Research On Wireless Digital Communication System Based On Laser Carrier

The open atmospheric laser communication system has many advantages,such as no cochannel interference,high bandwidth,and convenient use.It has the characteristics that traditional wireless microwave communication does not have,and has been widely studied by scholars.In order to make up for the shortcomings of microwave communication in open space and increase the reliability of open atmosphere laser communication,this paper refers to a large number of domestic and foreign literatures,and studies the modulation and demodulation and coding and decoding techniques suitable for laser communication.Finally,the hardware platform was built and the system was verified.In terms of modulation and demodulation technology,this paper focuses on PPM modulation and PPM soft demodulation algorithm,and simulates PPM modulation.In the simulation,the performance of PPM modulation and OOK modulation are compared,PPM modulation and demodulation has better performance has been verified.In terms of coding and decoding,this thesis focuses on the LDPC coding technology in channel coding and decoding technology,and studies the construction,coding method and soft decision decoding technology of LDPC code check matrix.In order to further improve the transmission performance of laser communication,the LDPC coding code is fused with PPM modulation and demodulation.The simulation results show that the fusion of LDPC and PPM can effectively resist channel noise and significantly reduce the bit error rate.Based on LDPC coding and decoding technology and PPM modulation and demodulation technology,this thesis builds a physical platform for open atmospheric laser communication system and demonstrates the hardware scheme.The laser and laser driver at the transmitting end,the photoelectric converter at the receiving end,the transimpedance amplifier,the inverse proportional adder,the automatic gain control circuit,and the AD sampling circuit are introduced in detail,and the schematic diagrams of the corresponding devices are given,and some circuits are also performed.The rationality of the circuit design has been verified by simulation.Finally,the PCB is drawn according to the schematic diagram,and the physical production of the demodulation board and the main control board at the receiving end is completed,and the physical platform is verified.Both the transmitter and receiver in the physical platform use the FPGA as the core control circuit.Based on the physical platform of the communication system,the Verilog language is used to write programs to verify the physical platform and algorithms.For the transmitting end,the sending function is completed by the control module,the LDPC encoding module,and the modulation module.For the receiving end,the receiving function of the receiving end is completed by the AD sampling module,the frame header detecting module,the PPM demodulating module,and the LDPC decoding module.Finally,the system is verified by oscilloscope and SignalTapII tool.The verification results show that the transmitter can correctly complete LDPC coding and PPM modulation,and the receiver can correctly perform PPM demodulation and LDPC decoding.