Legitimate Eavesdropping Scheme for Suspicious Relay Communication Networks

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 probability 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 probability 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.