Research And Countermeasure Of Mechanism On Pilot Contamination In Multi-cell Massive MIMO Systems

Massive MIMO(Multiple-Input Multiple-Output)systems have been widely studied due to their potential advantages.Research results have shown that the accurate information of Channel State Information(CSI)is one of the important factors for full exploit of the technical advantages.Generally,the receiver estimates CSI based on pilot signals,and thus,pilot signals can effect the accuracy of CSI directly.In fact,in order to ensure the quality of the CSI,the pilot signals of all the the users should be orthogonal each other.However,it will consume a lot of resources to satisfy this orthogonality.Because the limitation of spectrum resources and channel coherent time,scheme of pilot reuse has been adopted usually,where users in different cells use the same pilot signals,while users in the same cell use the orthogonal pilot signals.In the pilot reuse case,because the pilot signals used by users in target cell and interference cells cannot be distinguished accurately by the receiver,the quality of channel estimation will be reduced largely,and the performance of system will be degraded greatly.This phenomenon is called as “pilot contamination”.One of the direct consequences of the pilot contamination is existence of error floor.In this thesis,pilot contamination in a massive MIMO uplink system is analysed detailedly and the corresponding methods to reduce its effects are proposed.Specific research includes the following aspects: the first,this thesis reveals the intrinsic reasons of the error floor generated.Specifically,based on analysis on events,so-called as “center-overturned”,the thesis shows that the probability of the events is equal to the error floor,and then,gives the analytic formulas of calculating the error floor.The second,based on an equivalent transmit-receive equation,the corresponding parameters,so-called as “indicated numbers of the system”,are defined,and the influence of these parameters on error floor is demonstrated through the theoretical analysis.The last,in order to mitigate effects of pilot contamination,this thesis proposes an iterative channel algorithm.In this algorithm,the decoded signals are screened first,and selected based on criteria,then,added selected signals to the pilot sequences.After that,channel estimation is re-performed.According to the signal constellations,two selection criteria are presented.Simulation results show that the proposed algorithm can improve the channel estimation accuracy and the system performance efficiently.