  Wireless surveillance, in which untrusted communications links are
proactively monitored by legitimate agencies, has started to garner a lot of
interest for enhancing the national security. In this paper, we propose a new
cell-free massive multiple-input multiple-output (CF-mMIMO) wireless
surveillance system, where a large number of distributed multi-antenna aided
legitimate monitoring nodes (MNs) embark on either observing or jamming
untrusted communication links. To facilitate concurrent observing and jamming,
a subset of the MNs is selected for monitoring the untrusted transmitters
(UTs), while the remaining MNs are selected for jamming the untrusted receivers
(URs). We analyze the performance of CF-mMIMO wireless surveillance and derive
a closed-form expression for the monitoring success probability of MNs. We then
propose a greedy algorithm for the observing vs, jamming mode assignment of
MNs, followed by the conception of a jamming transmit power allocation
algorithm for maximizing the minimum monitoring success probability concerning
all the UT and UR pairs based on the associated long-term channel state
information knowledge. In conclusion, our proposed CF-mMIMO system is capable
of significantly improving the performance of the MNs compared to that of the
state-of-the-art baseline. In scenarios of a mediocre number of MNs, our
proposed scheme provides an 11-fold improvement in the minimum monitoring
success probability compared to its co-located mMIMO benchmarker.
