Free Space Optical Communication

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Free Space Optical Communication
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tm
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Wireless Optics?
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tm

• Fiber replaced by free space
• Channel characteristics not in control
• Transmitter and Receiver essentially the same
• Indoor and Outdoor implementations differ
• Three basic configurations
• Line of Sight (max. bandwidth)
• Directed -- Non-Line of Sight Hybrid
• Non directed -- Non-Line of Sight Diffused (min.
  bandwidth)
• Thus Wireless need not imply Roaming

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Indoor links
Picture Optical Wireless- The Promise and
Reality , Heatly and Neild

• Interference
• Incandescent Light ( 2800 K) Max. interference
• Sunlight ( 6000 K)
• Fluorescent lamps
• Attenuation
• Free Space Loss (due to beam divergence) --
  important
• Atmospheric Loss (not much indoors)
• Eye Safety Most Important
• Should be class I safe (lt 0.5 mW, 880 nm, LASER)
• Restricts system power (though LEDs can be used
  at higher powers, but Bandwidth limited)

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Outdoor links
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b21.htm

• Attenuation Most Important
• Atmospheric Loss (varies with weather)
• 0.2 dB/km in exceptionally clear weather
• 300 dB/km in very dense fog
• Restricts the range (500m in most commercial
  systems)
• May need low capacity back-up RF links
• Free Space Loss (due to beam divergence)
• Scintillation Noise (atmospheric turbulence
  induced intensity fluctuations) speckled
  pattern
• Alignment Issues Line of sight
• Interference
• Sunlight ( 6000 K)

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Attenuation Outdoor links

• PR PT . Areceiver . e s.R/(Div-range)2
• PR PT e s.R
• Free Space losses beam divergence
• Atmospheric losses exponential term dominates
• Scattering Absorption
• Scattering dominates in s
• Does Attenuation depend on wavelength?

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Attenuation Scattering

• Depends on particle size
• Size parameter a 2p r/?
• r varies with atmospheric composition
• r ltlt ? gt s ?-4 Rayleigh Scattering
• r ? gt s ?-1.6 to 0 Mie Scattering
• r gtgt ? gt s ?0 Geometric Scattering
• Thus, larger ? gt lower attenuation
• Belief that 1550 nm is less attenuated than 785
  nm in fog.
• Does this apply always?

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Attenuation Scattering contd
Type Radius (µm) 785 nm a(size paremeter) 1550 nm a(size paremeter)
Air Molecules 0.0001 0.0008 0.0004
Haze Particle 0.01-1 0.08-8 0.04-4
Fog Particle 1 to 20 8 to 160 4 to 80
Rain 100 to 10000 800 to 80000 400-400000
Snow 1000 to 5000 8000 to 400000 4000 to 20000
Hail 5000 to 50000 40000 to 800000 20000 to 400000
Table Comparison of beam propagation in haze and
fog, Kim, McArthur and Koreevar
The authors, studied the FOGGY weather conditions
which were showing a discrepancy between
analytical and empirical data.
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Attenuation Scattering contd

• The particle size distribution is difficult to
  obtain. so we express in terms of Visibility (V)
• s (3.91/V) x (?/550 nm)-q
• V visibility (km) light falls off to 2 of
  initial value
• q Size distribution of scattering particles
• 1.6 (Vgt50 km)
• 1.3 (6 km ltVlt 50 km)
• 0.16 V0.34 (1 km ltVlt 6 km) Haze
• V - 0.5 (0.5 km ltVlt 1 km) Mist
• 0 ( V lt 0.5 km) Fog
• The authors, proposed a new wavelength dependence
  through Mie Scattering calculations

Earlier 0.585 V1/3 (V lt 6 km)
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Scintillation Noise

• Inhomogenities in Temp. and Pressure

Variations in Refractive Index along the
transmission path
Speckled pattern (both in time and space) at the
receiver Can be removed by time and space
averaging. But problems arise with restrictions
on size of receiver and high bit rates.
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Some images
Pictures http//www.cablefreesolutions.com/imagel
ib.htm
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Web Resources

• www.freespaceoptic.com
• www.cablefreesolutions.com/casestudies.htm
• www.engalco.co.uk/fso_report.htm
• www.fiberwork.com.br/site/english/fso_e.htm
• www.ieeexplore.ieee.org