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Understanding DPIM: The Future of Free Space Optics

Explore how Digital Pulse Interval Modulation (DPIM) enhances optical wireless communications. Learn about its benefits, limitations, and solutions for efficient data transmission. Discover the potential of DPIM in overcoming the Last Mile Problem in FSO technology.

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Understanding DPIM: The Future of Free Space Optics

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

  2. Fiber Optic Cable Intro to FSO • Intra-city Fiber Optic Links

  3. The Reasoning • High-speed Access • The Last Mile Problem? Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam propagation @ 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego

  4. The Solution • Free Space Optics Picture taken from: I. I. Kim, and E. Korevaar, Availability of Free Space Optics (FSO) and hybrid FSO/RF systems, Optical Access Incorporated, San Diego

  5. The Solution (cont’d) • High-speed Access (cont’d) Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam propagation @ 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego

  6. The Solution (cont’d) • Typical FSO Laser/Photodiode Systems Photos taken from: http://www.systemsupportsolutions.com

  7. 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)

  8. FSO Limitations (cont’d) • 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. Picture taken from: TD. A. Rockwell, and G. S. Mecherle, Optical Wireless: Low-cost, Broadband, Optical Access, Fsona Communication Corporation, Richmond, BC FSO Limitations (cont’d) • TX/RX Alignment • TX/RX Misalignment

  10. RF Back-up (Hybrid FSO/RF) Limitation Solutions • Active Beam Tracking

  11. Limitation Solutions (cont’d) • Increase Laser Power • Higher power received • Higher power per unit area • Operating @ 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

  12. Limitation Solutions (cont’d) • 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)

  13. 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

  14. Table taken from: A.R. Hayes, Z. Ghassemlooy, and N.L. See, The Effect of Baseline Wander on the Performance of Digital Pulse Interval Modulation, 1999 IEEE Pulse Time Modulation (cont’d)

  15. Pulse Time Modulation (cont’d) • M = log2L Picture Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland

  16. Pulse Time Modulation (cont’d) • 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 @ the cost of increased BW. • Higher power means more power received @ the receiver @ 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

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