6 주차 통과대역디지털변조 q 목표 Ÿ Digital passand modulation 이해 Ÿ ASK, FSK, PSK, QAM의특성비교 - Error proaility - Power spectrum - Bandwidth efficiency ( 대역효율 ) - 그외 : implementation 디지털통신 1 충북대학교
내용 q Introduction q Binary passand modulation Ÿ ASK (Amplitude Shift Keying) Ÿ FSK (Frequency Shift Keying) Ÿ PSK (Phase Shift Keying) q Comparison of inary modulation schemes 디지털통신 2 충북대학교
Modulation Def) Modulation The process y which some characteristic of a carrier is varied in accordance with source information (signal) demodulation Continuous modulation (passand) Def) Linear modulation Pulse modulation (aseand) AM PAM ASK PM PPM PSK FM PWM (PDM) FSK : 전송신호를다음과같이표시할수있는변조방식 s( t) = s ( t)cos(2 p f t) - s ( t)sin(2 p f t) 정보신호 : I c Q c s ( t) : in-phase component of s( t) I s ( t) : quadrature component s( t) Q Digital modulation (passand) QAM eg) M-PSK, M-QAM 디지털통신 3 충북대학교
변조 (passand modulation) 하는이유? 1 채널의효율성 (à multiplexing) - FDM ( 대역의효율적사용 ) 2 안테나구현의용이성 - 안테나의크기문제 3 Filtering, amplification 용이 4 Interference 영향최소화, SNR 개선 - spread spectrum, FM,... 5 DC 또는 DC 부근의신호전송불가능 Analog modulation vs. Digital modulation 디지털통신 4 충북대학교
Digital Modulations q Carrier phase recovery 여부에따라 Ÿ Coherent : ASK, PSK, FSK, QAM, CPM, Hyrids Ÿ Noncoherent : DPSK,... (ASK, FSK, CPM, Hyrids) q -ary signaling à 11 장 Ÿ 가지의 signal 사용 (, -it symol) Ÿ M-PSK, M-QAM, M-FSK, M-ASK,... q Digital modulation 에서고려해야할요소 1) BER (or SER) à power 에의해결정 2) Spectrum (power spectral density) 3) Bandwidth efficiency q Two factors of andwidth efficiency data rate @ à modulation level, spectrum에의해결정 andwidth 1) Multilevel encoding : M-PAM, M-PSK,... 2) Spectral shaping : RC filtering, duoinary signaling 디지털통신 5 충북대학교
Basic Binary Digital Modulations q ASK (Binary ASK) '0' : 0 '1' : Acos(2 p f t) c q PSK (BPSK) '0' : Acos(2 p f t) c '1' : Acos(2 p f t + p ) = -Acos(2 p f t) c c q FSK (Binary FSK) '0' : Acos(2 p f0t) '1' : Acos(2 p f1t ) 참고 : analog FM, PM à 파형구별불가 디지털통신 6 충북대학교
통과대역디지털변조 (1) Amplitude Shift Keying (ASK) (2) Frequency Shift Keying (FSK) (3) Phase Shift Keying (PSK) (4) Differential Phase Shift Keying (DPSK) 디지털통신 7 충북대학교
ASK (Amplitude Shift Keying) q ASK signal ìs1( t) = A cos(2 p fct), 0 < t T ('1') s( t) = í îs2( t) = 0, 0 < t T ('0') q ASK signal generation Ÿ Mixer 이용 : 0 1 1 0 1 0 1 1 0 1 cos(2 pfct) Ÿ Switch 이용 : cos(2 pfct) 0 Switch à also called 'On-Off keying (OOK)' 디지털통신 8 충북대학교
Power Spectrum of ASK q Power spectrum Data rate : R = 1/ T [it/sec] S( f ) 1) No pulse shaping (null-to-null BW) Tx andwidth : B = 2 / T BW efficiency : T 1/ T h = = 0.5 [its/sec/hz] 2 / T f c 1 - T f c f c 1 + T f 2) Pulse shaping with Nyquist filter Tx andwidth : B = 1/ T BW efficiency : T 1/ T h = = 1 [its/sec/hz] ß maximum efficiency 1/ T 디지털통신 9 충북대학교
Transmission of Band-Limited ASK Signal q Conditions for andwidth limitation Ÿ Minimize transmission(occupied) andwidth Ÿ Suppress sideloes Ÿ Minimize ISI ideal OOK waveform 0 1 1 0 1 t andlimited OOK waveform t q Bandwidth limitation 방법 Ÿ Bandpass filtering Ÿ Lowpass filtering à called 'pulse shaping' 디지털통신 10 충북대학교
1) Band-Limitation y BPF (at Passand) cos(2 pfct ) 0 Switch BPF Bandlimited OOK signal q 문제점 Ÿ Requires high-q BPF Ÿ Requires accurate filters à 高價 (SAW filter) à 제작어려움 디지털통신 11 충북대학교
2) Band-Limitation y LPF (at Baseand) 0 1 1 0 1 LPF 또는 Pulse shaping filter cos(2 pfct) Bandlimited OOK signal q LPF Ÿ Pulse shaping 역할 Ÿ 보통 filter 사용 디지털통신 12 충북대학교
Practical ASK Transmission System 0 1 1 0 1 G(f) LPF, Pulse shaping filter cos 2 ( p f t) Bandlimited ASK signal c BPF Suppress sideloes 디지털통신 13 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 Rx signal r( t) = s( t) + n( t) ìs1( t) = A cos (2 p fct), 0 < t T ('1') s( t) = í îs2( t) = 0, 0 < t T ('0') 0 1 1 0 1 cos(2 pfct) 디지털통신 14 충북대학교
Demodulation of ASK Signal (2) q Coherent detection for inary ASK signals - Use matched filter or correlator r( t) = s( t) + n( t) ìs1 ( t) = Acos( wct) s( t) = í îs2( t) = 0 Ks ( t) = Acos( w t) d c T ò dt 0 y( t) T y = y( T ) 2 ì ïa T / 2 + n y = í ï îno o 1 > < h 0 ˆi : 2 2 c c c v( t) = r( t) Acos(2 p f t) = A cos (2 p f t) + Acos(2 p f t) n( t) à 2 A T y = a1 + no = T + no, n o = ò Acos(2 p fct) n( t) dt o 2 : y = a2 + no = no 디지털통신 15 충북대학교
BER of ASK q BER of ASK Average it energy : Noise variance at sampler output : E E1 + E2 a1 + a2 A T = = = 2 2 4 2 2 2 A N0 n = E no = T s é ù o ë û 2 2 2 æ dmin / 2 ö æ E ö P = Q = Q ç var of n ç è o ø è N o ø f ( r ) = f ( r s ( t) sent) 0 o o 2 s 2 f ( r ) = f ( r s ( t) sent) 1 o o 1 s 2 a 2 h a 1 r o Note: 2-ASK 의 BER 은 RZ line code 경우와동일 P ( error s ( t ) sent) P ( error s ( t) sent) 1 2 디지털통신 16 충북대학교
Noncoherent Detection of ASK Signals (1) q Noncoherent detection r( t) = s( t) + n( t) BPF Envelope detector y( t) $ i t i = it Noise at BPF output (narrow-and noise) n ( t) = n ( t)cos2 p f t - n ( t)sin 2p f t o c c s c r( t) = A + n cos(2 p f t) - n ( t)sin 2p f t Rx signal: ( ) : ( ) 2 2 c c s c y( t) = A + nc + ns» A + n à Ricean distriution c 2 2 : y( t) = nc + ns à Rayleigh distriution 디지털통신 17 충북대학교
Noncoherent Detection of ASK Signals (2) q BER of ASK 2 y é y ù f0( y) = exp, y 0 2 ê- 2 ú ³ s ë 2s û 2 2 y æ y + A ö æ Ay ö f1( y) = exp I 2 ç - 2 0 ç 2 s è 2s ø è s ø f ( y) 0 f ( y) 1 1 P 0 A h A y 2 0.5 10-1 Noncoherent ASK P ( error = 1) P ( error = 0) 10-2 10-3 Coherent ASK 10-4 10-5 2 4 6 8 10 12 E / N 0 [db] 디지털통신 18 충북대학교
10.2. Frequency Shift Keying (FSK) ASK 의특징 q 장점 - 변조회로간단 ( 반송파 on-off 회로 ) à 저가로회로구현가능 q 단점 - 잡음의영향을크게받음 디지털통신 19 충북대학교