DIGITAL PULSE INTERVAL MODULATION DPIM AS AN ALTERNATIVE

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

Intro to FSO n Intra-city Fiber Optic Links Fiber Optic Cable

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

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

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

FSO Limitations n Power Link Budget Equation n n n PTX – Power Transmitted PRX – Power Received d. TX – Transmit Aperture Diameter (m) d. RX – Receive Aperture Diameter (m) D – Beam Divergence (mrad) R – Range (km) – atmospheric attenuation factor (d. B/km)

FSO Limitations (cont’d) n 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) n TX/RX Alignment n 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 n RF Back-up (Hybrid FSO/RF) n Active Beam Tracking

Limitation Solutions (cont’d) n Increase Laser Power n n n Higher power received Higher power per unit area Operating @ 1550 nm instead of 800 nm 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) n On-Off Keying (OOK) n n n Simplest solution based on intensity modulation ‘ 0’ – zero intensity, ‘ 1’ positive intensity Popular Pulse Time Modulation Schemes for OC n n Pulse Position Modulation (PPM) Pulse Interval Modulation (PIM)

Pulse Time Modulation n n PPM n Higher average power efficiency than OOK n Increases system complexity due to symbol-level synchronization. DPIM n Higher APE than OOK but a bit lower than PPM n No symbol-level synchronization required n Higher Information capacity n Data encoded as a number of time intervals between successive pulses n 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) n M = log 2 L Picture Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland

Pulse Time Modulation (cont’d) n Bandwidth and Power Efficiency Comparisons Table Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland

Conclusion n 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