Implementation of li_fi_using_arduino

Department of Electrical Engineering and Information Technology
Chair of Measurement and Sensor Technology
Project Documentation
„Project Lab Embedded Systems“
Group: 10
Members: Hrishkesh Pattepu
Mu Zhang
Ugur Bolat
Vivek Maru
Project: Implementation of Li-Fi technology using Arduino
Date: 2017-06-28

II
Table of Contents
1. Abstract
2. Members Responsibilities
3. Introduction to Li-Fi
3.1 Working principle of Li-Fi Technology
3.1.1 VLC Transmitter
3.1.2 VLC Receiver
4. Functional Description
4.1 Overview of the design
4.2 Software
4.2.1 Transmitter
4.2.2 Receiver
4.3 Hardware
4.3.1 Simulation using LTSpice
4.3.2 Experimental Setup
4.4 Interaction with Users as well as Hard/Soft ware
5. Description of files
6. References

Technische Universität Chemnitz
Prof. Dr.‐Ing. Olfa Kanoun

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1 Abstract
Demand on wireless communication has always been increasing since the available radio
spectrum becomes insufficient due to huge number of computing devices. Therefore, visible
light communication (VLC) brings an alternative and compelling solution by exploiting a
wider chunk of electromagnetic spectrum from 380 to 780 nm. Visible light communication is
a potentially ground-breaking form of wireless communication that can supplement radio
frequency communication and also can enable unique wireless device use cases.
This project is aimed at creating a communication channel using Visible light spectrum.
The communication will be supported through interfaces with Arduino UNO boards and LT
spice simulations. With this report, we present both Numerical and experimental results for
our prototype based on Arduino.
 Keywords – Visible light communication, Light emitting diode, Optical system, Arduino,
Photo diodes
2 Member Responsibilities

Mohammed Al-Ubaedi Supervisor
Hrishkesh Pattepu Circuit simulation, Component requirement
declaration, Literature research on Arduino
UNO board capabilities for Li-Fi
Mu Zhang Circuit simulation, Calculations of required
electrical quantities (Current, Voltage, SNR)
Ugur Bolat Prototype design, Coding in C with Arduino
IDE, Literature research on Li-Fi technology
and related terms
Vivek Maru Prototype design, Coding in C with Arduino
IDE, Literature research on improvements of
communication rate.


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3 Introduction to Li-Fi Technology
There has been significant research in recent years in Visible light communication
technology. The driving force behind this research is the possibility of using general
illumination i.e. Light emitting diodes (LEDs) for data communication. Visual light
communication using LEDs offer many advantages including high data rate and high data
security. Professor Harald Haas is a pioneer introducing Light Fidelity (Li-Fi), which refers to
data transmission through light illumination [1]. This approach gives rise to many optical
wireless application cases like Li-Fi office in Paris [2] or LiFi-X [3] for internet connectivity.
Still, there are few standards, e.g. IEEE 802.15.7 [9] and many will come soon because recent
researches prove that VLC is a promising technique for high-speed wireless communication
[4]. In this report, we will be discussing high level definitions of working principle of Li-Fi in
this chapter. In the following chapter 4, we will be discussing functional area of our prototype
including software and hardware parts following with the test results of simulation and
experiments.
3.1 Working principle of Li-Fi Technology

The objective of these systems is to use visible light as a media for the communication.
The visible light spectrum has hundreds of terahertz of bandwidth (Fig.1) which can be used
to transmit data. Basic functionality of LEDs is to illuminate the desired area. Apart from very
obvious advantages, LEDs have some significant plus points like long lifespan, compact form
factor, lower head generation. Due to these advantages, adoption of these devices is on an
exponential rise. On the other hand, LEDs are able to switch to different light intensity at very
fast rate (around 1- 5 MHz) [5] so that human eye cannot perceive the flickering effect of
light. This functionality is perfectly sufficient for building data transmission. It can increase
the available wireless channel and bandwidth significantly.
Visible light communication comes with some downsides to it. One of the most
discussed is the data rate of transmission falls very rapidly with the increasing distance of the
communication channel or line of sight. The path loss here is inversely proportional to the
distance raised to the power of four [8]. Also, the data can be corrupted if the receiver is
directly exposed to Sun or some other significantly high illumination source.
Fig. 1. The Visible light spectrum ranging from 430 THz to 790 THz [5].


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A driver circuit can drive the LED by changing light level to transfer encoded data in
the emitting light. A photo detector, having an ability to receive light and to turn it into
current, can receive the encoded data and provide output in alternating current form. Then,
digitalization of the signal is needed to be performed like in any other communication
phenomena. In this case, system design of a basic Li-Fi application requires one transmitter
circuit and one receiver circuit as shown in Fig.2 to build the communication network. We
will be discussing further details about both the part of the diagram shown in further sub-
sections.
Fig. 2. The VLC radio transmitter and receiver based on intensity modulation/direct detection [6].
3.1.1 VLC Transmitter
To control brightness of a LED, a driver circuit is necessary. Design of the driver
circuit may differ depending on the application but one can include Digital-to-Analog
Converter (DAC) for converting encoded digital signal to analog, Trans conductance
Amplifier (TCA) for controlling output current via differential input voltages and a LED (or
more than one LED such array of LEDs to create parallel communication channel). For
simplicity, modulation can be performed in ON/OFF mode but other modulation techniques
[7] have to be used when aiming high-speed data rate. Also, there are techniques developed to
mitigate flickering and dimming [7] as defined in the IEEE 802.15.7 [9] visible light
communication standard. In a simple ON/OFF mode, we can transmit bits “0” and “1” by
deciding two different intensities of light. Even though red LEDs provides much better
luminance, which can lead to more distance for the communication, white LEDs might be a
better option since they are commonly used due to the fact that human eye perceives true
colors under white light. Coming to the device requirement, it is very important to know with
which technology the LED is manufactured. Commercially, white light LEDs are produced by
two different ways. 1) Blue LED with Phosphor and 2) RGB combination. Because of low
cost and ease of implementation the former one is mostly used but in terms of
communication, the phosphor coating limits the speed of the data transmission [5]. On the
other hand, RGB combination LEDs are preferred for communication purposes as it
eliminates the drawback of phosphor coating. Also, we can use color shift keying to modulate
and encode data in 3 different colors with the same illumination source.
To notice here that using white light can have some downside as it can easily merge
with normal day light and other light sources which are around and it can cause a loss of data.
There are methods of modulation available to avoid these kinds of issues but it is out of the
scope of this report here.


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3.1.2 VLC Receiver
A photodiode is a semiconductor device which can convert the received light into
current. Mobile devices like smartphones or cameras include photodiodes in an array form to
capture images. It has the potential to convert any mobile device into a very good VLC
receiver. These sensors are only suitable for low data rate applications because having large
number of photodiodes to capture high resolution picture decreases number of frame per
second (fps), which leads to very low data rate. There are many methods to increase fps, e.g.
rolling shutter process where the received light will be read row by row or column by column
in a matrix form. Although performing all methods for image sensors, it still can reach only
up to certain kbps. However, designing a VLC receiver with single high-quality photodiode in
terms of perception and emission will provide a better solution which can reach up to
hundreds of mbps. However, the disadvantage of using less number of photodiode may limit
the range and the angle so line of sight becomes an issue.
After converting the received light into current, amplification of AC signal might be
needed, as can be performed by Trans-conductance Amplifier (TCA). We also need to
consider the noise received because of ambient light or solar radiation which can be mitigated
by applying a high pass filter at the receiver side. Most of the previous studies show that these
noises remain stationary over space and time. Then, Analog to Digital Converter (ADC) is
used to obtain modulated signal so that demodulation can be perform on digital signal.


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4 Functional Description
4.1 Overview
This section demonstrates the experimental unit built for the realization of visual light
communication. The hardware components contain mainly two Arduino UNO development
kits, one for transmission purpose and the other for receiving purpose. These development
boards are integrated with a single LED as a transmitter and a solar panel as a receiver. The
specification details of the components will be discussed in the following sub-sections. The
exact circuit connections are showed in the later section 4.4. To realize Li-Fi, one transmitter
and one receiver software are coded in C. All the tasks of the signal processing and
modulation of data are being realized using an Arduino software library which will be
discussed in following section 4.2.

4.2 Software
4.2.1 Transmitter
In VLC, communication must be implemented in a way the naked eye should not
perceive the flickering effect. One of the simplest ways to overcome this issue is to encode
the data bits. Here, the most striking difference between VLC and RF is that in VLC, the data
cannot be encoded in phase or amplitude of the light signal. This means that those modulation
techniques cannot be applied here and the data for VLC has to be encoded in varying light
intensity of the emitting light wave. In this project, Manchester Encoding techniques are used
and encoding a bit is not represented as a steady state of the signal but as a variation. For
example a bit with value one will be encoded by the signal going from the low state to the
high state and a zero will be encoded by the signal going from the high state to the low state.
For a light source this mean that the light quantity averaged over a period of time will no vary
over time (for a light source blinking faster than 25Hz, the eye acts as a low-pass filter that
average the light quantity). However, IEEE 802.15.7 standard [9] suggests that flickering or
the change in light intensity should be faster than 200 Hz to avoid any harmful effects on
humans.
In this setup, the data to be sent are serialized (with a start and a stop bit) and are
encoded to drive the LED. As a consequence, the LED appears to be ON all the time for the
user. The data are sent with a symbol rate of 1200 baud which corresponds to a bit rate of 500
bit/s. Arduino’s PWM generation frequency can be a bottleneck to achieve higher bit rate than
500 bit/s.
To produce bit stream at the digital output of the Ardiuno, internal interrupt is
activated at every 500 ms. However, before sending the data, the data frame must be prepared
like shown in Fig. 3. The data frame consists of 5 different types of data packages:
Fig. 3. Data frame architecture for a Manchester encoded data.


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1. Threshold stimulator: 0xAA
2. Synchronization symbol: 0xD5
3. Start bit: 0x02
4. Data
5. End bit: 0x03
By doing so, data can be safely read on the receiver side. To help with the synchronization
of the receiver and transmitter, the data to be sent are encapsulated into a frame. This frame is
composed of a synchronization preamble (0xAA) that helps to compute the binarization
threshold. A 0x5D then breaks the synchronization (helps detecting the Manchester encoding
phase), and a STX (0x02) symbol indicate the start of the frame. We will be sending 8 bits of
data after the start bit. An ETX (0x03) ends the transmission.
In transmitter software, there are two main processes; one is responsible of creating data
frames for the next cycle of data transmission when MCU is not busy with writing bit stream
to the digital output, the other one is responsible of serializing the data frame and writing bits
on the digital output. The data flow diagram of the transmitter software is shown in Fig. 4.
Fig. 4 Software processes responsible for transmission of data


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4.2.2 Receiver
In this setup, the voltage of the LED is captured four times faster than the transmission
rate with the ADC supported by an internal interrupt timer and the voltage is averaged over
time to compute a threshold to distinguish high and low state of the incoming signal. This
threshold is used to binarize the signal and then decode the Manchester encoding. The serial
signal is then decoded (detection of start and stop bit) into a byte stream. To notice, all the
communication is being done here by converting the analog signal in to PWM signal using
Arduino’s inbuilt timer mechanism.
After the initialization of the ADC is done, the calculation of binarization threshold is
performed recursively until synchronization data (0xD5) is detected. After the
synchronization data is detected, the data including start bit and stop bit is being read and then
stored into a buffer. There is also a check for the data frame where if he frame or the package
is corrupted, it will not be store in the buffer and will be skipped. The buffered data streams
can play the actual audio signal transmitted. There are various libraries and functions
available which can take this stream as an input and give us an audio output on the speaker,
which are out of the scope of this report. The data flow diagram of the transmitter software is
shown in Fig. 5.
Fig. 5. Software process responsible on receiver side


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4.3 Hardware
4.3.1 Simulation using LTSpice
To realize the Li-Fi communication, we have attempted to build a circuit using the
conventional communication techniques in LTSpice simulation tool. LTspice provides a
schematic capture and waveform viewer with enhancements and models to speed the
simulation. Most of the signal conversions and signal processing is being done by Arduino in
our experimental setup but to show what exactly goes behind the C code, we have built this
simulation setup. Fig. 6 provides the full circuit definition of the simulation setup. Detailed
circuit analysis is described below.
Fig. 6. The Simulation setup for VLC communication.
The input for this simulation is an audio source from 3.5 mm jack which is an alternating
signal between +ve and –ve voltage as can be expected. A LED is a semiconductor device and
providing a –ve voltage can have back currents in the device and it can damage the LED. To
mitigate this effect we need to apply a DC offset so that the applied stimuli to the LED are
always positive. There are two different methods to apply a DC offset to an analog signal. 1)
Bridge structure and 2) Summing amplifier. The later one is more effective compared to the
former one which is shown in the following figure Fig. 7. We chose the same for our
simulation as the former one is not supported by LTSpice.
Fig. 7. Summing amplifier circuit using LM 386.


9
For this purpose, we have an input of 10kOhm which is not over drive amplifier. The
summing amplifier is used to combine the voltage present on two or more inputs into a single
output voltage. Here, we have one DC offset input and the other one is input audio signal.
This summing amplifier is followed by a LM386 IC which is a power amplifier designed
for low voltage application. It is an 8 pin dual in-line package which can take 4V-12V or 5V-
18V depending on the wiring and can pump up the gain up to 200. In addition to that, we have
a bypass pin in LM386 which can help to eliminate the extra noise from the voltage source or
unamplified audio signal. This whole setup of amplifying a signal is repeated twice in the
receiver side of the circuit. The gain calculation of this setup is as below.
2
150
R
R



1350
.1350
150
30000
LM386ofGain.
Substitute R=650 we get,
= 75.588
30000
=50.9554
Amplitude of noise= v GainBandwidthrms 
81/20  nv 19980 2500 10
9
=7.06 ∗ 10
Amplitude of signal = 0.7V







)_(
)_(
log20
signalnoisepower
signalinputpower
SNR
SNR =20 log 





)_(
)_(
signalnoiseamplitude
signalinputamplitude
SNR =2 log
.
. ∗ =39.91 dB
To establish the communication channel in simulation, an Opto-isolator is used which
transfers electrical signals between to isolated circuits using light. With LTSpice reference, it
is a package consisting both LED and a photodiode. The electrical input signal is converted
into light in a closed loop channel and a sensor photodiode which detects incoming light and
generates electrical signals on the receiver side of the circuit. Opto-isolator used here in
simulation can withstand an input to output voltages up to 10KV.


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With the above mentioned devices and simulation setup, we have the following test results
with respect to the applied input condition as follows,
Applied Input:
 The Input signal which is source from 3.5 mm jack is alternating between +ve and -ve
voltage which to be given to DC offset.
 The DC offset is given through Vdc which raises the input above ‘0’, the signals is
transmitted Via LED which is received in the receiver through photodiode.
 The Received stage is amplified in two stages. The first stage amplifies with the gain of
50. The second stage also amplifies with Vopto of input this output is Vam1 amplifies it
again with a gain factor of 50 and the output voltage as Vam2 where the analog signal
received.
Fig. 8. Simulation results using LTSpice
So far, the simulation results are in acceptable range. We are able to receive the
transmitted audio at Vopto as shown in the Fig. 8. This received signal is amplified twice to
make it audible on a speaker at Vamp1 and Vamp2 as shown in the figure. Initially we are
using two audio files for testing; the audio signal which is given at the input is compared to
output Vam2 of the original sound signal.


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As mentioned in previous sub-section 4.3.1, most of the signal processing and modulation
is being achieved using Arduino in our experimental setup. Detailed description of the setup
is discussed in the following sub-section 4.3.2.
4.3.2 Experimental Setup
The setup consists of the very basic development components mentioned in the list below.
1. Arduino UNO development kits – 2 (One for transmitter, One for receiver)
2. 1 watt Blue LED
3. Solar panel
4. Current limiting resistors
Some of the very important technical specification which leaded us to consider Arduino
Uno development kit for our setup is shown in the Fig. 9. In addition to that, Arduino is open
source and many of the complex task libraries are made available for developers by Arduino
community itself which eases the complex task set coding.
`
Fig. 9. Technical specification of Arduino Uno board [10]
For demonstration purpose, below is the exact circuit diagram used for our experimental
setup shown in Fig. 10. Transmitter circuit on the left side includes a 1 watt power LED and a
current limiting resistor. Receiver side circuit on the right side includes a photodiode (Solar
panel for better reception, considered in actual prototype) connected to ADC pin of Arduino
to receive the generated electrical signals by photodiode with respect to changing intensity of
light. In addition, receiver side also includes a speaker connected to PWM pin of Arduino to
play the transmitted audio signal through VLC. In our setup, we have used a Buzzer to check
the audio which was not capable of accommodating high frequency audio but it gave
acceptable reading. For start, we have used the 32 KB of memory from ATmega328p itself to
store an 8-bit mono audio sample with duration of 4 seconds.


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Fig. 10. VLC setup using two Arduino modules (Transmitter located on Left side and
Receiver on right)
To improve the transmission or the data rate, the intensity and power of the LED should
be increased. By replacing the 1 watt LED with 3 watt LED can improve the design. Note
here that replacing 1 watt LED with a 3 watt one need extra driving circuit as Arduino itself
cannot produce the amount of current needed to drive a 3 watt LED. Using 3 watt LED can
help in mitigating the effect of ambient noise and it can also provide data transmission at
different angles. With our experimental setup, the angle limitation is a restricting parameter
for transmission which can be seen in following section 4.4.
Fig. 11 demonstrates the actual setup used for all the test cases present in the section 4.4.
Fig 11. Experimental setup to realize Li-Fi using VLC


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4.4 Test Cases
For the setup described in Fig. 11, we have experimented with different distances
between transmitter LED and receiving Solar panel and also with different angles at the same
time. Graphical representation of all the test cases is as below.
1. Distance : 17 cm, Angle : 0 Degree
Fig. 12.
Fig. 13.


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3. Distance : 20 cm, Angle : 8.5 Degree
Fig. 14.
4. Distance : 20 cm, Angle : 11.3 Degree
Fig. 15.


15
Fig. 16
Fig. 17


16
Fig.18
Analyzing all the results and test cases, we concluded that in the distance between 17 cm
to 20 cm, there is no loss of data which can be seen in Fig. 12 and Fig.13. We were able to
receive the exact same audio signal that was transmitted. Keeping the distance same and
changing the angle to 8.5 degrees showed some changes. The packages sent were received at
the receiver but in a different order and after around 1200 packets, we lost the data completely
afterwards as shown in Fig. 14. Increasing the angle to 11.3 degrees the number of received
data packets decreased to 500 and then complete loss of data as per Fig. 15. Further increasing
the angle to 14 and 16 degrees, number of received packets decreased to approximately 100 as
per Fig. 16 and Fig.17. The next test case was to increase the distance to 23 cm and as can be
expected, the number of data packets received were less and out of order as shown in Fig 18.
Here to note that, using an Arduino development kit we can only achieve reliable visual
light communication at the distance of around 20 cm as the current generated from Arduino
board can be enough to drive only small power LEDs. To increase the distance and angle, we
need to replace the LED with a higher power one like 3 watt or so which will also require
additional LED driving circuit.


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5 Description of files

 Project_Documentation.pdf Documentation containing description
of all different sections of the project,
test results and Conclusions
Additional Files
 Source Code : Transmitter_lifi.txt C code developed for transmission of
data using Arduino IDE
 Source Code : Receiver_lifi.txt C code developed for receiving of data
using Arduino IDE
 Input_audio.wav Audio file used for circuit simulation as
an Input
 Output_audio.wav Audio file received after circuit
simulation as an Output
 Original.csv File containing the 8-bit mono audio
data packets
 17cm_0cmdeg.csv Data received at Distance : 17 cm,
Degree : 0
Degree : 0
Degree : 8.5
Degree : 11.3
Degree : 14
Degree : 16
Degree : 0


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6 References
[1] H. Haas, L. Yin, Y. Wang, and C. Chen, “What is LiFi ?” Journal of Light ware Technology, vol. 34, no. 6,
pp. 1533–1544, 2015.
[2] http://luxreview.com/article/2016/06/world-s-first-li-fioffice-to-open-in-paris, “World’s first Li-fi office to
open in Paris.”
[3] http://purelifi.com/lifi-products/lifi-x/, “LiFi-X.”
[4] S. Rajbhandari, H. Chun, G. Faulkner, K. Cameron, A. V. N. Jalajakumari, R. Henderson, D. Tsonev, M.
Ijaz, Z. Chen, H. Haas, E. Xie, J. J. D. McKendry, J. Herrnsdorf, E. Gu, M. D. Dawson, and D. O’Brien, “High-
Speed Integrated Visible Light Communication System: Device Constraints and Design Considerations,” IEEE
Journal on Selected Areas in Communications, vol. 33, no. 9, pp. 1750–1757, 2015.
[5] P. Pathak, X. Feng, P. Hu, and P. Mohapatra, “Visible Light Communication, Networking and Sensing:
Potential and Challenges,” IEEE Communications Surveys & Tutorials, vol. 17, no. c, pp. 1–1, 2015.
[6] A. Jovicic, J. Li, and T. Richardson, “Visible light communication: Opportunities, challenges and the path to
market,” IEEE Communications Magazine, vol. 51, no. 12, pp. 26–32, 2013.
[7] S. Rajagopal, R. D. Roberts, and S. K. Lim, “IEEE 802.15.7 visible light communication: Modulation
schemes
[8] J. M. Kahn and J. R. Barry, “Wireless Infrared Communications,” Proc. IEEE, vol. 85, no. 2, Feb. 1997, pp.
265–98.
[9] IEEE Standard for Local and Metropolitan Area Networks-Part 15.7: Short-Range Wireless Optical
Communication Using Visible Light, IEEE Std. 802.15.7, Sep. 2011.
[10] https://store.arduino.cc/arduino-uno-rev3

Implementation of li_fi_using_arduino

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