free-space optical networking using the spectrum of visible light
TRANSCRIPT
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Free-Space Optical Networking Using the Spectrum of
Visible Light
Nitin Chacko1
1Department of Electronics and Communication Engineering
Rajagiri School of Engineering and Technology, Cochin
Swapna Davies2
2Department of Electronics and Communication Engineering
Rajagiri School of Engineering and Technology, Cochin
Abstract— Radio frequency technology suffers from limited bandwidth and electromagnetic interference. The recent developments in solid-state Light Emitting Diode (LED) materials and devices are driving resurgence into the use of Free-Space Optical (FSO) wireless communication. LED-based network transceivers have a variety of competitive advantages over RF including high bandwidth density, security, energy consumption, and aesthetics. They also use a highly reusable unregulated part of the spectrum (visible light). Many opportunities exist to exploit low-cost nature of LEDs and lighting units for widespread deployment of optical communication. The prime focus is to reducing cost, and for that, we have to make appropriate selection of system’s components, e.g. modulation, coding, filtering. The objective is to describe the viability of an optical free-space visible light transceiver as a basis for indoor wireless networking and to achieve acceptable bit error rate (BER) performance for indoor use, with a low cost system.
Index Terms— Free-space optics, Light Emitting Diode, Optical communication, Optical modulation techniques, Visible light spectrum, Wireless communication .
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1 INTRODUCTION
Optical wireless communication (OWC) refers to a free-space optical
(FSO) link, where the transmitter and receiver are not necessarily aligned to each other. OWC in general addresses quite different
applications, starting from chip-to-chip interconnects and ending in intra-satellite data links. OWC links can be realized with quite
different optical sources and detectors. For low data rates, traditional light bulbs, liquid crystal displays (LCDs), or plasma display panels
(PDPs) can be used. In the receiver end, low-cost digital cameras are used as they are currently featured in every mobile device. As
societal dependence upon wireless systems continues to grow, wireless technology needs to expand to meet the demand. Phones,
laptops, and global positioning systems are all devices that implement certain forms of wireless communication to send
information to another location. However, the availability of current forms of wireless is very limited, and it is not necessarily safe to
implement wireless radio, making it necessary to explore other alternatives to wireless communication to allow continued expansion
upon communication systems and to ensure safe use. The radio spectrum is highly congested and the demand for wireless data
communication is increasing day-by-day. The bandwidth required for the radio frequency communication is rapidly getting exhausted.[1,2]
The introduction of multiple nodes and cell splitting can be done to overcome this, but it is expensive. Also, two nodes do not provide
double the capacity of one due to the interference issue. Moreover,
doubling the infrastructure will not double the revenue. Recent studies on the hazards of radio frequency have found that extreme
radio frequency radiation causes adverse effect on the environment.
Optical Wireless Communication (OWC) refers to a free-space
optical (FSO) link, where both transmitter and receiver are not
necessarily aligned to each other. OWC in general addresses quite
different applications, starting from chip-to-chip interconnects and
ending in intra-satellite data links. OWC links can be realized with
quite different optical sources and detectors. For low data rates,
traditional light bulbs, liquid crystal displays (LCDs), or plasma
display panels (PDPs) can be used. In the receiver end, low-cost
digital cameras are used as they are currently featured in every
mobile device. The new LED-based luminaries will be omnipresent a
few years from now. Besides their original lighting function, their
light can be modulated at high speed. In this way, we can realize
significantly higher data rates over moderate distances.[3]
When compared with the traditional incandescent and fluorescent
lamps, LEDs have a number of advantages such as a longer life
expectancy, a higher tolerance to humidity, a smaller size and lower
power consumption. As the cost of manufacturing decreases, LEDs
become affordable and popular for color displays, traffic signals, and
for illumination applications.[4] In recent years, LEDs have been
used to transmit data at higher rates over a short-range optical
wireless communication link.
For dual purpose of illumination and data communications, white
LEDs are ideal sources for future applications. With the availability
of highly efficient white LEDs or by using a blue emitter in
combination with a phosphor, we are witnessing a surge in research
and development in indoor visible light communication systems.
Light Emitting Diode (LED) Visible Light Communication (VLC)
system is creating a possible valuable addition to future generations
of technology, which have the potential to utilize light for the
purposes of advanced technological communication at ultra high
speed surpassing that of current wireless systems.[5]
The most common link configurations for indoor OWC systems
are the line-of-sight (LOS) and the diffuse or a hybrid LOS-diffuse.
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Normally, the diffuse system provides a larger coverage area and an
excellent mobility, but at the cost of lower data rates, higher path
losses and multipath induced inter-symbol interference (ISI) caused
by the signal reflections from walls and other objects within the
room. On the other hand, LOS links, where the beam is confined
within a narrow field-of-view (FOV), offer a much higher channel
capacity and a longer range. However, LOS links offer a limited
coverage area as well as requiring alignment and tracking to
maintain link availability. In order to protect the data integrity during
transmission the input data should be framed, so as to detect lost
signals and to ensure correct transmission and reception of the data.
Computer network protocols like stop and wait algorithms are
employed to solve this problem.
2 VISIBLE LIGHT COMMUNICATION
The radio spectrum is highly congested and the demand for wireless
data communication is increasing day-by-day. Bandwidth required
for the radio frequency communication is rapidly getting exhausted.
Introduction of multiple nodes and cell splitting can be done to
overcome this, but it is expensive. Also, two nodes do not provide
double the capacity of one due to the interference issue. Moreover,
doubling the infrastructure will not double the revenue. Recent
studies on hazards of radio frequency have found that extreme radio
frequency radiation causes adverse effect on environment.[2,3]
Optical wireless communication (OWC) systems operating in the
visible band (390–750 nm) are commonly referred to as visible light
communication (VLC). The history of Visible Light
Communications (VLC) dates back to the 1880s in Washington, D.C.
when the Scottish-born scientist Alexander Graham Bell invented
the photophone, which transmitted speech on modulated sunlight
over several hundred meters. This pre-dates the transmission of
speech by radio.
Visible light communication is a subset of wireless optic
technology. Specially designed electronic device containing
a photodiode receives signals from light sources. The image sensor
used in these devices is in fact an array of photodiodes and in some
applications its use may be preferred over a single photodiode. Such
a sensor may provide either multi-channel communication or a
spatial awareness of multiple light sources.
3 SYSTEM DESCRIPTION
Precise dimming appears to be challenging for incandescent and gas-
discharge lamps, whereas in the LEDs it is quite convenient to
accurately control the dimming level. This is because, the LED
response time during on and off switch operation is very short.
Therefore, by modulating the driver current at a relatively high
frequency, it is thus possible to switch LEDs on and off without this
being perceived by the human eyes.
3.1 VLC System Overview
LEDs are used both for lighting as well as communications. LED
access points are connected to the backbone wired network.[1]
Communications for the entire room in the system is covered by four
optical cells, each of which has a wide divergence angle LED source.
At the receiving end, the optical receivers, mounted on a mobile
terminal, has a dedicated FOV of 30° to ensure seamless
connectivity as well as alleviating the need for using pointing and
tracking schemes. In addition, the suitable modulation scheme can
also be adopted to improve the overall system capacity. The
separation between the source and receiver will be a few meters.
Each compartment or cell consists of an LED transmitter, a diffuser
and an optical receiver.
Fig. 1. Indoor cellular visible light communication with four
compartments (cells)
3.2 Transmitter End
The network transmission elements and lighting are very often used
in the same space, and thus combining the two devices into one
would save on overall component and power cost. Similarly, light
sources such as traffic lights can be retrofitted with VLC capabilities
to enable vehicular communications, or at the very least, road-to-
vehicle communications, where traffic lights can be used to transmit
information about upcoming traffic. All of these use cases rely on the
implementation of a modulated light source for communication.
Given the ease of modulating LEDs electrically, the extension of
LED lighting towards communications seems a natural next step.
Additionally, research has demonstrated that white LEDs are a viable
low-cost next step with respect to power efficient lighting. The
comparative assessment of the luminous efficacy of different light
sources is shown in table I.
LEDs with no shaping lenses can be essentially considered
as the Lambertian source. A surface which obeys Lambert's law is
said to be Lambertian, and exhibits Lambertian reflectance. Such a
surface has the same radiance when viewed from any angle. This
means that, to the human eye it has the same apparent brightness
(or luminance). It has the same radiance because, although the
emitted power from a given area element is reduced by the cosine of
the emission angle, the apparent size (solid angle) of the observed
area, as seen by a viewer, is decreased by a corresponding amount.
Therefore, its radiance is the same.
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TABLE 1 APPROXIMATE LUMINOUS EFFICACY OF DIFFERENT LIGHT SOURCES
lm/W = Lumens per Watt, hrs = Hours
In many applications, there are requirements for specific
radiation distributions to ensure a full coverage and an optimum link
performance. In such cases shaping lenses are used at the
transmitter.[3,4] The light source position at the center of a
compartment or cell is composed of an LED and an optical lens. To
achieve a wider coverage area with a uniform radiation distribution
pattern, a luminit holographic LSD is employed at the transmitter
end. Figure 2 depicts the system block diagram of a signal cell VLC
system which includes a transmitter, a concentrator, filter and a
detector.
Fig. 2. Block diagram representation of visible light communication system
3.3 Extension of Divergence Angle of Transmitter
The holographic light shaping diffusers provide extended
effective divergence angle.
𝜃 𝑜𝑢𝑡𝑝𝑢𝑡 = 𝜃(𝐹𝑂𝑉)2 + 𝜃(𝐿𝑆𝐷)2
(1)
where, θ (output) is the effective output angle of the light, θ (FOV) is
the light source field-of-view and θ (LSD) is the angle of
holographic light shaping diffuser.
Fig. 3. Visible light communication using holographic light shaping
diffuser
Holographic Diffusers are used to control the diffuse area of
illumination and increase transmission efficiency to greater than 90
percentage from filament lamps, LEDs, arc lamps, and other sources.
Standard ground glass and opal glass will produce diffuse
illumination, but the diffuse light area will often exceed the
requirements of the system.[1] This over-illumination, associated
with traditional diffusers, reduces efficiency and can often lead to
added costs by requiring higher power illumination sources, lenses,
and possibly filters. It is important to note that diffusing angles are
given for a collimated input beam and angular divergence will vary
for different incidence angles.
Unlike many holographic elements, these specific polycarbonate
components can be used throughout the visible and near-infrared.
The hologram is a two level surface relief diffractive element that
affects only the phase of light passing through it. The far-field
radiation pattern passing through the hologram is approximately the
Fourier transform of the surface relief structure. In order to simplify
the calculation of the beam intensity through the holographic LSD, it
is divided into an array of ``pixels,'' and the MATLAB platform is
used to simulate the beam profile for every pixel. For a very tiny
beam profile, the intensity of light can be considered as uniform after
passing through the single pixel. Finally, the overall coverage area is
could be the sum of individual foot prints per pixel.
3.4 Receiver End
A typical indoor optical wireless communication receiver front-
end usually consists of a concentrator, an optical filter, a
photodetector, a pre-amplifier, a post-equalizer, and an electrical
filter. A schematic diagram is shown in figure. The specifications for
the receiver are given in Table II.
TABLE 2
SPECIFICATION OF INDOOR VLC SYSTEM
MHz = Mega Hertz, nm = Nanometer, mW =Milli Watt, m = meter, mm =
millimeter, A/W = Ampere/Watt, ns=nanosecond
For Non-LOS channel or non-directed channel such as a diffuse
channel, using non-imaging hemispherical or compound parabolic
concentrator (CPC) and corresponding optical filter could effectively
enlarge the active receiving area and broaden the FOV to increase
the received optical power. However, for the directed LOS channel,
the FOV should be designed to be small to reduce the received
ambient light noise power because the ambient light noise is usually
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diffused inside the whole room as background light. Generally there
are two kinds of photodetectors that can be adopted in an indoor
LOS VLC system design - the photodiode (PD) and the image
sensor.
The photodiode has been widely adopted in optical
communication systems with relatively large received optical power.
The advantages of the photodiode include its low price and possible
high reception bandwidth. The bandwidth of the photodiode is
usually inversely proportional to its active receiving area due to the
internal capacitance along with the receiving area.[5,6]
Fig. 3. Receiver front-end for the VLC system
Compared with the photodiode, the image sensor is able to
provide receiver spatial diversity to enhance detection performance
and additional source location information for location-aware
services. For application scenarios where multiple LED arrays in a
room send different signals to multiple users, using a large FOV PD
detector may lead to large interference that degrades received SNR.
In this case an image sensor would better serve as a photodetector
that could effectively discriminate different LED arrays and reduce
inter-array interference.
Besides, using an image sensor to realize high data rate MIMO
optical wireless communication has also been proved to be feasible.
The major noise sources present in an indoor VLC system include
ambient light noise (background solar radiation through windows,
incandescent radiation, and fluorescent radiation), signal and
ambient light induced shot noise in the photodetector, and the
electrical preamplifier noise. The ambient light noise induced by
background solar radiation and incandescent lamps represents
essentially a DC interference that could be easily eliminated using an
electrical high pass filter. The noise induced by fluorescent lamps
needs to be determined in different application scenarios based on
what kind of driving circuit is used.
4 MODULATION TECHNIQUES
The eye safety introduces a limitation on the amount of optical
power being transmitted For indoor applications, the eye safety limit
on transmit optical power is even more stringent. The optical channel
differs significantly from the RF channels. Unlike RF systems where
the amplitude, frequency and phase of the carrier signal are
modulated, in optical systems, it is the intensity of the optical carrier
that is modulated in most systems operating below 2.5 Gbps data
rates. For data rates greater than 2.5 Gbps, external modulation is
normally adopted. Additionally, the use of photodetectors with a
surface area many times larger than the optical wavelength facilitates
the averaging of thousands of wavelength of the incident wave.[3]
On-Off Keying (OOK)
Among all modulation techniques based on intensity
modulation with direct detection, on-off keying (OOK) is the most
used scheme for digital optical transmission due to its simplicity. A
bit one is simply represented by an optical pulse that occupies the
entire or part of the bit duration while a bit zero is represented by the
absence of an optical pulse. Both return-to-zero and non-return-to-
zero schemes can be applied. In the NRZ scheme, a pulse with
duration equal to the bit duration is transmitted to represent 1 while
in the RZ scheme, the pulse occupies only the partial duration of bit.
The electrical power spectral densities of OOK-NRZ and
OOK-RZ (Duty cycle = 0.5) assuming independently and identically
distributed (IID) ones and zeros are given by
𝑠𝑂𝑂𝐾−𝑁𝑅𝑍 𝑓 = (𝑃𝑟𝑅)2𝑇𝑏 sin 𝜋𝑓𝑇𝑏
𝜋𝑓𝑇𝑏
2
1 +𝛿(𝑓)
𝑇𝑏 (2)
𝑠𝑂𝑂𝐾−𝑅𝑍(𝐷𝑈𝑇𝑌 𝐶𝑌𝐶𝐿𝐸=0.5) 𝑓
= (𝑃𝑟𝑅)2𝑇𝑏 sin 𝜋𝑓𝑇𝑏/2
𝜋𝑓𝑇𝑏/2
2
1 + 𝛿(𝑓 −
𝑛𝑇𝑏
)
𝑇𝑏
∞
𝑛=−∞
(3)
where, 𝛿 𝑓 is the Dirac delta function, f = Frequency, Tb = Bit
duration, Rb = Bit rate, Pr = Average optical power, R =
Responsivity
Pulse Position Modulation (PPM)
In PPM, each symbol interval of duration T = log2 L/Rb is
partitioned into L subintervals, or chips, each of duration T/L, and
the transmitter sends an optical pulse during one and only one of
these chips. For any L greater than 2, PPM requires less optical
power than OOK. In principle, the optical power requirement can be
made arbitrarily small by making L suitably large, at the expense of
increased bandwidth. The bandwidth required by PPM to achieve a
bit rate of Rb is approximately the inverse of one chip duration, B =
L/T. In addition to the increased bandwidth requirement, PPM needs
(compared to OOK) more transmitter peak power and both chip- and
symbol-level synchronization.
𝑠𝑃𝑃𝑀 𝑓 = P f 2 + [𝑠𝐶,𝑃𝑃𝑀 𝑓 + 𝑠𝐷,𝑃𝑃𝑀 𝑓 ] (4)
𝑠𝐶,𝐷𝑃𝐼𝑀 𝑓 = Rk −1
L2 e−j2𝜋𝑘𝑓𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀
5𝐿
𝑘=−5𝐿 (5)
𝑠𝐷 ,𝐷𝑃𝐼𝑀 𝑓 =2𝜋
𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀2L2
δ f −2𝜋k
𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀
∞
𝑘=−∞ (6)
f = Frequency, L = Symbol length, Tsym = Symbol duration, Sc =
Continuous component, Sd = Discrete component, P(f) = Fourier
transform of pulse shape
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Digital Pulse Interval Modulation
In DPIM, the information is encoded by inserting empty
slots between two pulses. The DPIM offers a reduced complexity
compared to PPM due to its built-in symbol synchronization. Guard
slots can also be inserted.
𝑠𝐷𝑃𝐼𝑀 𝑓 =1
𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀
P f 2 + [𝑠𝐶,𝐷𝑃𝐼𝑀 𝑓 + 𝑠𝐷 ,𝐷𝑃𝐼𝑀 𝑓 ] (7)
𝑠𝐶,𝐷𝑃𝐼𝑀 𝑓 = Rk −1
L2 e−j2𝜋𝑘𝑓𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀
5𝐿
𝑘=−5𝐿 (8)
𝑠𝐷 ,𝐷𝑃𝐼𝑀 𝑓 =2𝜋
𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀2L2
δ f −2𝜋k
𝑇𝑠𝑦𝑚 _𝐷𝑃𝐼𝑀
∞
𝑘=−∞ (9)
f = Frequency, L = Symbol length, Tsym_DPIM = Symbol duration, Sc =
Continuous component, Sd = Discrete component, P(f) = Fourier
transform of pulse shape
5 IMPLEMENTATION AND SIMULATION RESULTS
The simulation is done using MATLAB software. MATLAB
(MATrix LABoratory) is a numerical computing environment and
fourth-generation programming language. The software is developed
by MathWorks Incorporated. The matrix manipulations, plotting of
functions and data and implementation of algorithms can be done
using this platform. These are often used in physical and
mathematical problems and are most useful when it is difficult or
impossible to obtain a closed-form expression, or infeasible to apply
a deterministic algorithm. The goal of conducting simulations is to
verify and validate the selection of parameters as well as to visualize
the intermediate results not obtainable from experimental results.
Fig. 4. Normalized power distribution at the receiving plane
Fig.5. Power contour plot at the receiving plane
Fig. 6. Analytical power spectral density of OOK modulation
Fig. 7. Bit error probability curve for OOK modulation
Fig. 8. Analytical power spectral density of pulse position modulation
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Fig. 9. Symbol error probability curve pulse position modulation
Fig. 10. Analytical power spectral density of digital pulse interval
modulation
Fig. 11. Symbol error probability curve for digital pulse interval modulation
The normalized power distribution graph for a four-cell structure
is shown in the figure 4.. It is can be seen that most of the power is
concentrated near the centre of each cell decreasing sharply towards
the cell edges. In a four-cell configuration with a circular foot print,
the area within dotted line circle, see figure 5, is defined as the 3-dB
power attenuation area from the centre of a cell. The rest of the area
is defined as the no coverage area or the ―dead zones‖ with no
optical illumination.[1,2].
To design, implement and operate efficient optical communication
systems, it is imperative that the characteristics of the channel are
well understood. The characterization of a communication channel is
performed by its channel impulse response, which is then used to
analyse and combat the effects of channel distortions. Two types of
configurations are considered in VLC channel. They are LOS (Line-
of-Sight) and non-LOS channels. For directed LOS and tracked
configurations, reflections do not need to be taken into consideration.
Consequently, the path loss is easily calculated from knowledge of
the transmitter beam divergence, receiver size and separation
distance.
However, a non-LOS configuration, also known as diffuse
systems uses reflection from the room surfaces and furniture. These
reflections could be seen as unwanted signals or multipath
distortions which make the prediction of the path loss more complex.
The delay spread is a measure of the multipath richness of a
communications channel. In general, it can be interpreted as the
difference between the time of arrival of the earliest significant
multipath component (line-of-sight component) and the time of
arrival of the latest multipath components.[2,3]
LOS Communication Link
Line-of-Sight propagation is the characteristic of light waves
traveling in a straight line. The fundamental equation for finding the
DC gain of a line-of-sight optical wireless system is given by,
G = A m + 1
2πd2
′
cosᵐ Φ cos(ψ) 0 ≤ ψ ≤ ψₐ
0 0 ≥ ψₐ
(10)
where G is the channel gain, A is the photodetector surface area, m is
order of Lambertian emission, d is the distance vector, Φ is the
incidence angle, ψ is the irradiance angle and ψa is the field-of-view
(semiangle) at the receiver.
The received power is the product of transmitted power and the
channel gain.
P received = P transmitted X Channel gain (G) (11)
Diffuse (Non-LOS) Communication Link
For non-directed LOS and diffuse links, the optical path
loss is more complex to predict since it is dependent on a multitude
of factors, such as room dimensions, the reflectivity of the ceiling,
walls and objects within the room, and the position and orientation of
the transmitter and receiver, window size and place and other
physical matters within a room. The reflection characteristics of
object surfaces within a room depend on several factors including,
the transmission wavelength, surface material, the angle of incidence
and roughness of the surface relative to the wavelength. The latter
mainly determines the shape of the optical reflection pattern.
The three digital modulation schemes popular in optical wireless
communication systems (OOK, PPM and DPIM) are compared
based on bandwidth requirement, power efficiency and transmission
capacity. In OOK, the bandwidth requirement is roughly equivalent
to the data rate. PPM achieves higher average power efficiency than
OOK at the expense of an increased bandwidth compared to OOK.
Besides, the use of PPM imposes more system complexity compared
to OOK at the receiver.
Unlike PPM, DPIM does not require symbol synchronization
since each symbol is initiated with a pulse. Furthermore, DPIM
displays a higher transmission capacity by eliminating all the unused
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time slots from within each symbol.
The mean delay spread and RMS delay spread for a diffuse link
is shown in the figure 12 a and b.
Fig. 12. Channel delay spread for a diffuse communication link a) Mean
delay spread b) RMS delay spread
6 CONCLUSION
The development of wireless communications technology over the
last few decades has brought with it an explosion of new applications
for consumers. Convenience of access to the internet is
unprecedented, with indoor wireless local area network (WLAN).
Large-area, high-speed network coverage through metropolitan
access networks (MANs) is now being realized, and indeed, such
networks are the current state-of-the-art with respect to developing
standards, enabling next generation mobile applications and high-
speed wireless municipal access networks. Moving forward, the
natural inclination is towards faster, more reliable wireless
communications. As a result, development of complex schemes that
allow for high symbol rate and high signal-to-noise ratio is an open
research topic in both academia and industry, complicated not just by
the difficulties of transmitting in multipath fading channels, but also
by interference from other users of the same frequency band. The
latter is of increasing concern, especially as more and more wireless
applications are refined. Visible light communication offers a real
alternative to radio based communications. The spectrum is free,
plentiful, and the cost of implementation is actually less than
equivalent radio technology. This technology also saves a lot of
energy.
An indoor visible light communication is mathematically modeled
and the system is simulated with the help of MATLAB software. The
received power distributions and power contour plots for a practical
indoor VLC link is obtained. By employing a holographic light
shaping diffuser, the power distribution is made uniform. Thus the
coverage area is extended further in indoor VLC environment.
Visible light communication system provides advantages including
ubiquitous computing, highly secure data transmission, very high
data, dual functionality of illumination and communication, low cost
of maintenance, low power consumption, safety and reliability. The
visible light communication systems can serve either as a disruptive
technology, as optical fiber was to the traditional all-copper long
distance backbone, or as a system to be used in tandem with the
existing wireless infrastructure to provide additional bandwidth.
ACKNOWLEDGMENT
I express my sincere gratitude to The Almighty who
empowered me to successfully complete the M.Tech. project work,
by showering his abundant grace and mercy. I would like to add
heartfelt words for the people who helped me a lot in the completion
of my project. I express my honor, respect, deep gratitude and
regards to my guide Mrs. Swapna Davies for her kind guidance and
constant supervision as well as for providing necessary information
regarding the seminar. I am highly indebted to Mr. Jaison Jacob,
Head of Department Electronics and Communication Engineering,
for guiding me all throughout the process. I would like to express my
heartfelt gratitude to Mr. Walter Joseph and Dr. Deepti Das Krishna
for their valuable advice and timely help. I would like to
acknowledge Mrs. Anu Mathew for the inspiration and warm
encouragement. I would like to express the deepest appreciation to
my family and friends for their abiding love and prayers. Their
unconditional support is invaluable.
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Author Profile:
Nitin Chacko is currently pursuing M.Tech. degree in Electronics and Communication Engineering (Specialization in Communication Engineering) at Rajagiri School of Engineering and Technology, Cochin. Phone: +919995975963 E-mail: [email protected]
Swapna Davies is currently Assistant Professor in the Department of Electronics and Communication Engineering at Rajagiri School of Engineering and Technology, Cochin. E-mail: [email protected]