Research On Non-orthogonal Multiple Access Technology Based On Cell-Free Massive MIMO

Due to the frequency resources are expensive and scarce,the improvement of spectral efficiency has become an important research direction of wireless mobile communications.On the one hand,Cell-Free Massive MIMO(CF-mMIMO),as a distributed Massive MIMO technology,has stronger anti-shadow fading capabilities than collected Massive MIMO.It provides higher throughput and spectral efficiency.On the other hand,as one of the research hotspots in recent years,non-orthogonal multiple access(NOMA)is able to serve multiple users on the same time-frequency resource by power multiplexing.It is considered to be one of the key technologies to improve spectral efficiency in the 5G era.Therefore,applying NOMA technology to the CF-mMIMO system can further improve the spectral efficiency and meet the current increasing communication demands.The pilot transmission and channel estimation methods of CFmMIMO,as well as the implementation scheme of the downlink data transmission stage are described.Firstly,this paper introduces the theories of CF-mMIMO and NOMA.The pilot transmission and channel estimation methods of CF-mMIMO,as well as the implementation scheme of the downlink data transmission stage are described.Superposition coding(SC)and successive interference cancellation(SIC)algorithm,which are the two key technologies in power-domain NOMA,are briefly introduced in this paper.The process of NOMA in the downlink transmitting is analyzed,and the achieved sum rate of NOMA is derived.Then,this paper explains the implemetation scheme of the combination of these two technology.The peformance analysis and simulation results are shown.Secondly,NOMA based on CF-mMIMO over Rician fading channel is studied.We introduced the channel model,implemetnation scheme over Rician fading channel and the derivation process of spectral efficiency formula.Aiming at the poor fairness of NOMA based on CF-mMIMO over Rician fading channel,a power allocation algorithm for maximizing the minimum spectral efficiency is proposed.The minimum spectral efficiency of users in the system is greatly improved.Thirdly,the use of NOMA and OMA in CF-mMIMO currently both has their own advantages and disadvantages.As a result of it,a more flexible scheme is proposed in this paper,that is,the combination of OMA and NOMA based on CF-mMIMO.The transmitting signal,received signal,signal-to-interference and noise ratio(SINR)and spectral efficiency in this scheme is derived.We also explains the principles and theories of the max-min bandwidth efficiency power allocation algorithm.The simulation results are given,which proves that this scheme has better performance than using OMA or NOMA alone in some scenarios.Finally,we summarize the paper,and point out the unresolved problems and future research directions about the system of NOMA based on the CF-mMIMO.