Research On Non-Stationary Channel Modeling For 6G Unmanned Aerial Vehicle Communications

With the commercial deployment of the Fifth Generation(5G)mobile communication system,the research and development of the Sixth Generation(6G)mobile communication system is on the agenda in various countries.In order to meet the increasingly extensive communication application scenarios and needs,the research and development of 6G mobile communication technology will focus on four major trends:full coverage,full spectrum,full application,and strong security.The future 6G mobile network is committed to achieving seamless coverage of wireless networks worldwide.As the key enabling technology and important application scenario of 6G mobile communication system,Unmanned Aerial Vehicle(UAV)communications have received extensive attention.UAV communication channels have significant differences compared to traditional terrestrial communication channels,such as the non-stationary characteristics in the time domain due to the high maneuverability of arbitrary UAVs trajectories,the channel variability due to flight altitude changes,as well as the new features introduced in combination with other key 6G technologies,such as ultra-massive MIMO(Multiple-Input Multiple-Output)brings non-stationarity in the antenna array domain and high delay resolution due to millimeter wave(mmWave)and terahertz band applications,etc.These new features make the traditional channel models no longer applicable to UAV channel characterization and bring great challenges to 6G UAV channel modeling research.Considering the shortcomings of the existing channel models,this paper conducts an in-depth study of 6G UAV channels,and the innovative work is summarized as follows.(1)For the UAV-to-ground communication scenario,a single-user UAV regular shaped-geometry based stochastic channel model(RS-GBSM)based on the effective scattering region is proposed,which considers the near-ground scatterer distribution characteristics of realistic air-to-ground scenarios to depict the 3D spatial UAV channel propagation environment more accurately.Key channel model parameters,including the time delay,power,phase,and angle of the cluster are derived based on the geometric relationship between the transceiver and the scatterer.A Markov process is applied to model the cluster generation and recombination phenomenon brought by the transceiver movement,and the transition region is introduced to realize the smooth evolution of the cluster generation and recombination process.Considering the simulation efficiency,the corresponding simulation model is obtained by discretizing the angular parameters based on the theoretical model.Important statistical properties,including the temporal autocorrelation function(ACF),spatial cross-correlation function(CCF),Doppler power spectral density(PSD),root-mean-square(RMS)time delay,and stationary interval,are derived and calculated.By analyzing the statistical characteristics,the influence of channel environment and non-stationary characteristics of the UAV channel is investigated.Finally,the accuracy of the proposed model is verified by comparing the fitting simulation result with the UAV channel measurement data.(2)For the arbitrary trajectory UAV-to-ground communication scenario,general single-user UAV irregular shaped-GBSM(IS-GBSM)supporting arbitrary 3D trajectories is proposed.For more general UAV communication scenarios,a novel channel model supporting multiple UAV channel characteristics is established leveraging the twin cluster channel modeling framework.The application conditions of temporal trajectory sampling and spatial trajectory sampling are discussed,and a grid based large scale parameter(LSP)generation method considering non-stationary and 3D spatial consistency is proposed.In addition,combined with the key technologies to be applied in the 6G communication system,the proposed model applies the space-time joint cluster generation and evolution mechanism to support the ultra-massive MIMO antenna configuration.Besides,the model framework supports high delay resolution and large bandwidth applications in the mm Wave frequency band.The key statistical properties of the model,such as space-time correlation function(STCF),time delay PSD,Doppler PSD,RMS delay/Doppler spread,stationary interval,and coherent bandwidth,are derived and simulated.At last,the validity and advancement of the proposed model are verified by comparison using measurement data.(3)For multi-user applications of UAV communication,the proposed UAV channel model is extended to multi-user scenarios by characterizing the channel correlation of multi-links.The inter-link correlations of LSPs and small scale parameters(SSPs)are realized using different parameter generation methods.The model is also extended to support Uniform Planar Antenna(UPA)and Uniform Circular Antenna(UCA)configurations at the transceiver to improve the generality of the model.Simulations are conducted on the multi-user statistical properties,such as correlation matrix collinearity(CMC),wideband spectral dispersion(WSD),multi-user channel capacity,and relative channel capacity.The influences of the number of antennas,user density,and UAV height on the channel are analyzed in the UAV massive MIMO multiuser communication scenario.(4)Wideband channel measurements at the 2.5 GHz/3.5 GHz bands are conducted for the underground UAV communication scenario,and a single-user channel model applicable to different underground space structures is proposed based on underground space channel characteristics.The proposed model considers the waveguide effect and near-field/far-field effect in the tunnel scenario,and the enclosed space and rich scatterer distribution in the room-pillar scenario.By fitting with the field underground channel measurement results,the simulation parameters under different frequency bands are obtained.The statistical properties of channel simulation and channel measurement are well fitted,which proves the practicality of the proposed model.In summary,this paper has conducted the extensive and in-depth study in 6G UAV communication channel research,using different modeling approaches to establish UAV channel models applicable to different scenarios and applications.These research works can provide important references as well as theoretical and technical support for the design,evaluation,and testing of 6G UAV communication systems.