Opportunistic Transmission of Nonregenerative Network With Untrusted Relay

In this paper, we consider an opportunistic transmission scheme of nonregenerative relay networks consisting of a source, a destination, and an untrusted relay, where the relay has a different security clearance from the source-destination pair. In this network, the information is transmitted through the direct link if the secrecy rate of the direct link is larger than or equal to that of the triangular link, and the information is transmitted through the triangular link if the secrecy rate of the direct link is smaller than that of the triangular link. For the network, we derive the exact secrecy outage probability in integral form and its closed-form lower bound. When the required secrecy rate is larger than a certain threshold value (0.6942 b/s/Hz), we show that the help of the untrusted relay is unnecessary if the source broadcasts the information and the half-duplex mode is considered.