Chapter -6Contents

Introduction to Mobile CommunicationIntroduction to Optical CommunicationIntroduction to TV Systems :CATV,DTH,CCTV & HDTV.Introduction to Modern TV Systems: Plasma TV,LCD TV,LED TV

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Introduction to Mobile Communication

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Mobile Communication

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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Concept of Cell

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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.

Each cell is allocated a fixed number of cellular voice channels (frequencies).

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.

Within a cell,each radio frequency channel can support up to 20 mobile telephone users at one time.

Frequency reuse 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.

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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Components of Cellular Systems

A cellular system consists mainly of: Mobile station (MS). Base station (BS). Mobile Switching Center (MSC).

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Global System for Mobile Communication [GSM]

Globally accepted standard for digital cellular communication.GSM is a second generation cellular telephone system.

GSM Features Short message service (SMS) Subscriber identity module (SIM), on-the-air privacy

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GSM Services

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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GSM-Advantages

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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GSM Architecture

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Introduction to Optical Communication System

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Optical Communication

Fiber-optic communication systems are light wave systems that employ optical fibers for information transmission.

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Advantages & Disadvantages of Optical Communication System

Advantages

Enormous potential bandwidth Small size and weight Electrical Isolation Signal security Potential low cost

Disadvantage

Higher initial cost in installation. Requires specialized and sophisticated tools for

maintenance and repairing.www.infonics.co.nr

Block Diagram of Optical Communication System

An optical communication system mainly consists of three parts—transmitter, optical fiber, and receiver.

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Optical Fiber 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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Structure of Optical Fiber 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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Propagation of Light through Optical Fiber

The concept of light propagation, the transmission of light along an optical fiber, can be described by two theories.

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.

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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Ray theory of Light Propagation

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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Snell’s Law 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 1 is called angle of incidence and 2, is the angle of refraction.θ θ

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Critical Angle 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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Total Internal Reflection (TIR)

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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Light Propagation

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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Acceptance Angle

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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Light Propagation through Fiber

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By multiple total internal reflections at the core-cladding interface the light propagates throughout the fiber over very long distances with low attenuation.

Television Systems

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Cable Television

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

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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Cable TV System

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Cable TV System

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 frequency-multiplexed onto a single cable. The main output cable is called the trunk cable.

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Cable TV System

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.

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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Cable TV System

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 (CCTV)

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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CCTV- Various Link Arrangements

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CCTV- Various Link Arrangements

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CCTV Applications

Education. Medicine. Business. Surveillance.

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Digital TV (DTV) or High-definition TV (HDTV)

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. www.infonics.co.nr

HDTV Vs Conventional TV

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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Modern TV Systems

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Plasma TV 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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Plasma-Structure

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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Plasma TV-Working Principle 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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Plasma TV-Working Principle

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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Plasma TV-Advantages & Disadvantages

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.www.infonics.co.nr

LCD TV

LCD TV is a television display technology  based on a liquid crystal display.

LCD TVs consume much less power than plasma displays because they work on the principle of blocking light rather than emitting it.

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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LCD TV-Working Principle 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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LCD TV-Structure

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LED TV

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 TV-Advantages

LED TVs are incredibly slim when compared to the other kinds and come in variety of sizes.

Due to the use of smaller LEDs to illuminate the display they consume less power.

They have high response time without any motion delay and lags.

LED television has a wider viewing angle around 175 degrees,

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LCD Vs LED

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‘Direct to Home’ Service (DTH)

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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Components of DTH

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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Block Diagram of DTH

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Advantages of DTH

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 value-added services such as the Internet, e-mail,e-commerce, and interactive multimedia.

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