Modulation Techniques

Analog modulation

why modulate analog signals?

• higher frequency can give more efficient transmission

• Practical Hight of antenna
• permits frequency division multiplexing
• types of modulation
• Amplitude
• Frequency
• Phase
Analog
Modulation
Techniques
• Amplitude Modulation
• Frequency Modulation
• Phase Modulation
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Digital – process of changing one of the characteristic

Modulation of an analog signal (typically a sinewave) based
on the information in a digital signal
 sinewave is defined by 3 characteristics (amplitude,
frequency, and phase)  digital data (binary 0 & 1)
can be represented by varying any of the three
 application: transmission of digital data over
telephone wire (modem)

Types of Digital
Moddulation
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Modulation of Digital Data: ASK
ASK – strength of carrier signal is varied to represent binary 1 or 0
 both frequency & phase remain constant while amplitude changes
 commonly, one of the amplitudes is zero

A cos(2πfc t), binary 0 0, binary 0

s(t)  0  
A1cos(2πf ct), binary 1 Acos(2πfc t), binary 1

+A

-A

 advantage: simplicity
 disadvantage: ASK is very susceptible to noise interference –
noise usually (only) affects the amplitude, therefore ASK is the
modulation technique most affected by noise
 application: ASK is used to transmit digital data over optical fiber
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Modulation of Digital Data: ASK (cont.)

Example [ ASK ]

vd(t)

vc(t)

vASK(t)
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Modulation of Digital Data: FSK

FSK – frequency of carrier signal is varied to represent binary 1 or 0

 peak amplitude & phase remain constant during each bit interval

Acos(2πf1t), binary 0
s(t)  
Acos(2πf 2t), binary 1

f1<f2
+A

-A

 demodulation: demodulator must be able to determine which of

two possible frequencies is present at a given time
 advantage: FSK is less susceptible to errors than ASK – receiver
looks for specific frequency changes over a number
of intervals, so voltage (noise) spikes can be ignored
 disadvantage: FSK spectrum is 2 x ASK spectrum
 application: over voice lines, in high-freq. radio transmission, etc.
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Modulation of Digital Data: FSK (cont.)

Example [ FSK ]

vd(t)

vc1(t)

vc2(t)

vFSK(t)
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Modulation of Digital Data: PSK
PSK – phase of carrier signal is varied to represent binary 1 or 0
 peak amplitude & freq. remain constant during each bit interval
 example: binary 1 = 0º phase, binary 0 = 180º (rad) phase
 PSK is equivalent to multiplying carrier signal by +1 when the
information is 1, and by -1 when the information is 0

2-PSK, or Acos(2πfc t), binary 1

s(t)  +A
Binary PSK, Acos(2πf ct  π), binary 0
since only 2
different phases
Acos(2πf ct), binary 1 -A
are used.
s(t)  
-Acos(2πfc t), binary 0

 demodulation: demodulator must determine the phase of received

sinusoid with respect to some reference phase
 advantage:  PSK is less susceptible to errors than ASK, while it
requires/occupies the same bandwidth as ASK
 more efficient use of bandwidth (higher data-rate)
are possible, compared to FSK !!!
 disadvantage: more complex signal detection / recovery process,
than in ASK and FSK
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Modulation of Digital Data: PSK (cont.)

Example [ PSK ]

vd(t)

vc(t)

vPSK(t)
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Modulation of Digital Data: PSK (cont.)
QPSK = 4-PSK – PSK that uses phase shifts of 90º=/2 rad  4
different signals generated, each representing 2 bits
Acos(2πfc t), binary 00
 π
Acos(2πfc t  ), binary 01
s(t)   2
Acos(2πf t  π),
 c binary 10
 3π
Acos(2πf t  ), binary 11

c
2

 advantage: higher data rate than in PSK (2 bits per bit

interval), while bandwidth occupancy remains the same
 4-PSK can easily be extended to 8-PSK, i.e. n-PSK
 however, higher rate PSK schemes are limited by the
ability of equipment to distinguish small differences in
phase
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Modulation of Digital Data: QAM
Quadrature – uses “two-dimensional” signalling
Amplitude  original information stream is split into two sequences that
Modulation consist of odd and even symbols, e.g. Bk and Ak
(QAM) 1 0 1 1 0 1 …
1 -1 1 1 -1 1 …
B1 A1 B2 A2 B3 A3 …
 Ak sequence (in-phase comp.) is modulated by cos(2πfc t)
Bk sequence (quadrature-phase comp.) is modulated by
sin(2πfc t)
 composite signal Akcos(2πfc t)  Bk sin(2πfc t) is sent
through the channel

Ak x Yi(t) = Ak cos(2fct)

cos(2fct) + Y(t) = Ak cos(2fct) + Bk sin(2fct)

Transmitted
Bk x Yq(t) = Bk sin(2fct) Signal

sin(2fct)

 advantage: data rate = 2 bits per bit-interval!

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Modulation of Digital Data: QAM (cont.)

Example [ QAM ]

vd(t)
Bk

sin(ct)

Ak

cos(ct)
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Signal Constellation
Constellation Diagram – used to represents possible symbols that may
be selected by a given modulation scheme as
points in 2-D plane
 X-axis is related to in-phase carrier: cos(ct)
 the projection of the point on the X-axis defines
the peak amplitude of the in-phase component
 Y-axis is related to quadrature carrier: sin(ct)
 the projection of the point on the Y-axis defines
the peak amplitude of the quadrature component
 the length of line that connects the point to
the origin is the peak amplitude of the signal
element (combination of X & Y components)
 the angle the line makes with the X-axis is the
phase of the signal element
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Modulation of Digital Data: QAM
QAM cont. – QAM can also be seen as a combination of ASK & PSK
  cos(2πf t  tan
1
Bk
Y(t)  A k cos(2πfc t)  Bk sin(2πfc t)  A k  Bk
2 2 2 -1
c )
Ak

Bk
(-A,A) (A, A)

4-level QAM Ak

(-A,-A) (A,-A)
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Modulation of Digital Data: QAM
16level QAM – the number of bits transmitted per T [sec] interval
can be further increased by increasing the number of levels used
 in case of 16-level QAM, Ak and Bk individually can
assume 4 different levels: -1, -1/3, 1/3, 1
 data rate: 4 bits/pulse  4W bits/second

  cos(2πf t  tan
1
Bk
Y(t)  A k cos(2πfc t)  Bk sin(2πfc t)  A k  Bk
2 2 2 -1
c )
Ak
Bk

Ak and Bk individually
can take on 4 different values;
Ak the resultant signal can take
on (only) 3 different values!!!

In QAM various combinations of amplitude and phase are employed

to achieve higher digital data rates.

Amplitude changes are susceptible to noise  the number of phase shifts used
by a QAM system is always greater than the number of amplitude shifts.
OFDM stands for Orthogonal Frequency Division
Multiplexing.

It's a technique used to transmit data over radio waves or optical fibers.

To understand OFDM, let's break it down into its parts:

•Frequency Division Multiplexing (FDM) is a technique that allows multiple signals to
be sent simultaneously over a single communication channel by dividing the available
frequency range into sub-channels.
•Orthogonal means that the sub-channels are perfectly spaced and don't interfere with
each other.
•So, OFDM is a way of using FDM with orthogonal sub-channels.
By using OFDM, a single communication channel can be divided into multiple sub-
channels, each of which can carry its own data. This means that multiple users or
devices can use the same channel simultaneously, without interfering with each other.
OFDM is commonly used in technologies like Wi-Fi, digital television broadcasting,
and 4G/5G cellular networks to increase the data transmission rates and improve the
quality of the signal.
OFDM
is a technique in which
several message signals are
combined into a composite
signal for transmission over a
common channel.
Simple Digital OFDM system Implementation using FFT transforms
The concepts used in the simple analog OFDM implementation can be extended to
the digital domain by using a combination of Fast Fourier Transform (FFT) and
Inverse Fast Fourier Transform (IFFT) digital signal processing. These transforms
are important from the OFDM perspective because they can be viewed as mapping
digitally modulated input data (data symbols) onto orthogonal subcarriers. In
principle, the IFFT takes frequency-domain input data (complex numbers
representing the modulated subcarriers) and converts it to the time-domain output
data (analog OFDM symbol waveform).
In a digitally implemented OFDM system, the input bits are grouped and mapped to
source data symbols that are a complex number representing the modulation
constellation point (e.g., the QPSK or QAM symbols that would be present in a
single subcarrier system). These complex source symbols are treated by the
transmitter as though they are in the frequency-domain and are the inputs to an
IFFT block that transforms the data into the time-domain. The IFFT takes in N
source symbols at a time where N is the number of subcarriers in the system. Each
of these N input symbols has a symbol period of T seconds. Recall that the output
of the IFFT is N orthogonal sinusoids. These orthogonal sinusoids each have a
different frequency and the lowest frequency is DC.