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DIGITAL
COMMUNICATION
By
K.Eugine Raj
Assistant Professor/ECE
SCAD Engg College
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Memory less modulation
methods The digital pass band transmission has two types
of modulation technique.
1. Memory less modulation
eg: ASK, PSK, QPSK.
2. Memory modulation
eg: MSK, DPSK
All the Receiver uses either Coherent on nonCoherent method.
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Amplitude shift keying(ASK)
ASK is also called as on-off keying.
Binary 1 represents a pulse and Binary 0represents no pulse.
It is a memory less modulation method.
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Representation of signal
The ASK waveform can be represented by,
gm(t)=Re(Amf(t)ej2fct)
gm(t)=Am f(t) cos(2fct), m=1,2,M. Am=(2m-1-M)(d/2)
d--> distance between adjacent signal.
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Signal space diagram
For ASK signal, M Number of message points,(M=2,4 &8).
N=1,Dimension.
So it has one basic function, (t).
Minimum distance between these two point is given by,
dmin=
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Spectral characteristics The complex envelope of an ASK modulated waveform is
The PSD of the complex envelope of ASK is,
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Generation of ASK
Here the input binary signal is given to an
unipolar NRZ encoder which is the multipliedwith the carrier signal.
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Phase shift keying(PSK)
Here phase of the career is shifted between twobinary input value.
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Representation of signal
For symbol-1, S1(t)= cos(2 )
For symbol-0, S2(t)= cos(2 + )
The modulated signal is given by,
S(t)=f(t) cos 2 +
The energy present in the wave form is,
Sm=
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Signal space diagram
For a Binary PSK it consist of N=2 dimensional.
So it consist of two basic function, 1(t) & 2(t).
The Euclidean distance is given by,
dmin= (1 cos )
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Generation of PSK
Here the input binary signal is given to anPolar NRZ encoder which is the multiplied
with the carrier signal.
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It has two quadrature carrier as, Cos2fct &
Sin2fct.
QAM wave is represented as,
gm(t)=Vmf(t)cos(2fct+m)
Vm= +
m=tan-1(Ams/Amc)
Where, Ams & Amc are the amplitude of
quadrature carrier.
Representation of signal
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Signal space diagram
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Signal space diagram
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Quadrature phase shift
keying(QPSK) By grouping bits in symbol, the
transmission bandwidth can be
reduced.
One symbol represented in twobits in which successive bits are
taken.
This reduce bit rate, hence BW.
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Binary PSK and ASK signal combined to producea signal constellation.
M=M1M2=4
Log2(4)=2 bit symbol
Advantage of QPSK is its BW conservation.
Si(t)=
2 + 2 1 ,0
0,
QPSK transmit N bits for NTs/2 seconds
Representation of signal
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Signal Space Diagram
Here M=4 and N=2 dimensional.
Therefore it has two basic function 1(t) & 2(t).
The Euclidean distance between adjacent point is,
dmin=
Basic coefficient are,Sm=[Sm1 Sm2]
Sm= 1 1
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Frequency shift keying(FSK)
In binary FSK system symbol 1 and 0 are
distinguished from each other by transmitting
two sinusoidal wave that have different
frequency.
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Signal Space Diagram
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