Study of the Atmospheric Turbulence in Free
Space Optical Communications
M. Ijaz, Shan Wu, Zhe Fan, W.O. Popoola
and Z. Ghassemlooy
Optical Communications Research Group, NCRLab,
School of Computing, Engineering and Information
Sciences, Northumbria University, UK
http://soe.unn.ac.uk/ncrlab/
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Contents
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Introduction
Free Space Optical Communication
Atmospheric Turbulence
Refractive Index Fluctuations
Experimental Work and Procedure
Results and Discussions
Conclusions
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Introduction- Research Aim
•
Free space optical communications is currently seen as a promising
alternative technology for bandwidth hungry applications, particularly within
the last mile access networks.
•
The applications of FSO includes base station to base station in cellular
networks, building to building, multicampus university networks, airports,
hospitals, a high-speed, high-capacity back up link and disaster recovery
links
•
FSO systems offer rapid deployment with no need for trenches and its
spectrum is licence free unlike the radio communication spectrum
•
Despite the absorption and scattering from the constituents of the
atmosphere, FSOC can be severely affected by the inhomogenities in the
temperature(Turbulence) on the clear day
•
In this research work the affect of atmospheric turbulence on FSOC link is
studied experimentally under controlled environment.
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Introduction-Free Space Optical
Communication
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Atmospheric Turbulence
• Atmospheric turbulence results from thermal
gradients within the optical path caused by
the variation in air temperature and density
• Random distributed cells are formed.
• They have variable size (10 cm - 1 km) and
different temperature.
• These various cells have different refractive
indexes thus causing scattering, multipath
variation of the arriving signal
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Refractive Index fluctuations
• Refractive index is highly dependent on the small scale
temperature fluctuations in air defined by
n(R,t) = no + n1(R,t)
• Where no is mean index of refraction (no = 1) and
n1(R,t) is the random deviation of index from its mean
value.
• Where R is the vector position in three dimension and
t is the time.
• n1(R,t) is dependent on the temperature and pressure
and is given by
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Refractive Index Fluctuations-cont
• The differentiation of the above equation tell
us about the dependence of temperature
small variations in the temperature gives us
large change in the refraction index
• Where P and T are absolute temperature and
Pressure respectively.
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Laser Beam Deformation
• Laser beam wander due to turbulence cells which are
larger than the beam diameter.
• Scintillation or fluctuations in beam intensity at the
receiver due to turbulent cells that are smaller than the
beam diameter.
Isaac I. Kim et al (1998)
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Experimental Work and
Procedure
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Experimental Work and Procedure-cont
Main simulation parameters used in the
experiment
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Parameter
Value
Optical source laser diode
(Beta Tx)
Optical Beta-Tx
wavelength
Maximum optical power
Class IIIb
Maximum data rate
PIN photo detector
SFH203PFA switching
time
Modulation type
1Mbps
0.5μs
Optical band-pass filter
Turbulence simulation
chamber
Temperature range
800nm-1100nm
140×30×30cm
850nm
3mW
OOK
20℃-80℃
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Results and DiscussionThe Strength of the Turbulence
• In order to characterize the strength of turbulence generated
within the simulated turbulence chamber, the received
average signal with and without the turbulence was studied.
• The signal distribution without and with scintillation are fitted
to a Gaussian distributions and log normal respectively
X
• The turbulence model discussed thus far is valid for the weak
turbulence with small values of σx2
Strength of Fluctuations
Weak
Intermediate
Strong
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Rytov Variance
σx2 < 0.3
σx2 ≈1
σx2>>1
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The Strength of the Turbulence-cont
The value of the log intensity variance was calculated to
be 0.002 Results in weak turbulence while without
scintillation; the noise variance was 10-5.
50
60
Experimental
Theoretical
Experimental
Theoretical
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Number of occurencs
Number of Occurences
50
45
40
30
20
35
30
25
20
15
10
10
5
0
0
0.005
0.01
0.015
Value of Sample
0.02
The received average signal (without scintillation)
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0.025
0
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Value of Sample
0.45
0.5
0.55
0.6
The received average signal (with scintillation)
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BER Evaluation
Binary signal with additive noise and PDFs for the binary signal with the threshold
•
where a0 and a1 are probabilities of transmission for binary ones and zeros
respectively and P0 and P1 are given by .
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BER Evaluation-cont
The received signal distribution(without scintillation)
Dotted lines -Theoretical fit solid line –experimental
data
The received signal distribution(with scintillation)
Dotted lines -Theoretical fit solid line –experimental
data
Temperature (°C)
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BER
T4
T1
36
30
6.84×10 -4
39
34
3.94×10 -4
45
39
3.24×10 -4
55
49
2.74×10 -4
59
53
6.63×10 -5
60
54
1.93×10 -4
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Optical Power Loss vs. Temperature
• The measured variance of the power fluctuation was
0.012
• This also confirms that the turbulence generated was
indeed very weak during our study.
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Conclusion
• In this research work the effect of turbulence in
FSOC is studied experimentally.
• The experimental data showed that if
scintillation effect is not mitigated, it can cause a
serious impairment to the performance and
availability of an FSO link.
• From an error free link, the simulated turbulence
(weak in strength) caused the BER to degrade to
about 10-4.
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Special Thanks for
Prof. Z. Ghassemlooy
Mr. W. Popoola
All colleagues in NCRL
&
Your Attention
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Thank You !
Question, please ?
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