  This paper studies a new and challenging wireless surveillance problem where
a legitimate monitor attempts to eavesdrop two suspicious communication links
simultaneously. To facilitate concurrent eavesdropping, our multi-antenna
legitimate monitor employs a proactive eavesdropping via jamming approach, by
selectively jamming suspicious receivers to lower the transmission rates of the
target links. In particular, we are interested in characterizing the achievable
eavesdropping rate region for the minimum-mean-squared-error (MMSE) receiver
case, by optimizing the legitimate monitor's jamming transmit covariance matrix
subject to its power budget. As the monitor cannot hear more than what
suspicious links transmit, the achievable eavesdropping rate region is
essentially the intersection of the achievable rate region for the two
suspicious links and that for the two eavesdropping links. The former region
can be purposely altered by the monitor's jamming transmit covariance matrix,
whereas the latter region is fixed when the MMSE receiver is employed.
Therefore, we first analytically characterize the achievable rate region for
the two suspicious links via optimizing the jamming transmit covariance matrix
and then obtain the achievable eavesdropping rate region for the MMSE receiver
case. Furthermore, we also extend our study to the MMSE with successive
interference cancellation (MMSE-SIC) receiver case and characterize the
corresponding achievable eavesdropping rate region by jointly optimizing the
time-sharing factor between different decoding orders. Finally, numerical
results are provided to corroborate our analysis and examine the eavesdropping
performance.
