Secure Optimal Precoding for User-Centric Cell-Free Massive MIMO System

The security of user-centric cell-free massive multiple-input multiple-output (UC-CF mMIMO) system in the presence of multiple collusive eavesdroppers (Eves) is investigated in this letter. The access points (APs) share the estimated channel state informations (CSIs) of serving users to central processing unit (CPU). Utilizing the received partial CSIs, the CPU optimizes precoding via formulating a secrecy rate maximization problem under minimum rate requirements of users and power constraints of APs. The formulated problem is non-convex, and the optimum solution is acquired by iteratively solving a sequence of second-order cone programming problems. The numerical simulations present the comparison of secrecy performance achieved in different AP selection methods of UC-CF system, and CF system.