PRINCIPLES OF
COMMUNICATIONS
Prepared By: MEng. Samah A. Massadeh
Communication Systems Block Diagram
(Analog or Digital)
MEng. Samah A. Massadeh
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 Source:
Originates a message (e.g., human voice, TV
picture, email message) and converts it to an electrical
waveform, referred to as a message signal.
 Transmitter
(Modulator): Modifies the message signal
for efficient transmission.
 Channel:
A physical medium of choice that can convey
the electrical signals at the transmitter output over a
distance.
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 Receiver
(Demodulator): Processes the signal received from
the channel by reversing the signal modifications made at
the transmitter and removing the distortion made by the
channel.
 Sink:
Converts the electrical signal at the output of the
receiver to its original form – the message.
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Transmission Modes

There are three modes of transmission simplex,
half duplex, and full duplex. Transmission mode
describes the direction, of flow of signal between
two connected devices.
Transmission
Modes
Simplex
MEng. Samah A. Massadeh
Half Duplex
Full Duplex
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 The
main difference between simplex, half duplex, and
full duplex is that in a simplex mode of transmission the
communication is unidirectional whereas, in the halfduplex mode of transmission the communication is two
directional but the channel is alternately used by the both
the connected device.
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Definition of Simplex

In a simplex transmission mode, the communication between
sender and receiver occur only in one direction. That means only the
sender can transmit the data, and receiver can only receive the
data. The receiver can not reply in reverse to the sender.

Simplex is like a one-way road in which the traffic travels only in
one direction, no vehicle from opposite direction is allowed to
enter. The entire channel capacity is only utilized by the sender.

You can better understand the simplex transmission mode with an
example of keyboard and monitor. The Keyboard can only transmit
the input to the monitor, and the monitor can only receive the input
and display it on the screen. The monitor can not transmit any
information back to the keyboard.
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Definition of Half Duplex

In a half-duplex transmission mode, the communication between sender
and receiver occurs in both the directions but, one at a time. The sender
and receiver both can transmit and receive the information but, only one
is allowed to transmit at a time.

Half duplex is still a one way road, in which a vehicle traveling in
opposite direction of the traffic has to wait till the road is empty. The
entire channel capacity is utilized by the transmitter, transmitting at
that particular time.

Half duplex can be understood with an example of walkie-talkies. As the
speaker at both the end of walkie-talkies can speak but they have to
speak one by one. Both can not speak simultaneously.
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Definition of Full Duplex

In a full duplex transmission mode, the communication between sender
and receiver can occur simultaneously. Sender and receiver both can
transmit and receive simultaneously at the same time. The full duplex
transmission mode is like a two way road in which traffic can flow in both
the direction at the same time. The entire capacity of the channel is shared
by both the transmitted signal traveling in opposite direction.

Sharing of the channel capacity can be achieved in two different ways.
First, either you physically separate the link in two parts one for sending
and other for receiving. Second, or you let the capacity of a channel to be
shared by the two signals traveling in opposite direction.

Full duplex can be understood best, with an example of a telephone. When
two people communicate over a telephone both are free to speak and listen
at the same time.
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Key Differences Between Simplex, Half
Duplex and Full Duplex

In a Simplex mode of transmission, the signal can be sent only in
one direction; hence, it is unidirectional. On the other hand, in
half duplex, both the sender and receiver can transmit the signal
but, only one at a time, whereas, in full duplex, the sender and
receiver can transmit the signal simultaneously at the same time.

In a simplex mode of transmission, only one of the two devices on
the link can transmit the signal, and the other can only receive
but can not send back the signal in reverse. In a half duplex
mode, both the devices connected on the link can transmit the
signal but only one device can transmit at a time. In a full-duplex
mode, both the device on the link can transmit simultaneously.
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 The
performance of full duplex is better than half duplex
and simplex because it better utilizes the bandwidth, as
compared to half duplex and simplex.
 If
we take the example of keyboard and monitor, it is
observed that keyboard inputs the command and monitor
displays it, monitor never reply back to the keyboard;
hence, it is an example of the simplex transmission mode. In
a walkie-talkie, only one person can communicate at a time
so; it represents an example of half duplex mode of
transmission. In a telephone, both the person on the either
side of a telephone can communicate parallelly at the same
time; hence, it represents an example of a full-duplex mode
of transmission.
MEng. Samah A. Massadeh

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BASIS FOR COMPARISON
SIMPLEX
Direction of Communication Communication is
unidirectional.
HALF DUPLEX
FULL DUPLEX
Communication is twodirectional but, one at a
time.
Communication is two
directional and done
simultaneously.
Send/Receive
A sender can send data but, A sender can send as well as A sender can send as well as
can not receive.
receive the data but one at receive the data
a time.
simultaneously.
Performance
The half duplex and full
duplex yields better
performance than the
Simplex.
The full duplex mode yields Full duplex has better
higher performance than
performance as it doubles
half duplex.
the utilization of
bandwidth.
MEng. Samah A. Massadeh
Example
Keyboard and monitor.
Walkie-Talkies.
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Telephone.
Signals and Systems
Review
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Signals-Basics

A signal is a function which represents a physical quantity
A
function of one, two or more variables
 For
example,
Temperature
Your
Depends upon Rainfall during the year
voice signals is a function of time and frequency.
14
Signals-Basics

Various types of signals in communications
 Electrical
 Voltages
 Acoustic
signals
and currents
signals
 Sound
 Video
signals
 Intensity
level of a pixel (camera, video) over time
 But,
for enabling an electronic communication system, we
convert all signals into electrical signals
 Using
various kinds of transducers
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Signals-Basics
Information signal
1
0.8
0.6

Consider a signal
x(t , f ,  )  A  sin( 2  f  t   )
Amplitude
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0
2
4
6
Time
Amplitude

Frequency
Phase
So all information is represented by one or more of these parameters
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8
10
Some key terms

Amplitude
 Measure

of the strength of the signal
Frequency
 Number
of oscillations per second
 Hertz

Bandwidth
 Range
 Our
of frequencies occupied by a signal
voice signal occupies the frequencies between 300-3400 Hz
17
Signals-Basics

A microphone converts the voice signal from mechanical signal
into an electrical signal

Similarly, a digital camera converts a picture into digital images
 Sensors
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Analogue and Digital Signals
 Analogue
signals
Continuous
in time and continuous in
amplitude
Most
signals in the real world are
continuous time
x(t
)
t
19
Analogue and Digital Signals

Digital signals

Discrete in time and usually discrete in terms of amplitude too
 Some
real world signals are discrete, computer
signals are always discrete
x[n]
n
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Analog, Digital, Continuous-Time,
Discrete-Time?
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 In
signal classification, the adjectives “analog” and
“digital” refer to the amplitude property of the signals,
while “continuous-time” and “discrete-time” refer to
the time property.
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System-Basics
 “Systems”
process input signals to produce output signals
 Manipulate
signals
 Example,
 An
MP3 player is a system because it converts data
(MP3 song) into audio signals
A
communication system
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Sine and Cosine signal
10
 Start
signals
from “0”
sin(2πf
6
4
Amplitude (volts)
 Sinusoidal
8
2
0
-2
-4
x 0) = 0, @t=0
-6
-8
-10
0
2
4
6
Time (seconds)
8
10
0
2
4
6
Time (seconds)
8
10
10
8
6
 Start
(cosine) signals
from “1”
cos(2πf
x 0) = 1, @t=0
Amplitude (volts)
 Cosinusoidal
4
2
0
-2
-4
-6
-8
-10
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Some important terms
Wavelength
Crest
10
8
6
Time
Period
Amplitude (volts)
4
2
0
-2
-4
Trough
-6
-8
-10
0
2
4
6
Time (seconds)
8
10
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Some important terms

Crest
 Indicates

Trough
 Indicates

largest value of a signal
smallest value of a signal
Time period
 Time
taken by the signal to complete one cycle
 Measured in seconds

Frequency
 Number
of cycles completed by the signal in one
second
 Measured in Hertz
26
Some important terms
Wavelength
 Distance
(spatial i.e. in meters) between two
successive crests or troughs
 Distance
travelled by a signal in one cycle
10
Wavelength
8
6
4
Amplitude (volts)

2
0
-2
-4
-6
27
-8
-10
0
2
4
6
Time (seconds)
8
10
Some important terms
1
frequency( Hz ) 
time period ( s)
velocity(m / s)
wavelength (m) 
frequency(Hz)
wavelength (m)  velocity(m / s)  time period (s)
For wireless communication, velocity is taken as 3x108 m/s
For wireline communication (electrical), velocity is taken as
2x108 m/s
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Find the amplitude, freq & phase of the following
signals
x(t , f ,  )  A  sin( 2  f  t   )
Amplitude
a(t )  sin( 4t )
Frequency
Phase
b(t )  7.5 sin(6t  3)
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Noise
 Noise
 It
is a random signal (no particular shape or pattern)
is an undesirable signal
 Because
it corrupts information!
 Noise
usually enters the communication system via the
communicating medium
 But
it could be locally generated as well.
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Time-domain & frequency-domain plots

Most signals of our interest are dependent on time and
frequency At the same time
x(t , f ,  )  A  sin( 2  f  t   )

Therefore, we have two different kinds of plots
 Time-domain
plots
Give
information about the time dependent
properties
 Frequency-domain
Give
plots
information about the frequency dependent
properties
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Example-1
Frequency-domain plot
1
0.8
0.9
0.6
0.8
0.4
0.7
I Complete cycle in 1 sec
0.2
Magntude
Amplitude (volts)
Time-domain plot
1
0
-0.2
0.6
0.5
0.4
-0.4
0.3
-0.6
0.2
-0.8
0.1
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
0.7
0.8
0.9
1
0
0
1
2
3
4
5
6
Frequency (Hertz)
7
x(t )  sin( 2 1 t )  sin( 2t )
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8
9
10
Example-2
Frequency-domain plot
1
0.8
0.9
0.6
0.8
0.4
0.7
I Complete cycle in 0.5 sec
0.2
0
-0.2
Magntude
Amplitude (volts)
Time-domain plot
1
0.6
0.5
0.4
-0.4
0.3
-0.6
0.2
-0.8
0.1
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
0.7
0.8
0.9
1
0
0
1
2
3
4
5
6
Frequency (Hertz)
7
x(t )  sin( 2  2  t )  sin( 4t )
33
8
9
10
Example-3
Frequency-domain plot
1
0.8
0.9
0.6
0.8
0.4
0.7
0.2
0.6
Magntude
Amplitude (volts)
Time-domain plot
1
0
-0.2
0.5
0.4
-0.4
0.3
-0.6
0.2
-0.8
0.1
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
0.7
0.8
0.9
1
0
0
1
2
3
4
5
6
Frequency (Hertz)
7
x(t )  sin( 2  4  t )  sin(8t )
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8
9
10
Example-4
Frequency-domain plot
1
0.8
0.9
0.6
0.8
0.4
0.7
0.2
0.6
Magntude
Amplitude (volts)
Time-domain plot
1
0
-0.2
0.5
0.4
-0.4
0.3
-0.6
0.2
-0.8
0.1
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
0.7
0.8
0.9
1
0
0
1
2
3
4
5
6
Frequency (Hertz)
7
x(t )  sin( 2  9  t )  sin(18t )
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8
9
10
Signals and Spectra
 Electrical
communication signals are time-varying
quantities such as voltage or current.
 Although
a signal physically exists in the time
domain, we can also represent it in the
frequency domain where we view the signal as
consisting of sinusoidal components at various
frequencies.
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Signals and Spectra

This frequency-domain description is called the spectrum.
37
Spectrum & Bandwidth
 Spectrum
 range
of frequencies contained in signal
 Bandwidth
 width
of spectrum
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Spectrum & Bandwidth
•
•
Range of Frequencies?
Bandwidth?
Bandwidth (BW)=(6000-1000)=5000Hz=5KHz
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Bandwidth:
 Bandwidth
(BW) of a system h(t) is defined as the interval of
positive frequencies over which the magnitude |H(f)|
remains within a given numerical factor.
 Bandwidth
of a signal x(t) is defined as the interval of
positive frequencies over which the magnitude |X(f)| remains
within a given numerical factor.
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Spectrum of Pure Sine wave
Power Line Frequency
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Ideal Frequency
Selective Filter
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What is an Electric Filter?
 An
electric filter is usually a frequencyselective network that passes a specified band
of frequencies and blocks or attenuates signals
of frequencies outside this band.
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Ideal Low-pass Filter (LPF)

A filter that provides a constant output from dc up to a cut-off frequency fc and then passes no
signal above that frequency is called an ideal low-pass filter.

The ideal response of a low-pass filter
1, 𝑓 < 𝑓𝑐
𝐻 𝑓 =ቊ
0, 𝑓 > 𝑓𝑐
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 Note
that:
 The
voltage gain (the ration of output voltage and input
voltage i.e. (Vout/Vin) is constant over a frequency range
from zero to cut off frequency fc .
 The
output of any signal having a frequency exceeding fc will
be attenuated i.e there will be no output voltage for
frequencies exceeding cut-off frequency .
 The
output will be available faithfully from 0 to fc with
constant gain, and is 0 from fc onward.
 The
frequencies between 0 and fc are, therefore, called the
passband frequencies, while the range of frequencies, those
beyond fc , that are attenuated include the stopband
frequencies. The bandwidth (BW) is, therefore fc .
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Ideal High Pass Filter (HPF)

A filter that provides or passes signals above a cut-off frequency fc is a
high-pass filter
0, 𝑓 < 𝑓𝑐
𝐻 𝑓 =ቊ
1, 𝑓 > 𝑓𝑐
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 frequency
fc , called the cut-off frequency, and above this
frequency, the gain is constant.
 Thus
signal of any frequency beyond fc is reproduced with a
constant gain, and frequencies from 0 to fc will be
attenuated.
 In
the optical domain, high-pass and low-pass have the
opposite meanings, with a "high-pass" filter (more commonly
"long-pass") passing only longer wavelengths (lower
frequencies), and vice versa for "low-pass" (more commonly
"short-pass").
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Ideal Band-Pass Filter (BPF)
 When
the filter circuit passes signals that are above one cutoff frequency and below a second cut-off frequency, it is
called a band-pass filter.
 Thus
a band-pass filter has a passband between two cut-off
frequencies fc2 and fc1 ,where fc2 >fc1 and two stopbands:0<f<fc1 and f>fc2 are lower and higher cut-off
frequencies respectively.
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1, 𝑓𝑐1 < 𝑓 < 𝑓𝑐2
𝐻 𝑓 =ቊ
0, 𝑜𝑡ℎ𝑒𝑟 𝑤𝑖𝑠𝑒
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Ideal Band-Stop Filter (BSF)
 The
band-stop or band-reject filter performs exactly
opposite to the band-pass i.e. it has a bandstop between
two cut-off frequencies fc1 and fc2 and two passbands :
0<f<fc1 and f>fc2.
0, 𝑓𝑐1 < 𝑓 < 𝑓𝑐2
𝐻 𝑓 =ቊ
1, 𝑜𝑡ℎ𝑒𝑟 𝑤𝑖𝑠𝑒
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Real Frequency
Selective Filter
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Low-pass Filter (LPF)
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High Pass Filter (HPF)
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Band-Pass Filter (BPF)
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Band-Stop Filter (BSF)
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