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7/27/2019 Topic 4 - Modulation Techniques
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UCCN2023
Fundamentals of Wireless
Communications
Topic 4 : Modulation Techniques
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Topic 4: Modulation Techniques
4.1 Modulation
4.2 Analog Modulation Techniques
4.3 Digital Modulation Techniques
4.4 Constellation Diagram
4.5 Digital Modulation & Constellation Diagram
4.6 Gray Mapping
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4.1 Modulation
Modulation is the process ofencoding information from a messagesource in a mannersuitable for transmission through the chosenchannel.
It generally involves translating a baseband message signal (calledthe source) to a bandpass signal at frequencies that are very high
when compared to the baseband frequency. The bandpass signal is called the modulated signal
The baseband message signal is called the modulating signal.
Why modulation? Why not transmit the waveforms directly over theradio channel? Main reason = To reduce antenna size.
The typical antenna size is /4. Assuming that the baseband messageis a sinusoid with frequency f = 1000 Hz; then the antenna size would
be 75 km!
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4.1 Modulation
Modulation may be done by varying the amplitude, phase, or
frequency of a high frequency carrierin accordance with theamplitude of the message signal.
Demodulation is the process of extracting the baseband messagefrom the carrierso that it may be processed and interpreted by the
intended receiver.
The ultimate goal of a modulation technique is to transport themessage signal through a radio channel with the best possible quality
while occupying the least amount of radio spectrum.
First generation mobile radio systems employs analog modulation
schemes.
Digital modulation schemes are used in present systems and willdominate the future systems since digital modulation offers numerous
benefits.
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Message signal
x(t)
Carrier signal
AM signal
s(t)
Time
Time
Time
5
4.2 Analog Modulation Techniques - Amplitude
Modulation (AM) Schemes
Amplitude of carrier signal is varied as the message signal to be transmitted.
Frequency of carrier signal is kept constant.
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4.2 Analog Modulation Techniques FrequencyModulation (AM) Schemes
FM integrates message signal with carrier signal by varying the
instantaneous frequency. Amplitude of carrier signal is kept constant.
Carrier signal
Message signal
x(t)
FM signal
s(t)
Time
Time
Time
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4.2 Analog Modulation Techniques AM and FM
Sourced from:
Wireless Communications & Network, Stallings, pg 151
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4.2 Analog Modulation Techniques AM vs FM
In amplitude modulation (AM) schemes, there is a linear relationshipbetween the quality of the received signal and the powerof the
received signal since AM signals superimpose the exact relative
amplitudes of the modulating signal onto the carrier. Thus, AM signals
have all theirinformation in the amplitude of the carrier.
Frequency modulation (FM) is the most popular analog modulationtechnique used in mobile radio systems. In FM, the amplitude of themodulated carrier signal is kept constant while its frequency is varied
by the modulating message signal.
FM offers many advantages over amplitude modulation (AM), which
makes it a better choice for many mobile radio applications.
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4.2 Analog Modulation Techniques AM vs FM
FM signals have all theirinformation in the phase or frequency of thecarrier. As shown subsequently, this provides a nonlinear and very
rapid improvement in reception quality once a certain minimum
received signal level, called the FM threshold, is achieved.
Frequency modulation has betternoise immunity when compared to
amplitude modulation. Since signals are represented as frequencyvariations rather than amplitude variations, FM signals are less
susceptible to atmospheric and impulse noise, which tend to cause
rapid fluctuations in the amplitude of the received radio signal.
Also, message amplitude variations do not carry information in FM, so
burst noise does not affect FM system performance as much as AMsystems, provided that the FM received signal is above the FM
threshold.
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4.3 Digital Modulation Techniques
Modern mobile communication systems use digital modulationtechniques.
Advancements in very large-scale integration (VLSI) and digital
signal processing (DSP) technology have made digital modulation
more cost effective than analog transmission systems.
Digital modulation offers many advantages over analog modulation. Greater noise immunity and robustness to channel impairments.
Easier multiplexing of various forms of information (e.g., voice,
data, and video).
Greater security.
Digital transmissions accommodate digital error-control codes
which detect and/or correct transmission errors
Support complex signal conditioning and processing techniques
such as source coding, encryption, and equalization to improve
the performance of the overall communication link.
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4.3 Digital Modulation Techniques
New multipurpose programmable digital signal processors have madeit possible to implement digital modulators and demodulators
completely in software.
Instead of having a particular modem design permanently frozen
as hardware, embedded software implementations now allow
alterations and improvements without having to redesign or
replace the modem.
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4.3 Digital Modulation Techniques - Amplitude Shift
Keying (ASK)
Represents digital data as variations in the amplitude of a carrierwave.
In Binary ASK, where only two
symbol states are needed, the
carrier is simply turned on or off,
and the process is sometimes
referred to as ON-OFF Keying
(OOK).
If more than two symbol states
are used, then an M-ary ASKprocess is adopted, an example
being the 8-ASK format shown
here.
Sourced from:
Digital Communications, Bateman, pg 105
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4.3 Digital Modulation Techniques - Frequency Shift
Keying (FSK)
The most common form of FSK is binary FSK (BFSK), the two binaryvalues are represented by two different frequencies near the carrier
frequency.
Message signal
x(t)
BFSK signal
y(t)
1 0 1 1 0 1
Time
Time
Time
Time
To represent Binary 1
To represent Binary 0
)2cos()( 11 tfAtS
)2cos()( 22 tfAtS
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4.3 Digital Modulation Techniques - Frequency Shift
Keying (FSK)
In Multiple FSK (MFSK), more than two frequencies are used. Eachsignaling element represents more than one bit. The transmitted
MFSK signal for one signal element time can be defined as follows:.
Sourced from:
Digital Communications, Bateman, pg 143
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4.3 Digital Modulation Techniques - Frequency Shift
Keying (FSK)
Example : With fc=250KHz, fd=25KHz, and M=8(L=3 bits), we havethe following frequency assignment for each of the 8 possible 3-bit
data combinations:
KHzMfWbandwidth
KHzf
KHzfKHzf
KHzf
KHzf
KHzf
KHzf
KHzf
ds 4002
425111
375110325101
275100
225011
175010
125001
75000
8
7
6
5
4
3
2
1
This scheme can support a data rate of:
KbpsHzbitsLfT db 150)25)(3(22/1
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4.3 Digital Modulation Techniques - Phase Shift
Keying (PSK)
In PSK, the phase of the carrier signal is shifted to represent data.BPSK, the simplest scheme uses two phases to represent the two
binary digits
To represent Binary 1
To represent Binary 0
)2cos()(2 tfAtS c
Message signal
x(t)
)2cos()(1 tfAtS c
1 0 1 1 0 1
BPSK signal
y(t)
Time
Time
Time
Time
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4.3 Digital Modulation Techniques - Phase Shift
Keying (PSK)
Four-Level PSK - More efficient use of bandwidth can be achieved ifeach signaling element represents more than one bit.
For example, instead of a phase shift of 180, a common encodingtechnique, known as Quadrature Phase-Shift Keying (QPSK), uses
phase shifts separated by multiples of (90).2/
10)4
2cos(
00)4
32cos(
01)
4
32cos(
11)4
2cos(
)(
tfA
tfA
tfA
tfA
ts
c
c
c
c
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4.3 Digital Modulation Techniques ASK, FSK, PSK
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4.3 Digital Modulation Techniques - Combined
Amplitude and Phase Keying (QAM/APK)
The most commonly used combination is ASK and PSK signalling.Depending on the constraints put on the amplitude/phase relationship:
Sometimes classified as M-ary APK
Sometimes as Quadrature Amplitude Modulation (QAM),
QAM is a logical extension of QPSK.
In QAM scheme, the transmitter send two different signals simultaneouslyon same carrier frequency
Use two copies of carrier, one shifted by 90
Each carrier isASK modulated
QAM is used on asymmetric digital subscriber line (ADSL) and somewireless standards.
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4.4 Constellation Diagram
A constellation diagram is a graphical representation of a signalmodulated by a digital modulation scheme
It displays the signal as a two-dimensional scatter diagram in the
complex plane at symbol sampling instants.
It shows the complex envelope of each possible symbol state.
The power efficiency is related to the minimum distance between thepoints in the constellation.
The bandwidth efficiency is related to the number of points in theconstellation.
Gray coding is used to assign groups of bits to each constellationpoint where adjacent constellation points differ by a single bit.
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4.4 Constellation Diagram
A constellation diagram helps us to define the amplitude and phase of a
signal when we are using two carriers, one in quadrature of the other. The X-axis represents the in-phase carrierand the Y-axis represents
quadrature carrier.
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4.4 Constellation Diagram Examples
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4.5 Digital Modulation & Constellation Diagram - ASK
Pure ASK: carrieramplitude is used to carry symbol information
An example of4-ASK with constellation diagram and modulationsignal set. Note: quadrature branch is not used.
41)2cos()( iwheretfAtS cii
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4.5 Digital Modulation & Constellation Diagram - PSK
BPSK: One bit per symbol, note the mapping from bits to symbols inconstellation diagram
Modulation signal set
2,1)2cos()( iwheretfAtS ici
Phase separation:
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4.5 Digital Modulation & Constellation Diagram - PSK
QPSK: Two bits per symbol with a minimum phase separation of
Modulation signal set
41
)2cos()(
iwhere
tfAtS ici
2/
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4.5 Digital Modulation & Constellation DiagramASK / PSK
PSK and ASK can be combined. Here is an example of4-ary or4-PAM (pulse amplitude modulation) with constellation pattern andtransmitted signal s(t):
2 amplitude levels and phase shift ofare combined to represent 4-ary symbols
Note in M-ary or M-PAM, quadrature component is not used, a moregeneric scheme of combining PSK/ASK is QAM, which uses both I
and Q branches.
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4.5 Digital Modulation & Constellation Diagram QAM
QAM: combines features of PSK and ASK, uses both I and Qcomponents, and is very bandwidth efficient.
An example of (squared) 16-QAM.
Note for squared M-QAM, I andQ branches are both M-ary.
Depending on the channelquality, 64-QAM, 128-QAM, or
256-QAM are possible.
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4.6 Gray Mapping
Gray coding: adjacent constellation points only differ in a single bit
(minimum Hamming distance). If noise or distortions cause mis-classification in the receiver, Gray
coding can minimise the bit error rate.
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Questions for Quizzes and Exams:
QPSK: Slide 17 Draw constellation diagram and waveform for each symbol as in
slide 25
8-PSK: (Exam) State the formula and draw the waveform for each symbol as in
slide 25. Modulate the sequence 001101110001011.
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Questions for Quizzes and Exams: