Research On Relay Selection Based On Non- Orthogonal Multiple Access Cognitive Network

With the continuous advancement of technology,people's lives are changing with each passing day,which prompted the rapid development of wireless communication technology.However,with the increase of user equipments,the demanding of communication quality is getting higher,as well as spectrum resources are becoming scarcer.As a result,existing communication technologies are gradually unable to meet people's needs.Non-Orthogonal Multiple Access(NOMA)is a new generation technology of wireless communication,which uses superposition coding(SC)to simultaneously transmit multiple user signals.At the receiving end,it uses Successive Interference Cancellation(SIC)to separate user signals,which makes the spectrum able to accept more users,improving spectrum efficiency.On the other hand,Cognitive Radio(CR)is also a technology that can improve spectrum efficiency and improve fairness among users.Significant progresses have been made in the study of the two technologies,and in further research,combining it with NOMA technology is a research hotspot because it can further improve spectral efficiency and improve network performance.The existing research tend to combine the two technologies in a cooperative transmission manner.Generally,users who are closer to the base station are used as secondary users,and users farther from the base station are used as primary users.The secondary users assist the primary users to perform signal transmission,which not only ensures the quality of service of the primary users,but also provides access to the spectrum for secondary users.However,these technologies also rely on other factors for improving network performance,such as appropriate relay selection strategies and power allocation.This paper focuses on user cooperation and relay selection in Cooperative CR-NOMA networks.In order to maximize network performance,the most sensible approach is to use all secondary users as candidate relay users and adopt appropriate relay selection strategies to achieve a "win-win" situation for primary and secondary users.Therefore,this paper proposes two different relay selection strategies.One is a network-based relay selection strategy,which aims to improve the interrupt performance for primary users and secondary users at the same time.The second is based on the primary user's relay selection strategy,which tend to improve the performance for secondary users under the premise of ensuring the performance of the primary users.At the same time,this paper gives the complete outage probability expressionsof two different strategies for different users,and analyzes the outage probability of large SNR,and then calculates the diversity gain of the network under different strategies.The results show that under the two strategies,the primary users have achieved full diversity gain.Finally,the correctness of the outage probability expression is verified by simulation.Under the two strategies,the network has achieved different degrees of performance improvement,which also proves that increasing the number of relay users can further improve user performance when the full diversity gain is obtained.