Research On Modulation And Demodulation Technology Of Free Space Optical Communication

Optical communication modulation technology and networking technology are two important technologies for building FSO(Free Space Optical)communication.The current point-to-point optical communication system cannot complete the data transmission of multiple targets,and at the same time the laser transmitter and receiver need to be aligned with high precision,otherwise,the data transmission will fail,which greatly increases the difficulty of using FSO communication technology in life.Therefore,the research of optical communication modulation technology and networking technology is of great significance for the wide application of FSO communication technology.In this thesis,the basic principles of optical communication modulation techniques are used to build a point-to-multipoint FSO communication system,with the aim of providing a design solution for the implementation of FSO communication networking technology.This thesis first introduces the research background and development status of FSO communication technology explains the modulation and demodulation methods commonly used in FSO communication technology and introduces the beam scanning technology and optical communication networking technology.For signal demodulation,this thesis proposes the use of ADC(Analog to Digital Converter)to sample the signal and then further demodulate the signal through FPGA(Field Programmable Gate Array).The advantages of using ADCs in the demodulation process are also analyzed,providing an important theoretical basis for the implementation of FSO communication systems for high-quality communication.In order to realize point-to-multipoint FSO communication,this thesis designs an FSO communication relay to act as a relay station for data information in optical communication and builds an FSO communication system by combining it with a transmitter and a receiver.The transmitter uses an LD(Laser Diode)to emit laser signals and the receiver uses an APD(Avalanche Photodiode)to detect the laser signals in the channel.The system uses a combination of FPGA and MCU(Microcontroller Unit)as the controller,which enables laser transmission,signal reception,and data modulation and demodulation.In this thesis,different modulation methods are used to test the FSO communication system,including OOK(On-Off Keying)method,LPPM(Level-Pulse Position Modulation)method,PWM(Pulse Width Modulation)method,and DPIM(Dual-Pulse Interval Modulation)method.The data transmission experiments of the FSO communication system were carried out by sending two-dimensional color pictures,analyzing the signal waveforms of the transmitter,receiver,and repeater,collecting the BER of the data transmission under different modulation methods,and comparing the data transmission effect by ADC demodulation and direct demodulation by FPGA.The experimental results show that the system can achieve point-to-multipoint FSO communication,and all four modulation methods demonstrate higher communication quality using the ADC demodulation communication system,while the DPIM method has poorer communication quality with a fixed modulation period(1.2μs)for each bit of binary data,and the LPPM method has slightly better communication quality than the OOK method for direct FPGA demodulation.The LPPM method is slightly better than the OOK method for direct demodulation by the FPGA,and the PWM method can make the communication system more stable in a comprehensive comparison.The system built in this thesis successfully implements point-to-multipoint FSO communication and can deliver data information to different targets at different locations with high quality.The system provides a prototype system for the design of FSO communication networking technology and provides an important theoretical basis for the wide application of FSO communication technology in the future.The system built in this thesis successfully implements point-to-multipoint FSO communication and can deliver data information to different targets at different locations with high quality.The system provides a prototype system for the design of FSO communication networking technology and provides an important theoretical basis for the wide application of FSO communication technology in the future.