Introduction to Communication Engineering

Communication Engineering
Electronics
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Introduction to CommSim
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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.