Secrecy Performance of Cell-Free Massive MIMO with Hardware Impairments in Spatially Correlated Fading Channels

This paper investigates the effects of hardware impairments and spatial correlation of fading channels on the secrecy performance of cell-free massive multiple-input multiple-output (MIMO) systems in the presence of the pilot spoofing attack. Specifically, the additive hardware distortion model is introduced to model the hardware impairments, and the minimum mean square error-based channel estimator in the uplink training phase is utilized to realize channel estimation. Besides, the maximum ratio combining is used for downlink data transmission, and a novel closed expression of the secrecy rate is derived. To improve secrecy performance, a heuristic power allocation scheme is further proposed. The analysis and simulation results show that hardware impairments would worsen the secrecy performance, which can be improved by increasing the number of antennas of access point. It is also shown that the accuracy of the channel estimation can be improved by enhancing the spatial correlation among the access point antennas. In addition, the pilot attack of the eavesdropper can be significantly suppressed by enhancing the spatial correlation and increasing the degree of spatial separation between the users and the eavesdropper.