Intelligent reflecting surface (IRS) is a promising 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 with the presence of eavesdropper. Two types of sparse RF chain antenna structures are adopted, i.e., sub-connected structure and fully connected structure. First, cluster heads are selected for each beam, and analog precoding based on discrete phase is designed. Then, users are clustered based on channel correlation, and NOMA technology is employed to serve the users. In addition, a low-complexity forced-zero method is utilized to design digital precoding in order to eliminate inter-cluster interference. 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 subject to the total transmit power budget, minimal achievable rate requirement of each user, and IRS reflection coefficients. 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 verify the convergence of the proposed algorithm, and deploying IRS can bring significant beamforming gains to suppress the eavesdropping.

翻译：智能反射面（IRS）是一种能够扩展网络覆盖范围并提升频谱效率的前沿技术。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，并考虑了存在窃听者的场景。系统采用了两种稀疏射频链路天线结构，即子连接结构与全连接结构。首先，为每个波束选取簇头，并设计基于离散相位的模拟预编码；随后基于信道相关性进行用户分簇，并利用NOMA技术服务用户。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，通过联合优化功率分配与IRS相移（在总发射功率预算、各用户最小可达速率需求及IRS反射系数约束下），提出了一种最大化系统和保密速率的安全传输方案。由于存在多个耦合变量，所构建的问题呈现非凸特性。我们利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，并且部署IRS能够带来显著的波束成形增益以抑制窃听。