topic 4 - modulation techniques

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

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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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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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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


)2cos()( 11 tfAtS

)2cos()( 22 tfAtS


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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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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



)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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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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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:

topic 4 - modulation techniques

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