Power Control Of Cell-free Massive MIMO System

With the rapid development of information technology in today's era,users' traffic demand is also continuously increasing,and mobile communication technology is also continuously updated iteratively.In the development process of mobile communication technology from 1G to 5G,the network architecture of the cellular network is basically adopted.The cell-free massive MIMO system is based on the distributed antenna system and introduces the "user-centered" idea,eliminates the concept of cells,and effectively solves the mutual interference between cells and a series problem of frequent cell switching.The system integrates the advantages of distributed antenna system and traditional massive MIMO.It enhances the user experience and improves the overall performance of the system by providing huge throughput and coverage probability for all users in the entire system.It has gradually become a 5G key technology in the development of communication technology.This paper mainly studies the power control problem of cell-free massive MIMO system based on Rayleigh fading channel model.According to the channel hardening characteristics,we divide a coherent interval into three stages:uplink pilot training,uplink payload transmission and downlink payload transmission,and respectively derive and calculate the closed expression of the uplink and downlink user achievable rates formula.In the payload transmission stage,by adjusting the transmit power at the user or at the access point,co-channel interference can be reduced and the overall performance of the system can be improved.In most of the previous studies,in order to provide all users with a unified high-quality service,the max-min power control target was basically selected,and the dichotomy was used to obtain a set of optimal power control coefficients,so that the minimum rate users The speed can be guaranteed.However,since max-min only pays attention to the user with the smallest rate,it will inevitably sacrifice the rates of other users,thereby reducing the total energy efficiency and spectrum efficiency of the system.Therefore,this paper proposes a power control objective based on maximizing the product of the minimum rate user rate and the average rate,and introduces the artificial fish school algorithm to achieve power control.Due to the characteristics of the artificial fish school algorithm itself,we can flexibly adjust the objective function to achieve the desired result.Desired control target.The simulation results verify that this method can increase the average user rate while achieving the max-min power control target,and effectively improve the total spectrum efficiency and energy efficiency of the system.In the uplink pilot training phase,under ideal circumstances,the system randomly allocates a mutually orthogonal pilot sequence to each user.However,in an actual system,the number of orthogonal pilot sequences is not infinite.Therefore,the channel estimation value obtained by a user will be interfered by non-orthogonal pilot sequences sent by other users,which is pilot pollution.In order to solve the pilot pollution,this article introduces the pilot power control coefficient,through the min-max power control target,and through the fish school algorithm to obtain a set of optimal pilot power control coefficients,so as to ensure the normalization of the channel estimation value power of the user with the largest mean squared error.Finally,through simulation analysis,we verify that after using pilot power control,the user's rate and the total energy effect of the system have been improved to a certain extent.