  In this paper, we investigate secure and reliable transmission strategies for
multi-cell multi-user massive multiple-input multiple-output (MIMO) systems
with a multi-antenna active eavesdropper. We consider a time-division duplex
system where uplink training is required and an active eavesdropper can attack
the training phase to cause pilot contamination at the transmitter. This forces
the precoder used in the subsequent downlink transmission phase to implicitly
beamform towards the eavesdropper, thus increasing its received signal power.
Assuming matched filter precoding and artificial noise (AN) generation at the
transmitter, we derive an asymptotic achievable secrecy rate when the number of
transmit antennas approaches infinity. For the case of a single-antenna active
eavesdropper, we obtain a closed-form expression for the optimal power
allocation policy for the transmit signal and the AN, and find the minimum
transmit power required to ensure reliable secure communication. Furthermore,
we show that the transmit antenna correlation diversity of the intended users
and the eavesdropper can be exploited in order to improve the secrecy rate. In
fact, under certain orthogonality conditions of the channel covariance
matrices, the secrecy rate loss introduced by the eavesdropper can be
completely mitigated.
