Research And Implementation Of OFDM Synchronization Technology In Flying Ad Hoc Networks

Orthogonal Frequency Division Multiplexing(OFDM)technology has the advantages of high spectrum utilization,resistance to frequency selective fading,and low implementation complexity,and is widely used in the field of wireless communication.However,in the flight ad hoc network communication scenario,the signal-to-noise ratio is low,and there is a large Doppler frequency offset,which brings great challenges to the timing synchronization and frequency offset estimation of the OFDM system.In a multipath channel with a low signalto-noise ratio,it is difficult for the receiver to accurately estimate the start position of each OFDM frame,resulting in timing synchronization deviation.At the same time,a large Doppler frequency deviation will cause a carrier frequency deviation and affect the correct reception of the signal.In order to recover data correctly at the receiver,it is necessary to compensate for timing deviation and frequency deviation.Aiming at the application scenarios of flying ad hoc networks,this paper focuses on the timing synchronization and frequency synchronization technology of OFDM,and uses FPGA to complete the realization of the synchronization system.The main tasks include:1.The technical principles of OFDM and the characteristics of wireless channels are introduced,and the channel model,link structure and transmission frame structure of the OFDM system in the application scenario of the flying ad hoc networks are given.2.Summarize and analyze the types and causes of synchronization deviation in OFDM system,and introduce the impact of synchronization deviation on the system.Then the algorithms are mathematically deduced from theory,some classic timing synchronization algorithms and frequency offset estimation algorithms are studied,and the advantages and disadvantages of each algorithm are analyzed by MATLAB simulation.3.On the basis of the classic timing synchronization and frequency offset estimation algorithms,a new synchronization scheme is proposed to solve the problems of low signalto-noise ratio and large frequency offset in the OFDM system of the flying ad hoc networks.Timing synchronization of this scheme includes coarse synchronization and fine synchronization,the coarse synchronization includes a timing synchronization algorithm based on time domain search and a capture confirmation algorithm based on energy detection,ensuring the real-time synchronization and the correctness of the coarse synchronization position,and then based on the coarse synchronization position perform fine timing synchronization,reduce the deviation of the synchronization position,and improve the accuracy of timing synchronization.For the integer frequency offset caused by the Doppler frequency shift of the aircraft in high-speed motion,this paper proposes a frequency domain differential integer frequency offset estimation algorithm for estimation.By performing differential processing on the cross-correlation operation results,timing synchronization can be reduced.The influence of synchronization deviation on the estimation of integer multiples of frequency deviation,which improves the accuracy of frequency deviation estimation.4.Based on the synchronization scheme proposed in this paper,FPGA implementation is completed,the hardware structure of each module is introduced in detail.Then use ModelSim to simulate and verify each module of the synchronization system,and Finally,use the development board to test the synchronization system to verify the correctness of the synchronization scheme.