As an excellent tool for aiding communication, intelligent reflecting surface (IRS) can extend the coverage area, remove blind area, and achieve a dramatic rate improvement. In this paper, we improve the secret rate (SR) performance at directional modulation (DM) networks using IRS. To fully explore the benefits of IRS, two efficient methods are proposed to enhance SR performance. The first approach computes the confidential message (CM) beamforming vector by maximizing the SR, and the signal-to-leakage-noise ratio (SLNR) method is used to optimize the IRS phase shift matrix, which is called Max-SR-SLNR. Here, Eve is maximally interfered by transmitting artificial noise (AN) along the direct path and null-space projection (NSP) on the remaining two channels. To reduce the computational complexity, the CM, AN beamforming and IRS phase shift design are independently designed in the following methods. The CM beamforming vector is constructed based on maximum ratio transmission (MRT) criteria along the channel from Alice-to-IRS, and phase shift matrix of IRS is directly given by phase alignment (PA) method. This method is called MRT-NSP-PA. Simulation results show that the SR performance of the Max-SR-SLNR method outperforms the MRT-NSP-PA method in the cases of small-scale and medium-scale IRSs, and the latter approaches the former in performance as IRS tends to lager-scale.

翻译：作为辅助通信的卓越工具，智能反射面（IRS）可扩展覆盖区域、消除盲区并实现显著的速率提升。本文利用IRS改善定向调制（DM）网络的保密速率（SR）性能。为充分发掘IRS的优势，本文提出两种高效方法来增强SR性能。第一种方法通过最大化SR计算保密消息（CM）波束成形向量，并采用信号-泄漏-噪声比（SLNR）方法优化IRS相移矩阵，称为Max-SR-SLNR法。该方法中，窃听者受到沿直达路径发射人工噪声（AN）以及剩余两条信道采用零空间投影（NSP）的最大干扰。为降低计算复杂度，后续方法分别独立设计CM、AN波束成形及IRS相移矩阵：基于Alice到IRS信道上的最大比传输（MRT）准则构建CM波束成形向量，而IRS相移矩阵直接由相位对齐（PA）方法给出。该方案称为MRT-NSP-PA法。仿真结果表明，当IRS为小规模和中规模时，Max-SR-SLNR方法的SR性能优于MRT-NSP-PA方法；当IRS趋向大规模时，后者性能接近前者。