Cognitive DF Generalized Order Relay Selection Networks with Interference from Primary User

The generalized order relay selection scheme is efficient in situations where the opportunistic relaying scheme could fail. Such conditions include; the presence of imperfect channel-state-information (CSI) where the scheduling unit could fail in error in selecting the best relay among the available relays, the presence of outdated channel information (OCI) where the relay which was the best at the selection time instant could not be the best at the transmission time instant, and in situations where the best relay is busy in some scheduling duties in other parts of the network. Also, this scheme is efficient in situations where the best relay is busy in load balancing or scheduling duties in other parts of the network. In this paper, we introduce the generalized order relaying scheme for cognitive decode-and-forward (DF) relay networks and evaluate its performance in the presence of interference from primary user (PU) on the secondary user (SU). Closed-form and asymptotic expressions are derived for the outage probability in addition to deriving the diversity order and coding gain of the system. Monte-Carlo simulations are given to verify the achieved results. Main findings illustrate that with fixed interference power, the diversity gain of the secondary system linearly decreases with decreasing the number of relays, and vice versa increases. Furthermore, the diversity gain linearly increases with decreasing the order of the selected relay, and vice versa decreases. Also, results show that the interference from the PU transmitter at the SU relays is more severe on the system performance compared to that at the SU destination.