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

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

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)

The Solution (cont’d) Typical FSO Laser/Photodiode Systems

Photos taken from: http://www.systemsupportsolutions.com

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

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)

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 theexpense 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

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)

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