Digital Communication V.pdf

8/14/2019 Digital Communication V.pdf

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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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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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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.



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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



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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

Digital Communication V.pdf

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