111 lecture 5 basic modulation techniques (i) principles of communications fall 2008 nctu ee...
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111
Lecture 5Basic Modulation Techniques (I)
Principles of Communications
Fall 2008
NCTU EE Tzu-Hsien Sang
2
Outlines
• Linear Modulation
• Angle Modulation
• Interference
• Feedback Demodulators
• Analog Pulse Modulation
• Delta Modulation and PCM
• Multiplexing
2
3
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.
4
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
5
• Coherent (Synchronous) Demodulator (Detector): The receiver knows exactly the phase and frequency of the received signal.
ttmAtmA
tttmAttxtd
cCC
ccCcc
2cos)()(
cos2]cos)([cos2)()(
6
• 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
7
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
8
• 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
9
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.
10
• 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
11
• 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
13
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).
16
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
17
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
18
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
22
222
Efficiency
tmA
twtmAtx
C
cCc
19
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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