Secure Communication with a Relay Helping the Wire-tapper

A four terminal Gaussian network, composed of a single source-destination pair, a relay and a wire-tapper is considered. Unlike the relay channel with a wire-tapper, it is assumed that the relay assists the wire-tapper, not the destination. The relay's objective is to decrease the achievable secrecy rates. However, since the destination is also allowed to listen to the relay's transmission, it also benefits from the relay in terms of achievable rates. Direct transmission, amplify-and-forward (AF), decode-and-forward (DF) and compress-and-forward (CF) relaying schemes are compared in terms of secrecy rates. It is shown that the best relaying strategy depends on relay's location. Comparison of relaying protocols and best power allocation schemes, when the relay assists the source-destination communication, do not readily extend to the case when the relay assists the wire-tapper.