Research On The Pilot Decontamination Techniques For Massive MIMO-OFDM Systems

Ultra-dense network,based on the intensive deployment of small base stations for ameliorating network coverage,is one of the most effective 5G network technologies to improve the data traffic in the future.However,the inter-cell interference problem caused by the deployment of ultra-dense network becomes the main factor to restrict the system capacity.Due to the frequency multiplexing among multi-cell and limited channel coherence time,the adjacent cells in the ultra-dense network can only reuse the orthogonal pilot training sequences to estimate channel,resulting in pilot contamination problem which becomes the main interference problem among the cells.Multi-cell large-scale multi-input multiple-output(MIMO)systems with the large-scale antennas configuration,which could significantly improve the energy efficiency and spectrum efficiency of communication systems because of its high gain based on large-scale antennas,is considered as the important alternative technology in the future 5G wireless communications.However,increasing the number of base station antennas cannot eliminate the influence of pilot contamination,a bottleneck problem limiting the performance of multi-cell large-scale MIMO systems.At present,there are numerous pilot decontamination technology achievements for multi-cell large-scale MIMO systems,among which the pilot decontamination technology based on pilot assignation strategy is a research hotspot.Because different communication mechanisms are corresponding to different designs of orthogonal pilot,the dissertation will combine the advantages of orthogonal frequency division multiplexing(OFDM)technology and large-scale MIMO technology to constitute a promising multi-cell large-scale MIMO-OFDM communication mechanism.In this dissertation,the design of the optimal pilot symbols based on the mechanism combined with the different pilot decontamination schemes are gradually studied to reducing pilot contamination.The main work and innovation are as follows:1.Aiming at pilot contamination problem caused by frequency reuse among multi-cell and limited channel coherence time in multi-cell large-scale MIMO-OFDM systems,this dissertation takes advantage of multi-carrier resources in frequency domain(FD)of OFDM technology to carry out signal modeling,then to design the optimal pilot symbol set for the users in the frequency reuse cells.When the subcarrier resources of the system are sufficient to make the transmitting and receiving optimal pilot symbol equation of base station possess the time domain(TD)channel impulse response(CIR)solutions,it is theoretically proved that the set of product symbols formed by the optimal pilot symbol set and the Fast Fourier Transform(FFT)matrix satisfies the complete orthogonality.At this time,the TD CIRs can be estimated accurately through the time synchronization transmission of optimal pilot symbol scheme,which results in the complete elimination of pilot contamination.2.For the problem that the subcarrier resources of the system is not sufficient to make the product symbol set meet the complete orthogonality,and that the time synchronization transmission of optimal pilot symbol scheme will generate the pilot contamination,this dissertation will propose a solution which is a combination of uplink and downlink two-stage pilot transmission scheme,in which the "pure" TD CIRs are able to be estimated through the downlink transmission of the optimal pilot symbols which is encapsulated in the uplink optimal pilot symbols so that the base station can extract the "pure" TD CIRs from the uplink channel estimation.The simulation results show that the proposed scheme has obvious advantages in pilot training overhead,channel estimation accuracy,and system sum rate over existing pilot decontamination schemes.In addition,this scheme can make the base station avoid the problem of high complexity such as collaboration among cells or requirement of second order statistical information of channels.3.In order to further reduce the pilot training overhead and improve the spectral efficiency of the system,this dissertation proposes a user grouping scheme based on the optimal pilot symbol set under the condition that the subcarrier resources of the system are not sufficient.Although the product symbol set does not satisfy the complete orthogonality when the subcarrier resources are not sufficient,the subsets of certain elements in the product symbol set satisfy the partial orthogonality.Utilizing the above characteristic,this dissertation can theoretically prove that there are optimal user groups for users in frequency reuse cells,so that the product symbols of each user are orthogonal to each other.The design of the user grouping scheme depends on the Channel's Coherence Interval(COHI)of the system.If the COHI is not less than the number of optimal user groups,the user grouping scheme is exactly the optimal user grouping,or the user grouping scheme is determined by the value of the COHI,and the time staggered transmission of optimal pilot symbols of users in different groups is able to reduce the pilot contamination.The simulation results show that the proposed scheme can obtain the ideal pilot training overhead,channel estimation accuracy and system sum rate.In addition,the program is simple and with practical value.4.As for the uplink power allocation of pilot-data and the effective aggregation rate(AR)performance of multi-cell large-scale MIMO-OFDM system under the premise of a given uplink total power,uplink pilot power allocation optimization evaluation function based on pilot decontamination schemes is given in this dissertation,moreover,the conventional simultaneous pilot training sequence transmission scheme and user grouping scheme are optimized respectively based on this evaluation function.The simulation results show that the effective AR performance of the system is a convex function of the uplink pilot power allocation factor.In addition,in order to obtain the maximum effective AR of the system,the dissertation establishes one-variable optimization function based on the uplink pilot power allocation factor.The optimal uplink pilot power allocation factor is then solved by the golden section method and the effective AR of the system is achieved accordingly.The simulation results also show that the user grouping scheme can not only improve the spectral efficiency of the system because of the pilot training overhead,and can improve the energy efficiency of the system through the optimization of the uplink pilot power allocation further.