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디지털통신 9

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WMCS Lab Digi Communication lecture note [Digi Comm]

WMCS Lab Digi Communication lecture note [Digi Comm]


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  • 1. 9장 디지털 변조 디지털통신(Digital Comm.)
  • 2. Contents  ASK (Amplitude Shift Keying)  FSK (Frequency Shift Keying)  PSK (Phase Shift Keying)  MFSK (M-ary FSK)  MPSK (M-ary PSK)  MSK (Minimum Shift Keying)  Modem (Modulator-Demodulator) 디지털통신(Digital Comm.)
  • 3. Transmission of digital communication system Bandwidth in phone line Digital modulator Digital demodulator modem Digital info. 디지털통신(Digital Comm.) modem Analog info. Digital info.
  • 4. Modulation  Digital modulation  digital data is translated into an analog signal (baseband)  ASK, FSK, PSK  differences in spectral efficiency, power efficiency, robustness  Analog modulation  shifts center frequency of baseband signal up to the radio carrier  Motivation  smaller antennas (e.g., /4)  Frequency Division Multiplexing  medium characteristics  Basic schemes  Amplitude Modulation (AM)  Frequency Modulation (FM)  Phase Modulation (PM) 디지털통신(Digital Comm.)
  • 5. Modulation and demodulation in wireless communication digital data 101101001 digital modulation analog baseband signal analog modulation radio transmitter radio carrier analog demodulation radio carrier 디지털통신(Digital Comm.) analog baseband signal synchronization decision digital data 101101001 radio receiver
  • 6. Digital modulation  변조(Modulation)  Digital data bits → analog carrier waves  Using carrier frequency 원거리 통신에 적당한 주파수와 진폭을 가진 아날로그(교류) 신호  Types of modulation  진폭 편이 변조(ASK: Amplitude Shift Keying) 반송파로 사용하는 정현파의 진폭에 정보를 싣는 변조  주파수 편이 변주(FSK: Frequency Shift Keying) 반송파로 사용하는 정현파의 주파수에 정보를 싣는 변조  위상 편이 변조(PSK: Phase Shift Keying) 반송파로 사용하는 정현파의 위상에 정보를 싣는 변조 디지털통신(Digital Comm.)
  • 7. Digital modulation  Modulation of digital signals known as Shift Keying  Amplitude Shift Keying (ASK):  very simple  low bandwidth requirements  very susceptible to interference  Frequency Shift Keying (FSK): 1 0 1 t 1 0 1  needs larger bandwidth t  Phase Shift Keying (PSK):  more complex  robust against interference 디지털통신(Digital Comm.) 1 0 1 t
  • 8. Digital modulation Analysis (a) ASK si (t )  Waves Vector 2 Ei  t  cos 0t    T M=2 0t T s1 s2 i  1,2,......., M T T T  (b) FSK si (t )  2E co s(  i t   ) T i  1, 2 ,......., M 0  t T 디지털통신(Digital Comm.)  1 t  2 t  M=3 s1 s3 T T T  3 t  1 t 
  • 9. Digital modulation Analysis (c) PSK si (t )  Vector 2E cos(0t  2 i / M ) T i  1, 2,......., M 0t T (d) QAM Waves M=2  1 t  T T T M=8 2Ei  t  si (t )  cos 0t  i  t     T 디지털통신(Digital Comm.)  2 t   1 t  i  1,2,......., M 0t T s1 s2 T T T
  • 10. Digital modulation  진폭 편이 키잉 (ASK: Amplitude Shift Keying) si ( t )  2 Ei (t ) cos( 0 t   ), T 0  t  T , i  1,, M E : symbol energy   si t dt T 2 0 T : symbol duration time Ex) Binary ASK (on-off keying; OOK) : 무선 전신에서 사용되었던 디지털 변조의 초기 형태중의 하나  주파수 편이 키잉 (FSK: Frequency Shift Keying) si  t   디지털통신(Digital Comm.) 2E cos(i t   ), T 0  t  T , i  1,, M
  • 11. Digital modulation  위상 편이 키잉 (PSK: Phase Shift Keying) si t   2E cos0t  i t , T 여기서,  i (t )  0  t  T , i  1,, M 2 i , i  1,, M M  직교 진폭 변조 (QAM: Quadrature Amplitude Modulation) si (t )  디지털통신 (Digital Comm.) 2 Ei (t ) cos(0t  i (t )), T 0  t  T , i  1,, M
  • 12. ASK(Amplitude Shift Keying)  진폭 편이 변조(ASK)  데이터의 신호와 전압 변화에 따라 반송파의 진폭을 변화  Data bit 1 : amplitude A1 Data bit 0 : amplitude A2 - Special case of ASK : OOK (on-off keying) Data bit 1 : carrier transmitted Data bit 0 : no carrier  비효율적 갑작스런 변화에 민감  음성급 회선에서 최고 1200bps OOK modulation 디지털통신(Digital Comm.)
  • 13. Generation of ASK signal DSB method PCM signal Switching method ASK signal Carrier OOK modulation 1 (t )  A cos  ct , 0  t  Tb OOK modulation 디지털통신(Digital Comm.) 1 (t ) (for the bit 1) ASK (t )   (for the bit 0) 0
  • 14. ASK modulated signal 디지털통신(Digital Comm.)
  • 15. ASK modulated signal Ideal OOK waves Bandlimitted OOK waves Power spectral density of OOK signal 디지털통신(Digital Comm.) 버스트(burst)
  • 16. ASK demodulation s0 (t )  0 s1 (t )  A cos wc t  A cos wc t OOK demodulation by using matched filter (MF) detector 수신신호의 평균에너지 1 E 2 Correlation coefficient 디지털통신(Digital Comm.)  Tb 0 2 {s0 (t )  s12 (t )}dt    Tb 0 1 2 s0 (t )s1(t )dt E  A2Tb E1 A cos ct dt   4 2 Tb 2 0   Tb 0 2 0 1(t )dt E  0.
  • 17. ASK demodulation  A2T   E  1  E (1   )  1 1 b    erfc   erfc . Pe  erfc  2  2   2  8  2 2       E1  E0 A2Tb yo  E 2 4    E  2  Tb 0 2  1 (t )n(t )dt   A2    Tb Tb 0 A 2 0 OOK signal PSD 디지털통신(Digital Comm.)    Tb Tb 0 0 E[n(t )n( )] cos  c t cos  c dtd A2  (t   ) cos  c t cos  c dtd  2 2  Tb 0 A2Tb cos  c t dt  4 2
  • 18. ASK demodulation Coherent detector : matched filter detector, Incoherent detector : envelope detector, quadrature detector, square detector << square detector >> 1 2 1 2 y (t )   PCM (t ) 2 2 2  PCM (t ) cos 2  ct   PCM (t )(1  cos 2 ct ) 디지털통신(Digital Comm.)
  • 19. ASK demodulation 1   yc (t )   PCM (t ) cos   2   1   y (t )   s PCM (t ) sin    2   << quadrature detector >> 1 2 1 2 y (t )   PCM (t )(cos 2   sin 2  )   PCM (t ). 4 4 디지털통신(Digital Comm.)
  • 20. ASK demodulation << envelope detector >> p1 ( y )   y 2  A2  y exp  , 2 2  2Ac 2    y2  p0 ( y )  2 exp  2 ,   2  y A   y0  E  1 Pe  exp   2 , E   1.  2   디지털통신(Digital Comm.)
  • 21. ASK demodulation  E  1 Pe  exp   2 , E   1.  2   Incoherent detector Coherent detector  E  1  Pe  erfc  2  2   디지털통신(Digital Comm.)
  • 22. FSK(Frequency Shift Keying)  주파수 편이 변주(FSK)  두개의 이진 값에 서로 다른 반송파의 주파수를 적용(진폭은 동일)  데이터 표현 낮은 주파수 : 0 높은 주파수 : 1     ASK에 비해 잡음 등의 레벨 변동을 받지 않음 저속의 데이터 통신방식에 많이 이용 대역폭을 넓게 차지 동축케이블을 이용한 근거리 통신망 디지털통신(Digital Comm.)
  • 23. FSK (Frequency Shift Keying) 0 (t )  A cos 0t , FSK (t )   1 (t )  A cos 1t , Ideal FSK waves Tb 디지털통신(Digital Comm.) 2 2 0 1 0  t  Tb (for data bit 0) 0  t  Tb (for data bit 1)
  • 24. FSK (Frequency Shift Keying) = Required bandwidth for BFSK modulation 디지털통신(Digital Comm.)
  • 25. FSK (Frequency Shift Keying) 디지털통신(Digital Comm.)
  • 26. FSK demodulation << 정합필터 검파기 >> Average energy per bit 1 Tb 2 [0 (t )  12 (t )]dt 2 0 A2 Tb  [cos 2 0t  cos 2 1t ]dt 2 0 A2 Tb  [2  cos 20t  cos 21t ]dt. 4 0 E A2Tb E 2   Tb 0 (Tb  2 o ) 0 (t )1 (t )dt E Tb   A2  cos 0t cos 1t dt 0 A2Tb 2 sin(1  0 )Tb sin(1  0 )Tb  (1  0 )Tb (1  0 )Tb ≈0 디지털통신(Digital Comm.) 2n   1  0  Tb (   0)
  • 27. FSK demodulation  A Tb   E (1   )  1  E  1 1     erfc   erfc Pe  erfc  2  2  2 2   2  2      (if ,   0) Output signal variance in MF receiver  y2    2  2 Tb 0 Tb 0 E  {s1 (t )  s0 (t )}2 dt 2 {s12 (t )  s0 (t )  2 s1 (t ) s0 (t )}dt  2E  2   E (1   ). 디지털통신(Digital Comm.)
  • 28. FSK demodulation << incoherent detector >> 고주파 잡음을 증가시킨다 디지털통신(Digital Comm.)
  • 29. FSK demodulation 10>>4/Tb BPF Pe   E  1 exp   2   2   A 2Tb E 2 디지털통신(Digital Comm.)
  • 30. PSK (Phase Shift Keying)  위상 편이 변조(PSK)  디지털 신호에 대응 반송파의 위상을 각각 다르게 전송  2,4,8 등분으로 위상을 나누어 데이터를 표현  다른 위상에 1비트 또는 2, 3비트를 한꺼번에 할당  2위상 편이 변조방식 (BPSK)  2등분된 각각의 서로 다는 위상에 0또는 1을 할당  데이터 표현 1 : 180도의 위상변화 0 : 위상의 변화 없음 디지털통신(Digital Comm.)
  • 31. BPSK (Binary Phase Shift Keying) 0 (t )  A cos(c t   0 ),  0  t  T (for data bit 0)  b PSK (t )   1 (t )  A cos(c t  1 ),  0  t  Tb (for data bit 1)  Ideal PSK waves Data bit 1 Data bit 0 디지털통신(Digital Comm.)
  • 32. BPSK (Binary Phase Shift Keying)  위상이 2진 1 또는 0에 따라 변경 성운 다이아그램(또는 성상도) 디지털통신(Digital Comm.)
  • 33. BPSK (Binary Phase Shift Keying)  BPSK (t )   A cos  c t , 디지털통신(Digital Comm.) 0  t  Tb
  • 34. BPSK demodulation << coherent detector >> LPF BPF A2 A2  cos(2c t  2 ) A cos (c t   )  2 2 2 디지털통신(Digital Comm.) 2 Selected by BPF
  • 35. BPSK demodulation << matched filter detector >> A2Tb E 2 2  2 A Tb  Tb 0 A cos( c t   0 ) A cos( c t  1 ) dt  cos(1   0 ) = -1   E (1   )  1   1    erfc E[1  cos(1   0 )]   1 erfc E  Pe  erfc   2 2 2  2 2     디지털통신(Digital Comm.) 1   0  180 
  • 36. BPSK demodulation  E  2 E 1 BPSK : Pe  erfc     ,  2  2    E  2 1 BFSK : Pe  erfc   2  ,    E  2   디지털통신(Digital Comm.) 3dB increased BPSK의 확률밀도: 2진 FSK(BFSK)에 비해 분산이 1/2
  • 37. DPSK (Differential PSK)  DPSK (t )  Ab (t ) cos  c t   A cos  c t 디지털통신(Digital Comm.)
  • 38. DPSK demodulation A 2b(t )b(t  Tb ) cos( c t   ) cos{ c (t  Tb )   }      A2  Tb  b(t )b(t  Tb ) cos  cTb  cos 2 c  t    2   2     2      적분기에의해 제거   A2 y (t )  b(t )b(t  Tb ) (if , cTb  2 n) 2 디지털통신(Digital Comm.) d (t )  0, b(t )  1  b(t )  1 or  b(t  Tb)  1 b(t  Tb)  1 b(t )  1  b(t )  1 d (t )  1,  or b(t  Tb)  1 b(t  Tb)  1
  • 39. DPSK demodulation 3dB (a) Coherent BPSK (b) DPSK (c) Coherent OOK, FSK (d) Incoherent FSK (e) Incoherent OOK DPSK error prob.  E 1 Pe  exp     2   디지털통신(Digital Comm.)
  • 40. QPSK (Quadrature Phase Shift Keying)  4위상 편이 변조방식(QPSK)  연속되는 2 비트의 조합으로 분할  바로 직전에 전송한 반송파의 위상을 기준  다음에 전송할 2비트의 종료(4종류)에 의해서 결정되는 각도만큼 위상 변화 권고안 DIBIT ITU-T V.26 00 01 11 10 BELL 201 B/C 00 01 11 10 디지털통신(Digital Comm.) 위상 편이각 A 방식 0˚ 90˚ 180˚ 270˚ B방식 45˚ 135˚ 225˚ 315˚ 45˚ 135˚ 225˚ 315˚
  • 41. QPSK (Quadrature Phase Shift Keying)  BPSK1 (t )   A cos  c t ,       QPSK (t )           2 A cos  c t   4  3   2 A cos  c t   4   5   2 A cos  c t   4   7   2 A cos  c t   4   sin(   )  sin  cos   cos  sin  cos(   )  cos  cos   sin  sin  디지털통신(Digital Comm.)  BPSK2 (t )   A sin  c t  A cos c t  A sin c t  A cos  t  A sin  t  c c QPSK (t )    A cos c t  A sin c t  A cos c t  A sin c t    A(cos c t  sin c t )
  • 42. QPSK (Quadrature Phase Shift Keying)  BPSK1 (t )   A cos  c t , 디지털통신(Digital Comm.)  BPSK2 (t )   A sin  c t
  • 43. QPSK (Quadrature Phase Shift Keying) Even bit Odd bit QPSK modulator 디지털통신(Digital Comm.) QPSK demodulator
  • 44. QPSK (Quadrature Phase Shift Keying) 0 → -1 1 → +1 (0,0) (1,0) 디지털통신(Digital Comm.) (0,1) (1,1)
  • 45. OQPSK (Offset Quadrature Phase Shift Keying) • 180o 반송파 위상 변화가 배제됨 • 위상 변화는 매 Tb 초 마다 0°, 90° 로 제한됨 • 진폭 변화가 QPSK보다 작음 디지털통신(Digital Comm.)
  • 46. 대역통과 디지털 변조 - QPSK와 OQPSK의 신호파형 d0  1 DIBIT 0 0 1 1 0 1 1 0 Phase shift d 4  1 d6  1 d1  1 0 → -1 1 → +1 d 2  1 d 3  1 d5  1 d7  1 t s(t) 270° 0° 90° 180° 0 2T 4T 6T 8T (a) QPSK d0  1 d 3  1 d1  1 d6  1 d 4  1 d 2  1 d7  1 d5  1 s(t) t 0 T 2T 3T 4T 5T (b) OQPSK 디지털통신(Digital Comm.) 6T 7T
  • 47. 대역통과 디지털 변조  QPSK 신호의 문제점 - 전력 증폭기의 비선형 영역에서 부엽(sidelobe) 다시 증가 QPSK 출력 스펙트럼 디지털통신(Digital Comm.) OQPSK 출력 스펙트럼
  • 48. 8-PSK 디지털통신(Digital Comm.)
  • 49. MFSK (M-ary Frequency Shift Keying) MFSK 변조 디지털통신(Digital Comm.) M  2N  MFSK (t )  A cos  i t , 0  t  Ts , i  0, 2, , M  1
  • 50. MFSK (M-ary Frequency Shift Keying)  0  n s , 1  (n  2) s ,  2  (n  4) s , WT  2M s  (2 M )( b N )  (2 N 1 N )b (Required bandwidth for FSK modulation) 디지털통신(Digital Comm.) Symbol period Ts NTb Symbol frequency s  2 / Ts  2 / NTb  b / N
  • 51. MPSK (M-ary Phase Shift Keying)  MPSK (t )  A cos( c t   i ), 0  t  Ts , i  0, 1, , M  1   i  (2i  1) M  2N M MPSK 변조  MPSK (t )  ( A cos i ) cos  c t  ( A sin  i ) sin  c t be  A cos i , bo  A sin  i  MPSK (t )  be cos  ct  bo sin  ct 2 WT  2s    b N (Required bandwidth for PSK modulation) 디지털통신(Digital Comm.)
  • 52. special pre-computation avoids sudden phase shifts  MSK (Minimum Shift Keying) MSK (Minimum Shift Keying) MSK 변조 CPM (Continuous Phase Modulation) -> 구형파를 완만하게 만드는 과정 필요    b  t  cos  c t  4   MSK (t )  Abe (t ) sin        Abo (t ) cos b t  sin  c t  4    bo (t )  be (t )   sin( c  )t    2    b  4    b (t )  be (t )   A o sin( c  )t  2   bo (t )  be (t ) b (t )  be (t )  , bL (t )  o bH (t )  2 2   H   c  ,  L  c     MSK (t )  A   AbH (t ) sin  H t , (bo (t )  be (t )일 때)  MSK (t )  또는  Ab (t ) sin  t , (b (t )  b (t )일 때). L o e  L 디지털통신(Digital Comm.)
  • 53. MSK (Minimum Shift Keying)  n  n     b (n은 정수) H  L   Tb  2        m   m  (m은 정수)   b L  H Tb  2    H  c  b 4 ,  L  c  b 4   n  H   L  b    b  n  1  2  2      m     c 4 b   H  (m  1) 디지털통신(Digital Comm.) b 4 ,  L  (m  1) b 4 .  Tb 0 sin  H t sin  L t dt  0
  • 54. MSK (Minimum Shift Keying)  T  Sb ( )  2 A2Tbsinc 2  b .       pe (t )  be (t ) sin  b t , 0  t  Tb    4    p (t )  b (t ) cos  b t ,  T  t  T   o b b  o 4      (   b 4)Tb   (   b 4)Tb  S p ( )  A Tb sinc     sinc         2  16 A2Tb  cos Tb     2 1  (2Tb  ) 2  2   (   c )Tb  S QPSK ( )  2 A2Tb sinc 2         2 16 A 2Tb  cos{(   c )Tb }  S  MSK ( )   2 1  {2(   )T  }2  c b    디지털통신(Digital Comm.) 2 MSK 전력 스펙트럼
  • 55. MSK (Minimum Shift Keying) 디지털통신(Digital Comm.)
  • 56. MSK (Minimum Shift Keying) 2  디지털통신(Digital Comm.) b 2
  • 57. Example of MSK 1 0 1 1 0 1 0 bit data even 0101 even bits odd 0011 odd bits signal value hnnh - - ++ low frequency h: high frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t No phase shifts! 디지털통신(Digital Comm.)
  • 58. MSK (Minimum Shift Keying)  각 변조 방식의 정규화된 전력 스펙트럼 밀도 0 BPSK OQ P S K and Q P S K MSK Normalized Power Spectal Density G(f)[dB] -10 -20 -30 -40 -50 -60 -70 -3 -2 -1 0 1 2 BPSK,QPSK,OQPSK, MSK 에 대한 정규화된 전력 스펙트럼 밀도 디지털통신(Digital Comm.) 3
  • 59. QAM (Quadrature Amplitude Modulation)  다중 레벨 신호 방식  진폭,위상 편이변조 주파수의 진폭과 위상 모두에 변화를 주어 신호를 할당 대부분의 최근 모뎀들은 이 변조방식을 사용함 디지털통신(Digital Comm.)
  • 60. 구상진폭변조(QAM)  8-QAM의 시간영역 표시 디지털통신(Digital Comm.)
  • 61. 구상진폭변조(QAM)  16-QAM 디지털통신(Digital Comm.)
  • 62. QAM (Quadrature Amplitude Modulation) QAM 모뎀 QAM (t )  Ai (t ) cos( c t   i ) (i  0, 1, ,15) 16진 QAM 위상 선도 디지털통신(Digital Comm.)
  • 63. Channel Capacity  Impairments, such as noise, limit data rate that can be achieved  For digital data, to what extent do impairments limit data rate?  Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions 디지털통신(Digital Comm.)
  • 64. Concepts Related to Channel Capacity  Data rate - rate at which data can be communicated (bps)  Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)  Noise - average level of noise over the communications path  Error rate - rate at which errors occur  Error = transmit 1 and receive 0; transmit 0 and receive 1 디지털통신(Digital Comm.)
  • 65. Nyquist Bandwidth  Consider the case of a channel that is noise free  The limitation on data rate is simply the bandwidth of the signal  Nyquist states that if the rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate.  For binary signals (two voltage levels)  C = 2B  ex) BW = 3100 Hz, capacity of the channel = 2B=6200 Hz  With multilevel signaling (multi voltage levels)  C = 2B log2 M M = number of discrete signal or voltage levels BW 1 W  (1   ) Rs 2 ; Baseband transmission W  (1   ) Rs ;Bandpass transmission 디지털통신(Digital Comm.) if )   0 (no ISI) theoretical min BW W  1/ 2T  Rs / 2 ( Hz )
  • 66. Signal-to-Noise Ratio  Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission  Typically measured at a receiver  Signal-to-noise ratio (SNR, or S/N) ( SNR ) dB signal power  10 log10 noise power  A high SNR means a high-quality signal, low number of required intermediate repeaters  SNR sets upper bound on achievable data rate  For a given level of noise, we would expect that a greater signal strength would improve the ability to receive data correctly in the presence of noise 디지털통신(Digital Comm.)
  • 67. Shannon Capacity Formula  Equation: C  B log 2 1  SNR   Represents theoretical maximum that can be achieved  In practice, only much lower rates achieved  Formula assumes white noise (thermal noise)  Impulse noise is not accounted for  Attenuation distortion or delay distortion not accounted for  Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB B  4 MHz  3 MHz  1 MHz SNR dB  24 dB  10 log10 SNR  SNR  251 디지털통신(Digital Comm.)
  • 68. Example of Nyquist and Shannon Formulations  Using Shannon’s formula C  B log 2 1  SNR  C  106  log 2 1  251  106  8  8Mbps  How many signaling levels are required? C  2 B log 2 M   8 106  2  106  log 2 M 4  log 2 M M  16 디지털통신(Digital Comm.)
  • 69. MODEM의 분류(1)  Classification  ASK modulator 구조 간단 전송과정에서의 레벨 변동에 약함 근거리 전송 및 소량의 전송에만 이용  FSK modulator 1200bps 이하의 비동기식 모뎀으로 널리 이용 원거리 전송용 전송 과정에서의 잡음 및 간섭에 강하고 레벨 및 주파수 변동도 작기 때문  PSK modulator 2400, 4800bps 등의 중속 데이터 전송을 위한 모뎀  QAM modulator 9600bps의 중속 데이터 전송을 위해 사용 디지털통신(Digital Comm.)
  • 70. ITU-T Modem Standards  V.22bis  Two-speed modem: 1200 or 2400 bps  1200bps mode: DPSK (Differential Phase Shift Keying) Bit pattern defines the phase change  2400bps mode: 16-QAM  V.32, V.33  TCM (Trellis Coded Modulation) QAM + redundant bit For error detection and correction  V.42  LAPM (Link Access Procedure for Modems) 디지털통신(Digital Comm.)
  • 71. TCM (Trellis Coded Modulation)  Block code  Linear equation, RS, BCH  To reduce PB, bandwidth increased  Convolutional code  Nonlinear system, viterbi decoding  To reduce PB, bandwidth increased  TCM : Coding + Modulation  Coding gain without B.W expansion (3~6 dB) but complex decoder 디지털통신(Digital Comm.)
  • 72. TCM(Trellis Coded Modulation) Increase of signal set size for Trellis coded modulation 디지털통신(Digital Comm.)
  • 73. 128 symbols = 27 Data 전송율 14400bps = 2400 baud rate x 7bits x (6/7) 디지털통신(Digital Comm.)
  • 74. 모뎀 규격 비교 ITU-T 규격 Bell 규격 전송속도(bps) V.21 103A 회선 동기형태 전송형태 300 변조방식 FSK FDX V.22 212A 300/1200 V.23 202A 201B 2400 V.27 208A 4800 V.29 209A 비동기 600/1200 V.26 PSTN 9600 현재 MODEM 동향 디지털통신(Digital Comm.) PSK HDX FSK QPSK 전용회선 동기 FDX 8-PSK QAM V.34 : 28.8k 표준 V.34+ : 33.6k 표준 V.90 : 56k 표준
  • 75. V-Series Modem: Constellation  V.22bis, V.32, and V.33 디지털통신(Digital Comm.)
  • 76. Traditional Modems   Data rate of traditional modem is limited by 33.6Kbps Data rate  Signal-to-noise ratio, (shannon의정리 C=Blog2(1+S/N)) 디지털통신(Digital Comm.)
  • 77. 56K Modems   전송속도 : 하향(최대 56Kbps), 상향(최대 33.6Kbps) 기존의 모뎀 접근 방식과 다름  기존의 방식 PC A → modem → PCM(Inverse PCM) → PSTN → Inverse PCM(PCM) → modem → PC B     양방향으로 PCM을 사용하는 양자화의 단계에서 제한을 받음 양자화 잡음(quantization noise)이 발생하여 대역폭의 손실 발생 상하향 최대 33.6Kbps 56K 모뎀 방식     PC A → modem → PCM(Inverse PCM) → PSTN → ISP 인터넷(digital link) 업로드 방향으로만 PCM을 사용하는 양자화의 단계에서 제한을 받음 다운로드시는 양자화 과정이 없으므로 양자화 잡음이 발생하지 않음 하향(최대 56Kbps), 상향(최대 33.6Kbps)  Why 56Kbps?  Switching stations of the telephone company use PCM/Inverse PCM for digitizing voice  8000 samples/sec * 7 bits/sample = 56 Kbps 디지털통신(Digital Comm.)
  • 78. 56K Modems 디지털통신(Digital Comm.)

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