| Medium-and long-range subsonic UAVs have broad application prospects in the military and civilian fields.In the of network formation flying application,subsonic UAVs have become the mainstream of future development by virtue of their long endurance and low fuel consumption.The physical layer communication link carries out the information transmission process in the form of digital signal processing and plays an important role in information interaction in the formation of subsonic UAVs.On a common hardware platform,the realization of a link communication system has become the core technology of UAV formation flying.In this thesis,by studying the performance index,main technology and theoretical basis of physical layer link and general physical layer link of UAV data chain,the link communication system based on π/4-DQPSK modulation and demodulation technology is built.The performance analysis is carried out through MATLAB software simulation,and the development and design of FPGA is completed to realize the signal processing circuit of physical layer.Aiming at the problems of fast movement speed,large Doppler frequency shift and fast channel change of the link terminal in the physical layer of subsonic UAV formation flight,this thesis carried out a research on the basic theory of physical layer link and selected the key technologies suitable for the system.The main contents are :1.The(2,1,7)convolutional coding in the signal transmitter and Viterbi decoding algorithm in the receiver for decoding are accepted.By introducing the principle of Viterbi algorithm in detail,and comparing hard decision and soft decision,the soft decision method is considered suitable for the system,and the solution to the overflow problem in the branch iterative operation of the decoding algorithm is given as well.2.The π/4-DQPSK modulation technology based on the phase selection realization method is adopted for the communication system for the formation of subsonic UAVs.The π/4-DQPSK modulation technology can overcome the Doppler frequency shift phenomenon and phase ambiguity within a certain range.3.In the design of the filter,square root raised cosine roll-off filter and comb integral filter are selected in this thesis.The square root raised cosine roll-off filter is of great significance for reducing inter-symbol interference.The comb-shaped integral filter can adjust the sampling rate within a unit symbol period while saving the computing unit in hardware resources.4.A closed-loop feedback method based on zero-crossing detection algorithm is used in the bit synchronization module in this thesis.The bit synchronizer is composed of an interpolation filter,a timing error detector,a loop filter and an interpolation control module.Compared with other error detection algorithms,the zero-crossing detection algorithm only requires two sampling points and does not require the participation of carrier synchronization,which is suitable for high-speed communication.The Simulink simulation platform in MATLAB is used to build the module and simulate the performance of the link system.The results show that under the condition of 3 d B signal-to-noise ratio,the system has a maximum bit error rate of 2.02%.Under the condition(>5d B),the system error rate is tending to 0.The FPGA hardware circuit design is completed and the joint simulation test of Simulink and Modelsim simulator is performed.The test results show that the communication link system implemented by FPGA can meet the communication performance requirements of the subsonic UAV link. |
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