DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN ALTERNATIVE MODULATION SCHEME FOR FREE SPACE OPTICS (FSO)

1
DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN
ALTERNATIVE MODULATION SCHEME FOR FREE SPACE
OPTICS (FSO)
2
Intro to FSO

• Intra-city Fiber Optic Links

3
The Reasoning

• High-speed Access
• The Last Mile Problem?

4
The Solution

• Free Space Optics

5
The Solution (contd)

• High-speed Access (contd)

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The Solution (contd)

• Typical FSO Laser/Photodiode Systems
• Photos taken from http//www.systemsupportsoluti
  ons.com

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FSO Limitations

• Power Link Budget Equation
• PTX Power Transmitted
• PRX Power Received
• dTX Transmit Aperture Diameter (m)
• dRX Receive Aperture Diameter (m)
• D Beam Divergence (mrad)
• R Range (km)
• ? atmospheric attenuation factor (dB/km)

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FSO Limitations (contd)

• Atmospheric Attenuation
• Table taken from I. I. Kim, and E. Korevaar,
  Availability of Free Space Optics (FSO) and
  hybrid FSO/RF systems,
  Optical Access Incorporated, San Diego

9
FSO Limitations (contd)

• TX/RX Alignment

• TX/RX Misalignment

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Limitation Solutions

• Active Beam Tracking

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Limitation Solutions (contd)

• Increase Laser Power
• Higher power received
• Higher power per unit area
• Operating _at_ 1550nm instead of 800nm
• Increase Average Power Efficiency (APE)
• Pulse Modulation Schemes can provide
• higher average power efficiency at the
• expense of higher BW requirement
• Hence, increase Peak-APE

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Limitation Solutions (contd)

• On-Off Keying (OOK)
• Simplest solution based on intensity modulation
• 0 zero intensity, 1 positive intensity
• Popular Pulse Time Modulation Schemes for OC
• Pulse Position Modulation (PPM)
• Pulse Interval Modulation (PIM)

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Pulse Time Modulation

• PPM
• Higher average power efficiency than OOK
• Increases system complexity due to symbol-level
• synchronization.
• DPIM
• Higher APE than OOK but a bit lower than PPM
• No symbol-level synchronization required
• Higher Information capacity
• Data encoded as a number of time intervals
  between
• successive pulses
• Simplified receiver structure

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Pulse Time Modulation (contd)
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Pulse Time Modulation (contd)

• M log2L
• Picture Taken form J. Zhang, Modulation Analysis
  for Outdoors Applications of Optical Wireless
  Communications, Nokia Networks Oy, Finland

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Pulse Time Modulation (contd)

• Bandwidth and Power Efficiency Comparisons
• Table Taken form J. Zhang, Modulation Analysis
  for Outdoors Applications of Optical Wireless
  Communications, Nokia Networks Oy, Finland

17
Conclusion

• Power Increased by DPIM _at_ the cost of
• increased BW.
• Higher power means more power received _at_ the
  receiver _at_ high levels of attenuation and
  misalignment between TX/RX
• Major FSO benefit reliable link connection
  and/or increased distance between TX/RX for
  certain cities