Beamforming is a powerful tool for physical layer security, as it can be used for steering signals towards legitimate receivers and away from eavesdroppers. An active eavesdropper, however, can interfere with the pilot phase that the transmitter needs to acquire the channel knowledge necessary for beamforming. By doing so, the eavesdropper can make the transmitter form beams towards the eavesdropper rather than towards the legitimate receiver. To mitigate active eavesdroppers, we propose VILLAIN, a novel channel estimator that uses secret pilots. When an eavesdropper interferes with the pilot phase, VILLAIN produces a channel estimate that is orthogonal to the eavesdropper's channel (in the noiseless case). We prove that beamforming based on this channel estimate delivers the highest possible signal power to the legitimate receiver without delivering any signal power to the eavesdropper. Simulations show that VILLAIN mitigates active eavesdroppers also in the noisy case.

翻译：波束赋形是物理层安全的有力工具，可用于将信号导向合法接收方并避开窃听者。然而，主动窃听者能够干扰发射机获取信道知识（波束赋形所需）的导频阶段。通过这种干扰，窃听者可使发射机将波束指向自身而非合法接收方。为抑制主动窃听者，我们提出了一种名为VILLAIN的新型信道估计方法，该方法采用秘密导频。当窃听者干扰导频阶段时，VILLAIN会生成与窃听者信道正交（在无噪声情况下）的信道估计值。我们证明，基于该信道估计值的波束赋形能在不向窃听者输送任何信号功率的前提下，向合法接收方传递最大可能的信号功率。仿真结果表明，即使在有噪声情况下，VILLAIN也能有效抑制主动窃听者。