With fast growth of wireless services, secrecy has become an increasingly important issue for wireless networks. In this paper, we investigate the secrecy capacity of the primary system in a cognitive radio system based on artificial noise, which has been proposed for dealing with the eavesdropper. We first consider a special case of one eavesdropper and two regimes of the eavesdropping channel condition. Specifically, we analyze the impact of interference generated by a secondary system toward the primary system in a cognitive radio system. The channel state information (CSI) of the primary channel is assumed to be perfectly known at both the primary transmitter and receiver, whereas that of the eavesdropper is partially known. Under these assumptions, we derive analytical expressions for the ergodic secrecy capacity in the cases of strong eavesdropping channel and weak eavesdropping channel and analyze the impact of the secondary system on the primary ergodic secrecy capacity. Moreover, we extend the analysis to the general case of arbitrary eavesdropping channel condition and arbitrary number of eavesdroppers. Some numerical results will be also presented to verify the analysis
