  To enhance the national security, there is a growing need for government
agencies to legitimately monitor suspicious communication links for preventing
intended crimes and terror attacks. In this paper, we propose a new wireless
information surveillance paradigm by investigating a scenario where a
legitimate monitor aims to intercept a suspicious wireless communication link
over fading channels. The legitimate monitor can successfully eavesdrop
(decode) the information of the suspicious link at each fading state only when
its achievable data rate is no smaller than that at the suspicious receiver. In
practice, such legitimate eavesdropping is particularly challenging, since the
legitimate monitor may be far away from the suspicious transmitter and cannot
eavesdrop efficiently. To overcome this issue, we propose a new approach,
namely proactive eavesdropping via cognitive jamming, in which the legitimate
monitor purposely jams the receiver so as to change the suspicious
communication (e.g., to a smaller data rate) for overhearing more efficiently.
In particular, we consider delay-sensitive and delay-tolerant applications for
the suspicious data communications, under which the legitimate monitor
maximizes the eavesdropping non-outage probability for event-based monitoring
and the relative eavesdropping rate for content analysis, respectively, by
optimizing its jamming power allocation over different fading states subject to
an average power constraint. Numerical results show that the proposed proactive
eavesdropping via cognitive jamming approach greatly outperforms the
conventional passive eavesdropping without jamming and the proactive
eavesdropping with constant-power jamming.
