Research On Massive Mimo And Non-orthogonal Multiple Access In Fifth-Gerneration Communication System

Massive multiple-input multiple-output(MIMO)technology can enhance spectral efficiency and energy efficiency of wireless communication systems.It is one of the key technologies of the next-generation mobile communication system(5G),and can greatly improve the system performance.However,it brings several challenging problems.Distributed large-scale antennas system(DLSAS)can further improve system reliability and energy efficiency of the system.However,due to the long distance between users and some base station antennas,there exists significant energy attenuation when the users communicate with base station.For this problem,this thesis analyzes how to improve the energy efficiency of DLSAS.On the other hand,non-orthogonal multiple access(NOMA)technology plays an important role in improving the multiple access efficiency of 5G so that a large number of smart terminals can access the system.This thesis introduces one of the mainstream NOMA technologies,namely sparse code multiple access(SCMA).The SCMA codebook is usually characterized by high design complexity and is closely related to the complexity of the receiver.This thesis focuses on the pilot contamination(PC)of centralized massive MIMO,the energy efficiency problem of DLSAS,and the design of SCMA codebook.Chapter 2 analyzes the transmission performance of a massive MIMO system with PC.A pilot allocation method is proposed to reduce PC.For the classic time division duplex(TDD)multi-cell massive MIMO system model,the thesis analyzes the system performance loss caused by PC,and then proposes an optimized pilot assignment method,which can reduce the influence of PC.The pilot allocation method firstly groups users based on the angle of arrival of the user signals and then allocates the pilot sequences to users according to the result of user grouping.The purpose of this method is to reduce the interference between users sharing the same pilot sequence,and improve the accuracy of channel estimation.Besides,the thesis does a theoretical analysis of the channel estimation and the sum rate of the system,and obtains closed-form solution of channel estimation and sum rate respectively.Finally,simulation results under multiple pilot allocation methods show that this optimized pilot allocation method brings better performance to the system.Chapter 3 studies the high energy efficiency transmission under two system models namely the base station antennas are deployed in a circular array and randomly distributed in the cell.Energy efficiency optimization methods of the two system models are proposed to solve the energy attenuation by selecting some appropriate antennas for the user so as to avoid energy loss between the communication of the user and base station antennas,which are far away from the user.The result of the antenna selecting algorithm shows that only a small part of base station antennas are selected as active antennas.Finally,the energy efficiency and spectrum efficiency of DLSAS are analyzed by computer simulations,and a comparison with the previous method is made.Simulation results prove that the proposed energy efficiency optimization method greatly improves the energy efficiency of the system with little loss in spectrum efficiency compared to the previous method.Chapter 4 studies one of the NOMA technologies,namely sparse code multiple access(SCMA)technology.Firstly,the thesis introduces the uplink transceiver of SCMA.In the transmission part,low-density parity-check(LDPC)coding,the mapping of users to carrier resources and the SCMA muti-dimensional codebook design are introduced.In the receiving part,the decoding algrithm based on belief propagation(BP)in the logarithmic domain and the detector based on message passing algorithm(MPA)are discussed.To improve system performance,the idea of bit interleaved coded modulation with iterative decoding(BICM-ID)is adopted and bit interleaver is introduced between encoder and modulator,meanwhile iterative receiver is adopted in order to make full use of demodulating and decoding information.Finally,the function modules of the uplink SCMA are implemented by simulations,and the performance of the system under different system parameters is analyzed.The complexity of hardware implementation is considered at the same time.Chapter 5 proposes an optimized design method of the SCMA codebook.The design of codebook is a challenging problem,because of its high complexity and its close relation to the complexity of the detector.According to these characteristics of the codebook design,this thesis proposes an optimized codebook design method which is easy to implement and can also improve the system transmission performanc.The codebook design method is optimized from the amplitude and phase of high-dimensional constellation points.It can not only increase the euclidean distance and can reduce the interference between conflict users,but also reduce the projection of the constellation points in the coordinate system and reduce the computational complexity of the detector.In the end,by computing and comparing the computational complexity of the receiver detector under several different codebook design methods,it is observed that the optimized method proposed in this thesis greatly reduces the addition operations,multiplication operations and comparison of the detector.Furthermore,simulation results confirm that the optimized codebook design method achieves better performance than other methods.