Characterization of fog attenuation for free space optical

International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 3, May – June (2013), © IAEME
244
CHARACTERIZATION OF FOG ATTENUATION FOR FREE SPACE
OPTICAL COMMUNICATION LINK
Mazin Ali A. Ali
The University of Mustansiriyah / College of Science / Department of Physics, Baghdad-Iraq.
ABSTRACT
This paper presents the effect of fog attenuation on free space optical (FSO)
communication. The analysis focuses on Al-Naboulsi model (advection, radiation) fog
coefficient and effects on visibility. The performance comparison is also done for visibilities
(0.5, 1, 5, 10, 25, and 50) km. The result shows that the performance of specific attenuation
50 km is better than the other visibilities in both performance parameters (advection,
radiation). On the other the effect of fog attenuation on optical communication link is
performance, receiver signal power, link margin, data rate and signal to noise ratio (S/N) are
the major interesting design parameters in the current study.
Keywords: fog attenuation, optical communication, receiver signal power, link margin, data
rate, signal to noise ratio
1. INTRODUCTION
An optical wireless (OW) or Free Space Optics (FSO) link can be established using
Lasers or light emitting diode (LED) between any two line of sight points in free space for a
certain link distance, enabling point-to-point data links at rates exceeding 1 Gbits/s. Lasers
work in the visible and near infrared spectrum of the electromagnetic radiations. The inherent
advantage of using lasers for establishing connection between two geographically separated
line of sight points provides a well-focused narrow beam that on one hand is secured and on
the other hand is less scattered as it traverses the free space mostly the earth atmosphere [1].
OW is now finding niche applications both in military as well as commercial services sectors
and is being researched for scenarios involving communication between fixed as well as
mobile platforms. Few of the potential application scenarios of OW links are transmission
links between satellites, links for deep space missions, links between unmanned aerial
INTERNATIONAL JOURNAL OF ELECTRONICS AND
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 3, May – June, 2013, pp. 244-255
© IAEME: www.iaeme.com/ijecet.asp
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245
vehicles (UAV), high altitude platforms (HAP), data links from earth to satellites and re-
establishing high speed connections in case of emergency or disaster recovery situations [2].
The optical beam traversing through the earth atmosphere is attenuated by absorption and
scattering of radiation from fog, clouds, snow, rain, sleet and dust etc. This attenuation is
typically dominated by fog, clouds and snow. However, the attenuation due to snow, rain and
sleet etc., is generally less significant as compared to signal transmission through fog and
clouds, such that the optical signal becomes weak enough that the communication system will
cease to operate [3]. The real challenge to these optical wireless links arise in the presence of
different fog conditions: as the size of the fog particles is comparable to the optical
wavelengths used for transmission [4]. The most commonly used wavelengths (650 nm, 750
nm, 850 nm, 950 nm, 1050 nm, 1550 nm) in FSO fall inside the transmission window such
that the contribution of attenuations from phenomena like absorption to total extinction are
almost negligible as compared to scattering, the most dominant factor of optical signal
attenuation in free space[5].
2. THEORETICAL BACKGROUND
2.1 Fog and Fog Formation
Fogs are composed of very fine water droplets of water, smoke, ice or combination of
these suspended in the air near the Earth's surface [6]. The presences of these droplets act to
scatter the light and so reduce the visibility near the ground. A fog layer is reported whenever
the horizontal visibility at the surface is less than 1 km [7, 8]. Normally, after sunset a strong
cooling takes place near the earth surface through the divergence effect of long wave
radiation. As the cooling increases, the relative humidity (the ratio of absolute humidity to
saturation) increases until fog droplets are activated. Typically, fog formation takes place as
the difference (∆) between temperature and dew point becomes (5 °F) 3 °C, or less and as a
result water vapors in the air begin to condense into liquid water form while relative humidity
reaches to 100% [9].
Another way is to use visibility data to predict specific attenuation. The models
Kruse, Kim and Al Naboulsi [10, 11, 12, and 13] use this approach and predict specific
attenuation using visibility. The specific attenuation for both Kim and Kruse model is given
by
)/(
0
)(
%log10
kmdB
q
kmV
V
speca
−
=








λ
λ
(1)
Here V(km) stands for visibility, V% stands for transmission of air drops to percentage of
clear sky, λ in nm stands for wavelength and λ0 as visibility reference (550 nm). For Kruse
model [10].
1.6 if V> 50 km
q = 1.3 if 6 km > V > 50 km (2)
0.585V1/3
if V< 50 km

246
Equation (2) implies that for any meteorological condition, there will be less
attenuation for higher wavelengths. The attenuation of 10 µm is expected to be less than
attenuation of shorter wavelengths. Kim rejected such wavelength dependent attenuation for
low visibility in dense fog. The q variable in equation (1) for Kim model [11] is given by
1.6 if V> 50 km
1.3 if 6 km < V < 50 km (3)
q= 0.16V+0.34 if 1 km < V < 6 km
V-0.5 if 0.5 km < V < 1 km
0 if V< 0.5 km
Al Naboulsi et al. (France Telecom model) [12, 13] has provided relations to predict
fog attenuation by characterizing advection and radiation fog separately. Generally radiation
(convection) and advection fog are the most usually encountered types in nature [14], [15].
Advection fog is formed by the movement of wet and warm air masses above colder
maritime or terrestrial surfaces. It is characterized by liquid water content higher than 0.20
g/m3 and a particle diameter close to 20 µm [16]. Al Naboulsi provides the advection fog
attenuation coefficients as
( )
Vadv
8367.311478.0 +
=
λ
λγ (4)
Radiation fog is related to the ground cooling by radiation. It appears when the air is
sufficiently cool and becomes saturated. This fog generally appears during the night and at
the end of the day. Particle diameter is around 4 µm and the liquid water content varies
between 0.01 and 0.1 g/m3
[16]. Al Naboulsi provides the radiation fog attenuation
coefficients as
( )
Vrad
7502.313709.0218126.0 ++
=
λλ
λγ (5)
The specific attenuation in dB/km for both types of fog is given by Al Naboulsi as follows
)(
)10ln(
10
λγ=





km
dB
speca (6)
2.2. Received Power for optical communications link
Consider a laser transmitting a total power Prec at the wavelength 650nm. The signal
power received at the communications detector can be expressed as [17]

247
rectrans
L
L
D
transPrecP ττ
γ
θ
10/
10
22
2
−
= (7)
Where D is the receiver diameter, θ is the divergence angle, γ is the atmospheric
attenuation factor (dB/km), τtrans,τrec are the transmitter and receiver optical efficiency
respectively
2.3. Link Margin for optical communications link
Another important parameter in optical communications link analysis is "Link
Margin", which is the ratio of available received power to the receiver power required to
achieve a specified BER at a given data rate. Note that the "required" power at the receiver
PREQ (watts) to achieve a given data rate, R (bits/sec), we can define the link margin LM as
[18]:
rectrans
L
LDRhc
b
NtransPLM ττ
γ
θλ
10/
10]
)22/(2[*]/([
−
= (8)
Where R is a data rate, h is a plank constant and c is the light velocity.
2.4. Data Rate for optical communications link
Given a laser transmitter power Ptrans, with transmitter divergence of θ, receiver
diameter D, transmit and receive optical efficiency τtrans, τrec the achievable data rate R can be
obtained from [19]
b
NpEL
D
L
recPtransP
R
22)2/(
210/
10
θπ
γ−
= (9)
Where EP=hc/λ is the photon energy at wavelength λ and Nb is the receiver sensitivity
(photons/bit).
2.5. Signal to Noise ratio (S/N) for optical communications link
To consider the design of optical communications systems for propagation of light in
free space, the light noise, and the basic light in fog attenuation parameters as analyzed
above. The main parameters in free space communications as shown the evaluation criteria
for optical communication signal to noise ratio (S/N). The signal to noise ratio (S/N) of a
signal power to the noise power corrupting the signal [20]
2
2
2
cos
4
))(3exp(
tan
/



















 −






=
RX
r
mediumm
m
TXdiv
T
NEP
D
L
LP
NS
ϕλα
ϑ
(10)

248
This equation assumes the beam pattern of the transmitter is a constant for angles up
to the 3-dB (halfway) point and zero beyond that angle. Where φ is the angle between the
optical axis of the receiver and the line of sight between transmitter and receiver, and noise
equivalent power (NEP) is defined as the incident optical power at a particular wavelength or
with a specified spectral content required to reduce a photo detector current equal to root
mean square noise current and can be expressed as [21]
ηλ
hc
NEP
2
= , Watt (11)
Where η is the quantum efficiency and λ is the operation optical signal wavelength.
3. SIMULATION RESULTS
Simulation carried out by matlab to show the effect of fog on optical communication.
We have investigation the high quality and best performance of optical communications for
(high, medium, low) visibility between transmitter and receiver. So the receiver power, link
margin, data rate and signal to noise ratio due to the effect of fog can be evaluated.
Table (1): Proposed operating parameters for optical communications links
Operating parameter value
Laser wavelength 650 nm
Transmitter power 100mw
Transmitter divergence angle 1.5mrad
Transmitter efficiency 0.95
Receiver efficiency 0.95
Receiver sensitivity -20dBm
Receiver diameter 10cm
Receiver angle 5.16 degree
Based on the modeling equations analysis and the assumed set of the operating system
parameters as shown in table (1).
Simulations were performed to compare the measured value of fog FSO specific
attenuation under visibility conditions and attenuation prediction by Al Naboulsi model. Fig.
(1) shows this comparison but from fig. (1) it cannot be inferred that which model performs
better than others. Al Naboulsi models seem to be closer but still it cannot be said certainly
that this model is better.

249
Figure (1) comparing the specific attenuation of different visibility using Al Naboulsi model
The receiver signal power depends upon the different parameters. The received power
is achieved for advection and radiation fog at a link distance less than 1km under different
visibility as shown in fig. (2). The received signal power increases with increasing visibility

250
for both advection and radiation fog. It is also observed that received signal power decreases
with increasing path length under different values of visibility. Also observed that the
advection and radiation fog have closer behavior for low and medium visibility.
(a) Advection fog
(b) Radiation fog
Figure (2) receiver signal power versus path length (km) for fog attenuation under different
visibility
The link margin for receiver sensitivity -20dBm is achieved for data rate 100Mb/s
operating under fog condition (advection, radiation) at a distance 1km. The link margin
increases with increasing visibility for both advection and radiation fog. Also noted that the
link margin have a close behavior for low and medium visibility.

251
(a) Advection fog
(b) Radiation fog
Figure (3) link margin versus link distance for fog attenuation under different visibility
The range equation can be used to generate the communication data rate versus link
range for varying atmospheric visibility under fog conditions. Receiver sensitivity -20dBm
which is equivalent to 327000 photons/bit at wavelength 650nm. The data rate decreasing
with decrease visibility as shown in Fig (4), As can be seen a maximum data rate of 100 Mb/s
can be achieved for a range large than 1km under advection and radiation fog for different
visibility.

252
(a) Advection fog
(b) Radiation fog
Fig. (4) Data rate versus path length for fog attenuation under different visibility
The signal no noise ratio(S/N) is increases with increasing visibility for both fog
conditions. It is also observed that the signal to noise ratio (S/N) for advection fog have very
close behavior compared to radiation fog. As well as high (S/N) has presented high visibility
compared with low and medium visibility.

253
(a) Advection fog
(b) Radiation fog
Fig. (5) Signal to noise ratio (S/N) for fog attenuation under different visibility
CONCLUSION
This paper explains the effect of fog attenuation based on Al Naboulsi model on the
propagation of optical communication link in free space. High, medium and low visibility
have presented to observed effects on receiver signal power, link margin, data rate and signal
to noise ratio at different distances. Theoretically found that the receiver signal power, lank
margin, data rate and signal to noise ration decreasing with increasing path link but
increasing with increasing visibility under fog attenuation conditions. On the other hand we
observed that the advection and radiation fog have very close behavior at high visibility but
has simple difference at low and medium visibilities.

254
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Characterization of fog attenuation for free space optical

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Characterization of fog attenuation for free space optical