Channel Modeling And Two-stage Precoding Design For Stratospheric Multi-antenna Communication Systems

In recent years,with the rapid development of wireless communication,the requirements of large volumes of wireless communication,high rate,and high spectral efficiency have become increasingly urgent,and the spectrum resources have become increasingly scarce.Because of its wide coverage,flexible configuration,and high transmission quality,the stratospheric communication has been considered as emerging alternative technology,which has attracted attention worldwide from industry and academia.Multiple-input multiple-output(MIMO)technology could increase spectral efficiency and user sum rate by exploting spatial multiplexing.As a new technology,the challenge is to investigate the application of MIMO technology to stratospheric high altitude platform(HAP)communication systems.Three-dimensional(3-D)HAP-MIMO channel modeling is the basic work of key technology research of stratospheric communication systems.The accurate channel model can provide basis for the performance analysis and the precoding algorithm design of stratospheric communication systems.The existing 3-D HAP-MIMO channel models are extended from the traditional vehicle-to-vehicle(V2V)channel model,and they ignore the influence of the motion of transmitter,receiver,and scatterers on the statistical characteristics of HAP-MIMO channel.Based on the existing3-D HAP-MIMO channel model,the literature uses correlation matrix to design a two-stage precoding for unmanned aerial vehicle(UAV)massive MIMO system.However,the dimension of the correlation matrix is high,and the two singular value decompositions also bring high algorithm complexity.Based on the different signal propagation characteristics,this paper studies 3-D HAP-MIMO channel model based on different time-varying parameters.Based on the proposed HAP-MIMO channel model,this paper further investigates a user grouping and two-stage precoding scheme based on statistical-eigenmode(SE)in HAP massive MIMO communication systems.1.Channel Modeling of 3-D HAP-MIMO Channel Based on Dynamic Evolution of NLOS ComponentsWe assume that the multipath components(MPCs)scattering from the far scatterers could appear again after they have vanished,and we propose a 3-D HAP-MIMO channel model based on dynamic evolution of NLOS components and corresponding simulation model.Firstly,we use two-state discrete-time Markov chain(DTMC)to model the dynamic properties of NLOS components,i.e.,appearance and disappearance,and survival and death probabilities of MPCs are derived by using Chapman-Kolmogorov(C-K)equations.Secondly,in order to reduce the complexity of the algorithm,eigenvalue decomposition algorithm is used instead of multiple iterations of C-K equations,and the complexity of the algorithm is reduced effectively.2.Channel Modeling of 3-D HAP-MIMO Channel Based on Dynamic Evolution of LOS ComponentsConsidering the dynamic characteristics of the LOS components,such as disappearance and reappearance,the 3-D HAP-MIMO channel model based on dynamic evolution of LOS components is proposed.Firstly,a two-state continuous-time Markov chain(CTMC)is used to model the dynamic evolution of LOS components,and the closed-form expressions of survival probabilities are derived by C-K equations.Secondly,we use the measurement results of spatial correlation function of LOS components to validate that the proposed 3-D HAP-MIMO channel could accurately describe the dynamic properties of LOS components.3.Channel Modeling of 3-D HAP-MIMO Channel Based on Isotropic and Non-Isotropic ScatteringBecause of the rich scatterers in urban scenario,we propose a 3-D HAP-MIMO channel model based on isotropic scattering and non-isotropic scattering.Firstly,the volume of inner cylinder and that between the inner cylinder and outer cylinder are used to model isotropic scattering and non-isotropic scattering in urban environment,respectively.Secondly,the propose channel model uses time-varying parameters including distances between the scatterers and receiver,azimuth angle of departure(AAOD),and elevation angle of departure(EAOD)to investigate the statistical characteristics of the 3-D HAP-MIMO channel.In the proposed model,the Laplacian probability density function(PDF)is used to describe the elevatin angle distribution of scatterers.Finally,we use the spatial correlation function of measurement results to validate the proposed 3-D HAP-MIMO channel,and the numerical results show that the proposed 3-D HAP-MIMO channel model is applicable to describe the statistical propeties of HAP-MIMO channel.4.Channel Modeling of Non-Stationary 3-D Wideband HAP-MIMO ChannelThis paper investigates a non-stationary HAP-MIMO channel model and corresponding simulation model.Firstly,we use 3-D multi-cylinders and two-dimensional(2-D)one-ring to model stationary scatterers and moving scatterers,respectively.The proposed model assumes that the azimuth and elevation angle of the scatterers depend on each other,and the von Mises Fisher PDF is used to jointly consider azimuth and elevation angles.Secondly,we assume that the multipath components(MPCs)scattering from the far stationary scatterers could appear again after their disappearance,and a CTMC is used to model the dynamic properties of MPCs.Thirdly,long distance and small-scale time-variant parameters are used to investigate the non-stationarities of HAP-MIMO channel.Forthly,we use the spatial correlation function and correlation matrix distance(CMD)of measurement results to validate the proposed nonstationary HAP-MIMO channel.The numerical results demonstrate that the proposed 3-D HAP-MIMO channel model is more practical to characterize the non-stationary properties of HAP-MIMO channel.5.User Grouping and Two-Stage Precoding Scheme Based on Statistical Channel State InformationIn this paper,we use the proposed correlation-based model to investigate stratospheric massive MIMO communication systems,and user grouping scheme and beamformer including outer beamformer and inner beamformer based on SE are investigated.In the proposed precoding scheme,we first exploit that the signal power mainly concentrates on statistical-eigenmode(SE),and second we use SE to design user grouping scheme and outer beamformer scheme.The user grouping scheme is designed using the average chordal distances between the SEs of users in the same group.The outer beamformer is designed using reduced-dimensional SE instead of using statistical correlation matrix.Using reduced-dimensional instantaneous effective channel,the inner beamformer is derived using regularized zero forcing(RZF).When the number of transmit antennas is 16 × 16,the sum rate performance of the proposed two-stage precoding scheme fits the scheme based on correlation matrix,and it demonstrates that the proposed precoding scheme is suitable to exploit HAP massive MIMO systems.