Title: Proactive Eavesdropping in Relaying Systems,
Abstract: This paper investigates the performance of a legitimate surveillance system,
where a legitimate monitor aims to eavesdrop on a dubious decode-and-forward
relaying communication link. In order to maximize the effective eavesdropping
rate, two strategies are proposed, where the legitimate monitor adaptively acts
as an eavesdropper, a jammer or a helper. In addition, the corresponding
optimal jamming beamformer and jamming power are presented. Numerical results
demonstrate that the proposed strategies attain better performance compared
with intuitive benchmark schemes. Moreover, it is revealed that the position of
the legitimate monitor plays an important role on the eavesdropping performance
for the two strategies.