doc.: IEEE 802.15-15-08-0131-01-vlc

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area NProject: IEEE P802.15 Working Group for Wireless Personal Area Networks (etworks (WPANsWPANs))

Submission Title: [Some challenges for visible light communications]Date Submitted: [Revised version July 24th 2008]Source: [Dominic O’Brien] Company [University of Oxford]Address [Parks Road, Oxford, OX1 3PJ UK]Voice:[+441865273916], FAX: [+441865273906], E-Mail:[dominic.obrien@eng.ox.ac.uk]

Abstract: [VLC has a number of technical challenges, which are discussed in the presentation]Purpose: [Informing those interested in VLC of some of the technical challenges facedNotice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

doc.: IEEE 802.15-15-08-0131-01-vlc

Submission

Some Challenges for Visible Light Communications

Dominic O’BrienLubin ZengHoa Le MinhGrahame FaulknerDepartment of Engineering Science, University of Oxford

doc.: IEEE 802.15-15-08-0131-01-vlc

Submission

Introduction

• Typical VLC link characteristics• Challenges

– Technical• Bandwidth limitations• Providing an uplink

– Regulatory• Compatibility with Lighting Control systems• Illumination systems

• Conclusions

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Submission

Typical link characteristics

• Source• Channel • Receiver

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Submission

LED Modulation

• Opto-electronic response

0 10 20 30 40 50-25

-20

-15

-10

-5

0

freq (MHz)

Relative response (

dB)

White response

Blue response

Measured LED small-signal bandwidth

sR dVL

sC

dC

V

I

Luxeon LED

Rs = 0.9727 ΩL = 33.342 nHCs = 2.8 nFCd = 2.567 nFtt = 1.09 ns

SPICE Model

3

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Submission

Improvement of LED Response• Using blue-response only (blue filtering)

350 400 450 500 550 600 650 700 750 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength(nm)

Intensity (normalised)

~130 ns

~25 ns

Measured optical spectrum Measured impulse response

• Issue: Only 10% of signal power is recovered⇒ Reducing SNR, link distance

• LEDs with more blue energy [1] could be used to gain more filtered power, however the balance of white colour is shifted

Blue filtering

[1] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. ECOC 07 (PDS 3.6), pp. 1-2. ECO [06.11], 16-20 Sep. 2007, Berlin, Germany

4

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Submission

VLC Channel

8

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Submission

Room Power Distribution

• Assume – 1% modulation of

typical illumination power

– Typical receiver performance

• Conclusions– Very high SNR

available• SNRmin = 38.50dB• SNRmax = 49.41dB

– Modulation limited by source bandwidth

9

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Submission

Optical Receiver• Receiver consists of

– Optical filter• Rejects ‘out-of-band’ ambient

illumination noise– Lens system or concentrator

• Collects and focuses radiation – Photodetector (or array of detectors)

• Converts optical power to photocurrent– Incoherent detection

– Preamplifier (or number of preamplifiers)• Determines system noise performance

– Post-amplifier and subsequent processing

Optical filter

Optical system

Photodetector

Amplifier

Output

Input radiation

11

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Submission

Optical Receiver: Constant Radiance Theorem

• Optical ‘gain’ of receiver limited by required field of view

Ωi

Ωο

Ai

Ao

AiΩi<=AoΩο

AiΩi<=Ao2π

12

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Submission

Receiver Performance: Figure of Merit

• Receiver Figure of Merit (FOM)– Fibre systems

• Performance determined by sensitivity (given sufficient detector area)

• FOV usually not relevant

– Free space systems• Etendue crucial determinant

min

2P

ARFOM bπ=

Detector Area A

Receiver sensitivity

Pmin

Field of view 2π Sr

Bit rate Rb

13

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Submission

Improving data rate: equalisation

• Transmitter equalisation– High bandwidth– Energy efficiency

• Blue filtering– Lose low frequency energy from phosphor

• Receiver– Simple analogue equalisation– More complex also

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Submission

Typical waveforms for RX equalisation

Data rate 33Mb/s

0 500 1000 1500 2000 2500 3000 3500-0.2

0

0.2

0.4

0.6

0.8

1

1.2

time(ns)

Sig

nal

Recovered datatransmitted data

Data rate 14Mb/s

0 500 1000 1500 2000 2500 3000 3500 4000-0.2

0

0.2

0.4

0.6

0.8

1

1.2

time(ns)S

igna

l

Recovered datatransmitted data

NRZ data Manchester data

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Submission

Bandwidth Improvement: Post Equalisation

Pre-equalisation: experiment

Post-equalisation: simulation17

• Pre- and post-equalization: single LED link

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Submission

Improving data rate: complex modulation

• High SNR channel– Complex modulation attractive

• OFDM– 100Mb/s over 20MHz channel [1]

• PAM – Simulations show LED characteristics not

optimal

[1] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. ECOC 07 (PDS 3.6), pp. 1-2. ECO [06.11], 16-20 Sep. 2007, Berlin, Germany

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Submission

Improving data rate: PAM

• Simulation uses measured LED impulse response

• Simple 1st order RX equaliser

• 4-PAM• 24Mb/s (33Mb/s NRZ)

Data rate 24Mb/s (4-PAM)

time(ns)0 2000 4000 6000 8000 10000

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Sig

nal

Recovered datatransmitted data

Further work required

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Submission

Improving data rate: MIMO

• Parallel ‘alignment free’ data links• Simulations show linear capacity growth• Experimental results for a simple IR

system• Simulations of in-room VLC system

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Submission

Simple IR system

1x2 Laser array

3x3 photodiode array

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 10 -6

0

0.5

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 10 -6

0

0.5

1

Nor

mal

ised

Am

plitu

deN

orm

alis

ed A

mpl

itude

Channel 1

Channel 2

Experimental system

Recovered data Transmitted data

x

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MIMO VLC: Simulation System

24

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MIMO VLC: Preliminary Results

Position of the receiverAggregate data rate is linearly proportional to the

number of channels and channel rate

25

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Providing an uplink• VLC good at broadcast• Uplink difficult to achieve

– Retro-reflectors• Low speed• Low cost

– IR uplink• Separate system• Infrastructure complex and expensive

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Submission

Retro-Reflecting Link• Novel optical communications between reader and tag• Low power (tag has no source)• Long range (determined by illumination source )• Visibly secure (user can see beam of light)

θIlluminating Source

Beamsplitter

ReceiverRetroreflectingTransceiver showing angle of rotation

Reader

Tag

θIlluminating Source

Beamsplitter

ReceiverRetroreflectingTransceiver showing angle of rotation

θIlluminating Source

Beamsplitter

ReceiverRetroreflectingTransceiver showing angle of rotation

Reader

Tag

18

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Submission

Cooperative communications

O'Brien, D.C.: ‘Cooperation and cognition in optical wireless communications’, in Fitzek, M.K.a.F. (Ed.): ‘Cognitive Wireless Networks: Concepts, Methodologies and Visions

- Inspiring the Age of Enlightenment of Wireless Communications -’ (Springer, 2007)

RF transceiver

Base station

VLC transmitter

RF communications

Optical com

munications

RF transceiver

Terminal

VLC receiver

Terminal outside hotspot Terminal within hotspot

RF transceiver 1

Terminal

VLC receive r

RF com

munications

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Submission

Providing an uplink: Cooperative systems

• Combine VLC with RF• Optical downlink only• RF uplink/downlink

– 100Mb/s downlink/10Mb/s RF LAN– Fuzzy logic decision making– Typical traffic asymmetry– Significant performance benefits using combination

Hou-J, and O'Brien-Dc: ‘Vertical handover-decision-making algorithm using fuzzy logic for the integrated Radio-and-OW system’, IEEE Transactions on Wireless Communications, 2006, 5, (1), pp. 176-185

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Submission

Compatibility with lighting• Most modern systems use PWM dimming

– Channel does not exist when light is dimmed• Solutions

– Use modulation scheme that ‘incorporates’ PWM dimming (PPM-like)

– Use sensing to only transmit in active regions– But both reduce overall data rate

• Requirement for closer collaboration with lighting industry.

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Submission

Conclusions

• VLC offers high SNR low bandwidth channel– Naturally suited to broadcast

• Challenges– Data rate– Uplink– Compatibility

• If overcome possibility of low cost method to augment wireless capacity