Detection of active eavesdroppers in massive MIMO

We consider physical layer security of massive MIMO systems in TDD mode. We show that with massive MIMO a passive eavesdropper is not very dangerous and must therefore be active and attack the training phase. An attack on the training phase is potentially very harmful to the physical layer security, and we therefore investigate three different schemes for detecting the presence of an active eavesdropper. The three schemes differ in the location where the detection is done (base station, intended user, or jointly), and also in the level of system parameters that are assumed known to the base station and/or intended user.