Full-Duplex Decode-and-Forward Relaying: Secrecy Rates and Optimal Power Allocation

This paper investigates the secrecy rates and optimal power allocation schemes of a decode-and-forward (DF) wiretap relay channel where a relay operates in a full-duplex (FD) mode. A practical self-interference model is adopted to take into account the effect of residual self-interference. At first, we demonstrate that while the optimal power allocation schemes between the source and the relay are non-convex, closed-form solutions can still be established under different power constraints. An asymptotic behavior of the solutions is then provided to shed important insights on the derived power allocation schemes. Specifically, by using the method of dominant balance, we show that the relay should use full power when its power budget is sufficiently small as compared to power budget at the source. Otherwise, the optimal power consumed at the relay approaches a constant to effectively handle the residual self-interference. The analysis is also helpful to demonstrate that the secrecy capacity of the considered full-duplex relay system is twice as much as that of the half-duplex system. In addition, numerical results reveal that DF relaying provides significantly higher secrecy rate over Amplify-and-Forward (AF) relaying.