Research On Outage Performance Of Cooperative Non-orthogonal Multiple Access Systems

With the explosive growth in the number of internet of things devices in recent years,existing Orthogonal Multiple Access(OMA)techniques,such as time division multiple access,frequency division multiple access,code division multiple access,etc.,can no longer meet the requirements for massive device connections in future wireless communication networks.Therefore,new multiple access techniques need to be developed for massive connections and high spectral efficiency.Compared with OMA,Non-Orthogonal Multiple Access(NOMA)allows multiple users to exploit the same time/frequency/code resource block,so it can support more device connections and has more high spectral efficiency.In order to improve signal reception reliability and expand network coverage,cooperative relaying can be further integrated into NOMA to form cooperative NOMA.Although NOMA has received extensive attention and research from scientists and engineers around the world in recent years,the research on cooperative NOMA is insufficient,especially for the key question of how to implement a low-complexity cooperative NOMA system needs to be further answered.In view of the above problems,this dissertation investigates the new NOMA system integrating user cooperative relaying and the Simultaneously Wireless Information and Power Transfer(SWIPT)technique.Compared with the cooperative NOMA system based on dedicated relays,the proposed new cooperative NOMA system can avoid the deployment of dedicated relays and reduce the implementation complexity.However,the corresponding performance analysis and optimization of the proposed system need to be established urgently.This dissertation mainly analyzes and optimizes the outage performance of the proposed system.The research results are expected to further improve the cooperative NOMA and provide theoretical support for the design and deployment of low-complexity cooperative NOMA systems.The specific research content and innovation points of this dissertation are as follows:Firstly,for the high complexity of deploying dedicated relays to implement cooperative NOMA,this dissertation proposes the idea of user cooperative relaying,which enables a near user to act as a half-duplex cooperative relay for a far user in downlink NOMA systems.At the same time,this dissertation considers that the near user adopts the SWIPT technique to harvest energy from the wireless signals transmitted by the Base Station(BS),and uses the harvested energy to forward signals for the far user,further alleviating the relaying energy consumption problem of the near user.Specifically,this dissertation considers both power splitting and time switching protocols,as well as the non-linear energy harvesting model matching the actual situation,to derive the outage probability of the considered system.On this basis,this dissertation utilizes the GaussChebyshev approximation technique to obtain the closed expression of outage probability.By analyzing the closed expression,a low-complexity algorithm is proposed to minimize system outage probability and verified in simulation.This part of the research results can provide theoretical guidance for the design of low-complexity and low-outage-probability cooperative NOMA systems.Secondly,in view of the problems of both high system outage probability and high BS energy consumption caused by difficultly matching fixed system resource allocation to dynamically changing wireless channels,this dissertation achieves on-demand user services by adaptively optimizing system resource allocation,while reducing both system outage probability and BS energy consumption.Specifically,this dissertation considers both power splitting and time switching protocols respectively,and constructs multidimensional resource joint optimization problems aiming at minimum energy consumption and conditioned on success transmission and available resource constraints,by building the relationship among channel conditions and user requirements as well as system resources.Since it is very challenging to directly solve the optimization problem,this dissertation proposes an optimization algorithm of low-complexity power-time on-demand resource allocation by in-depth analysis of the structure of the optimization problem,which is verified in simulation.The research results in this part can provide theoretical guidance for the resource allocation of cooperative NOMA system aiming at reducing the system outage probability and the BS energy consumption.Then,for the limited spectral efficiency of the half-duplex technique,this dissertation considers to adopt cooperative full-duplex relay in the NOMA system integrating user cooperative relaying and the SWIPT technique,and uses the self-interference cancellation technique combining the analog domain and the digital domain at the near user to eliminate the self-interference generated by the full-duplex technique due to transceiving signals simultaneously.By properly modelling the residual self-interference,this dissertation analyzes the influence of residual self-interference on the system outage performance.Specifically,this dissertation considers the time switching protocol and the non-linear energy harvesting model to derive the system outage probability,and adopts the GaussChebyshev approximation technique to obtain the closed expression of the outage probability.By analyzing the closed expression,a low-complexity algorithm is proposed to minimize system outage probability and verified in simulation.This part of the results can provide theoretical guidance for the design of low-complexity and low-outage-probability cooperative NOMA systems with full-duplex relaying.Finally,in view of the unclear impact of user cooperative relaying on the physical layer security of the NOMA system,this dissertation initially explores a downlink cooperative NOMA system with user full-duplex relaying,and accurately quantifies the effect of full-duplex relay cooperation on the secrecy outage performance of the NOMA system.Specifically,this dissertation considers a scenario where a near user acts as a full-duplex relay in downlink cooperative NOMA system coexisting with an eavesdropper,and uses the secrecy outage probability as an indicator to respectively character the secrecy outage performance of the near user and far user in the NOMA system.It is found that the NOMA system based on user full-duplex cooperation has higher secrecy performance than the corresponding OMA system,and the stronger the ability to eliminate self-interference,the better the secrecy outage performance.This part of the research results can provide a reference for the physical layer security design in the future cooperative NOMA system.