Vlc term paper

1. 1
Indoor Visible Light Communication
INTRODUCTION
Visible light communications (VLC) has emerged as a potential technology for ubiquitous indoor wireless
broadband access. It refers to the transmission of information by modulating the intensity of light-
emitting diodes (LEDs) at high frequencies making instantaneous changes in light intensity unnoticeable
to the human eye. Therefore, VLC technology can exploit the existing lighting infrastructure where
legacy tungsten and florescent-based lamps are being replaced by high brightness LEDs with longer
lifetime, lower power consumption, and higher efficiency.
Figure 1. Visible Light Spectrum
The opto-electronic devices used in VLC are cheaper as compared to RF equipment as well as wireline
systems. Further, VLC transmission does not interfere with existing RF systems and is not governed by
Federal Communications Commission (FCC) regulations. The Visible light signal does not penetrate walls,
thus providing a degree of privacy within the office area. In addition to privacy, this feature of VLC
systems makes it easier to build a cell-based network. For example, in an office building each room
would be a cell and there would be no interference between the cells. Therefore, all units can be
identical in a cellular architecture as compared to RF configuration in which the operating frequencies of
neighboring cells have to be different. Due to the above reasons, optical wireless systems are becoming
more popular in various operating environments.

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MOTIVATION
The number of personal computers and personal digital assistants for indoor use are rapidly growing in
offices, manufacturing floors, shopping areas and warehouses. This will result in the need for flexible
interconnection through the distributed or centralized data communication systems. [1]
The traditional way to meet this requirement is to use wired physical connections. But, wired physical
connections have some inherent problems, in setting up and in its expansion. Further, these need more
space, time to setup, monetary investment in copper, maintenance etc. Wireless systems offer an
attractive alternative. While the radio spectrum is limited, the demand for wireless data transmission
keeps increasing. There is a pressing need for new kinds of wireless communication systems. Visible light
communication (VLC) has been proposed as an alternative means of wireless communication. The idea is
to modulate LEDs transmitting electromagnetic waves in the visible light frequencies to communicate
between devices within the same room.
HISTORY
The VLC first developed by Dr. Alexander Graham Bell on 21st June, 1880. He had demonstrated a
system which is capable of transmitting voice signals using sun light as a carrier and it was named as
Photophone. The transmitter used sunlight reflected off a vibrating mirror to send voice signals. The
receiver receives the light signal modulated with voice signal and demodulates it using selenium photo
cell. The distance between the transmitter and receiver was around 213 meters in this demonstration.
More new work started in 2003 at Nakagawa Laboratory, in Keio University, Japan, utilizing LEDs to
convey information by noticeable light. Since then there have been countless study doings focused on
VLC
Figure 2. Photophone: Transmitter and Receiver [2]

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INDOOR VLC SYSTEM
Like any other communication system Visible light communication system also consists of three major
components:
1. Optical Transmitter
2. Optical Receiver
3. Channel
Figure 3. A VLC system
A transmitter consists of an Information source, a driver circuit (which converts information signal into
appropriate current variations to drive LED/LASER), LED/LASER, an optical antenna with other optical
components such as lens and a tracking machine to direct the beam towards receiver side.
On the receiver side, the receiving antennas (lens) collect all the power falling on it and focus it on the
receiver detector. Detector converts optical signal to electrical signal, which will be demodulated to
obtain the required message signal.
Indoor atmosphere (channel for indoor VLC) is free of environmental degradation, such as mist, fog,
particulate matter, clouds etc., indoor optical wireless systems encounter only free space loss and signal
fading.
 Free Space Loss: It is that part of the transmitted power, which is lost or not captured by the
receiver’s aperture
 Signal Fading: The reason for this is reception of signals via different paths by the receiver. Some
of these interfere destructively (i.e. they are out of phase), so that the received signal power
effectively decreases. This type of degradation is also known as multi-path signal fading.

4. 4
Transmission Techniques
Several transmission techniques are possible for indoor optical wireless systems; these
techniques may be classified according to the degree of directionality of transmitter and receiver:
1. Directed beam infrared (DBIR) radiation: In DBIR (Fig.4a) system, the optical beam travels
directly without any reflection from the transmitter to the receiver. The optical wireless link
using this technique is established between two fixed data terminals with highly directional
transmitter and receiver at both ends of the link.
Figure 4a. Directed beam infrared (DBIR) radiation [1]
2. Diffuse infrared (DFIR) radiation: In DFIR (Fig.4b) system, the transmitters send optical signals in
a wide angle to the ceiling and after one or several reflections the signals arrive at the receivers.
This is the most desirable configuration from a users’ point of view, since no alignment is
required prior to use, and the systems do not require a line-of-sight path for transmission. In this
configuration, the data rate depends on the room size and the reflection coefficients of the
surfaces inside the room.
Figure 4b. Diffuse infrared (DFIR) radiation [1]

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3. Quasi-diffuse infrared (QDIR) radiation: In QDIR (Fig.4c) system, there is a base station (BS) with
a relatively broad coverage made of passive or active reflector. The BS is usually mounted on the
ceiling. The BS transmits (receives) the signal power to (from) the remote terminals (RTs).
Figure 4c. Quasi-diffuse infrared (QDIR) radiation [1]
Modulation Techniques:
In optical wireless communications, modulation takes place in two stages. First the transmitted
information is coded as waveforms and then these waveforms modulate emitted visible light.
Several modulation and detection schemes have been considered for use in optical wireless systems in
the past. Most common schemes suitable for indoor optical wireless are the On-Off Keying (OOK), Pulse
Modulation (PM) and Sub-carrier Modulation.
 OOK is the simplest technique to implement in wireless infrared transmission. Prior to
transmission, the information is translated to a specific code such as Manchester, RZ, or NRZ
codes, to get a stream of pulses. In OOK, a pulse is transmitted if the code bit is ‘one’ during a
fixed time slot and a ‘zero’ is represented by the absence of the pulse during the time slot.
 Pulse Modulation (PM): Indoor optical wireless communication systems require modulation
techniques, which make high-speed digital transmission possible with less average transmitter
power. Higher average power efficiency can be achieved by employing pulse modulation
schemes in which a range of time-dependent features of a pulse carrier may be used to convey
information. Examples of such modulation schemes are:
1. PPM and its variants are widely considered as the best modulation techniques for
power-limited intensity modulation with direct detection (IM/DD) communication
systems. PPM has been widely used in optical communication systems.
2. Differential PPM (DPPM) is a simple modification of PPM that can achieve improved
power and/or bandwidth efficiency in applications where low cost dictates the use of
hard-decision detection, and multipath ISI is minimal (e.g., at low bit rates or in
directed-LOS links).
 Sub-carrier Modulation High-speed single-carrier modulation schemes such as OOK and L-PPM
are wide band and suffer from ISI due to multipath dispersion when the symbol rate exceeds
100 Mbps. When a bit stream modulates a single radio frequency, which is then further used to
modulate optical carriers; the modulation is called single-sub-carrier modulation (SSM).

6. 6
ADVANTAGES
Visible light communication has following advantages over other competing radio communication
technologies such as Wi-Fi and cellular phone wireless communication:
 The first reason to consider is visible light’s frequency spectrum bandwidth, which ranges from
430 THz to 750 THz. The bandwidth is much larger than the radio frequency bandwidth, which
ranges from 3 kHz to 300 GHz.
 There is no EMI.
 Visible light communication requires much less power compared to RF communication
 Visible light cannot penetrate through the walls, so Indoor Visible light communication is
comparatively secure.
 Visible light usually poses no health hazards to human body and eyes.
 Light sources are everywhere and can be more efficiently used by increasing its simultaneous
functionality by transmitting data in addition to lighting an area.
DISADVANTAGES
 The most obvious is the fact that it will only work in places where there are electronic lights.
 The bandwidth of LED is limited and so are the data rates.
 It is LOS type of communication and thus blocking could interrupt the signal reception.
 Range of the communication link is limited due to the effect of turbulence, attenuation and
background noise sources.
 Speed limitations of the opto-electronic devices.
 The interference induced by the artificial light sources.
 Shot noise induced by the background ambient light & aspects due to receiver noise.
APPLICATION
Lights in the visible spectrum are used everywhere, providing several opportunities to apply visible light
communications. Some of the potential applications of visible light communication are:
1. Mobile-connectivity: By pointing a visible light at another device you can create a very high
speed data link with inherent security. This overcomes the problems of having to pair or
connect and provides a much higher data rate than Bluetooth or WiFi.
2. Positioning and communications: To obtain the position of a mobile user in indoors is
challenging and VLC allows the transmission of positioning information from a lighting fixture so
that a user knows their location in a building. There have been a number of schemes proposed
that use either triangulation or proximity to a beacon or a combination to provide position
estimation.
3. Illumination and communications: White LEDs can be used for both illumination and
communications so that information can be broadcast within a room (LiFi) or transmitted via a
car headlight with the necessary illumination provided at the same time. This may be a wide
area of application and there is considerable interest in building systems that do this.
4. Information display and communications: This is one of area of interest in VLC. Displays, such
as signboards and indicator boards, are often fabricated from arrays of LEDs, and these can be
modulated to broadcast the signboard information to a handheld terminal. This might find
application in airports, museums and other environments where location dependent broadcast
of data is required.

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CHALLENGES IN VLC SYSTEM:
1. Increasing data rate: White light LED’s manufactured using Phosphor is unable to support high
data rate due to slow component of Yellow Phosphor. The simplest way of mitigating the low
bandwidth is to block the phosphor component at the receiver by using a blue filter. Increase
bandwidth using Blue filter is shown in [7]. It can also possible to improve the response by the
use of bandwidth-efficient modulation schemes that take advantage of the high available signal
to noise ratio. In addition by using Multi Input Multi Output (MIMO) using LED’s can get higher
data rates.
2. Achieving a high electrical signal to noise ratio (SNR): The difficulty arises due to two reasons.
Firstly, the SNR of an IM/DD system depends upon the square of the average power of the
received optical signal. Secondly, in many environments there exists intense ambient infrared
noise.
3. Eye Safety: Eye safety consideration puts limit on the amount of optical power that should be
emitted by the transmitter, thus limiting the coverage of an optical wireless system. Both indoor
and outdoor optical wireless systems can pose a hazard if LDs are operated at high output
power.
4. Flickering: If the modulated optical carrier is changing with a very less frequency (<100Hz), then
the user could see the change in the intensity of light with respect to time. This could be taken
care if we use any of the sub-carrier modulation technique.
5. Uplink: VLC using illumination sources (LED’s) is naturally suited to broadcast applications, and
providing an uplink to the distributed transmitter structures can be problematic.
CONCOLUSION
Indoor VLC offers the advantage of utilizing existing lighting infrastructure and visible light for indoor
communication. It can provide both high speed and secure connection and the same time. There are a
number of technical and regulatory challenges to be overcome, rapid technical progress is being made
but the challenges of standardization will require cooperation and agreement from a number of
different bodies. However, it is expected to provide an alternative means of wireless communication to
accommodate the fast growing need for high speed communication.
REFERENCES
1. Singh, Chaturi, et al. "A review of indoor optical wireless systems." IETE Technical Review 19.1-2(2002): 3-17
2. A. G. Bell, “Selenium and the Photophone,” in Nature vol. 22, ed, 1880, pp. 500-503
3. O'Brien, Dominic, et al. "Indoor visible light communications: challenges and prospects." OpticalEngineering+
Applications. International Society for Optics and Photonics, 2008.
4. Salian, Punith P., et al. "Visible light communication." India Educators' Conference (TIIEC), 2013 Texas
Instruments. IEEE, 2013.
5. Sridhar Rajagopal and Richard D. Roberts, “IEEE 802.15.7 Visible Light Communication: Modulation Schemes
and Dimming Support” IEEE Communications Magazine, March 2012, DOI: 0163-6804, pp. 72-82.
6. IEEE 802.15 WPAN Visual Light Communication Study Group (lGvlc)
http://www.ieee802.org/15/pub/IGvlc.html, 2008
7. Grubor, J, Lee, S. C. J, Langer, K. D., Koonen, T., and Walewski, J. W., "Wireless High- Speed Data Transmission
with Phosphorescent White-Light LEOs'. Post deadline session at European Conference on Optical
Communications.