Secrecy outage analysis and power allocation for decode‐and‐forward relay systems

Abstract

The decode-and-forward multi-relay system with the presence of an eavesdropper is researched in this study. Two widely used transmission schemes are considered, namely, the best-relay selection scheme and the all-relay-based beamforming scheme. To be specific, the former scheme adopts only the single best relay among all the candidate relays that have succeeded in decoding the source signal to assist the transmission, whereas the all-relay-based beamforming scheme allows all the candidates to assist the source simultaneously in a beamforming manner. The authors reveal that in both schemes, if the total transmit power of the two hops is constrained, contradiction exists between the two means of improving physical-layer security: (i) letting more relays succeed in decoding the source signal; (ii) increasing the transmit power of the selected relay(s). The authors thus analyse the secrecy outage behaviour of the two schemes and derive expressions of secrecy outage probability (SOP), based on which, the optimal power allocation solutions are obtained. It is demonstrated that in both schemes, the analysis results match well with the simulation results. Moreover, the proposed power allocation solutions surpass the conventional average power solution in SOP in both schemes.