## 1 Introduction  
Anticipating future advancements in mobile communications, unmanned aerial vehicle (UAV) communication systems are expected to play a pivotal role in the development of space-air-ground integrated networks (SAGINs) for sixth-generation (6G) communications [1, 2, 3]. As emphasized in [1], UAVs have become indispensable for the realization of SAGINs due to their remarkable versatility and adaptability. In particular, references [4, 5] highlight their extensive applications across various domains, such as surveillance and disaster management. In the context of secure communication, UAVs offer significant potential as nodes for enhancing data confidentiality, providing innovative solutions to safeguard sensitive information during transmission. Nevertheless, the openness inherent in wireless communication environments--especially in air-to-ground channels--introduces significant vulnerabilities. In particular, waveform leakage increases the risk of signal interception, making secure and reliable communication within UAV networks an urgent research priority. To mitigate interception risks, significant advancements have been made in the realm of physical layer security (PLS) technologies [6]. Unlike conventional encryption techniques, these approaches leverage the unique characteristics of wireless channels to enhance data security [7]. Specifically, the use of artificial noise (AN) and directional modulation (DM) has garnered considerable attention as effective strategies for enhancing the security of wireless communication systems. For instance, AN, as proposed in [8], manipulates the null space of the legitimate receiver's channel to effectively jam the eavesdropper's reception without imposing an additional burden on the legitimate receiver. Additionally, the integration of UAV trajectory design and AN to thwart eavesdropping from multiple colluding curious users was introduced in [9]. Another significant contribution was presented in [10], where an optimal power allocation strategy was investigated to maximize secrecy rates in the presence of AN, ensuring that the legitimate receiver's signal strength remains adequately protected. Despite these advancements, several challenges persist in the effective application of AN, particularly regarding the efficiency of AN in dynamic, multi-user environments. The fundamental trade-off between interference power and effective signal energy remains a critical area of exploration, underscoring the need for further optimization to improve the practical implementation of AN in secure communication systems. On the other hand, DM has attracted considerable attention for its potential to enhance communication security [11]. In particular, the study in [12] introduced a novel approach by combining DM with beamforming to focus the transmitted signal toward a specific receiver, thus reducing the likelihood of interception by unauthorized users. Additionally, the integration of DM with multiple antennas was explored in [13] to improve the robustness of secure communications in multipath environments. This strategy enhanced the transmission's security by mitigating the effects of eavesdroppers situated in diverse spatial regions. Furthermore, the work presented in [14] proposed a dynamic directional modulation (DDM) technique to enhance the privacy of wireless transmissions, which was evaluated using information leakage measures. Despite the effectiveness of these techniques, the security performance of DM approaches is significantly compromised when eavesdroppers are in close proximity to legitimate users. This limitation underscores the ongoing challenges in optimizing DM strategies to ensure robust security in dynamic and adversarial environments. While both AN and DM techniques have shown promise in enhancing communication security, their combined application has also been investigated.