Secrecy Rate of Two-Hop AF Relaying Networks with an Untrusted Relay

Two cooperative communication protocols, two-way and one-way half-duplex relayings, are investigated and then compared in the amplify-and-forward (AF) mode. The relay is assumed to play the roles of eavesdropper and relay (i.e., an untrusted relay). With a fair power constraint at each node, the secrecy rate of the one-way relaying protocol is proved to be zero strictly. For the two-way relaying protocol, the signal-to-noise ratio (SNR) threshold is derived, above which the secrecy rate is non-zero. Defining a parameter, namely mistrust level, and non-zero secrecy rate can be achieved for the one-way relaying protocol in certain range of the mistrust level at high SNR. With a fair total power constraint, no matter how unreliable the relay is, the two-way AF half-duplex relaying protocol is proved to be a better choice under high SNR.