[show abstract][hide abstract] ABSTRACT: The error characteristics of a free-space optical (FSO) channel are signiﬁcantly different from the ﬁber based optical links and thus require a deep physical understanding of the propagation channel. In particular different fog conditions greatly inﬂuence the optical transmissions and thus a channel model is required to estimate the detrimental fog effects. In this paper we shall present the probabilistic model for radiation fog from the measured data over a 80 m FSO link installed at Graz, Austria. The fog events are classiﬁed into thick fog, moderate fog, light fog and general fog based on the international code of visibility range. We applied some probability distribution functions (PDFs) such as Kumaraswamy, Johnson SB and Logistic distribution, to the actual measured optical attenuations. The performance of each distribution is evaluated by Q-Q and P-P plots. It is found that Kumaraswamy distribution is the best ﬁt for general fog, while Logistic distribution is the optimum choice for thick fog. On the other hand, Johnson SB distribution best ﬁts the moderate and light fog related measured attenuation data. The difference in these probabilistic models and the resultant variation in the received signal strength under different fog types needs to be considered in designing an efﬁcient FSO system.
[show abstract][hide abstract] ABSTRACT: A technique of modeling the fog droplet size distributions using exponential distribution has been demonstrated. In order to compute the two parameters of the exponential distribution, two experimental measurements are required, which are the measurement of liquid water content (LWC) and the optical attenuations at visible wavelength. The two measurements are used to form two equations, which are treated as nonlinear equations using a numerical technique to obtain the two parameters of the exponential distribution. From the performance analysis of the newly computed parameters, it is found that the two parameters are quite efficient in modeling the optical attenuations in continental fog conditions.
Proceedings of the 7th International Symposium on Communication Systems Networks and Digital Signal Processing, CSNDSP 2010, University of Northumbria at Newcastle, UK, 21-23 July 2010; 01/2010
[show abstract][hide abstract] ABSTRACT: Free Space Optics (FSO) is now a well established access technology, better known for its robustness in transmitting large data volumes in an energy efficient manner. However the BER performance of a FSO ground-link is adversely affected by cloud coverage, harsh weather conditions, and atmospheric turbulence. Fog, clouds and dry snow play a detrimental role by attenuating optical energy transmitted in terrestrial free-space and thus decrease the link availability and reliability. We measured the time variation of received optical signal level during continental fog and dry snowfall over a link distance of 80 m. We perform a detailed analysis of the continental fog and dry snow attenuation results and further characterise them by presenting some useful attenuation statistics and also showing their comparison with the corresponding measured density values collected by a particle sensor device. We propose also an empirical relationship between temperature, relative humidity and optical attenuation values for the continental fog case based on standard curve fitting technique.