Spectrum-sharing DF generalized order relay selection with interference and multiple primary users using orthogonal spectrums

In this paper, we propose and evaluate the performance of cognitive decode-and-forward (DF) generalized order relay selection network where the primary user (PU) receivers utilize orthogonal spectrums in the presence of interference. We consider a new scenario where the primary receivers utilize orthogonal spectrum bands and the spectrum of the primary 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 (BS) in transmitting the data for the different users. The generalized order relay selection scheme is efficient in situations where the opportunistic relaying scheme could fail such as in conditions of imperfect channel-state-information, outdated channel information, and in cases where the best relay is busy in some scheduling and load balancing duties in other parts of the network. Closed-from expression is derived for the end-to-end (e2e) outage probability assuming Rayleigh fading channels. Furthermore, to get more insights about the system behavior, the performance is studied at the high signal-to-noise ratio (SNR) regime where the diversity order and coding gain are derived. Monte-Carlo simulations are given to verify the achieved results. The main results show that the number of primary receivers affects the system performance through affecting the coding gain and not the diversity order. Unlike the existing papers where the same spectrum band is assumed to be shared by the primary receivers with the secondary users, our findings demonstrate that increasing the number of primary receivers in the proposed scenario enhances the system performance. Finally, results illustrate that when the interference at the secondary relays or 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.