li-fi.pdf

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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

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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.

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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.

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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.

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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.

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Fig 3.3, Data stream from an IR remote control.

Fig 3.4, Data streams of a typical Li-Fi system

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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

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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)

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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.

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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

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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

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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.

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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

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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.

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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

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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.

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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.

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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..

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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

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the availability of receiving devices that require arrays of photodiodes is still

limited.

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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.

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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.

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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