Research On Non-Orthogonal Multiple Access (NOMA) Technology In Massive MIMO System

With the rapid development of mobile Internet technology,people's lives are more comfortable and convenient.At the same time,it also challenges the next generation of 5G communications.In order to solve the problem of insufficient bandwidth resources,it is particularly important to find technologies to improve spectrum utilization.As the key technologies of next generation wireless communication,massive multiple input multiple output(MIMO)technology and non orthogonal multiple access(NOMA)technology achieve higher spectral efficiency through multiplexing of airspace and power domain respectively.On this basis,the combination of massive MIMO technology and NOMA technology can further improve the spectrum utilizat ion rate and realize multi-user access and large-capacity wireless transmission.This thesis introduces the system model of massive MIMO and NOMA,and considers the practical problems encountered a fter the combination.It mainly studies from three aspects: massive MIMO system based on NOMA,user pairing of joint power allocation in massive MIMO NOMA system,and massive MIMO NOMA system based on clusters.Firstly,NOMA technology is introduced into massive MIMO system,and the performance of using NOMA in massive MIMO system is analyzed.Pilot transmission is used to obtain the estimated channel state information(CSI).In order to save pilot resources,users are divided into NOMA groups.Two users in the same group share the same pilot,and the pilot is sent on the uplink to facilitate the base station to estimate all channels.Time division duplex(TDD)operation is adopted,and the downlink channel is available based on the channel reciprocity.Two users in the same group share the same beam.Under these conditions,the strict capacity limit based on NOMA is derived.Simulation results show that NOMA is better than orthogonal multiple access(OMA)in massive MIMO system.Then,in the massive MIMO NOMA system,the problem of user pairing is studied.Base stations complete user pairing according to large-scale fading coefficients.Due to the interweaving of user pairing and power allocation,this problem becomes a mixed integer programming problem.In order to solve this nonconvex problem,the alternating optimization method is used to obtain the optimal power distribution matrix.Based on the Hungarian algorithm,the optimal user matching scheme is proposed.Considering the competition between users in the Hungarian algorithm,a low complexity user matching algorithm is proposed.The simulation analysis of the algorithm is carried out.Finally,the massive MIMO NOMA system is extended to include more than two users in each cluster.In the channel estimation period,users in the same cluster are assigned the same pilot.Base station(BS)estimates the linear combination of channels from all users in the same cluster.BS uses the estimated CSI for downlink beamforming,and users in the same cluster share the same beam.The effects of different user cluster modes on system rate are compared by simulation.