Research On Pilot Assignment And AP Selection Technology In Cell-Free Massive MIMO System

Due to the rapid increase in the number of global intelligent terminals and the gradual popularization of the Internet of Everything,the existing communication rate is not enough to meet the needs of the current information explosion era.The deployment of 5G is now in full swing in countries all over the world,and some communication powerhouses have also initiated the design and research and development of 6G projects.As early as October 2018,China has already started 6G-related research and development.The entire 1G to 5G system technology is based on the cellular network.As a breakthrough point,the "Cell-Free" network architecture has become one of the main directions of 6G transformation,and has received more and more attention and research from the academic community.Based on the distributed antenna system,a very promising Cell-Free massive multi-input multi-output(MIMO)technology is produced.This paper mainly studies the security issue,pilot allocation,and access point(AP)selection issues in the Cell-Free massive MIMO system.For the research of security issues in Cell-Free massive MIMO systems,this paper analyzes the Cell-Free massive MIMO system model with an active eavesdropping user.This model considers AP selection,pilot contamination,and power allocation,and there are multiple antennas at the AP.This model is more universal.In addition,this paper also focuses on the analysis and derivation of the uplink pilot transmission and downlink data transmission phases under this model,and derives the transmission expression and the closed-form expression of the user's achievable rate.This paper also analyzes the confidentiality performance of the system,deduces the lower limit of mutual information between the signal sent by the AP and the signal received by the normal user,and the upper limit of the mutual information of the attacker,defines and analyzes the Confidentiality rate of the eavesdropped user.In terms of pilot allocation,this article analyzes a Cell-Free massive MIMO system that considers AP selection,pilot contamination,and power allocation,but does not consider active eavesdropping.The channel estimation and uplink and downlink data transmission stages under this model are analyzed,and the closed-form expression of the user achievable rate under this model is given.At the same time,the theoretical performance of the system when the number of APs tends to infinity is also analyzed.Analyze and derive the pilot power control algorithm and Max-Min power control algorithm under this model to improve the lower limit of user experience.A pilot allocation algorithm based on weighted counting is proposed to reduce pilot contamination and improve system performance.The main idea of the algorithm is to assign different pilots to users in close proximity as much as possible,and at the same time give priority to assigning pilots of users with poor channel quality,and weighting the number of times the pilots are used.The simulation results show that the proposed algorithm improves the system performance significantly and has low complexity,which is feasible in practical applications.For the study of AP selection,this paper proposes an AP selection algorithm based on large-scale fading and weaker priority.For specific selection problems,the main idea of the algorithm is based on large-scale fading.Considering the limited power of each AP,in order to avoid an AP from being overused,the algorithm maintains an array of AP usage times.When performing AP selection,users with poor channel quality are given priority,while maintaining a minimum value of the sum of the large-scale fading of the AP that the user has selected.When the channel gain of an AP selected by a user is greater than the minimum value and the number of uses of the AP being selected has exceeded the average value,the AP is skipped.When the number of uses of the AP being selected is less than a certain threshold or no better value is found among APs that should not be selected later,this AP is selected for the user.The simulation results show that the algorithm proposed in this paper has a significant effect on system performance improvement compared with mainstream algorithms,and it also has a lower complexity,making its practical application possible.