Lighting and Full-Duplex Communication using Optical technology
Charles Lim
1
Department of Electrical Engineering, Ryerson University
Optical Communication – EE 8114
Canada due to aging power grids unable to
support the increasing power demand, cutting
electric consumption through a more efficient
lighting source scheme would save both
countries millions if not billions of dollars. LEDs
are also the main lighting source used in
multimode fibers for optical communications
meaning this ability of LEDs to transmit data can
also be exploited as a means to transfer optical
wireless data while still satisfying its main
purpose as a lighting system.
Abstract
This paper discusses possible ways of data
communication and lighting from the same source,
factors affecting it and a very simple project created to
demonstrate the true possibility of wireless
communications with LEDs. Some of the critical
issues in designing and creating an efficient lighting
and optical communications system are discussed as
well. An array of white and infrared LEDs and
photodiodes are used as transmitters and receivers to
enable lighting and full-duplex communication as well
as communication. Proposals for driving and powering
LEDs can be achieved through new power line
technologies for existing homes or through installed
optical fiber for newer pre-built homes. The lighting
and downlink communication are achieved through
white LEDs as transmitters and photodetectors as
receivers. On the other hand, the uplink
communication uses infrared technology to enable
operation at totally different wavelengths. As more
research occurs, clearly full-duplex communication is
becoming more feasible for in-door wireless
communication systems.
LEDs can be used for communications
in places such as hospitals and aircrafts where
RF electromagnetic interferences are prohibited.
Some major advantages of using LEDs for
communication are no interference with Radio
Frequency (RF) bands, better security
capabilities, fast data rates, long life.
Communications using white LEDs are very
secure compared to RF communication in that it
is confined to an area and can be controlled
separately. With very little maintenance required,
long life as well as dual purpose of lighting and
communication, the initial cost of an LED
system can be offset quickly. For these reasons,
lighting and communication using LEDs as a
source (white LED) are discussed in this
proposal.
Methods to carry data to the LED system:
PLC and LED System
Power lines can be used to power the
LED as well as provide modulating medium. In
existing houses, already installed power lines
and outlets behave as data networks and ports
which can be used to power and modulate LED
lighting systems. In PLC (power line
communications) existing devices must connect
to an outlet to enable communication. With the
combination of LED systems and PLC
equipment, wireless communications anywhere
in the house can be achieved with or without
plugging the device onto an outlet.
Figure 1: Plan of the Room
Introduction
White LEDs, a much more power
efficient device than that of an incandescent
bulb, brings lighting to areas where there is little
power source or efficient use of power is
required. Under properly built and operated LED
systems, with roughly 50,000 or more
continuous hours of life, LEDs easily outlast the
lighting sources such as fluorescent and
incandescent bulbs eliminating material and
maintenance cost. With the recent blackout or
power crisis that occurred in the USA and
The data signal is sent along with power
through the power line and is received and
filtered at the LED circuitry. The filtered data is
then used to modulate the power coming in from
the power line through a power supply circuit
References 1 2
1
that modulates the DC voltage going to the
LEDs. The array of LEDs used to light up the
room will all switch at a high bit rates thus,
preventing human eyes from detecting any
switching. This would allow for easy installation
of wireless LED systems into already built and
even better for pre-built houses.
path is poor then the temperature of the LED
device and the package will be high. If the
junction temperature of the LED goes over 75°C
then the long lifetime doesn’t apply. Typically
for the LED to achieve 25,000 to 50,000 hours
lifetime the junction temperature has to be lower
than 75°C. Most LED devices are limited to a die
junction temperature of roughly 125°C and
anything over that will cause problems to
become common.
The main factor limiting LED
communication speed using PLC technology for
implementation for lighting and communication
are the existing speed of power line modems.
Current modem speeds are in the range of a few
hundred kbps which aren’t exactly the fastest
way to go. The issues limiting the speeds of
PLC include noise mostly generated by electric
appliances. The 3 main classes of noise are:
stationary continuous noise, cyclic stationary
continuous noise, and cyclic impulsive noise
synchronous to mains. Once the PLC speed
increases, higher data rates can be achieved for
LED wireless communication as well.
Packaging material and Integration
Unlike filament based bulbs which
radiate a good deal of the heat away from the
source, LEDs do not. Thus, this requires
conductive cooling as well as proper LED
packaging material. Current LED packages have
a thermal resistance of around 300°C/W which is
terrible for the limitations of 75°C for long
lifetime. If we assume room temperature to be
25°C, then the package must be able to support
at least 50°C. System integrators must also be
careful when designing the system as they have
to balance not only light output, fixture design
but also thermal path design.
Optical Fiber and LED system
Another way to modulate data to power
the LEDs is to route fiber optic cables around the
house beside or with the power cables
themselves. The fiber carries the data stream to
all the power supplies driving the LEDs and this
data stream is used to modulate the power supply
to turn the LED on and off. This is very
advantageous as high data rates can be achieved
everywhere in the house with just one fiber optic
cable running around. No EMI interference is
induced in the cable and since the bandwidth of
the fiber is very high, the extra bandwidth can
also be used to carry not only data signals but
also video, audio, telephone and television
signals to the corresponding devices to serve as
the main data carrier attaching all the relevant
devices in the home.
Multipath dispersion
Multipath dispersion is mainly due to
the large amount of LEDs required to light up
the entire room. For low data rates, multipath
dispersion may not be an issue as the speed of
light and the distance is minimal for lower rate
applications. Multipath dispersion can be easily
taken care of by adaptive digital signal
processing techniques with the complexity of the
algorithm depending on the communication
system requirements. Also, interference from the
white LEDs and IR LED transmitters will be
minimized due to spectral separation. The
narrow linewidth of the LED for downlink and
the IR for uplink allows for good spectral
separation between both the downlink and uplink
systems. Signals from white LEDs will only
mainly cause multipath dispersion with the white
light photo receivers as well as the IR signals for
the IR photodiode.
The disadvantage of routing fiber optic
cables is that it is expensive for already existing
houses to place the cables and unnecessary. This
type of method is more suited for custom houses
or designs where in the houses are still in the
process of being built and additional installation
of fiber optics into the houses will not cause too
much trouble.2
Reflectivity of the room
Increasing the amount of reflective
materials in the room will increase the power of
the received reflected signals increasing BER.
Even if we only assume we take into account the
first reflection(as the second reflections will be
of much lower intensity) this could still be a
problem or issue causing dispersion for high
speed systems. This problem can also be easily
taken care of by the adaptive digital signal
processing techniques as well.
Factors affecting LED lighting and
communication systems:
Thermal temperature
LEDs are generally low power devices
and typically consume 1W, thus if the thermal
References 1 2 4 5
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uplink design is to use diffused IR to allow for
spectral separation of the downlink and uplink
carriers. The IR transmitter will be on the device
while a single IR receiver will be installed along
with the array of white LEDs for every ceiling
section of the room to ensure full coverage of the
area. Depending on how large the room is, it will
determine how many IR detectors are needed to
be installed.
Infrared and visible light are free of
government regulations, immune to radio
interference and secure (cannot penetrate walls).
The key factor in infrared is its spectral region; it
offers a virtually unlimited bandwidth while
another key factor for white LEDs is the
abundance of optical power throughout the room
and the ability to modulate it at high data rates.
Downlink/Uplink bit rate
The maximum downlink bit rate is
determined by the LED power and receiver
circuitry which should not be an issue. The
uplink data rate needs to be minimal due to the
fact that the uplink system consisting of a few
diffused IR transmitters cannot support high data
rates unlike the vast number of LEDs used to
light up the room. For most home usage
purposes like the internet and television, this
should not be a problem as data is usually
downloaded to the home user. Uplink data rate
can afford to be minimal as possible since for
normal home purposes, uplink is not that critical
unless it is used for a server.
LED Power
LED power is not only critical in
lighting the room but also in ensuring the optical
receiver is able to receive enough power to
support the current data rate. The optical transfer
function is given by
Optical TF = (PR / PE)
where
PE = PR(h2)/(Acos) * sin2()
 = angle between the radiation
axis and the normal to the receiver area.
 = /2 for a lambertian source
A= area exposed at the rx
emitted from the transmitter light.
h[m] = distance of the rx to tx
With all this in mind, optical signals(IR
and visible light) like other light sources also
have some interference when transmitting or
receiving signals. The major sources of
interference affecting infrared are; sunlight, and
indoor light. Assuming we are operating an LED
system throughout the house, IR interference
will be minimal unless a large amount of
sunlight is allowed inside the area. As the
lighting mechanism is powered by the LED with
a narrow linewidth limited to the visible band, IR
wavelengths
should
encounter
minimal
interference. Same can be said of the white LEDs
and as long as the visible light receiver is not
placed directly on the sunlight there should be
minimal interference with the downlink signal as
the optical power generated by the white LEDs
are good. To ensure all possible cases of
interferences are checked and to increase the
reliability of the system an LMS adaptive digital
signal processing technique is proposed.
3
In the Lambertian emission, radiant
intensity Pointer depends on the angle of
radiance .
Ptr() = (m+1)Ps [Cosm() / 2]
Ps = total power transmitted
m = directivity of emission
pattern.
Also,
PR = wAsin2(FOV)
FOV = receiver field of view
0 o < FOV  90o
w = uniform radiant emittance
The received optical power is independent of
position and angular orientation of the
photodetector with respect to the radiating
surface. (Surface should cover the entire FOV).
DOWNLINK USING WHITE LEDs UPLINK
USING INFRARED:
Figure (2): Optical Power Spectral Densities
of common light sources.
The design of the downlink is to use the
existing white LEDs used primarily for
luminance to modulate the transmitted signal and
get received on the receiver of the device. The
Simple LED Downlink System using RS232
References
To prove the theory of LED lighting
and communication, a simple circuit was
designed utilizing RS232 serial communications,
1
3
TX
LED
a couple of superbright white LEDs as well as
visible light photodiodes. RS232 is a well known
serial
communications
technology
and
components to interface to this technology are
easier to find, as such, this has been chosen to be
the interface to the computer.
Figure 3: Block diagram of simple LED system
Conclusion
Depending on the nature and
requirements of the LED system, a PLC or
optical fiber can be used in conjunction with
LEDs to bring about a wireless home or business
network. There are several key factors affecting
the design and use of LED systems which
include thermal issues, packaging, dispersion,
reflectivity, bit rate, adaptive DSP techniques,
receiver and transmitters as well as area. Indoor
lighting and downlink transmission can be
achieved through the installation of an
appropriate amount of LEDs into the room. The
mobile computer will have a visible light
photodiode and a near IR LED to enable fullduplex operation anywhere within the room. As
such, the design of this optical wireless network
is plausible and advantageous to the average
home user and even small LANs as the cost is
cheap yet provides full-duplex communication
and lighting. Optical wireless technology is
finally becoming a real contender to RF
technologies. The need for separate lighting and
communication equipment is no longer required
as well as RF interference and restrictions. As
the costs become cheaper and the efficiency of
LEDs increase, it is only a matter of time before
optical wireless technology becomes a reality for
everyone.
TX Circuit
The transmitter circuit was realized by
driving the white LEDs according to their
forward bias current using the correct resistor
values all attached to the emitter of the same
transistor. The transistor gets turned on and off
by the incoming RS232 signal generated by the
computer serial port.
RX Circuit
The receiver circuit was realized by
using 3 visible light photodiodes and an op-amp
used as a comparator circuit. The comparator
inverting terminal is referenced by a known
voltage level with the non-inverting terminal
voltage being controlled by the photodiode
circuit.
Any changes in the photodiode
voltage(i.e. data from light received) causes the
comparator to switch high or low.
Glue Logic
To interface both the transmitter circuit
to the serial port circuit. A serial RS232 line
transceiver was used to convert the input of the
RS232 to logic that can drive the LED transistor
circuit. On the other side, the receive input of the
transceiver was attached to the output of the
photo-detector circuit and is used to drive the
connecting receiver pin on the RS232 port.
References:
[1] Bala Balasingam, Charles Lim, Gajendran Wignarajah,
Jeyacanthan Nesarajah, Sarmila Selvaratnam,
“Lighting and Communication from the same source”,
2003 ICUE (International Conference for Upcoming
Engineers)
Observations
The simple design works although at a
limited data rate. This is due to the very simple
design circuitry on the receiver section block.
The distance between the LED and the receiver
is4 limited as well due to the fact only 10 LEDs
were used for the transmitter section block. To
increase data rate, the receiver design as well as
components need to be high speed and
optimized. To increase the distance between the
LED and photo-detector, additional LEDs must
be added to increase the total power output to
allow for more range as well as illumination.
[2] Toshihiko Komine, Masao Nakagawa, “Integrated system
of white LED visible-light communication and power-line
communication”, 2002 IEEE
[3] Yuichi Tanaka, Shinichirou Haruyama, Masao
Nakagawa, “Wireless Optical Transmissions with white
colored LED for Wireless Home Links” , 2000 IEEE
[4] Tsunemasa Taguchi, Yamaguchi University,
“Light Gets Solid”, OE Magazine, The Monthly
Publication of SPIE October 2003
[5] Srinath Aanegola, Jim Petroski, Emil Radkov,
GELcore LLC, “Let There Be Light”, OE Magazine,
The Monthly Publication of SPIE October 2003
References 1 2 3 4 5
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