chapter 4e_pulse txn
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CHAPTER 4: PULSE TRANSMISSION
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 2
Each analog modulation method leads to its corresponding binarymethod.
The basebands involved are all digital and the simplest of these is
the binary digit which will be used to develop the basic theory ofthe methods.
There are three basic methods, each corresponding to one of thethree analog modulation methods. They are: Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Nowadays, keying methods are used in many different areas andthe theoretical requirements differ in some respects between them.
INTRODUCTION
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ASK is the simplest and the oldest method but it is the main method for opticalfiber transmission.
In ASK, the strength of the carrier signal is varied to represent binary 1 or 0.Both frequency and phase remain constant while the amplitude changes.
The amplitude of a sinusoidal carrier is shifted from 0 volts for a logic 0 to somefixed value, V for a logic 1.
This method gives 100% amplitude modulation and often called on-off keying(OOK)
OOK is one of the value that represented by no voltage. The advantage of thismethod is a reduction in the amount of energy required to transmit
information.The main disadvantage of ASK is easily corrupted by unwanted noise. Theterm noise refers to unintentional voltages introduced onto a line by variousphenomena such as heat or electromagnetic induction by other sources.
AMPLITUDE SHIFT KEYING (ASK)
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ASK can be done by multiplying the sinusoidal carrier by the digital
baseband signal as shown below.
ASK GENERATION
1 0 1 1 1 0 0 1 0 1 1
Carrier signal
Baseband signal
X
ASK signal
=
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ASK EXPRESSION
ASK can be expressed as:
The Fourier series of the baseband signal is given by:
signalcarrierthe-cos
signalbasebandthe-)(
Where,
cos)()(
tE
tv
tEtvtv
cc
m
ccmASK
[
[y!
.....
)5cos513cos
31(cos2
21)( 000 ! ttttvm [[[
T
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 6
The ASK equation is:
ASK EXPRESSION
......)3cos(3
)3cos(3
)cos()cos(cos2
.....3coscos3
2coscos
2cos
2
...
5cos513cos
31(cos2
21cos
)(cos)(
00
00
00
000
!
!
-
y!
y!
tE
tE
tE
tE
tE
ttE
ttE
tE
ttttE
tvtEtv
cc
cc
cc
cc
cc
cc
cc
cc
cc
mccASK
[[T
[[T
[[T[[T[
[[T
[[T
[
[[[T
[
[
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 7
Frequency spectrum of ASK can be shown as follow:
FREQUENCY SPECTRUM OF ASK
f0 3f0 5f0 f
Baseband signal|V|
2/T
2/3T
2/5T
fc f
Carrier signal|V|
Ec/2
Ec /T Ec /TEc/3T Ec/3T
3 x bit rate
|V|
f
ASK signal
Ec
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 8
Bandwidth of ASK is depends on the requirements and the
specifications required.
The BW is given by:
Example:
Obtain the bandwidth of ASK if the required signal to be sent is up
to the 3rd harmonics.
BANDWIDTH OF ASK
AMm
ASK
BWf
BW
!!
!
2
frequencymodulatingx2
0
0
0
00
222
11
ratebitx336)3(22
fff
TTf
ffffBW
bb
b
bmASK
!@!!!
!!!!!
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 9
In FSK, the frequency of the carrier signal is varied to representbinary 0 or 1.
FSK avoids most of the noise problem of ASK. Because the
receiving device is looking for specific changes over a givennumber of periods, it can ignore voltage spikes.
There are two main areas of FSK used:
Audio frequency multiplexing on to 4kHz telephone channels(voice frequency) for teletype or data.
Radio teletype (RTTY) transmission at HF and VHF.FSK is also being used in computer modem.
FREQUENCY SHIFT KEYING (FSK)
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 10
ASK can be obtained by the sum of two ASK signals as shown below.
FSK GENERATION
Carrier signal 1 Carrier signal 2
Baseband zeros
signal
Baseband ones
signal
X X
ASK signal
1
ASK signal
2
= =
+
=
FSK signal
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 11
FSK can be expressed as:
Usually fc1 < fc2 and E1 = E2.
FSK EXPRESSION
)()()()()( 2211 tvtvtvtvtv cmcmFSK yy!
tEtv
tEtv
tvtv
ttttv
c
c
mm
m
222
111
12
0001
cos)(
cos)(
)(1)(
.....)5cos5
13cos
3
1(cos
2
2
1)(
Where,
[
[
[[[T
!
!
!
!
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 12
FSK EXPRESSION
? A ? A
.....3cos3
3cos3
coscoscos2
.....3cos
3
3cos
3
coscoscos2
...5cos5
13cos
3
1cos
2
2
1cos
...5cos5
13cos
3
1cos
2
2
1cos
)(cos)(cos)(
22
22
22
22
22
11
11
11
11
11
22
11
222111
!
-
y
-
y!
yy!
tE
tE
tE
tE
tE
tE
tE
tE
tE
tE
ttttE
ttttE
tvtEtvtEtv
oo
oo
oo
oo
ooo
ooo
mmFSK
[[T
[[T
[[T
[[T
[
[[
T
[[
T
[[T
[[T
[
[[[T
[
[[[T
[
[[
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 13
FREQUENCY SPECTRUM OF FSK
Ec/2
Ec /T Ec /TEc/3T Ec/3T
fc1
|V|
f
ASK signal 1
fc1 f
Carrier signal 1|V|
Ec
f0 3f0 5f0 f
Baseband signal|V|
2/T
2/3T
2/5T
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FREQUENCY SPECTRUM OF FSK
Ec/2
Ec /T Ec /TEc/3T Ec/3T
fc2
|V|
f
ASK signal 2
fc2 f
Carrier signal 2|V|
Ec
f0 3f0 5f0 f
Baseband signal|V|
2/T
2/3T
2/5T
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 15
FREQUENCY SPECTRUM OF FSK
Ec/2
Ec/T Ec/T
Ec/3T
Ec/3T
fc2
|V|
f
ASK signal 2
Ec/2
Ec/T Ec/T
Ec/3T
Ec/3T
fc1
|V|
f
ASK signal 1
+
Ec/2
Ec/T Ec /TEc/3T Ec/3T
fc1
|V| E c/2
Ec /T Ec/TEc/3T Ec /3T
fc2f
=
FSK signal
Deviation, 2(f
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 16
Bandwidth of FSK is given by:
Example:
Obtain the bandwidth of FSK if the required signal to be sent is up
to the 3rd harmonics.
BANDWIDTH OF FSK
m
FSK
ff
BW
(!
!
2
frequency)modulatingdeviation(frequencyx2
bb
mFSK
fff
f
ffffffBW
322
62
62)3(2)(2
0
0
(!(!
(!
(!(!
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 17
The phase of the carrier is varied to represent binary 0 or 1.
Both peak amplitude and frequency amplitude remain constant asthe phase changes.
Advantages: Less susceptible to noise degradation
Less bandwidth than FSK (same as ASK)
Bandwidth reduced by multilevel scheme.
Applications very desirable method because of high bit rate and
high carrier frequency which it becomes the only method used fordigital transmission on microwave radio.
PHASE SHIFT KEYING (PSK)
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 18
PSK can be done by multiplying the sinusoidal carrier by the
bipolar baseband signal in order the phase changing happen as
shown below.
PSK GENERATION
Carrier signal
Bipolar baseband
signal
PSK signal
x
=
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 19
The PSK equation can be expressed as:
PSK EXPRESSION
?
!
-
!
-
!
......)3cos(3
1)3cos(
3
1
)cos()cos(2
.....5coscos5
13coscos
3
1coscos
4
...5cos
5
13cos
3
1(cos
4cos)(
00
00
000
000
tt
ttE
ttttttE
ttttEtv
cc
ccc
cccc
ccPSK
[[[[
[[[[T
[[[[[[T
[[[
T
[
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FREQUENCY SPECTRUM OF PSK
From the equation of PSK, it can be shown that the spectrum of
PSK is more or less like ASK spectrum but without the carrier.
It can be shown as follow:
2Ec /T 2Ec/T
2Ec/3T 2Ec /3T
fc3f0 fcf0 fc fc +f0 fc +3f0
|V|
f
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 21
BANDWIDTH OF PSK
Bandwidth of PSK is given by:
Example:
Obtain the bandwidth of PSK if the required signal to be sent is up
to the 5th harmonics.
ASKm
PSK
BWf
BW
!!
!
2
frequencymodulatingx2
0
00
2ratebitx5510)5(22
ffffffBW
b
bmASK
!!!!!!
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 22
Multiplexing is a process of combining two or more informationsignals and sending them simultaneously down the sametransmission system or communication channel.
The method used is depending on both the type of signal and thetransmission medium being used.
Two main methods are: Frequency Division Multiplexing (FDM) usually deal with analog
information.
Time Division Multiplexing (TDM) usually deal with digital information intime domain.
Advantages of multiplexing: The communication system more efficient and reliable.
Can save cost and bandwidth.
MULTIPLEXING
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Using modulated carriers, several carriers can be used at differentfrequencies.
Frequency Division Multiplexing is the process when the available
frequency range is divided among the signals.Individual signals are assigned a different frequency within acommon bandwidth.
Radio and television signals are examples of FDM.
There are limitations to the number of signals that can be crowdedinto a given frequency range Each signal requires a certain bandwidth
At the receiving end, the signals are then separated out again byfiltering
FREQUENCY DIVISION MULTIPLEXING (FDM)
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FDM EXAMPLE
If a medium between a transmitter and a receiver has a bandwidth of
at least 5 times from a bandwidth of an information signal, thus the
medium can be multiplexed for carrying 5 information signals at a
time.The FDM spectrum can be drawn as follow:
vm1(t) vm2(t) vm3(t) vm4(t) vm5(t)
v
f(Hz)
BW of the information signal
BW of the medium
Each signal will be assigned
at different carrier frequency.
They must not overlapping to
avoid crosstalk.
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 25
FDM EXAMPLE
4 channels with 100 kHz of bandwidth are to be
multiplexed together in a link. Calculate the
minimum bandwidth of the link if the guard band of10 kHz is required to prevent interference. Draw the
FDM spectrum.
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This method is suitable when a signal is in the form of a pulse train
(as in PCM).
The pulses are made narrower and the spaces that are left between
pulses are used for pulses from another signals.
The transmission time is shared by a number of signals by
interleaving the pulse trains of various signals in a specified order
where each signal must be sampled at a Nyquist rate.
At the receiver, the pulse trains corresponding to various signals areseparated.
Digital telephone systems use TDM.
TIME DIVISION MULTIPLEXING (TDM)
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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 27
TDM EXAMPLE
Three signals have to be multiplexed together using TDM. The three
signals have the same of bandwidth, B. Draw the diagram that
showing the transmission of these signals if they are represented by
S1, S2 and S3.
S1 S1 S1 S1S3 S3 S3 S3
S2 S2 S2 S2
.. tTs = sampling time Tn = Ts/n = sampling interval
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