  In this paper, we study a cell-free multiple-input multiple-output network
equipped with integrated sensing and communication (ISAC) access points (APs).
The distributed APs are used to jointly serve the communication needs of user
equipments (UEs) while sensing a target, assumed to be an eavesdropper (Eve).
To increase the system's robustness towards said Eve, we develop an ISAC
waveform model that includes artificial noise (AN) aimed at degrading the Eve
channel quality. The central processing unit receives the observations from
each AP and calculates the optimal precoding and AN covariance matrices by
solving a semi-definite relaxation of a constrained Cramer-Rao bound (CRB)
minimization problem. Simulation results highlight an underlying trade-off
between sensing and communication performances: in particular, the UEs
signal-to-noise and interference ratio and the maximum Eve's signal to noise
ratio are directly proportional to the CRB. Furthermore, the optimal AN
covariance matrix is rank-1 and has a peak in the eve's direction, leading to a
surprising inverse-proportionality between the UEs-Eve distance and optimal-CRB
magnitude.
