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DIGITAL PULSE INTERVAL
MODULATION (DPIM) AS AN
ALTERNATIVE MODULATION
SCHEME FOR FREE SPACE
OPTICS (FSO)
Intro to FSO

Intra-city Fiber Optic Links
Fiber Optic
Cable
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
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
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
The Solution (cont’d)

Typical FSO Laser/Photodiode Systems
Photos taken from: http://www.systemsupportsolutions.com
FSO Limitations

Power Link Budget Equation
2
RXA
(  10R )
d 10
PRX  PTX
2
dTXA  DR 







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)
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
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
Limitation Solutions

RF Back-up (Hybrid FSO/RF)

Active Beam Tracking
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
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)
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
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
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
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
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