  This paper considers the physical layer security of a pilot-based massive
multiple-input multiple-output (MaMIMO) system in presence of a multi-antenna
jammer. To improve security of the network, we propose a new jamming detection
method that makes use of a generalized likelihood ratio test over some
coherence blocks. Our proposed method utilizes intentionally unused pilots in
the network. The performance of the proposed detector improves by increasing
the number of antennas at the base station, the number of unused pilots and
also by the number of the coherence blocks that are utilized. Simulation
results confirm our analyses and show that in the MaMIMO regime, perfect
detection (i.e., correct detection probability is one) is achievable even with
a small number of unused pilots.
