Amplitude modulation and demodulation

digital bandpass modulation and demodulation and amplitude modulation and frequency modulation difference
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Dr.NavneetSingh,India,Teacher
Published Date:21-07-2017
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1 Lecture 7. Digital Communications Part II. Digital Modulation • Digital Baseband Modulation • Digital Bandpass Modulation Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 75 Digital Baseband Modulation • Pulse Amplitude Modulation (PAM) • Pulse Shaping Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 76 Digital Baseband Modulation • Choose baseband signals to carry the digits. – Each baseband signal can carry multiple bits. • Each baseband signal carries 1 bit. Binary • Bit Rate: R1/ b • Totally 2 baseband signals are required. • Each baseband signal carries a symbol (with log M bits). 2 M-ary •Symbol Rate: RM log / R1/ Bit Rate: b 2 s • Totally M baseband signals are required. Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 77 Digital Baseband Modulation • Focus on “amplitude modulation” – The baseband signals have the same shape, but different amplitudes. – Time-domain representation of the modulated signal:  s()tZv(tn)  n n  Z is a discrete random variable with where PrZaM  1/ , 1i ,...,M, n ni v(t) is a unit baseband signal. – Power spectrum of the modulated signal: Read the 2  supplemental  1 2 m  2 Z Gf() V()f  f material for  sZ     m   details. Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 78 Pulse Amplitude Modulation (PAM) • Binary PAM • Binary On-Off Keying (OOK) • 4-ary PAM Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 79 Binary PAM a negative rectangular pulse 1: a positive rectangular pulse 0: with amplitude -A and width  with amplitude A and width  A  110 100 11… s()tZv(tn)  n s(t) n  PrZ 1 1/ 2  n At , 0   vt ()  0, otherwise  Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 710 Power Spectrum of Binary PAM 2   1 m 2  2 Z Gf() V()f  f sZ    m   22 With Binary PAM: Vf()Asinc(f) Gf()Asinc(f) BPAM 2  0,1 Z Z Gf() BPAM 2 A f -  0 See Textbook (Sec. 3.2) or Reference Proakis & Salehi (Sec. 8.2) for more details. Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 711 Effective Bandwidth of Binary PAM 22 G (f) Afsinc ( ) BPAM 2 A f - 0  90% bandwidth: 1/ 90% power 95% bandwidth: 2/ 95% power • Suppose 90% of signal power must pass through the channel (90% in-band power): B1/ Required Channel Bandwidth: h_90% B R hb _ 90% R1/ Bit rate: b • Suppose 95% of signal power must pass through the channel (95% in-band power): B 2/ 2R Required Channel Bandwidth: h_95% b Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 712 Bandwidth Efficiency of Binary PAM Information Bit Rate R b  • Bandwidth Efficiency : Required Channel Bandwidth B h • Bandwidth Efficiency of Binary PAM: R1/ b 1 with 90% in-band power BPAM B1/ h_90%  0.5 with 95% in-band power BPAM B 2/ h_95% What if the two pulses have unsymmetrical amplitudes? Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 713 Binary On-Off Keying (OOK) 1: a positive rectangular pulse 0: nothing (can be regarded as a with amplitude A and width  pulse with amplitude 0) A  110 100 11… s()tZv(tn)  n s(t) n  PrZZ1 Pr 01/ 2  nn At , 0   vt ()  0, otherwise  Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 714 Power Spectrum of Binary OOK 2   1 m 2  2 Z Gf() V()f  f sZ    m   1 2 Gf() A sinc(f )  BOOK With Binary OOK: Vf()Asinc(f)  2  1/ 2,1/ 4  11 m  Z Z  f    44   m  Gf() BOOK 1 2 A 4 … … f   0 Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 715 Bandwidth Efficiency of Binary OOK Information Bit Rate R b • Bandwidth Efficiency :  Required Channel Bandwidth B h • Bandwidth Efficiency of Binary OOK: R1/ b  1 with 90% in-band power BOOK B1/ h_90%  0.5 with 95% in-band power BOOK B 2/ h_95% Can we improve the bandwidth efficiency without sacrificing the in-band power? Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 716 4-ary PAM Each waveform carries 2-bit information. • 4-ary PAM: 11: 01: 00: 10:  s()tZv(tn)  n 1 1 0 1 0 0 1 0 0 0 … n  s(t)  PrZZ1 Pr 1/ 3 nn  PrZZ 1 Pr1/ 3 nn 1/ 4 At , 0   vt ()  0, otherwise  Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 717 Power Spectrum of 4-ary PAM 2   1 m 2  2 Z Gf() V()f  f sZ    m   22 5 Gf()Asinc(f) With 4-ary PAM: Vf()Asinc(f) 4PAM 9 2  0, 5/9 Z Z G (f) 4PAM 2 5 A 9 f -  0 B1/ • Required channel bandwidth with 90% in-band power: h_90% B 2/ • Required channel bandwidth with 95% in-band power: h_95% Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 718 Bandwidth Efficiency of 4-ary PAM •Symbol rate: R1/ S RR22 / •Bit rate: bS • Require channel bandwidth: 1 with 90% in-band power: B1/  R R h_90% S b 2 with 95% in-band power:  R B 2/ 2R b h_95% S  2 with 90% in-band power 4PAM  1 with 95% in-band power 4PAM 4-ary PAM achieves higher bandwidth efficiency than binary PAM Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 719 Bandwidth Efficiency of M-ary PAM • Suppose there are totally M distinct amplitude (power) levels. • How many bits are carried by each symbol? k M 2 kM log 2 • What is the relationship between symbol rate R and bit rate R ? S b RRk / R kR or Sb bS • What is the required channel bandwidth with 90% in-band power? B RRk / hS _90% b Tradeoff between bandwidth efficiency and fidelity performance • Bandwidth Efficiency of M-ary PAM kM log with 90% in-band power MPAM 2 • A larger M also leads to a smaller minimal amplitude difference – higher error probability (to be discussed). Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 720 Pulse Shaping • Inter-Symbol Interference (ISI) • Sinc-Shaped Pulse and Raised-Cosine Pulse Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 721 Transmission over Bandlimited Channel • Frequency domain Baseband Channel PAM signal H(f) 2 Gf()G ()f H(f) YPAM G (f) PAM The signal distortion -B 0 B f h h incurred by channel is f 0 always non-zero • Time domain  y()ts ()t h()t Zx(tn)  n PAM signal Baseband Channel n   h(t) xt ()vt ()h() t st () Z v(tn)  n n  Inter-symbol Sample y(t) at m, m=1,2,…, we have  Interference y() m Z xm(n)Zx(0) Zxm(n)  nm n (ISI) nn  m Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 722 ISI and Eye Diagram • An eye diagram is constructed by plotting overlapping k-symbol segments of a baseband signal. • An eye diagram can be displayed on an oscillo- scope by triggering the time sweep of the oscilloscope. See Reference Ziemer & Tranter (Sec. 4.6) for more details about eye diagram. • ISI is caused by insufficient channel bandwidth. • Any better choice than rectangular pulse? Sinc-Shaped pulse Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 723 Sinc-Shaped Pulse v(t)  Asinc( f ) V(f) A A f -/2 t 0/2 1/ -2/ -1/ 2/  Rectangular Pulse V(f) v(t)=Asinc(t/) A A t -1/(2)1/(2)  0 f  Sinc-Shaped Pulse Lin Dai (City University of Hong Kong) EE3008 Principles of Communications Lecture 7