Research And Implementation On Key Technologies Of Ground-Air Communication System Based On OFDM

Ground-air wireless communication refers to the communication between ground base station and near-ground air vehicle(aircraft,unmanned aircraft,etc.).With the continuous development of aircraft,higher requirements are put forward for the data transmission capacity,rate and quality of ground-air communication.Orthogonal frequency division multiplexing(OFDM),as a multicarrier modulation technology,is widely used because of its high spectral efficiency and strong anti-multipath interference ability.Based on OFDM system,this paper studies the downlink of ground-air communication,designs the ground-air OFDM communication system.The key technologies of peak-to-average power ratio reduction,timefrequency synchronization and channel estimation are studied and implemented on the FPGA.The specific work is as follows:Firstly,according to the characteristics of severe multipath and Doppler effects in the ground-air channel,the structure of the ground-air OFDM system and the frame structure of the signal are designed based on typical OFDM systems.The PAPR reduction algorithm studies and simulates limited-amplitude filter algorithm and signal scrambling algorithm.Timefrequency synchronization algorithm studies and simulates S&C and Minn time synchronization algorithm,as well as the frequency synchronization algorithm based on cyclic prefix and training sequence;channel estimation algorithm simulates least squares(LS),minimum mean square error channel estimation algorithm,interpolation and equalization algorithm.To solve the problem of time-frequency synchronization caused by multipath delay and Doppler frequency shift,the overall time-frequency synchronization is designed on the basis of basic research and simulation.Time-frequency synchronization uses the ZC sequence as the synchronization training sequence.First,coarse time synchronization and integer frequency offset estimation are performed through correlation operations,then performs fractional frequency offset estimation after frequency offset compensation,and fine time synchronization is finally performed.Aiming at the channel estimation problem,the ground-air channel is analyzed and modeled.Based on LS estimation and using the idea of Kalman estimation for reference,a joint algorithm is proposed to improve the channel estimation performance of grid pilot under large Doppler frequency shift.The joint algorithm first uses the LS algorithm for rough estimation,and then uses the Kalman estimation to further optimize the time selective fading to improve performance.Finally,the ground-air OFDM system is implemented on FPGA,and various sub-modules are designed,including scrambling/de-scrambling,interlacing/de-interlacing,QPSK maping/de-mapping,pilot insertion,IFFT,cyclic prefix insertion,etc.The accuracy of each sub-module is verified by functional simulation.The design and implementation of some key technologies,including PAPR reduction algorithm of transmission sequence,time-frequency synchronization algorithm based on training sequence and least squares channel estimation algorithm based on pilot.The feasibility and accuracy of corresponding modules are verified by functional simulation.