Secrecy Improvement of Cognitive Relay Networks using Modulation Diversity

In this paper, physical-layer security of cognitive relay network under spectrum sharing environment using a decode-and-forward (DF) relay is studied and improved by incorporating modulation diversity into the secondary system. Modulation diversity rotates the signal constellation to make each component of the signal point unique and then interleaves quadrature components of the signal points to affect each component by independent fading channel. Modulation diversity facilitates source and intermediate relay to transmit different symbols to the destination and thus the proposed system transmits two symbols in two time slots compared to the conventional underlay cognitive relay network which transmits one symbol in two time slots. Hence, the secrecy capacity of the proposed system is greater than that of the conventional system. Therefore, the secrecy performance analysis and results of the proposed system are completely different than those of the conventional system. Moreover, it is shown that the secrecy outage probability of the proposed system is improved by 10 times compared to the conventional system. Finally, the closed-form expressions for the secrecy outage probability, probability of non-zero secrecy capacity and probability of intercept of the proposed system are derived and discussed.