| Near space is the region of earth’s atmosphere that lies between20and100km above sea level,encompassing the stratosphere, mesosphere, and thermosphere. This is above where airliners fly butbelow orbiting satellites. The area is of interest for military purposes as well as commercial interests,especially in its potential in communications support, intelligence gathering, surveillance,reconnaissance, early warning and navigation. Development and validation of the near space vehicleare very prosperous at home and abroad, and recently various vehicles are being developed inprogress, including low dynamic high-altitude balloons, high altitude airship, suborbital, solar UAV,and high dynamic hypersonic vehicle and other types. However, low signal-to-noise ratio, largedynamic doppler, plasma sheath blackout impairment problem existed in near space vehicle tracking,telemetry, and command (TT&C) and communication systems, which have not been studiedelaboratively. A deep understanding of wireless channel is a must in wireless communication systemdesign. In order to solve the near space vehicle TT&C and communication problems, it is necessaryto research the channel characteristics of the near space vehicle in-depth.For all kinds of near-space vehicle’s TT&C and communication problem, this paper is mainlycarried out the study of channel characteristics for near space vehicles, First, atmospheric radio wavepropagation link characteristics of near-space high dynamic and low dynamic vehicle, atmosphericpropagation channel characteristics, plasma sheath radio wave propagation characteristics andplasma sheath channel characteristics are studied. Second, atmospheric propagation channel model,plasma sheath channel model, Nearspace-Ground/Nearspace-Space integrated channel models areestablished on the basis of previous channel characteristics study. Then the performance of typicalTT&C and communication system under complex integrated channels are evaluated. Finally, somereasonable advices are given to against those bad channel environments. The content researched inthis paper provides the theoretical basis of the channel support for the establishment of the domesticnearspace vehicle TT&C and communication system. The specific content and author’s maincontribution are summarized as follows:1. A frequency selective slow fading nearspace low dynamic channel model is proposed underthe study of communication link characteristics and its channel characteristics of thenearspace vehicle. Firstly, starting from the communication link view, we analysis andpredict the near-space radio wave propagation loss according to ITU standards andattenuation models, including free space attenuation, gas absorption, rain attenuation, cloudloss and the effects of atmospheric scintillation. Simulation results show that the impact ofrain attenuation is the largest and the other effects are small even can be ignored at high frequencies. Secondly, from the view of wireless communication channel theory, qualitativeanalysis of the coherence time and coherence bandwidth of the near space atmosphericpropagation channel are made, and it is modeled as a frequency selective slow fadingchannel. Then simulation analysis of the frequency selective slow fading channel and Karain attenuation channel for typical TT&C system are conducted. Finally, recommendationsagainst rain attenuation at Ka high band communications are given. It is concluded that thenear space low dynamic vehicles have great advantages in high-speed telemetry/data relay.2. An OFDM based near space low dynamic platform relay communication system solution ispresented. Based on the analysis of the lackage of the existing satellite communications andthe defects of United States TDRSS, we put forward a low dynamic platform based nearspace communications relay system solution taking into account the lingering characteristicsof low dynamic vehicle and its potential high-speed communication capacity. While OFDMtechnology has unique advantages in multipath confrontation capabilities and asymmetricbusiness supporting ability, we first raise OFDM technology as a core physical layerhigh-speed transmission technology in the proposed relay communication system.3. A novel low-complexity LMMSE channel estimation algorithm is proposed for the nearspace OFDM relay communication system. The proposed low complexity LMMSE channelestimation algrithom is designed to reduce computational complexity caused by matrixinverse operation in MMSE estimator. Correlation bandwidth was used as a criterion todivide large auto-correlation matrix into numbers of sub-matrixs, including non-overlap andoverlap method. The diaglog sub-matrix blocks representing low-frequency importantchannel information in channel autocorrelation matrix were preserved while the othersub-matrix blocks ignored, thus reducing autocorrelation matrix inversion computationalcomplexity. BER and MSE performance of the new algrithom was evaluated in frequencyselective flat fading channel with comparison to LMMSE, SVD algorithm, simulationresults and complexity anaysis shows that low complexity obtained at cost of performancedegradation slightly.4. A frequency selective fast fading nearspace high dynamic channel model and a plasmasheath channel model are established based on the study of high dynamic vehicle near spaceatmospheric propagation channel characteristics and plasma sheath channel characteristics.We focus on near-space high dynamic vehicle unique plasma sheath channel environmentand study the propagation characteristics of radio waves in the plasma in-depth starting fromthe radio wave propagation mechanism. The way to obtain Plasma electrical parameters ispresent, and the relationship between wave propagation properties among the electron density distribution, the collision frequency and the incident wave frequency is given clearly.The feasibility of FDTD algorithm in the calculation of electromagnetic wave innon-uniform plasma is validated, and PLJERC-FDTD algorithm is used to obtain the plasmasheath transmission/reflection power coefficient and phase shift characteristics. Themathematical model of the plasma channel is proposed, and the large-scale and small-scalechannel characteristics of the plasma sheath channel characteristics are studied andre-defined thoroughly.5. Nearspace–Ground high/low dynamic integrated channel models and Nearspace–Spacehigh/low dynamic integrated channel models are presented. On the foundation of near spaceatmospheric radio wave propagation channel characteristics and plasma sheath channelcharacteristics study, considering the complexity of the channel environment whennearspace vehicles communicate with ground TT&C platforms and space-based TT&Cplatforms, and according to the difference of communication objects and vehicle dynamicproperty, four integrated channel models are presented，including Nearspace–Ground highdynamic integrated channel model, Nearspace–Ground low dynamic integrated channelmodel, Nearspace–Space high dynamic integrated channel model and Nearspace–Space lowdynamic integrated channel model. The simulation implement method of the four integratedchannel models are given, and simulation analysis of typical TT&C and communicationsystem performance under the four integrated channel models are made as well.Confrontation recommendations to improve the quality of communication performanceunder integrated channel are given finally. |
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