Transcript Chapter 6 - WordPress.com

Contents
Introduction to Mobile Communication
Introduction to Optical Communication
Introduction to TV Systems :CATV,DTH,CCTV & HDTV.
Introduction to Modern TV Systems: Plasma TV,LCD
TV,LED TV
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Any wireless communication link between
two terminals, two stations or persons of
which one or both are in motion.
The cellular telephone system replaced
mobile systems serving large areas
operating with a single base station and a
single high power transmitter with many
smaller areas (cells), each with its own base
station and low-power transmitter.
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With the cellular concept, dividing a large geographic area called a
coverage zone,into small sections called cells. The cell is the basic
geographic unit of a cellular system.
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Each cell is allocated a fixed number of cellular voice channels
(frequencies).
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Large cells(macrocells)typically have a radious between 1 mile and 15
miles with base station transmit powers between 1W and 6W.The
smallest cells (microcells) typically have a radious of 1500 feet or less
with base station transmit powers between .1W and 1W.
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Within a cell,each radio frequency channel can support up to 20
mobile telephone users at one time.
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Frequency reuse is the process in which the same set of
frequencies (channels) can be allocated to more than
one cell, provided the cells are separated by sufficient
distance.
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Each cellular base station is allocated a group of radio
channels to be used within cell. Base stations in adjacent
cells are assigned channel group which contain
completely different channels than neighboring cells.
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A cellular system consists mainly of:
 Mobile station (MS).
 Base station (BS).
 Mobile Switching Center (MSC).
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Globally accepted standard for digital
cellular communication.GSM is a second
generation cellular telephone system.
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GSM Features
 Short message service (SMS)
 Subscriber identity module (SIM),
 on-the-air privacy
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 Teleservices.
 Bearer services.
 Supplementary services.
▪ Caller identification.
▪ Call forwarding
▪ Call waiting.
▪ Multiparty conversations.
▪ Barring of outgoing (international) calls.
▪ Call Waiting (CW)
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It is a wireless system. So mobile
equipment (cell phone) can be on move.
High secrecy in the system. So information
cannot be tapped easily.
Easy to carry Message services and
consumes less power.
GSM provides more voice channels in
limited bandwidth.
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Fiber-optic communication systems are
light wave systems that employ optical
fibers for information transmission.
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Advantages
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Enormous potential bandwidth
Small size and weight
Electrical Isolation
Signal security
Potential low cost
Disadvantage
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Higher initial cost in installation.
Requires specialized and sophisticated tools for
maintenance and repairing.
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An optical communication system mainly consists
of three parts—transmitter, optical fiber, and
receiver.
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An optical fiber is a very thin strand of silica
glass in geometry quite like a human hair. In
reality it is a very narrow, very long glass
cylinder with special characteristics. When
light enters one end of the fiber it travels until
it leaves the fiber at the other end.
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An optical fiber consists of two parts:
 the core and the cladding.
The core is a narrow cylindrical strand of glass and the
cladding is a tubular jacket surrounding it. The core has
a (slightly) higher refractive index than the cladding.
This means that the boundary (interface) between the
core and the cladding acts as a perfect mirror.
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The concept of light propagation, the transmission of
light along an optical fiber, can be described by two
theories.
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Ray theory
 Uses the concepts of light reflection and refraction.
The
advantage of the ray approach is that we get a clearer
picture of the propagation of light along a fiber.
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Mode theory
 Light is described as an electromagnetic wave. The mode
theory describes the behavior of light within an optical fiber.
The mode theory is useful in describing the optical fiber
properties of absorption, attenuation, and dispersion.
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When a light ray encounters a boundary
separating two different media, part of the ray
is reflected back into the first medium and the
remainder is bent (or refracted) as it enters the
second medium.
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If light passes from a medium with a lower refractive index n1 to one with a
higher refractive index n2, the light is bent toward the normal. If the light
passes from n2 to n1 it is bent away from the normal.
 Snell’s Law determines the amount of this bending. It is expressed as:
n1 sinθ1 = n2 sinθ2
Where θ1 is called angle of incidence and θ2, is the angle of refraction.
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If n2 > n1 , then the angle of refraction is greater than the angle
of incidence and the refracted ray is said to have moved away
from the normal. If the angle of incidence (θ1) is increased
further, the angle of refraction (θ2) also increases in
accordance with the Snell’s law and at a particular angle of
incidence the angle of refraction becomes 90o and the refracted
ray grazes along the media interface. This angle of incidence is
called the critical angle of incidence (θc) of medium 2 with
respect to medium 1.
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When the angle of incidence of the light ray is greater than
the critical angle then no refraction takes place. Instead, all
the light is reflected back into the denser material. This is
called total internal reflection.
 For total internal reflection to occur, the index of
refraction of the core must be higher than that of the
cladding, and the incidence angle is larger than the critical
angle.
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The light energy in the form of optical signals
propagates
inside
the
core-cladding
arrangement and throughout the length of the
fiber by Total Internal Reflection (TIR) of light.
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The angle of acceptance is defined as the
maximum angle with which a ray can be sent
in to the optical fiber such that it suffers total
internal reflection and reaches the other end of
the fiber.
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By multiple total internal reflections at the corecladding interface the light propagates throughout
the fiber over very long distances with low
attenuation.
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Cable TV, sometimes called CATV("community
antenna television," now often "community
access television”), is a system of delivering the
TV signal to home receivers by way of a coaxial
cable rather than over the air by radio wave
propagation.
A cable TV company collects all the available
signals and programs and frequency-multiplexes
them on a single coaxial cable that is fed to the
homes of subscribers.
A special cable decoder box is used to receive the
cable signals, select the desired channel, and
feed a signal to the TV set.
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Cable TV companies, generally referred to
as multiple (cable) systems operators
(MSOs), collect signals and programs from
many sources, multiplex them, and
distribute them to subscribers.
The main building or facility is called the
headend. The antennas receive local TV
stations and other nearby stations plus the
special cable channel signals distributed by
satellite.
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The cable companies use parabolic dishes
to pick up the so-called premium cable
channels. A cable TV company uses many
TV antennas and receivers to pick up the
stations whose programming it will
redistribute. These signals are then
processed and combined or frequencymultiplexed onto a single cable. The main
output cable is called the trunk cable.
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The trunk cable is usually buried and extended to
surrounding areas. A junction box containing
amplifiers takes the signal and redistributes it to
smaller cables, called feeders, which go to specific
areas and neighbour hoods.
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From there the signals are again rejuvenated with
amplifiers and sent to individual homes by
coaxial cables called drops. The overall system is
referred to as a hybrid fiber cable (HFC) system.
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The coaxial cable comes into a home and is
connected to a cable decoder box, which is
essentially a special TV tuner that picks up
the cable channels and provides a
frequency synthesizer and mixer to select
the desired channel. The cable TV box is
thus a tuner that can select the special cable
TV channels and convert them to a
frequency that any TV set can pick up.
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Closed circuit television is a special
application in which camera signals are
made available only to a limited number of
monitors or receivers.
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 Education.
 Medicine.
 Business.
 Surveillance.
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HDTV screen made up of thousands of tiny dots of
light, called pixels. Each pixel can be any of 256
colors. These pixels can be used to create any
image. The greater the number of pixels on the
screen, the greater the resolution and the finer
the detail that can be represented.
Each horizontal scan line is divided into hundreds
of pixels. The format of a HDTV screen is
described in terms of the numbers of pixels per
horizontal line by the number of vertical pixels.
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One major difference between conventional
analog TV and HDTV is that HDTV can use
progressive line scanning rather than
interlaced scanning. In progressive scanning
each line is scanned one at a time from top
to bottom.
In the interlaced scanning the picture is
divided into two fields. Here scanning of
every alternate line instead of every
successive line.
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Plasma TVs are created by ionized gas (plasma) that lights up when
an electrical current flowing through it.
Plasma-Structure
Plasma is a gas including ions and electrons. Under normal conditions,
the gas has only uncharged particles. When a voltage is applied onto
the gas, the number of electrons increases and causes an unbalance.
These free electrons hit the atoms, knocking loose other electrons.
Thus, with the missing electron, the component gets a more positive
charge and so becomes an ion.
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Both the electrons and ions get attracted to
each other causing inter collision. This
collision causes the energy to be produced.
When the energy is liberated during
collision, light is produced by them. These
light photons are mostly ultraviolet in
nature. These UV rays react with phosphor
to give a coloured light.
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The xenon and neon gas in a plasma television is
contained in hundreds of thousands of tiny cells
positioned between two plates of glass. Long electrodes
are also sandwiched between the glass plates, on both
sides of the cells.
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An electrode applies an electrical current to
the small cells filled with a noble gas mixture.
This excites the gas, ionizing it and
transforming it into a plasma. This plasma
emits ultraviolet light .When the UV light hits
a phosphor coating, it causes the phosphor to
glow. Depending on which particular
phosphor the cell is coated with, it will create
a red, green, or blue glow, creating the image
seen on the television screen.
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Advantages
The slimmest of all displays
Very high contrast ratios [1:2,000,000]
Weighs less and is less bulky than CRT’s.
High clarity and hence better colour reproduction.
Disadvantages
 Cost is much higher compared to other displays.
 Energy consumption is more.
 Produces glares due to reflection.
 These displays are not available in smaller sizes than
32 inches.
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LCD TV is a television display technology based
on a liquid crystal display.
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LCD TVs consume much less power than plasma
displays because they work on the principle of
blocking light rather than emitting it.
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An important difference between plasma and
LCD technology is that an LCD screen doesn't
have a coating of phosphor dots (colors are
created through the use of filters).
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Liquid-crystal displays use special chemicals sandwiched
between pieces of glass. These chemicals are designed to
be electrically activated so that they block light or pass
light. A bright white light is placed behind the screen. This
light passes through the liquid crystal then through color
filter.
 The red, blue, and green sections of the color filter are
enabled to pass the desired amount of light. Controlled
red, green, and blue shades are thus emitted to reproduce
the picture. Each cluster of red green and blue makes up
one pixel on the screen. By selectively illuminating the
colors within each pixel, a wide range of hues can be
produced on the larger display.
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An LED TV is a type of LCD TV that uses
LEDs to backlight the display, rather than
the cold cathode fluorescent lights (CCFLs)
used in most LCD TVs.
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LED TVs are incredibly slim when compared to
the other kinds and come in variety of sizes.
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Due to the use of smaller LEDs to illuminate
the display they consume less power.
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They have high response time without any
motion delay and lags.
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LED television has a wider viewing angle
around 175 degrees,
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Direct to Home is a term used to refer to
satellite television broadcasts intended for
home reception, also referred to more broadly
as Direct broadcast satellite (DBS) signals.
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There are five major components involved in a direct
to home (DTH) satellite system: The programming
source, the broadcast center, the satellite, the satellite
dish and the receiver.
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Availability of satellite broadcast in rural and
semi-urban areas where cable is difficult to
install.
 Due to digital compression techniques, many
more channels are available than can be
provided by cable operators.
 DTH also offers digital quality signals which do
not degrade the picture or sound quality.
 The DTH system can also provide many valueadded services such as the Internet, e-mail,ecommerce, and interactive multimedia.
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