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1
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
(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
Beam reflection (directional)
Source: Discovery Channel
Flame
Probably the first ever applications in visible light communications
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Network Evolution
6
Direct Fiber
Source: NTT
HFC
DS3/E3
Bonded Copper
Bonded T1/E1
Carrier 2
TDM
SONET/ SDH
PON
Wireless[FSO/RF]
Carrier 1
MSO/ Cable
Ethernet
Ethernet
Ethernet
Ethernet
Ethernet
Ethernet
Ethernet
Ethernet
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
Office
Lounge
BedRoom
Indoor Free space Optics and/or Radio
Home Gateway
PLC
cellular
ADSL
FTTH
RLL
Bridge
(Mesh) radio
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
• BluetoothR ~ tens of Mbit/s
• Optical wireless– Infra-red communications – R ~ Gbit/s– Visible light communications – R ~ hundreds of Mbit/s
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OW Apps: Broadband VLC
Indoor broadband broadcasting in Hospital / Supermarket / University / Office
Source: Boston University
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OW Apps: Indoor Broadband
Source: Oxford University (OMEGA project)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Apps: Traffic Communications
FSO
M Kavehrad PSU, USA
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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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
IRUV
Wavelength (m)
No
rma
lise
d p
ow
er/u
nit
wa
vele
ng
th
0
0.2
0.4
0.6
0.8
1
1.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Sun
Fluorescent
Incandescent
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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What is LED?
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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LED – Fundamental
Light Emitting Diode (LED)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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White-Light LED
• LED types:
RGB Blue chip + Phosphor OLED
Well-known technology, limited use, problem with balancing each R, G, B component to create white light
Popular for today general lighting, efficient and cheap
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
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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VLC System
High Signal to Noise Ratio
Signal to Noise ratio: how good signal is!
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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VLC Transceivers
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
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
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)
Inte
nsity
(no
rmal
ised
)
LED frequency response
Blue light
LED temporal impulse response
100ns/div
50ns/div
White light(1) Intrinsic LED modulation
bandwidth is narrow (3MHz)
(2) Blue-part provides wider bandwidth (20 MHz)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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How can we improve the LED frequency response?
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Pre-Equalisation
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)
Res
pons
e (d
B)
Driver 1
Driver 2Driver 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• 80 Mbit/s OOK-NRZ transmission
VLC link configuration
Equalization
BER performance
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Post-Equalisation
LED
DC source
Bias-Tee
Data
PIN
Oscilloscope G > 1
Lens
Blue filter
White-Light Blue-
Light
C
R
1st order equaliser RL
0 10 20 30 40 50 60 70 80-70
-60
-50
-40
-30
Frequency (MHz)
Nor
mal
ized
gai
n (d
B)
Yellow component
Blue componentFitted blue component
White
0 10 20 30 40 50 60 70 80-80
-70
-60
-50
-40
-30
Frequency (MHz)N
orm
aliz
ed g
ain
(dB
)
Blue-filtered bandwidth
Equalised bandwidth
0 20 40 60 80 100 120 14010
-10
10-8
10-6
10-4
10-2
100
Data rate (Mbit/s)
BE
R
White-light
Blue-filteringEqualizer
Simple RC equalisation circuit
3-time BW improvement
Natural BW
Equalised BW
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Complex Modulation - Code
• Pulse Amplitude Modulation (PAM)
• Orthogonal Frequency-Division Multiplexing (OFDM)Orthogonal Subcarriers are used + M-QAMLikely achieved hundreds of Mbit/s
Tx
Rx50 Msym/s 4-PAM
“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
Board with core
Indoor channel
receiver
d1
H
r1
r2
θaφ
receiver
- User is highly mobile- Cellular structure and cell handover strategy are being developed- Cell size and transmit power are optimised
-36
-34
-33-32
-30
-28
-28-28
-28
-28
-28-26
-26
-26
-26
-26
-26
-24
receiver plane coordinate(mm)
rece
iver
pla
ne
co
ord
inat
e(m
m)
-1000 -500 0 500 1000
-1000
-500
0
500
1000
“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)
Pre-equalisation
Post-equalisation
Modulation scheme
Modulation bandwidth
Demonstrated data rate
White channel 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)
Blue channel x 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)
- 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
If the channel matrix H is full rank, it is possible to transmit data in parallel
Parallel transmission: Multiple-Input-Multiple-Output
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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MIMO VLC Channel Matrix
Tx1 Tx2 Tx3 Tx4
4Rx
44434241
34333231
24232221
14131211
HHHH
HHHH
HHHH
HHHH
HIssue: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 channelWhite channel and
equalisation Blue channel
Number of channels 4 16 36 4 16 36 4 16 36
Data rate (Mbit/s) 48 192 432 120 480 1080 160 640 1440
Lens diameter (cm) 0.2 0.44 0.71 0.44 0.8 1.38 1.6 3.6 7.14
Detector size (cm)
0.74x0.74
1.68x1.68
3.05x3.05
1.65x1.65
3.08x3.08
5.91x5.91
6.0x6.0
13.7x13.7
31.4x31.4
Source: Oxford University (Samsung’s project)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Organic LED (OLED)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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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)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OLED structure
Source: Lumiblade
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OLED
Source: Lumiblade, Korea Institute of Industrial Technology
LightingLarge panel better for illumination
larger capacitor value
CommunicationsLarger capacitor value slow response
Electrical modelling (equivalent circuit)
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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OLED
Equalisation approach
Merit: total value of serial capacitors is smaller than individual capacitor value
The external Ceq minimises the effect of OLED capacitance
200 400 600 800 1000
-12
-10
-8
-6
-4
-2
0
Frequency (kHz)
Response (
dB
)
unequalised
Eq1 (390 Ohm, 15nF)Eq2 (820 Ohm, 3.9nF)
OLED: experimentally transmit data at 2 Mbit/s over the original BW of 0.15 MHz
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Other Projects in VLC
• Smart VLC receiver and MIMO• Portable device/Smartphone VLC• Dimming and VLC
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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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
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Acknowledgements
• OCRG group• School of CEIS• Oxford University• OMEGA project• Samsung Electronics
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011
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Thank you
“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011