Analysis of Secure User-Centric Cell-Free Massive MIMO Network for Multi-UAV Communications

This paper investigates security aspect in cell-free massive MIMO communication network, where numerous access points (APs) equipped with multi-antenna are employed to serve multi unmanned aerial vehicles (UAVs). Considering the pilot contamination, active pilot attack and user-centric (UC) association rule, minimum mean square error (MMSE) approach and distributed conjugate beamforming are performed for uplink channel estimation and downlink data transmission. Significantly, the closed-form lower bound on achievable secrecy rate has been derived, which facilitates the analysis of the effects of various system parameters, such as UC rules, pilot allocation scheme, transmit power, AP antenna configuration, UAVs' height, etc. The resultants indicate that UC approach performs better than the conventional cell-free architecture, while pilot contamination and active pilot attacks degrade the achievable secrecy rate. In addition, it is shown that the secrecy rate is dominantly affected by the height of legitimate UAV. Finally, the derivations and findings are numerically verified via simulations.