Research On Adaptive Transmission Of Air-ground Broadband Communication System

With increasing the high demand of data transmission capacity,improving the spectral efficiency is the core issue of air-ground broadband transmission system under the circumstance of limited spectrum resource.Adaptive transmission technology enables robust and spectrally-efficient transmission,which has been applied in land-based wireless communications successfully.However,it cannot effectively apply in the link of air-ground broadband,because different kind of transmission service is corresponding to the different channel characteristic of air-ground link,and different stages of flight(e.g.arrival,takeoff,en-route and parking.etc.)is corresponding to the different delay and data transmission requirements.These factors put forward a great challenge in adaptive modulation of air-ground broadband communications system,there has not yet been an effective solution in adaptive transmission of air-ground broadband system.How to model an adaptive modulation mechanism to fit for the different transmission services and different transmission link characteristic,and how to improve the lower improve the spectral efficiency that are the main difficulties in adaptive transmission of air-ground broadband.As one of the key technologies of adaptive transmission,adaptive modulation requires an accuracy evaluation of channel quality,which can be adapted the transmission scheme to match channel changes.However,the high speed movement between aircraft and base station leads the Doppler Effect in the air-ground link,which has great influence on QoS(Quality of Service)of Air Traffic Service/ Aeronautical Operational Control(ATS/AOC)service.Moreover,AAC/APC(Aeronautical Administrative Communication / Aeronautical Passenger Communications)service involved mass data transmission,which will increase processing delay and lead to a difficulty in channel quality prediction.This thesis unites the project “Research and testing on prototype of data link in air-ground broadband communication systems”,which sponsored by Commercial Aircraft Corporation of China Ltd(COMAC),and based on LTE-A transmission system.A novel nondata-aided error vector magnitude(NDA-EVM)was proposed to evaluate the channel quality.The adaptation strategies based on NDA-EVM were designed for ATS/AOC and AAC/APC transmission links respectively,which can guarantee QoS and effectively improve the spectral efficiency.The algorithms presented in this thesis have been constructed the technical scheme and been approved in COMAC,and that will be apply in the flight test in prototype of data link in air-ground broadband communication systems.The detail of the study and main contributions of this thesis contain the following aspects.(1)Adaptive modulation requires an evaluation of channel quality to trace channel changes accurately in real time.But the traditional evaluation cannot solve the problem effectively.NDA-EVM was considered as a metric use in adaptive modulation to evaluate the channel quality of air-ground link.The unified model to calculate NDA-EVM over fading channels was analytically derived,and the performance bounds were also derived,which provides a quantitative analysis method in evaluating channel quality in air-ground link.The sensitivity analysis was taken to reveal the impacts of the channel parameters on NDA-EVM estimation.Simulation results illustrate that: Comparing with the traditional metric,NDA-EVM can reflect the change of air-ground link in real-time.It approaches theory value and also be sensitive to channel changes.(2)ATS/AOC transmission link is characterized by rapidly change of time-varying channels and lower delay in transmission,the traditional adaptive modulation cannot improve spectral efficiency in ATS/AOC.To solve the issue,adaptation strategy based on NDA-EVM was proposed.The threshold of each modulation mode can be determined by deriving the expression between NDA-EVM and SER.Base on those,the adaptation strategy based on state machine of NDA-EVM was designed,which made the modulation order can be adjusted quickly and precisely.By simulating “en-route scenario”,“taxi scenario” and “arrival and takeoff scenario” in the lower delay link,the performance of adaptive modulation in ATS/AOC transmission link between proposed algorithm and traditional algorithms were analyzed and compared.Simulation results illustrate that: with the same time complexity,the proposed algorithm has better performance in aspects of real-time in channel tracking and modulation modes adjustment.In the stage of en-route,it improves the spectral efficiency by 0.77bit/s/Hz,the accuracy in modulation modes adjustment improves by 19.1% compare with traditional adaptive modulation.In the stage of arrival and takeoff / taxi,it improves the spectral efficiency by 1.1bit/s/Hz,the accuracy improves by 14.9% compare with traditional adaptive modulation.It can satisfy the demand of real-time monitoring in flight control and high speed transmission in air traffic management.(3)There has not yet been an effective adaptive modulation in AAC/APC.To solve the issue of that the traditional adaptive modulation cannot guarantee QoS and has a lower spectral efficiency,which is caused by the delay of AAC/APC transmission links,the adaptation strategy based on NDA-EVM for delay link was Cross-layer designed.Considering adaptive modulation in single layer cannot apply in the link with long delay,the process of data packet in MAC was analyzed and the FSMC(finite state Markov chain)model was built in PHY.Based on those,the adaptation strategy based on FSMC channel model of NDA-EVM was designed.It simplifies the adaptive modulation in AAC/APC to the search for the optimal transfer probability in FSMC matrix in two typical states,which can be called “resident state” and “transient state”.Simulation results illustrate that: the proposed algorithm can guarantee QoS and improve the spectral efficiency.In LOS scenario,the spectral efficiency can reach 7.4bit/s/Hz,and in NLOS scenario,it can reach 6.1bit/s/Hz.It can meet the demand of high-speed internet access of passengers and high speed transmission in AOC servers and it also improve an effective technical approach of adaptive modulation in AAC/APC links.