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Performance of Free Space Optical Communication Link under Foggy Weather

Article in Journal of Communications · May 2019

DOI: 10.12720/jcm.14.6.518-523

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 Journal of Communications Vol. 14, No. 6, June 2019

Performance of Free Space Optical Communication Link

under Foggy Weather
Farouk Kh. Shaker and Mazin Ali A. Ali
Physics Department, College of Science, Mustansiriyah University, Iraq
Email: mazin79phy@yahoo.com

Abstract—In recent years, free space optical communication more, directing blunders can likewise prompt a
(FSO) has received considerable attention as a cost-effective, debasement in the execution of FSO frameworks. Warm
free-to-license and broadband approach to high data rate extensions, powerless quakes and dynamic breeze loads
applications. However, the performance of the FSO connection cause building motions, which cause mechanical
suffers greatly from fading caused by the disturbance caused by
vibration of the transmitted beams, resulting in
different weather conditions. The weather considers a worthy
communication problems between transmitter and
influence on the laser beam to transmit over the atmosphere.
The attenuation weather results in the quality of the receiver receiver. FSO contains three components: a transmitter,
signal. This paper investigates the quality of the FSO an air channel, and receiver. Transmitter is considered as
communication system under foggy weather conditions. The an optical source Laser Diode (LD) or Light Emitting
analysis of the system based on the measured receiver signal, Diode (LED) to transmit optical radiation through the
bit rate error and signal to noise ratio in an optical atmosphere channel follows the Beer-Lambert‘s law [3]
communications link.  but wireless communication is a technology that has its
own limits [4], a proper understanding of optical signal
Index Terms—FSO, optical communications, OptiSystem 7.0, propagation in different atmospheric conditions has
atmospheric attenuation, SNR, BER become essential, and thus arises the need to rationalize
the effects of atmospheric channel on terrestrial FSO
links. Which FSO link has become poor performance
I. INTRODUCTION even link failure by a bad weather and losing in the
Free Space Optics (FSO) is a type of communications atmospheric along the path link, it becomes a great
technology which uses light for wireless transmission of challenge in FSO communications. This includes (fog,
data through the air as the same of fiber cable. Free Space rain, dust, smoke, and turbulence, etc) [5], [6], also the
Optics has the similar ability to fiber optics, but there is a intense scintillation effects causing the most severe
very big difference in low cost and Very high spread. impairment to the FSO links [7]. One of the solutions
They are characterized by great speed, relatively low cost, proposed to address the turbulence is by using hybrid
very large frequency range, fast manumitting and system (FSO switches to RF) [8], or using Multiple-Input
installation, very safe and very large protection, as well Multiple-Output (MIMO) FSO systems [9], or by
as long range spectrum which is free of licenses. Free decreased divergence angle and increasing receiver
Space Optics (FSO) anticipate the laser technique, which diameter [10].
employs light sources to transmit data through the air
under an indistinguishable climate. The motivation of the II. MODELS OF FOG SPECIFIC ATTENUATION
FSO is to discard the cost, effort and time anticipated that The atmosphere contents many particles such as (rain,
would present the fiber optic connection, but then to keep fog, dust, etc.) effect on the signal and causes attenuation
up high information rates (up to 1 GB/s and past) for but the fog is dominant. It is considered a harsh weather
exchanging video cuts, sound, information, and pictures. and reduces the visibility. Mie scattering theory was the
Sending utilizing FSO is generally straightforward. accurate way to calculate the attenuation by fog. This
Contrasted with Radio Frequency (RF) system, FSO theory requires more information about particle size,
system works have a high optical transmission capacity,
refractive index, and particle size distribution. The Fog
which encourages fast information exchange [1].
was inhomogeneous phenomena and change from time to
Regardless of the benefits of FSO, these systems
another [11]. Another way used to calculate fog
experience the ill effects of a few weaknesses. Where air
attenuation based on visibility data. These ways modeled
unsettling influences and system blunder mistakes are the
by Kruse, Kim and Al-Naboulsi [12]–[14] use the
most harming wonders in these systems. Where climatic
approach to estimate the attenuation for the collecting
unsettling influence causes vacillations in the power of
data of visibility. For Kruse model the attenuation [12] is
the getting sign and its optical stage, prompting a serious
corruption of optical connection execution [2]. What's written as:
q
3.912  c 
    *  (1)
Manuscript received September 5, 2018; revised April 24, 2019. V  550 
doi:10.12720/jcm.14.6.518-523

©2019 Journal of Communications 518

 Journal of Communications Vol. 14, No. 6, June 2019

Here γ(λ) is specific attenuation, V(km) stands for 

visibility, λ in nm stands for wavelength and the quantity L  10 20
(6)
coefficient q is depends on: The transmitter GT and receiver GR antenna gains are
given by the following:
1.6, if V 50km
 (2)
q  1.3, if 6kmV 50 km 32
0.585V 1 / 3 , if V  6km
GT  (7)
  div
2

Equation (2) obey for any meteorological condition, 2

  .Dr 
this means that for higher wavelengths, the attenuation G R    (8)
becomes lower. Kim revealed that a case of low visibility,  c 
the wavelength dependent on the attenuation under the
dense fog. The parameter q for the Kim model [13] is where Dr is the receiver aperture diameter and is
given by: the divergence beam which can be written as follows:
1.6, if V 50km 4c
1.3, if 6km V 50 km  div
2
 (9)
 (3)  .Dt
q  0.16V  0.34, if 1km V  6km
V  0.5, if 0.5kmV 1km where Dt is the transmitter aperture diameter and λc is the
 wavelength
0, if V  0.5km
C. Bit Error Rate (BER)
III. OPTICAL COMMUNICATION LINK MODEL For the optical link, the basic formula obeys to an
exponential behavior of the length of the path (L) as eqs.
We explore three significant parameters to show the
(4-6) [18], [19]. Also, the Bit Error Rate (ƁER) can
performance of optical communication links:
basically determine the average probability of selecting
A. Received Signal Power the wrong bits. BER is inversely related to the Signal to
Suppose the situation of laser beam propagation Noise Ratio (SNR) and can be written as: [20], [21].
between two points in FSO. Consider a laser transmission
 2    SNR 
with a power Ptrans at the wavelength λ. The received BER   . exp   (10)
signal power can be detected by the photodetector and   .SNR   8 
formulated as [15]
IV. DESIGN AND SIMULATIONS
D2    .L 
Pr  Pt * exp   t r (4) The first step in designing a wireless communication,
 .L 2  10  such as FSO communication system in different media
channels is to know what happens to an optical wave or a
where Dr is the receiver diameter, θ is the divergence signal as it travels through that medium. The tremendous
angle, γ is the attenuation factor (dB/m), τt, τr are the bandwidth favorable by FSO communications is
transmitter and receiver optical efficiency respectively. available only under clear weather case. Where there is
no dispersion and power loss is practically zero. However,
B. Signal to Noise Ratio (SNR)
this is not a realistic situation and to exploit is the large
The free space optical propagation link model can be potentials of FSO communications, appropriate measures
suggested to evaluate the performance optical should be used in transmitter and receiver devices. Using
wavelengths with Line of Sight (LOS) link. The (SNR) (OptiSystem version 7.0) simulation software, a design of
requirements are produced from the formula [16]: free space optical Link at 1km. This system is equipped
 4  with a wavelength of 1550 nm and the ability of about 30
SNR  Pt  30  GT  GR  20 log   10 logk B .BW .T     NF  FM (5)
 c  dBm. The transmitter contents the PRBS (Pseudo
Random Bit Sequence) generator, NRZ pulse generator,
where: laser source and the Mach Zehnder Modulator (MZM).
PT: the transmitter power; GT: the transmitter antenna Using the simulation system, the data generated by the
gain;GR: the receiver antenna gain; λc : wavelength; kB : PRBS generator is encrypted at 2.5 Gbps, and the
the Boltzmann’s constant (1.38*10-34 J/K). The receiver transmitted light is encrypted using MZM, where the
Bandwidth (B.W=1 MHz), T: the ambient temperature in laser source is considered as the main source of
K,NF: is the receiver noise figure, information, the laser is Pass through the one air channels
Fm: is the Fade margin, and γ: is the total attenuation in free space, to be provided the air channel capacity of
in dB/km. The maximum link distance (L) is given by approximately 30dBm. Where the opening of the active
[17]: area for receiver are determined for (1≤ D(cm)≤ 10). The

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beam divergence is (1≤ θ(mrad) ≤ 3), and the calculation Fig. 2 represents the receiver power signal with the
contents all the loss effect on the link and we can show diameter of the receiver under the influence of weather
the simplex main design model can be shown in Fig. 1. case. The power of the signal for (1550 nm) has been
increased with increasing receiver diameter. It is also
noted that 1550nm wavelength is very high signal power
received when there is haze compared to other conditions.

Fig. 1. Simulation layout of 1TX/1 RX FSO system.

In the link design, the loss effect on the main link of a

free space optical communication system resulting from
different weather condition (Haze, Light fog, Heavy Fog)
is studied. The optical signals propagate along of FSO
channel are received by (APD) photodetector. These
simulations use three visualizers namely optical power
meter to measure the power received, the quality of
signal power, finally a Bit Error Rate (BER) analyzer.
Fig. 2. Receiver signal power versus receiver diameter aperture.
V. RESULTS AND DISCUSSIONS
Consider a case of optical laser diffusion between two Fig. 3 represents the intensity of the receiver signal
points in ground applications. Therefore, the attenuation with the change of divergence angle of the laser same
coefficient, optical receiver power, SNR and ƁER of the conditions, where optical signal power suffers from
1550nm laser beam were studied in this simulation. FSO problematic with increased transmitter divergence angle.
systems are analyzed with power (30 dBm), a range of 1 It is also noted that the wavelength of 1550 nm is a very
km under different weather conditions. The infrared laser high reception power in haze weather compared to other
was used with the parameters contained in the table (1) conditions.
together with other parameters assumed in this simulation.
The achievement of the FSO communication system can
be evaluated and examined by the quality of the receiver
signal power and signal to noise ratio. This work focus
on the best performance of FSO communication link and
the quality of the signal under the different foggy weather.
The investigating carried out based on the computer
simulation to model the weather effect on the optical link
and operation parameters are shown in the Table I.
TABLE I: SYSTEM OPERATING PARAMETERS WHICH USED IN THIS
SIMULATION [22]-[23]
Operating parameter value
Wavelength 1550 (nm)
Transmitter power 30mw
Transmitter divergence angle 1≤ θ (mrad) ≤ 3
Efficiency of transmitter 0.8 Fig. 3. Receiver signal power versus transmitter divergence angle.
Efficiency of receiver 1
Sensitivity of receiver -20dBm To study the quality of the signal to noise ratio under
Diameter of receiver 1≤ D (c m)≤ 10 the foggy weather, the analyzed and the simulation
Range 1≤ L (m) ≤ 1000 results as shown in Fig. 4. It is observed that when the
The APD gain 100 diameter of the receiver increases, the signal to noise is
System temperature, T 290K
increasing directly under various weather case, especially
Noise figure, FT 4dB
Weather Haze 7.76 in the case of the haze, which is as high to the same
Light Fog 15.98 diameter of the receiver. In the case of heavy fog, the
condition
Heavy Fog 34.95 proportion of the signal to noise ratio was low.

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Fig. 4. Signal to Noise Ratio (SNR) verses receiver diameter aperture.

The signal-to-Noise Ratio (SNR) was plotted against ratio is decreased, especially in the case of heavy fog.
the transmitter divergence angle as shown in Fig. 5. Where SNR about 38 dB for heavy fog. While for haze
When the divergence angle increased the signal to noise about 65 dB.

Fig. 5. Signal to Noise Ratio (SNR) versus transmitter divergence angle

Fig. (6) represents the Bit Error Rate (BER) versus conditions, especially in the case of heavy fog. While for
receiver diameter. The (BER) is a decline when the haze weather the BER has linear behavior.
diameter of the receiver increases under different weather

Fig. 6. Bit Error Rate (BER) verses receiver diameter aperture

The Bit Error Rate (BER) was plotted as a function of is noted that the highest error in the received bits was
transmitter divergence angle for different weather recorded in the case of heavy fog, while the lowest error
conditions as shown in Fig. (7). The bit error rate of the received bit error rate was recorded in haze
increases with increasing transmitter divergence angle, it weather at the same transmitter divergence angle.

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