Transcript ICC05Jaya - Shivkumar Kalyanaraman

Error Analysis of Multi-Hop
Free-Space Optical Communication
Jayasri Akella, Murat Yuksel, Shiv Kalyanaraman
Department of Electrical, Computer and Systems Engineering
Rensselaer Polytechnic Institute
Troy, New York 12180
Email: [email protected]
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Motivation

To improve quality of Free Space Optical link


Communication medium being open space the
link suffers from the vagaries of atmosphere
impairing the link SNR, causing high end-toend BER and high error variance.
Multihop approach reduces both end-to-end
error and its variance and enables the design of
efficient FEC schemes to improve the link
reliability.
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Outline

Introduction to FSO communication system

Effect of atmospheric on a single hop FSO link

Effect of atmosphere on multihop FSO link

Comparison

Conclusions
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Free space optical communication - A brief
introduction
Line-of-sight communication technology using
optical range (IR- Blue) of the EM spectrum
Medium of transmission is free space/air.
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Pros and Cons of FSO Communication

Pros




Easy to deploy in terms of cost and time
Very high bandwidth
Low power per bit
Cons


Should always maintain line of sight
Adverse atmospheric effects
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Channel Behavior
FSO channel behaves like a time varying
attenuator.
 Causes of attenuation



Fixed geometric spreading
Atmospheric attenuation
• Fog: Can cause up to 300dB/KM
• Rain/Snow/Hail: Can cause up to 6db/KM (much less!!)

Causes of noise:
―
―
Scintillation: Due to pockets of varying refractive
index in atmosphere.
Ambient light and thermal noise
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Link Power Budget
PTotal  PRcvd  PLens  PGeometric spread  PAtmospheri c Attn
PTotal
PRcvd
PLens
PGeometric
PAtmospheric
Transmitted Optical Power at the transmitter
Received Optical Power
Losses at the lenses on both ends of the
communication
Spread due to the finite divergence of the light
beam
Attenuation caused by the suspended particles in
atmosphere.
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Effect of Atmosphere on FSO link


Rain/Snow
Fog


Size of optical wavelength is comparable to the size of fog particles. So
the maximum attenuation experienced for fog ~300DB/Km (in contrast
to RF, where rain causes the maximum damage to the signal.)
sometimes leading to total loss.
Turbulence and Scintillation are the sources of noise.
Effects of Rain/Snow and Fog can be can be captured in
“Visibility”.
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Effect of Atmosphere on FSO link
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Effect of Atmosphere on FSO link
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Error Probability due to Attenuation



For each packet, can model channel as constant
since FSO channel is slowly varying.
For On-OFF keying the error probability is given
by:
Pe  Q(av SNR)
Where av is atmospheric attenuation of channel
av  e R
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3.91 
 
 


V  550nm 
q
Error Probability over Single Hop
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Visibility versus Number Hops
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Reliability of the FSO link

To increase the reliability of an FSO link, two
important methods have been proposed in the
literature

Hybrid Approach: Provide hybrid link
protection using an RF link [1]

Multi-hop approach: Scaling the hop length
down between the transmitter and receiver
using multi-hop routing[2].
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Multihop Increases Efficiency of
FEC schemes

FEC (forward error correction codes) can be
used on top of multi-hop approach to improve
link reliability.


If we manage to tightly bound error variance within
certain limits, we can design more efficient error
control codes for a given FSO link.
We show through simulations that multi-hop
end-to-end error is lower and also has a smaller
variance than single hop.
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Channel Model
For small errors Pe <10e-2 , the channel is approximated as:
N
 1  P 
1
ie
i 1
1
N
P
i 1
0
N
P
ie
i 1
N
 1  P 
ie
i 1
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ie
0
Error Accumulation with Hop Length
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Bit Error Rate versus Number of Hops
Assume fixed link range
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Transmitted Power versus Hop Length.
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Simulation Details for Multi-hop scenario
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Clear weather conditions:
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Adverse weather conditions:
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Visibility is modeled as a Gaussian N~(10,3) Kms and
variance 3 Kms (rough approximation from Albany,
NY visibility data from the past 30 years.)
Visibility is modeled as a Gaussian with mean 3 Kms
and variance 1.5 Kms (rough approximation from
Albany, NY visibility data from the past 30 years.)
Hop Length is 500 meters for multi-hop scenario,
end-to-end range is 2.5 Kms (5 hops)
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Single Hop and Multi-hop Error comparison
Clear Weather Conditions
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Single Hop and Multi-hop Error comparison
Adverse Weather Conditions
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Comparison
Multi-hop significantly outperforms single hop
Number of
Hops
Mean error
Mean error
Variance
Variance
Clear
Weather
Adverse
Weather
Clear
Weather
Adverse
Weather
1
1.5e-3
0.27
0.02
0.1176
5
9e-27
5e-3
8e-50
4.5e-3
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Conclusions

The mean error is smaller over multiple hops
compared to single hop for the same link range.

The variance is also smaller for the multi-hop case.
Small variance helps to design efficient FEC schemes

Future Work:


Design suitable FEC schemes over multi-hop FSO link
Optimization of cost versus reliability for multiple hops
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