Optimal power allocation to improve secrecy performance of non-regenerative cooperative systems using an untrusted relay

To protect data communication from eavesdropper nodes, different techniques have been developed to improve the physical-layer security (PLS) of communications systems. Destination-based cooperative signalling (DBCS) is one of such techniques where the destination sends an intended artificial noise to the untrusted listeners helping to protect the source message from being captured reliably at eavesdroppers. In this study, the authors investigate the application of DBCS to improve the PLS, and as a consequence the secrecy performance of a two-hop amplify-and-forward cooperative system with an untrusted relay. To get the best performance out of DBCS, the transmit power of the source's signal as well as the artificial noise should be carefully adjusted. To address this, they have introduced the optimal power allocation to maximise the secrecy rate of the system under a sum-power constraint at the network nodes. For a system with large-scale antenna arrays at the base station, then then find the closed-form solution for the secrecy outage probability and the ergodic secrecy rate of the optimised system for both uplink and downlink. The presented simulation results validate the authors' theoretical analysis and reveal that the proposed DBCS with optimal power allocation significantly improves the secrecy performance of the system.