  In this paper, we enhance physical layer security for downlink heterogeneous
networks (HetNets) by using friendly jammers and full-duplex users. The jammers
are selected to transmit jamming signal if their interfering power on the
scheduled users is below a threshold, meanwhile the scheduled users confound
the eavesdroppers using artificial noise by full-duplexing. Using the tools of
stochastic geometry, we derive the expressions of connection probability and
secrecy probability. In particular, the locations of active jammers are modeled
by a modified Poisson hole process (PHP). Determining the jammer selection
threshold is further investigated for connection probability maximization
subject to the security constraints. A greedy algorithm is proposed to
efficiently solve this problem. The accuracy of the theoretical analysis and
the efficiency of the proposed algorithm are evaluated by numerical
simulations.
