This letter studies a challenging physical layer security problem for downlink transmissions when facing a randomly located active eavesdropper. Artificial noise (AN) is transmitted in conjunction with a confidential signal to defend against the eavesdropper. However, the active eavesdropper with changeable location can perform both eavesdropping and malicious jamming simultaneously, which yields the secrecy outage probability as an unsuitable performance metric. In this case, a more appropriate metric, the hybrid outage probability which takes both the transmission outage probability and secrecy outage probability into consideration, is derived to evaluate the security performance. Based on the hybrid outage probability, we can optimize the power allocation to the confidential and AN signals. Furthermore, an insecure region is defined to restrict the location of the eavesdropper. Additionally, the insecure range is defined as a valuable indicator of a circular insecure region. Simulation results are provided to depict the region around receiver is insecure.

中文翻译：

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随机定位的主动窃听者下行链路传输接收机周围的不安全区域

这封信研究了当面对随机定位的主动窃听者时下行链路传输的具有挑战性的物理层安全问题。人工噪声 (AN) 与机密信号一起传输以防御窃听者。然而，位置可变的主动窃听者可以同时执行窃听和恶意干扰，这导致保密中断概率作为不合适的性能指标。在这种情况下，推导出一个更合适的度量，即综合考虑传输中断概率和保密中断概率的混合中断概率来评估安全性能。基于混合中断概率，我们可以优化机密和 AN 信号的功率分配。此外，不安全区域被定义为限制窃听者的位置。此外，不安全范围被定义为圆形不安全区域的一个有价值的指标。提供了模拟结果来描述接收器周围的区域是不安全的。