Exact Outage Probability of Cooperative Secrecy Transmission: Impact of Unavailable Relays

The impact of unavailable relays is a serious issue in cooperative communication schemes, where the operation state of relays affects optimal relay selection. In addition, secure signal transmission is an interesting approach to prevent the wiretapping actions of eavesdropper nodes in a wireless environment. In this paper, we propose two cooperative secrecy transmission schemes: a secrecy decode-and-forward protocol and a hybrid of direct secrecy and cooperative secrecy transmission schemes to enhance the spectrum utilization efficiency. In the proposed schemes, a source transmits data signals to a destination with the assistance of relays. At the same time, these data signals are eavesdropped by another node. The best relays are successfully selected in a safe manner based on both decoding from a source to a destination and the available state of cooperative relays. The secrecy performance of each scheme is analyzed and evaluated by the exact outage probability over Rayleigh fading channels. Monte Carlo simulations were performed to verify the theoretical analysis. Finally, a discussion and comparison of the cooperative secrecy transmission schemes and the direct secrecy transmission approach are presented.