Secrecy performance of threshold-based cognitive relay network with diversity combining

In the spectrum sharing mode, the transmitting power of the secondary user (SU) is optimally controlled, such that no additional interference occurs at the primary user (PU). In this paper, the secrecy outage performance is analyzed for such cognitive underlay decode-and-forward (DF) threshold-based relay network. The relayed and the direct signals are combined at the secondary eavesdropper and destination. Unlike other works to date, we have used both selection combining (SC) and maximal ratio combining (MRC) diversity schemes at the destination and eavesdropper with all possible permutations, and the best possible combination for cognitive system secrecy has been validated. Further, we consider threshold-based relaying where, the relay can successfully decode and re-transmit, only if the received signal-to-noise ratio (SNR) meets the particular threshold. We show that the diversity scheme, the threshold SNR, the secrecy rate, and the interference power all have a significant impact on cognitive system secrecy. We have also shown that the secrecy performance is more sensitive to changes in the quality of direct link between source to eavesdropper than source to destination, especially at the higher secrecy rates. The accuracy of the analytical expressions is corroborated by the computer simulation results.