digital modulation technique

Digital Modulation Technique

Presented By:Amit Degada.Teaching Assistant,SV NIT, Surat.

Goal of Today’s Lecture

Differential Phase Shift keying Quadrature Phase Shift Keying Minimum Phase Shift Keying Introduction To Information Theory Information Measure

Differential Phase Shift Keying (DPSK)

Why We Require?• To Have Non-coherent Detection• That Makes Receiver Design

How can we do?• 0 may be used represent transition• 1 indicate No Transition

DPSK Transmitter

dK

dK-1

bK

AcCos(2Πfct)

S(t)=AcCos(2Πfct)Encoder

Delay Tb

Product Modulator

What Should We Do to make Encoder?

DPSK Transmitter…………Modified

dK

dK-1

bK

AcCos(2Πfct)

S(t)=±AcCos(2Πfct)

Delay Tb

Product Modulator

Ex- NOR Gate

Differentially Encoded SequenceBinary Data 0 0 1 0 0 1 0 0 1 1

Differentially Encoded Data

1 0 1 1 0 1 1 0 1 1 1

Phase of DPSK 0 π 0 0 π 0 0 π 0 0 0

Shifted Differentially encoded Data dk-1

1 0 1 1 0 1 1 0 1 1

Phase of shifted Data

0 π 0 0 π 0 0 π 0 0

Phase Comparision Output

- - + - - + - - + +

Detected Binary Seq.

0 0 1 0 0 1 0 0 1 1

DPSK Receiver

Quadrature Phase Shift Keying (QPSK)

Extension of Binary-PSK Spectrum Efficient Technique In M-ary Transmission it is Possible to Transmit M Possible

Signal M = 2n where, n= no of Bits that we Combine

signaling Interval T= nTb

In QPSK n=2 === > So M =4 and signaling Interval T= 2Tb


M=4 so we have possible signal are 00,01,10,11

Or In Natural Coded Form 00,10,11,01

3( ) cos(2 )4

c cs t A f t

cos(2 )4

c cA f t

cos(2 )4

c cA f t

3cos(2 )4

c cA f t

-135

-45

45

135

Binary Dibit 00

Binary Dibit 10

Binary Dibit 11

Binary Dibit 01

QPSK Waveform

00 11 00 11 10 10

QPSK Signal Phase

Constellation Diagram


( ) cos(2 ( ))c cs t A f t t

The QPSK Formula

Where, ϕ(t)=135,45,-45,-135

( ) cos ( ).cos(2 ) sin ( )sin(2 )c c c cS t A t f t A t f t

………………(1)

Simplifying Equation 1

This Gives the Idea about Transmitter design

QPSK Transmitter

QPSK Receiver

Synchronization Circuit

Minimum Shift Keying (MSK)

In Binary FSK the Phase Continuity is maintained at the transition Point. This type of Modulated wave is referred as Continuous Phase Frequency Shift Keying (CPFSK)

In MSK there is phase change equals to one half Bit Rate when the bit Changes 0 to 1 and 1 to 0.

12 b

fT


1 2 1 21

2 2c c c c

cf f f ff

2c

ff

1 2

1 2

2c c

c c

f ffc

f f f

1 2 1 22

2 2c c c c

cf f f ff

2c

ff

Let’s take fc1 and fc2 represents binary 1 and 0 Respectively

Where

Similarly


The MSK Equation

where

( ) cos(2 ( ))s t Ac fct t

( )t ft

For Symbol 1

( )t ft

2 b

tT

For Symbol 0

( )t ft

2 b

tT

Carrier Phase Coding

For dibit 00

Φ(t)

tTb 2Tb

-π/2

-π


For dibit 10

Tb 2Tb

π/2

π


Tb 2Tb

π/2

π

For dibit 11


For dibit 01

Φ(t)

tTb 2Tb

-π/2

-π

Information Theory

It is a study of Communication Engineering plus Maths

A Communication Engineer has to Fight with

Limited Power Inevitable Background Noise Limited Bandwidth

Information Theory deals with

The Measure of Source Information

The Information Capacity of the channel

Coding

If The rate of Information from a source does not exceed the capacity of the Channel, then there exist a Coding Scheme such that Information can be transmitted over the Communication Channel with arbitrary small amount of errors despite the presence of Noise

Source Encoder

Channel Encoder

Noisy Channel

Channel Decoder

Source Decoder

Equivalent noiseless Channel

Information Measure

This is utilized to determine the information rate of discrete Sources

Consider Two Messages

A Dog bites a man

A man bites a dog

Thank You

digital modulation technique

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