Proactive Eavesdropping of Two Suspicious Communication Links via Jamming

This paper studies a new and challenging wireless surveillance problem where a legitimate monitor (e.g., the National Security Agency in the USA) attempts to eavesdrop more than one suspicious communication links simultaneously to maximally protect public security. To facilitate concurrent eavesdropping, our multi-antenna monitor employs a proactive eavesdropping via jamming approach, by purposely jamming suspicious receivers to lower the transmission rates of the target links. In particular, we are interested in characterizing the monitor's achievable eavesdropping signal-to-interference-plus-noise ratio (SINR) region of two suspicious links, by optimizing the legitimate monitor's transmit covariance matrix for jamming the two suspicious receivers. As the monitor cannot hear more than what suspicious links transmit, the achievable eavesdropping SINR region is essentially the intersection of the achievable region for the two suspicious links and that for the two eavesdropping links, and the former region can be purposely altered by the monitor's jamming transmit covariance matrix subject to its power budget. As both suspicious links' SINRs are affected by monitor's jamming covariance matrix, we first analyze the achievable region bounds for both suspicious links and then characterize the achievable eavesdropping SINR region. Finally, numerical results are provided to corroborate our analysis.