Optical Wireless
Communications
Prof. Brandt-Pearce
Lecture 4
Visible Light Communications
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Visible Light Communications (VLC)
 Introduction
 Applications
 White LED
Illuminance Distribution
 Channel Model
Challenges and Solutions
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Visible Light Communications (VLC)
 Visible light is only a small portion of the electromagnetic spectrum.
(Ref: Wikipedia)
 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
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Home
Office
Shopping Center
Plane
Hospital
Convention Centers
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Indoor VLC
Advantages
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Safe for health
Secure
No interference on RF signals
High speed
Confined to small geographical area
Challenging Problems
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Connectivity while moving
Multiuser support
Dimming
Shadowing
Confined to small geographical area
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Indoor VLC
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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Rival Technologies
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.
 E.g., 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?
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VLC vs. Infrared (IR) and Radio-frequency(RF)
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White LEDs
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)
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White LEDs
 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.
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Signal Distribution
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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Indoor VLC Configurations
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
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.
1
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Illuminance Distribution in VLC
 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
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Illuminance Distribution in VLC
 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
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An example of a VLC system
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Illuminance Distribution in VLC
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°
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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
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1
L Kwonhyung, P Hyuncheol and J R Barry, IEEE Communications Letters, 15, 217–219, 2011.
Channel Model
 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
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Performance Limits
 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
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Signal Processing
 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
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Challenges and Solutions
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
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Challenges and Solutions
Solution for connectivity
 This problem is similar to the connectivity problem in cellular
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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
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Challenges and Solutions
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
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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
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Challenges and Solutions
Solution for multiuser support
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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)
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Challenges and Solutions
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
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Challenges and Solutions
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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