Secrecy transmission with a helper: To relay or not to relay

In this paper, we consider secure communications between a source and a destination helped by a relay, in the presence of an eavesdropper. We consider both path-loss and fading in our channel models. We investigate such a question that whether the relay operates as a tradition relay or a jammer. A one time slot cooperative jamming and two time slots cooperative relaying transmission scheme are proposed. For fairness, we consider the total power constraint and assume that the two proposed schemes are with the same power budget. Optimal power allocation among the information signal and jamming signal are derived. For the proposed schemes, we give the performance comparison in the high and low transmitted power regimes, which gives some insight into the choice of transmission schemes to achieve a better performance. Furthermore, We analyze the impact of eavesdropper's position on security in our simulation.