Research On Distribution Of Base Station In Massive MIMO System

The massive Multiple-Input Multiple-Output(MIMO)technology,as one of the key technologies of the fifth generation mobile communication(5G),has attracted widespread attentions in academia and industry.In the massive MIMO system,the base station is equipped with a large number of antennas,which leads to the asymptotic orthogonality of the channels from different users to the base station.Hence,the base station can use simple signal processing to distinguish different signals from different users on the same frequency simultaneously.It is the feature that make the massive MIMO system serve multiple users on the same frequency at the same time,which making full use of the space diversity technology and greatly improving the spectral efficiency and transmission reliability.The above-mentioned advantages of the massive MIMO system are based on the assumption that the base station can acquire accurate Channel State Information(CSI).In practical,CSI is unknown to the base station.In order to acquire CSI,a common method is training-based channel estimation.Specifically,each user transmits pre-designed pilot sequences,and the base station estimates the CSI according to the received signals.In order to ensure the estimating performance,usually,different users need to send mutually orthogonal pilot sequences.However,on the one hand,for the massive MIMO system,a long pilot sequence is needed to ensure the orthogonality.On the other hand,the micro base stations are gradually being used,and the coverage of the cell is shrinking.As a result,the number of the required cells is increasing and the distance between adjacent cells becomes short,which cause inter-cell interference.Based on the above factors,the conventional method is that the users in the same cell use the orthogonal pilot sequences,while the users in the adjacent cells use the same pilot sequences,referred to as Pilot Reuse.Unfortunately,the pilot reuse causes inter-cell interference,which greatly degrades the accuracy of the channel estimation.This phenomenon is called as pilot contamination.Therefore,to find effective methods of alleviating pilot contamination is one of the focuses of research on the massive MIMO systems.For the massive MIMO uplink system with pilot contamination,this thesis studies the maximum of signals that can be supported by the system and a scheme of reasonably distributing base stations,aiming at mitigating pilot contamination.Specially,main contributions of this thesis consist of two parts.In the first part,based on an equivalent system model,corresponding constraints on the maximum are presented.It is shown that,when the constellation sizes satisfy the constraints,the decoding error probability will go to zero,as the number of antennas and the pilot signal-to-noise ratio(SNR)tend to infinity although the pilot contamination exists.In the second part,still based on analysis on the equivalent system model,a method of judging the reasonable distribution of base stations is proposed.Based on this method,the specific requirements of reasonable distribution of base stations are calculated under the indoor millimeter wave environments.Finally,the effectiveness of this method is verified by simulations.