1. introduction of telecommunication
TRANSCRIPT
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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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Elements of Communication Systems
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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Elements of Communication Systems
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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Elements of Communication Systems
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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Elements of Communication Systems
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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Elements of Communication Systems
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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Elements of Communication Systems
Signal with distortion
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Elements of Communication Systems
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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Elements of Communication Systems
Signal with noise
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Elements of Communication Systems
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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Fundamental Limitations
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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Fundamental Limitations
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