jayasri akella error analysis of multi-hop free-space optical communication jayasri akella, murat...
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Jayasri Akella
Error Analysis of Multi-Hop Free-Space Optical Communication
Jayasri Akella, Murat Yuksel, Shiv KalyanaramanDepartment of Electrical, Computer and Systems Engineering
Rensselaer Polytechnic InstituteTroy, New York 12180
Email: [email protected]
Jayasri Akella
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-to-end 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
c AttnAtmospherispreadGeometric LensRcvdTotal PPPPP
PTotal Transmitted Optical Power at the transmitter
PRcvd Received Optical Power
PLens Losses at the lenses on both ends of the communication
PGeometric Spread due to the finite divergence of the light beam
PAtmospheric 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:
Where av is atmospheric attenuation of channel
)( SNRaQP ve
q
Rv
nmV
ea
550
91.3
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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:
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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
Clear weather conditions: 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.)
Adverse weather conditions: 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
Number of Hops
Mean errorClear
Weather
Mean errorAdverse Weather
VarianceClear
Weather
VarianceAdverse Weather
1 1.5e-3 0.27 0.02 0.1176
5 9e-27 5e-3 8e-50 4.5e-3
Multi-hop significantly outperforms single hop
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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