Coherent OFDM Multihop System For Free Space Optical Communication In Strong Turbulence Environment

FSO is generally considered the best way to solve the "last mile" problem.It has the advantages of high transmission rate,low power consumption,bandwidth and convenient deployment.Therefore,it is considered that its application potential is huge by the researchers at home and abroad.As the requirement of people on the quality of communication is getting higher and higher,the development of relay technology is gradually being known to solve the problem that the coverage area of the transmitter is not broad enough.In actual application,in order to adapt to different communication environments,relay transmission technology is generally divided into serial trunk and parallel trunk according to the structure.Multi-hop transmission is a type of relay transmission,which means that a number of relay nodes are added to the serial communication link.Moreover,Multi-hop system can effectively increase the transmitter coverage and improve system deployment diversity and flexibility.OFDM has the advantages of fast transmission rate,high-frequency band utilization,and strong anti-frequency selective fading,and has been used in free-space optical communication systems.In addition,the coherent OFDM detection method has the advantages of receiving sensitivity,high anti-dispersion and strong dispersion capability of the polarizing film through the coherent processing of the local oscillation light and the signal light.Therefore,at present,it is a better performance detection method.Due to the presence of various particles in the atmosphere,light signals are scattered or absorbed as they propagate.The extent of these effects always depends on lots of factors,such as the size and density of the particles.In order to better study the communication system performance,different atmospheric environments are usually analyzed by different mathematical models.The turbulence intensity is generally divided into weak turbulence and moderate turbulence.The lognormal distribution is often used to describe the atmospheric channel model in weak turbulence environment.Gamma-Gamma distribution model is commonly used to describe strong turbulence environment through two parameters.While the negative exponential distribution model is often used in atmospheric channels with very strong turbulence and long communication links.K distribution is often used to describe theatmospheric channel model in the case of strong turbulence and moderate communication distance.Based on the above conditions,the contents of this report are as follows:In free-space optical communications,coherent OFDM and multi-hop transmission techniques are adopted.This paper presents a theoretical model of coherent OFDM serial relay system under negative exponential distribution atmospheric channel model,in the case of very strong turbulent atmosphere and long-distance transmission.Moreover,the report also deduces the joint attenuation model,the outage probability and the error symbol Rate MeijerG closed expression.In addition,simulation independently analyzes the influences of the number of subcarriers,mapping order and other factors on the interrupt probability and error symbol rate of impact,respectively in the single-hop,double-hop and triple-hop time.This study provides a practical approach in the strong turbulent atmosphere and the long-distance transmission.In the case of moderate-strong turbulence and moderate transmission distance,this report proposes a theoretical model of coherent OFDM multi-hop system under K-distributed atmospheric channel model,as well as deuces the joint attenuation model,the outage probability and the error symbol Rate of MeijerG closed expression in this system.The simulation analyzes the contrast of the single-hop and three-hop and the comparison of interrupt probability between three-hop and four-hop.Moreover,this paper separately analyzes the performance of error symbol rate in single-hop,triple-hop and quad-hop,which can obtain the mapping order,the number of subcarriers,the number of relay nodes and other factors on the interrupt probability and error symbol rate.This system model can directly describe the radial correlation in the process of atmospheric scattering and establish an effective theoretical model for the practical application in a strong turbulence communication system.Among the multi-hop transmissions,the forwarding mode adopts a better performance of DF.In the joint attenuation channel,the atmospheric effect,the path loss,and the aiming error are considered in combination.And,the mapping method adopts the QAM method.