【作者】 陈纯毅；

【导师】 杨华民；

【作者基本信息】 长春理工大学 ， 物理电子学， 2009， 博士

【摘要】 随着信息技术的发展,传统的无线射频通信已经越来越满足不了人们对通信速率的要求。近年来,无线光通信成为无线通信领域的一个研究热点。当无线光通信链路经过大气信道时,大气会对激光信号的传输产生一系列影响,造成通信系统性能变坏。大气影响已成为无线光通信推广应用的一大障碍。本文针对无线光通信应用,研究大气对激光传输的影响机理及抑制技术,全文主要包括四个部分。第一部分主要从理论上研究大气对无线光通信中激光传输的影响机理。首先从大气吸收和散射理论出发,研究了大气消光造成的激光功率衰减问题,并对各种能见度下的大气信道衰减特性进行了计算和分析;在大气多次散射模型基础上,基于辐射传输理论推导出接收端的漫射强度分量和消弱强度分量计算模型;使用蒙特卡洛光线跟踪方法计算了不同光学厚度下的激光脉冲传输时间响应,给出了多次散射大气信道的传输带宽模型;系统地研究了大气湍流对激光传输的影响,给出了影响通信系统性能的五种大气湍流效应及对应的理论计算模型,通过仿真分析了不同条件下的大气湍流影响结果。第二部分设计了一个野外激光传输实验系统,并在三条不同路径上开展了激光传输实验。在实验测量数据的基础上,分析了不同条件下的光强起伏、到达角起伏和焦平面光斑面积的变化情况。通过分析发现,大孔径接收、多光束发射都能够有效地降低接收光强的衰落深度、衰落概率和衰落频率。第三部分建立了大气信道中的激光功率传输模型,分析了不同大气湍流条件下的光纤耦合效率,为通信系统的功率预算提供了模型支持;给出了不同近似条件下的APD信号探测模型,为分析通信系统的性能奠定了基础。第四部分针对大气对激光探测信号产生的三种最终影响,研究了解决方法。为了解决激光脉冲时间展宽导致的码间串扰问题,研究了均衡滤波和小波调制发射速率分集技术,通过仿真对系统性能进行了分析;针对大气湍流导致的探测信号随机起伏问题,研究了自适应最优阈值判决技术,给出了相应的系统结构和预测流程,通过仿真对比了最小均方误差阈值自适应和Kalman滤波阈值自适应的性能;对部分相干光传输信道的性能进行了分析,提出了一种新的大气湍流影响抑制技术——优化光源初始相干度技术,给出了光源初始相干度的优化准则及相应的自适应系统结构;针对接收分集模式,推导出多子孔径接收器的等效孔径平均因子,对比分析发现相同接收面积条件下多子孔径接收优于单个大孔径接收,给出了最大似然合并接收分集和等增益合并接收分集的误码率计算模型,通过仿真对比了二者的性能。本文的研究成果对大气无线光通信系统的设计具有重要的指导意义和参考价值。更多还原

【Abstract】 With the rapid development of the information technology, conventional radio frequency (RF) systems can not meet the increasing demand for the transmission rate. Recently, the optical wirelesss communication (OWC) becomes a research focus in the field of wirless communication. However, the atmosphere has a series of effects on the propagaton of a laser signal and can degrade the performance of OWC systems. The atmospheric effects have become an obstacle that limits the wide application of OWC technology.Focusing on the OWC application, this dissertation investigated the mechanism of atmospheric effects on the laser propagation and its mitigation technology. The dissertation consists of four important parts. The main contents are as follows:Firstly, the mechanism of atmospheric effects on the laser propagation in OWC was theoretically studied. One, based on the theory of atmospheric absorption and scattering, the problem of power attenuation caused by atmospheric extinction was researched, and the attenuation characteristics of the atmospheric channel with various visibility distances were caculated and analyzed. Two, based on the atmospheric multi-scattering model, the formula of the diffuse intensity and the reduced intensity at the receiver side were derived using radiative transfer theory. Three, the time-domain responses of a laser pulse through atmospheric channels with various optical thicknesses were computed using Monte Carlo ray tracing method, and then the transmission bandwidth model was proposed. Four, the turbulence-induced effects on the the laser propagation were systematically researched, and five kinds of turbulence effects and their mathematical models were discussed. Then, simulation results of turbulence effects under various conditions were analyzed.Secondly, a field experiment system of laser propagation was designed, and the experiments over three different paths were conducted. Based on the data collected during the experiments, the intensity fluctuation, the angle-of-arrival fluctuation and the spot size on the focus plane were analyzed. The results show that the large aperture receiving and the multi-beam transmission can reduce the depth, probability and frequency of the received intensity degradation.Thirdly, the power model of the laser propagation through the atmosphere was established. And, the fiber coupling efficiency under various conditions was computed. This model can give a support for the power budget of OWC systems. In addition, the detection models of avalanche photodiodes (APD) with different approximation conditions were given, which layed a good foundation for analyzing the performance of OWC systems.Fourthly, the solutions to the three kinds of problems in OWC, caused by atmospheric effects on a detected signal, were studied in detail. One, focusing on the intersymbol interference caused by the time-domain extending of a laser pulse, the equalization filering and the wavelet-modualtion-based transimission-rate diversity were researched and the system performance was evaluated and analyzed by simulation. Two, the adaptive optimal thresholding method was investigated to overcome the turbulence-induced fluctuations of the detected signal; the system architecheture and the adaptive prediction process were described in detail; a comparision of the adaptive thresholding performance between the least-mean-square method and Kalman method was presented based on the simulation results. Three, the performance analysis of a link with a partially coherent beam was presented, and then a novel technology, called the optimal initial coherence technology of a laser source, for reducing the turbulence-induced effects was proposed; the optimization criterion and the system architecheture were also discussed in detail. Four, considering the receiving diversity scheme, the formula of the equivalent aperture averaging factor for a multi-subaperture receiver was derived; it shows by analysis that with the same receiving area, the aperture averaging performance of a multi-subaperture receiver is better than a single-large-aperture receiver; then, the bit-error-rate models for both the maximum-likelihood combining and the equal-gain combining were suggested, and a performance comparision between the two combining scheme was also presented.The results obtained in this dissertation are very benefical and valuable for the design of atmospheric OWC systems.更多还原