  We consider a cognitive wireless powered communication network (CWPCN)
sharing the spectrum with a primary network who faces security threats from
eavesdroppers (EAVs). We propose a new cooperative protocol for the wireless
powered secondary users (SU) to cooperate with the primary user (PU). In the
protocol, the SUs first harvest energy from the power signals transmitted by
the cognitive hybrid access point during the wireless power transfer (WPT)
phase, and then use the harvested energy to interfere with the EAVs and gain
transmission opportunities at the same time during the wireless information
transfer (WIT) phase. Taking the maximization of the SU ergodic rate as the
design objective, resource allocation algorithms based on the dual optimization
method and the block coordinate descent method are proposed for the cases of
perfect channel state information (CSI) and collusive/non-collusive EAVs under
the PU secrecy constraint. More PU favorable greedy algorithms aimed at
minimizing the PU secrecy outage probability are also proposed. We furthermore
consider the unknown EAVs' CSI case and propose an efficient algorithm to
improve the PU security performance. Extensive simulations show that our
proposed protocol and corresponding resource allocation algorithms can not only
let the SU gain transmission opportunities but also improve the PU security
performance even with unknown EAVs' CSI.
