This presentation gives introduction about Visible Light Communication (VLC) Technology

1. 1
Vinayagam Mariappan
Optical Wireless Communications
eSILICON LABS
(An Embedded Network and Multimedia Solutions Provider)

2. Agenda
Introduction
Applications
White LED
Illuminance Distribution
Channel Model
Challenges and Solutions
2

3. 3
• VLC (Visible Light Communication)
– New communication technology using “Visible Light”
– Wavelength between ~400nm (750THz) and ~700nm (428THz)
• General characteristic
– Visibility : Aesthetically pleasing
– Security : What You See Is What You Send
– Unregulated : No regulation in optical frequency
– Health : Harmless for human body
– Non-interference with other devices
Visible Light Communications (VLC)
visibleIR UV
700nm 400nm100μm
RF
3cm1m 1nm
428THz 750THz3THz10GHz300MHz 300PHz
1mm
300GHz
IrDA
TC 48
802.15.3c
THz
VLC802.11
802.16

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(Ref: Wikipedia)
• Visible light is only a small portion of the electromagnetic spectrum.
Visible Light Communications (VLC)
• Dates back to 1880, when Alexander Graham Bell
invented the photophone
• VLC is used for
– Vehicle to vehicle communication
– Networking in indoor environments

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Indoor VLC
• Can provide network access at
– Home
– Office
– Shopping Center
– Plane
– Hospital
– Convention Centers

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• Advantages
– Safe for health
– Secure
– No interference on RF signals
– High speed
– Confined to small geographical area
• Challenging Problems
– Connectivity while moving
– Multiuser support
– Dimming
– Shadowing
– Confined to small geographical area
Indoor VLC

7. Indoor VLC
7
• Light-emitting-diodes (LED) are preferred sources for dual purpose of lighting
and data communications
– Eye-safety regulations (compared to Laser)
– Longevity
– Lower cost
– Are mercury free
– Less consumption
– High speed
– Have smaller and compact size
– Minimum heat generation
– higher tolerance to humidity
– A much higher energy conversion efficiency (white LEDs with luminous efficacy
greater that 200 lm/W are now available)

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• WiFi
– Has limited capacity, and cannot increase it easily, because it covers a wide
area, services potentially many users, and limited bandwidth. Higher order
modulation of limited use since SNR limited.
– EX., office buildings, conference centers, stadiums
• Bluetooth
– Single user system for personal area communications. Very small range and
low data rate. Less shadowing so good around moving people.
– E.g., wireless microphones
• Millimeter Wave? UWB?
Rival Technologies

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VLC vs. Infrared (IR) and Radio-frequency(RF)

10. White LEDs
10
• Two technologies in white LEDs
– Phosphor-based LEDs
▫ This technique involves the use of blue LED coated with a phosphor layer that emits
yellow light
▫ The phosphor layer absorbs a portion of a short wavelength light emitted by the blue LED
and then the emitted light from the absorber experiences wavelength shift to a longer
wavelength of yellow light
▫ Are cheap and are less complex

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White LEDs
• Trichromatic
– Generates white light by combining red (~625 nm), green (525 nm), and blue
(470 nm) (RGB) in a correct proportion
– Are high-speed
– Enables color control
– Typically, these triplet devices consist of a single package with three emitters
and combining optics
– Are attractive for VLC as they offer the possibility of wavelength division
multiplexing (WDM)

12. White LEDs
12
• The most important factor in VLC is the switching properties of the
visible LEDs
• They have the ability to be switched on and off very rapidly thereby
making it possible to impress data on their radiated optical
power/intensity
• Modulation speed of white LEDs
– is limited due to the relaxation time of the LEDs
– BW of trichromatic LEDs < 20 MHz
– BW of phosphor-based LEDs < 5 MHz

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An illustration of the VLC concept
• This is for the downlink
only, and a parallel
similar system is needed
for the uplink.

14. Signal Distribution
14
• Three main options
– Electrical network – extension of Internet
– Passive optical network (PON)
– Wireless-over-fiber
– Power-line communication system

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A block diagram of a VLC system
• Precise dimming appears to be challenging for incandescent and gas-
discharge lamps
• With LEDs it is quite convenient to accurately control the dimming level
• The illumination requirement is that the illuminance must be 200–1000 lx
for a typical office environment

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• Generally there are 4 configurations for indoor optical links 1
– (a) Directed – line-of-sight (LOS) link
– (b) Non-directed LOS link
– (c) Diffuse link
– (d) Quasi diffuse link
Indoor VLC Configurations
1 H. Elgala, R. Mesleh, and H. Haas, “Indoor
optical wireless communication: Potential
and state-of-the-art,” IEEE Commun. Mag.,
vol. 49, no. 9, pp. 56 – 62, Sep 2011.

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• Since LEDs are used for the dual propose of illumination and communication,
it is necessary to define the luminous intensity and transmitted optical power
• Transmitted optical power indicates the total energy radiated from an LED
• Luminous intensity is used for expressing the brightness of an LED
• Luminous intensity is the luminous flux per solid angle and is given as
– where Φ is the luminous flux and Ω is the spatial angle
• Φ can be calculated from the energy flux Φe as
– where V(λ) is the standard luminosity curve, and Km is the maximum visibility,
which is ~683 lm/W at 555 nm wavelength
Illuminance Distribution in VLC

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• In fulfilling the lighting requirements, a single high luminous efficiency LED
can only provide limited luminous flux and over a limited area
• To illuminate a much larger environment, spatially distributed LED clusters
would be needed
• LED array, and illuminance distribution for
– (b) one transmitter and
– (c) four transmitters
Illuminance Distribution in VLC

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An example of a VLC system

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• Optical power distribution in received optical plane for using four sources and a
FWHM of
– (a) 70°
– (b) 12.5°
– (c) with reflection from walls for 70°
Illuminance Distribution in VLC

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Channel Model
• The output PSD of a white phosphor-based LED (solid line, which
corresponds to the left axis) is compared to the measured spectral reflectance
(which corresponds to the right axis) of plaster and plastic wall (dash-dot line),
floor (dash line) and ceiling (dot line) 1
1 L Kwonhyung, P Hyuncheol and J R
Barry, IEEE Communications Letters, 15,
217–219, 2011.

22. Channel Model
22
• Multipath effect limits the channel bandwidth
• The impulse response of the channel is modeled as a short impulse
(caused by LOS path) followed by a broad pulse (multipath effect/NLOS
path)
• The delay between the two parts is determined by room geometry and
size
• The NLOS part is usually modeled as a Gaussian pulse

23. Performance Limits
23
• System is typically either
– bandwidth limited
– background-light limited (if daylight falls on PD)
• If channel-bandwidth limited, use higher-order modulation or equalizer
• If background-noise limited:
– Shot noise due to receive intensity – nothing can be done
– Decrease symbol rate
• Channel state information (CSI) is needed at the transmitter

24. Signal Processing
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• Optical and electrical filtering:
– Block out-of-band background light
– Remove electrical harmonics
• Equalization required:
– Bandwidth limited by LED response and by multipath
– Types of equalizers:
▫ FIR filters, adapted using an LMS algorithm
▫ Decision-feedback equalizer
▫ MLSE – very complex
• Remove multiuser interference

25. Challenges and Solutions
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• As discussed before, main challenges for indoor VLC systems are
– Connectivity while moving: users need to be connected when they move inside
the indoor environment
– Multiuser support: in large areas is vital, many users need to have access to the
network at the same time
– Dimming: is an important feature in VLC when communications is integrated
with lighting
– Shadowing: happens when the direct paths from user to all sources are blocked
• Some solution has been proposed for each one

26. Challenges and Solutions
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• Solution for connectivity
– This problem is similar to the connectivity problem in cellular network when you
move from one area of the city to another area while speaking with cell-phone
– The solution is called “handover”, using which the user is transferred from one BS
to another
– Handover is done in the area that two BS’s have common coverage
– Similar solution can be used in signal processing domain for VLC
– The user can be transferred from one light source to another in the area that is
under the coverage of both

27. Challenges and Solutions
27
• Solution for multiuser support
– One solution is time division multiplexing
(TDM)
– Each frame is divided into equal time slots
– Each user transmit data in one time slot in a
predefined order
– The other solution is code division multiple
access (CDMA)
– Codes are assigned to users
– Each user transmit its data using the assigned
signature pattern
– It is used in 3G and 4G cellular networks
– CDMA has been adopted and developed for
optical systems
– Optical orthogonal codes (OOC) are used as
signature pattern for users

28. Challenges and Solutions
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• Solution for multiuser support
– Last solution is spatial multiplexing
– Can use to increase data rate or to add users
– Rely on LED arrays and multiple receivers
– Or can use an imaging receiver (camera)

29. Challenges and Solutions
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• Solution for Dimming
– Two main solutions are proposed for solving dimming problem in VLC systems
– Pulse width modulation (PWM) is combined with other modulation schemes in
order to control the duty cycle of the transmitter signal
– By controlling the width of the PWM signaling, the dimming level can be
controlled
– The other solution is using modified forms of PPM
– In these schemes multiple pulses are transmitted instead of one pulse
– By controlling and changing the ratio between the number of pulses and the
length, the dimming level can be altered

30. Challenges and Solutions
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• Solution for shadowing
– As shown before, the impulse
response in VLC systems has two
parts
– When the line-of-sight (LOS) part
(which is received via direct path) is
blocked, the impulse response is
only the second part
– Then the data can be recovered
using the second part which is
indeed the received data from the
indirect paths (multipath signal)

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Thank You