Error%20Analysis%20of%20Multi-Hop%20Free-Space%20Optical%20Communication

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Error Analysis of Multi-Hop Free-Space Optical
Communication

• Jayasri Akella, Murat Yuksel, Shiv Kalyanaraman
• Department of Electrical, Computer and Systems
  EngineeringRensselaer Polytechnic
  InstituteTroy, New York 12180
• Email akellj_at_rpi.edu

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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-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

• 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

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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 routing2.

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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 lt10e-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
Multi-hop significantly outperforms single hop
Number of Hops Mean error Clear Weather Mean error Adverse Weather Variance Clear Weather Variance 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