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  1. 1. Free Space Optical Communication Picture:
  2. 2. ‘ Wireless’ Optics? <ul><li>Fiber replaced by free space </li></ul><ul><ul><li>Channel characteristics not in control </li></ul></ul><ul><ul><li>Transmitter and Receiver essentially the same </li></ul></ul><ul><li>Indoor and Outdoor implementations differ </li></ul><ul><li>Three basic configurations </li></ul><ul><ul><li>Line of Sight (max. bandwidth) </li></ul></ul><ul><ul><li>Directed -- Non-Line of Sight Hybrid </li></ul></ul><ul><ul><li>Non directed -- Non-Line of Sight Diffused (min. bandwidth) </li></ul></ul><ul><li>Thus ‘Wireless’ need not imply Roaming </li></ul>Picture:
  3. 3. Indoor links… <ul><li>Interference </li></ul><ul><ul><li>Incandescent Light (~ 2800 K) – Max. interference </li></ul></ul><ul><ul><li>Sunlight (~ 6000 K) </li></ul></ul><ul><ul><li>Fluorescent lamps </li></ul></ul><ul><li>Attenuation </li></ul><ul><ul><li>Free Space Loss (due to beam divergence) -- important </li></ul></ul><ul><ul><li>Atmospheric Loss (not much indoors) </li></ul></ul><ul><li>Eye Safety – Most Important </li></ul><ul><ul><li>Should be class I safe (< 0.5 mW, 880 nm, LASER) </li></ul></ul><ul><ul><li>Restricts system power (though LEDs can be used at higher powers, but Bandwidth limited) </li></ul></ul>Picture: Optical Wireless- The Promise and Reality , Heatly and Neild
  4. 4. Outdoor links… <ul><li>Attenuation – Most Important </li></ul><ul><ul><li>Atmospheric Loss (varies with weather) </li></ul></ul><ul><ul><ul><li>0.2 dB/km in exceptionally clear weather </li></ul></ul></ul><ul><ul><ul><li>300 dB/km in very dense fog </li></ul></ul></ul><ul><ul><ul><li>Restricts the range (~500m in most commercial systems) </li></ul></ul></ul><ul><ul><ul><li>May need low capacity back-up RF links </li></ul></ul></ul><ul><ul><li>Free Space Loss (due to beam divergence) </li></ul></ul><ul><li>Scintillation Noise ( atmospheric turbulence induced intensity fluctuations ) – speckled pattern </li></ul><ul><li>Alignment Issues – Line of sight </li></ul><ul><li>Interference </li></ul><ul><ul><li>Sunlight (~ 6000 K) </li></ul></ul>Picture:
  5. 5. Attenuation :: Outdoor links <ul><ul><li>P R = P T . A receiver . e – σ .R /(Div-range) 2 </li></ul></ul><ul><li>P R ~ P T e – σ .R </li></ul><ul><li>Free Space losses beam divergence </li></ul><ul><li>Atmospheric losses exponential term– dominates </li></ul><ul><ul><li>Scattering + Absorption </li></ul></ul><ul><ul><li>Scattering dominates in σ </li></ul></ul><ul><li>Does Attenuation depend on wavelength? </li></ul>
  6. 6. Attenuation :: Scattering <ul><li>Depends on particle size </li></ul><ul><ul><li>Size parameter α = 2 π r/ λ </li></ul></ul><ul><ul><li>‘ r ’ varies with atmospheric composition </li></ul></ul><ul><li>r << λ => σ ~ λ -4 Rayleigh Scattering </li></ul><ul><li>r ~ λ => σ ~ λ -1.6 to 0 Mie Scattering </li></ul><ul><li>r >> λ => σ ~ λ 0 Geometric Scattering </li></ul><ul><li>Thus, larger λ => lower attenuation </li></ul><ul><li>Belief that 1550 nm is less attenuated than 785 nm in fog. </li></ul><ul><li>Does this apply always? </li></ul>
  7. 7. Attenuation :: Scattering …contd 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. 20000 to 400000 40000 to 800000 5000 to 50000 Hail 4000 to 20000 8000 to 400000 1000 to 5000 Snow 400-400000 800 to 80000 100 to 10000 Rain 4 to 80 8 to 160 1 to 20 Fog Particle 0.04-4 0.08-8 0.01-1 Haze Particle 0.0004 0.0008 0.0001 Air Molecules 1550 nm α (size paremeter ) 785 nm α (size paremeter ) Radius ( μ m) Type
  8. 8. Attenuation :: Scattering …contd <ul><li>The particle size distribution is difficult to obtain. so we express in terms of Visibility (V) </li></ul><ul><ul><li>σ = (3.91/V) x ( λ /550 nm) -q </li></ul></ul><ul><ul><li>V= visibility (km) light falls off to 2% of initial value </li></ul></ul><ul><ul><li>q= Size distribution of scattering particles </li></ul></ul><ul><ul><li>= 1.6 (V>50 km) </li></ul></ul><ul><ul><li>= 1.3 (6 km <V< 50 km) </li></ul></ul><ul><ul><li>= 0.16 V+0.34 (1 km <V< 6 km) Haze </li></ul></ul><ul><ul><li>= V - 0.5 (0.5 km <V< 1 km) Mist </li></ul></ul><ul><ul><li>= 0 ( V < 0.5 km) Fog </li></ul></ul><ul><li>The authors, proposed a new wavelength dependence through Mie Scattering calculations </li></ul>Earlier = 0.585 V 1/3 (V < 6 km)
  9. 9. Scintillation Noise <ul><li>Inhomogenities in Temp. and Pressure </li></ul>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.
  10. 10. Some images Pictures:
  11. 11. Web Resources <ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul><ul><li> </li></ul>