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Free space optical (FSO) link design
under diverse weather conditions
Z. Salem, N. Khan, W. Ishaq,
zahid_saleem@comsats.edu.pk
nasrullahk@yahoo.com
wishaq@gmu.edu

Out line
Link design
Impact of atmosphere on FSO
Absorption & Scattering
Turbulent media
Turbid media
Link margin
Results
Limitations
Conclusions
Question

Link design

Link design
The main objective of FSO link design is to get as much
light as possible from one end to the other, in order to
receive a stronger signal that would result in higher link
margin and greater link availability
Link design parameters consists of two parts
Internal system parameters ( system related parameters)
External system parameters( Link related parameters)

It consists of …
Link design
Internal system parameter
• Transmitted Power (P T )
• Transmit Beam Divergence (θ(
in rad)
• Surface Area of Receiver Aperture (A R )
• Receiver Sensitivity (S R )
• Transmitter & Receiver optical losses (η)(
These parameters collectively form Generalized link margin ( GLM).
Mathematically
GLM
PT
* η * AR
= ⎡ A ( * ) 2
T
+ R θ ⎤
⎣
⎦

Link design
Power transmitted (P T )
(two transmitters)
Wavelength (λ)(
Transmitter beam
divergence
Receiver diameter (D R )
Range
Data Rate
Receiver sensitivity (S R )
Transmitter and receiver
losses (η)(
100 mW
(each 50 mW)
1550nm
2 mRad
10cm
2500 m
180 Mbps
-33dBm
50%

Link design
External system parameter
Deal with the environmental factors which
attenuates the signal strength.
It includes…
• Atmospheric attenuation
• Range of deployment
• Visibility
• Scintillation

Impact of atmosphere on FSO

Impact of atmosphere
• Absorption and Scattering
• Turbulent media
• Turbid media

Impact of atmosphere
Absorption and Scattering
Absorption is mainly caused by the water
vapor (H 2 O) and carbon dioxide (CO 2 )
present in the air along the transmission
path.
Scattering caused by particles ( rain, fog
or dust ) that are large as compared to the
wavelength of the light being transmitted is
referred as Mie scattering.

Impact of atmosphere
Turbulent media effects
In-homogeneities in the pressure and
temperature lead to change in the
refractive index along optical path, results
in turbulence
Affects of turbulent media are
• Beam divergence,
• Beam wandering
• Scintillation

Impact of atmosphere
Turbulent media effects
Beam Divergence
When laser beam passes through the
atmosphere which has lens like nature
results in spreading the beam.

Impact of atmosphere
Turbulent media effects
Beam Wandering
When the size of the turbulence cells is
larger than the beam diameter, the laser
beam randomly bends .

Scintillation
Impact of atmosphere
Turbulent media effects
As laser beam propagates through atmosphere,
tiny changes in refractive index results in
creating a multi-path effect that result in
fluctuations at the optical receiver. This
phenomenon is called scintillation.

Impact of atmosphere
Turbid media effects
It refers to beam passing through a
medium consisting of large number of
discrete scatters or aerosol particles (e.g.
rain, fog or dust), which give rise to strong
scattering effects.

Impact of atmosphere
Turbid media effect

Link Margin

Link Margin
It is a ratio of the available received power on a clear day (at a given
range) to the receiver power sensitivity required to meet the bit
error rate specification. This is typically measured in dB.
Following are various parameters that contribute to the
link margin.
• Transmitted Power ( P T )
• Beam width of transmitter (θ(
in rad)
• Surface Area of transmit Beam at Range R (A T ).
• Surface Area of Receiver aperture (A R )
• Range of deployment (R)
• Receiver Sensitivity (S R )
• Atmospheric Attenuation Coefficient (σ(
in dB per KM)
=
R
LM PT
e
R
2
[ + ( θ * ) ]
T
A
S A R
( −σ
R)

Results

Results
Rain Attenuation
Attenuation (dB/Km)
40
35
30
25
20
15
10
5
0
0 50 100 150 200
Rain Rate (mm/hr)

Results
Visibility and Attenuation for different
degree of fog
Attenuation (dB/Km)
500
400
300
200
100
0
0 1 2 3 4
Visibility (Km)

Results
Power per transmitter in mW 50
No of transmitters 2
Transmitter beam divergence in (milli-rad) 2
Receiver sensitivity for BER of 1*10 -11 in dBm -33
Range in (km) 2.5
Geometrical losses in dB -34
Weather losses in dB/km -3
Losses due to inefficiency in dB -3
Total losses in dB -44.5
Actual received power in dBm -25.1
System dynamic range in dB 53
System link margin in dB 7.9

Limitations
FSO link has poor performance, even link
failure under adverse weather conditions.
Reasons are
Meteorological data is missing
May be error in data collection
Don’t t show lightening conditions
FSO commercial limitation is dense fog

Conclusion
FSO system performance is degraded under
adverse weather conditions. It can be
handled by the following techniques.
Smaller hops( short distance FSO nodes)
Use hybrid system ( FSO switches to RF)
Use adoptive optics.

References
P.L.Eardley and D.R Wiseley, , IEE Proc.Optoelectron, , 143, 330
(1996).
Wireless channel characterization and modeling by Vijayalakshmi
Vasudevan.
Laser beam propagation in the atmosphere By J.W. Strohbehn
Electro-Optics Handbook, pp.82-87, 87, RCA Commercial
Engineering, Technical Series EOH-11, Harrison, N.J. (1974).
H. Weichel, , Laser Beam Propagation in the Atmosphere, SPIE,
Bellingham, WA ~1990
W. K. Pratt, Laser Communication Systems, J. Wiley & Sons,
New York ~1969.
“The last-mile solution: Hybrid FSO Radio” by Scott Bloom Chief
technical officer & W. Seth Hartley Optical propagation specialist,
st,
AirFiber, , Inc.

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