Theoretical And Experimental Researches On Influences Of Atmospheric Turbulence In The Satellite-to-ground Laser Communicaition Link

Comparing to the traditional microwave communication system, satellite laser communication systems have many potential advantages, such as higher data rate, low probability of intercept, no restriction for frequency use, less volume, less mass, less power consumption and so on. Recently, satellite laser communication has been concerned by many countries in the world and it has turned to be a hot topic in the field of satellite communication research. Space experiments have been performed in some countries or area, which have advanced technology on satellite laser communication, such as Japan, American and Europe. Intersatellite and ground-to-satellite laser communications have been realized in these experiments. Based on the achievements of the space experiments, these countries are preparing to promote this technology to real utility. In the near future, a new generation of the satellite communication network, which use laser as the carrier of the information, will appear.In ground-to-satellite laser communications, laser beam, as the information carrier, will propagates through the aerosphere of the earth. In this process, the refractive index fluctuation caused by atmospheric turbulence will influence the propagation of the laser beam, destroy the coherence of the optical field and result in a series of optical effects on the laser beam, such as intensity fluctuation, angle of arrival fluctuation, beam wander and spread, etc.Error performance is a importance parameter to indicate the communication performance of the system. The level of the error performance can be directly used to judge the quality of the communication system. Generally, error performance of a OOK system is indicated by bit error rate and error performances of PPM and DPIM systems are indicated by packet error rates.In a ground-to-satellite laser link, atmospheric turbulence introduced optical effects will influence the communication performance and increase the error probability of the information transmission. So with consideration of the turbulence effects, this dissertation is concerned with the researh on the communication performance of the ground-to-satellite laser link.First of all, the research status of ground-to-satellite laser communication is summarized. The characteristic of the atmospheric turbulence is described. Basic theory of laser propagation in turbulence and influeces of turbulence effects on the system are represented. The main configuration and principle of the ground-to-satellite laser communication system are introduced. Based on weak fluctuation theory and the beam-wander model, considering the combined effect of scintillation and beam wander, the probability density of the received intensity for uplink is derived. Bit error rate for uplink is analyzed, in comparison with the condition in which beam wander is not taken into account. Based on the bit error analysis, optimum divergence angle and transmitter beam radius for a communication system are researched.Packet error rates of OOK, DPIM and PPM modulation schemes are researched for ground-to-satellite laser links. Optimum threshold for DPIM system is deduced. Analysis process is based on threshold detection for OOK and DPIM, and optimal detection for PPM. Packet error rates of these three modulation schemes are compared for both uplink and downlink, with consideration of the turbulence effects in the ground-to-satellite laser link, and numerical results are given.An 11.8km urban laser link is established to perform the long term examination of the laser beam propagation characteristics through atmospheric turbulence. Far field distribution of Gaussian beam, scintillation index of the received intensity, probability density function, fade statistic, and high-frequency spectrum are researched based on the analysis of the experimental data. Besides, daily variation curve of scintillation index is given, compared with the variation of refractive-index structure parameter.An 11.16km bi-directional urban laser link is established by two satellite laser communication terminal, to perform the simulated experiment of the ground-to-satellite laser communication. Relations between scintillation index and the differences of temperature and humidity at two ends of the link are researched. Bit error rates are measured for two channels with different modulation rates and the results are given, with consideration of scintillation index. Relations between bit error rate and the differences of temperature and humidity at two ends of the link are also researched. In addition, long distance video transmission is performed and pictures of the transmitted video are presented.This dissertation can benefit the error perpormance analysis and modulation scheme choice for the ground-to-satellite laser communication system, and provide experimental foundation for the system design.