In this letter, we consider secure communications in multi-hop relaying systems, where full-duplex relays (FDRs) operate to enhance wireless physical layer security. Each FDR is designed to transmit jamming signals to the eavesdropper when it receives information signals from the previous adjacent node.

The achievable secrecy rate with the proposed decode-and-forward (DF) FDRs are analyzed with a total transmit power constraint. The transmit power allocation problem is solved by using the geometric programming (GP) method. Numerical results present that the proposed FDRs significantly enhance the secrecy rate compared to the conventional half-duplex relays (HDRs).