10 May 2008
D CO’Brien/University of OxfordSlide 1
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 2
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 3
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 4
doc.: IEEE 802.15-15-08 0265-00-0vlc
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Sources
• Blue LED & Phosphor
– Low cost
– Phosphor limits bandwidth
• RGB triplet
– Higher cost
– Potentially higher bandwidth
– Potential for WDM
10 May 2008
D CO’Brien/University of OxfordSlide 5
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 6
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 7
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 8
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 9
doc.: IEEE 802.15-15-08 0265-00-0vlc
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Propagation: conclusions
• Very high SNR available
• Bandwidth of channel >~88MHz
10 May 2008
D CO’Brien/University of OxfordSlide 10
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 11
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 12
doc.: IEEE 802.15-15-08 0265-00-0vlc
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Strategies for High-speed VLC
• Equalization
– Transmitter (pre-) equalization
– Receiver (post-) equalization
• Complex modulation
• Multiple-Input-Multiple-Output (MIMO)
10 May 2008
D CO’Brien/University of OxfordSlide 13
doc.: IEEE 802.15-15-08 0265-00-0vlc
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(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
D CO’Brien/University of OxfordSlide 14
doc.: IEEE 802.15-15-08 0265-00-0vlc
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-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
D CO’Brien/University of OxfordSlide 15
doc.: IEEE 802.15-15-08 0265-00-0vlc
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(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
D CO’Brien/University of OxfordSlide 16
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 17
doc.: IEEE 802.15-15-08 0265-00-0vlc
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(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
D CO’Brien/University of OxfordSlide 18
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 19
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 20
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 21
doc.: IEEE 802.15-15-08 0265-00-0vlc
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MIMO System: Room Test Performance
(Aggregate) 80 Mbit/s parallel transmission
Challenges:
• Non-geometric symmetry
• Channel estimation
10 May 2008
D CO’Brien/University of OxfordSlide 22
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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
D CO’Brien/University of OxfordSlide 23
doc.: IEEE 802.15-15-08 0265-00-0vlc
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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