Design Of Transmission Strategies And Outage Performance Analysis For Cooperative Non-orthogonal Multiple Access Systems

As one of the candidate technologies for the next generation mobile communication systems,non-orthogonal multiple access(NOMA)has the ability of efficient spectrum utilization and massive connection by making different users reuse the same time-frequency resources to jointly transmit data.On the other hand,in order to expand the coverage of signal or improve the reliability of received signal,cooperative communication technology has been introduced into NOMA,leading to a new technology named cooperative NOMA.At present,there are many literature regarding power allocation,user ordering,relay selection and other aspects in cooperative NOMA.However,these existing literature do not carry out in-depth analysis on the concept of priority,or fail to explore the characteristics of the concerning system models(e.g.shared relays based cooperative NOMA)as much as possible.In view of the above problems,this paper carries out a series of studies,and the main contributions can be summarized as follows:1.In the downlink multi-relay cooperative NOMA transmission scenario,the relay selection strategies are proposed for the cases where only user priority and only service priority are considered,respectively:(1)To solve the problem that only limited outage performance can be harvested by the existing relay selection strategies,where user priority based user ordering criterion is adopted,a dynamic user ordering criterion based on the ascending relationship of instantaneous channel gain of different users is proposed in this paper.The closed-form approximate expression of overall outage probability and the asymptotic expression as well as the corresponding diversity gain are derived by using Gauss-Chebyshev quadrature.Furthermore,the advantages of the proposed strategy with respect to outage performance are verified by simulations.(2)To tackle the issue that the existing relay selection strategies do not take into account the fact that users may have multiple services with different priorities,a relay selection strategy with the aim to maximize the system’s sum rate is designed,where one user can have different target rates according to the change of instantaneous channel state information(CSI).The strategy realizes flexible service for different users by adopting the dynamic user ordering criterion and power allocation scheme.In order to prove that the proposed strategy can further reduce the overall outage probability,the closed-form approximate expression of overall outage probability and the asymptotic expression as well as the corresponding diversity gain are derived under Nakagami-m fading by using Gauss-Chebyshev quadrature.Moreover,the correctness of the analysis and the advantages of the proposed strategy with respect to outage performance are verified by simulations.2.In the uplink multi-relay cooperative NOMA transmission scenario,to solve the problem that no scholars have studied relay selection strategies in this scenario,and the existing strategies proposed for downlink cooperative NOMA cannot be directly applied to this problem because of the difference in signal’s transmission and processing between uplink and downlink NOMA,based on the degree of the use for CSI,this paper proposes a novel relay selection strategy that takes user priority as consideration,which also involves different user ordering criterion and power allocation schemes,and achieves different tradeoff in implementation complexity and performance.The closed-form outage probability expression and corresponding diversity gain for each strategy are derived,respectively,and the conclusion that the system can achieve full diversity gain by using instantaneous CSI is obtained.Furthermore,the simulations verify the correctness of the theoretical analysis and show that the outage performance gain caused by the dynamic power allocation scheme is greater than that caused by the dynamic user ordering criterion.3.In the shared relay based two-way cooperative NOMA transmission scenario,the transmission strategies involving a decode-and-forward(DF)/amplify-and-forward(AF)relay are proposed,respectively:(1)To solve the problem that the existing shared DF relays based cooperative NOMA transmission strategies adopt dynamic power allocation scheme to improve outage performance,which has high signaling overhead and high requirements on the transmitting hardware processing capability,a max-min criterion based dynamic user ordering scheme with low overhead and implementation complexity are proposed in this paper,where the instantaneous CSI of all users can be used to select the optimal user ordering mode that minimizes the overall outage probability.The closed-form expression of the overall outage probability is derived,and it is proved that the strategy can obtain full diversity gain.Moreover,the simulations show that the proposed strategy can achieve outage performance similar to the existing transmission strategy using dynamic power allocation scheme.(2)For the scenario where the signal processing capacity of relay node is limited or only AF relay can be used for the need of secure communication,a shared AF relay based cooperative NOMA transmission strategy is proposed,where the outage performance of each user is analysed and the approximate expression of the outage probability is derived using some approximate conclusions which are valid under high SNR.Furthermore,the correctness of the theoretical analysis and the advantages of the proposed transmission strategy in terms of outage performance are verified by simulations.