Research On PPM Modulation And Demodulation Technology Of Atmospheric Laser Communication System

Atmospheric laser communication uses laser to transmit information,which has the advantages of high bandwidth,large capacity,low cost,and strong anti-electromagnetic interference ability.It is one of the research hotspots in the field of optical communication.However,the actual atmospheric channel has atmospheric attenuation and atmospheric turbulence effects,which will cause strong interference to signal transmission.The most basic method to combat turbulence interference is to use modulation technology with high energy efficiency ratio.As a modulation method with high energy efficiency ratio,PPM shows significant advantages in atmospheric laser communication,but it is prone to problems such as symbol crosstalk and clock desynchronization during transmission.Therefore,research on stable and efficient PPM modulation and demodulation technology is of great significance to its application in atmospheric laser communication systems.This article has carried out research on PPM modulation and demodulation technology,and verified the designed algorithm by building an actual laser communication system.The main research contents are as follows:1.In the theoretical part,first analyze and compare the modulation principle,bandwidth requirement and power efficiency between OOK and PPM,then study the characteristics of the atmospheric channel and its influence on PPM signal transmission,finally derive the bit error rate formula of the PPM signal in the weakly turbulent channel.Prove the superiority of PPM modulation applied in the atmospheric laser communication system.2.Researched and designed the key algorithms of PPM modulation and demodulation,including PPM modulation,bit synchronization,symbol synchronization and PPM decoding,developed a PPM modulation and demodulation module based on FPGA,and verified the effectiveness and reliability of related algorithms through simulation and experiments.3.A PPM modulation and demodulation atmospheric laser communication system is designed.The system consists of three parts:transmitter,optical antenna,and receiver.By analyzing the functional characteristics and working principles of the components required by each part,the selection criteria were determined,which laid the hardware foundation for the subsequent algorithm verification experiments.Complete the comparative experiment of PPM and OOK modulation.Experimental results show that when the received optical power is large enough,both PPM modulation and OOK modulation systems can achieve zero error transmission.As the received optical power decreases,the bit error rate of the communication system gradually rises.Under the condition that the transmission rate of 20Mbps and the bit error rate is not higher than 10-6,compared with OOK modulation,the receiver sensitivity of 4-PPM modulation is improved by 6dB.The experimental results are consistent with theoretical research,which proves the reliability of the PPM modulation and demodulation algorithm designed in this paper,and provides experimental data support for related research.