Visible Light Communications
Hoa Le Minh and Zabih Ghassemlooy
Optical Communications Research Group (OCRG)
School of Computing, Engineering and Information Sciences
Northumbria University, United Kingdom
hoa.le-minh@northumbria.ac.uk
(ERASMUS Framework)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Presentation Outline
• Optical wireless communications backgrounds
• Visible Light Communications
– Light Sources
– Current technologies
– Challenges
• Organic Light Source
• Summary
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Why Optical Wireless?
RF spectrum:
crowded, expensive
OW spectrum:
free, large bandwidth
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Optical Wireless Applications
(short range)
• Traffic Communications
• Public data broadcasting
• Indoor broadband broadcasting in Hospital
/ Supermarket / University / Office
• Home Access Networks
• Military Communications
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Applications
Probably the first ever applications in visible light communications
Flame
Beam reflection (directional)
Source: Discovery Channel
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Network Evolution
HFC
Direct Fiber
Ethernet
MSO/ Cable
Ethernet
Ethernet
Wireless
[FSO/RF]
SONET/
SDH
Carrier 1
Ethernet
Bonded Copper
Ethernet
TDM
PON
Carrier 2
Ethernet
Source: NTT
Ethernet
Bonded
T1/E1
DS3/E3
Ethernet
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Ethernet
High speed data delivered to home/office/premise  need ultrafast home access networks
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Apps: Home Access Network
Bridge
Office
(Mesh)
radio
BedRoom
Lounge
PLC
cellular
Indoor Free space Optics
and/or Radio
Home
Gateway
ADSL
RL
L
FTTH
Power line, radio, visible light and infrared communications
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Home/Office Wireless Network
• WiFi
a/b/g/n – data rate R up to hundreds of Mbit/s
• Bluetooth
R ~ tens of Mbit/s
• Optical wireless
– Infra-red communications – R ~ Gbit/s
– Visible light communications – R ~ hundreds of Mbit/s
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OW Apps: Broadband VLC
Source: Boston University
Indoor broadband broadcasting in Hospital / Supermarket / University / Office
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OW Apps: Indoor Broadband
Source: Oxford University
(OMEGA project)
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Apps: Traffic Communications
FSO
M Kavehrad
PSU, USA
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Research in VLC
• VLCC (Casio, NEC, Panasonic Electric Works,
Samsung, Sharp, Toshiba, NTT, Docomo)
• OMEGA (EU Framework 7)
• IEEE 802.15 Wireless Personal Area Network standards
• Boston University
• Siemens
• France Telecom
• Oxford University
• Edinburgh University
• Northumbria University
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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VISIBLE LIGHT COMMUNICATIONS
Main purpose: General Lighting
Added Value: Communications
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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General Lighting Sources
• Incandescent bulb
– First industrial light source
– 5% light, 95% heat
– Few thousand hours of life
• Fluorescent lamp
– White light
– 25% light
– 10,000s hours
• Solid-state light emitting diode (LED)
– Compact
– 50% light
– More than 50,000 hours lifespan
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Light Source Spectrum
Normalised power/unit wavelength
1.2
Sun
Incandescent
1
0.8
Fluorescent
0.6
0.4
0.2
0
UV
IR
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
Wavelength (m)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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What is LED?
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LED – Fundamental
Light Emitting Diode (LED)
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White-Light LED
• LED types:
RGB
Well-known technology, limited
use, problem with balancing
each R, G, B component to
create white light
Blue chip + Phosphor
Popular for today general
lighting, efficient and cheap
OLED
New technology,
expensive and short life
time. It is, however, very
potential
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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VLC System
Key Attributes
- Secured communications: “you
receive what you see”
- Immunity to RF interference
- Signals are easily confined
- Unlicensed spectrum
- Visible light meets eye-safe
regulation
- Green communications
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VLC System
High Signal to Noise Ratio
Signal to Noise ratio: how good signal is!
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VLC Transceivers
Transmitter
Optical receiver
Concentrator PD
Input
data signal
Preamplifier
LPF
L
DC
R
Recovered
data signal
Modulators LED array
Resonant
Capacitor (C) Bias Tee
A
High-speed
buffer
Inductance (Lseries)
DC arm
Signal
Luxeon LED, R
Z
DC bias current
from Laser driver
Individual LED driving circuit
DC current: for illumination (provide sufficient brightness)
Signal: Data for communications
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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LED Frequency Response
LED frequency response
LED temporal impulse response
100ns/div
(1) Intrinsic LED modulation
bandwidth is narrow (3MHz)
(2) Blue-part provides wider
bandwidth (20 MHz)
50ns/div
White
light
Blue
light
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How can we improve the LED
frequency response?
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Pre-Equalisation
Equalization
driver 1
driver 2 C
1
Data
driver 3
R1
R2
BiasTee LED
C2
Pre- Equalizer
Beamshaping lens
DC
current
source
White
light
Blue
light
PIN Amplifier
Concentrator
Blue
filter
VLC link configuration
Oscilloscope
BER performance
-1
10
Blue-filtering
Pre-Equalization
-2
10
-3
10
BER
• 45 MHz equalized bandwidth achieved
• 80 Mbit/s OOK-NRZ transmission
-4
10
-5
10
-6
10
10
20
30
40
50
60
70
Data rate (Mbit/s)
80
90
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Post-Equalisation
Bias-Tee
LED
Blue
filter
-30
BlueLight G > 1
Oscilloscope
Data
R
st
PIN
1 order
equaliser
Lens
DC
source
Yellow component
Blue component
Fitted blue component
White
C
RL
Normalized gain (dB)
WhiteLight
-40
-50
-60
-70
0
10
20
30
40
50
Frequency (MHz)
Simple RC equalisation circuit
0
80
-30
-2
Normalized gain (dB)
White-light
Blue-filtering
Equalizer
10
-4
10
BER
70
Natural BW
10
-6
10
-8
-40
-50
-60
-70
Blue-filtered bandwidth
Equalised bandwidth
10
-80
-10
10
60
0
20
40
60
80
Data rate (Mbit/s)
100
120
140
0
10
20
30
40
50
Frequency (MHz)
60
70
80
Equalised BW
3-time BW improvement
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Complex Modulation - Code
• Pulse Amplitude Modulation (PAM)
Tx
Rx
50 Msym/s 4-PAM
• Orthogonal Frequency-Division Multiplexing (OFDM)
Orthogonal Subcarriers are used + M-QAM
Likely achieved hundreds of Mbit/s
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Complex Modulation - Diversity
• Space Pulse Amplitude Modulation (SPAM)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Cellular VLC
Transmitter
θa
d1
φ
100 0
-34
H
Indoor channel
re ceiver p lan e co o rd in ate(m m )
r1
Board with core
-33
-3 2
500
-28
-26
-28
-28
0
-2 6
-2 8
-24
-26
-26-28
-28 -2 6
-500
-26
-30
-36
-1000
receiver
receiver
r2
-1000
-500
0
50 0
receiver p lan e co o rd in ate(m m )
10 00
- User is highly mobile
- Cellular structure and cell handover strategy are being developed
- Cell size and transmit power are optimised
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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High speed VLC
• Summary of strategies to achieve high
speed VLC (single channel)
Preequalisation
Postequalisation
White channel
Modulation
scheme
Modulation
bandwidth
Demonstrated data rate
OOK-NRZ
2 MHz
10 Mbit/s (BER < 10-6)
White channel
x
OOK-NRZ
25 MHz
40 Mbit/s (BER < 10-6)
Blue channel
x
OOK-NRZ
45 MHz
80 Mbit/s (BER < 10-6)
OOK-NRZ
50 MHz
100 Mbit/s (BER < 10-9)
Blue channel
DMT-QAM
25 MHz
100 Mbit/s (BER < 10-6)
Blue channel
DMT-QAM
50 MHz
231 Mbit/s (BER < 10-3)
Blue channel
x
- Bandwidth expansion: equalisation
- High bandwidth efficiency: complex modulation
- SNR and system dynamic range must be large to support both approaches
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Gigabit VLC
Parallel transmission: Multiple-Input-Multiple-Output
If the channel matrix H is full rank, it is possible to transmit data in parallel
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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MIMO VLC Channel Matrix
Tx1
Tx2
Tx3
Tx4
4Rx
 H 11

H 21
H  
 H 31

 H 41
H 12
H 13
H 22
H 23
H 32
H 33
H 42
H 43
H 14 

H 24

H 34 

H 44 
Issue:
If there is a geometry symmetry
rank(H) < 4
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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MIMO VLC Performance
White channel
Number of
4
16
channels
Data rate
48
192
(Mbit/s)
Lens diameter
0.2
0.44
(cm)
Detector size 0.74x 1.68x
(cm)
0.74 1.68
White channel and
equalisation
Blue channel
36
4
16
36
4
16
36
432
120
480
1080
160
640
1440
0.71
0.44
0.8
1.38
1.6
3.6
7.14
3.05x
3.05
1.65x
1.65
3.08x
3.08
5.91x
5.91
6.0x
6.0
13.7x
13.7
31.4x
31.4
Source: Oxford University
(Samsung’s project)
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Organic LED (OLED)
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OLED
• OLEDs:
– Invented by Kodak in the 1980s
– Intended for use in screens (brighter, thinner, faster, lighter and
less power consumption than LCDs)
– Produced in large panels that illuminate a broad area.
– Can be flexible with the relevant plastic substrate (create
different shape)
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OLED structure
Source: Lumiblade
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OLED
Electrical modelling (equivalent circuit)
Lighting
Large panel  better for illumination
 larger capacitor value
Communications
Larger capacitor value  slow response
Source: Lumiblade, Korea Institute of Industrial Technology
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OLED
0
-2
Response (dB)
Equalisation approach
-4
-6
-8
-10
-12
unequalised
Eq1 (390 Ohm, 15nF)
Eq2 (820 Ohm, 3.9nF)
200
400
600
Frequency (kHz)
800
1000
Merit: total value of serial capacitors is smaller than individual capacitor value
 The external Ceq minimises the effect of OLED capacitance
OLED: experimentally transmit data at 2 Mbit/s over the original BW of 0.15 MHz
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Other Projects in VLC
• Smart VLC receiver and MIMO
• Portable device/Smartphone VLC
• Dimming and VLC
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Remaining Challenges
•
•
•
•
Higher data rate?
Uplink communications?
Light dimming (asynchronous transmission)?
Heat dissipation?
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Conclusions
Optical Wireless Communications is an emerging
technology that truly delivers data at very high rate with
fibre-like quality
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Acknowledgements
•
•
•
•
•
OCRG group
School of CEIS
Oxford University
OMEGA project
Samsung Electronics
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Thank you
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