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: akellj@rpi.edu

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

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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.

Jayasri Akella

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