1
INTRODUCTION
WHAT IS LI-FI?
Li-Fi is a VLC, visible light communication, technology developed by a
team of scientists including Dr Gordon Povey, Prof. Harald Haas and Dr Mostafa
Afgani at the University of Edinburgh. The term Li-Fi was coined by Prof. Haas
when he amazed people by streaming high-definition video from a standard
LED lamp, at TED Global in July 2011. Li-Fi is now part of the Visible Light
Communications (VLC) PAN IEEE 802.15.7 standard.“Li-Fi is typically
implemented using white LED light bulbs. These devices are normally used for
illumination by applying a constant current through the LED. However, by fast
and subtle variations of the current, the optical output can be made to vary at
extremely high speeds. Unseen by the human eye, this variation is used to carry
high-speed data, “says Dr Povey, , Product Manager of the University of
Edinburgh's Li-Fi Program „D-Light Project‟.In simple terms, Li-Fi can be thought
of as a light-based Wi-Fi. That is, it uses light instead of radio waves to transmit
information. And instead of Wi-Fi modems, Li-Fi would use transceiver-fitted
LED lamps that can light a room as well as transmit and receive information.
Since simple light bulbs are used, there can technically be any number of access
points. This technology uses a part of the electromagnetic spectrum that is still
not greatly utilized- The Visible Spectrum. Light is in fact very much part of our
lives for millions and millions of years and does not have any major ill effect.
Moreover there is 10,000 times more space available in this spectrum and just
counting on the bulbs in use, it also multiplies to 10,000 times more availability
2
as an infrastructure; globally. It is possible to encode data in the light by varying
the rate at which the LEDs flicker on and off to give different strings of 1s and 0s.
The LED intensity is modulated so rapidly that human eyes cannot notice, so the
output appears constant.More sophisticated techniques could dramatically
increase VLC data rates. Teams at the University of Oxford and the University of
Edinburgh are focusing on parallel data transmission using arrays of LEDs,
where each LED transmits a different data stream. Other groups are using
mixtures of red, green and blue LEDs to alter the light's frequency, with each
frequency encoding a different data channel.Li-Fi, as it has been dubbed, has
already achieved blisteringly high speeds in the lab. Researchers at the Heinrich
Hertz Institute in Berlin, Germany, have reached data rates of over 500
megabytes per second using a standard white-light LED. Haas has set up a spin-
off firm to sell a consumer VLC transmitter that is due for launch next year. It is
capable of transmitting data at 100 MB/s - faster than most UK broadband
connections.
3
GENESIS OF LI-FI
DR. Harald Hass, at TED Talks July 2011
Harald Haas, a professor at the University of Edinburgh who began his research
in the field in 2004, gave a debut demonstration of what he called a Li-Fi
prototype at the TEDGlobal conference in Edinburgh on 12th July 2011. He used
a table lamp with an LED bulb to transmit a video of blooming flowers that was
then projected onto a screen behind him. During the event he periodically
blocked the light from lamp to prove that the lamp was indeed the source of
incoming data. At TEDGlobal, Haas demonstrated a data rate of transmission of
around 10Mbps -- comparable to a fairly good UK broadband connection. Two
months later he achieved 123Mbps.
4
HOW LI-FI WORKS?
fig 3.1 Data transfer using Li-Fi.
Very quickly, which gives nice opportunities for transmitting data Hence all that
is required is some LEDs and a controller that code data into those LEDs. All one
has to do is to vary the rate at which the LED‟s flicker depending upon the data
we want to encode. Further enhancements can be made in this method, like
using an array of LEDs for parallel data transmission, or using mixtures of red,
green and blue LEDs to alter the light‟s frequency with each frequency encoding
a different data channel. Such advancements promise a theoretical speed of 10
Gbps – meaning one can download a full high-definition film in just 30 seconds.
5
Fig 3.2 An artistic future vision of Li-Fi system at work.
To further get a grasp of Li-Fi consider an IR remote.(fig 3.3). It sends a single
data stream of bits at the rate of 10,000-20,000 bps.
Now replace the IR LED with a Light Box containing a large LED array. This
system, fig 3.4, is capable of sending thousands of such streams at very fast rate.
6
Fig 3.3, Data stream from an IR remote control.
Fig 3.4, Data streams of a typical Li-Fi system
7
TECHNOLOGY
Transmitters
Every kind of light source can theoretically be used as transmitting device
for VLC. However, some are better suited than others. For instance, incandescent
lights quickly break down when switched on and o_ frequently. These are thus
not recommended as VLC transmitters. More promising alternatives are
uourescent lights and LEDs. VLC transmitters are usually also used for
providing illumination of the rooms in which they are used. This makes
uorescent lights a particularly popular choice, because they can icker quickly
enough to transmit a meaningful amount of data and are already widely used for
illumination purposes. However, with an ever-rising market share of LEDs and
further technological improvements such as higher brightness and spectral
clarity [Won et al. 2008], LEDs are expected to replace uorescent lights as
illumination sources and VLC transmitters. The simplest form of LEDs are those
which consist of a bluish to ultraviolet LED surrounded by phosphorus which is
then stimulated by the actual LED and emits white light. This leads to data rates
up to 40 Mbit/s [Won et al. 2008]. RGB LEDs do not rely on phosphorus any
more to generate white light. They come with three distinct LEDs (a red, a blue
and a green one) which, when lighting up at the same time, emit light that
humans perceive as white. Because there is no delay by stimulating phosphorus
_rst, Data rates of up to 100 MBit/s can be achieved using RGB LEDs ([Won et al.
2008]). In recent years the development of resonant cavity LEDs (RCLEDs) has
advanced considerably. These are similar to RGB LEDs in that they are
8
comprised of three distinct LEDs, but in addition they are _tted with Bragg
mirrors which enhance the spectral clarity to such a degree that emitted light can
be modulated at very high frequencies. In early 2010, Siemens has shown that
data transmission at a rate of 500MBit/s is possible with this approach [Siemens
2010]. It should be noted that VLC will probably not be used for massive data
transmis- sion. High data rates as the ones referred to above, were reached under
meticulous setups which cannot be expected to be reproduced in real-life
scenarios. One can expect to see data rates of about 5 kbit/s in average
applications, such as location estimation [Haruyama et al. 2008]. The distance in
which VLC can be expected to be reasonably used ranges up to about 6 meters
[Won et al. 2008].
Receivers
The most common choice of receivers are photodiodes which turn light
into electrical pulses. The signal retrieved in this way can then be demodulated
into actual data. In more complex VLC-based scenarios, such as Image Sensor
Communication [Iizuka and Wang 2008], even CMOS or CCD sensors are used
(which are usually built into digital cameras)
9
MODULATION
In order to actually send out data via LEDs, such as pictures or audio _les, it is
necessary to modulate these into a carrier signal. In the context of visible light
communication, this carrier signal consists of light pulses sent out in short
intervals. How these are exactly interpreted depends on the chosen modulation
scheme, two of which will be presented in this section. At _rest, a scheme called
subcarrier pulse-position modulation is presented which is already established as
VLC-standard by the VLCC. The second modulation scheme to be addressed is
called frequency shift keying, commonly referred to as FSK. A detailed account
on modulation can be found in Sugiyama et al. [2007]. They also explore how to
combine pulse-position modulation with illumination control.
Pulse-position modulation
To successfully carry out subcarrier pulse position modulation (SC-PPM) a time
window T is chosen in which exactly one pulse of length T/k is expected. Thus,
subcarrier pulse-position modulation can also be described as parameterized
form, i.e. SC-kPPM. k has to be a power of two, i.e. k = 2 ℓ for some ℓ. Then there
are k = 2ℓ different points of time for the pulse to occur. Suppose a pulse is
registered at some point k‟≤ k. The data represented by this pulse is then simply
the number k0 written as k{digit binary number.
Figure 4 exemplifies pulse-phase modulation by showing how the data 1,
0, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 1 is modulated into a succession of pulses with
SC-4PPM and SC-2PPM. The standard JEITA CP-1222 [Haruyama et al. 2008]
which is promoted by the VLCC, recommends using a SC-4PPM modulation
scheme.
10
Data is represented by presence and absence of the carrier wave which is a
scheme generally referred to as On-Off Keying (OOK). An alternative scheme is
presented in the upcoming section.
Frequency-shift keying
In frequency shift keying (FSK) data is represented by varying frequencies
of the carrier wave. In order to transmit two distinct values (0 and 1), there need
to be two distinct frequencies. This is also the simplest form of frequency-shift
keying, called binary frequency-shift keying (BFSK). Figure shows an example of
frequency-shift keying by modulating of the same data string that was used in
the SC-PPM example.
Examples for sub-carrier pulse position modulation in context of VLC: SC-4PPM
and SC-2PPM
11
Example for binary frequency-shift keying in VLC
At this point it is important to clarify a common source of confusion: In
none of the modulation schemes it is the actual light frequency that is changed.
That would lead to undesired effects as changing the light frequency also means
changing the wave length of the light. Since VLC transmitters also serve general
illumination purposes, ongoing variation of the color of surrounding light is
unacceptable in most circumstances.
In subcarrier pulse position modulation it is the occurrence of light pulses
that defines the frequency whereas in frequency shift keying the actual pulse
frequency is changed depending on the data that is to be sent. In FSK, there is no
“position” of pulses, because light pulses are sent uninterruptedly
12
WHY LI-FI?
PRESENT SCENARIO IN WIRELESS
COMMUNICATION
-fi devices present.
devices, we transmit more than 600 terabytes of data every
month.
Wireless communications has become a utility like electricity and water. We use
it every day. We use it in our everyday lives now -- in our private lives, in our
business lives. And we even have to be asked sometimes, very kindly, to switch
off the mobile phone at events like this for good reasons. And , therefore , it is
very important to look into the issues that this technology has, because it's so
fundamental to our lives.
ISSUES WITH WI-FI USING RADIO WAVES
There are four issues with the current wi-fi scenario , which are :-
1. CAPACITY
We transmit wireless data is by using electromagnetic waves -- in particular,
radio waves.
Radio waves are scarce, expensive and we only have a certain range of it.
13
wireless data transmissions and the number of bytes and data which are
transmitted every month.
2. EFFICIENCY
yed worldwide.
used to cool the base stations.
The efficiency of such a base station is only at about five percent.
3. HEALTH ISSUES
associated with radio waves.
-phones in places like
hospitals.
4. SECURITY
14
ALTERNATIVES TO RADIO WAVES IN EM
SPECTRUM
The issues concerning radio waves beg a close inspection at EM Spectrum
for some alternative. The EM Spectrum is as given below:-
The Electromagnetic Spectrum
dangerous.
-Rays have similar health issues.
human body.
Hence we are left with only the Visible Light Spectrum.
15
LIGHT FOR WIRELESS COMMUNICATION
Light is inherently safe and can be used in places where radio frequency
communication is often deemed problematic, such as in aircraft cabins or
hospitals. So visible light communication not only has the potential to solve the
problem of lack of spectrum space, but can also enable novel application. The
visible light spectrum is unused; it's not regulated, and can be used for
communication at very high speeds.
HOW LI-FI OVERCOMES ISSUES ATTACHED WITH RADIO
WAVES:-
1. CAPACITY
waves region.
2. EFFICIENCY
transmission is very
efficient.
3. SAFETY
4. SECURITY
16
POTENTIAL APPLICATIONS OF LI-FI
Li-Fi technology is still in its infancy. However some areas where it seems
perfectly applicable are:-
Smart Lighting. Any private or public lighting including street lamps can
be used to provide Li-Fi
hotspots and the same communications and sensor infrastructure can be
used to monitor and control lighting and data.
Indoor Positioning. Transmission of a unique ID is all that is required for
basic positioning. Multiple LED light bulbs can be used with trilateration
for more accurate indoor positioning and navigation.
Mobile Connectivity. Laptops, smart phones, tablets and other mobile
devices can interconnect directly using VLC. Short range links give very
high data rates and also provides security via the visible pairing method.
Hazardous Environments. VLC provides a safe alternative to
electromagnetic interference from RF communications in environments
such as mines and petrochemical plants.
Vehicles & Transportation. LED headlights and tail-lights are being
introduced. Street lamps, signage and traffic signals are also moving to
LED. This can be used for vehicle-to-vehicle and vehicle-to-roadside
communications. This can be applied for road safety and traffic
management.
Hospital & Healthcare. VLC emits no electromagnetic interference and so
does not interfere with medical instruments, nor is it interfered with by
MRI scanners.
17
Wi-Fi Spectrum Relief. Excess capacity demands of Wi-Fi networks can be
off-loaded to VLC networks where available. This is especially effective on
the downlink where bottlenecks tend to occur.
Aviation. LEDs are being used in aircraft passenger cabins. VLC can be
used to reduce weight and cabling and adding flexibility to seating
layouts. The in-flight entertainment systems can be supported by VLC.
Underwater Communications. Due to strong signal absorption in water,
RF use is impractical. Acoustic waves have extremely low bandwidth and
disturb marine life. VLC provides a solution for short-range
communications.
RF Avoidance. Some people claim they are hypersensitive to radio
frequencies and are looking for an alternative. VLC is a good solution to
this problem.
Toys. Many toys incorporate LED lights and these can be used to enable
extremely low-cost communication between interactive toys.
18
ADVANTAGES/DISADVATAGES OF LI-FI
ADVANTAGES
1. SUPERIORITY OVER RF WAVES
As was demonstrated earlier, the visible light has considerable edge over RF
waves in many fields.
2. LITTLE INFRASTRUCTURE REQUIREMENTS
There are an estimated 14 billion bulbs in the world today. Since Li-Fi can
operate
on conventional LEDs infrastructure is pretty much present already.
3. SIMPLE SYSTEM STRUCTURE
A typical Li-Fi system consists of an LED array, a photoreciever , a de/modulator
pair.
DISADVANTAGES
The biggest disadvantage is that it needs direct line of sight to transmit data, so
one wouldn't be able to have a single router in his/her house and the data goes
through walls etc..
19
FUTURE
In 2009, the US Federal Communications Commission warned of a
looming spectrum crisis: because our mobile devices are so data-hungry we will
soon run out of radio-frequency bandwidth. Li-Fi could free up bandwidth,
especially as much of the infrastructure is already in place.The solution might be
Li-Fi. Direct modulation of LED devices is a low cost, secure, and safe way to
transmit data, and there is an abundance of free visible light spectrum. High
intensity LEDs used in light bulbs, flash lights and cameras can transmit very
high data rates, faster than Wi-Fi.And the technique looks good not only on
paper. At Heinrich Hertz Institute in Berlin, Germany researchers have achieved
a data rate of 500 megabytes per second using a standard white LED. This
year‟s,2012, Consumers Electronics Show in Las Vegas demonstrated VLC in full
vigour when a pair of Casio smartphones exchanged data using light of varying
intensity given off from theirscreens. In October, 2011 a number of companies
and industry groups formed the Li-Fi Consortium to work towards and promote
Light Fidelity (Li-Fi) in order to overcome the rapidly diminishing bandwidth for
Wireless Fidelity (Wi-Fi).However everyone is not so optimistic. Dr Suresh
Borkar , a trend-watcher, consultant and communications expert who teaches at
the Illinois Institute of Technology, opines that at the current stage of maturity,
Li-Fi usage will be limited to in-house and proximity applications. The use of
very high frequency (400-800 THz) limits it to very short distances and more of
point-to-point communications.
Li-Fi, according to Dr Borkar, is still in the experimental laboratory stage.
Standards have to be defined and devices identified and made available along
with the infrastructure and related entities before it can be used widely. Some
limited prototypefriendly deployments have taken place in the last year or so but
20
the availability of receiving devices that require arrays of photodiodes is still
limited.
21
CONCLUSION
The fact that Li-Fi is being considered as one of the IEEE 802.xx standards bodes
well for its potential success. Like other 802.xx standards, it is definedonly at
layers 1 and 2 (physical and media access control (MAC) layers) of the Open
Systems Interconnection (OSI) model. Layer 3 and higher layers need to be
designed using the Internet Engineering Task Force (IETF) packet transport
standards.
Li-Fi is certainly not useless, but it has certain inherent limits for the technology.
LiFi may not be able to replace conventional radios altogether, but it could
turbocharge the development of wireless television and make it easier to throw a
wireless signal across an entire house. At present, finding the ideal position for a
wireless router is something of a divine art. If the signal could be passed via VLC
from Point A to Point B inside a home, small local routers at both points could
create local fields with less chance of overlapping and interfering with each
other. Large scale areas that are saturated with radio signals or that don‟t permit
them for security reasons could use LiFi as an alternate high-speed wireless
network solution.
22
BIBLIOGRAPHY
REFERENCES
[1] Project: IEEE P802.15 Working Group for Wireless Personal Area Networks
(WPANs)
Submission Title: [Visible Light Communication : Tutorial]
Date Submitted: [9 March 2008]
Source: [(1)Eun Tae Won, Dongjae Shin, D.K. Jung, Y.J. Oh, Taehan Bae, Hyuk-
Choon Kwon, Chihong
Cho, Jaeseung Son, (2) Dominic O‟Brien (3)Tae-Gyu Kang (4) Tom Matsumura]
Company [(1)Samsung
Electronics Co.,LTD, (2)University of Oxford, (3)ETRI (4) VLCC (28 Members)]
[2] Design and Implementation of an Ethernet-VLC Interface for Broadcast
Transmissions
Thispaper appears in: Communications Letters, IEEE Date of Publication:
December 2010 Author(s): Delgado, F. Dept. de Ingeniera Telematica, Univ. de
Las Palmas de Gran Canada, Las Palmas de Gran, Spain Quintana, I. ; Rufo, J. ;
Rabadan, J.A. ; Quintana, C. ; Perez-Jimenez, R.
23
Websites
http://www.ed.ac.uk
http://www.visiblelightcomm.com
http://new.electronicsforu.com
http://blog.ted.com
http://www.newscientist.com
http://purevlc.com