  Intelligent reflecting surface (IRS) is of low-cost and energy-efficiency and
will be a promising technology for the future wireless communications like
sixth generation. To address the problem of conventional directional modulation
(DM) that Alice only transmits single confidential bit stream (CBS) to Bob with
multiple antennas in a line-of-sight channel, IRS is proposed to create
friendly multipaths for DM such that two CBSs can be transmitted from Alice to
Bob. This will significantly enhance the secrecy rate (SR) of DM. To maximize
the SR (Max-SR), a general non-convex optimization problem is formulated with
the unit-modulus constraint of IRS phase-shift matrix (PSM), and the general
alternating iterative (GAI) algorithm is proposed to jointly obtain the
transmit beamforming vectors (TBVs) and PSM by alternately optimizing one and
fixing another. To reduce its high complexity, a low-complexity iterative
algorithm for Max-SR is proposed by placing the constraint of null-space (NS)
on the TBVs, called NS projection (NSP). Here, each CBS is transmitted
separately in the NSs of other CBS and AN channels. Simulation results show
that the SRs of the proposed GAI and NSP can approximately double that of
IRS-based DM with single CBS for massive IRS in the high signal-to-noise ratio
region.
