Legitimate Monitoring via Cooperative Relay and Proactive Jamming.

With the development of communication technologies, many suspicious communication technologies have also been greatly improved. Therefore, one single monitor may not be able to afford in a monitoring process. In this paper, we propose a wireless communication system using two legitimate monitors for cooperative monitoring program, in which the signals sent by two monitors are respectively designed. One monitor is used as the cooperative relay, which is mainly used to improve the overall information transmission rate. The other monitor, which simultaneously receiving suspicious information and sending jamming signals, is used to ensure the successful eavesdropping process. Our purpose is to obtain the maximum eavesdropping rate by jointly design the transmit beamforming of two monitors. Two cases that the channel state information (CSI) is perfectly and imperfectly known by the monitors are considered. For the perfect CSI, several cases corresponding to the different conditions about the CSI from the suspicious transmitter to two monitors are discussed. In each case, a closed-form solution of the appropriate sending strategy is obtained, in which the physical meaning is also proposed. Then, for the imperfect CSI case, by employing the S-procedure method, we reformulate the robust beamforming design problem and solve it optimally. Finally, in the simulation results, the comparison between the cooperative monitoring scheme and the single monitoring scheme demonstrates that the proposed scheme is better in terms of eavesdropping rate.