Measurement of instantaneous-fading correlation in bidirectional optical channels through atmospheric turbulence
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摘要: 为证明双向光传输系统中大气湍流信道的互易性，提出一种测量瞬时接收信号衰落相关性的方法，并建立了分析测量数据的数学模型。在相距883 m的两栋高楼之间进行双向光传输测量实验，根据测得的光斑图像数据，对信道的瞬时衰落相关性进行验证，并分析了实际场景下归一化接收信号起伏方差对相关系数的影响。结果表明：两个相反方向的光信道瞬时衰落相关系数大部分均在0.85以上，最高可达0.95，证明了双向大气湍流光信道之间存在良好的互易性；且随着归一化接收信号起伏方差的增大，相关系数呈轻微下降的趋势。Abstract: To prove the reciprocity of an atmospheric turbulent channel in bidirectional optical transmission systems, we propose a method for measuring the correlation between the fading of instantaneously received signals and establish a mathematical model for analyzing the measurement data. Experiments of bidirectional optical transmission measurements were carried out between two tall buildings separated by 883 m. According to the measured speckle image data, we verified the instantaneous-fading correlation of the channel and analyzed the effect of the normalized received signal fluctuation variance on the correlation coefficient in practical scenarios. It was shown that most of the instantaneous-fading correlation coefficients of optical channels in the two counter-directions were above 0.85 and even up to 0.95, which proves that reciprocity can be well maintained for a bidirectional turbulent optical channel. With an increasing fluctuation variance of the normalized received optical signal, the correlation coefficient is slightly descending.
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图 1 双向光瞬时衰落特性相关测量原理图
Figure 1. Schematic diagram of correlation measurement of bidirectional optical instantaneous fading
图 2 双向大气湍流光信道瞬时衰落特性相关测量系统图
Figure 2. Measurement system diagram for instantaneous fading characteristic of bidirectional atmospheric turbulent optical channel
图 3 (a)、(b)和(c)分别为根据不同时间段的测量数据作图。从左至右依次为归一化接收信号随时间变化图、A端归一化接收信号频数分布直方图、B端归一化接收信号频数分布直方图，其中红色代表A端测量数据，绿色代表B端测量数据
Figure 3. (a), (b), and (c) are plotted according to measurement data at different time. From left to right, the normalized received signal changes with time, the normalized received signal frequency distribution histogram of A-end, and the normalized received signal frequency distribution histogram of B-end, where the red represents measurement data at A-end, and green represents the measurement data at B-end
图 5 信号瞬时衰落相关系数随归一化接收信号起伏方差的变化趋势图
Figure 5. Change of correlation coefficient of signal instantaneous fading with normalized received signal fluctuation variance
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