LEDs For Free Space Optical Communications Systems€¦ · LEDs For Free-Space Optical...

LEDs For Free-Space Optical Communications SystemsChristine EversPhotonics and Optical CommunicationsSpring Semester 2005Instructor: Professor Dr. Dietmar Knipp

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Table of Contents

Overview

IntroductionWireless Home Link (WHL)White LED optical communications systemsInfrared communications systems

Analysis of data transmission utilizing white LEDsComparison White LED with InfraredConclusions

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Introduction

Wireless Home LinkIncrease in demand for office data communication systems

Wireless Home Link (WHL) to pursue internet access from anywhere at home

requires high speed wireless link

does not require any license for use

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Introduction

White Light Emitting Diodesnext generation of lighting

bright output, high power efficiency, long lifetime

faster photo-response than fluorescent light

light can be detected in photodiode at receiver

resistance to humidity and shadowing

low power consumption, minimal heat generation

easy to install, aesthetically pleasing unlike fluorescent light

eventually replace incandescent / fluorescent lights4

Introduction

Infraredshort-range, indoor applications

emitters and detectors available at low cost

virtually unlimited bandwidth of spectral region

infrared and visible light close together in wavelength w/ qualitatively similar behavior

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Table of Contents

Overview



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White Light Emitting Diodes

System Setupdata arriving in office / home, flowing into network, and being taken out from optical access point

transmitted optical pulses received at user terminal equipped with photo diodes convert optical pulse into electric signal

access points: white LEDs w/ functioning of lighting simultaneously

modulate electric signals into visible lightwave signals, emitting into airassuming non-directed line of sight (LOS)assuming optical wireless channelassuming Additive White Gaussian Noise (AWGN)

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System Setup8


Types of White LEDs1. One-Chip type white LEDs

Yttrium Aluminium Garnet (YAG) phosphor layer on top of InGaN-based blue LED chip

electric current supplied to blue LED chip, blue light emitted, which excites phosphor, which emits yellow fluorescence white light emission

2. Multi-Chip type white LEDs

mixing light from LEDs of three primary colors (emit each color simultaneously)

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White Light Emitting DiodesOne-Chip Type vs. Multi-Chip Type LED

(a) One-chip type white LED (b) Multi-chip type white LED

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Chip-type comparison1. One-Chip type white LEDs

low cost, bright outputdifficulty w.r.t. high-speed data trafficphosphor emitting light after blue emission of blue LED chip response speed lower than multi-chip typedeficient in color-rendering output (measure of faithfulness of color reproduction)

2. Multi-Chip type white LEDsemit various colors by changing mixture raio of three primary colorsmore suitable for wireless data transmission

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Evaluation Criteria

Two criteria

1. luminous intensity brightness of LED

2. transmitted optical power (energy flux per solid angle illuminance at illuminated surface) total energy radiated from LED

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EvaluationLED Considerations:

numberconfigurationdirectivity

Profile Considerations:IlluminanceSNR DistributionDelayModulation SchemeMulti-Chip Type

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Evaluation ctd.illuminance influenced by number of LEDs and their directivity and distribution over ceiling

SNR dependent on number of LEDs and directivitysmall SNR in corners of room

Delay spread: number of LEDs proportional to delaydistance between LEDs proportional to delaynarrow directivity of LED effect of distant LEDs

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Evaluation ctd.Modulation scheme: evaluation of intensity modulation and direct detection

PAM performing worse than on-off keying (OOK)OOK-NRZ (non return to zero) better than OOK-RZ (return to zero) for small delay and hence small inter-symbol interference (ISI)OOK-NRZ worse than OOK-RZ for large delay and hence large ISI

Multi-chip type: to observe white color: each primary color LED must transmit at different power

the more distinct color combinations are, the higher the degradation of BERthe more distinct color combinations are, the better the E/O conversion efficiency

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Modulation Techniques16


Number of LEDs17


Distribution of LEDs18

Table of Contents

Overview



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Conclusion

InfraredPerformance:

short-range applicationsavailability of wide, worldwide unregulated bandwidthability to achieve high bit rateslow signal-processing complexitypotentially very low cost

Challenges:achieving high SNRmultipath propagation causing significant ISI in non-directed links at bit rates about 10Mb/s

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Conclusion


Performance:arrangement of LEDs in roomnumber of LEDs distributed in room (dependent on directivity)

small directivity: large number of LEDswide directivity: small number of LEDs

influence of ISI caused by receiving a signal with two or more paths changes with modulation scheme

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Comparison

SimilaritiesPros:

qualitatively similar behavior of infrared and visible lightdesign may be simplified since transmissions in different rooms need not be coordinatedprevent multipath fading simplification of user equipment designmay achieve very high aggregate capacity

Cons: access points require as no penetration of walls possibleambient noise interfering with receiver

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ConclusionLEDs:

high luminous intensityhalves infrared current consumption and enhances transmission distances by 1.8m

sizecost-effectivenesspower consumptionintensityruggednessresistance to interference no separate access points required but implemented in light equipment that is installed anyways

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Bibliography

[1] Y. Tanaka, T. Komine, S. Haruyama and M. Nakagawa, “Indoor Visible Light Data Transmission System Utilizing White LED Lights”, in IEICE Transactions on Communications, v. E86-B, no. 8, August 2003.

[2] J. M. Kahn and J. R. Barry, “Wireless Infrared Communications”, in Proceedings of the IEEE, v. 85, no.2, February 1992.

[3] T. Douseki, “A batteryless optical-wireless system with white LED-illumination”, in Proceedings of the 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), v. 4, September 2004.

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LEDs For Free Space Optical Communications Systems€¦ · LEDs For Free-Space Optical...

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