Introduction to Communication Engineering

This document provides an introduction to communication engineering and modulation schemes. It explains the concept of modulation and its role in transmitting digital data over analog carriers. The document covers amplitude modulation, frequency modulation, and angle modulation, discussing their working principles and characteristics. It also explores the advantages and disadvantages of each modulation scheme. The document concludes with a brief discussion on the applications and limitations of communication systems.

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Communication Engineering
Electronics
Course name:
Module name/code:
Title of CW:
Introduction to CommSim
Student name
Student Registration Number
Date of Submission

INTRODUCTION
Modulation Schemes allow the digital data obtained from a digital source to be transmitted
across analog carriers. The modulation involves the process of varying the amplitude, frequency,
or phase. The carrier signal is a basic continuous time signal which is a function of time. The
schemes seek to vary any of the three parameters of the signal.
For the amplitude modulation, the carrier signal and the message signal are summed to result in
the amplitude modulated signal such that,
V c ( t ) =V co sin ( 2 π f c t )
V AM (t )= {V co +V m ( t ) } sin ( 2 πft +f )
The modulation scheme varies the amplitude of the carrier signal referencing the modulating
signal. The method impressed the message signal on the carrier signal. The overall amplitude of
the modulated signal varied depending on the instantaneous amplitude of the modulating signal.
y ( t ) =A sin ωc t+ M
2 [ [ sin ( ωc +ωm ) t +φ ] +sin ( ωc+ωt ) ( t−φ ) ]
The amplitude of the message signal determines the modulation index. The modulation index is
obtained as,
m= V m ( t )max
V co
=V mo
V co
Other parameters that can be varied are the frequency and phase of a signal. The frequency
modulation spectra contain the carrier frequency and the sideband components whose amplitudes
depend on the Bessel functions of the first order.

The Bessel function looks like damped sine wave and the order of the function is obtained as,
Ji ( I ) =J i ( I )∗( −1 )i
The negative frequencies fold up to corresponding positive harmonic frequencies. The
modulation index determines the sidebands with larger amplitudes where the spectrum gets
brighter. It is important to consider the aliasing to ensure the Nyquist frequency1. The
instantaneous frequency is varied linearly for the message signal being modulated,
f i ( t )=f c +k f m(t )
The FM signal is a non-linear function of the modulating signal that makes frequency
modulation a nonlinear modulation process. The modulation presents two situation cases namely
1 A.B. Carlson, P.B. Crilly, J.C. Rutledge, Communication Systems, McGraw-Hill, Singapore, International Edition, 2002, (4th
Ed.).

the single tone modulation for the narrow FM signal and the single tone modulation for the
wideband FM signal.
m ( t ) =Am cos ( 2 π f m )
f i ( t )=f c +k f Am cos ( 2 π f m t )
The frequency deviation is obtained as,
∆ f =kf Am
The angle of the carrier wave is varied according to the base-band signal. When the phase
is varied, other parameters remain constant. The angle modulation provides better discrimination
against noise and interference than amplitude modulation2. It demonstrates an increased
transmission bandwidth.
2 B.P. LATHI, Modern Digital and Analog Communication Systems, Oxford University Press, New Delhi, India, (3rd Ed.), 2009.

s ( t ) =Ac cos ( θi ( t ) )
f i ( t ) = 1
2 π
dθ ( t )
dt
Wi ( t )=2 π f i ( t )= d θi ( t )
dt
d θi ( t )
dt =2 π f c
The form of angle modulation in which the angle is varied linearly with the message signal,
θi ( t ) =2 π f c t +k p m(t)
S(t )= Ac cos [ 2 π f c t +k p m ( t ) ]
θi ( t ) =2 π f c+2 π kf ∫
0
t
m ( τ ) dτ
S(t )= Ac cos [ 2 π f c t +2 π k f ∫
0
t
m ( c ) dc ]

AIM OF THE EXPERIMENT
(i) To familiarize oneself with communication system simulation package
(ii) To perform a variety of communication system functions and record results
EXPERIMENT PROCEDURE
(i) A 4kHz sine wave generator and a summing unction was added to the diagram and
the results output was set to orange trace 2.
(ii) The 1kHz input sine wave was displayed using the blue trace and the simulation was
run.
(iii) A different layer of proof by determining the spectrum of the product signal was
added and the simulation properties were adjusted to 8msec at 4.096MHz.
(iv) The spectrum analyzer block on the comm>operators>spectrum section was added to
the diagram.
(v) A trigger was setup to perform the analysis and it was connected to the Trg input of
the analysis block.
(vi) The plot properties were selected for the XY plot where the simulation was run and
the spectrum display did not have appropriate axes which may have needed change.

(vii) A filter was added to the diagram at the FIR option and a number of taps were
selected with the 1500 or 1700 Hz filter type being a low-pass or high-pass filter.
(viii) The AM spectrum of the signal was plotted on the XY coordinate graphical plot
section.
(ix) The modulation signal was recovered from the AM using the synchronous detection
where the AM signal was multiplied by an in-phase carrier and passed through a low
pass filter.
(x) Another attempt was made at the synchronous detection with a quadrature carrier
which made the carrier sine wave.
(xi) The SSBSC was generated using the in-phase and quadrature carrier and modulation.
The AM wae was filtered using a filter of 4th order Chebyshev II bandpass filter. The
lower and upper cutoff frequencies were selected to lower the sidebands.
(xii) Setting up a carrier signal and a square wave data signal to generate an OOK signal.
(xiii) The data signal was recovered using the synchronous detection including the filtering
and squaring.
(xiv) The data signal was recovered using the rectification, filtering, and squaring to repeat
the pseudo-random bit stream.
EXPERIMENT RESULTS & DATA ANALYSIS
Exercise 1
The results of the frequencies are multiplied together to provide the resultant as the sum of the
cosine waves at frequencies,
2 cos ωl t cos ω2 t=cos ( ω1 +ω2 ) t+ cos ( ω1−ω2 ) t
Two frequencies are used and appropriate values are provided,

The results were obtained as,
Exercise 2
AM Generation and Detection
To generate the AM by multiplying a carrier by a suitable signal,
e ( t ) AM =V c { I +mcos ( ωm t ) } sin ωc t
¿ {2+1 cos ( 2 π∗1∗103 t ) } sin 2 π∗16∗103 t

Exercise 3
SSBSC Generation and Detection
The single sideband suppressed carrier signal is generated using the in-phase and quadrature
carrier and modulation. In the experiment, the DSBSC wave was initially generated and the band
pass filter was used to produce the SSBSC wave. The band pass filter is tuned to meet the upper
or lower sideband frequencies.
For the generation,

For the upper sideband frequencies,
For the detection,
For the local oscillator,

Exercise 4
FM Generation and Detection

DISCUSSION
The amplitude modulation is implemented using a simple circuit, easy to generate and
tune for the short wave broadcasting. The AM covers a greater area than FM although it is quite
inefficient and it exposes the message signal to electromagnetic interference and other noises.
The frequency modulation finds application in media as it provides for audio quality and
immunity to noise. The audio quality increases as the frequency deviation value of a signal
increases. Unfortunately, the frequency modulation scheme requires a large bandwidth for
implementation.
CONCLUSION
In a nutshell, a communication system has a transmitter, channel, message, and a receiver
block. The baseband signals transmitted over the system contain frequency within a given range

and the transmission of digital data on analog equipment is enabled using a modulation at the
transmission block. The modulator modulates the carrier wave with a given frequency by
impressing a low-frequency information signal onto a higher frequency carrier signal.
BIBLIOGRAPHY
A.B. Carlson, P.B. Crilly, J.C. Rutledge. Communication Systems, McGraw-Hill, Singapore,
International Edition, 2002, (4th Ed.).
B.P. LATHI. Modern Digital and Analog Communication Systems, Oxford University Press,
New Delhi, India, 2009, (3rd Ed.).