통과대역 디지털 변조 - kocwelearning.kocw.net/contents4/document/lec/2013/chungbuk/... ·...
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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
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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
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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
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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
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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
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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
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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)
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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)'
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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
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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'
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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
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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
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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
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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
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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= + =
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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
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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
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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
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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