为了提高面向可疑中继网络的合法窃听性能，增强合法监测中心对通信网络的监管能力，针对由单根天线的可疑节点和多根天线的合法监听者组成的合法监听模型，提出了先窃听后干扰窃听（EJS）、两时隙持续窃听（CES）和先干扰后窃听（JES）3种窃听方案，推导了相应的可疑中断概率、窃听非中断概率和平均窃听速率的闭式表达式。特别地，在EJS和JES方案中，求解了最大化平均窃听速率的优化问题，给出了合法监听者采用单天线发射干扰信号的发射信噪比（SNR）的近似最优解表达式。结合前述3种方案的优势，在合法监听者与可疑节点之间的链路部分可用和全部可用的场景下，设计了基于最大化窃听非中断概率的最优窃听模式选择方案（OEMS），通过比较上述3种方案的窃听非中断概率，选择最高窃听非中断概率值对应的方案作为窃听手段，从而实现最佳的监听性能。仿真结果表明，在合法监听者更靠近可疑中继时，与EJS方案相比，CES和JES方案在低信噪比区域分别具有更高的窃听非中断概率和窃听速率，而且所提的OEMS能达到最优的窃听性能，同时具有更好的场景适应性。在同等条件下，与采用单天线的监听者相比，采用双天线的监听者具有如下优势：为实现持续监听，其干扰信号发射功率可降低15 dB左右；EJS和JES方案的平均窃听速率在低SNR区域最高可提升0.8 bits/（s·Hz）左右。因此，增加合法监听者的天线数量不仅能节省总的发射功耗，还可增强其监听能力。

In order to improve the legal eavesdropping performance against the suspicious relay networks and enhance the monitoring ability of the legal monitoring center in communication networks, the study proposed an eavesdropping-then-jamming scheme (EJS), a two-slot continuous eavesdropping scheme (CES) and a jamming-then-eavesdropping scheme (JES) for a legal interception model consisting of suspicious nodes with a single antenna and legitimate listeners with multiple antennas, and derived the closed-form expressions, including the outage probabi-lity of suspicious communications, the non-outage probability of eavesdropping and the average eavesdropping rate of each scheme. In particular, for EJS and JES schemes, the optimization problem of maximizing the average eavesdropping rate was solved, and the approximate optimal solution expression of the transmitting signal-to-noise ratio (SNR) of the legitimate monitor with a single antenna was given. By integrating the advantages of the previous three schemes, the optimal-eavesdropping-mode selection scheme (OEMS) based on maximizing the non-outage probabi-lity of eavesdropping was designed in the scenario where part or all of the links between the legitimate surveillance and the suspicious nodes are available. Specifically, by comparing the non-outage probability of eavesdropping of the above three schemes, the scheme corresponding to the highest value was selected as the eavesdropping means. Simulation results show that when the legitimate monitor is closer to the suspicious relay, compared with the EJS, CES has higher non-outage probability of eavesdropping and JES has higher eavesdropping rate in low SNR areas. The proposed OEMS can achieve the optimal secrecy performance and has better adaptability for more scenes. Compared to the legitimate monitor with single antenna, the legitimate monitor with double antennas has more advantages as following: in order to achieve continuous monitoring, the transmitting power of interference signal can be reduced by about 15 dB; and the average eavesdropping rate of EJS scheme and JES scheme can be increased about 0.8 bits/(s·Hz) in low SNR region. Therefore, increasing the antenna number of legitimate monitor can not only save the power cost, but also enhance the monitoring ability.