Secure Power Control for Downlink Cell-Free Massive MIMO With Passive Eavesdroppers

This work studies secure communications for a cell-free massive multiple-input multiple-output (CF-mMIMO) network which is attacked by multiple passive eavesdroppers overhearing communications between access points (APs) and users in the network. It will be revealed that the distributed APs in CF-mMIMO allow not only legitimate users but also eavesdroppers to reap the diversity gain, which seriously degrades secrecy performance. In addition, by the principle of order statistics, the secrecy performance is further degraded as the number of eavesdroppers increases. Motivated by these, this work proposes an artificial noise-aided secure power control scheme for CF-mMIMO under passive eavesdropping aiming to achieve a higher secrecy rate and/or guaranteed secrecy performance, i.e., minimum secrecy rate. In particular, it will be demonstrated that the secure power control is especially important to guarantee the secrecy fairness. The performance of the proposed power control scheme is evaluated and compared with various power control schemes via numerical experiments, which clearly shows that the proposed power control scheme outperforms all the competing schemes.