In this paper, we investigate secure and reliable transmission strategies formulti-cell multi-user massive multiple-input multiple-output (MIMO) systems inthe presence of an active eavesdropper. We consider a time-division duplexsystem where uplink training is required and an active eavesdropper can attackthe training phase to cause pilot contamination at the transmitter. This forcesthe precoder used in the subsequent downlink transmission phase to implicitlybeamform towards the eavesdropper, thus increasing its received signal power.We derive an asymptotic achievable secrecy rate for matched filter precodingand artificial noise (AN) generation at the transmitter when the number oftransmit antennas goes to infinity. For the achievability scheme at hand, weobtain the optimal power allocation policy for the transmit signal and the ANin closed form. For the case of correlated fading channels, we show that theimpact of the active eavesdropper can be completely removed if the transmitcorrelation matrices of the users and the eavesdropper are orthogonal. Inspiredby this result, we propose a precoder null space design exploiting the low rankproperty of the transmit correlation matrices of massive MIMO channels, whichcan significantly degrade the eavesdropping capabilities of the activeeavesdropper.
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