Passive eavesdropping compromises confidentiality in wireless networks, especially in resource-constrained environments where heavyweight cryptography is impractical. Physical layer security (PLS) exploits channel randomness and spatial selectivity to confine information to an intended receiver with modest overhead. However, typical PLS techniques, such as using beamforming, artificial noise, and reconfigurable intelligent surfaces, often involve added active power or specialized deployment, and, in many designs, rely on precise time synchronization and perfect CSI estimation, which limits their practicality. To this end, we propose AmbShield, an AmBD-assisted PLS scheme that leverages naturally distributed AmBDs to simultaneously strengthen the legitimate channel and degrade eavesdroppers' without requiring extra transmit power and with minimal deployment overhead. In AmbShield, AmBDs are exploited as friendly jammers that randomly backscatter to create interference at eavesdroppers, and as passive relays that backscatter the desired signal to enhance the capacity of legitimate devices. We further develop a unified analytical framework that analyzes the exact probability density function (PDF) and cumulative distribution function (CDF) of legitimate and eavesdropper signal-to-interference-noise ratio (SINR), and a closed-form secrecy outage probability (SOP). The analysis provides clear design guidelines on various practical system parameters to minimize SOP. Extensive experiments that include Monte Carlo simulations, theoretical derivations, and high-SNR asymptotic analysis demonstrate the security gains of AmbShield across diverse system parameters under imperfect synchronization and CSI estimation.

翻译：被动窃听会破坏无线网络的机密性，尤其在资源受限环境中，重量级密码学方案往往不切实际。物理层安全利用信道随机性和空间选择性，以适度的开销将信息限定在目标接收者处。然而，典型的物理层安全技术，如使用波束成形、人工噪声和可重构智能表面，通常需要额外的主动功率或专门部署，并且在许多设计中依赖于精确的时间同步和完美的信道状态信息估计，这限制了其实用性。为此，我们提出AmbShield，一种环境反向散射设备辅助的物理层安全方案，该方案利用自然分布的环境反向散射设备，在不增加额外发射功率且部署开销最小的情况下，同时增强合法信道并劣化窃听信道。在AmbShield中，环境反向散射设备被用作友好干扰器，通过随机反向散射在窃听者处产生干扰；同时作为无源中继，通过反向散射期望信号以增强合法设备的信道容量。我们进一步建立了一个统一的分析框架，用于分析合法用户与窃听者的信号干扰噪声比的精确概率密度函数和累积分布函数，并推导出闭式的安全中断概率表达式。该分析为各种实际系统参数提供了清晰的设计指导，以最小化安全中断概率。包含蒙特卡洛仿真、理论推导以及高信噪比渐近分析在内的大量实验表明，在不完美同步和信道状态信息估计条件下，AmbShield在不同系统参数下均能实现显著的安全性能提升。