Performance Analysis Study Of Cooperative NOMA System Under Interference

The rapid development of the Internet and the Internet of Things has led to an exponential increase of the number of wireless connectivity.In order to enable enormous machines to attain highly reliable communication and achieve high data rate and low delay experiences,nonorthogonal multiple access(NOMA)which is one of the candidate technologies of 5G communication is studied in depth.Compared with orthogonal multiple access(OMA),NOMA not only has higher spectral efficiency,but also meets the demand of massive connectivity.Therefore,the performance study of cooperative NOMA systems is significant for system performance optimization.However,the broadcast characteristic of wireless communication will lead system to be affected by eavesdropping and interference,the study of physical layer security for cooperative NOMA systems is important for enhancing the system's secrecy performance.Firstly,a cooperative NOMA system with both co-channel interference and feedback delay is studied,where the best relay is selected using the partial relay selection(PRS)scheme.The performance of multiple-relay cooperative NOMA systems is studied for decode-andforward(DF)protocol and amplify-and-forward(AF)protocol,respectively.In particular,the closed-form expression of outage probability is derived and the asymptotic expression and diversity gain at high signal-to-noise ratio(SNR)are obtained.Simulation results illustrate that the performance of cooperative NOMA systems with lower feedback delay becomes much better.The asymptotic results at high SNR are consistent with the simulation results.The cooperative NOMA system with AF protocol has better performance at low SNR.At high SNR,cooperative NOMA system with low feedback delay shows the same performance for DF protocol and AF protocol.Secondly,the physical layer security performance of a single-relay cooperative NOMA system with DF protocol is studied when an illegal eavesdropper has multiuser detection capability.Specifically,closed-form expressions of secrecy outage probability(SOP)and strictly positive secrecy capacity(SPSC)are derived,and the asymptotic expression is obtained at high SNR.Simulation results demonstrate that the physical layer security performance of cooperative NOMA system is better for less co-channel interference.The asymptotic results match well with the simulation results at high SNR.Finally,this thesis proposes a two-stage relay selection scheme in multiple-relay cooperative NOMA system for improving the physical layer security performance of cooperative NOMA systems.A closed-form expression of outage probability is derived,and the asymptotic expression and diversity gain are also obtained at high SNR.The theoretical analysis shows that the two-stage relay selection scheme proposed can achieve full diversity order.Simulation results show that the performance of cooperative NOMA system applying the two-stage relay selection scheme is obviously better than that of single-relay scheme and Max-min relay selection scheme.