Jamming strategy and security performance analysis for the cooperative system with untrusted relay

In this paper, we investigate the cooperative jamming (CJ) strategy in a three-node cooperative system, where the relay, which forwards the received signals in a amplify-and-forward (AF) protocol, is untrusted, furthermore, there is an external malicious eavesdropper. A two-phase CJ scheme, which merely using internal nodes of the cooperative system without the help of external nodes, is proposed to enhance the secrecy information transmission against the malicious eavesdropper and the untrusted relay. And then, we provide the optimal power allocation with maximum instantaneous secrecy rate (OPA-MISR) and the suboptimal power allocation with max-min instantaneous secrecy rate (SPA-MMISR). Finally, the simulation results show that the cooperative jamming proposed is effective to improve the achievable instantaneous secrecy rate and to resist both internal untrusted relay and external malicious eavesdropper.