Research On Superimposed Pilot Optimization Design In Multi-cell Massive MIMO Systems

As the candidate key technology of the Fifth generation mobile communication systems,massive multiple input multiple output(MIMO)technology has attracted widespread attention in the industry due to its advantages of high spectral efficiency and low transmited power.The performance of the system mainly depends on whether the base station obtains accurate channel state information(CSI).Affected by the channel coherence time and the dramatical increase of the number of users,the bandwidth utilization rate which is based on the conventional multiplexed pilot(MP)configuration mode inevitably decreased.Therefore,it is very important to study the channel estimation scheme based on the superimposed pilot(SP)configuration mode,which is suitable for multiuser massive MIMO systems.In this mode,the performance of the SP-based channel estimation is closely related with the specific SPs.Then the thesis,mainly focuses on the SP opitimization in the multiuser massive MIMO systems,which includes1.For massive MIMO systems,pilot contamination(PC)and data interference(DI)are two main factors that affect the accuracy of the SP based channel estimation,provided that the channel coherence time is less than the total number of users.To tackle the effect of PC,a block-diagonal grassmannian line packing(BD-GLP)approach is proposed,in which the specific sequences for different cells are first designed based on GLP,and then are block-diagonal extended to form the SP matrix for the users in different cells.To also alleviate the effect of DI on the channel estimation with the designed SP matrix,an iterative channel estimation method based on Tikhonov regularization(TR)is presented.Numerical simulation results show that in the case of full utilization of bandwidth resources,the proposed method mitigates the effects of the PC and DI,hence improves the accuracy of channel estimation.2.Considered the mutuality of PC in multiuser massive MIMO systems,a soft superimposed pilot optimization design is put forward in view of approximate equiangular tight frame(A-ETF).It perfoms the soft SP allocation among the total users in the system by measuring the whole PC factor.Meanwhile,due to the fact that users with large channel coefficients will bring a serious DI to other users with small channel coefficients in the channel estimation,an iterative least squares(LS)channel estimation method on the basis of the ordering successive interference cancellation(OSIC)is presented.Finally,combining the SP optimization design and the iterative LS channel estimation,the spectral efficiency(SE)of different users at different iterations are theoretically derived.