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Lecture 5Basic Modulation Techniques (I)

Principles of Communications

Fall 2008

NCTU EE Tzu-Hsien Sang

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Outlines

• Linear Modulation

• Angle Modulation

• Interference

• Feedback Demodulators

• Analog Pulse Modulation

• Delta Modulation and PCM

• Multiplexing

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To modulate or not to modulate, that is the question…

• Recall the simple digital modulation for transmitting 0s and 1s with fiber optics.

• In some aspects, digital modulation is much simpler than analog modulation.

• Modulator alters a carrier’s shape so that the alteration the intended message (information).

• Modulation, historically, is done on the RF carrier wave. Thus the conversion from message signals to RF signals is also called modulation.

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Linear Modulatoin

• General form:

• The envelope A(t) has a one-to-one correspondence with the message m(t).

• Example I: DSB (Double SideBand) Suppressed Carrier (SC)

ttAtx cc cos)()(

)(2

1)(

2

1)(X

cos)()(

C CCCC

cCc

ffMAffMAf

ttmAtx

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• Coherent (Synchronous) Demodulator (Detector): The receiver knows exactly the phase and frequency of the received signal.

ttmAtmA

tttmAttxtd

cCC

ccCcc

2cos)()(

cos2]cos)([cos2)()(

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• What if the receiver reference is not coherent? ---- A phase error case ((t), unknown, random, time-varying, …)

2cos(ct+(t))

xc(t) d(t)

varying!- timeisIt

.1)(cos1 ),(cos)()(

))(2cos()()(cos)(

))(cos(cos)(2)(

tttmty

tttmAttmA

ttttmAtd

D

cCC

ccC

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Let’s synch up!

• Carrier recovery: Regenerate the carrier at the receiver ( fc and (t) )

• Example: Square circuit

( )2 Narrowband

BPF at 2fc

2f xr(t)

cos2ct cosct

ttmAtmAttmAtx cCCcCr cos2)(2

1)(

2

1cos)()( 22222222

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• What are we expecting? It will become much clearer if we look in the frequency domain.

FT of xr2(t):

0 2fc -2fc

FT(m2(t))

Narrow BPF

f

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Side Show: The importance of synchronization• In communications, many things heavily depend on

the synchronization between the Rx and the Tx. There are many usages of the word synchronization. So be careful…

• Example: The French composer Pierre Schaeffer (1910-1995) had done probably the first tele-synchronized act of art in 1944 to celebrate the liberation of Paris.

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• Example: Global Positioning System: Use the time differences of received signals from satellites to determine the location.

• Q: Can you do a coarse estimation of what it takes to have a 10m precision?

• The speed of light:

• The modulation scheme

m/s. 103 8

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• Example II: Amplitude Modulation (AM), it’s simply DSB with carrier

bias. dc theis one. than less bebetter had

index. modulation theis ,)(min

message. normalized the,)(min

)()(

cos)](1[)(

Aa

aA

tma

tm

tmtm

ttamAtx

n

cnCc

12

-fc fc 0 0

AM

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Demodulation

• Coherent detection: precise but requires carrier recovery circuit.

• Incoherent detection: envelope detection Simple receiver (LPF) but requires sufficient carrier power (a < 1) and fc >> W. ( In theory, fc > W is ok, but a “good” LPF is needed.)

• Think about the advantages and disadvantages of analog signal processing (against DSP).

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This is the way to figure out the power…

%)100()(1

)(%)100(

)(

)(

])([)'(2

1

])()(2[)'(2

1

2cos)'()]([2

1)'()]([

2

1

cos)'()]([)(

have weaveraging, time thedoing AM,For

22

22

22

2

222

222

2222

222

tma

tma

tmA

tmEfficiencyE

tmAA

tmtmAAA

tAtmAAtmA

tAtmAtx

n

n

C

C

cCC

cCc

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How much power is for the message?

%.3.331 If

5.0)( sinusoid. a is)( If II. Example

%.501 If

1)(m 1)(min

)()(m1-

case) (typical )(min)(max

:I Example

2

2nn

Ea

tm tm

Ea

ttm

tmt

tmtm

n

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carrier.) with (mixed recovered.

accurately becannot m(t) message ofcomponent dc The

:AM of gedisadvantaOther

? :Question

)(2

1

cos)]([)(

aspower signal average thecalculatecan weSC,-DSBFor

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222

Efficiency

tmA

twtmAtx

C

cCc

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Single Sideband (SSB) modulation: A good example of signal manipulation

• Method one of generation of SSB signals: Sideband filtering, easy to understand, but difficult to implement. Why?

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• Method 2: Phase-shift modulation: