Enhancing security in the cognitive relay assisted co-existing radio systems with interferences

We consider the problem of secret communication through co-existing radio systems assisted by cognitive relay (CR). Assuming a cognitive radio setup in which secondary base station (SBS) is allowed to transmit simultaneously with the primary base station (PBS) over the same channel, at first, we investigate the impact of interferences on the receive diversity of cognitive interference channels and quantify the loss of receive diversity order as well as the secrecy rate due to interferences. Then, we propose a scheme to improve the secrecy rate of secondary receiver (SR) making use of interference energy that already exists in the communication medium employing selective precoding at the CR. In the proposed scheme, interference provides an additional source of energy that enhances secrecy rate at the SR without the need to increase the transmitted power. We also calculate the secrecy rate of primary receiver (PR) in order to focus the performance of proposed scheme in enhancing security of the cognitive interference channels. Our results show that exploiting free interference power, the performance of secondary downlink can be improved without affecting the performance of PR.