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.

翻译：智能反射面（IRS）是一种由大量低成本可重构参数无源器件组成的新兴无线通信技术，能够以特定相位偏移反射信号，并构建可编程的通信环境。为避免空间调制（SM）中高昂的硬件成本与能量消耗，本文提出了一种基于混合预编码器的IRS辅助混合安全空间调制（SSM）系统。为提升安全性能，我们通过联合优化IRS处的波束赋形与发射端处的混合预编码，构建了以最大化安全速率（SR）为目标的优化问题。鉴于SR无闭式表达式，推导了近似安全速率（ASR）表达式作为目标函数。为改善SR性能，提出了三种IRS波束赋形方法：IRS交替方向乘子法（IRS-ADMM）、IRS块坐标上升法（IRS-BCA）及IRS半定松弛法（IRS-SDR）。在混合预编码设计方面，提出了近似安全速率-逐次凸近似（ASR-SCA）方法与截止速率-梯度上升（COR-GA）方法。仿真结果表明，所提IRS-SDR与IRS-ADMM波束赋形方案较IRS-BCA获得了显著的安全速率性能增益。特别地，所提IRS-ADMM与IRS-BCA具有低复杂度特性，但相较于IRS-SDR存在少量性能损失。对于混合预编码，所提ASR-SCA方法在高发射功率区域表现优于COR-GA。