  Intelligent reflecting surface (IRS) is an emerging technology for wireless
communication composed of a large number of low-cost passive devices with
reconfigurable parameters, which can reflect signals with a certain phase shift
and is capable of building programmable communication environment. In this
paper, to avoid the high hardware cost and energy consumption in spatial
modulation (SM), an IRS-aided hybrid secure SM (SSM) system with a hybrid
precoder is proposed. To improve the security performance, we formulate an
optimization problem to maximize the secrecy rate (SR) by jointly optimizing
the beamforming at IRS and hybrid precoding at the transmitter. Considering
that the SR has no closed form expression, an approximate SR (ASR) expression
is derived as the objective function. To improve the SR performance, three IRS
beamforming methods, called IRS alternating direction method of multipliers
(IRS-ADMM), IRS block coordinate ascend (IRS-BCA) and IRS semi-definite
relaxation (IRS-SDR), are proposed. As for the hybrid precoding design,
approximated secrecy rate-successive convex approximation (ASR-SCA) method and
cut-off rate-gradient ascend (COR-GA) method are proposed. Simulation results
demonstrate that the proposed IRS-SDR and IRS-ADMM beamformers harvest
substantial SR performance gains over IRS-BCA. Particularly, the proposed
IRS-ADMM and IRS-BCA are of low-complexity at the expense of a little
performance loss compared with IRS-SDR. For hybrid precoding, the proposed
ASR-SCA performs better than COR-GA in the high transmit power region.
