Analysis And Design Of Cell-Free Massive MIMO Systems

Cell-Free massive MIMO system deploys hundreds of thousands of APs,uses TDD operating mode,uses the same time-frequency resources to serve all UE,and connects CPU for uplink and downlink user transmission and signal processing.It has the advantages of wide coverage,balanced quality of service and flexible deployment.It eliminates the cell boundaries of traditional cellular networks,and significantly improve the total spectrum efficiency and energy efficiency.However,there are still some challenges in the practical application of Cell-Free massive MIMO system,such as hardware impairments caused by low-precision radio frequency links,channel aging caused by user mobility,and how to improve system performance through power optimization.Based on this,this paper studies the above three problems with theoretical research and simulation verification,as follows:Firstly,considering the deployment cost of the radio frequency link and power consumption of Cell-Free massive MIMO system,we propose a Cell-Free massive MIMO system model based on hardware impairment,and deduce accurate closed expressions of spectral efficiency and energy efficiency of uplink and downlink based on the large number law,and propose the hardware scaling law of the uplink and downlink,and reveal the impact of hardware impairment on the performance of system uplink and downlink.Simulation results illustrate the correctness of theoretical research and point out that the low-precision radio frequency link can be deployed to reduce the deployment cost with no loss of the system performance.Secondly,considering the power optimization problem of Cell-Free massive MIMO system based on hardware impairment,we propose a power optimization model with the objective of maximizing the minimum spectral efficiency of UE,and design a max-min power optimization algorithm by using convex optimization theory and bisection algorithm.Simulation results illustrate that the spectral efficiency of the system using the power optimization algorithm is 20.1%higher than the system using the average power allocation when transmitter and receiver have hardware impairment and the number of APs and UE are 100,10 respectively.Thirdly,considering the channel aging problem in medium and high-speed mobile scene,we propose a Cell-Free massive MIMO system model considering channel aging,and deduce accurate closed expressions of spectrum efficiency and energy efficiency of uplink,and study the power scaling law under practical system.Simulation results illustrate that the power scaling law can reduce power consumption of the system without loss of the system performance.In conclusion,this paper reveals the impact of key factors such as the number of APs,the number of UE,hardware impairment,power optimization and channel aging on the performance of Cell-Free massive MIMO system.Research shows that Cell-Free massive MIMO system can achieve better performance with lower cost and power consumption,and effectively combat the effect of channel aging.Cell-Free massive MIMO system has strong robustness and is expected to become one of the candidate transmission technologies for future mobile communication networks.