Research On Dual-layer Precoding Techniques For Massive MIMO Systems

Massive multi-input multi-output(MIMO)technology can greatly improve the spectrum efficiency and energy efficiency of system by deploying a large number of antenna elements at the base station.Therefore massive MIMO technology is regarded as a key alternative technology of the candidate of the next generation mobile communication system and attracts tremendous attention.However,in order to take full of advantage of massive MIMO technology,the inter-cell interference(ICI)and inter-user interference(IUI)must be completely eliminated.As a kind of signal preprocessing technology,precoding is widely studied that can effectively eliminate the interference and reduce the complexity of the receiver designs.On one hand,the interference problem and pilot contamination problems coexist in massive MIMO systems for time division duplexing(TDD)mode.Hence how to eliminate the interference and pilot contamination is taken as one of the problem to be studied in this thesis.On the other hand,the user usually needs to feed the channel state information(CSI)back to the base station through the feedback link resulting to a higher feedback overhead problem for the massive MIMO system for frequency division duplexing(FDD)mode.Hence how to reduce the feedback overhead with the reduced interferences of inter-cell and IUI in intra-cell of the system is taken as another problem to be studied in this thesis.In the point of these,this thesis focuses on the research on the dual-layer precoding design scheme in massive MIMO system.The corresponding contents can be outlined as follow.Dual-layer precoding scheme design combined with pilot contamination elimination.In order to effectively eliminate IUI,ICI and pilot contamination in the system,a dual-layer precoding scheme with pilot contamination elimination based on TDD mode is presented.Firstly,the scheme designs the outer precoding matrix based on regularized zero-forcing(RZF)technology to eliminate the IUI.Then based on minimization of mean square error criterion(MMSE),the actual channel and the outer precoding matrix is regarded as the equivalent channel to designs the inner precoding matrix in order to eliminate ICI and the interference caused by pilot contamination.The numerical simulation results show that the proposed scheme not only effectively eliminates the interference in the system,but also makes the sum rate of the system be improved significantly.Dual-layer precoding scheme design combined with channel covariance matrix.In order to eliminate the IUI and reduce the feedback overhead,two kinds of dual-layer precoding design schemes are given for FDD mode.The first scheme is based on the gradient ascent algorithm to achieve dual-layer precoding design.According to the channel covariance matrix of each group,the outer precoding matrix of this scheme is designed by utilizing gradient ascent algorithm to eliminate the IUI between the groups,and then the actual channel and the outer precoding matrix is taken as the equivalent channel to further reduce the channel matrix dimension.Finally,based on the equivalent channel,the zero-forcing(ZF)technique is utilized to eliminate the IUI.Thus,this scheme reduces the system feedback overhead to a certain extent.The other scheme design the dual-tier precoding scheme based on the taylor series expansion.In this scheme,according to each group of channel covariance matrices,the IUI between the groups is eliminated by the taylor series expansion,then the actual channel and the outer precoding matrix is regarded as the equivalent channel to further decrease the channel matrix dimension.Finally,the inner precoding matrix is designed to eliminate IUI based on the ZF criterion.The numerical simulation results show that the performance of the precoding scheme based on the taylor series expansion is better than the precoding scheme based on the gradient ascent algorithm.