Secure Multigroup Multicast Cell-Free Massive MIMO Transmission in the Presence of Phase Noise

This paper aims to explore the secrecy analysis and power resource allocation in multigroup multicast cell-free massive MIMO system with multi-antenna users in the presence of phase noise. Considering the presence of an active multiple-antenna eavesdropper, minimum mean squared error (MMSE) and distributed conjugate beamforming are exploited for channel estimation and downlink multicast transmission. Leveraging on random matrix theory, the analytical closed-form expression of the achievable secrecy rate is provided. The derived results enable to evaluate the effects of various parameters, which reveals the secrecy performance is degraded by phase noise and enhanced by adopting multi-antenna configuration. Analytical results illustrate that phase noise at users imposes more severe influence than that on access points (APs). In addition, it reveals that security of the concerned network is also restricted by the transmit power of the eavesdropper. Specifically, this work formulates the power allocation as a nonconvex optimization problem which enables to maximize the minimum achievable rate. Then, one iterative optimization algorithm is adopted to solve this problem based on continuous convex approximation. Finally, numerical results corroborate the derived expression and findings, and also verify the effectiveness of the proposed power control algorithm.