Data transmission through visible light communication (li fi)

Digital Communication EED 350 Project Report
1
Data transmission through the visual medium
Aditya Gupta (ag794@snu.edu.in)
Aayush Kulshrestha (ak235@snu.edu.in)
Anmol Raj (ar498@snu.edu.in)
Omkar (os847@snu.edu.in)
Prabhat Goel (pg953@snu.edu.in)
Monika Singh (ms391@snu.edu.in)
Kirthana Sukumaran (ks242@snu.edu.in)
Maliha Taj (mt468@snu.edu.in)
Jhayashree Ramakrishnan (jr214@snu.edu.in)
Surya naga chanakya Peddada (sp282@snu.edu.in)

2
Division of Work:
1. Anmol and Omkar – Hardware implementation
2. Maliha and Kirthana – Characteristics of LED and its limitations
3. Aayush and Prabhat – Modulation Schemes
4. Monika and Jhayashree – Channel Modeling Techniques
5. Aditya and Chanakya – Study of Audio Transmission
Timeline of Implementation
Our idea was to divide the project into two parts – research and planning, implementation.
We decided to use the time till the mid semester break to all try and find a good project that is
not feasible, but also of some real world value. Once we had decided on the project, we began
with the division of work and will now begin with the implementation.
Important Dates
 26th
August – First Meeting – Introductions
 2nd
September – Presentation of Various Topic Ideas
 9th
September – Final decision on Project Topic
 17th
September – Presentation and explanation of project to everyone
 22nd
September – Division of Work
 19th
October – Report discussion and beginning of work
 18th
November – Division of material
 21st
November – Progress Report
 23rd
November – Meeting Sir
 25th
November – Confirmation of first Prototype aWorkging
 29th
November – Decision to use Audio as well
 2nd
December – Audio implemented
 9th
December – Presentation Preperation

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Abstract:
To transfer data from one device to another using the visual medium as the route of
transmission. Our project involves transferring data through Visible light communication
(VLC). We designed and implemented VLC transceiver circuit and verified the transmission
of data using DSO in Prototype I. We implemented the Audio transmission of data through
VLC in Prototype II. We studied the characteristics of LED, its limitations and studied the
different modulation techniques, discussed the issues and challenges faced in implementing
the hardware design. We have defined the problem of nonlinearity of LED’s and have studied
to mitigate this problem. Based on change in the signal characteristics due to several
phenomena from LED to photo receiver, introduced the channel modelling techniques.
Keywords: VLC, LED, Photo receiver, two-stage amplifier circuit, Solar cell
.
1. Introduction
Our Digital Communication project revolves around the most important feature of modern
technology: data transmission. With the introduction of Wi-Fi and other means of wireless
communication, the way we access information on a daily basis has changed profoundly. We
now heavily rely on immediate reception of what we are searching on and have become
increasingly demanding of faster connections. This has caused the wavelengths of
transmission to become more and more cluttered. Soon, it will become very difficult to find a
wavelength that does not interfere with already existing connections.
This is where our project comes in. The visual medium is relatively untapped, with
transmission rarely taking place in those wavelengths. This creates a scenario where if there is
a direct line of sight between two devices, we can create a connection through the visual
medium and transmit data between those two devices quickly and easily. The benefits of this
design are vast reaching but here is a simple example. If you are visiting someone’s home
these days, you have to connect to their Wi-Fi to use the internet in their home. This can be
tedious especially with Wi-Fi’s with extra security. However, with our project, if the other
person’s computer is connected to the internet, you can simply connect to their computer
through the visual medium and access the internet remotely. This is still a very new concept,
and better and more efficient methods of implementation will be developed very soon. Our
project is relatively straight forward.
We have also developed a prototype which can send audio signals from one device to
another as a practical example of where our project can be used. We will be transmitting a
song from VLC Media Player on a computer to a speaker using Visual Light Communication
where the receiver is a solar panel.

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Section I: Hardware implementation
Prototype 1: Experiment setup for testing VLC transceiver
Challenges faced:
1. Designing in the circuit of Prototype I: The VLC transmitter and Receiver circuit that
we intended to design (2) was made to work using simple comparators and amplifiers.
The chips that they used for their comparator circuit and amplifiers were unknown to
us. The chips weren’t available in the market either. We analyzed the functioning of
the OPA354 as amplifiers and LT1715 as comparator circuit.
We found out that LM335 as comparator and LM339 as quad core amplifier can
function the same way as LT1715 and OPA354 do. We then made modifications
accordingly with the proper values of resistances and capacitances to work as per
requirement of our VLC transmitter and receiver.
2. Communication Problem:
To communicate the VLC transceiver with the PC, we need to have a protocol so that
it can communicate with it. We first tried using the USB protocol (1) using
PL2303HXA (USB-TTL module). For using the COM port of PC and interfacing it
with the computer USB interface which is being provided by USB module. At the
software level, the connection can be seen as a direct serial connection.
We found that this serial connection is directly made by USB-TTL converter
and we can easily send and receive data using serial terminal which is widely
available on net. The application like “PL2303 USB-UART” can be downloaded to
interface two Android phones with the transmitter and receiver of VLC to transmit
and receive data. But then the data were transmitting and receiving was all done by
using serial terminal. It was an interface which was being used. We just needed to
interface the USB-TTL converter with the PC and start using that. The USB module
which we were using was outdated and needed a newer of PL2303HXA to transmit
data serially. So, we had to switch to RS-232 protocol. (3) The same problem we
came up with while using RS232 protocol that we were transmitting data through
HyperTerminal (Serial terminal) and it can send data up to 120kbit/s.
We want to transmit data by using different modulation techniques, error
encoding and decoding schemes and higher data rates for which we need to learn that
how can we either interface it with familiar Arduino or ARM based microprocessors
to give a programming environment.
Implementation: At the transmitter side of our prototype, we have a LED driving circuit
which drives the LED. The LED is being driven by ON-Off Keying (OOK) signals coming
from PL2303HXA (USB converter that converts USB to TTL logic) connected to the laptop
which is needed in our project to switch the LED ON and OFF. The waveforms are applied at
both higher and lower frequencies across the LED through frequency generator which we
observed in digital signal oscilloscope at the receiver circuit.
At the receiver side, we again are interested in converting the optical signals received by
photodiode into the waveforms that we transmitted through LED. The photodiode is able to

5
receive a voltage only up to in range of 100-150mv which is very less to receive the
waveforms sent by VLC communication. Therefore, we implemented two-stage amplifier
circuit through amplification using Op-amp.
Results:
1. Even though the waveforms sent at higher frequencies are getting distorted, it still
roughly was able to maintain the square shape.
2. The LEDs used in this prototype isn’t bright enough for long distance transmission.
3. The received data sent through the LED matches with the transmitted data from the
output. Therefore, we tested the VLC transceiver circuit and it is also able to transmit data of
variety such as text, information data and sound sources.
Prototype 2: Audio transmission of data using VLC transceiver
Challenges in audio transmission: In design of VLC receiver, the major task is to sense the
LED flickering so to generate the Analog voltage, to do so initially we tried and used a
photodiode. But the area that is being illuminated through LED was much greater than that of
the optical sensor area i.e. photodiode. So, to tackle down this problem we have searched out
the alternative for reception of visible light using a solar cell receiver [5]. VLC using a solar
cell as a receiver can be implemented without a photodiode receiver and power supplies.
This technique enables us to implement an eco-friendly visible light receiver.
Secondly, the solar cell is directly connected to the speaker. When no valid input is given to
the solar cell the speaker is producing a random noise (in presence of CFL bulb), to remove

6
the noise we switch off the bulb and focused the LED on the solar cell panel and start
receiving the audio that is being transmitted through LED.
The LED is connected to the transmitter circuit, which consist a voltage comparator (Lm741)
followed by a transistor. The reference voltage to the voltage comparator is provided through
the voltage divider circuit [6]. The reference voltage is compared to the audio input being
played from the laptop. The audio signal is given to the circuit through a 3.5mm audio jack
connected to the laptop. The transistor work as modulator and amplifier, the LED is
connected to the emitter of the transistor. Whenever signals interrupt through op-amp, there is
very little fluctuations and amplified by transistor which gives flickering effect to LED and
makes visible light communication.
Implementation: At the transmitter side, the low analog audio signal is being taken from any
mobile or PC or music enabled device. We need AC inputs so a series capacitance is used to
filter out the DC components from the circuit. Later on, the audio signals are being applied at
the non-inverting pin and it is getting compared with the voltage at the inverting pin which is
basically 2.5V. Whenever the analog signals are more than 2.5V, the output of comparator
circuit gives positive values of signals which make our transistor Q1 active and thus the LED
ON. While the analog signals which are lesser than 2.5V makes the output of comparator as
0v which makes the LED OFF. That’s how we are controlling the flickering of LED. At the
receiver side; we used a low power Solar panel for receiving the optical signals transmitted
through the transmitter and connecting it to general speaker which gives audio as the output.
Results: A VLC system for transmission of audio signals is successfully implemented and
demonstrated using LED and solar cell.

7
Section II: Characteristics of LED and its limitations
LEDs will be the future of modern lighting system as they enjoy many advantages over
conventional lighting devices such as Mean Time before Failure (MTBF), high lighting
efficiency, specific spectrum and environmental friendliness. Data transmission in VLC is
done by changing the light intensity. Change in amplitude is so small for a naked human eye
that it is unnoticeable. LED can be modulated at higher speeds which make it a suitable
candidate for data transmission. Right choice of modulation scheme, selection of line coding
scheme, use of equalizer at transmitter and receiver can further improve the performance of
LED. It has been observed that nearly 33% of the total electrical energy consumed is for
lighting purpose Therefore it is a must that efficient lighting source must be used in order to
reduce this proportion. Other than water, electrical energy is also generated from coal;
therefore it is a must that efficient lighting source must be used in order to reduce this
proportion. Other than water, electrical energy is also generated from coal.
NONLINEAR DISTORTION MITIGATION IN VISIBLE LIGHT COMMUNICATIONS
While performing the experiment we observed that the signal received at the receiver side
was distorted (The problem faced). For a sine wave input which we generated from the
function generator the output obtained was clipped. These distortions are due to the
nonlinearity of the components in the system.
The major contributor to this nonlinear response is the LED.
To mitigate this problem, we first need to understand what exactly the problem is. Hence this
nonlinearity is mathematically modelled to study its impacts on the system and deduce
mitigation methods.
Nonlinear models can be divided into memory models and memory-less models, based on
whether we take into account the memory effects of nonlinearities. On the other hand,
according to whether or not the dynamic characteristics of LEDs are considered, we can also
categorize the nonlinear models into dynamic and static models.
Memory Based & Memory less models:
Memory based systems the input of the system depends on the output of previous inputs
resulting in the generation of a closed loop system. For
example: y= mx + n(x-1) +c has a nonlinear response. Volterra series is a good example to
model nonlinear system with memory.
A power series or a polynomial model is a good example to describe a memory-less system.
For example y = ax^2 + b*x + c.
Static & Dynamic models:
Static indicates that the LED nonlinearity does not change over time. This kind of
nonlinearity is relatively easy to deal with. (Input is constant with time)

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However, LED nonlinearity may change over time due to component aging and temperature
drift. Hence, the dynamic model is required to capture these changes.(Input keeps changing
with time)
The mitigation measures that can be applied are:
Waveform shaping:
In waveform-specific mitigation techniques, the overall system nonlinear mapping is not
modified. Instead, the signaling waveform is changed such that it is less sensitive to the
nonlinear effect. Each signal has its own characteristics associated with the waveform shape
and with different types of modulation we can avoid nonlinear distortion. For example: PPM
instead of generic amplitude modulation.
Cited:photonselectrons.wordpress.com Cited: codeandlife.com
In our project we inplemented this analysis by using a square wave as the signal waveform
and performed an OOK modulation to reduce the distortions caused.
Non linearity correction:
Waveform-agnostic mitigation techniques mainly work by linearizing the overall system
nonlinearity so that the input signal can be passed approximately undistorted within the
dynamic range. This linearization can take place at either the transmitter
(PREDISTORTER) or the receiver (POSTDISTORTER). The distorter can be static or
adaptive based on the dynamic characteristics of the LED.
In this method we try to manipulate the output response of the system by changing the
parameters in the system .The basic protocol is to adjust the system response in such a way
that the operating point will be restricted to a range such that it provides a linear output, i.e.
without clipping. This job is done by the distorter which was mentioned above.

9
Foot Note: with reference to a research paper on NONLINEAR DISTORTION MITIGATION IN VISIBLE LIGHT
COMMUNICATIONS. Authors: KAI YING, ZHENHUA YU, ROBERT J. BAXLEY, HUA QIAN, GEE-KUNG
CHANG, AND G. TONG ZHOU
In our project, we have used White Led which has a power consumption of 1W-very less
power consumption. Also compared with conventional lighting methods, white LED has
lower power consumption and lower voltage, longer lifetime, smaller size, and cooler
operation.

10
Section III: Modulation Schemes
1) ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM):
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier
modulation method. A large number of closely spaced orthogonal sub-carrier signals are used
to carry data on several parallel data streams or channels. OFDM can be used to implement
VLC because of its several advantages. OFDM allows equalization to be performed
efficiently with single-tap equalizers in the frequency domain. More importantly, it allows
information to be adaptively encoded into different frequency bands such that the available
communication resources are always fully-utilized. At a system level, OFDM provides a
straightforward multiple access technique, which needs to be implemented additionally if an
alternative modulation format is employed. Conventional OFDM generates complex-valued
bipolar signals, which need to be modified in order to become suitable for VLC. This
operation effectively maps the information symbols onto sub-carriers in different frequency
bands. A real OFDM signal can be obtained but reduces the system bandwidth by a half. This
approach has been widely accepted in the literature for the generation of a real OFDM signal.
The resulting waveform, however, is still bipolar in nature (it has a positive and negative
part). A number of approaches have been proposed for the creation of a unipolar signal.
Meanwhile, OFDM is also known for its disadvantage of high PAPR. VLC-OFDM inherits
the disadvantage of high PAPR from RF-OFDM. High PAPR makes VLC-OFDM very
sensitive to the nonlinearity of LEDs .Moreover, high PAPR requires large biasing to convert
the bipolar OFDM signal into a unipolar version, which causes serious degradation of optical
power efficiency. Therefore, PAPR reduction is an indispensable module in VLC-OFDM
systems. Among PAPR reduction algorithms proposed for OFDM, distortion based methods
are particularly favored for practical scenarios because the modification of the receiver is
avoided. Clipping is the simplest distortion-based method to reduce the PAPR of OFDM
signals. However, there are some differences between RF-OFDM and VLC-OFDM to
prevent applying conventional PAPR reduction methods to VLC-OFDM systems directly. In
VLC-OFDM systems, since the LED acts as a low-pass filter, the out of-band subcarriers
cannot be leveraged to transmit information to other users. Thus, we do not need to consider
the out-of-band interference. This actually gives us additional degrees of freedom to develop
PAPR reduction schemes for VLC-OFDM systems.
Due to these disadvantages, we conclude that OFDM is not an efficient method for
implementing VLC.
1) QUADRATURE AMPLITUDE MODULATION:
Quadrature amplitude modulation (QAM) is both an analog and a
digital modulation scheme. It conveys two analog message signals, by changing
the amplitudes of two carrier waves, using the amplitude shift keying (ASK) digital
modulation scheme or amplitude modulation (AM) analog modulation scheme.
The two carrier waves, usually sinusoids, are out of phase with each other by 90° and
are thus called quadrature carriers or quadrature components.

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QAM is of many types (higher orders) like: 8-QAM, 16-QAM, 32-QAM etc. In real
life applications, 64-QAM and 128-QAM are used in overhead broadcast transmission
techniques.
QAM can be implemented for VLC because of its many advantages. By using higher order
modulation (QAM) formats, i.e. more points on the constellation, it is possible to transmit
more bits per symbol. However the points are closer together and they are therefore more
susceptible to noise and data errors. Normally a QAM constellation is square and therefore
the most common forms of QAM 16QAM, 64QAM and 256QAM.
The advantage of moving to the higher order formats is that there are more points within the
constellation and therefore it is possible to transmit more bits per symbol. The downside is
that the constellation points are closer together and therefore the link is more susceptible to
noise. As a result, higher order versions of QAM are only used when there is a sufficiently
high signal to noise ratio.
Typically it is found that if data rates above those that can be achieved using 8-PSK are
required, it is more usual to use quadrature amplitude modulation. This is because it has a
greater distance between adjacent points in the plane and this improves its noise immunity.
As a result it can achieve the same data rate at a lower signal level.
The second limitation is also associated with the amplitude component of the signal. When a
phase or frequency modulated signal is amplified in a radio transmitter, there is no need to
use linear amplifiers, whereas when using QAM that contains an amplitude component,
linearity must be maintained. Unfortunately linear amplifiers are less efficient and consume
more power, and this makes them less attractive for mobile applications.
OOK modulation is a very popular modulation used in control applications .This is in part
due to its simplicity and low implementation costs. OOK modulation has the advantage of
allowing the transmitter to remain idle during the transmission of a “zero”, therefore
conserving power. This modulation is based on intensity modulation with direct detection.
The generation of optical pulse is achieved by opening and breaking of led, when sending
information “1”, the light pulses are sent; when sending information “0”, the LED is shut
down completely. The bit rate is denoted as Rb=1/Tb, where Tb is the bit duration; and is
directly related to the rate at which the source can be switched on and off.

12
Pulse-position modulation (PPM) is a form of signal modulation in which M message bits
are encoded by transmitting a single pulse in one of (2^M) possible required time-shifts. This
is repeated every T seconds, such that the transmitted bit rate is (M/T) bits per second.
Single-pulse position modulation converts a binary M-bit data group to a single pulse signal
at a particular time slot in time segment which are composed by L=2M time slots, each time
slot is called chip. Optical block encoding is achieved by converting each word of M bits into
one of L = 2M optical fields for transmission.
Digital Pulse Interval Modulation(DPIM) the symbol length of the DPIM is unfixed and can
be divided into unprotected slots and protected slots, protected DPIM modulation mostly
adopt one protected slot to reduce the impact of inter symbol interference effectively. The
modulation
symbols Sk (k is the decimal number expressed by the symbol) contain k+2 time slots, after
each starting time slot L , the pulse adds a protected empty slot and adds k empty slots for
expressing information . When demodulation in the receiver, after determining the pulse time
slot received, it only needs to count the empty time slot and subtract one for them.
For using perfect modulation technique we took analysis of Bandwidth demand,
Transmission capacity, availability of circuit components and cost required to implement.
PERFORMANCE ANALYSIS AND COMPARISON OF BANDWIDTH DEMAND
1. For the OOK system, bandwidth=Rb(Bit Rate)
2. For PPM ,bandwidth=2^M*Rb/M
3. For DPIM, bandwidth=(2^M+3)Rb/2M
4. For DPPM, bandwidth=(2^M+1)Rb/2M
PERFORMANCE COMPARISON OF TRANSMISSION CAPACITY (Ability to transfer
per unit time,C)
1. C(OOK)=(1/t),where t=slot width
2. C(PPM)=(M/2^M)*C(OOK)
3. C(DPIM)=2M/(2^M-3)*C(OOK)
4. C(DPPM)=2M/(2^M+1)*C(OOK)
(a) (b)
The comparison of (a) Bandwidth and (b) Transmission capacity on OOK, PPM, DPPM,
DPIM and DH-PIM

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Challenges Faced
1) What factors we should consider for choosing modulation schemes to implement VLC.
After some research, we finally decided to ponder upon following points:
(i) Ease of design
(ii) Bandwidth requirements
(iii) Implementation costs
2) After Deciding upon above points, we observed that there is no possible modulation
scheme that inherits all features mentioned above. There is always a tradeoff between some
of the factors.
3) Implementation of on-off-key (OOK) modulation in hardware to get optimal voltage to
drive the LED (transmitter). (7) (8) (9)
Section III: Channel Modeling Techniques
A channel is the medium between the transmitter and the receiver through which the
information travels. This channel can be a copper wire, optical fiber etc. In visible light
communication the channel is nothing but light. The characteristics of the signal changes as it
travel from the transmitter to the receiver. The change in the signal characteristics are due to
several phenomena: 1) existence of line of sight path between the antennas 2) reflection,
refraction and diffraction of the signal due to the objects in between the antennas 3) The
relative motion between the transmitter and receiver and the objects in between them 4) The
signal attenuation as it travels through the medium 5) Noise LOS Channel modelling Line-of-
sight propagationis a characteristic of electromagnetic radiation. It is a transmission which
includes light emissions traveling in a straight line.
x(t)- input signal
y(t)- output signal
y(t)=x(t)*h(t) + n(t)
h(t)-impulse response of the channel
usually dirac delta function δ(t- て)
1. Types of losses occurring in Channel Modeling
Slow Fading or Shadowing- Loss in the power of the received signal due to
distortion caused due to reflection, absorption and scattering of transmitted signal. It
is also called slow fading or long- term fading because the effect of this fading does
not bring about a change in the received signal immediately. The phase and
magnitude changes remain fairly constant over a long period of time
.
Fast Fading or Multipath Fading : A signal travelling in an environment may get
reflected by several objects on the path. This gives rise to several reflected signals.
The reflected signals arrive at the receiver at different time instants and with different

14
intensities leading to multipath propagation. Depending on the phase of the each
individual reflected signal, the received signal power may increase or decrease. A
small variation in the phase of the each reflected signal from each multipath may lead
to significant difference in the total received power. This phenomenon is also referred
as fast fading or short-term fading.
Source: Gaussianwaves.com
2. LOS - Line Of Sight Propagation
● Line-of-sight propagation is a characteristic of electromagnetic radiation. It is a
transmission which includes light emissions traveling in a straight line.
● Maximizes power efficiency and reduces multipath distortion.
● Point - to- point link - transceivers communicate with each other by a thin light beam.
Source:FREE SPACE OPTICAL NETWORKING WITH VISIBLE LIGHT: A MULTI-HOP MULTI-
ACCESS SOLUTION ZEYU WU Dissertation submitted in partial fulfillment of the
requirements for the degree of Doctor of Philosophy (12) (13)

15
● A directed beam enhances point- to - point link.
● Fewer disturbances due to ambient light.
3. Challenges Faced
● Possible only if there is an uninterrupted LOS path between the transmitter and
receiver.
● Cannot be implemented for mobile devices
● Short range
All the above three problems can be tackled by increasing the field of view (FOV) for the
transmitter. This can be done by implementing an array of LEDs on the transmitter side. This
will allow us to propagate the signal over a longer distance and also be able to provide us
mobility, though this mobility will be restricted to within the room or within the FOV of the
transmitting LEDs. Also multiple receivers can be used in combination with a single
transmitter.
Also multipath fading will increase in case of a larger FOV. This can be tackled by making
sure that the △乙=乙(i-1)-乙(i) is greater than inverse of the bandwidth i.e △乙> 1/B.W.
Here △乙 is the time difference between the propagation of ith
wave and the (i-1)th
wave.
Also torch reflectors were used in the audio transmission to enhance the intensity of the
transmitted light signal.
Section V: Study of Audio Transmission
As our initial plan of transmitting data in the form of an alphabetical code was facing many
problems which we were unable to overcome, we decided to try and come up with a different
prototype. As we were researching we found that there were quite a few papers online about
audio transmission through VLC and that they were not as difficult to implement as some
other concepts. We found one particular circuit which suited level of sophistication as well as
being possible to implement on a simple budget. Once we decided on using this circuit we
started implementing it and faced a few challenges.
1. The main difference between audio transmission and the regular data transmission is the
necessity of a car jack at the transmitter, which is used to convert data from analog to digital
for playing the music.
2. We observed in the circuit certain elements which were basically functioning as
comparators and amplifiers and decided to use our own simpler versions of these components
so that we could have greater control over the design.
3. We made the mistake of passing 9V to our LED instead of 5V which caused it to burn and
this is why our presentation on the final day took a little longer.
4. We noticed with our first prototype that photodiodes (which were recommended as the
receiver of this prototype) were weaker at sensing our sources of light and therefore decided
to switch to using a solar panel which did a much better job of sensing the signal.(16)

16
References:
1. The Demonstration of Wireless Access via Visible Light Communications - Liwei
Ding, Fang Liu, Zhiqiang Yu, Yongjin Wang Institute of Communication
Technology, Nanjing University of Posts and Telecommunications Jiangsu, China
2. Design of Wireless Optical Access System using LED - Liwei Ding, Fang Liu,
Zhiqiang Yu, Yongjin Wang Institute of Communication Technology, Nanjing
University of Posts and Telecommunications Jiangsu, China
3. VISIBLE LIGHT COMMUNICATION - Durgesh Gujjari, Dalhousie University,
Halifax, Nova Scotia, August 2012
4. Design of Wireless Optical Access System using LED - Liwei Ding, Fang Liu,
Zhiqiang Yu, Yongjin Wang Institute of Communication Technology, Nanjing
University of Posts and Telecommunications Jiangsu, China
5. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=667551
6. http://electronics.stackexchange.com/questions/199806/help-understanding-negative-
feedback-bias-design-on-audio-amplifier/199820#199820
7. International Journal of Advances in Engineering & Technology, Jan. 2013
8. Visible Light Communication Using OFDM Mostafa Z. Afgani, Harald Haas, Hany
Elgala, and Dietmar Knipp,n 28759 Bremen, Germany
9. 4 th Student Conference on Research and Development (SCOReD 2006), Shah Alam,
Selangor, MALAYSIA, 27-28 June, 2006
10. http://www.cennser.org/IJEI/eiV02N01/ei020101
11. Visible Light Communication:Opportunities, challenges and Channel models by
M.Saadi, L wattisuttikulkij, Y.Zhao, P.sangwongngam
12. www.gaussianwaves.com
13. http://ece.tamu.edu/~cui/ECEN478/lecture2_channel.pdf
14. FREE SPACE OPTICAL NETWORKING WITH VISIBLE LIGHT: A MULTI-HOP
MULTI-ACCESS SOLUTION ZEYU WU Dissertation submitted in partial
fulfillment of the requirements for the degree of Doctor of Philosophy
15. Visible Light Communication:Opportunities, challenges and Channel models by
M.Saadi, L wattisuttikulkij, Y.Zhao, P.sangwongngam
16. Son, Do, Cho, Eun Byeol, Moon, Inkyu, Ghassemlooy, Zabih, Kim, Soeun and Lee,
Chung (2013) Simultaneous transmission of audio and video signals using visible
light communications. EURASIP Journal on Wireless Communications and
Networking, 2013 (1). p. 250. ISSN 1687-1499
17. T. Komine and M. Nakagawa, “Fundamental Analysis for Visible-Light
Communication System Using LED Lights”, IEEE Transactions on Consumer
Electronics, Vol. 50, no. 1, pp. 100-107, February 2004.

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