in the
presence of an eavesdropper. To maximize the secrecy rate, both the beamformer
at the transmitter and the IRS phase shifts are jointly optimized. Based on the
block coordinate descent (BCD) and minorization maximization (MM) techniques,
two efficient algorithms are developed to solve the resulting non-convex
optimization problem for small- and large-scale IRSs, respectively. Simulation
results show that IRSs can significantly improve physical layer security if the
proposed algorithms are employed. Furthermore, we reveal that deploying
large-scale IRSs is more efficient than enlarging the antenna array size of the
transmitter for both boosting the secrecy rate and enhancing the energy
efficiency.
