Intro to FSO
description
Transcript of Intro to FSO
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DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN ALTERNATIVE MODULATION SCHEME FOR FREE SPACE OPTICS (FSO)
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Intro to FSO Intra-city Fiber Optic Links
Fiber Optic Cable
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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
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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
The Solution
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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
The Solution (cont’d)
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The Solution (cont’d) Typical FSO Laser/Photodiode Systems
Photos taken from: http://www.systemsupportsolutions.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)
2)(2 1010
DRd
dPP
TXA
RXATXRX
R
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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
FSO Limitations (cont’d)
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FSO Limitations (cont’d) TX/RX Alignment
TX/RX Misalignment
Picture taken from: TD. A. Rockwell, and G. S. Mecherle, Optical Wireless: Low-cost, Broadband, Optical Access,
Fsona Communication Corporation, Richmond, BC
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Limitation Solutions RF Back-up (Hybrid FSO/RF)
Active Beam Tracking
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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 theexpense of higher BW requirement
Hence, increase Peak-APE
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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)
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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 (cont’d)
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
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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
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Bandwidth and Power Efficiency Comparisons
Table Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland
Pulse Time Modulation (cont’d)
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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