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

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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)(

%log10kmdB

q

kmV

Vspeca

−

=

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



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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 +=

λλγ (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

7502.313709.02

18126.0 ++=

λλλγ (5)

The specific attenuation in dB/km for both types of fog is given by Al Naboulsi as follows

)()10ln(

10λγ=

km

dBspeca (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

rectransL

L

DtransPrecP 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]:

rectransL

LDRhcb

NtransPLM ττγ

θλ10/

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]

bNpEL

DL

recPtransPR

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

LPNS

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

hcNEP

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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