实际地面无线光通信中信道衰落特性的研究

With the advantages of both wireless communications and optical fiber communications, terrestrial wireless optical communication has become the key technology of broadband communications in developed countries. However,turbulence-induced scintillation will lead to channel fading, and eventually to diminishing the performance of wireless optical communication systems. In order to mitigate atmospheric scintillation, large aperture receiver and spatial diversity technology have been widely used in optical antenna design of wireless optical communication system. Large aperture receiver utilizes the effects of aperture averaging, while spatial diversity is closely related to channel correlation. However, the current theoretical formulas of aperture averaging factor and scintillation correlation coefficient cannot be applied to the actual terrestrial wireless optical communication link. In this respect, the project intends to use theoretical analysis, numerical simulation and experimental combined methods to study the actual terrestrial wireless optical communication channel fading characteristics, in particular aperture averaging factor and scintillation correlation coefficient for turbulent non-uniform path and small aperture receiver; and then provide a more appropriate basis for optical antenna design optimization and design examples. The study can not only deepen our understanding of the channel fading characteristics, improve the accuracy of analysis of the BER performance of communication systems, but also provide guidance to the design of optical antenna for high-performance terrestrial wireless optical communication products.

由于具有无线通信与光纤通信的优点，地面无线光通信已成为发达国家宽带通信的关键技术。但是，大气湍流引起的闪烁会导致信道衰落，进而降低无线光通信系统的性能。为了抑制大气闪烁，大孔径接收与空间分集技术被广泛应用在无线光通信系统的光学天线设计中。大孔径接收利用的是孔径平滑效应；空间分集与信道相关性密切相关。然而，目前的孔径平滑因子和闪烁相关系数的理论公式并不适用于实际地面无线光通信链路中。为此，本项目拟利用理论分析、数值仿真及实验相结合的方法，研究实际地面无线光通信中的信道衰落特性，具体为湍流非均匀路径中、小孔径情况下的孔径平滑因子和闪烁相关系数；并在此基础上给出更恰当的光学天线设计优化依据及设计方案实例。该研究不仅可以深化对信道衰落特性的认识，提高对通信系统误码性能分析的准确性，而且为高性能的地面无线光通信产品在光学天线设计上提供指导。

目前对自由空间光通信信道衰落特性的研究大都限定在理想的水平湍流均匀路径假设下，开展实际地面无线光通信链路中信道衰落特性的基础研究对在湍流介质中工作的光学系统的性能评估与收发天线的优化设计具有重要意义。.本项目获得的重要结果有：对于实际地面无线光通信链路，孔径平滑因子随孔径大小的变化符合指数下降关系模型。闪烁相关系数曲线存在一个负值区，负值区的宽度约为一个菲涅尔尺度。球面波的相关系数总是略大于平面波的相关系数。平面波与球面波闪烁强弱主要受菲涅尔数（表征孔径平滑效应）、Rytov方差（表征湍流强度）影响。.本项目的关键数据及科学意义有：在弱起伏下，相关系数变化曲线存在负值区，平面波孔径平滑效应比球面波更强，且当菲涅尔数>4.6（即D>2.15*菲涅尔尺度）时，对于湍流均匀路径，平面波闪烁将小于球面波，对于更小孔径，平面波闪烁大于球面波。在非弱起伏下，当Rytov方差> 4.8，平面波闪烁将小于球面波。这些结论对无线光通信收发天线子系统的优化设计具有重要参考价值。