Cognitive Opportunistic DF Relay Networks with Interference and Multiple Primary Receivers Using Orthogonal Spectrums

Most of existing papers on cognitive relay networks with multiple primary user (PU) receivers consider the scenario where the PU receivers utilize the same spectrum band. In this paper, we consider a new scenario where the PU receivers utilize orthogonal spectrum bands and the spectrum of a PU receiver whose channel enhances the performance of the secondary system is shared with the secondary user (SU) nodes. Using orthogonal spectrum bands in cellular networks aims to reduce the interference between users as in the downlink transmission where orthogonal frequency bands are used by the base station in transmitting the data for the different users. In this paper, we study the outage performance of cognitive opportunistic decode-and-forward relaying operating in the secondary network with multiple PU receivers and in the presence of interference from a PU transmitter. A closed-form expression is derived for the outage probability with all system links following Rayleigh distribution. Furthermore, to get more insights about the system behavior, the performance is studied at the high signal-to-noise ratio (SNR) regime where approximate expressions for the outage probability, diversity order, and coding gain are obtained. Monte Carlo simulations are given to validate the achieved results. Main findings illustrate that with fixed interference power, the diversity order of the secondary system equals the number of relays and it is not affected by the number of PU receivers. Also, results show that the number of PU receivers affects the system performance through affecting only the coding gain. Unlike the existing papers where the same spectrum band is assumed to be shared by the PU receivers, our findings demonstrate that increasing the number of PU receivers in the proposed scenario enhances the system performance. Finally, results illustrate that when the interference at the SU relays or at SU destination or at both scales with SNR, the system achieves a zero diversity order and a noise floor appears in the results due to the effect of interference on the system performance.