In this paper, the physical layer security is investigated in a dual-hop wireless communication scenario, where an energy-constrained relay not only assists the source-to-destination information transmission, but also acts as a potential eavesdropper wiretapping the source information. We take into account both antenna mode switching and destination selection for the amplify and forward relaying network. The untrusted relay (UR) is equipped with two antennas, where one antenna that maximizes the source-to-relay channel gain will be selected for reception and the other one is exploited to forward the processed information. Moreover, the $N$th best destination is considered as the legitimate destination, which aims to achieve a better secrecy rate. Specially, in general signal-to-noise ratio regime, a low-complexity Gaussian Q-function is introduced to derive the exact closed-form expression of ergodic secrecy rate (ESR). In addition, when the UR works in a non-linear energy harvesting mode, the achievable secrecy rate maximization problem is formulated and solved by an alternative optimization method. The simulation results reveal the correctness of the theoretical derivation and also proves that much higher secrecy performance can be achieved by the proposed scheme than other benchmark schemes.

在本文中，在双跳无线通信方案中研究了物理层安全性，其中能量受限的中继不仅协助源到目的地的信息传输，而且还充当窃听源信息的潜在窃听者。我们同时考虑了放大和正向中继网络的天线模式切换和目的地选择。不受信任的中继（UR）配备有两个天线，其中将选择一个将源到中继信道增益最大化的天线进行接收，并利用另一根天线转发已处理的信息。而且，$ñ$最佳目的地被认为是合法目的地，旨在达到更高的保密率。特别地，在一般的信噪比方案中，引入了一种低复杂度的高斯Q函数来得出遍历保密率（ESR）的精确闭合形式。此外，当UR以非线性能量收集模式工作时，可替代的优化方法提出并解决了可达到的保密率最大化问题。仿真结果证明了理论推导的正确性，并且证明了所提出的方案比其他基准方案能够实现更高的保密性能。