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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 4 — Feb. 14, 2011
  • pp: 3379–3386
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The wavelength dependent model of extinction in fog and haze for free space optical communication

Martin Grabner and Vaclav Kvicera  »View Author Affiliations


Optics Express, Vol. 19, Issue 4, pp. 3379-3386 (2011)
http://dx.doi.org/10.1364/OE.19.003379


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Abstract

The wavelength dependence of the extinction coefficient in fog and haze is investigated using Mie single scattering theory. It is shown that the effective radius of drop size distribution determines the slope of the log-log dependence of the extinction on wavelengths in the interval between 0.2 and 2 microns. The relation between the atmospheric visibility and the effective radius is derived from the empirical relationship of liquid water content and extinction. Based on these results, the model of the relationship between visibility and the extinction coefficient with different effective radii for fog and for haze conditions is proposed.

© 2011 OSA

1. Introduction

Free-space optical (FSO) communication systems are influenced by the propagation impairments due to adverse weather conditions. Particularly, the scattering on the atmospheric aerosols/hydrometeors may cause significant extinction of an optical signal resulting in system outage.

It was pointed out by several authors, e. g., [6

6. I. I. Kim, B. McArthur, and E. J. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. SPIE 4214, 26–37 (2001). [CrossRef]

], that it is mainly the different radius of scatterers that is responsible for different wavelength dependence of light extinction in dense fogs in comparison with the extinction in haze conditions. Nevertheless no attempts to date are known to authors that would reveal this difference explicitly in the form of relatively simple model equations. Therefore the wavelength dependence is analyzed using Mie scattering theory and a new model that takes the radius of scatterers into account is proposed for a limited range of wavelengths.

1.1 Visibility and Kruse model

The atmospheric visibility is defined as a distance where the 550 nm collimated light beam is attenuated to a fraction (5% or 2%) of an original power. Several models relate an extinction coefficient γ (1/km) and visibility V (km). The specific optical attenuation is obtained from the extinction coefficient by A (dB/km) = 10 log(e) γ.

The model derived in [4

4. P. W. Kruse, L. D. McGlauchlin, and R. B. McQuistan, Elements of Infrared Technology: Generation, Transmission and Detection (Jonh Wiley & Sons, New York, 1962), Chap. 5.

] has served a long time to the infrared communication community [8

8. P. Corrigan, R. Martini, E. A. Whittaker, and C. Bethea, “Quantum cascade lasers and the Kruse model in free space optical communication,” Opt. Express 17(6), 4355–4359 (2009). [CrossRef] [PubMed]

] as the only model providing a wavelength dependent relation between the atmospheric visibility and the extinction coefficient. For visibilities lower than 6 km it reads as follows:
γ(λ)=3.91V(λ0.55)q  where: q=0.585V1/3  for  V<6  km.
(1)
Here γ (1/km) is the extinction coefficient, λ (µm) is the wavelength and V (km) is the visibility. Beside the wavelength dependence term (λ/0.55)-q, the basic relation between γ and V is expressed via an inverse proportional term 3.91/V. This can be obtained directly from the 2% definition of visibility:
II0=0.02=eγ(0.55μm)Vγ(0.55μm)=ln0.02V
(2)
where I/I0 is the fraction of the attenuated power intensity to the original intensity. It is clear from the context of the section 5.6 in [4

4. P. W. Kruse, L. D. McGlauchlin, and R. B. McQuistan, Elements of Infrared Technology: Generation, Transmission and Detection (Jonh Wiley & Sons, New York, 1962), Chap. 5.

] where the model was introduced that the model is assumed to apply for wavelengths at least in the range from 0.55 to 6 um. However, as pointed out by several authors [6

6. I. I. Kim, B. McArthur, and E. J. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. SPIE 4214, 26–37 (2001). [CrossRef]

,9

9. K. W. Fischer, M. R. Witiw, and E. Eisenberg, “Optical attenuation in fog at a wavelength of 1.55 micrometers,” Atmos. Res. 87(3-4), 252–258 (2008). [CrossRef]

], the wavelength dependence of the Kruse model was derived from data gathered during haze conditions and probably not during fog. Therefore, for the lowest visibilities (V < 0.5 km), the formula (1) is less reliable.

1.2 Other models

Kim et al. argued in [6

6. I. I. Kim, B. McArthur, and E. J. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. SPIE 4214, 26–37 (2001). [CrossRef]

] that the recent empirical data suggests no wavelength dependence of optical attenuation during low visibility conditions at least for wavelengths in the range from 0.55 to 1.55 µm. The modification of the Kruse model was proposed such that a wavelength dependence is removed altogether (q = 0) at the lowest visibilities (V < 0.5 km).

Al Naboulsi et al. introduced in [7

7. M. Al Naboulsi, H. Sizun, and F. de Fornel, “Fog attenuation prediction for optical and infrared waves,” Opt. Eng. 43(2), 319–329 (2004). [CrossRef]

] a slightly different approach to fog attenuation modeling. They considered two specific types of fog – advection and convection (also called radiation) fog. Then using FASCOD software, they derived interpolation formulas applicable at the wavelengths from 0.69 µm to 1.55 µm for visibilities between 0.05 and 1 km. Al Naboulsi models were compared with experimental data measured on the site “La Turbie” at Nice, France where the transmission of light with the wavelengths 0.85 and 0.95 µm through dense fog was investigated.

Ferdinandov et al. recently introduced in [5

5. E. Ferdinandov, K. Dimitrov, A. Dandarov, and I. Bakalski, “A general model of the atmospheric scattering in the wavelength interval 300 – 1100 nm,” Radioengineering 18, 517–521 (2009).

] a model that is claimed to be valid for the visibilities 0.1 < V(km) < 50 and in the range of wavelengths between 0.3 and 1.1 µm. It was derived by fitting the experimental data published in literature. It seems however that the used experimental data was not obtained during dense fog conditions.

Nebuloni analyzed in [10

10. R. Nebuloni, “Empirical relationships between extinction coefficient and visibility in fog,” Appl. Opt. 44(18), 3795–3804 (2005). [CrossRef] [PubMed]

] extinction data available in three infrared regions to obtain the coefficients of a power law model γ = aVb fitting best the measured data at 1.2, 3.7 and 10.6 microns.

The general trend of the wavelength dependence is different for different models. The Kruse and Ferdinandov models describe decreasing trend for all visibilities, the Al Naboulsi models describe increasing attenuation with wavelength for visibilities up to 1 km and the Kim model gives no wavelength dependence for the lowest visibilities V < 0.5 km and it is similar to the Kruse model for higher visibilities V > 1 km.

2. Scattering theory: extinction vs. effective radius

An extinction coefficient γ of the hydrometeor can be calculated (within the first order multiple scattering approximation) as:
γ=0πr2n(r)Qext  dr
(3)
where r is the particle radius, n(r) is the particle (drop) size distribution and Qext is the extinction efficiency factor [11

11. H. C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, New York, 1981).

]. A gamma type distribution is widely used for representing the microphysical properties of clouds and fogs [12

12. D. Deirmendjian, Electromagnetic Scattering on Spherical Polydispersions, (American Elsevier Pub. Co., New York, 1969), Chap. 4.

]:
n(r)=arαebr
(4)
where a, b and α are adjustable parameters of the distribution. It is appropriate to interpret these model parameters in terms of physical quantities. A particularly fruitful approach is to define an effective radius re [11

11. H. C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, New York, 1981).

] of the distribution that well characterizes the whole distribution from the point of view of an integrated geometrical cross section of all particles. The effective radius is proportional to a ratio of the total volume and the total geometrical cross section of particles in the distribution:
re=0r3n(r) dr0r2n(r) dr=34total volumetotal geom . cross section.
(5)
Substituting (4) into (5), one can find for the effective radius thatre=(α+3)/b.

The integral in (3) has been evaluated numerically (by an adaptive Simpson rule with relative accuracy better than 10−3) with the extinction efficiency factor Qext calculated using full Mie formulae [11

11. H. C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, New York, 1981).

]. The complex refractive index of water was taken from [13

13. D. Segelstein, The Complex Refractive Index of Water, (University of Missouri, Kansas City, 1981).

]. Figure 1
Fig. 1 Extinction coefficient for different effective radii, LWC = 1 g/m3, α = 5
shows the wavelength dependence of the extinction coefficient for fixed LWC, α and different values of re. In the short wavelength limit, one can check in Fig. 1 that the extinction coefficient is given by γ(km−1) = 1500/re(µm) corresponding to (6) with LWC = ρ = 1. In the region where applicable (see Fig. 1), a simple relation between the extinction coefficient, the liquid water content and the effective radius is so obtained:
γ=1500LWCre
(7)
where γ (1/km), LWC (g/m3) and re (µm). The region of sufficient accuracy of (7) with a relative error less than 10% is determined by the condition: λ < re/5. In order to analyze the extinction dependence on visibility, the relative extinction coefficient measured relatively to the extinction coefficient at the 0.55 μm wavelength is of interest. The relative extinction is independent of LWC provided re and α are fixed. It is also clear from Fig. 1 that there is no generally monotonic wavelength dependence except in some interval around 0.55 μm. It seems therefore that the exponent q in (1) may serve as some approximation only in the interval 0.2 μm < λ < 2 μm that is focused on in the following.

The wavelength dependence of the relative extinction coefficient in the mentioned interval is depicted in Fig. 2
Fig. 2 Relative extinction and the linear approximation of its log-log wavelength dependence, α = 5
. The whole interval was divided into two subintervals: λ < 0.55 μm and λ ≥ 0.55 μm and a linear approximation of the log-log wavelength dependence was found in both of them, see the straight lines in Fig. 2.

The slope of the log-log dependence s (the exponent in (10) below) as a function of the effective radius was obtained. The s(re) function can be approximated by the following expression:
s=2(tanh(p1(w+p4))1)+p2exp(p3(w+p5)2)
(8)
w=log10re
(9)
where re (μm) and the parameters pi for the two wavelength subintervals are summarized in the Table 1

Table 1. Parameters of the model (8) for two wavelength subintervals

table-icon
View This Table
.

Figure 3
Fig. 3 The exponent of wavelength dependence γ ~λs, α = 5
shows the exponent s obtained for different re (points) and the interpolating functions (8) (lines) with the parameter values stated in Table 1. The function (8) was chosen so that the limit of s as re → 0 is s = −4 which corresponds to the Rayleigh scattering. The limit of s as re becomes large is s = 0 which corresponds to the geometrical optics approximation. Similarly as in (1), the wavelength dependence of the extinction coefficient can be expressed as:
γ(λ)=γ(0.55μm)(λ0.55)s
(10)
where γ (1/km), λ (μm). The extinction coefficient at the wavelength of 0.55 μm, γ(0.55μm), is to be obtained from the visibility using its definition as in (2).

3. Relation of the effective radius and atmospheric visibility

The model expressed by Eqs. (8)-(10) is based on the underlying physics as it takes the dependence on the effective radius of the particular hydrometeor into account. Both the visibility and the effective radius are the inputs of the model. Nevertheless it may be difficult in practice to get relevant information about the size of scattering particles. It is therefore desirable to estimate the relationship between the effective radius and visibility in order to express the extinction coefficient as a function of visibility only.

For that purpose, let us consider the following facts:
  • a) The relation between the extinction coefficient γ and the liquid water content LWC is often (e.g [14

    14. R. G. Eldridge, “Haze and fog aerosol distributions,” J. Atmos. Sci. 23(5), 605–613 (1966). [CrossRef]

    ].) empirically modeled by a power-law equation:
    γ=γ0(LWCLWC0)c
    (11)
where γ 0 stands for the extinction coefficient corresponding to the particular liquid water content LWC 0. The value of the exponent c is usually between 0.5 and 1, but it is wavelength dependent generally.
  • b) The extinction coefficient γ is linearly proportional to LWC when both the effective radius re and the shape parameter α are fixed. It means c = 1 in such a case.
  • c) For a fixed LWC (and fixed α) and λ = 0.55 μm, the effective radius re is inversely proportional to the extinction coefficient provided re > re min where re min is about 0.5 μm.
  • d) It is from the definition of visibility that γ/γ 0 = V 0/V for the wavelength λ = 0.55 μm where V 0 stands for the visibility corresponding to γ 0 and LWC 0.
Now, let us keep α constant, since it seems to be the least sensitive parameter in our context. Then it is evident from the points a) and b) above that if the empirical exponent c ≠ 1 then re has to vary with LWC. Consider the effective radius re 0 corresponding to the LWC 0. The further step is to find the values of effective radius re corresponding to the values of LWC different from LWC 0. Figure 4
Fig. 4 The scheme of the relation between the effective radius and the liquid water content
shows that (for example) for c = 2/3 and LWC = 0.01 g/m3, the effective radius re < re 0 (see the point c) above) and it depends on the ratio of extinction coefficients γ and γ 1.

So using a), b) and c), one finds the following equation (see parameters in Fig. 4):
rere0=γ1γ=(LWCLWC0)1c  where  γ1=γ0(LWCLWC0)1
(12)
from which one has:
re=re0(LWCLWC0)1c=re0((γγ0)1/c)1c=re0(γγ0)(1/c)1=re0(V0V)(1/c)1.
(13)
In the last equality in (13), the point d) was applied, because it was assumed that the empirical exponent c was obtained for the wavelength λ = 0.55 μm. The relation (13) gives the possibility to deduce the effective radius from the visibility if the particular re 0, V 0 and c are known from experiments. For example, consider typical values c = 2/3 (from [14

14. R. G. Eldridge, “Haze and fog aerosol distributions,” J. Atmos. Sci. 23(5), 605–613 (1966). [CrossRef]

,15

15. P. Chýlek, “Extinction and liquid water content of fogs and clouds,” J. Atmos. Sci. 35, 296–300 (1978).

]), re 0 = 10 μm ([16

16. M. Grabner, and V. Kvicera, “On the relation between atmospheric visibility and the drop size distribution of fog for FSO link planning,” in Proceedings of the 35th European Conference on Optical Communication (VDE VERLAG GMBH, Vienna, 2009), pp. 1–2.

]), V 0 = 0.05 km. The Eq. (13) then reads:
re=10(0.05V)1/2
(14)
and it can be substituted to (9) to calculate the exponent s as a function of visibility V.

4. Model summary

The new proposed model is applied as follows. First, the effective radius re is estimated from visibility using (13). Second, the exponent of wavelength dependence s is calculated using (8) and (9). Finally the extinction coefficient γ is obtained using (10).

5. Comparison with measured data

Figure 5
Fig. 5 Liquid water content, LWC, particulate surface area, PSA, and effective radius of fog measured during a fog event observed on 20-21 January 2009 in Prague, the Czech Republic.
shows the example of a fog event observed in Prague, the Czech Republic. The two parameters of fog are measured [16

16. M. Grabner, and V. Kvicera, “On the relation between atmospheric visibility and the drop size distribution of fog for FSO link planning,” in Proceedings of the 35th European Conference on Optical Communication (VDE VERLAG GMBH, Vienna, 2009), pp. 1–2.

]: liquid water content LWC (g/m3) and particulate surface area PSA (cm2/m3). The effective radius, re (μm) is then calculated using its definition (5) as: re = 30000∙LWC/PSA. It can be seen in Fig. 5 that the re fluctuates around about 10 microns during the developed fog phase. It is expected, however, that the typical effective radius depends on local climatic conditions [17

17. M. S. Awan, R. Nebuloni, C. Capsoni, L. Csurgai-Horváth, S. S. Muhammad, F. Nadeem, M. S. Khan, and E. Leitgeb, “Prediction of drop size distribution parameters for optical wireless communications through moderate continental fog,” Int. J. Satell. Commun. Network. 29(1), 97–116 (2011). [CrossRef]

] and so the Eq. (14) is to be modified accordingly.

Figure 6
Fig. 6 Specific optical attenuation vs visibility predicted by different models and measured data [18], the wavelength 1.55 μm shifted downwards (multiplied by 1/10) for better clarity.
shows a comparison of different models applied at two typical FSO wavelengths. The measured (two-year) data [18

18. M. Grabner, and V. Kvicera, “Fog attenuation dependence on atmospheric visibility at two wavelengths for FSO link planning,” in Proceedings of Loughborough Antennas & Propagation Conference (Loughborough University, Loughborough, 2010), pp. 193–196.

] obtained on two FSO paths located in Prague are also shown in Fig. 6 as an empirical reference. A reasonable agreement of the proposed model with the measurement is achieved at least for visibilities lower than 2 km.

6. Conclusion

The wavelength dependent model of the relation between atmospheric visibility and the extinction coefficient of fog and haze was proposed based on the results of applied Mie scattering theory. The model is applicable in the interval of wavelengths 0.2 μm < λ < 2 μm and for visibilities lower than 10 km. The presented modeling approach reveals more explicitly the connection between the extinction coefficient and the microphysical parameters of fog and haze. It also provides some degree of flexibility to adapt its properties according to locally specific atmospheric conditions.

Acknowledgement

This research is supported by the Czech Science Foundation under the project No. P102/11/1376.

References and links

1.

H. Willebrand, and B. S. Ghuman, Free-Space Optics: Enabling Optical Connectivity in Today’s Networks (SAMS, Indianapolis, 2002), Chap. 3.

2.

O. Bouchet, H. Sizun, C. Boisrobert, F. de Fornel, and P. Favennec, Free-Space Optics, Propagation and Communication (ISTE, London, 2006), Chap. 4.

3.

A. K. Majumdar, and J. C. Ricklin, eds., Free-Space Laser Communications (Springer, New York, 2008).

4.

P. W. Kruse, L. D. McGlauchlin, and R. B. McQuistan, Elements of Infrared Technology: Generation, Transmission and Detection (Jonh Wiley & Sons, New York, 1962), Chap. 5.

5.

E. Ferdinandov, K. Dimitrov, A. Dandarov, and I. Bakalski, “A general model of the atmospheric scattering in the wavelength interval 300 – 1100 nm,” Radioengineering 18, 517–521 (2009).

6.

I. I. Kim, B. McArthur, and E. J. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. SPIE 4214, 26–37 (2001). [CrossRef]

7.

M. Al Naboulsi, H. Sizun, and F. de Fornel, “Fog attenuation prediction for optical and infrared waves,” Opt. Eng. 43(2), 319–329 (2004). [CrossRef]

8.

P. Corrigan, R. Martini, E. A. Whittaker, and C. Bethea, “Quantum cascade lasers and the Kruse model in free space optical communication,” Opt. Express 17(6), 4355–4359 (2009). [CrossRef] [PubMed]

9.

K. W. Fischer, M. R. Witiw, and E. Eisenberg, “Optical attenuation in fog at a wavelength of 1.55 micrometers,” Atmos. Res. 87(3-4), 252–258 (2008). [CrossRef]

10.

R. Nebuloni, “Empirical relationships between extinction coefficient and visibility in fog,” Appl. Opt. 44(18), 3795–3804 (2005). [CrossRef] [PubMed]

11.

H. C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, New York, 1981).

12.

D. Deirmendjian, Electromagnetic Scattering on Spherical Polydispersions, (American Elsevier Pub. Co., New York, 1969), Chap. 4.

13.

D. Segelstein, The Complex Refractive Index of Water, (University of Missouri, Kansas City, 1981).

14.

R. G. Eldridge, “Haze and fog aerosol distributions,” J. Atmos. Sci. 23(5), 605–613 (1966). [CrossRef]

15.

P. Chýlek, “Extinction and liquid water content of fogs and clouds,” J. Atmos. Sci. 35, 296–300 (1978).

16.

M. Grabner, and V. Kvicera, “On the relation between atmospheric visibility and the drop size distribution of fog for FSO link planning,” in Proceedings of the 35th European Conference on Optical Communication (VDE VERLAG GMBH, Vienna, 2009), pp. 1–2.

17.

M. S. Awan, R. Nebuloni, C. Capsoni, L. Csurgai-Horváth, S. S. Muhammad, F. Nadeem, M. S. Khan, and E. Leitgeb, “Prediction of drop size distribution parameters for optical wireless communications through moderate continental fog,” Int. J. Satell. Commun. Network. 29(1), 97–116 (2011). [CrossRef]

18.

M. Grabner, and V. Kvicera, “Fog attenuation dependence on atmospheric visibility at two wavelengths for FSO link planning,” in Proceedings of Loughborough Antennas & Propagation Conference (Loughborough University, Loughborough, 2010), pp. 193–196.

OCIS Codes
(010.1310) Atmospheric and oceanic optics : Atmospheric scattering
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Atmospheric and Oceanic Optics

History
Original Manuscript: December 13, 2010
Revised Manuscript: February 1, 2011
Manuscript Accepted: February 2, 2011
Published: February 4, 2011

Citation
Martin Grabner and Vaclav Kvicera, "The wavelength dependent model of extinction in fog and haze for free space optical communication," Opt. Express 19, 3379-3386 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-4-3379


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References

  1. H. Willebrand, and B. S. Ghuman, Free-Space Optics: Enabling Optical Connectivity in Today’s Networks (SAMS, Indianapolis, 2002), Chap. 3.
  2. O. Bouchet, H. Sizun, C. Boisrobert, F. de Fornel, and P. Favennec, Free-Space Optics, Propagation and Communication (ISTE, London, 2006), Chap. 4.
  3. A. K. Majumdar, and J. C. Ricklin, eds., Free-Space Laser Communications (Springer, New York, 2008).
  4. P. W. Kruse, L. D. McGlauchlin, and R. B. McQuistan, Elements of Infrared Technology: Generation, Transmission and Detection (Jonh Wiley & Sons, New York, 1962), Chap. 5.
  5. E. Ferdinandov, K. Dimitrov, A. Dandarov, and I. Bakalski, “A general model of the atmospheric scattering in the wavelength interval 300 – 1100 nm,” Radioengineering 18, 517–521 (2009).
  6. I. I. Kim, B. McArthur, and E. J. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications,” Proc. SPIE 4214, 26–37 (2001). [CrossRef]
  7. M. Al Naboulsi, H. Sizun, and F. de Fornel, “Fog attenuation prediction for optical and infrared waves,” Opt. Eng. 43(2), 319–329 (2004). [CrossRef]
  8. P. Corrigan, R. Martini, E. A. Whittaker, and C. Bethea, “Quantum cascade lasers and the Kruse model in free space optical communication,” Opt. Express 17(6), 4355–4359 (2009). [CrossRef] [PubMed]
  9. K. W. Fischer, M. R. Witiw, and E. Eisenberg, “Optical attenuation in fog at a wavelength of 1.55 micrometers,” Atmos. Res. 87(3-4), 252–258 (2008). [CrossRef]
  10. R. Nebuloni, “Empirical relationships between extinction coefficient and visibility in fog,” Appl. Opt. 44(18), 3795–3804 (2005). [CrossRef] [PubMed]
  11. H. C. van de Hulst, Light Scattering by Small Particles, (Dover Publications, New York, 1981).
  12. D. Deirmendjian, Electromagnetic Scattering on Spherical Polydispersions, (American Elsevier Pub. Co., New York, 1969), Chap. 4.
  13. D. Segelstein, The Complex Refractive Index of Water, (University of Missouri, Kansas City, 1981).
  14. R. G. Eldridge, “Haze and fog aerosol distributions,” J. Atmos. Sci. 23(5), 605–613 (1966). [CrossRef]
  15. P. Chýlek, “Extinction and liquid water content of fogs and clouds,” J. Atmos. Sci. 35, 296–300 (1978).
  16. M. Grabner, and V. Kvicera, “On the relation between atmospheric visibility and the drop size distribution of fog for FSO link planning,” in Proceedings of the 35th European Conference on Optical Communication (VDE VERLAG GMBH, Vienna, 2009), pp. 1–2.
  17. M. S. Awan, R. Nebuloni, C. Capsoni, L. Csurgai-Horváth, S. S. Muhammad, F. Nadeem, M. S. Khan, and E. Leitgeb, “Prediction of drop size distribution parameters for optical wireless communications through moderate continental fog,” Int. J. Satell. Commun. Network. 29(1), 97–116 (2011). [CrossRef]
  18. M. Grabner, and V. Kvicera, “Fog attenuation dependence on atmospheric visibility at two wavelengths for FSO link planning,” in Proceedings of Loughborough Antennas & Propagation Conference (Loughborough University, Loughborough, 2010), pp. 193–196.

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