I. Introduction
Physical layer security has been considered as a viable alternative and complement to conventional cryptographic technologies [1], [2], [3], [4], and can be especially taken as a cooperative relay and jammer to enlist friendly neighboring nodes for improved security [5], [6], [7], [8], [9]. More precisely, a cooperative jammer sends artificial noise (AN) to selectively obstruct the receiver of a eavesdropper (Eve), protecting transmitted messages from being decoded by Eve.