Chapter 1(Edit)

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CHAPTER 1

INTRODUCTION TO

COMMUNICATION SYSTEM



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Introduction

Communication is the basic process of exchanging information.The flow of information is as shown in Figure 1.1.

Source

Flow of information

Destination

Figure 1.1 A simple communication system



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Electronic Communication System : the whole mechanism ofsending and receiving as well as processing of information electronically from source to a destination.

Examples : Telephone, radio and television, radar and satellitesystems.

The original source could be in analog form (human voice,music) or digital form (binary-coded numbers).

Analog signals are the time-varying voltages or currents that are

continuously changing (sine or cosine waves) whereas digitalsignals are the time-varying voltages or currents that change insteps or levels (binary). All forms of information must beconverted to electromagnetic energy before being propagatedthrough an electronic communications system.



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Elements Of A Communication System

Figure 1.2 shows the basic elements of an electroniccommunication system. The basic elements are:

1. Transmitter

2. Communications channel

3. Receiver

Messageinput

(Voice, data,pictures, etc)

CommunicationsChannel(Medium)

Transmitter Receiver Destination

Noise

Figure 1.2 Basic block diagram of a communication system



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Transmitter

The transmitter : collection of electronic components and circuitsdesigned to convert the information into a signal suitable for

transmission over a given communications medium. It may be assimple as a microphone or as complex as a microwave radiotransmitter.

The primary function of the transmitter : to convert the inputmessage or information into electrical signals ( voltage or

current) or into electromagnetic waves (radio waves,microwaves, or light waves) such that it is suitable fortransmission and compatible with the channel.

The main process in the transmitter is modulation and encoding (for digital signal).

Figure 1.3 describes a basic block diagram of a transmitter. Thetypes of circuits that depend on the type of communicationsystem adopted.



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ModulatingSignal/

InformationSignal

ModulatorAudio

Amplifier RF Amplifier

CarrierSignal

TransmittingAntenna

Figure 1.3 Basic block diagram of a transmitter



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Communications channel or transmission medium

The communications channel : medium by which the electronicsignal is sent from one place to another either using line (orconducted media) or free space (or radio).

Examples of conducted media are pair of wires that carry a voicesignal from a microphone to a headset or a fiber-optic.

Free space or radio is the general term of wireless communicationwhich makes use of the electromagnetic spectrum where signals arecommunicated from one point to another by converting them intoelectric and magnetic fields that propagate readily over long

distances. There is normally no signal processing in the transmission medium,

it is just the medium where the transmitter is connected to thereceiver. However, each medium introduces losses termed asattenuation, distortion and adds noise to some degree to thetransmitted signal. The amount of attenuation, distortion, and noise

depends on the type of transmission medium used.

Noise Noise is random, undesirable electrical energy that enters the

communications system via the communication medium andinterferes with the transmitted message. Noise may be produced

internally or externally.



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Receiver

The receiver is a collection of electronic components and circuits that

accept the transmitted message from the channel and convert it backinto a form understandable by humans. The primary purpose of thereceiver is to separate the information from the received signal orwave and sent the information to the user/ destination. Receivernormally compensates for the attenuation and distortion and reducesnoise caused by the transmission medium. An example of thereceiver is earphone or a complex electronic receiver.

The circuits used in a receiver again depend on the type ofcommunication system used. Basically, a receiver consists of severalstages of amplification, frequency conversion, and filtering. The mainprocess is demodulation or detection of the received signal. Thereceiver must always match the transmitter and the transmissionmedium. Figure 1.4 illustrates a typical block diagram of a receiver.



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RF Amplifier

Demodulator

IntermediateFrequency

Amplifier &Filter

DestinationAudio

Amplifier

Receiving Antenna

Mixer

LocalOscillator

Figure 1.4 Block diagram of a typical receiver



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Modulation and Demodulation

Baseband signal is the original information signal

either in a digital or analog form. Putting the originalsignal directly into the medium is referred to asbaseband transmission. However, there are manyinstances when the baseband signals are incompatiblewith the media and cannot be transmitted directly. Thisis when the modulation techniques must be used.

Baseband Signals



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Modulation

Modulation is the process of modifying or changing one ormore of the properties of the analog carrier by the modulatingsignal (baseband signal or information signal). The carrier isusually a sine wave having a higher frequency than themodulating signal. The changing one or more properties of the

analog carrier is in proportion with the information signal. Thebasic properties of the carrier that can be changed are eitheramplitude, frequency, or phase. A modulator is a circuit whichperforms modulation in a transmitter.



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The process of modulation is needed in any communicationsystem. The reasons why modulation is needed are:

To generate modulated signal that is suitable fortransmission and compatible with the channel.

To allow efficient transmission

By using a high frequency carrier signal, the informationsignal e.g. voice can travel and propagate through the air atgreater distances, and shorter transmission time. Also, highfrequency signal is less prone to noise and interference.Certain types of modulation have the useful property ofsuppressing both noise and interference.

For example, FM systems use limiter to reduce noise andkeep the signal amplitude constant. PCM systems userepeaters to regenerate the signal along the transmissionpath.



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To overcome hardware limitations

The physical sizes of some electronic components depend

on the range of frequencies that are used in the circuit. Thehigher the frequencies, the physical size of the componentsmay be reduced.

Example 1:

Calculate the length of the antenna for a baseband signal offrequency 300Hz.

Solution:

Minimum antenna length, And

Therefore, L =

10

1 L

f

c

km x

x f

c x L 100

300

100.3

10

1

10

18



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Example 2:

Based on example 1, the antenna length required is 100km.

After modulation process, if assuming the carrier frequency is

100MHz, then the antenna length becomes:

New antenna length,

To allow frequency assignment

Frequency assignment is where, several channels whichcarry different messages are assigned with different carrier

frequencies to be transmitted simultaneously using acommon communication medium.

m x

x x L 3.0

10100

100.3

10

1

6

8



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Multiplexing

Multiplexing is a process of sending multiple signalssimultaneously through the same channel facility. By

choosing appropriate carrier frequencies, all the signals canbe multiplexed together and the channel can be shared, asshown in Figure 1.5

By modulation techniques, transmission speed and distancecan be increased

MultiplexerTransmission

medium Demultiplexer

Different inputsignals

Different outputsignals

All the signal are transmitted through the sametransmission facility

Figure 1.5 Multiplexing in a communication system



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Demodulation

Demodulation is the reverse process of modulation.

Demodulation will extract the original baseband information signaland transmitted message.

The demodulation process will convert the modulated carrier back tooriginal information (detection of information from the carrier).

A demodulator is a circuit which performs demodulation in areceiver.



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Transmission Media

Guided transmission media is a form of conductor that provides aconduit in which electromagnetic signals are contained. Onlydevices physically connected to the medium can receive the signals.

Examples of guided transmission media is metallic cables which

transport signals using electrical current and optical fiber whichpropagating electromagnetic waves through a nonconductivematerial.

There are two basic types of transmission line:

Guided transmission media



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i. Two wire/ balanced line

Balanced line is made up of two parallel conductorsspaced from one another by a distance of ½ inch up toseveral inches. The same current flows in each wire withrespect to ground, but the direction is 180 out of phasewith the current in the other wire. The wires are notconnected to ground.

Insulating spacers normally used to keep the wires

separated, or sometimes the spacing is maintained by acontinuous plastic insulator that is part of the conductorinsulation (called twin lead).

Main application of two-wire lines is in transmission of lowfrequency signal such as in transmitting telephone signal

or low data rate transmission.

For higher frequency signal, two-wire line is not suitabledue to energy loss by radiation from the wire. So, coaxialcable is suitable for higher frequency applications.



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(a) (b)

Figure 1.6 (a) Open-wire, (b) twin-lead two wire transmission line



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Unbalanced Line

In an unbalanced line, one conductor is connected toground. Coaxial cable is an example of unbalancedline. It consists of a plastic insulator such as Teflon andover the insulator is a second conductor called shield.An outer plastic sheath protects and insulates the braid.Coax is unbalanced line since the current in the centerconductor is referenced to the braid which is connectedto ground.

Figure 1.7

Structure of Coaxial cable



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Unguided transmission media

Unguided transmission media is a wireless system. Signals are

emitted then radiated through air or a vacuum (sometimes water).The direction of propagation depends on the direction in which thesignal was emitted and any obstacles the signal may encounterwhile propagating. It is available to anyone who has a devicecapable of receiving them.

Examples of an unguided transmission media are air (Earthatmosphere) and free space (vacuum).



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Transmission Modes

Electronic communication systems can be designed to handle

transmission in two transmission modes. They are:

1. Simplex (Sx)

In simplex communications, the information travels inone direction only. Simplex systems are sometimes

called one-way only, receive only, or transmit onlysystems.

Example: radio and television (TV) broadcasting,telemetry system of satellite to earth, pager services



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2. Duplex

Full Duplex

Full duplex transmission is when individualscommunicate with one another and each cantransmit and receive simultaneously. Full duplexsystem is sometimes called two-way simultaneous,duplex, or both way lines. With full duplex system,simultaneous transmission is not necessarilybetween the same two locations. One station cantransmit to a second station and receive from a thirdstation at the same time.

Example: telephone



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Half Duplex

Half duplex operation is another form of two-waycommunications but not simultaneously. With halfduplex system, the direction alternates which meanstransmission can occur in both directions but not at thesame time.

Example: walkie-talkie



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Electromagnetic Spectrum And TransmissionFrequencies

Electromagnetic signals are also referred to as radio-frequency (RF)waves. Electromagnetic waves are waves that travel at the speed oflight and oscillate such that it is made up of an electric field andmagnetic field at right angles to one another and to the direction ofpropagation. These oscillations may occur at a very low frequencyor at an extremely high frequency. This entire range of frequenciesis called electromagnetic spectrum. Figure 1.8 shows the entireelectromagnetic spectrum.

The frequency spectrum is divided into segments for the purpose ofclassifying the various portions and how they are used as shown in

Figure 1.9.



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Figure 1.8 Electromagnetic spectrum used in electronic communications



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Transmission Frequencies and wavelength

Frequency is the number of times a particular phenomenon occursin a given period or time. The frequency is measured in hertz (Hz).

Wavelength is the distance traveled by an electromagnetic waveduring one period. Period is the distance between two points ofsimilar cycles of a periodic wave.

For the electromagnetic waves, the relationship betweenwavelength and frequency is given by the equation,

where :c – the speed of light

f – the frequency of the signal (Hz)

– the wavelength of the signal (meters)

f c



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Example 3:

If given f = 21 MHz, find the wavelength.

Solution:

If given the wavelength, =2.4m, find the frequency.

Solution:

m M

x

f

c29.14

21

100.38

MHz xc

f 1254.2

100.38



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Bandwidth

f1 = 300 Hz f2 = 3000 Hz

Bandwidth

(BW)

Figure 1.10 Bandwidth of the voice frequency



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The bandwidth is then,

In a modulated signal, bandwidth is the range of frequencies thatcontain the information. For higher frequency operations, there ismore spectrum space for information signals. It also permits widerbandwidth signals to be used. Since the electromagnetic spectrum isone of the most precious natural resources, techniques must bedeveloped to minimize the bandwidth required to transmit giveninformation.

Hz

f f BW

2700

3003000

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Information Capacity

Information capacity is a measure of how much information can bepropagated through a communication system. It is expressed in bitrate. Bit rate is the number of bits transmitted within one second andis expressed in bits per second (bps).



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Hartley’s Law

Another important aspect of information theory is the impact of noise

on the signal. The presence of noise reduces the amount ofinformation that can be transmitted in a given bandwidth. If thebandwidth is increases, the information rate will also increase but itwill also allows more noise to pass.

The above relationship is summarized below, known as Shannon-

Hartley theorem:

I B x t

Where:

I = Information capacity (bps) B = Bandwidth (Hz)

T = transmission time (s)



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Shannon-Hartley Theorem

Another important aspect of information theory is the impact of noiseon the signal. The presence of noise reduces the amount of

information that can be transmitted in a given bandwidth. If thebandwidth is increases, the information rate will also increase but itwill also allows more noise to pass.

The above relationship is summarized below, known as Shannon-

Hartley theorem:I = B log2 ( 1 + S/N ) or I = 3.32 B log10 ( 1 + S/N )

Where:

I = Information capacity (bps)B = Bandwidth (Hz)T = transmission time (s)S/N = Signal-to-noise power ratio



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Noise

Noise is random, undesirable electric energy that enters thecommunication systems via the communication medium and

interferes with the transmitted message. However, some noiseis also produced in the receiver. It is unavoidable and thus,becomes a problem in communications systems because thereceived signals are so low in amplitude.

Noise comes from many sources. They are:

1. AtmosphereAtmospheric noise originated from electrical disturbances thatoccur naturally in the earth’s atmosphere, referred to as static.Static usually comes from lightning. It has greatest impact atfrequencies less than 30MHz.



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2. Cosmic noiseSun and other stars emit various kinds of radiation, whichcauses the greatest disruption in the 15 to 150MHz range.

3. Electrical interferenceIt is created by manufactured equipment such as automotiveignition and electric motors and generators.

4. Electric ignition systems of cars, electric motors, fluorescent

lights, and other types of equipment generate signals.

5. Thermal agitationThermal agitation refers to the random motion of the atoms andelectrons in an electronic component caused by heat.Increasing the temperature causes this motion to increase.

Usually noise signal are low but has great impact on extremelylow-level signals being transmitted over long distance.

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