Cell-Free Massive MIMO with Protective Partial Zero-Forcing and Active Eavesdropping

We consider a cell-free massive MIMO (CF-mMIMO) system with multi-antenna access points (APs) and distributed protective partial zero-forcing (PPZF) precoding, which is prone to an active eavesdropping attack during uplink training. We develop a tractable analytical framework to derive a novel closed-form expression for the spectral efficiency (SE) at the users and eavesdropper (Eve), and, hence, the secrecy SE. These closed-form expressions are of particular importance for enabling further system design. Our findings show that PPZF can substantially outperform the conventional maximum-ratio transmission (MRT) scheme especially when the ratio of number of AP antennas to the number of users is high. Moreover, the secrecy enhancement obtained by using a higher number of AP antennas is more pronounced when Eve is located farther away from the legitimate user. Finally, simulation results validate the accuracy of the derived theoretical analysis.