Multiuser cognitive generalized order user selection with multiple primary receivers using orthogonal spectrums and imperfect channel estimation

In this paper, we propose a new scenario of multiuser cognitive generalized order user selection network with multiple primary receivers using orthogonal spectrums and imperfect channel estimation. Using orthogonal spectrum bands aims to mitigate the interference between users as in the downlink transmission in cellular networks. The generalized order user selection scheme is efficient in situations where a user other than the best user is erroneously selected by the scheduling unit for data reception as in imperfect channel estimation or outdated channel information conditions. In this scheme, the secondary user with the second or even the N^th best signal-to-noise ratio (SNR) is scheduled for data reception. The spectrum of the primary receiver whose channel results in the best performance for the secondary system is shared with the secondary users. Closed-form expressions are derived for the outage probability, average symbol error probability (ASEP), and ergodic channel capacity assuming Rayleigh fading channels. Furthermore, to get more insights about the system performance, the system is studied at the high SNR regime where the diversity order and coding gain are derived. The achieved analytical and asymptotic results are verified by Monte-Carlo simulations. Main results show that the number of primary receivers affects the system performance through affecting only the coding gain. Unlike the existing studies where the same spectrum band is shared by the primary receivers, our findings show that increasing the number of primary receivers in the proposed scenario enhances the system performance. Also, findings illustrate that a zero diversity gain is achieved by the system and a noise floor appears in the results when the secondary users' channels are imperfectly estimated.