1. Introduction of Telecommunication

8/3/2019 1. Introduction of Telecommunication

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Introduction of Telecommunication

Telecommunication Engineering

www.ee.ui.ac.id/wasp


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Elements of Communication Systems

A communication system conveys information from itssource to a destination some distance away

Basic function of comm system is to transfer information

in electrical form

Information is represented by message Message is defined as the physical manifestation of

information as produced by the source


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The goal of a comm system is to reproduce at thedestination an acceptable replica of the source message

Two kinds of source: analog and digital

An analog message is a physical quantity that varies with

time, usually in a smooth and continuous fashion Most of the sources are analog, e.g. voice, light intensity


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A digital message is an ordered sequence of symbolsselected from a finite set of discrete elements

Examples: hourly temperature readings, keys on a

komputer keyboard

Most comm systems have input and output transducers. The input transducer converts the message to an

electrical signal (voltage or current)

The output transducer conducts reverse process


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We will learn about the signal transmission A signal contains messages (compare with a wave)

Therefore, the terms signal and message will be used

interchangeably


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Figure below depicts the elements of comm system


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Transmitter processes the input signal to produce atransmitted signal suited to characteristics of the

transmission channel

Signal processing for transmission: modulation and coding

The transmission channel is the electrical medium thatbridges the distance from source to destination.

Transmission channel: wire or wireless (radio wave)


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Every channel introduces some amount of transmissionloss or attenuation the signal power decreases with

increasing distance

The receiver operates on the output signal from the

channel in preparation for delivery to the transducer atthe destination

Receivers signal processing: amplification, demodulation,

decoding, and filtering


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Attenuation is undesireable since it reduces signalstrength at the receiver

Distortion, interference, and noise appear as alterations

of the signal shape

Distortion is waveform pertubation caused by imperfectresponse of the system to the desired signal itself

Distortion disappears when the signal is turned off


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If the channel has a linear but distorting response, thenthe distortion may be corrected (or reduced), with the

help of special filters called equalizers

Interference is contamination by extraneous signals from

human sources (other transmitters, power lines andmachinary, etc.)

Interference occurs most often in radio systems

antennas receive several signals at the same time


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Signal with distortion


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Noise refers to random and unpredictable electricalsignals produced by natural processes both internal and

external to the system

The signal with noise will cause the message be partially

corrupted or totally obliterated Filtering can reduce noise contamination, but some noise

cannot be eliminated

Two way (duplex) communication requires a transmitter

and receiver at each end.

Otherwise, it is called simplex (SX) transmission


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Signal with noise


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A full-duplex (FDX) system has a channel that allowssimultaneous transmission in both directions

A half-duplex (HDX) system allows transmission in either

direction but not at the same time


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Fundamental Limitations

Two constraints in comm system design: technologicalproblems and physical limitations

Technological problems: hardware availability, economic

factors, regulations, etc.

Physical limitations: bandwidth and noise Bandwidth measure ofspeed

When signal changes rapidly with time the frequency

(spectrum) extends over a wide range large bandwidth

Every comm system has a finite bandwidth that limits the

rate of signal transmission


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The most common noise is thermal noise

Thermal noise is the caused by random motion ofcharged particles that generates random currents orvoltages

We measure noise relative to an information signal interms of the signal-to-noise power ratio S/N

Thermal noise is quite small S/N can be large

At lower value of S/N the noise can create someproblems: degrades fidelity in analog comm and produceserrors in digital comm

We cannot amplify the received signal to avoid noisesince the noise will also be amplified


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Shannon stated the relations between channel capacityand rate of information

The Hartley-Shannon law says that the rate of

information cannot exceed the channel capacity

It sets an upper limit on the performance of a commsystem with a given bandwidth and S/N

log 1C B S N


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Modulation

Modulation and coding are operations performed at thetransmitter to achieve efficient and reliable information

transmission

(Analog) Modulation involves two signals: modulating

signal (message) and carrier wave A modulator alters the carrier wave in correspondence

with the variations of the modulating signal

At the receiver it needs demodulation, so that the

message can be extracted


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Modulation An example ofamplitude

modulation (AM) -> Fig (a), (b) AM is used for radio broadcasting

Continuous-wave modulation(CW):

Amplitude modulation (AM)

Frequency modulation (FM) Phase modulation (PM)

CW modulation producesfrequency translation

E.g. In AM broadcasting, themessage spectrum has bandwidth

from 100 Hz to 5 kHz; if thecarrier frequency is 600 kHz, thenthe modulated carrier will be at595605 kHz


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Modulation

Another modulation method is pulse modulation Pulse modulation has a periodic train of short pulses as

the carrier wave

Pulse modulation does not produce frequency translation

One example of pulse modulation is pulse amplitudemodulation (PAM) fig (c)

PAM consist ofshort samples extracted from the analog

signal

Sampling is an important signal-processing technique

Sampled signal should be able to be reconstructed


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Modulation

Modulation Benefits: Modulation for efficient transmission

Modulation to overcome hardware limitations

Modulation to reduce noise and interference

Modulation for frequency assignment Modulation for multiplexing


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Coding

Coding is a symbol-processing operation for improvedcomm when the information is digital

Both coding and modulation may be necessary for long-

distance digital transmission

Encoding transforms a digital message into a newsequence of symbols

Decoding converts an encoded sequence back to the

original message (with a few errors caused by

transmission)


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Coding

Coding: source coding and channel coding Source coding reduces redundancy to achieve the desired

efficiency

Channel coding is a technique to control redundancy to

further improve the performance reliability in a noisychannel

Channel coding sometimes called error-control coding

1. Introduction of Telecommunication

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