Physical Layer Security in Cell- Free Massive MIMO

Abstract

The achievable secrecy rates for a cell-free massive multiple-input multiple-output (MIMO) in the presence of a single-antenna active eavesdropper are derived and compared with that of co-located massive MIMO. The active eavesdropper contaminates the uplink channel estimates at the access points (APs), and the downlink precoders at the APs are constructed based on this contaminated channel estimates. Hence, the APs inadvertently beamform confidential information towards the active eavesdropper during the downlink transmission. The achievable rates at the legitimate user nodes and the rates leaked into the active eavesdropper are derived for estimated/imperfect channel state information in the finite and infinite regimes of the number of APs. A transmit power allocation algorithm for maximizing the achievable secrecy rate is developed. Our analysis and numerical results reveal that the rate leaked into the eavesdropper can be of significance in the presence of active pilot attacks in cell-free massive MIMO.