# Hybrid Cooperative Spectrum Sharing Protocols Based on OMA and NOMA in Cognitive Radio Networks
## 6 Conclusions  
In this paper, we proposed a new hybrid cooperative CRNs based on OMA and NOMA technology, if the SU shares the spectrum of the PU directly without any constraints, the PU may be adversely affected. Therefore, the PU is transmitted once in the first time slot, and then the transmission mode is switched according to the ACK/NACK feedback information of PR to improve spectrum utilization. As a reward, the PU can retransmit in overlay NOMA mode or transfer new packets in underlay OMA mode. The proposed cooperative CRNs is divided into three modes: non-CR transmission mode, underlay mode, and overlay cooperative transmission mode. In the non-CR transmission mode, PT sends a packet to PR, ST, and SR, and ACK/NACK feedback from PR. If PR sends ACK, the transmission switches to underlay OMA mode. In this case, the PU and SU accessed the spectrum simultaneously. If PR sends a NACK feedback, the transmission mode switches to the overlay mode. The ST either sends a superimposed signal consisting of the decoded signal from PT and its own signal intending to SR, or acts as a relay to forward the decoded signal from PR. Accordingly, we established a one-dimensional Markov model according to the switching states between cognitive modes and the state of the PU, and formulized the transition probabilities and steady-state probabilities between states. We obtained the end-to-end outage probability, the probability of PU success in each state, and the throughput performance of the PU and SU. Finally, From the numerical results and simulation results, we validated that the performance of the proposed hybrid CRNs based on NOMA and OMA is better than that of the hybrid CRNs based on pure OMA. For future work, we can consider multi-antenna NOMA or next generation wireless communication technologies.  
Figure 10: Throughput vs. interference threshold (\(I_{th}\)).  
**Author Contributions**: [PERSON] researched the literatures, conceived the study concepts, designed the algorithm, and took charge of the original draft preparation; [PERSON] improved the mathematical models, provided systematic research and analysis methodology and edited the manuscript, and the refinement of the paper is completed under the supervision of [PERSON]; [PERSON] completed some numerical simulations, and checked formula deducing and English grammar. [PERSON] is responsible for project administration. All authors have read and agreed to the published version of the manuscript.  
**Funding**: This research was funded in part by the National Natural Science Foundation of China under grant 61663024, in part by the Erasmus+ Programme of European Commission under grant 573879-EPP-1-2016-1-FR-EPPKA2-CBHE-JP, and The APC was funded by the Hongliu First Class Discipline Development Project of Lanzhou University of Technology under grant 225305.  
**Institutional Review Board Statement**: Not applicable.  
**Informed Consent Statement**: Not applicable.  
**Data Availability Statement**: The data supporting this article are from previously reported studies and datasets, which have been cited.  
**Conflicts of Interest**: The authors declare no conflict of interest.