PDF Digital Modulation - Seoul National University

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Digital Modulation

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Contents

• Why digital?

• Comparison of modulation schemes

• Comm. system block diagram

• Modulation types

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Why digital ?

• Noise robust

• Error correction

• Easy integration of various services

• Low cost

• Easy manufacturing

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Comparison of modulation schemes

• Power efficiency

• Bandwidth efficiency

• Implementation complexity

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Comm. system block diagram

Channel Encoding Source

Encoding

Tx Filtering

Channel b(t)

c

t

 cos

Rx Filtering Sampler

Slicer Channel

Decoding Sourcel

Decoding

A_k

g(t)

s(t)

N(t) f(t)

Q_k Q(t) A_k

c

t

 cos

Decoding

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s t A g t mT

Q t A p t mT z t

z t N t f t

p t g t b t f t

Q Q kT A p kT mT z kT

A p A p kT mT z kT

If p t at non zero multiples of T then there is no ISI

m m

k m

m

k m

m k

( ) ( )

( ) ( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( )

( ( ) ,

 

  

 

  

   

   

 





)

0

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• Nyquist Criterion

- Raised cosine filter

p t t kT t

T P j jm

T

k

m

( ) ( ) ( )

( )

 

 

 











1  2

1

1 2T

1 2



 1 T 2T

T P( )

1 2

 T

P

T T

T T T

T

( ) sin

  

       

  



  



F



HG I

F KJ

HG I

L KJ

NM O

QP

^ ^ ^{ }

 

R S ||

| T ||

|

0 1

2 1

2 1 1

0 1

b g

b g b g

b g

- Tx,Rx filter SRC ( Square-Root Raised Cosine ) filter

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Modulation types

• BPSK

• QPSK

P Q E

e

N

 F

b

HG I

2 KJ

0

P Q E

e

N

 F

b

HG I

2 KJ

0

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• DPSK(Differentially Encoded PSK)

D

MOD XOR

m_k

d_k d_k-1

M k : 1 0 0 1 0 d k-1 : 1 1 0 1 1 d k : 1 1 0 1 1 0

P E

e DPSK

N

b O

,

 exp F 

HG I

1 KJ

2 : Differential decoding

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- Transmitter

PSF S/P

_Mapping^Signal

PSF

- Receiver

Received signal at t= kT :

Phase change : V V

_k _k^*_1

V

_k

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• Implementation 1

Decision Device

LPF LPF

W W_k _k_₁Z Z_k _k_₁

Z W

_k _k_₁

 W Z

_k _k_₁

V V

_k _k1^*

W

_k

Z

_k

Implementation 2 ( LDI Receiver )

Decision Device

LNA BPF Lim

^Discrimina_-tor

I.D.

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• Constant envelope modulation

- FSK

- MSK, GMSK - GFSK

- Advantages

• Power Amp. Efficiency

• simple receiver implementations

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• FSK

- Coherent demodulation

- Noncoherent demodulation 1

Decision Device

BPF BPF

f

₁

f

₂

+ -

Env.Det Env.Det

+ -

0

z

T 0

z

T

cos 2  f t

_₁

cos 2  f t

₁

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- Noncoherent demodulation 2 ( LDI detector )

Lim Discrimina -tor I.D.

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• MSK

s = cos 2 s = cos 2

: s , s orthogonal MSK

= 2

If , then s , s are orthogonal.

2

-1 1

1 2

 

f f t

f

h f

R h

f R

T

c c

b

 

 





  



b g

d i

b g

d i

b g

1 2

2

1 2

 

  



t = 2

t 2

b g b g

b g

f f t

ft f t h

t

c

base

T

b



  



 2 

 h

 b g t

T

_b ^2T_b

1

-1

t

base

1

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• GMSK

Gaussian pulse shaping

Effects of Gaussian filter BW: Spectrum, ISI

Data Source

Gaussian Filter

Freq Mod.

• GFSK

• Bluetooth: GFSK & PSK

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Fading channel

performance