Intelligent reflecting surface (IRS) is a promising and disruptive technique to extend the network coverage and improve spectral efficiency. This paper investigates an IRS-assisted Terahertz (THz) multiple-input multiple-output (MIMO)-nonorthogonal multiple access (NOMA) system based on hybrid precoding in the presence of eavesdropper. Two types of sparse RF chain antenna structures are adopted, i.e., sub-connected structure and fully connected structure. Cluster heads are firstly selected for transmissions, and discrete phase-based analog precoding is designed for the transmit beamforming. Subsequently, based on the channel conditions, the users are grouped into multiple clusters, and each cluster is transmitted by using the NOMA technique. In addition, a low complexity zero-forcing method is employed to design digital precoding so as to eliminate interference between clusters. On this basis, we propose a secure transmission scheme to maximize the sum secrecy rate by jointly optimizing the power allocation and phase shifts of IRS under the constraints of system transmission power, achievable rate requirement of each user, and IRS phase shifts. Due to multiple coupled variables, the formulated problem leads to a non-convex issue. We apply the Taylor series expansion and semidefinite programming to convert the original non-convex problem into a convex one. Then, an alternating optimization algorithm is developed to obtain a feasible solution of the original problem. Simulation results are demonstrated to validate the convergence of the proposed algorithm, and confirm that the deployment of IRS can significantly improve the secrecy performance.

翻译：智能反射面(IRS)是一种具有颠覆性潜力的技术，能够扩展网络覆盖范围并提升频谱效率。本文研究了一种基于混合预编码的IRS辅助太赫兹(THz)多输入多输出(MIMO)-非正交多址接入(NOMA)系统，且系统存在窃听者。采用两种稀疏射频链天线结构：子连接结构与全连接结构。首先选择簇头进行传输，并针对发射波束成形设计了基于离散相位的模拟预编码。随后，根据信道条件将用户分组为多个簇，每个簇采用NOMA技术进行传输。此外，采用低复杂度的迫零方法设计数字预编码以消除簇间干扰。在此基础上，提出了一种安全传输方案，通过联合优化功率分配与IRS相移，在系统发射功率、各用户可达速率需求及IRS相移约束下最大化和保密率。由于多个耦合变量的存在，所建模问题呈现非凸特性。我们应用泰勒级数展开与半定规划将原始非凸问题转化为凸问题，进而开发了一种交替优化算法以获取原始问题的可行解。仿真结果验证了所提算法的收敛性，并证实IRS的部署能显著提升保密性能。