Research On CO-OFDM In Free Space Optical Communication System

Free space optical(FSO)communication belongs to wireless communication technology,which employs optical beam as carriers,mainly used in outdoor circumstance.Compared with the conventional radio frequency(RF)communication technology,it possesses many advantages,such as abundant resource of spectrum,better resistance to the electromagnetic interference,better security,larger channel capacity,no license of accessibility of wideband,easy equipment and so on.It has attracted lots of attentions of specialists and scholars worldwide since it was born in 1960s,especially from the new century.Over these years,the growing number of communication users requires the huge bandwidth urgently.Because of this case,it is becoming more and more difficult for the conventional RF communication technology to satisfy our demand.Therefore,FSO communication technology becomes an important role in the next-generation(5G)wireless communications.Despite of a great number of advantages,FSO communication technology possesses many disadvantages as well.Because the propagation media of the laser is the atmosphere,which makes the optical beam signal easily be interfered by the external conditions and makes the system performance became worse to a certain degree,when FSO communication system works in the near-earth aerosphere.FSO communication system can be classified into intensity modulation and direct detection(IM/DD)system and coherent optical communication system according to their modulations.In this paper,we adopt the coherent optical communication system since it has many advantages,such as higher sensitivity,better resistance to the bias light and noise,larger communication distance and so on,especially with the combination of coherent orthogonal frequency division multiplexing(CO-OFDM),which makes a larger communication distance and higher data rates.Despite of that,there exists inter-carrier interference(ICI)in CO-OFDM systems due to the laser phase noise.In order to alleviate the phenomenon mentioned above and improve the system performance,the main research of the thesis mainly focuses on the following several parts.Firstly,because of the vulnerability to the atmospheric factors of the FSO system when it works in near-earth aerosphere,the multiple input and multiple output(MIMO)technology is introduced to CO-OFDM systems and we design the MIMO based CO-OFDM FSO system.It can make full use of the diversity gain and thus mitigate the system performance degradation caused by the path loss,atmospheric turbulence and pointing error.Secondly,the channel model of joint effect of atmospheric turbulence,path loss and pointing error is established.The analysis of the system bit error rate and outage probability under this channel model and their closed form expressions are obtained.Matlab numerical results of the system performance with different number of transmitters and receivers and different signal combining schemes are compared as well.Finally,CO-OFDM scheme based on Polar coding and decoding theories has been designed.The corresponding signal processing procedure of the Polar code based CO-OFDM system and also the performance analysis and simulations are given.It guarantees the high data rates,mitigates the influence of ICI to the system performance,and makes better resistance to the linear and nonlinear impairments,which makes CO-OFDM systems more suitable to the transmission with larger distance and higher data rates.Besides,it has low complexity and better efficiency.The simulation results under different coding length and different number of OFDM subcarriers as well as their influence to the system performance are obtained.