ICERIE 2017_104
Proceedings of the
International Conference on Engineering Research, Innovation and Education 2017
ICERIE 2017, 13 15 January, SUST, Sylhet, Bangladesh
Small Scale Wireless Data Transmission via Light Using Light Source
Tahmid H. Talukdar1,*, Shahadat H. Parvez1
1
Department of Electrical and Electronic Engineering (EEE), Shahjalal University of Science and
Technology, Sylhet-3114, Bangladesh.
Keywords:
 Wireless;
 Visible
communication;
 Visible light
communication
(VLC);
 Li-Fi;
 Wireless
fidelitynetwork.
Abstract:Communication is one of the intrinsic parts of our life. In this age of
information the need for wireless communication is booming to a new height.
Radio Frequency is mainly utilized to satisfy the hunger for our wireless
communication needs. But the problem with Radio Frequency is that it is becoming
crowded. Additionally Radio frequency cannot be used in certain places like
Hospitals, Underwater, Petrochemical plants or Planes. These problems with radio
frequency calls for an alternative wireless communication means. A good
alternative for Radio Frequency is to use a different band of the similar waves,
Light Waves.
In this article, wireless data transmission via means of light has been theorized and
tested. Light sources were employed to generate the signal where in the receiving
end ambient light sensors were used. The sending data after modulation resulted in
binary code which was sent by serial data transfer via a very high frequency
pulsating light. The light sensor generated a voltage which was proportional to the
intensity of light it received. From this waveform the sent data could be recovered.
The speed of transfer could be determined by the maximum frequency the system
could handle. Usage of light intensity sensor means for proper data transfer the
light source does not needs to be turned off completely. Which in turn mean the
light source can be used as a light source and as a wireless communication
medium.
If this technology is properly advanced, data transmission at high speeds is possible
and it can overcome the problems that Wi-Fi has in urban densely populated areas.
This technology will be more widely adopted if it can be built into laptops and
smartphones and also if the lighting infrastructure of the place in use can be
modified. Overall it offers high bandwidth which it can be increased in the future
by using more than one channel by means of RGB lights
1. INTRODUCTION
Public Wi-Fi can be a frustrating experience, as more and more people and their many devices access
wireless internet, the total transfer speeds slow down. One German physicist Harald Haas came up with a
solution which he calls “data through illumination” –taking the fiber out of fiber optic by sending data
through light that varies in properties faster than the human eye can follow. It’s the same idea behind
infrared remotes but with a lot more potential. Haas said his invention, which he called DLIGHT, could
produce data rates faster than the average broadband connections in his time. He envisioned a future
where data for laptops, smart phones, and tablets is transmitted through the light that illuminates the
* Correspondingauthor: tahmidhassan@live.com
335 | Tahmid Hassan Talukdar&ShahadatHussainParvez. ICERIE 2017
room. The security will also be top notch compared to Wi-Fi because if you are unable to see the light,
you have no way of capturing the transferred data and manipulating it (Rani. et. al., 2012).
The primary advantage of Li-Fi is that it can play a major role in relieving loads of other wireless systems
such as the Wi-Fi. It can work side by side with Wi-Fi offering a much larger frequency band (300 THz)
combined compared to that available in RF communications (300GHz) alone. Moreover, data
transmission via light makes sure less data transmission is done via radio frequency which can have an
adverse effect on human body. Bandwidth and security is the primary feature of Li-fi because Wi-fi is
prone to DOS, handshake and packet-capturing. This is why, it is ideal for use in high security military
areas where RF communication is exposed to eavesdropping. (Sharma et. al., 2014).
For a Li-Fi system to work, the very basic requirements are to have a transmitter which will transmit the
light and a receiver which will receive it. A known term is VLC, Visible Light Communication which is a
data communication medium. It uses visible light between 400 THz (780 nm) and 800 THz (375 nm) as
optical carrier for data transmission and illumination. It uses fast pulses of light to transmit information
across devices wirelessly. The primary components of a Li-Fi system are as follows:
a) A high brightness white LED which acts as transmission source.
b) A photodiode with a good enough response time to visible light as the receiving element.
LEDs can be switched on and off to generate digital strings of different combination of 1s and 0s. The sending
method will vary depending on the limitation of hardware and how fast the data needs to travel. For example, to
generate a new data stream, data can be transmitted via light by varying the flickering rate of the LED. The LEDs
can be used as a sender or source, by modulating the input signal to the LED with the data signal. The varied
frequency of the LED is not detectable to human eye because the flickering rate is way over 100 Hz but the
photodiode can easily detect the signal without any issues. Then it’s just a matter of decoding to get the data out of
the stream. Using multiplexing technique, very high speeds are possible. VLC data rate can also be increased by
parallel data transmission using an array of LEDs where each LED will transmit a different data stream, making the
effective bandwidth multiple times higher than one LED stream.
2. LIGHT FIDELITY CONVENTIONS
Conventional orthogonal frequency division multiplexing signals are complex-valued and bipolar in
nature. Therefore, the standard RF OFDM technique has to be modified in order to become suitable for
Intensity Modulation Direct Detection systems. A straightforward way to obtain a real-valued OFDM
signal is to impose a Hermitian symmetry constraint on the sub-carriers in the frequency domain.
However, the resulting time-domain signal is still bipolar. For intensity variation, unipolar system will be
preferred over bipolar in this case.
One way for obtaining a unipolar signal is to introduce a positive direct current (DC) bias around which
the amplitude of the OFDM signal can vary. The resulting unipolar modulation scheme is known as DCbiased optical OFDM (DCO-OFDM). The addition of the constant biasing level leads to a significant
increase in electrical energy consumption. However, if the light sources are used for illumination at the
same time, the light output as a result of the DC bias is not wasted as it is used to fulfill the illumination
function. Only if illumination is not required, such as in the uplink of a Li-Fi system, the DC bias can
significantly compromise energy efficiency. Therefore, researchers have devoted significant efforts to
336 | Tahmid Hassan Talukdar&ShahadatHussainParvez. ICERIE 2017
designing an OFDM-based modulation scheme which is purely unipolar. Some well-known solutions
include: asymmetrically clipped optical OFDM (ACO-OFDM), (Armstrong, 2006) pulse-amplitudemodulated discrete multitone modulation (PAM-DMT), (Lee, 2009) unipolar OFDM (U-OFDM),
(Tsonev, 2012) Flip-OFDM, (Fernando, 2011) spectrally-factorized optical OFDM (SFO-OFDM),
(Asadzadeh, 2011) etc. The general disadvantage of all these techniques is a 50% loss in spectral
efficiency, i.e., the data rates is halved.
This modulation technique is the prerequisite for duplex systems where multiple users are supported.
When it comes to the scope of this paper, the system is more of a one-way system, like a broadcaster or a
P.A. system in a public space. In this case, multiple user authentication and multiplexing matters less, and
speed and authenticity of data transfer matters more. The highest speeds depend on quality and response
time of photo detectors entirely. If the infrastructure is developed fully to support light based public
announcement systems it will not only ensure there is less data traffic clogging, but also secure
communications between sending and receiving end.
In this case only frequency modulation is used, the frequency of the pulse was set depending on which bit
the system was transferring. Even though it will reduce the complexity of the system compared to the
multiplexed system, it can be further optimized by specifying separate wavelengths as separate channels.
This will increase the data transfer capabilities of the system by a large margin. This approach focuses on
increasing the amount of data to be transmitted in parallel instead of increasing speed serially.
The experiment done here focuses on one channel stream only, and tries to maximize the serial transfer
speed of raw bit-wise data. This can be improved using photo detectors with a higher response rate than
the used one. Also, data modulation and manipulation before and after the transmission takes place can
increase the amount of data that can be sent per second, but that is beyond the scope of this paper.
3. METHODOLOGY
The data transfer is done between two nodes: a transmitter and a receiver. For transmission large LED
panel light was used, which was driven using MOSFET with adequate power handling capability. For
transmission the LED panel was modulated using Pulse Width Modulation technique. For bit 1 a duty
cycle and for bit 0 a different duty cycle was used and in both the case the frequency remained same. To
make the LED panel more useful the LED panel was kept on the whole time, so for data transmission the
waveform used was slightly aberrant. The wave started with initially being low and then going high (See
figure 1 for details). To speed up the data transmission the panel was modulated for one complete cycle
only. And on the receiving side PIN photodiode was used in reverse biased stage. The current was
allowed to flow through a resistor which allowed for a voltage drop to be measured.
The PIN diode used in the project was BPW34. The BPW34 is a very high speed and highly sensitive
PIN photodiode in a small flat plastic package. Its top view construction makes it ideal as a low cost
replacement of TO-5 devices in a lot of applications. Because of its water-clear epoxy the device is
sensitive to visible and infrared radiation. The large active area gives the photo detector a high sensitivity
at a wide viewing angle. This is ideal as a receiver because it has a rise time and a fall time of 100 ns each
337 | Tahmid Hassan Talukdar&ShahadatHussainParvez.
Talukdar&ShahadatHussainParvez ICERIE 2017
which is good enough for detecting high frequency light pulses making it a perfect candidate to be used as
a receiver
ceiver (Vishay Semiconductors, 2004).
The data stream was done serially so the data first had to be brought down to binary values to send them
bit by bit. After that, for every 1 bit the LED was turned off for 400 microseconds and then turned on for
600 microseconds.
icroseconds. On the other hand,
han in the case of 0 bit, the on-off times were reversed; it was kept off
for 600 microseconds and on for 400 microseconds. This meant half as much frequency. Figure 1 below
shows the timing diagram for data transmission used in this article.
Fig. 1: Timing diagram for LED input for each bit of data
In the receiving end the receiver waits for data transfer to start, which is indicated by a stream of 8 1 bits.
After the byte 255 was received, the receiver waits for the ASCII bytes to come. End of data is again
denoted by a stream of 8 1 bits. For the receiver to identify bit 0 or bit 1 calculating timing is essential.
ThePIN diode is connected in such a way that the voltage at sensing point is 0 when there is no or little
light. But the voltage is above 3.3 volt when there is light. So if the light is turned on and off at regular
interval there is a pulse in the sensing point. Sensing point is connected to microcontro
microcontroller which is
configured to measure the pulse length of the high pulse i.e. the time for which the li
light is on. If the time
is 8 milliseconds, then the bit counted as 0 and if the time is 4 milliseconds,
milliseconds then the bit is 1. After that,
the received signal iss converted to the desired output file format. Post-processing
Post processing is beyond the scope of
this experiment. The primary goal of this experiment was to send data across devices via light not only
with great speed but also with great reliability as well.
4. EXPERIMENTAL
IMENTAL SETUP
The whole setup consists of two sections the transmitter section and the receiver section. The transmitter
section contains a microcontroller connected to a computer via USB to serial adapter, which transmits
data by controlling the LED Pulse Width. Figure 2 below shows the circuit of the transm
transmitter used. The
LED panel used is a standard 18 watt overhead LED panel typically used for office space. The panel
works on 20 volt dc. The LED panel is switched on and off via MOSFET. Since it is not ideal to operate
MOSFETss directly using microcontroller,
microcontroller, a MOSFET driver is used to drive the MOSFET. The driver is
controlled directly by the microcontroller. This design additionally ensures that high power LED panel
can be used in future which can be turned on and off using IGBT. The microcontroller used in the
transmitter is ATMEGA328 running at 16 MHz The microcontroller is hooked to a computer via a USB
to serial adapter so that data can be sent to microcontroller from a serial monitor on the computer. The
microcontroller is communicating with the computer
computer at 115200 bits per seconds speed.
338 | Tahmid Hassan Talukdar&ShahadatHussainParvez. ICERIE 2017
Fig. 2 Transmission circuit
On the receiver section BPW34 PIN photodiode is used in reverse bias configuration with a resistor
connected to ground. Figure 3 below shows the circuit of the receiver used. The resistor value is chosen
for the configuration to provide above 3.3 volt at sensing point when light is shone to the photodiode and
around 0 volt when there is no light shone onto the photodiode. The sensing point is connected to the
microcontroller, which measures the timing necessary to know the bits passed. The bits are organized by a
stream of 8 i.e. bytes. The microcontroller decodes the stream after the whole byte is received and the
result is sent in the serial monitor of the computer the microcontroller is connected via USB to Serial
converter. The microcontroller is communicating with the computer at 115200 bits per seconds speed.
Fig. 3 Receiver circuit
339 | Tahmid Hassan Talukdar&ShahadatHussainParvez. ICERIE 2017
5 EXPERIMENT AND RESULTS
Before moving onto transferring actual files, we tried transferring bits of data to see if the transmission
system was working. We started from a very low frequency to make sure the data was passing through
and didn’t want room for errors caused by the photo detector not reacting fast enough.
The modulation frequency initially used was 10 Hz with the duty cycle for different bit as explained in
figure 1. With this setup data transmission transpired without any problems. There were some extra
delays for debugging that were then removed to test how fast the system can actually go. It performed
without any flaws even when external light and other disturbances were present in the system. The only
problem left was slow transfer speeds due to low frequency and the flickering issue that was also because
of the frequency being too low.
Increasing the frequency reduced the flickering and made the transmission faster, but worked only up to 1
KHz frequency. In this frequency data transmission was much faster than before and the flickering was
completely removed. Going above this frequency resulted in loss of bits and corrupted files.
A file sized exactly 1 Kilobyte took around 13 seconds, which roughly translates to a speed of 78 bytes
per second or 624 bits per second. This calculated time includes the processing time in the system which
encodes the information to bits and the receiver system which decodes the bits to actual data and the
communication delay in communicating with computer. This experiment uses off the shelf parts available
in the market. Using more specialized light sensor can result in a better transmission speed.
6. CONCLUSION
There are endless possibilities to be discovered in this field of technology. If this infrastructure is properly
developed, any controlled light source can turn into a Wi-Fi hotspot to transmit data wirelessly. This
experiment just sheds light on the idea that even without an infrastructure used worldwide, a reliable
system can be built to take advantage of this technology in which all the embedded devices will use this
method of communication to transfer data between themselves. Using this method, public announcement
systems can be built, possibly a hospital infrastructure getting rid of harmful Wi-Fi or changing street
signs in roadsides illuminated and informed by streetlights.
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