Relay selection scheme for amplify-and-forward cooperative communication system with artificial noise

Cooperative communication can improve the performance of communication system under multi-path fading conditions by relaying. If there is an eavesdropper in communication system, the secrecy capacity of the system will decrease. Sending artificial noise can enhance the secrecy capacity of communication system. A novel scheme combining relay selection with artificial noise for amplify-and-forward cooperative communication system in the presence of an eavesdropper is designed, which seeks the relay with the highest signal-to-noise ratio. The location of optimal relay node is found out in this scheme by the relay selection scheme based on distance. However, there is not always a relay node at the optimal location. If there is no relay at the optimal location, then the suboptimal relay node can be found out by drawing circles centered on the optimal location. After that, the optimal relay forwards the signals and sends the artificial noise in the null space of the legitimate channel to confuse the eavesdropper. A close-form expression for maximizing the secrecy capacity is derived, and it is used as the objective function to select the optimal or suboptimal relay node. The algorithm complexity of the relay selection scheme based on distance is lower than that of the relay selection scheme based on instantaneous channel states. Moreover, it can achieve higher secrecy capacity compared with the scheme without artificial noise. Simulations are conducted to validate the theoretical analyses, and the results demonstrate the validity and reliable security of the scheme.