This document discusses recent developments in optical wireless communications, including visible light communications (VLC). It provides an overview of basic components used in optical wireless systems as well as new developments like VLC using LEDs for simultaneous illumination and data transmission. Research activity in VLC is growing, especially in Japan, with potential applications in mobile devices, illumination systems, and traffic systems. Challenges remain in improving link budgets and integrating VLC into existing wireless networks, but it shows promise as a means of adding wireless capacity where light is already in use.

1. Visible Light Communications
and other developments in Optical Wireless
Dominic O’Brien, University of Oxford, dominic.obrien@eng.ox.ac.uk
Olivier Bouchet, France Telecom, olivier.bouchet@orange-ft.com
Eun Tae Won, Samsung, etwon@samsung.com
Dong Jae Shin, Samsung
Contributions from Communications Group at Oxford

2. Overview
• Optical wireless communications
- Basics
• Status of Optical Wireless (OW) in WWRF
• New developments
- Visible Light Communications
- Infra-red
- MIMO
- IRDA update
• Conclusions and future work
WWRF page 2

3. Basic components: transmitter
Source
• LED or Laser diode
Optics
- Eye safety regulation means that high
power requires sources to be modified
- Three ranges
- Visible (used for both illumination and data
transmission)
Output radiation
- Near infrared
700nm>Wavelength<1400nm- Low cost
Wavelength>1400nm- Eye safe
• Arrays of sources also used
- Used to provide multiple LOS paths,
multiplexed transmission
WWRF page 3

4. Basic components: receiver
• Receiver consists of
- Optical filter
- Rejects ‘out-of-band’ ambient illumination
Input radiation
noise
- Lens system or concentrator Optical
filter
- Collects and focuses radiation
Optical system
- Photodetector (or array of detectors)
Photodetector
- Converts optical power to photocurrent
Incoherent detection
- Preamplifier (or number of preamplifiers) Amplifier
- Determines system noise performance
- Post-amplifier and subsequent processing
Output
WWRF page 4

5. Optical wireless configurations
• Basic configurations either use diffuse or Line-of-Sight (LOS) paths
WWRF page 5

6. System characteristics
• Diffuse • Line-of-sight
- Many paths from Transmitter to - Single path from Transmitter to
Receiver Receiver
- Robust to blocking - Low path loss
Good coverage - No dispersion
- High path loss Very high bandwidth available
- Subject to multipath dispersion - Difficult to provide coverage
WWRF page 6

7. Attributes of optical wireless
• Advantages • Problems
- Bandwidth - Incoherent detection
- 200Thz carrier frequency - Less sensitive than radio
- Security - Available components optimised
- Wavelength allows directive links for fibre-optic applications
using low cost ‘antenna’ (lenses) - Higher performance available if
- Light confined by opaque barriers, optimised for OW
so leakage not a problem - Link blocking
- Interference - Geometrical solutions
- Suitable for RF sensitive - Diffuse channels
environments - Combine with RF
- Potential for low power - Noise from ambient light
- Optical filtering
- Electrical filtering in receiver
WWRF page 7

8. Status of Optical Wireless in WWRF
• Whitepaper on OW for short-range communications complete
- Book of visions 2006
• Opportunities
- Short term
- Point and shoot
• Medium term
- Augmentation of RF capacity
- Optical hotspots
- Similar in nature to 60GHz systems
• Challenges
- Improve link budget
- Optimal integration into 4G infrastructure
• Emerging areas
- Visible light communications
WWRF page 8

9. Visible light communications: background
• Some work using fluorescent lights
• Two emerging areas
- Ilumination moving from incandescent/fluorescent to solid state sources
(LEDs)
- Predicted to become predominant method for room illumination
- Used extensively in traffic systems (traffic signals,rear light cluster)
- Headlight will be possible in future
- Extra wireless capacity available at (potentially) low cost
- Where visible light offers advantage over current methods
- Point and shoot applications
- Offers ‘intuitive’ alignment
WWRF page 9

10. VLC: Activity
• Japanese consortium Visible Light Communications Consortium
(VLCC)[1]
- 21 members
- Phase 1 complete-Low speed demonstrators
- Phase 2 started-High speed communications
• Research
- Mostly in Japan (Keio University)
• Commercialisation
- Prototype systems
[1] www.vlcc.net
WWRF page 10

11. Introduction – Industry trend
Mobile Illumination ITS
PD
White LED
RGB LED
[Max 10 Mbps) [Max 5 Kbps) [Max 5 Kbps)
RGB Filter
Ear phone
Japan shop demo (Mar, 2006)
WWRF page 11

12. VLC Sources
• Blue LED & Phosphor • RGB triplet
- Low cost - Higher cost
- Phosphor limits bandwidth - Potentially higher bandwidth
- Modulation can cause colour shift - Potential for WDM
- Modulation without colour shift
Single chip LED spectrum RGB LED spectrum
WWRF page 12

13. VLC modelling
WWRF page 13

14. Room Power Distribution
• Assume
- 1% modulation of typical
illumination power
- Typical receiver performance
• Conclusions
- Very high SNR available
- SNRmin = 38.50dB
- SNRmax = 49.41dB
- Modulation limited by source
bandwidth
WWRF page 14

15. State of the art in VLC: Networking
• Optical hotspot
Connection to powerline ethernet
VLC
transmitter
IR uplink
High bandwidth
'Hotspot' coverage
area
[1] Komine-T and Nakagawa-M, "Integrated system of white LED visible-light communication and
power-line communication," IEEE Transactions on Consumer Electronics, vol. 49, pp. 71-9, 2003.
WWRF page 15

16. State of the art in VLC
• Short range ‘point and shoot’
- Intuitive alignment allows narrow beams
- Improved power budget
- Aesthetics
[1] Ceatec. exhibition,
"http://www.ceatec.com/en/2004/exhibitors/index.html."
WWRF page 16

17. Feasibility at 4 Mb/s – PC-PC visible link
7mm diameter lens
WWRF page 17

18. Music broadcast demonstrator
4x Transmitter
modulated
Luxeon
Star LEDs
~5Mb/s link
CD player Desklamp Receiver unit
Receiver
WWRF page 18

19. Visible retro-reflecting transceiver
• Light from reader strikes retro-reflecting tag
- returned to the reader over a wide range of
angles of incidence
- Modulating the returned beam allows data to be
transmitted from tag to reader
Tag Reader
• Tag has low power consumption as no source of
radiation required
• Long range
Illuminating Beamsplitter
Source θ
Tag
Receiver
Retroreflecting
Transceiver
Reader showing angle of
rotation
WWRF page 19

20. Potential applications for VLC
LED solid state lighting unit LED solid state lighting unit
Line of sight optical link Visible
Visible RF Wireless/optical optical link
optical link channel
Electronic
PC or portable Noticeboard
terminal
Indoor Visible Light Communications
Content retail
terminal
(DVD etc)
Gb/s content exchange Gb/s content download
Point to point Visible Light Communications
WWRF page 20

21. State of the art in VLC: telematics
• Telematics • Applications
- Visible signals used already - Traffic light-Car Communication [1]
- Vehicle lighting moving to solid- - Car-Car communication via
state sources
- Headlights[2]
Data communication
- Headlight to tail light
Data
communication
[1] Wook-Hbc, Haruyama-S, and Nakagawa-M, "Visible light communication with LED traffic lights using 2D
image sensor," IEICE Transactions on Fundamentals of Electronics, Communications and Computer
Sciences, vol. E89-A, pp. 654-9, 2006.
[2] ISLE, "http://www.pb.izm.fraunhofer.de/p2sa/030_Projects/Optik/Pr_isle.html," 2006.
WWRF page 21

22. New developments: IR
• Infrared data association (IrDa)
- 16Mb/s Very Fast IR (VFIR) products available
- Standards being finalised for
- IrBurst (100Mb/s)
- UFIR (>100Mb/s)
WWRF page 22

23. Improving the data rate: Optical MIMO
• Measure Channel (H) matrix using 'training data'
• Measure data sequence
• Process sequence to obtain estimate
1.5m
3x3
1x2 Laser sources photodiode
array
Schematic of experiment
WWRF page 23

24. Recovered data
• Processing
- Threshold
- 'Majority vote' over bit period
• No errors over 1.5m
1 1
0.9 0.9
Normalised signal level
0.8 0.8
0.7 0.7
0.6 0.6
0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2
0.1 0.1
0 0
2.5 3 3.5 4 4.5 5 5.5 6 2.6 2.8 3 3.2 3.4 3.6 3.8 4
time (s) x -6 -6
x
10 10
Data from source 1 Data from source 2
WWRF page 24

25. Applications comparison
Application
Standard Telematics Networking Point to Point
RF comms Intra-vehicle communications for Wide area coverage using IEEE Bluetooth <1Mb/s
collision avoidance 802.11(g) Ultra-Wide Band 110/480 Mb/s
Communications with infrastructure Bit rates up to 1Gb/s proposed
for tolling Hotspot coverage using 60GHz
systems-bit rates>1Gb/s
High Power consumption
ISO CALM standards under Complex systems required with line Complex modulation schemes and
of sight RF propagation processing
development
IR Intra-vehicle communications for High bandwidth optical ‘hotspots’ Infra-red Data Association (IrDa)
collision avoidance, Standards
Communications with VFIR 16Mb/s (product)
infrastructure for tolling IrBurst 100Mb/s(final vote)
UFIR >100Mb/s (final vote)
‘Luciole’ optical link ~1.5Gb/s(JVC)
ISO CALM standards under High modulation rates
IR has good directivity Low power available Simple baseband transmission
VLC Traffic light to vehicle comms. High bandwidth optical ‘hotspots’ Point to point links with improved link
Intra-vehicle comms for collision margins compared to IrDa
avoidance
Visible light aids intuitive alignment
Visible infrastructure already used High power available thus lowering link loss.
for signalling Low modulation rates. Simple baseband transmission
WWRF page 25

26. Summary and Conclusions
• Visible Light Communications
- High SNR wireless channel
- Intuitive communications
• IR communications
- MIMO techniques
- Faster point and shoot
• Challenges
- Improve link budget
- Integration into ‘4G’ wireless
- Level (Network, MAC)
- Low-cost systems
WWRF page 26