Research On Key Technologies Of Free Space Optical Communication In Different Weather Conditions

With the increasing shortage of spectrum resources in radio frequency communication,free space optical communication(FSO),as another scheme of wireless communication,has been widely concerned because of its strong ability of anti electromagnetic interference,no occupation of spectrum resources,large transmission capacity,large bandwidth and easy installation.Although FSO has great application potential,its extensive deployment still faces many obstacles,which are mainly reflected in the instability of atmospheric channel.The atmospheric channel is complex and changeable,and the laser is easily affected by various weather conditions during the transmission process.Therefore,it is of great significance to model and analyze the atmospheric channel under different weather conditions.In addition,applying multiplexing technology in FSO systems,further improving the system transmission rate and the ability to resist atmospheric effects,and realizing effective and reliable FSO systems is also one of the current research hotspots.In this thesis,the transmission characteristics of atmospheric channel under different weather conditions are analyzed,and the discrete wavelet transform-based orthogonal frequency division multiplexing(DWT-OFDM)technology is applied in FSO system to further improve the communication performance of FSO system.The main work of this thesis is as follows:1.The laser atmospheric attenuation models under different weather conditions are analyzed respectively,and the theoretical loss model of FSO link is given based on atmospheric attenuation and geometric attenuation.Based on this model,the path loss and bit error rate of FSO system under different weather conditions are analyzed.2.In order to reflect the random scattering characteristics of the atmospheric channel,Monte Carlo photon tracing method is also used to simulate the atmospheric channel.This method tracks the laser transmission path in the atmospheric channel.Through statistical analysis of the tracking results,the impulse response,path loss,and bit error rate characteristics of the atmospheric channel under different weather conditions are obtained.The results are compared with theoretical loss model.It is found that the Monte Carlo simulation model fully considers the energy of scattered photons,so its simulation of atmospheric channels is more in line with the actual situation.3.In order to further improve the transmission rate and anti atmospheric interference ability of FSO system,we propose to apply DWT-OFDM in FSO system.Due to the good orthogonality of wavelet transform,DWT-OFDM does not need to add cyclic prefix,and has strong ability to resist inter symbol and inter carrier interference.Through the simulation of FSO system based on DWT-OFDM,it is found that compared with FSO system based on traditional OFDM,the system has better bit error rate and peak to average power ratio performance.4.An experimental platform of FSO system based on DWT-OFDM is established,the feasibility of the system is verified,and a simple simulation of atmospheric channels under sunny,rainy and smog conditions is conducted.The experimental results show that under the three atmospheric channels simulated,the bit error rate performance of the FSO system based on DWT-OFDM is better than the system based on traditional OFDM.