Communication system

TC-307
Lecture 1, week 1
Course Instructor: Nida Nasir
Lecture 1
• Topics discussed are below:
• 1. basic definition of CS and examples
• 2. basic elements of CS: Tx, ch, RX
• 3. Info types: signal and message
• 4. simplex and duplex communication
• 5. repeaters, antenna, point to point, multicast, broadcast
• 6. baseband and broadband transmission

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Course Outline , books, sessional criteria
Topics
1. Introduction - CLO1 week1,2
2. Analog Modulation – CLO2 / CLO3 week 3,4,5,6,7
3. Noise – CLO2 week 8
4. Pulse Modulation – CLO2 / CLO3 week 10
5. Digital Modulation & Introduction to Information Theory – CLO2 week 10
6. Probability, Random Variables, Random Process – CLO2 week 9
Text Book
• Communication system by Bruce Carlson
Reference Books:
• Electronic Communication System by Frenzel
• Analog and digital Communication System by Simon Haykin
• Communication System by Proakis
• Advance electronic communication by Wayne Tomasi
Sessional criteria:
• Mid term =20 marks (MCQ) week 7
• Assignment= 10 marks week 4/ 10
• Quiz = 10 marks week 4 3
Week 1 Topics
Introduction to Communication System
1. Need and purpose of Communication
2. Elements of Communication system
3. Modes of communication
4. Types of channel and signals
5. ACS and DCS block diagram
6. Multiplexing and multiple access
7. Adverse effects of channel
8. Electromagnetic spectrum
9. Fundamental Limitations in CS
10. Signal to Noise Ratio
11. Hartley Shannon law and Nyquist criteria
12. Modulation and coding
13. Need and Benefits of modulation

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Week 1 Topics
Introduction to Communication System

•Transmitter
Elements of
Communication
system
•Channel
•Receiver

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Week 1 Topics
Introduction to Communication System
types of comm

With respect to input

With respect to
With respect to mode information/message
transmission
signals

Simplex Baseband Analog

Duplex Broadband digital

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Week 1 Topics
Introduction to Communication System
Types of channel
and signals

Channel Signals

Guided Analog

Unguided Digital
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Week 1 Topics
Introduction to Communication System Information source

Modulator
ACS
Channel

ACS and DCS block diagram

Demodulator

Information source

Source encoder

Channel encoder

Modulator
DCS
Channel

Demodulator

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Channel decoder
Week 1 Topics
Introduction to Communication System
Multiplexing and
multiple access

Multiplexing Multiple access

Frequency
Analog Digital division multiple
access

Frequency
Time Division Time division
division
Multiplexing multiple access
multiplexing

Wave Division Code division

Multiplexing multiple access
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Week 1 Topics
Introduction to Communication System

Adverse effects
of channel

Noise Interference Distortion Attenuation Fading

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Week 1 Topics
Introduction to Communication System

Electromagnetic spectrum
• Extremely low frequency
• Voice frequency
• Very low frequency
• Low frequency
• Medium frequency
• High frequency
• Very high frequency
• Ultra high frequency
• Super high frequency
• Extremely high frequency
• Infrared
• Visible/light spectrum

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Week 1 Topics
Introduction to Communication System
Signal bandwidth
Limitations of communication

Bandwidth
Channel
bandwidth
system

Thermal noise
Internal noise
Shot noise

Noise Atmospheric noise

Solar noise
External noise
Cosmic noise

Industrial/Man 12
made noise
Week 1 Topics
Introduction to Communication System
Continuous Wave Amplitude
Mod Modulation Coding
Analog Frequency
Modulation
Angle Modulation Source coding
(PCM,A to D Channel coding
Phase Modulation Conversion)
Amplitude Shift
Keying
Error detection &
Sampling
Frequency Shift correction
Modulation Digital
Keying

Pulse Amplitude
Phase shift Keying Quantization Encryption
Modulation

Analog pulse Pulse Duration

Modulation Modulation
Encoding
Pulse Position
Pulse Modulation
Modulation

Digitalpulse Pulse Code 13

Modulation Modulation
Basic definitions
Communication: is a process of exchanging information from source to destination. We
communicate to convey thoughts, ideas, feelings

Purpose of Communication: To establish a connection between two points that is transmitter and
receiver through a medium.

Communication system: is a process of conveying information from sender to a receiver at a distant

location. It can be classified into analog communication system and digital communication system

Goal of communication system: is to reproduce source message at the destination without loss of
information.

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Communication system examples

Common E.g. of CS are telephone, radio, TV, mobile, computer, satellite, internet communication
etc.

Telephone invented in 1876, first AM radio broadcast in 1920,

TV invented in 1923 and broadcast in 1928 and
first FM broadcast in 1935,
first cellular telephone network deployed in 1983.

The study of comm. Electronics is related to how we can make a system using electric and electronic
components while in Comm. Sys, only blocks and diagrams are discussed.

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Elements of Communication System:

All communication system have three basic elements,

1. Transmitter
2. communication channel SENDER
Transmitter Channel Reciever
DESTINATION
Input Outut Signal/
3. Receiver Signal/sour destination
ce
• Sender: is a person who wants to send information(audio/video) to another
person.
• Transmitter: it modulates the information/ message and pass it to channel.
• Receiver: it receives information after passing through the channel or medium,
demodulates and process it .
• destination: is another user who receives the audio/video from sender.
• communication channel: it is the path through which signal propagates from
transmitter to receiver.
• eg. TV news channel, FM Radio 16
Information types
• Transmission of information is done in the form of message or signal
• A message is a pure information that is speech audio, video input to
the communication system it is a non electrical quantity which is first
converted to discrete electrical form using transducers
• Signal is the suitable electrical quantity derive from message for
transmission. It is the electrical quantity that is it is in form of current
or voltage.

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Types of Electronic Communication
Electronic communications are classified according to below 3 methods

(1) According to modes: one-way(simplex) or two-way (full duplex or half duplex)

(2) According to transmission: baseband or broadband transmission

(3) According to input message/ information signal Either the Input is analog or digital signal, they
results in Analog Communication system(ACS) or Digital Communication system(DCS) respectively.
•An ACS is a system in which signal is transmitted and receive in analog form ( continuously varying
signal such as sine wave ). With ACS, both information and carrier are analog signals.
•In DCS, is a system in which signal is transmitted and receive in digital form ( means 2 states that is
either 0 or 1). In DCS, information signal is digital pulse while the carrier is an analog sine wave.

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Simplex Communication
The simplest way in which electronic communication is
conducted is one-way communications, normally
referred to as simplex communication.

It is unidirectional, that is we can send information only.

E.g of simplex communication are radio and TV

broadcasting. Another example of one-way
communication is transmission to a remotely controlled
vehicle like a toy car or an unmanned aerial vehicle (UAV
or drone).
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Duplex Communication
Full Duplex

Most of electronic communication is two-way, or duplex communication.

This is bidirectional. Here both Tx and Rx can send and receive information at same time.

Typical duplex applications are people communicating with one another over the telephone can talk and listen simultaneously. This is called

full duplex communication.

Half Duplex

The form of two-way communication in which only one party transmits at a time is known as half duplex communication. The communication

is two-way, but the direction alternates: the communicating parties take turns transmitting and receiving.

This is also bidirectional but Tx and Rx cannot send and receive message at one time.

Most radio transmissions, such as those used in the military, fire, police, aircraft, marine, and other services, are half duplex communication.
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Based on number of receivers, communication between
transmitter and receiver can be further categorized in

Point to point/ unicast: one transmitter, one receiver eg, telephone, intercom

Point to multipoint/broadcast: one transmitter, many receivers eg, radio, TV

When messages are intended for all users on the network, it is called broadcasting

When messages are intended for specific users on the network, it is called multicasting

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• Repeaters
Repeaters are placed at different locations in between the
transmitter and receiver. A repeater receives the
transmitted signal, amplifies it and send it to the next
repeater without distorting the original signal.
• Antenna
An Antenna is a structure or a device that radiates and
receive electromagnetic waves. So, they are used in both
transmitters and receivers. An antenna is basically a
metallic object, often a collection of wires. They can be
isotropic/omnidirectional and directional.
Omnidirectional antennas radiate and receive equally well
in all horizontal directions while Directional antennas focus
energy in a particular direction.

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Base band transmission
• Regardless of whether the original information or intelligence signals are analog or digital, they
are all referred to as baseband signals.

• In a communication system, baseband information signals can be sent directly and unmodified
over the medium or can be used to modulate a carrier for transmission over the medium.

• Putting the original voice, video, or digital signals directly into the medium is referred to as
baseband transmission.

• For example, in many telephone and intercom systems, it is the voice itself that is placed on the
wires and transmitted over some distance to the receiver. In most computer networks, the digital
signals are applied directly to coaxial or twisted-pair cables for transmission to another computer.
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Broadband/ Passband transmission
• In many instances, baseband signals are incompatible with the medium. Although it is
theoretically possible to transmit voice signals directly by radio, realistically it is
impractical.
• As a result, the baseband information signal, be it audio, video, or data, is normally used
to modulate a high-frequency signal called a carrier. The higher- frequency carriers
radiate into space more efficiently than the baseband signals themselves. Such wireless
signals consist of both electric and magnetic fields. These electromagnetic signals, which
are able to travel through space for long distances, are also referred to as radio-
frequency (RF) waves, or just radio waves.
• Modulation is the process of having a baseband voice, video, or digital signal modify
another, higher-frequency signal, the carrier. The process is illustrated in Fig. 1-7. The
information or message to be sent is said to be impressed upon the carrier. The carrier is
usually a sine wave generated by an oscillator. The carrier is fed to a circuit called a
modulator along with the baseband intelligence signal. The signal changes the carrier in a
unique way. The modulated carrier is amplified and sent to the antenna for transmission.
This process is called broadband transmission.
• Consider the common mathematical expression for a sine wave: v = Vp sin (2πft + φ )

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The three ways to make the baseband signal change the carrier sine wave are to vary its amplitude, vary its frequency, or vary
its phase angle.

• In amplitude modulation(AM), the baseband information signal called the modulating signal varies the amplitude of the
higher-frequency carrier signal, as shown in Fig. 1-8(a). It changes the Vp part while frequency and phase remains
constant.

• In frequency modulation (FM), the information signal varies the frequency of the carrier, as shown in Fig. 1-8(b). FM varies
the value of f in the first angle term inside the parentheses. The carrier amplitude and phase remains constant.

• Varying the phase angle produces phase modulation(PM). Here, the second term inside the parentheses (_) is made to
vary by the message signal. The frequency and amplitude are constant.

• Two common examples of transmitting digital data by modulation are given in Fig. 1-9. In Fig. 1-9(a), the data is converted
to frequency-varying tones. This is called frequency-shift keying (FSK). In Fig. 1-9(b), the data introduces a 180º-phase
shift. This is called phase-shift keying (PSK).

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Communication system
TC-307
Lecture 2, week 1
Course Instructor: Nida Nasir
Lecture 2
• Topics discussed are below:
• 1. analog signals and digital signals
• 2. wired and wireless
• Analog communication system explanation
• Disturbances in communication channel
• Digital communication system explanation
• Analog to digital conversion & pulse code modulation (PCM )

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Analog Signals
• An analog signal is a smoothly and
continuously varying voltage or
current. Some typical analog
signals are shown in Fig. 1-5. A sine
wave is a single-frequency analog
signal. Voice and video voltages are
analog signals that vary in
accordance with the sound or light
variations that are analogous to
the information being transmitted.
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Digital Signals
• Digital signals, in contrast to analog signals, do not vary continuously, but change in steps or in
discrete increments. Most digital signals use binary or two-state codes. Some examples are shown in
Fig. 1-6.

• The telegraph used Morse code, with its system of short and long signals (dots and dashes) to
designate letters and numbers. See Fig. 1-6(a).

• In radio telegraphy, also known as continuous-wave (CW) transmission, a sine wave signal is turned
off and on for short or long durations to represent the dots and dashes. Refer to Fig. 1-6(b).

• Data used in computers is also digital. Binary codes representing numbers, letters, and special
symbols are transmitted serially by wire, radio, or optical medium. The most commonly used digital
code in communications is the American Standard Code for Information Interchange (ASCII). Fig. 1-
6(c) shows a serial binary code.
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Types of channel
They are divided into 2 categories

1. Wired/ line communication channel:

• Also known as guided medium

• physical wires run between transmitter and receiver

• e.g. pair of wires, coaxial cable, twisted pair cables, fiber optic cable.

• ( telephone)

2. Wireless/ radio communication channel:

• Also known as unguided medium

• no physical wires, antennas are used in transmitter and receiver for transmission and reception

• e.g free space/air( radio, TV broadcast, satellite communication, IR-Infarred, bluetooth)

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Analog communication system block diagram
Blocks of ACS are
• Information source
• Transducer
• Transmitter
• Channel
• Receiver
• Noise

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1. Information source/ message is a non electrical quantity eg. musical sound, human voice, Picture, data etc.

2. Transducer

• When message produced by source is not electrical in nature, input transducer is used to convert it into time varying electrical signal like
current/ voltage called message signal/baseband signal. Output transducers convert output signal into desired form.

• Transducers convert physical characteristics (temperature, pressure, light intensity, and so on) into electrical signals.

• For voice messages, a microphone is used to translate the sound into an electronic audio signal.

• For TV, a camera converts the light information in the scene to a video signal.

• In computer systems, the message is typed on a keyboard and converted to binary codes that can be stored in memory or transmitted.

• From above examples,

Input transducer: microphone, tv camera, keyboard

Output transducer: loud speaker, picture tube, computer monitor display

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3. Transmitter
•The main function of transmitter is to modify the baseband signal and send this modulated signal for
transmission over channel to the desired receiver.
•The transmitter itself is a collection of electronic components and circuits. Mainly it consists of modulator.
Also Transmitters have electronic circuits like coders, oscillators, amplifiers, tuned circuits and filters etc
•A modulator converts/modulates input message signal (which is a low frequency signal) by a higher-
frequency carrier sine wave, then the modulated signal is raised in amplitude by power amplifiers,
resulting in a signal that is compatible with the selected transmission medium.

Modulator/ modulation :

Modulation is a process in which message signal is superimposed on carrier to change the parameters of
carrier signal wrt message signal. In modulator, 2 signals are mixed together known as message signal and
carrier signal for long distance communication.

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4. Receivers
• The function of receiver is to extract the message from distorted and noisy
signal received from transmission medium and then reproduce original
message signal by the use of demodulators.
• A receiver is a collection of electronic components and circuits that accepts
the transmitted message from the channel and converts it back to a form
understandable by humans. Main block of it is demodulator, or detector
that recovers the original intelligence signal from the modulated carrier.
Receivers also contain decoders, amplifiers, oscillators, mixers, tuned
circuits and filters.
• Demodulation/ demodulator is the reverse process of modulation and
converts modulated carrier back to original information

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Amplifiers and filters

• Amplification is done at receiver to compensate transmission loss( because signal is

degraded after passing through channel) . In this process, only the amplitude of the
signal increases.
• Filtering for the removal of unwanted signals.

Transceivers
• Most electronic communication is two-way, and so both parties must have both a
transmitter and a receiver. As a result, most communication equipment incorporates
circuits that both send and receive. These units are commonly referred to as
transceivers.
• All the transmitter and receiver circuits are packaged within a single housing and
usually share some common circuits such as the power supply.
• Telephones, handheld radios, cellular telephones, and computer modems are
examples of transceivers.
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 5. Communication Channel

• The function of comm. Ch. is to provide connection btw TX and RX

• The communication channel is the medium by which the electronic signal is sent from one place to another , from transmitter
to receiver. Many different types of media are used in communication systems, including wire conductors, fiber-optic cable, and
free space.

• Electrical Conductors. In its simplest form, the medium may simply be a pair of wires that carry a voice signal from a
microphone to a headset. It may be a coaxial cable such as that used to carry cable TV signals. Or it may be a twisted-pair cable
used in a local-area network (LAN).

• Optical Media. The communication medium may also be a fiber-optic cable or “lightpipe” that carries the message on a light
wave. These are widely used today to carry long-distance calls and all Internet communications.

• Free Space. When free space is the medium, the resulting system is known as radio. Also known as wireless radio is applied to
any form of wireless communication from one point to another. Radio makes use of the electromagnetic spectrum.

• In wireless communication, message signals are converted to electric and magnetic fields that propagate nearly instantaneously
through space over long distances.

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6. Noise
• Noise occurs in all electronic communications and is usually added in channel (known as
additive white Gaussian noise,AWGN). Its effect is experienced in the receiver part of
any communications system. While some noise can be filtered out, the general way to
minimize noise is to use components that contribute less noise and to lower their
temperatures.
• AWGN, ‘Addititive’ means it can be added, ‘white’ refers to uniform power across all
frequencies.
• The measure of noise is usually expressed in terms of the signal-to-noise (S/N) ratio
(SNR), which is the signal power divided by the noise power and can be stated
numerically or in terms of decibels (dB). Obviously, a very high SNR is preferred for best
performance. 38
 Disturbances in communication channel
• Every communication channel introduces some amount of transmission loss or attenuation, so signal power decreases with increase in distance. The disturbances in
communication channel are named as: Noise, attenuation, distortion, interference, multipath fading.
• Attenuation reduces signal strength at receiver but did not alter its shape
• Noise, interference, distortion are worst because they alters the shape of signal
• Multipath Fading is the reduction in received signal power level and usually occurs in wireless communication because of multipath propagation(signal reaching the
receiving antennas by two or more paths), weather disturbances like rainfall, snowfall, hailstorm, irregular earth surfaces/terrains etc
• Distortion is waveform perturbation caused by imperfect response of system to the desired signal. Distortion disappears, when signal is turned off. Distortion can be of 2
types:
• i. linear distortion/ amplitude distortion: If channel is linear but distorting response it can be corrected / reduced with the help of specialized filter called equalizers.
• ii. Non linear distortion. This is because distortion in frequency and phase of transmitting signal. Companding is used to cancel the effect of non linear distortion.
• Interference is contamination by extraneous signals from human sources like power lines machinery switching circuits etc. Interference occurs often in radio system whose
receiving antennas intercept several signal at same time. RFI (Radio frequency interference) also appears in cable system if transmission wires pick up signals from nearby
sources. Filtering removes interference to some extent. Interference problem occurs in broadcasting where 2 or more signals are picked up at same time by the receiver
which in turn will produce signal interference with each other. To avoid interference between signals a protective frequency band known as Guard band is used.
• Attenuation When a signal is transmitted through the electronic medium over long distances it loses its quality and strength due to dissipation and wastage of energy.
This is the ‘attenuation’ of the signal refers to any reduction in the strength of a signal. Sometimes called loss and is measured in dB. The loss due to attenuation can be
compensated by amplification, hence it is not a serious problem

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DIGITAL COMMUNICATION SYSTEM BLOCK DIAGRAM
blocks of DCS are

Information source
Transducer
Source encoder
Channel encoder
modulator
Channel
Digital demodulator
Channel decoder
Source decoder
Output transducer

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Following are the sections of the digital communication system.
• Source: The source can be an analog signal. Example: A Sound signal
• Input Transducer: This is a transducer which takes a physical input and converts it to an electrical signal (Example: microphone). This block also
consists of an analog to digital converter where a digital signal is needed for further processes. A digital signal is generally represented by a binary
sequence.
• Source Encoder (data compression) : The source encoder compresses the data into minimum number of bits. This process helps in effective
utilization of the bandwidth. It removes the redundant bits (unnecessary excess bits)
• The symbol produced by information source are first converted into digital form by the Source Encoder. This Encoder assign code words to the
symbol. The receiver side these signal are encoded by the use of Source decoder to obtain the signal in desired form.
• Channel Encoder (error correction) :The channel encoder, does the coding for error correction. During the transmission of the signal, due to the
noise in the channel, the signal may get altered and hence to avoid this, the channel encoder adds some redundant bits to the transmitted data.
These are the error correcting bits.
• After converting the message signal into binary sequence by the source encoder , the signal is ready to transmit through the channel. The
communication channel adds noise and interference to the signal being transmitted. To avoid these type of error Channel encoding is done.
Channel Encoder add some redundant bits(binary) to the input signal. Channel Decoder at the receiver is thus able to reconstruct error free
accurate bits and thus reduce the effect of channel noise and distortion at the receiver end.
• Digital Modulator: The signal to be transmitted is modulated here by a carrier. The signal is also converted to analog from the digital sequence, in
order to make it travel through the channel or medium.
• Channel: The channel or a medium, allows the analog signal to transmit from the transmitter end to the receiver end. It can be wired or wireless.
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• Digital Demodulator:This is the first step at the receiver end. The received signal is demodulated as well as
converted again from analog to digital. The signal gets reconstructed here.
• Channel Decoder:The channel decoder, after detecting the sequence, does some error corrections. The
distortions which might occur during the transmission, are corrected by adding some redundant bits. This
addition of bits helps in the complete recovery of the original signal.
• Source Decoder:The resultant signal is once again digitized by sampling and quantizing so that the pure digital
output is obtained without the loss of information. The source decoder recreates the source output.
• Output Transducer:This is the last block which converts the signal into the original physical form, which was at
the input of the transmitter. It converts the electrical signal into physical output (Example: loud speaker).
• Output Signal:This is the output which is produced after the whole process. Example − The sound signal
received.
• Encryption and decryption block is used in Tx and RX of DCS to provide communication privacy and prevents
unauthorized users from understanding messages and injecting false messages into system/channel.

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• Source Coding/encoding aims to convert information waveforms
(text, audio, image, video, etc.) into bits. The three major steps are:
1. Sampling: convert the continuous-time analog waveform to
discrete-time sequence (but still continuous-valued).
2. Quantization: convert each continuous-valued symbol to discrete-
valued representatives.
3. Data compression/discrete encoding: remove the redundancy in the
data and generate uniformly distributed bits.
• Source decoding does
the reverse of encoding

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Analog to digital conversion
PCM is widely used to convert Analog into digital
Which is a 3 step process.
1. Sampling
2. Quantization
3. Encoding
• Pulse-code-modulation (PCM) -- the scheme of transmitting data by digitizing and then using pulse codes to transmit the digitized data.
• Sampler: helps to collect the sample data at instantaneous values of message signal, so as to reconstruct the original signal. The sampling rate
must be greater than twice the highest frequency component W of the message signal, in accordance with the sampling theorem.
• Quantizer: Quantizing is a process of reducing the excessive bits and confining the data. The sampled output when given to Quantizer, reduces
the redundant bits and compresses the value.
• Encoder: The digitization of analog signal is done by the encoder. It designates each quantized level by a binary code. These three sections will
act as an analog to digital converter. Encoding minimizes the bandwidth used.

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• Channel encoding aims to convert information bits into passband waveforms
bits. The four major steps are
1. Error correcting codes: introduce redundancy into the information bits and
produce longer coded bits.
2. Symbol mapping: map the coded bits to constellation points, each of which is
a complex symbol.
3. Pulse shaping: modulate the symbol to suitable baseband waveforms.
4. Up conversion: convert the baseband waveform to passband waveform, so
that the effective frequency band follows the constraints from the physical
world.
• Channel decoding does the reverse of encoding.
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Communication system
TC-307
Lecture 3, week 1
Course Instructor: Nida Nasir
Lecture 3
• Topics discussed are below:
• 1. modulation and types
• 2. digital comm
• 3. need and benefits of modulation
• 4. Coding
• 5. Limitation of CS
• 6. SNR

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Modulation and its types

Modulation

Pulse
Analog Digital
Modulation

Continuous Angle Amplitude Frequency Phase shift Analog Digital

Wave Mod Modulation Shift Keying Shift Keying Keying Modulation Modulation

Pulse
Amplitude Frequency Phase Pulse Duration Pulse Position Pulse Code
Amplitude
Modulation Modulation Modulation Modulation Modulation Modulation
Modulation

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MODULATION
Modulation is a process in which message signal is superimposed on carrier to change the parameters of carrier
signal wrt message signal. In modulator, 2 signals are mixed together known as message signal and carrier signal
for long distance communication.

•Message signal/baseband signal/Modulating signal which is a low frequency signal denoted by m(t)
•Carrier signal is a high frequency signal denoted by c(t)
•Modulated signal is a combination of message and carrier signal, denoted by s(t)
M(t) S(t)
S(t) = m(t)c(t)
C(t)
•Any signal can be written as : v(t) = V Sin (2πft + φ) ;
Here V is the peak amplitude in volts, f = frequency in Hz, φ = phase angle/shift in radians
•Hence in modulation 3 parameters of carrier wave can be varied in accordance with the modulating signal.
Those are amplitude, frequency, phase.

When modulating signal is analog and carrier is also analog, various type of analog modulation exist, known as
Amplitude modulation(AM), frequency modulation(FM) and phase modulation(FM) respectively.
•If amplitude of carrier is varied proportional to information signal then AM is produced.
•If frequency of carrier is varied wrt to information signal, FM is produced.
•If phase of carrier is varied wrt to information signal then PM is produced. 50
When modulating signal is digital and carrier is analog, various type of Digital modulation exist and are known as Amplitude shift
keying(ASK), frequency shift keying(FSK) and phase shift keying(PSK).

•If amplitude of carrier is varied proportional to information signal then ASK is produced.

• If frequency is varied proportion to information signal then FSK is produced.

• If phase is varied proportion to information signal then PSK is produced.

When modulating signal is analog and carrier is digital/pulse, various type of pulse modulation come into being, known as pulse
Amplitude modulation(PAM), Pulse width modulation (PWM), Pulse Position modulation(PPM), Pulse code modulation(PCM)

• Digital modulation is the transmittal of digitally modulated analog signals (carriers) between two or more points in a
communications system. Digital modulation is sometimes called digital radio because digitally modulated signals can be propagated
through Earth’s atmosphere and used in wireless communications systems.

• Traditional electronic communications systems that use conventional analog modulation, such as amplitude modulation (AM),
frequency modulation (FM), and phase modulation (PM), are being replaced with more modern digital modulation systems that offer
several outstanding advantages over traditional analog systems, such as ease of processing, ease of multiplexing, and noise
immunity.
Analog, digital, pulse modulation waveforms

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Digital communication
• Digital communications include systems where relatively high-frequency analog carriers are modulated by
relatively low frequency digital information signals (digital radio) and systems involving the transmission of
digital pulses (digital transmission).
• Digital transmission systems transport information in digital form and therefore require a physical channel
between the transmitter and receiver, such as a metallic wire pair, a coaxial cable, or an optical fiber cable.
• In digital radio systems, the channel could be a physical cable, or it could be free space.
• Property that distinguishes digital radio systems from conventional analog modulation communications
systems is the nature of the modulating signal. Both analog and digital modulation systems use analog carriers
to transport the information through the system. However, with analog modulation systems, the information
signal is also analog, whereas with digital modulation, the information signal is digital, which could be computer
generated data or digitally encoded analog signal.
At the receiver, the carrier with the intelligence signal
is amplified and then demodulated
to extract the original baseband signal. Another
name for the demodulation process
is detection. (See Fig. 1-10
Need and Benefits of modulation
Modulation is needed basically to increase the bandwidth of signal and to multiplex
more signals.
Following are some of the advantages for implementing modulation in the
communication systems.
• Antenna size gets reduced.
• Modulation reduces noise and interference
• Avoid mixing of signals
• Communication range increases.
• Multiplexing of signals is possible
• Reception quality improves.
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1. Reduction in the height of antenna
• For the transmission of radio signals, the antenna height must be multiple of λ/4 ,where λ
is the wavelength .
• λ = c /f where c : is the velocity of light
f: is the frequency of the signal to be transmitted

• The minimum antenna height required to transmit a baseband signal of f = 10 kHz is

calculated as follows :
Min antenna height = λ/4 = c/4f = 3x108/ 4x 104 = 7500m or 7.5km
• The antenna of this height is practically impossible to install .
Now, let us consider a modulated signal at f = 1 MHz .
• The minimum antenna height is given by = λ/4 = c/4f = 3x108/ 4x 106 = 75m
• This antenna can be easily installed practically . Thus, modulation reduces the height of
the antenna .
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2. Modulation reduces noise and interference
• Modulation have property to suppress noise & interference with increase in
bandwidth known as Wideband noise reduction.
• It requires channel bw to be greater than signal bw, so a HF carrier is needed to
accommodate wideband noise reduction. So high bw is traded for noise
reduction.
3. Avoids mixing of signals
• If the baseband sound signals are transmitted without using the modulation by
more than one transmitter, then all the signals will be in the same frequency
range i.e. 0 to 20 kHz . Therefore, all the signals get mixed together and a
receiver can not separate them from each other .
• Hence, if each baseband sound signal is used to modulate a different carrier then
they will occupy different slots in the frequency domain (different
channels). Thus, modulation avoids mixing of signals .
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4. Increase the Range of Communication
• The frequency of baseband signal is low, and the low frequency signals can not travel
long distance when they are transmitted . They get heavily attenuated .
• The attenuation reduces with increase in frequency of the transmitted signal, and
they travel longer distance .
• The modulation process increases the frequency of the signal to be transmitted.
Therefore, it increases the range of communication.
5. Multiplexing is possible
• Multiplexing is a process in which two or more signals can be transmitted over the
same communication channel simultaneously .This is possible only with modulation.
• The multiplexing allows the same channel to be used by many signals . Hence, many
TV channels can use the same frequency range, without getting mixed with each
other or different frequency signals can be transmitted at the same time .
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6.Improves Quality of Reception
• With frequency modulation (FM) and the digital communication
techniques such as PCM, the effect of noise is reduced to a great
extent . This improves quality of reception .

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CODING
• Modulation and coding are performed at transmitter to achieve efficient and reliable information transmission
• Modulation is signal processing operation for effective transmission while coding is symbol processing operation for improved
communication when information is digital or inform of discrete symbols
• Both coding and modulation is necessary for long distance communication.
• Coding is method to convert data into non understandable form so that unauthorize people cannot access it its purpose to secure
the channel and the signal
• Coding is essential is digital technique for wide band noise reduction
• Encoding transforms digital message into new sequence of symbols while decoding converts encoded signals back to original
message with few errors caused by channel
• Channel coding is a technique used to improve reliability in a noisy channel
• Error control coding further reduces wide band noise reduction by appending extra bits to each binary code word it increases
bandwidth and hardware complexity but gives an error free digital communication.
• Source coding technique is dual of channel coding as it reduces redundancy to achieve desire efficiency
• Benefits of digital coding is used in analog communication with analog to digital conversion (PCM)
• A PCM is generated by sampling analog message quantizing sample values and encode sequence of digitize samples

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:
Limitations of communication system
The 2 most significant limitation on performance of comm system are
1. Noise 2. Bandwidth
Bandwidth limitation:
• Bandwidth (BW) is that portion of the electromagnetic spectrum occupied by a signal. It is also the frequency range over which a receiver or other electronic
circuit operates. Bandwidth is the difference between the upper and lower frequency limits of the signal or the equipment operation range.
• Ex: the bandwidth of the voice frequency range from 300 to 3000 Hz. The upper frequency is f2 and the lower frequency is f1. The bandwidth, then, is BW =
f2 - f1 = 3000-300 = 2700 Hz
• Example 1-5 A commonly used frequency range is 902 to 928 MHz. What is the width of this band?
BW = f2 - f1 = 928 - 902 = 26 MHz
• Example 1-6 A television signal occupies a 6-MHz bandwidth. If the low-frequency limit of channel 2 is 54 MHz, what is the upper-frequency limit?
• BW = f2 - f1 : f2 = BW + f1 = 6 + 54 = 60 MHz
• When information is modulated onto a carrier somewhere in the electromagnetic spectrum, the resulting signal occupies a small portion of the spectrum
surrounding the carrier frequency. The modulation process causes other signals, called sidebands, to be generated at frequencies above and below the
carrier frequency by an amount equal to the modulating frequency.
• For example, in AM broadcasting, audio signals up to 5 kHz can be transmitted. If the carrier frequency is 1000 kHz, or 1 MHz, and the modulating frequency
is 5 kHz, sidebands will be produced at
• Upper side band = Fc + Fm= 100 +5 = 1005 kHz
• Lower side band = Fc – Fm = 100 -5 = 995 kHz
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• Bandwidth of AM = USB – LSB = 1005 - 995 = 10 kHz
• The modulation process generates other signals that take up spectrum space. It is not just the carrier at 1000
kHz that is transmitted. Thus the term bandwidth refers to the range of frequencies that contain the
information. 2 types of bandwidth exist in communication
1. Message /Signal bandwidth
2. Channel / transmission bandwidth
• The bandwidth of info signal is difference btw highest and lowest freq contain in the information while the
bandwidth of communication channel is the difference btw highest and lowest frequencies the channel will
allow to pass through with satisfactory fidelity..
• Bw of comm channel must be large enough to pass all significant information frequencies.
• For error free transmission channel bandwidth should be larger than signal bandwidth.
• The term signal bandwidth refers to the frequency of desired input signal. Eg The bandwidth of audio signal is
the signal bandwidth which is 5 kHz.
• The term channel bandwidth refers to the range of frequencies required to transmit the desired information. Eg.
The bandwidth of the AM signal described above gives the channel bandwidth which is 10 kHz.
• The bw allocated to AM broadcast is 10 kHz, while FM broadcast has 200kHz bandwidth.
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2. Noise limitation:
• Noise refers to random and unpredictable electric signals produced by natural process both internal and external to the system. When these variations
are super imposed on information signals message becomes corrupted. Filtering reduces noise contamination to some amount but cannot be
eliminated. (Noise is one of the fundamental limitations of communication systems). There are 2 types of noise:
i. External noise is a noise that is generated outside the device or circuit. There primary sources are due to Atmospheric, solar, cosmic, industrial/ man
made noise
ii. Internal noise is generated within the device or circuit. They exist due to thermal noise, shot noise
• Atmospheric noise: lightning in rainy weather, thunderstorms
• Solar, cosmic noise: radiation from sun and stars
• Man made noise: electric motors, fluorescent lights, car, aircraft ignition etc.
• Shot noise: is due to random arrival of carriers(holes and electrons) in electronic components like diode, BJT, FET.
• Thermal noise: is due to rapid and random movement of carrier within a conductor due to thermal agitation of atoms. Thermal noise is present in all
electrical and electronic components in CS. Because thermal noise is random, continuous and occurs at all frequencies in the entire electromagnetic
frequency spectrum, it is often referred as White Noise. This noise is predictable, additive, therefore thermal noise is the most significant of all noise
sources. Also known as Johnson and Brownian noise.
• Johnson proves thermal noise power is proportional to the product of bandwidth and temperature. Noise power given by
• N = KTB N= noise power in Watts, B= bandwidth in Hz, K= Boltzmann constant= 1.38x10-23,
T = absolute temp in kelvin. To convert ⁰C in K; T = 273 + ⁰C
Noise power in dBm can be written as N = 10 log KT + 10 log10 B
0.001 63
• For electronic device operating at temp 17 ⁰C with a bandwidth of 10 kHz,
determine thermal noise power in watts and dBm.
Thermal noise given by: N=KTB
T = 273 + ⁰C = 273 + 17 = 290K
N= 1.38x 10-3 x 290 x 10 x 103 = 4x10-17 Watts
N in dbm =10 log10 4 x 10-17 / 0.001 = -134 dBm

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Signal to noise power ratio:
• Signal to noise power ratio is the ratio signal power level to noise power level. SNR is expressed as S/N=Ps / Pn
• SNR is expressed in log with S/N (dB) = 10 log10 Ps / Pn
• SNR is expressed in terms of voltage and resistance; S/N(dB) = 10 log10 (Vs2/ Rin)
• Vn2/ Rout
• If input and output resistance of amplifier receiver or network are equal then ; S/N(dB) = 20 log10 (Vs)
Vn
• At low values of SNR, noise degrades fidelity in ACS and produces error in DCS.
• Effect of noise in ACS is evaluated in terms of signal to noise ratio. In DCS noise performance is measured by receiver probability of error
Example: for an amplifier with input signal power of 10 watts an output noise power of 0.01 watts determine SNR ?
S/N=Ps / Pn = 10/0.01=1000
S/N(dB) = 10 log10 Ps / Pn = 10 log101000 = 30 dB
Example: for an amplifier with input signal voltage of 4V, an output noise voltage of 0.005V, input and output resistance of 50ohm, determine SNR ?
S/N(dB) = 20 log10 (Vs) ; S/N(dB) = 20 log10 (4 = 58.06dB
Vn 0.005)

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