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디지털통신 1 충북대학교

6주차

통과대역 디지털 변조

q 목표

Ÿ Digital passband modulation 이해

Ÿ ASK, FSK, PSK, QAM의 특성 비교

- Error probability

- Power spectrum

- Bandwidth efficiency (대역효율)

- 그 외: implementation

디지털통신 2 충북대학교

내용q Introduction

q Binary passband modulation Ÿ ASK (Amplitude Shift Keying)Ÿ FSK (Frequency Shift Keying)Ÿ PSK (Phase Shift Keying)

q Comparison of binary modulation schemes

디지털통신 3 충북대학교

ModulationDef) Modulation

The process by which some characteristic of a carrier is varied in accordance with source information (signal) ↔ demodulation

Continuous modulation(passband)

Pulse modulation(baseband)

Digital modulation(passband)

AM PAM ASKQAM

PM PPM PSKFM PWM (PDM) FSK

Def) Linear modulation: 전송신호를 다음과 같이 표시할 수 있는 변조 방식

( ) ( )cos(2 ) ( )sin(2 )I c Q cs t s t f t s t f tp p= -

정보신호 : ( ) : in-phase component of ( )( ) : quadrature component ( )

I

Q

s t s ts t s t

eg) M-PSK, M-QAM

디지털통신 4 충북대학교

변조(passband modulation) 하는 이유?

① 채널의 효율성 (à multiplexing) - FDM (대역의 효율적 사용)

② 안테나 구현의 용이성

- 안테나의 크기 문제

③ Filtering, amplification 용이

④ Interference 영향 최소화, SNR 개선

- spread spectrum, FM, ...

⑤ DC 또는 DC 부근의 신호 전송 불가능

…

※ Analog modulation vs. Digital modulation

디지털통신 5 충북대학교

Digital Modulationsq Carrier phase recovery 여부에 따라

Ÿ Coherent : ASK, PSK, FSK, QAM, CPM, HybridsŸ Noncoherent : DPSK, ... (ASK, FSK, CPM, Hybrids)

q -ary signaling à 11장

Ÿ 가지의 signal 사용 ( , -bit symbol)Ÿ M-PSK, M-QAM, M-FSK, M-ASK, ...

q Digital modulation에서 고려해야 할 요소

1) BER (or SER) à power에 의해 결정

2) Spectrum (power spectral density)

3) Bandwidth efficiency data rate

bandwidth@ à modulation level, spectrum에 의해 결정

q Two factors of bandwidth efficiency1) Multilevel encoding : M-PAM, M-PSK, ...2) Spectral shaping : RC filtering, duobinary signaling

디지털통신 6 충북대학교

Basic Binary Digital Modulationsq ASK (Binary ASK)

'0' : 0'1' : cos(2 )cA f tp

q PSK (BPSK)'0' : cos(2 )cA f tp

'1' : cos(2 ) cos(2 )c cA f t A f tp p p+ = -

q FSK (Binary FSK)'0' : 0cos(2 )A f tp

'1' : 1cos(2 )A f tp

※ 참고: analog FM, PM à 파형 구별 불가

디지털통신 7 충북대학교

통과대역 디지털 변조

(1) Amplitude Shift Keying (ASK)

(2) Frequency Shift Keying (FSK)

(3) Phase Shift Keying (PSK)

(4) Differential Phase Shift Keying (DPSK)

10.1 Amplitude Shift Keying (ASK)

디지털통신 8 충북대학교

ASK (Amplitude Shift Keying)

0 01 1 1 0 01 1 1

cos(2 )cf tp

cos(2 )cf tp

0

Switch

q ASK signal

1

2

( ) cos(2 ), 0 ('1')( )

( ) 0, 0 ('0 ')cs t A f t t T

s ts t t T

p= < £ì= í = < £î

q ASK signal generationŸ Mixer 이용:

Ÿ Switch 이용: à also called

'On-Off keying (OOK)'

10.1 Amplitude Shift Keying (ASK)

디지털통신 9 충북대학교

Power Spectrum of ASK

f1

cb

fT

+

( )S f

cf1

cb

fT

-

q Power spectrumData rate : 1/b bR T= [bit/sec]

1) No pulse shaping (null-to-null BW)

Tx bandwidth : 2 /T bB T=

BW efficiency : 1/ 0.52 /

b

b

TT

h = = [bits/sec/Hz]

2) Pulse shaping with Nyquist filter

Tx bandwidth : 1/T bB T=

BW efficiency : 1/ 11/

b

b

TT

h = = [bits/sec/Hz] ß maximum efficiency

10.1 Amplitude Shift Keying (ASK)

디지털통신 10 충북대학교

Transmission of Band-Limited ASK Signal

q Conditions for bandwidth limitation Ÿ Minimize transmission(occupied) bandwidthŸ Suppress sidelobesŸ Minimize ISI

t

0 1 1 0 1

t

ideal OOK waveform

bandlimited OOK waveform

q Bandwidth limitation 방법

Ÿ Bandpass filteringŸ Lowpass filtering à called 'pulse shaping'

10.1 Amplitude Shift Keying (ASK)

디지털통신 11 충북대학교

1) Band-Limitation by BPF (at Passband)

cos(2 )cf tp

0 SwitchBPF

Bandlimited OOKsignal

q 문제점

Ÿ Requires high-Q BPF à 高價 (SAW filter)Ÿ Requires accurate filters à 제작 어려움

10.1 Amplitude Shift Keying (ASK)

디지털통신 12 충북대학교

2) Band-Limitation by LPF (at Baseband)

0 01 1 1

cos(2 )cf tp

LPF 또는Pulse shaping

filter

Bandlimited OOKsignal

q LPFŸ Pulse shaping 역할

Ÿ 보통 filter 사용

10.1 Amplitude Shift Keying (ASK)

디지털통신 13 충북대학교

Practical ASK Transmission System

0 01 1 1

G(f)

LPF,Pulse shaping

filter( )cos 2 cf tp

Bandlimited ASK signal

BPF

Suppress sidelobes

10.1 Amplitude Shift Keying (ASK)

디지털통신 14 충북대학교

Demodulation of ASK Signal (1)q Demodulation of ASK signal

- Coherent detection : matched filter receiver, correlation receiver- Noncoherent detection : envelope detector

q Demodulation

Tx signal 1

2

( ) cos (2 ), 0 ('1')( )

( ) 0, 0 ('0 ')cs t A f t t T

s ts t t T

p= < £ì= í = < £î

Rx signal ( ) ( ) ( )r t s t n t= +

0 01 1 1

cos(2 )cf tp

10.1 Amplitude Shift Keying (ASK)

디지털통신 15 충북대학교

Demodulation of ASK Signal (2)

q Coherent detection for binary ASK signals- Use matched filter or correlator

1

2

( ) cos( )( )

( ) 0cs t A t

s ts t

w=ì= í =î

ibbT

>< h1

0òbT dt

0

( ) cos( )d cKs t A tw=

( ) ( ) ( )r t s t n t= +

2 / 2b o

o

A T ny

nì +ï= íïî

( )by y T=( )y t

: 2 2( ) ( ) cos(2 ) cos (2 ) cos(2 ) ( )c c cv t r t A f t A f t A f t n tp p p= × = + ×

à 2

1 2o b oAy a n T n= + = + , cos(2 ) ( )bT

o con A f t n t dtp= ×ò

: 2 o oy a n n= + =

10.1 Amplitude Shift Keying (ASK)

디지털통신 16 충북대학교

BER of ASK

or

1ah

1 1( ) ( ( ) sent)o of r f r s t=0 2( ) ( ( ) sent)o of r f r s t=

2s

2a

2s

( )1error ( ) sentP s t ( )2error ( ) sentP s t

q BER of ASK

Average bit energy : 2

1 2 1 2

2 2 4b

bA TE E a aE + +

= = =

Noise variance at sampler output : 2

2 2 0

2 2on o bNAE n Ts é ù= =ë û

min / 2 var of

bb

oo

EdP Q QNn

æ ö æ ö= =ç ÷ ç ÷ç ÷ è øè ø

※ Note: 2-ASK의 BER은 RZ line code 경우와 동일

10.1 Amplitude Shift Keying (ASK)

디지털통신 17 충북대학교

Noncoherent Detection of ASK Signals (1)

q Noncoherent detection

( ) ( ) ( )r t s t n t= +

BPFEnvelope detector

ib$( )y t

i bt iT=

Noise at BPF output (narrow-band noise)( ) ( )cos2 ( )sin 2o c c s cn t n t f t n t f tp p= -

Rx signal: ( )( ) cos(2 ) ( )sin 2c c s cr t A n f t n t f tp p= + -

: ( )2 2( ) c s cy t A n n A n= + + » + à Ricean distribution

: 2 2( ) c sy t n n= + à Rayleigh distribution

10.1 Amplitude Shift Keying (ASK)

디지털통신 18 충북대학교

Noncoherent Detection of ASK Signals (2)

0 yA2A h

1( )f y

0 ( )f y

( )error 1P b = ( )error 0P b =

2 4 6 8 10 12

bP

1

0.5110-

410-

310-

210-

510-

0/ [dB]E N

Coherent ASK

Noncoherent ASK

q BER of ASK

2

0 2 2( ) exp , 02

y yf y ys s

é ù= - ³ê ú

ë û2 2

1 02 2 2( ) exp2

y y A Ayf y Is s s

æ ö+ æ ö= - ×ç ÷ ç ÷è øè ø

10.2. Frequency Shift Keying (FSK)

디지털통신 19 충북대학교

ASK의 특징q 장점

- 변조회로 간단 (반송파 on-off 회로) à 저가로 회로구현 가능

q 단점

- 잡음의 영향을 크게 받음