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.