Research On Precoding Technique In 3D MIMO System Using Long-and Short-term Feedback

Three-dimensional multi-input multi-output(3D MIMO)systems,with 2D active antenna array system(AAS)at the base stations(BSs),can improve the system performance by exploiting both the horizontal and vertical degree of freedom.Moreover,with enough channel state information at the transmitter(CSI-T)in BSs,the 3D MIMO system performance can be further improved by 3D precoding technology.In frequency division duplex(FDD)systems without channel reciprocity,the BSs can only acquire CSI-T from uplink feedback.However,the instantaneous feedback has high feedback overhead,while the long-term feedback cannot fully reflect the channel characteristic.Consequently,taking precoding performance and feedback overhead into consideration,this paper makes a comprehensive utilization of long-and short-term CSI-T,and focuses on the research of codebook design and user scheduling in 3D MIMO limited feedback precoding systems.Specific work is as follows:Firstly,in view of the fact that existing 3D codebook designs cannot fully reflect the channel characteristics,a codebook feedback approach based on the long-and short-term CSI is proposed for 3D MIMO system,which is more suitable for the real 3D channel characteristics.The proposed codebook consists of two parts,namely strongly related global codebook and multiple local codebooks centered in the global codewords,of which global codebook is used to describe the users' best main directions,and the local codebook is used to further promote the codeword precision.Moreover,by making a comprehensive utilization of statistical and instantaneous CSI,the 3D precoding with proposed codebook feedback method can approach the optimal performance through long-term feedback of global codebook and instantaneous feedback of local codebook.The simulation results show that compared with the commonly used global Kronecker product codebook(KPC),the proposed scheme can either obtain better performance or reduce the feedback overhead on the premise of a similar performance.Further considering the multiuser scenarios,a space division multiple access(SDMA)scheme based on dual channel quality indicator(CQI)feedback is proposed to improve the performance of user scheduling.Firstly,based on the spatial correlation of 3D channel,a user clustering codebook is designed as the Kronecker product of rotated Discrete Fourier Transformation(DFT)matrices;then,to reduce the feedback overhead while guaranteeing theperformance,the users select the optimal cluster and suboptimal cluster and the corresponding main transmitting direction according to the transmit-side correlation matrix and feed them back as long-term information;finally,the BS performs the user scheduling by the two CQIs each user feeds back in a short time and realizes the downlink multiuser transmission,as a result,the instantaneous CSI is fully reflected at the BS and the scheduling opportunity of users is increased.The efficacy of the proposed SDMA scheme is demonstrated through simulations.Compared with the SDMA with single CQI feedback,the proposed method can increase the number of scheduled users,and further improve the system sum-rate.