Research On AP Deployment Scheme For High Energy Efficiency Cell-free Massive MIMO

Cell-free massive multiple-input multiple-output(MIMO)has become one promising technology due to its high frequency spectrum gain,high macro diversity and low path loss.Cell-free massive MIMO utilizes a large number of distributed access points(APs)to serve all users simultaneously over the same time-frequency resources,thus eliminating the cell boundary restriction,greatly alleviating or even eliminating the inter-cell interference,as well as eliminating the differences between center and edge users,ensuring seamless convergence of service quality under high-speed mobile.However,a large number of distributed APs can improve the performance of the system while also increasing the length of the wired backhaul link,which leads to a sharp increase in power consumption and circuitry dissipation.Therefore,how to improve the energy efficiency of system is a pressing issue.Based on the above discussion,this thesis has proposed the following research on the AP deployment of cell-free massive MIMO:Firstly,the basic model of cell-free massive MIMO is introduced.In view of its special system architecture,the characteristics of cell-free massive MIMO are introduced in detail.In addition,the random geometry theory needed in the research of cell-free massive MIMO system is summarized,the theoretical basis of random geometry theory is introduced,several classical point processes are introduced,and several common random geometry theory theorems are briefly described.Secondly,for the model of cell-free massive MIMO downlink transmission system based on line-of-sight(LoS)and non-line-of-sight(NLoS)transmissions,an AP deployment method is devised to increase the system energy efficiency.Considering the dual-slope path loss model with LoS and NLoS transmissions,assuming that APs and mobile terminals are both distributed according to Poisson point process.Using theoretical foundations from stochastic geometry,we obtain a tight lower bound for the user downlink achievable rate and then the energy efficiency is further obtained by estimating the total power consumption of the model proposed in this paper.An alternating optimization algorithm that designs the optimal AP density and antenna number is proposed by maximizing the energy efficiency.The simulation results show that the derived deterministic equivalence is highly consistent with the simulation values.It is also found that compared with the previous work that only considers NLoS transmissions,the actual optimal AP density should be much smaller,and the maximized EE is actually much higher.Finally,for the model of heterogeneous cell-free massive MIMO downlink communication system based on line-of-sight(LoS)and non-line-of-sight(NLoS)transmissions,an AP deployment method and quantization bit research scheme are proposed to increase the system energy efficiency.Considering the dual-slope path loss model with LoS and NLoS transmissions,assuming that APs and users are both distributed according to Poisson point process,and the APs are modeled as multi-level APs with different number and quantization bits,that is,some APs are equipped with low-resolution digital-to-analog converters(DACs)modules to reduce power consumption.Using tools from stochastic geometry,we derive the user downlink achievable rate and then the energy efficiency of the system is derived by calculating the spectral efficiency and estimating the total power consumption of the model proposed in this paper.An alternating optimization algorithm is developed to design the optimal number of AP density and quantization bits by maximizing the energy efficiency.The simulation results show that the derived deterministic equivalence is highly consistent with the simulation values and the proposed iterative alternative optimization algorithm can effectively enhance the energy efficiency of this system.