Research On Spectral Efficiency Improvement Of Uplink In Cell-free Massive MIMO System

Facing the requirements of high energy efficiency and high spectrum efficiency in next-generation mobile communications,Cell-Free Massive MIMO has strong macro-diversity and anti-multi-user interference capabilities.It also avoids frequent switching of cell services due to user movement and poor service quality at cellular boundaries.It has become one of the essential directions of future research on mobile communication systems.To effectively improve the system’s spectral efficiency,it is necessary to solve the problem of pilot pollution caused by the limited coherence time length and reasonably optimize the user’s transmit power to reduce signal interference.In recent years,the air-to-ground communication system combined with UAV communication technology has the advantages of flexible deployment and comprehensive coverage,which has become a significant research focus to improve the system’s spectrum efficiency.In this paper,from the perspective of pilot allocation,Max-min power optimization,and UAV-assisted communication technology,the research on increased system spectrum efficiency is carried out.The main research contents are as follows:In the general model of the Cell-Free Massive MIMO system,the research on uplink signal transmission is carried out.Then,the analysis of TDD and FDD duplex modes.When the system is running in TDD mode,the pilot pollution problem’s source and the effect function’s classification in power optimization are pointed out.Then,an air-ground heterogeneous communication system is constructed with the aid of UAV-assisted communication technology,and the model and signal transmission process of the heterogeneous system are analyzed.Based on the channel model of the Cell-Free Massive MIMO system and using part of the MMSE signal detection technology and access point selection technology,the spectral efficiency expression of the system uplink is derived.The pilot allocation algorithm is designed,and the Max-min optimization problem of pilot power and data power is modeled to improve the system’s spectrum efficiency.For the pilot allocation,the parts AP serving user is selected based on the large-scale fading matrix and user-centered idea.Then the pilot frequency allocation scheme is designed based on the AP repetition degree obtained by the AP selection.For pilot power optimization,the normalized mean square error method of maximizing the channel estimates of all users is used.The non-convex optimization problem in the scheme is transformed into a convex optimization problem by first-order Taylor approximation,and an algorithm to solve the problem is given.For data power optimization,the issue of maximizing the minimized spectral efficiency of users is proposed,and it is shown that the objective function of power optimization accords with linear programming.A dichotomy is given to solve the problem.Simulation results show that,compared with other pilot allocation algorithms,the proposed pilot allocation algorithm can improve the total spectrum efficiency of the system,and the solution of the two power optimization problems can further enhance the fairness of user communication performance.The uplink performance expression of Cell-Free Massive MIMO heterogeneous system under UAV-assisted communication is derived based on the channel model between UAV and user.At the same time,a UAV flight trajectory based on the K-means clustering algorithm is designed.Firstly,according to the user’s position information and the K-means clustering algorithm,the three particle coordinates of the cluster are obtained.Then the center coordinates and radius of the UAV flight trajectory are obtained through the particle coordinates.The simulation results show that the flight trajectory of UAV designed by the K-means clustering algorithm can communicate well through user-intensive areas and achieve the purpose of traffic unloading in dense regions.At the same time,it is proved that adding UAV-assisted communication can further improve the uplink spectral efficiency of users and can also better serve user-intensive scenarios and areas.