Abstract
Nonorthogonal multiple (NOMA) access using successive interference cancellation and cognitive radio are two promising techniques for enhancing the spectrum efficiency and utilization for future wireless communication systems. This paper presents a NOMAbased cooperative hybrid spectrum sharing protocol for cognitive radio networks. A two phase decodeand forward (DF) relaying scheme in a multirelay scenario is considered. Each secondary transmitter is grouped into one of the two clusters: a noncooperative cluster (NCC) and a cooperative cluster (CC). The cluster head (CH) of the CC working as the best DF relay for the primary system is permitted to transmit its own signal superimposed on the primary signal using a NOMA approach in exchange for cooperation. On the other hand, the CH of the NCC transmits in parallel with the primary system satisfying a predefined peak transmit power and peak interference power constraints that guarantee a given primary quality of the service requirement. It is demonstrated that the performances of both the primary and secondary systems increase with the increasing number of secondary nodes. The simulation and theoretical results affirm the efficacy of the proposed protocol compared to the traditional overlay and underlay models in terms of the outage probability and the ergodic capacity.
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Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01061075).
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Appendix
Appendix

1.
PDF and CDF of \(X=\frac{\alpha _0}{\alpha _1}\): Let \(\alpha _0\) and \(\alpha _1\), be two exponential distributed random variables with mean \(\lambda _0\) and \(\lambda _1\), respectively. Then the probability density function (PDF) and the cumulative density function (CDF) of \(X=\frac{\alpha _0}{\alpha _1}\) can be expressed as [23]:
$$\begin{aligned} f_{X}(x)&=\frac{\lambda _0\lambda _1}{(\lambda _0+\lambda _1x)^2},x>0 \end{aligned}$$(45)$$\begin{aligned} F_{X}(Z)&=\int ^{Z}_0 \frac{\lambda _0\lambda _1}{(\lambda _0+\lambda _1x)^2}dx \nonumber \\&=\frac{Z\lambda _1}{\lambda _0+Z\lambda _1} \end{aligned}$$(46) 
2.
Calculating the value of \(\alpha _{ST_iPR}\): Let \(R_P\) be the primary target rate.The value of \(\alpha _{ST_iPR}\) can be calculated as follows:
$$\begin{aligned} R_{P}= & {} \frac{1}{2}\log _2\left( 1+\frac{\rho \alpha _{ST_iPR}\varPhi P_{ST}}{\rho \alpha _{ST_iPR}(1\varPhi )P_{ST}+\rho P_{int}+1}\right) \nonumber \\ 2^{2R_P}1= & {} \frac{\rho \alpha _{ST_iPR} \varPhi P_{ST}}{\rho \alpha _{ST_iPR}(1\varPhi ) P_{ST}+\rho P_{int}+1}\nonumber \\&\alpha _{ST_iPR}=\frac{R_{th}(\rho P_{int}+1)}{\rho P_{ST} \{\varPhi (1\varPhi )R_{th}\}} \end{aligned}$$(47)where \(R_{th}=2^{2R_P}1\).
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Kader, M.F., Shin, S.Y. Cooperative Hybrid Spectrum Sharing: A NOMAbased Approach. Wireless Pers Commun 96, 1–21 (2017). https://doi.org/10.1007/s1127701741489
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Keywords
 Nonorthogonal multiple access
 Successive interference cancellation
 Cognitive radio
 Spectrum sharing
 Cluster head
 Outage probability
 Ergodic capacity