Terahertz Communication Performance In Atmospheric Turbulencewhile The High-speed Train Passes Through The Tunnel

Rail transports such as high-speed maglev and subway will usher in a new era of "smart rail mobility".At that time,train-ground communication system will need to provide a data rate up to 100 Gbps for the services including on-board high definition video,real-time train dispatching high definition video and so forth.Terahertz(THz)communication is considered to be a promising solution to realize ultra-high data rate of train-ground communication because of its strong penetration,high safety,good directivity and wide bandwidth.However,due to the decrease of wavelength,quality of communication suffers from atmospheric turbulence especially when the high-speed train passes through the tunnel.At present,there are few theoretical studies on the impact of turbulence on terahertz communication performance.This problem ranges over fluid dynamics and communication theory so that it is challenging.Therefore,this paper carries out the following research on this issue so as to provide a theoretical basis for the design of high-speed maglev or subway in the future.In order to analyze the atmospheric turbulence caused by the high-speed train passing through the tunnel,the approach which combines computational fluid dynamic(CFD)with communication theory is adopted.Temperature and pressure distributions in the tunnel are analyzed.Based on the refractive index structure constants deduced from Kolmogorov’s law,the theoretical expressions for the influences of atmospheric turbulence on amplitude and phase of terahertz signal are proposed.It is found that the reciprocals of amplitude variance and phase variance on the logarithmic scale are linear related to train speed,which simplifies cognition and is universal.It will provide a theoretical basis for further study of communication under the influences of tunnel turbulence.In order to study the performance of terahertz communication system under the influences of tunnel turbulence,the terahertz communication transmission model is established according to the three-dimensional geometric environment of the tunnel by Ray Tracing(RT).Via perceiving the turbulence as a multiplicative factor,the expressions of bit error rate(BER)containing turbulence parameters in binary phase shift keying(BPSK)modulation and quadrature phase shift keying(QPSK)modulation are derived,which are verified by Monte Carlo simulations.The quantitative predictions for the influences of tunnel turbulence on communication performance under different transmission frequencies,train speeds,tunnel diameters and vacuum degrees are obtained.In order to study the performance of Multiple-Input Multiple-Output(MIMO)spatial diversity terahertz communication system under the influences of tunnel turbulence,the probability density functions of channel gain are analyzed by padéapproximation.The BER expressions containing turbulence parameters of MIMO spatial diversity communication system are proposed.According to the central limit theorem,the performance of massive MIMO(M-MIMO)communication system in tunnel turbulence under different MIMO scales,transmission frequencies,train speeds and tunnel diameters is analyzed and predicted.In order to study the performance of MIMO spatial diversity wireless communication system under the influences of tunnel turbulence,the impacts of terahertz MIMO antenna array element spacings on conditional numbers are analyzed,and the optimal antenna array element spacings are deduced.The expressions of channel capacities of MIMO spatial multiplexing communication system containing turbulence factors are given.On this basis,the influences of tunnel turbulence on communication performance under different MIMO scales,train speeds,frequencies and tunnel diameters are further predicted.In addition,according to terahertz ultra-massive MIMO(UM-MIMO)antenna structure,a method for analyzing the influences of tunnel turbulence on the capacities of wireless communication system under different numbers of antenna sub-arrays is proposed,which puts forward a new idea for the study of UM-MIMO.The above theoretical study fills the gap for the lack of consideration of turbulence factors in the existing terahertz wireless communication research,which will provide a more accurate theoretical basis for the prediction of terahertz communication performance in future design of high-speed maglev or subway.Furthermore,the basic principles can also be extended to a variety of terahertz communication problems in other turbulent environments.