Superimposed Pilot Optimization Design And Channel Estimation In Cell-Free Massive MIMO System

As one of the key technologies of the fifth generation mobile communication system(5G),massive multiple input multiple output(MIMO)technology has aroused widespread attention in the industry due to its capability to greatly improve the spectrum efficiency(SE)of the system with lower power consumption.In recent years,Cell-Free massive MIMO,which developed from distributed massive MIMO,has gradually come into people's sight.It breaks the traditional base station centric feature of cellular cells,and turns to be user centric by deploying a large number of access points(AP)with one or more antennas in a wide area.These APs are connected to a central processing unit(CPU)through backhaul and serve all users in the same time-frequency resource block.This architecture is capable of combating shadow fading and interference suppression,and can significantly improve the SE under the premise of ensuring user quality of service(Qo S)fairness.Moreover,the APs have the characteristics of small size and light weight,which makes it very flexible to deploy.However,in the process of channel estimation,since the length of the channel coherence interval is smaller than the number of users in the system,it is impossible to allocate orthogonal pilot sequences to all uers,and some users have to reuse the same pilot sequences,which will result in serious pilot contamination(PC).In view of this,this thesis studies the channel estimation and pilot optimization design in Cell-Free massive MIMO system,and the specific research contents are as follows:1.Based on the linear minimum mean square error(LMMSE)criterion and the maximum ratio combining(MRC)technique,the channel estimation of Cell-Free massive MIMO system in the traditional multiplexed pilot(MP)configuration mode and the superimposed pilot(SP)configuration mode is investigated,and the closed-form expressions of user uplink SE in two configuration modes is derived theoretically.The simulation results show that,compared with MP configuration mode,SP configuration mode can effectively suppress PC,so as to significantly improve the SE of the system.2.In order to further reduce the PC of Cell-Free massive MIMO system in the SP configuration mode,an approximate optimal codebook,named as Approximate Maximum Welch Bound Equality(A-MWBE)codebook,is designed based on the codebook and frame theory,and the non-orthogonal pilot matrix is constructed by using A-MWBE codebook to obtain low correlation non-orthogonal pilot sequences.Moreover,a superimposed pilot optimization configuration scheme is proposed,its core idea is to use the PC factor as an indicator,and then assign orthogonal or non-orthogonal pilot sequences to users according to their PC factors,so as to minimize the total PC of the system.Finally,the expression of the user uplink SE under the optimization scheme is derived,and the simulation results show that the proposed scheme can further reduce the impact of PC on Cell-Free massive MIMO system in the SP configuration mode,and thereby improve the uplink SE of the system.