  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 allows not only legitimate users but also eavesdroppers to reap the
diversity gain, which seriously degrades secrecy performance. Motivated by
this, this work proposes an artificial noise (AN)-aided secure power control
scheme for CF-mMIMO under passive eavesdropping aiming to achieve a higher
secrecy rate and/or guarantee security. In particular, it will be demonstrated
that a careful use of AN signal in the power control is especially important to
improve the secrecy performance. 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.
