Research On Frequency Offset Estimation Over L-DACS1 Of Civil Aviation

With the increasing needs of communication for air passengers,it is imperative to research on the ground-to-air communication system of civil aviation.The L-DACS1(L-band Digital Aeronautical Communication System)based on OFDM(Orthogonal Frequency Division Multiplexing)technology is one of the important alternatives,which has high spectrum utilization,capability of anti-multipath and supports high-speed terminal access,etc.OFDM uses the orthogonal subcarriers to transmit data and increases spectrum utilization.However,the orthogonality among subcarriers is seriously influenced by the frequency offset,which would lead to a sharp decline in system performance.At the same time,in the L-DACS1,the relative speed between aircraft and ground station is very high,which would lead to high Doppler shift and fast changes of the channel environment.Therefore,it is of great significance to carry out researches on the frequency offset estimation of L-DACS1.Based on the project cooperated with Commercial Aircraft Corporation of China,namely "Development and testing of prototype system for ground-to-air broadband data link communication system",the thesis focus on the research of frequency offset estimation in the L-DACS1.The specific contents of this thesis are shown as follows:(1)The characteristics of mobile communication system and some major technologies in the L-DACS1 are analyzed.The advantages and difficulties of the OFDM technology are discussed.Then a system of signal model influenced by the frequency offset is established.In the following,the influence of frequency offset on the system performance is analyzed in detail.(2)A geometric model of motion for the cruise phase of civil aviation aircraft in the three-dimensional space is established and the formula of Doppler shift between aircraft and base station under arbitrary route is derived.Next,the formula is verified in different scenarios.This thesis simulates and analyzes Doppler frequency offset characteristics of civil aviation ground-to-air communication systems.(3)A frequency estimation algorithm based on position information is proposed to estimate the frequency in L-DACS1.The first step,the formula obtained from the thesis is used to pre-estimate the Doppler frequency offset between the aircraft and the base station based on the latitude and longitude of aircrafts and base stations,aircraft speed and flight altitude,etc.The second step,the improved FDKD(Frequency Domain Kronecker Delta)pilot is used to estimate the residual frequency offset after the frequency offset pre-compensation in the first step.