## 1 Introduction
The authors of [13] propose a low-complexity security precoding scheme based on GSVD for the visible light communication wiretap model. Secure transmission in relay communication wiretap models is studied in [14], where the base station uses GSVD to diagonalize the relay and user channels, generating independent parallel channels. In multi-user systems, a MIMO downlink communication scenario involving a base station and two users is considered in [15]. To make full use of spectral resources, a transmission protocol combining GSVD and non-orthogonal multiple access (NOMA) is proposed. In Ref. [16], a highly efficient precoding method leveraging GSVD and vector perturbation is proposed. In Ref. [17], a GSVD-based multi-user MIMO precoding scheme is studied, which transforms the multi-user MIMO channel into precoding weights to enhance the separability of effective channels among users. Compared to the microwave frequency band, the mmWave can provide a larger bandwidth to meet the communication demands for ultra-high transmission rates. Moreover, the hybrid beamforming (HBF) architecture used in mmWave communication enables a trade-off between system power consumption and spectral efficiency. The authors of [18] study secure transmission in unmanned aerial vehicle (UAV) scenarios and propose an HBF architecture based on generalized eigenvalue decomposition (GEVD) and SVD. When the ideal CSI of users is unknown, a neural network is constructed to learn the statistical features of the wireless channel for beamforming design. Even in the absence of the eavesdropper's CSI, this method can still achieve a positive secrecy rate. The secure design of intelligent reflecting surface (IRS)-assisted mmWave systems is studied in [19], where the base station simultaneously transmits information signals and AN. The IRS performs passive beamforming (PB) to reconstruct the wireless channel and optimize the secrecy rate. For IRS-assisted mmWave MISO systems, secure transmission in the scenario with multiple distributed eavesdroppers is investigated in [20]. Under the condition of unknown perfect cascaded eavesdropping channels, a robust secure beamforming design is proposed. It can be seen that, although some studies on the physical layer security of mmWave communication have been conducted, they mostly rely on methods such as beamforming or AN without exploring the inherent characteristics of mmWave channels. In fact, due to high path loss and limited propagation paths, mmWave channels exhibit inherent sparsity, which is perfectly reflected in the beamspace channel matrix. Additionally, wireless channels possess uniqueness, distinguishing legitimate user's channel from the eavesdropping channel, forming the foundation for secure beam design. On the other hand, unlike traditional microwave communication, mmWave communication relies on large-scale arrays to counteract path loss. To reduce the power consumption and hardware costs associated with large-scale arrays, hybrid precoding architectures are employed, requiring the design of both a baseband precoder in the digital domain and an RF precoder in the analog domain. Inspired by the sparsity of mmWave channels, we selected the array response vectors corresponding to the dominant paths in the beamspace channel representation to construct the RF precoder. The transmit beams generated by these array response vectors exhibit significant power. Furthermore, by linearly combining these array response vectors using a baseband precoder, the security performance of the hybrid precoder can approximate that of the optimal digital precoder obtained through the GSVD-based scheme. The main contributions of this paper are as follows:  
1. A mmWave massive MIMO analog-digital hybrid precoding model was constructed and the sparsity characteristic of mmWave channels was investigated. This sparsity was validated in the beamspace channel matrix, which contains only a few dominant propagation paths. Furthermore, we analyzed the eavesdropping channel model and provided its secrecy capacity expression. 2.