Analysis And Compensation Of The Influence Caused By Timing Errors On Free-Space Optical Communication

In the free space optical communication(FSO)technology,laser beams are used as information carrier to realize long-distance and high-speed communication in the atmosphere.The FSO system has plentiful advantages,such as large capacity,high data rate,small volume and mass,strong ability to resist electromagnetic interference,good security and confidentiality,no need for radio frequency license,no need to occupy the fiber channel and so on.The fluctuation of optical intensity caused by atmospheric turbulence is one of the main factors that damage atmospheric optical communication system.Due to the random fluctuation of optical refractive index caused by atmospheric turbulence,wavefront distortion and amplitude fluctuation will occur.It becomes an especial case when light wave is transmitted through turbulent atmosphere,which seriously affects the quality of transmitted beam.As a result,the bit error rate of communication system increases,which damages the performance of communication system.In addition to the main factor of atmospheric turbulence,the system is also restricted by the need for other non-atmospheric factors.Pointing error is major factor.The random sloshing of the platform at both ends of the transmitter and receiver will result in the misalignment of the beam,which makes the optical signal reaching the receiver attenuate randomly,thus affecting the performance of the communication link.Besides the power loss due to the fluctuation of received light intensity and aiming error caused by atmospheric turbulence,timing error also degrades the performance.The timing error consists of deterministic errors and random errors.In a high-speed communication system,timing error will bring the drift of decision point,the crosstalk between early codes,which results in introducing more error codes in the process of decision.In view of these,this paper combines these three effects and studies the effect and compensation of timing errors on atmospheric laser communication system under the condition of alignment error.To begin with,the background significance and research status of laser communication are described.The model of composite atmospheric channel is introduced.Then the model of time error and its damages on the indexes of atmospheric laser communication system are studied.Taking pulse position modulation as an example,efforts are devoted on points consisting of the error performance,the estimation methods of time slot error and the likelihood ratio compensation algorithm of time error.Finally the effects of timing error on the atmospheric laser communication system are verified in the equivalent experiment.The main research work of this paper is as follows.1.The effects of atmosphere are introduced,consisting of absorption,attenuation,scattering and atmospheric turbulence.In order to simulate the influence of the integrated atmosphere on the receiving optical power,an atmospheric channel simulation method based on random values and Markov chain is proposed.Compared with the traditional Markov chain method,less number of states is required,while the data approximately consistent with the 8.9km field experimental results.2.The structure and model of the system are studied.The signal to interference plus noise ratio(SINR)is introduced as the starting point of the analysis.In the process of analyzing channel capacity,it's found that the closed expression is more than complex to simplify.As a result,the upper and lower bounds of capacity are analyzed and derived as well as and the asymptotic bounds of capacity at infinite power.In the process of interrupt performance research,the interrupt performance is defined by the SINR threshold,and an approximate closed expression is given.In the study of channel capacity and interrupt performance,the simulation values are in consistent with the theoretical ones.3.The detector jitter model of photon counting receiver based on Poisson counting process in pulse position modulation is studied.In PPM system,the moment estimation and maximum likelihood estimation are used to estimate the time slot error,and the variance of these three estimations are derived and simulated,which are also compared with the Cramer-Rao Bound.The likelihood ratio compensation algorithm of time error is implemented by using the estimated value of time gap error.4.The establishment and analysis of the atmospheric laser communication system's equivalent experiment with timing errors are studied.The method of generating signal with time error and the scheme of analyzing timing error are given.So as to accomplish the simulation experiment of atmospheric turbulence,an atmospheric scintillation playback instrument is developed by the laboratory.The simulation data could be derived from either the actual experimental results or the simulation results created by computers.According to the characteristics of atmospheric fading,a custom bit error rate tester is designed and implemented for atmospheric laser communication system.Finally,the experiments are constructed and corresponding results are analyzed.The research content of this paper could be mainly included as the analysis and compensation algorithm of atmospheric laser communication system with timing errors.The former can provide effective design input in the actual system design,and the latter can optimize the system performance,which has certain reference significance.