Research On Bit Error Rate Characteristics Of OOK Modulation And PPM Modulation In Wireless Optical Communication System

With the continuous development of Free Space Optical Communication(FSO)technology,its advantages such as no need for frequency application,large transmission capacity,wide frequency band and easy installation make it more and more widely used in secure Communication and emergency support Communication.Although FSO has a broad application prospect,the instability of atmospheric channel will restrict the wide application of FSO,which is mainly manifested in that the laser is easily affected by various meteorological conditions in the transmission process.Therefore,it is of great significance to analyze the characteristics of atmospheric channel under different weather conditions and study the performance of different modulation modes in atmospheric channel transmission.Therefore,based on theoretical analysis and experimental measurement,this paper first analyzes the atmospheric channel characteristics under different weather,Secondly,the OOK and PPM modulation and demodulation programs based on FPGA were written.Finally,an experimental measurement system for the bit error rate of laser communication in the field is built to measure the bit error rate of different modulation modes in complex weather.The main work of this paper is as follows:(1)The influence of atmospheric attenuation caused by rain,snow and fog on the bit error rate of laser communication transmission is analyzed.At the same time,the bit error rate of OOK and PPM system is simulated and analyzed.The analysis results show that the bit error rate caused by rain,snow and fog on OOK and PPM modulation increases successively,and the bit error rate caused by rain is the smallest,while the bit error rate caused by fog is the largest.The BER of OOK and PPM are different with different rain,snow and fog models.(2)The OOK modulation and demodulation program and PPM modulation and demodulation program based on FPGA are written.The specific realization process of OOK modulation and demodulation bit synchronization is introduced,and the principle and realization process of PPM modulation and demodulation frame synchronization are mainly introduced.(3)Build an experimental system with a link length of 420m,and measure the bit error rate of different modulated modes in different weather.The bit error rate(BER)produced by the weather of sunny,cloudy,rain,snow and fog is analyzed,and it is found that there are great differences in BER under different weather conditions.Bit error rates of sunny and cloudy are lower,while those of rain and snow are higher,while those of fog are the highest.Bit error rates will also change with the change of rainfall,snow amount and fog visibility,which is mainly shown as low rainfall,low snow amount and high visibility.Bit error rates are lower,and vice versa.(4)The experimental results were compared with the theoretical model in rain and snow weather.After comparative analysis,the bit error rate in rainy days is closer to the bit error rate of Joss raindrop spectrum distribution.The bit error rate of snow model is closer to that of dry snow model.