【作者】 李永亮；

【导师】 王建宇； 傅忠谦；

【作者基本信息】 中国科学技术大学 ， 电路与系统， 2011， 博士

【摘要】 激光通信相对于微波通信而言,具有带宽高、容量大、能耗低、安全性好的优点,而且激光的通信端机有更小的体积和更轻的质量,因此以激光作为信息载体的通信方式是星地通信的重要发展方向。美国、日本、欧洲等国家和地区在星地激光通信领域已经进行过多次实际星地链路实验,并且正在积极向实用化、商业化发展,建立全球覆盖的卫星激光通信网络是其发展趋势。作为卫星激光通信网络关键组成部分的星地激光链路受大气湍流和大气折射率的影响,且链路天顶角存在上限。大气湍流造成的光束漂移、扩展、光斑破碎等现象会使光信号的强度发生起伏,同时给通信两端的捕获、对准和跟踪带来困难,要对通信性能进行分析就必须掌握光束在湍流中的传输特性。大气折射率随海拔高度的变化,导致星地通信光束的传输路径呈曲线,使通信两端的对准出现偏差,对折射率造成的偏差进行研究,可以为通信端机的设计提供参考。星地激光通信链路天顶角的上限制约了卫星的覆盖能力,最大天顶角由大气状况和通信端机性能共同决定,是影响星座性能和成本的重要因素,在星座设计中应予以考虑。本文对大气湍流影响下光束传输特性的仿真方法进行了总结和改进,并将仿真结果与实际的光传输实验结果进行对比研究;而在星地链路的关键技术方面,研究了大气折射率对星地光链路的影响,并分析了星地激光通信下行链路最大天顶角的特性。主要研究工作和创新点如下:(1)总结了基于标准抛物型方程的光传播多层相位屏法,并阐述对湍流和光场进行离散化时网格参数的选取原则;通过对实际链路进行仿真分析,指出已有的方法在光束发散角超过100μrad时无法保证可靠性;针对光束发散角较大的情况,研究了最大空间采样率的特性,在此基础上提出多层相位屏法的改进方案,并以光束发散角为100urad和1.35mrad的水平链路为例进行验证,验证结果表明,接收端仿真光场的特性与理论预期值、实验值相符,证明了新方法的可靠性和实用性。(2)在光束发散角较大的情况下,随着链路长度的增加,存放光场数据的矩阵规模将越来越大,甚至会导致计算机内存不足。本文对仿真精度分别与网格参数、链路条件的关系进行研究,提出一种最优网格参数的求解方法,并拟合出最优网格参数关于链路长度和光束发散角的经验公式,在仿真过程中使用该方法确定网格参数,可以在保证可靠性的前提下减少内存需求量。(3)从星地激光通信的捕获、对准、跟踪(Acquisition, Pointing, Tracking, APT)系统的设计需求出发,研究了大气折射率对APT和超前瞄准的影响。以折射率关于波长、大气温度、大气压力的公式为基础,结合大气温度和气压的高度轮廓线,推导出折射率关于海拔高度和波长的模型,并通过分层近似推导了大气偏折的求解方法。定义了捕获过程、通信过程以及超前瞄准的大气偏折角偏差,并通过仿真分析给出了各种角偏差与链路天顶角的关系。(4)从星地激光通信系统星座设计的需求出发,针对星地激光通信下行链路,推导出在大气湍流影响下的最大天顶角模型。以采用OOK调制、APD探测器的链路为例,基于Gamma-Gamma光强起伏概率密度函数、Webb-Gaussian APD输出模型以及Hufnagel-Valley湍流强度轮廓线,分析了星载激光载荷发射功率、发射端瞄准偏差角和背景光功率等因素对最大天顶角的影响,并给出了下行链路最大天顶角与近地面湍流强度和误码率上限的关系,为星地激光通信系统的星座设计提供了参考。更多还原

【Abstract】 The laser communication has the advantages of higher bandwidth, higher capacity, lower energy consumption, and better security compared with the microwave communication. Moreover, its termination is much smaller and lighter, and thus using it as a vehicle of information became the tendency of the development of satellite-to-ground laser communication. There are a lot of countries and areas such as America, Japan, Europe which have already done many satellite-to-ground communication experiments, and they are working actively in order to make it more practical and commercial. In the future the establishment of a global coverage network of satellite laser communication is its development tendency.As an essential part of the satellite-to-ground laser communication, the laser links are influenced by atmospheric turbulence and atmospheric refractivity. The atmospheric turbulence makes the light beams drift, expand or even breaks the light spots, and thus changes the intensity of the light signals and brings a lot of difficulties to acquisition, pointing and tracking from both terminals of the communication. Therefore, we have to know the characteristics of the transmission of light beam in atmospheric turbulence in order to analyze the performance of communication. The atmospheric refractivity changes according to the altitude, which curves the transmission path of the light beams and leads to deviation in the pointing from both terminals. The research on the deviation caused by the atmospheric refractivity can supply the reference for the design of the communication termination. There is a "maximal zenith angle" in the satellite-to-ground communication links, which restricts the overlay area of each single satellite. The "maximal zenith angle" is determined by the atmospheric condition and the performance of the communication termination. It is the one of the most important factors to determine the performance and cost of satellite, and needs to be considered carefully in the satellite constellation design.This dissertation improved the simulation method of light beam transmission in atmospheric turbulence and made a conclusion by comparing the result of simulation with the actual result of the experimentation. At the same time, the dissertation discussed the influence of the atmospheric refractivity to the satellite-to-ground links, and analyzed the characteristics of the maximal zenith angel in the satellite-to-ground laser communication downlinks. The main research work and innovation of this dissertation are as follows:(1) By summarizing the multi-phase screen simulation method of light beam transmission in atmospheric turbulence base on standard parabolic equation, this dissertation elucidated how to choose grid parameters when discretized the turbulence and optical field. By the numerical analysis of the practical links, we found out that the existing methods could not guarantee the reliability when the divergence angle was larger than 100μrad. For such kind of condition, the characteristics of the maximum spatial sampling rate was discussed, and an improved method using multi-phase screen was developed with the examples of divergence angles of 100urad and 1.35mrad in horizontal links. The results showed that by using the improved method, the characteristics of the light field and the theoretical expectation as well as the experimental result were almost the same, which proved the reliability and practicability of the new method.(2) When the light beam divergence angle is relatively large, with the links grow longer, the simulation needs very large memory of the computer, sometimes even larger than the capability of the experimental computer. By analyzing respectively the relationship of accuracy of simulation with the value of grid parameters and the conditions of different links, this dissertation not only proposed an optimal grid parameters solution, but also fitted out an empirical formula of the relationship of the optimal grid parameters with the divergence angles and the lengths of the links, which helped a lot to determine the value of grid parameter and lowed down the memory requirement while guaranteeing the reliability during the procedures of simulation.(3) For the design of the acquisition, pointing, tracking (APT) system, the influence of atmospheric refractivity on APT and pointing in advance. A formula based on the the wavelength of light, air temperature and atmospheric pressure, considering the air temperature and atmospheric pressure as functions of the altitude, an atmospheric refractivity model affected by altitude and wavelength of the light was induced by segregate approximation of the atmospheric refractivity. By defining the pointing error angles in the acquisition and communication process, the relationship of the pointing error angles and the zenith angel of the links was analysed by simulation.(4) Focusing on the demand of the constellation design of the satellite-to-ground laser communication system, a model of the maximal zenith angle was induced. Considering the links which used on-off-keying (OOK) modulation and direct detection of avalanche photodiode detector (APD), the factors including the transmitted power of the satellite borne laser, the aiming error angle and the background light power were analyzed,which all affected the maximal zenith angle. Relationship among the maximal zenith angel, the intensity of atmosphere turbulent close to ground, and the bit error rate was obtained, which could be the reference for the constellation design of the satellite-to-ground laser communication system.更多还原