Capacity Enhancement Of D2D Aided Coordinated Direct And Relay Transmission Using NOMA

In recent years,wireless communication technology has developed rapidly,and the use of mobile terminal equipment has become more common.Communication data has shown an explosive growth,which undoubtedly puts forward higher requirements for wireless communication systems.Although the current situation of frequency band resources is very severe,in order to ensure the user's communication experience,a faster rate and a small delay are often required during information transmission.The non-orthogonal multiple access(Non-Orthogonal Multiple Access,NOMA)technology satisfies the need for multiple users to simultaneously access the system to receive messages by superimposing and encoding multiple signals at the source end.This technology provides a solution to the access problem of massive users.In addition,D2D(Device-to-Device)technology has the characteristics of direct communication between short-distance users without passing through the base station,and has also received extensive attention from researchers.Because such communication method can offload part of the traffic for the base station and relieve network congestion,it can be well used in complex cellular communication networks.As an extension of cooperative relay technology,Coordinate Direct and Relay Transmission(CDRT)technology can not only expand the coverage of the central base station,but also improve the utilization of spectrum resources,so it is also one of the key technologies in the 5G network.In the existing research on CDRT,there are more scenarios that are combined with NOMA technology.This thesis has further expanded the traditional model,combined with the current background of the "Internet of Everything",and studied the D2D-assisted downlink transmission problem of the CDRT-NOMA scene.Due to the integration of D2D technology into the cellular network,this model can be better applied into the Internet of Vehicles.The main contributions of this thesis are as follows:This thesis proposes a NOMA scenario based on D2D-assisted direct and cooperative relay transmission.Specifically,the base station can directly send signals to the near users,but the communication link is poor due to the long distance from the far users.Therefore,there is no direct transmission link between the base station and the cell-edge user,and the information sent to the cell-edge user needs to be completed with the assistance of the relay user.In particular,when the relay user forwards the message of the far user in the second time slot,an additional D2D message is transmitted to the near user,which further fully utilizes frequency band resources and improves spectrum utilization.Theoretical analysis are carried out for the above scenario,and the ergodic capacity is used as an important indicator to measure the performance of the system,and the D-CDRT scheme under this model is compared with the system performance of the traditional CDRT scheme and the OMA scheme.This paper first derives the closed expression of the total traversal capacity of the system,and at the same time obtains the approximate expression of the total capacity in the case of high signal-to-noise ratio,and comes to a conclusion that the growth slope of the total traversal capacity of the D-CDRT scheme approaches.Then,a simple theoretical analysis of the comparison scheme are carried out,and the precise and approximate expression of the total traversal capacity are also derived.The last part of this thesis is the analysis and verification of the theoretical part of the model.Matlab is used to simulate the closed expression of the total traversal capacity of the system.At the same time,Monte Carlo simulation is performed on the model,and 10000 times experimental data are randomly generated.It is found that the curve of the real experiment and the curve of the theoretical analysis can be completely fitted.Moreover,the approximate expression of the total system traversal capacity can also be in good agreement with the real value in the high signal-to-noise ratio area,which also verifies the correctness of the theoretical analysis.Then,the proposed scheme is compared with the CDRT scheme and the OMA scheme under the same conditions.The simulation results show that the performance of the D-CDRT scheme is always better than the other two schemes.Finally,the power distribution factor and the distance between users are discussed separately because they are two factors which affect the system performance,and general conclusions are obtained from the two perspectives of theoretical analysis and simulation results.