Modulation

All About Modulation – Part I

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Intuitive Guide to Principles of Communications
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All About Modulation
Basic Concepts, Signal Space, Constellations and Phase Shift Keying
modulations (PSK, QPSK, OQPSK, M-PSK, π/4-QPSK, MSK, and GMSK)

Basic Concepts of modulation
Three kinds of modulations
Modulation is the process of facilitating the transfer of informationover a
medium. Sound transmission in air has limited range for the amount of power
your lungs can generate. To extend the range your voice can reach, we need to
transmit it through a medium other than air, such as a phone line or radio. The
process of converting information (voice in this case) so that it can be
successfully sent through a medium (wire or radio waves) is called modulation.
Webegin our discussion of digital modulation by starting with the three basic
types of digital modulation techniques. These are;
Amplitude-Shift Keying (ASK)
Frequency-Shift Keying (FSK)
Phase-Shift Keying (PSK)
All of these techniques vary a parameter of a sinusoid to represent the information
which we wish to send. A sinusoid has three different parameters than can be
varied. These are itsamplitude, phase and frequency. Modulation is a process of
mapping such that it takes your voice (as an example of a signal) converts it into
some aspect of a sine wave and then transmits the sine wave, leaving the actual
voice behind. The sine wave on the other side is remapped back to a near copy of
your sound.
The medium is the thing through which the sine wave travels. So wire is amedium and so are air, water and space. The sine wave is called the carrier. The
information to be sent, which can be voice or data is called the information signal.
Once the carrier is mapped with the information to be sent, it is no longer a sine

Copyright 2002 Charan Langton , revised Dec 2005

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All About Modulation – Part I

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wave and we call it the signal. Thesignal has the unfortunate luck of getting
corrupted by noise as it travels.
In ASK, the amplitude of the carrier is changed in response to information and all
else is kept fixed. Bit 1 is transmitted by a carrier of one particular amplitude. To
transmit 0, we change the amplitude keeping the frequency constant. On-Off
Keying (OOK) is a special form of ASK, where one of the amplitudes iszero as
shown below.

Figure 1 - Baseband information sequence – 0010110010

ASK(t ) = s(t ) sin(2πft )

Figure 2 - Binary ASK (OOK) signal
In FSK, we change the frequency in response to information, one particular
frequency for a 1 and another frequency for a 0 as shown below for the same bit
sequence as above. In the example below, frequency f1 for bit 1 is higher than f2
used for the 0bit.

⎧ sin( 2πf1t )
FSK ( t ) = ⎨
⎩sin( 2πf 2 t )

for bit 1
for bit 0

Copyright 2002 Charan Langton , revised Dec 2005

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All About Modulation – Part I

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Figure 3 - Binary FSK singnal
In PSK, we change the phase of the sinusoidal carrier to indicate information.
Phase in this context is the starting angle at which the sinusoid starts. To transmit0, we shift the phase of the sinusoid by 180°. Phase shift represents the change in
the state of the information in this case.
⎧ sin(2π f t ) for bit 1
PSK (t ) = ⎨
⎩sin(2π f t + π ) for bit 0

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Figure 4 - Binary PSK Carrier (Note the 180° phase shifts at bit edges)
ASK techniques are most susceptible to the effects of non-linear devices which
compress and distort signal amplitude. Toavoid such distortion, the system must
be operated in the linear range, away from the point of maximum power where
most of the non-linear behavior occurs. Despite this problem in high frequency
carrier systems, Amplitude Shift Keying is often used in wire-based radio
signaling, both with or without a carrier.
ASK is also combined with PSK to create hybrid systems such as Quadrature...