Research On Spectrum Efficiency Optimization In Cell-free Massive MIMO System

With the rapid development of the information industry,business scenarios that rely on significant connection,high reliability,and high data flow are increasing daily.Due to broader coverage,more vital anti-interference ability,and more flexible networking,the cell-free massive multiple-input multiple-output(MIMO)system has become a critical network architecture of the next generation of the mobile communication system.System resource allocation is an essential factor in giving play to the advantages of the cell-free massive MIMO system,and user spectral efficiency is the critical system performance index.Therefore,starting from optimizing the performance of system user spectral efficiency,this thesis studies the rational allocation of limited pilot resources and power resources in the system.Firstly,we propose a pilot allocation algorithm based on user grouping to address the pilot resource allocation problem in the cell-free massive MIMO system.Firstly,we establish a cell-free massive MIMO system with a pilot pollution model and analyze the channel estimation process under this model.To realize a non-multiplexing pilot for strong interference users,a user grouping algorithm based on K-means is proposed,a user grouping algorithm based on K-means is proposed.At the same time,to achieve the purpose of multiplexing the same pilot sequence between users with weak interference between groups and avoid the sharp increase of intergroup interference caused by multiplexing pilots between user groups,a pilot allocation algorithm based on user grouping is proposed.The simulation results show that compared with the three pilot allocation schemes,the pilot allocation scheme based on user grouping can provide better pilot pollution suppression ability to optimize the spectral efficiency of system users.Secondly,two power allocation schemes based on spectral efficiency maximization(max)and fairness(max-min)are proposed to address the power allocation problem of the cellular large-scale MIMO system based on channel spatial correlation and access point selection.Firstly,the cellular large-scale MIMO system model based on spatial correlation and access point selection is established,and the approximate closed expression of spectral efficiency based on maximum ratio transmission(MRT)is derived.Secondly,the problem of Max and max-min power allocation based on the closed face is studied,the constraint conditions under the limitation of transmission power are analyzed,and the two power allocation objective functions of Max and max-min based on this constraint are established.In the solution of Max nonconvex power allocation firstly,it is transformed into the equivalent problem of weighted minimum mean square error(MMSE).Then the local optimal value is approximated by block coordinate descent(BCD).In the max-min nonconvex power allocation problem the expression is relaxed,and the nonconvex problem is transformed into a second-order cone programming problem.Finally,the dichotomy method is used to solve it.The simulation results show that Max and max-min power allocation show their respective advantages compared with equal power allocation.Max power allocation improves the total spectral efficiency of the system,while maxmin power allocation reduces the spectral efficiency gap between system users and makes user services fairer.