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Error Analysis of Multi-Hop Free-Space Optical Communication

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: akellj@rpi.edu. Motivation.

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Error Analysis of Multi-Hop Free-Space Optical Communication

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  1. 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@rpi.edu

  2. 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.

  3. Outline • Introduction to FSO communication system • Effect of atmospheric on a single hop FSO link • Effect of atmosphere on multihop FSO link • Comparison • Conclusions

  4. 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.

  5. 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

  6. 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

  7. 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.

  8. 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”.

  9. Effect of Atmosphere on FSO link

  10. Effect of Atmosphere on FSO link

  11. 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

  12. Error Probability over Single Hop

  13. Visibility versus Number Hops

  14. 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].

  15. 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.

  16. Channel Model For small errors Pe <10e-2, the channel is approximated as:

  17. Error Accumulation with Hop Length

  18. Bit Error Rate versus Number of Hops Assume fixed link range

  19. Transmitted Power versus Hop Length.

  20. 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)

  21. Single Hop and Multi-hop Error comparison Clear Weather Conditions

  22. Single Hop and Multi-hop Error comparison Adverse Weather Conditions

  23. Comparison Multi-hop significantly outperforms single hop

  24. 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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