1 Visible Light Communications Hoa Le Minh and Zabih Ghassemlooy Optical Communications Research Group (OCRG) School of Computing, Engineering and Information.

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

[email protected]

(ERASMUS Framework)

“Gheorghe Asachi” Technical University of Iasi, Iasi, Romania, 13/06/2011

mailto:[email protected]

2

Presentation Outline

• Optical wireless communications backgrounds

• Visible Light Communications– Light Sources– Current technologies– Challenges

• Organic Light Source• Summary


3

Why Optical Wireless?

RF spectrum: crowded, expensive

OW spectrum:free, large bandwidth


4

Optical Wireless Applications(short range)

• Traffic Communications• Public data broadcasting• Indoor broadband broadcasting in

Hospital / Supermarket / University / Office

• Home Access Networks• Military Communications


5

Applications

Beam reflection (directional)

Source: Discovery Channel

Flame

Probably the first ever applications in visible light communications


6

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


7

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


8

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


9

OW Apps: Broadband VLC

Indoor broadband broadcasting in Hospital / Supermarket / University / Office

Source: Boston University


10

OW Apps: Indoor Broadband

Source: Oxford University (OMEGA project)


11

Apps: Traffic Communications

FSO

M Kavehrad PSU, USA


12

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


13

VISIBLE LIGHT COMMUNICATIONS

Main purpose: General Lighting Added Value: Communications


14

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


15

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


16

What is LED?


17

LED – Fundamental

Light Emitting Diode (LED)


18

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


19

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


20

VLC System

High Signal to Noise Ratio

Signal to Noise ratio: how good signal is!


21

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


22

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)


23

How can we improve the LED frequency response?


24

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


25

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


26

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


27

Complex Modulation - Diversity

• Space Pulse Amplitude Modulation (SPAM)


28

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


29

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


30

Gigabit VLC

If the channel matrix H is full rank, it is possible to transmit data in parallel

Parallel transmission: Multiple-Input-Multiple-Output


31

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


32

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)


33

Organic LED (OLED)


34

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)


35

OLED structure

Source: Lumiblade


36

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)


37

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


38

Other Projects in VLC

• Smart VLC receiver and MIMO• Portable device/Smartphone VLC• Dimming and VLC


39

Remaining Challenges

• Higher data rate?• Uplink communications?• Light dimming (asynchronous

transmission)?• Heat dissipation?


40

Conclusions

Optical Wireless Communications is an emerging

technology that truly delivers data at very high rate with fibre-like quality


41

Acknowledgements

• OCRG group• School of CEIS• Oxford University• OMEGA project• Samsung Electronics


42

Thank you


1 Visible Light Communications Hoa Le Minh and Zabih Ghassemlooy Optical Communications Research Group (OCRG) School of Computing, Engineering and Information.

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