Precoding Research Of Cell-Free Massive MIMO

With the development of mobile communication technology,the fifth generation mobile communication technology(5G)has entered our life.5G brings us from the Internet era to the Internet of Things era.5G requires high speed,low latency,and wide connection,and has been applied in all aspects of our lives.Technology is always improving,and people are currently researching on the sixth generation mobile communication system(6G),and 6G requires the real Internet of Everything.A large number of user accesses make the cellular network architecture unable to deal with them well,so updating the mobile network architecture has become a current research hotspot.Cell-free massive multiple-input multiple-output(MIMO)system is a potential architecture for B5 G and 6G technologies,and it has high spectral efficiency(SE)and energy efficiency(EE),the user-centered theory it uses can well meet the needs of the Internet of Everything in 6G.There are many factors affecting the performance of the cell free massive MIMO system,such as precoding,power allocation,and pilot allocation.This thesis studies the downlink precoding scheme in Cell-Free Massive MIMO system.Based on the precoding scheme,the effect of pilot contamination has been studied.The characteristics of different precoding schemes have been summarized.Based on these characteristics,a new scalable joint precoding scheme has been proposed innovatively.In this scheme,two different pilot assignment methods have been put forward in order to improve the system downlink spectral efficiency obviously.The specific contributions of this thesis are as follows:1)Several downlink spectral efficiency of different precoding scheme in cell-free Massive MIMO system has been introduced and analyzed,such as max-ratio transmission(MRT),full-pilot zero forcing(fp ZF),centralized zero forcing(CZF),Joint max-ratio and zero forcing(JMRZF).This paper also compares and analyzes these precoding schemes in terms of their implementation methods,computational complexity,and downlink spectrum efficiency.2)The advantages and disadvantages of these precoding schemes have been summarized,and finally proposed an innovative precoding scheme named scalable joint zero forcing(SJZF)precoding.The simulation results show that our SJZF has better downlink spectral efficiency than other schemes,approaching CZF,while maintaining scalability.In order to improve the downlink spectral efficiency of the system,the characteristic of the optimal pilot assignment scheme has been found by the exhaustive method.Different pilot assignment method has been analyzed.3)The advantages and disadvantages of several commonly used pilot assignment schemes are analyzed.Aiming at the random pilot assignment scheme,an average-criterion-based pilot assignment(SAPA)scheme which is based on the optimal pilot assignment characteristic has been proposed.In order to reduce the effect of pilot contamination obviously,a pilot assignment method named simulated annealing-based pilot assignment(SAPA)has been proposed.The simulation result shows that SAPA can almost find the optimal solution for pilot assignment,and has acceptable computational complexity.