Research On User Clustering And Power Allocation Scheme For Downlink In NOMA System

As one of the key technologies in the fifth-generation wireless mobile communication(5G),non-orthogonal multiple access can meet the requirements of high spectrum efficiency and high speed.Therefore,the research of non-orthogonal multiple access technology is very necessary.This paper studies the user clustering and power allocation in the specific non-orthogonal multiple access system model.The specific work is as follows:To solve the problem of low throughput of middle users after adopting the hybrid clustering scheme for the users in the cluster of the NOMA downlink system,an improved clustering scheme based on user groups is proposed.The system users are sorted in descending order according to the link gain,every two users in sorted sequence grouped into one user group and these user groups are numbered sequentially.Then,the users in the first and last user groups are clustered with traditional user clustering scheme and the remaining user groups are added to the existing two user clusters according to parity of user group number.When the number of system users increases and the throughput after using non-orthogonal multiple access in the same user cluster is less than the throughput after using orthogonal multiple access,the clustering result after primary clustering is performed again in order to further improve the system throughput performance under non-orthogonal multiple access.Downlink of non-orthogonal multiple access system performs power multiplexing at the base station end of users who share the same subchannel,that is,different transmission power is allocated to each user.Therefore,after the users in the cluster are determined,the power allocation of the users in the cluster is studied.Aiming at the problem that the relationship between channel conditions and user transmit power is not fully considered in the fixed power allocation scheme,an improved power allocation scheme based on signal-to-noise ratio is proposed in this paper.Taking the throughput of the user cluster as the objective function,the sum of power allocation factors at the base station,the minimum transmission rate of the users in the cluster and the power difference threshold for the users in the cluster to correctly perform successive interference cancellation at the receiving end as constraints,the Lagrange optimization method is used to solve the power allocation factor.Finally,the relationship between the signal-to-noise ratio,link gain and the transmission power of each user in the cluster is obtained,and the analytical expression is given.In order to further study the combination of the improved clustering scheme and the improved power allocation scheme,under which channel fading model,the system throughput improvement is the best.The improved clustering scheme and improved power allocation scheme are used for users in the dense urban center scene and high-speed railway viaduct scene,and the system throughput is obtained by simulation.The above throughput is compared with the throughput after using hybrid clustering scheme and fixed power allocation scheme,and the improvement of system throughput is obtained after the combination of the proposed clustering scheme and power allocation scheme under different channel fading model.