Research On Uplink Joint Reception Scheme Of Cell-Free Massive MIMO System

As the fifth generation mobile communication system is deploying globally,academia and industry have turned their attention to the sixth generation system to meet the demands for information and communications technology in 2030,where cell-free massive multiple-input multiple-output(MIMO)is considered to be a promising physical layer technology.Compared with the existing cellular mobile communication network,there is no concept of cell boundary in cell-free massive MIMO system.Specifically,a large number of randomly distributed access points(APs)serve all users simultaneously in a coordinated manner,and coherent user-centric transmission is implemented to overcome the inter-cell interference limitation in cellular networks.It provides strong spatial macro diversity gain and favorable propagation effect.This thesis focuses the research on the uplink joint reception scheme of the cell-free massive MIMO system which combines with the receiver design,pilot assignment,AP selection and AP switch ON/OFF(ASO)strategy.The chief content of this thesis is as follow:For the distributed uplink reception,we consider a novel and scalable protective weak partial fullpilot zero-forcing(PWPFZF)receiver.To solve the problem that the original implementation of this receiver will allocate weak users into strong group which violates the principle of PWPFZF,a distributed algorithm for joint pilot assignment and user grouping is proposed.The algorithm can be divided into two stages.In the first stage,the master AP is selected for each user,and the greedy algorithm is used for pilot assignment to minimize the pilot contamination of each user at their master AP,and the first user grouping is done with the pilot assignment.In the second stage,the second user grouping is performed with the goal of maximizing the total uplink SE.The proposed algorithm can ensure that each user experiences both least pilot contamination and inter-user interference among the APs that make major contributions to their signal detection.Besides,it only depends on long-term channel state information(CSI).Simulation results show that the proposed algorithm can effectively improve the average uplink SE.For the centralized uplink reception,a user-centric cell-free massive MIMO system model is deployed,and a centralized partial zero-forcing(ZF)receiver is proposed to reduce the computational complexity of traditional centralized ZF receiver.Then,a total uplink power consumption model considering both AP selection and ASO is developed.A novel tight approximate closed-form expression of uplink achievable rate for the proposed receiver is derived under the condition of imperfect CSI and pilot contamination,which is only related to large-scale fading coefficient.Finally,an AP selection algorithm based on the new rate expression derived is proposed,which jointly considers ASO to further improve energy efficiency(EE).Simulation results show that the proposed scheme can obtain a significant improvement in the total EE with a small loss of SE,and has a high feasibility.