10 May 2008 Project: IEEE P802.15 Working Group …...10 May 2008 Slide 2 D CO’Brien/University of Oxford doc.: IEEE 802.15-15-08 0265-00-0vlcSGVLC VLC with white-light LEDs: strategies

10 May 2008

D CO’Brien/University of OxfordSlide 1

doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

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: [VLC with white-light LEDs: strategies to increase data rate]

Date Submitted: [10 May 2008]

Source: [D C O’Brien] Company [University of Oxford]

Address [Department of Engineering Science, Parks Road, Oxford, OX1 3PJ,UK]

Voice:[+441865273916], FAX: [+441865273906], E-Mail:[[email protected]]

Abstract: [Presentation on techniques to improve transmission data-rate for VLC systems that use white-

light LEDs]

Purpose: [Information]

Notice: 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.

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

VLC with white-light LEDs: strategies to

increase data rate

Dominic O’Brien

Hoa Le Minh

Lubin Zeng

Grahame Faulkner

University of Oxford

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Contents

• The VLC link– Sources– Propagation

– Receiver

• Strategies to increase data rate– Pre-equalisation

– Post-equalisation

– Complex modulation– Parallel transmission (optical MIMO)

• Conclusions

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Sources

• Blue LED & Phosphor

– Low cost

– Phosphor limits bandwidth

• RGB triplet

– Higher cost

– Potentially higher bandwidth

– Potential for WDM

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doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Sources: Phosphor-based LED Emitter

Rs = 0.9727 ΩΩΩΩ, L = 33.342 nHCs = 2.8 nF, Cd = 2.567 nF, tt = 1.09 ns

sR d

VL

sC

dC

V

I

Spice model

0 10 20 30 40 50 60-30

-25

-20

-15

-10

-5

0

5

Frequency (MHz)

Norm

aliz

ed r

esponse (

dB

)

Blue response

White response

LED frequency response

Blue filtering

LED temporal impulse response

100ns/div

50ns/div

(1) Intrinsic LED modulation

bandwidth is narrow

(2) Blue component offers

wider bandwidth

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Sources: typical bandwidths

• Available bandwidth– LED modulation bandwidth is narrow ~3 MHz

– Blue-part has wider bandwidth ~12-20 MHz (dependent on devices)

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Propagation: modelling

A typical geometry for indoor VLC

- Transmitter: LEDs, lens and driver

- Channel: LOS and diffuse paths

- Receiver: Optics, PD and amplifiers

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Propagation: summary

• Power– Illumination levels ensure strong communications

signal

– Typical signal to noise ratio of >40dB

• Bandwidth– Channel bandwidth potentially affected by

• Inter-symbol interference from multiple line of sight paths

• Diffuse reflections from surfaces

– Modelling indicates bandwidth >88MHz[1] within ‘typical’ room

[1]J. Grubor et al., High-speed wireless indoor communication via visible light, ITG Fachbericht, Vol. 198

(2007), pp. 203-208.

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Propagation: conclusions

• Very high SNR available

• Bandwidth of channel >~88MHz

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doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Receiver

• Bandwidth set by photo-detector and preamplifier combination• Constraints

– Increasing area increases collected power• Increased capacitance therefore reduced bandwidth

• Examples– 20mm2 bootstrapped APD receiver (155Mb/s -40dBm OOK 1E-9

BER)[1]

– 14.4mm2 PIN diode receiver using commercial transimpedanceamplifer- bandwidth of 77MHz (100Mb/s -27dBm OOK 1E-9 BER)[2]

• Conclusion– Receiver bandwidths of up to 100MHz available with ‘reasonable’

collection areas

– Greater bandwidths more challenging

[1] McCullagh-Mj and Wisely-Dr, "155 Mbit/s optical wireless link using a bootstrapped

silicon APD receiver," Electronics Letters, vol. 30, pp. 430-2, 3 March 1994.

[2] Khoo-SH (DPhil Thesis, University of Oxford)

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Summary of VLC link properties

• Data rates limited by LED characteristics for bandwidths <100MHz

• Channel and receiver constraints need consideration for bandwidths>100MHz

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Strategies for High-speed VLC

• Equalization

– Transmitter (pre-) equalization

– Receiver (post-) equalization

• Complex modulation

• Multiple-Input-Multiple-Output (MIMO)

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

(Pre-) Equalization: Multiple Resonant LEDs

Input

data signal

Modulators LED array

LPF

Pre-

amplifier PD

L DC

R

Concentrator

Optical receiver

Transmitter

Recovered

data signal

DC arm

Inductance (Lseries) High-speed buffer

Resonant Capacitor (C)

DC bias current from Laser driver

Luxeon LED, R Signal

Bias Tee A

Z

Resonant modulation circuit

Multiple-resonant 16-LED VLC demonstration system

• Combination of the responses from multiple LED devices being driven at

different resonant frequencies larger VLC bandwidth

0 5 10 15 20 25 30 35 40-30

-25

-20

-15

-10

-5

0

5

Frequency (MHz)

Norm

aliz

ed response (dB

)

(1)

(2)

(3)

( ) ( )∑=

=N

iiiGaF

0

ωω

Equalized bandwidth

0 5 10 15 20 25 30 35 40-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

Frequency (MHz)

Norm

aliz

ed r

esponse (

dB

)

Measured resonant LED responses

Raw LED BW

Measured resonant peak curve

Fitted resonant peak curve

Resonant peaks

G1(ω) G2(ω) G3(ω)

G4(ω) G5(ω)

G6(ω) G7(ω)

G8(ω)

Resonant responses

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

-20

-25

-30

-35

-40

-25

x coordinate(m)

y c

oord

inate

(m)

Received power in dBm/cm2

0.5 1 1.5 2 2.5 3 3.5 4

0.5

1

1.5

2

2.5

3

3.5

4

• Link performance

30 35 40 45 5010

-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Data rate (Mbit/s)

BE

R

L = 2m

L = 2.5m

40 Mbit/s OOK-NRZ in standard room lighting condition [2]

Receiving power plane-distribution BER performance

[2] H. Le-Minh, D. C. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y. Oh, ”High-Speed Visible Light Communications Using

Multiple-Resonant Equalization”, accepted for publication in IEEE Photonics Technology Letters.

(Pre-) Equalization: Multiple Resonant LEDs

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

(Pre-) Equalization: Single LED Link

Data

LED

DC

current

source

Bias-

Tee C1

R1

PIN

Oscilloscope

Amplifier

Concentrator

Blue

filter Pre- Equalizer

C2 Beam-

shaping lens

R2

driver 1

driver 2

driver 3

White

light Blue

light

0 10 20 30 40 50 60-70

-65

-60

-55

-50

-45

-40

Frequency (MHz)

Response (

dB

)

Driver 1

Driver 2

Driver 3

LED bandwidth

10 20 30 40 50 60 70 80 90 10010

-6

10-5

10-4

10-3

10-2

10-1

Data rate (Mbit/s)

BE

R

Blue-filtering

Pre-Equalization

• 45 MHz equalized bandwidth achieved (3 drivers)

• 80 Mbit/s OOK-NRZ transmission [3]

[3] H. Le-Minh, D. C. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y.

Oh, ” 80 Mbit/s Visible Light Communications Using Pre-Equalized

White LED”, submitted to European Conference on Optical

Communications (ECOC 2008)

• Single LED is driven by multiple resonant driver branches + blue-filtering at receiver

VLC link configuration

Equalization

BER performance

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Complex Modulation• High SNR

– Potential for complex modulation– Driving devices potentially challenging

• DMT/OFDM– Link of (equivalent data-rate) 101-Mbit/s is

demonstrated using 20-MHz bandwidth [4]• M-PAM

– Initial demonstrations

[4] Grubor, J., et al., "Wireless high-speed data transmission with phosphorescent white-light LEDs", Proc. European

Conference on Optical Communications (ECOC 2007) (PDS 3.6), pp. 1-2. ECO [06.11], Sep. 2007, Berlin, Germany

50 Msymbol/s 4-PAM VLC link (from [3])

(100 Mbit/s equivalent NRZ rate)

Tx

Rx

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

(Post-) Equalization• LED Impulse response measured

– Fall time of devices >> Rise time– Equalization of exponential decay

Fitted response

Equalization

Equalization process Bandwidth performance

OOK-NRZ data rate is increased from 16 Mbit/s to 35 Mbit/s [1]

[1] L. Zeng, D. C. O’Brien, H. Le-Minh, K. Lee, D. Jung and Y. Oh, “Improvement of Data Rate by Using Equalization in an Indoor VLC System“,

IEEE International Conference on Circuits and Systems for Communications 2008 (IEEE ICCSC 2008), Shanghai, China, May 2008

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Equalisation summary

• Pre-equalisation– Possible with single or multiple LEDs

– Substantial bandwidth improvement

– Issues• Energy efficiency

• Driver complexity

• Effect of device variation

• Post-equalisation– Simulations indicate substantial improvement

– Preliminary experimental results promising

– Attractive as no complex LED drive circuitry

• Post-equalisation preferable from complexity point of view– Unclear as to which offers best performance

– Combination of pre-and post offers substantial improvements (in simulation)

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

MIMO using VLC

• Many sources offers the potential for parallel data transmission

– 1Gb/s parallel ‘proof-of concept’ by VLCC

• Would normally require careful alignment of sources and detectors

• MIMO processing allows signals to be recovered without precise alignment

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Multiple-Input-Multiple-Output System

Tx1 Tx2 Tx3 Tx4

4××××Rx

=

44434241

34333231

24232221

14131211

HHHH

HHHH

HHHH

HHHH

H

• Channel matrix H needs to be estimated at different receiver positions

• Simulation shows that data rate is linearly increased with the rank of H

• Geometric symmetry reduces the rank

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

MIMO System: Room Test Performance

(Aggregate) 80 Mbit/s parallel transmission

Challenges:

• Non-geometric symmetry

• Channel estimation

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

MIMO summary

• Initial results show linear capacity growth

• Possibility of increasing capacity by transmitting data

• Not possible at all locations due to symmetry of H-matrix– Work to develop a receiver optical system that

addresses this issue underway

10 May 2008


doc.: IEEE 802.15-15-08 0265-00-0vlc

SGVLC

Conclusions

• VLC has the potential to offer high data rates

– 100Mb/s either demonstrated or simulated using a number of different techniques

• Data rates of Gbit/s possible with more

advanced techniques

• Further work required on

– Development of each technique

– Comparison of alternatives

Documents

10 May 2008 Project: IEEE P802.15 Working Group …...10 May 2008 Slide 2 D CO’Brien/University of Oxford doc.: IEEE 802.15-15-08 0265-00-0vlcSGVLC VLC with white-light LEDs: strategies