On the Secrecy Capacity of Cooperative Wiretap Channel

Secure communications using physical layer characteristics has attracted many attentions in recent years. Secrecy capacity of communication systems can be increased by using multiple-antenna nodes. In this paper, we concentrate on the cooperative wiretap channel in which confidential messages are sent from a source to a legitimate receiver with the help of a relay in the presence of an eavesdropper. We study the secrecy capacity of Amplify and Forward (AF) relaying under peak power constraint at the relay node. To this end, we first evaluate an achievable secrecy rate for Gaussian input by solving a nonconvex optimization problem. Then, we prove the converse by using the secrecy capacity of genie-aided channel as an upper bound of the secrecy capacity of interested channel. We show that this upper bound is achievable and equal to achievable secrecy rate of Gaussian input.