FEDERAL TECHNICAL AND VOCATIONAL INSTITUTE

Faculty of Electrical-Electronics and ICT

Department of Electrical-Electronics Technology

COMMUNICATION SYSTEM II

ECTe 3032

Chapter two
Modulation
 Baseband vs. Passband Communication Systems

• Communication systems can be classified into two

groups depending on the range of frequencies they
use to transmit information.
• These communication systems are classified into
BASEBAND or PASSBAND system.
• Baseband transmission sends the information
signal as it is without modulation (without
frequency shifting).
• Passband transmission shifts the signal to be
transmitted in frequency to a higher frequency and
then transmits it, where at the receiver the signal is
shifted back to its original frequency.
• Almost all sources of information generate
baseband signals.
• Baseband signals are those that have frequencies
relatively close to zero such as the human voice
(20 Hz – 5 kHz) and the video signal from a TV
camera (0 Hz – 5.5MHz).
• The telephone system used for homes and offices,
for example, may transmit the baseband audio
signal as it is when the call is local (from your
home to your neighbor’s home).
• However, when the telephone call is a long–
distance call that is transmitted via microwave or
satellite links, the baseband audio signal becomes
unsuitable for transmission and the
communication system becomes a passband
system.
• Similarly, transmitting the video signal from
your camera to your TV using a wire
represents a baseband communication while
transmitting that video signal via satellites
passband transmission.
• Therefore, baseband transmission, which is
easier than passband transmission, is usually
used when communicating over wires, while
over the air transmission requires passband
transmission.
• Notice that even over
wires, the transmission may be passband
transmission in specific applications.
 Modulation

• Two signals are involved in the process of

modulation. Message signals also known
as baseband signals are the band of
frequencies representing the original
signal. This is the signal to be transmitted
to the receiver. The frequency of such a
signal is usually low. The other signal
involved with this is a high-frequency
sinusoidal wave. This signal is called the
carrier signal.
• The frequency of carrier signals is almost
always higher than that of the baseband
signal.
• The amplitude of the baseband signal is
transferred to the high-frequency carrier.
• Such a higher frequency carrier is able to travel
much farther than the baseband signal.
• Therefore, modulation can be defined as the
process of superimposing a low-frequency signal
on a high-frequency carrier signal.
• Modulation is a technique of imposing
information (analog or digital) contained
in a
lower frequency signal onto a higher
frequency signal.
• The lower frequency is called the
modulating signal, the higher frequency
signal is called the carrier, and the output
signal is called the modulated signal.
 Need for modulation

1.Reduction in the height of antenna

2.Avoids mixing of signals
3.Increases the range of
communication
4.Multiplexing is possible
5.Improves quality of reception
 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 :

• The antenna of this height is practically

• Now, let us consider a modulated signal at f = 1
MHz . The minimum antenna height is given by,

This antenna can be easily installed practically .

Thus, modulation reduces the height of the
antenna .
 2. 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 .
 3. 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.
 4. 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 .
5. 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 .
 Types of modulation
basically two types of modulation
 Analog Modulation:

In this modulation, a continuously varying

sine wave is used as a carrier wave that
modulates the message signal or data
signal.
The Sinusoidal wave’s general function is
shown in the figure below, in which, three
parameters can be altered to get
 Amplitude Modulation:

• Amplitude modulation was the earliest modulation

technique used to transmit voice by radio.
• This type of modulation technique is used in
electronic communication.
• In this modulation, the amplitude of the carrier
signal varies in accordance with the message signal,
and other factors like phase and frequency remain
constant.
• The modulated signal is shown in the below figure,
and its spectrum consists of the lower frequency
band, upper frequency band and carrier frequency
components.
• This type of modulation requires more power and
greater bandwidth; filtering is very difficult.
• Amplitude modulation is used in computer modems,
VHF aircraft radio, and in portable two-way radio
 Frequency Modulation:

• In this type of modulation, the frequency of the

carrier signal varies in accordance with the message
signal, and other parameters like amplitude and
phase remain constant.
• Frequency modulation is used in different
applications like radar, radio and telemetry etc.
• This type of modulation is commonly used for
broadcasting music and speech, magnetic tape
recording systems, two way radio systems and video
transmission systems.
• When noise occurs naturally in radio systems,
frequency modulation with sufficient bandwidth
provides an advantage in cancelling the noise
 Phase Modulation:

• In this type of modulation, the phase of the carrier

signal varies in accordance with the message signal.
• When the phase of the signal is changed, then it
affects the frequency.
• So, for this reason, this modulation is also comes
under the frequency modulation.
• Generally, phase modulation is used for transmitting
waves.
• It is an essential part of many digital
transmission coding schemes that underlie a wide
range of technologies like GSM, WiFi, and
satellite television.
 Digital Modulation
In digital wireless communication systems, the
modulating signal may be represented as a time
sequence of symbols or pulses, where each
symbol has m finite states.
Advantages of Digital over Analog:
 Greater noise immunity (due to its finite process)
 Robustness to channel impairments
 Easier multiplexing of various forms of
information like voice, data, video
It provides more information capacity, high data
security, quicker system availability with great quality
communication. It is of following types:
Amplitude Shift Keying: The amplitude of the carrier is
changed in response to the information and all else
are kept constant.
Frequency Shift Keying: Here, the frequency is
changed in response to the information. A
synchronous method is used for detection of FSK.
Phase Shift Keying: In PSK ,we change the sinusoidal
carrier to indicate information. These are mainly two
types Binary Phase Shift Keying and Quadrature Phase
Shift Keying