Research On Hybrid Beamforming Technology In Millimeter Wave Communication Systems

There are a large number of unlicensed continuous bands in the millimeter wave band,making millimeter wave technology one of the key technologies in fifth generation mobile communication systems(5G).Millimeter wave has a short wavelength which makes the signal experience more path loss,but the extremely short wavelength also reduces the size of large antenna arrays significantly.Therefore,large scale antenna array usually employed in millimeter wave communication systems.Unlike traditional full digital beamforming technology,hybrid beamforming technology can greatly reduce system complexity,energy consumption and cost with less performance loss.This thesis focuses on the related technologies of hybrid beamforming for multi-user millimeter-wave massive Multiple-Input Multiple-Output(MIMO)communication systems.The specific work is as follows:Firstly,this thesis introduces the development status of mobile communication technology,introduces the hybrid beamforming technology and its research status.Then we present the millimeter wave channel model and system model.Various hybrid beamforming techniques and their structures in the prior literature are described.Finally,combining with the existing research results,two novel hybrid beamforming schemes are proposed.In the first scheme,we divide the hybrid beamforming design process into two independent stages,respectively.In radio frequency(RF)domain,the analog RF combination matrix is reconstructed by using the discrete Fourier transform(DFT)matrix.Then the equal gain transmission(EGT)method is used to design the analog RF precoder which can maximize the array gain.The EGT algorithm can greatly reduce the computational complexity.In baseband digital domain,we aim to eliminate inter-user interference and intra-user interference,and an extended block diagonalization(BD)algorithm based on matrix vector space is proposed.The proposed extended BD algorithm considers the user's signal feature space while the traditional BD algorithm only considers the interference null space.Therefore,the signal power in the direction of target user is enhanced.The simulation results show that compared with other low complexity hybrid beamforming algorithms,the proposed algorithm performs better.Also,the proposed algorithm still has good performance in Rayleigh fading channel while most of existing algorithms only apply to sparse channels,showing wider applicability.The second scheme uses the iterative coordinate descent method to design the hybrid beamforming precoding matrix.The proposed algorithm is inspired by the beamforming algorithm in the point-to-point transmitter total power limited system.In this thesis,the algorithm is extended to the millimeter-wave multi-user downlink channel scene,and the design of the baseband digital precoder/combiner is improve.Further,we analyze the performance of the algorithm in practical systems with low-resolution phase shifters.The simulation results show that the proposed algorithm has good performance and is very suitable for practical systems with limited phase resolution of phase shifters.