《通信原理》课程教学资源（PPT课件讲稿，英文版）Chapter 5 AM,FM, and digital modulation systems

Chapter 5 AM, FM, and digital modulation systems

1 Chapter 5 AM,FM, and digital modulation systems

5.0 Introduction (Chapter objectives Amplitude modulation and single sideband Frequency and phase modulation Digitally modulated signals(ooK, BPSK, FSK, MSK MPSK, QAM, QPSK, a/4QPSK, and OFDM) Spread sprectrum and cdma system

2 5.0 Introduction (Chapter objectives) • Amplitude modulation and single sideband • Frequency and phase modulation • Digitally modulated signals(OOK, BPSK, FSK, MSK MPSK, QAM, QPSK, π/4QPSK, and OFDM) • Spread sprectrum and CDMA system

5.0 Introduction (the goal of this chapter) Study g(t)and s(t) for various types of analog and digital modulations Evaluate the spectrum for various types of analog and digital modulations Examine some transmitter and receiver structures Learn about spread spectrum systems

3 5.0 Introduction (the goal of this chapter) • Study g(t) and s(t) for various types of analog and digital modulations • Evaluate the spectrum for various types of analog and digital modulations • Examine some transmitter and receiver structures • Learn about spread spectrum systems

5.0 Introduction (the key of this chapter) a Grasping phase modulation and frequency modulation Grasping Binary modulation and Bandpass signaling Grasping Multilevel Modulated bandpass signaling Grasping Minimum-shift keying(MSK)and GMSK Knowing the principle of FDM, OFDM and Spread Spectrum Systems

4 5.0 Introduction (the key of this chapter) • Grasping phase modulation and frequency modulation • Grasping Binary modulation and Bandpass signaling • Grasping Multilevel Modulated bandpass signaling • Grasping Minimum-shift keying (MSK)and GMSK • Knowing the principle of FDM, OFDM and Spread Spectrum Systems

5.0 Introduction (band-pass signal and its spectra) Communication system constructure signal g(t) carrier s(t) Transmission Medium processing circuits (channel) transmitter r(t) carrier g(t) signal output circuits processing Modulation is the process of imparting the source information onto a Band-pass signal with a carrier frequency f by the introduction of amplitude or phase perturbations or both

5 5.0 Introduction (band-pass signal and its spectra) • Communication system constructure input signal processing carrier circuits Transmission Medium (channel) transmitter g(t) s(t) carrier circuits signal processing r(t) g(t) output • Modulation is the process of imparting the source information onto a Band-pass signal with a carrier frequency fc by the introduction of amplitude or phase perturbations or both

5.0 Introduction (band-pass signal and its spectra) Theorem: Any physical band-pass waveform can be epresentative b S(t)=Reig(t)e JOt j .. g(t) is called the complex envelope of s(t), and fc is carrier frequency(in hertz), Oc=2nfo Notice: The desired type of modulated signal, s(t) obtained by selecting the appropriate modulation mapping function glm(t)l, where m(t is the analog or digital base-band signal. The voltage(or current)spectrum of the band-pass signal iS S0)=1/2/G(-,+G*7(子-7 · and the psd is PO)=14/P0e2(子f)7

6 5.0 Introduction (band-pass signal and its spectra) Theorem:Any physical band-pass waveform can be representative by S(t) Re{g(t)e } jct = • g(t) is called the complex envelope of s(t),and f c is carrier frequency (in hertz), ωc=2πfc • Notice: The desired type of modulated signal,s(t),is obtained by selecting the appropriate modulation mapping function g[m(t)],where m(t) is the analog or digital base-band signal. • The voltage (or current)spectrum of the band-pass signal is S(f)=1/2[G(f- fc)+G*(-f- fc)] • and the PSD is (f) 1/4[ (f-f ) (-f- fc ) ] * g c Ps = Pg +P

5.1 Amplitude Modulation m() · The complex envelope of an AM signal i given by g()=A[+m1 (a)Sinusoidal Modulating Wave Ac[1+m(m)] → Where Ac specifies s0 ROIT. the power level and Ac m(t)is the modulating signal. The representation for am signal is (b)Resulting AM Signal given by: Figure 5-1 AM signal waveform s(D=Al+m(切cost 7

7 5.1 Amplitude Modulation • The complex envelope of an AM signal is given by • Where Ac specifies the power level and m(t) is the modulating signal. The representation for AM signal is given by: g(t) A [1 m(t)] = c + s(t) A m(t) t c c = [1+ ]cos • Fig.5-1

5.1 Amplitude Modulation (modulation percentage for AM signal 2. Definition %o positive modulation=(Amax-A/A X100=max m(t) 100 .%negative modulation=(Ac-Amin/Ac X100=-min(m(t)I 100 %overall modulation=(A max Amn)(2A)×100 {max[m(t)]-minm(t)}/2×100 where amax is the maximum value ofa [1+m(tl, and as is the level of the am envelope in the absence of modulation notice: The percentage of modulation can be over100 %o(Amin will have a negative value). If the transmitter uses a two-quadrant multiplier that produces a zero output when all+m(tl is negative, the output signal will be: 8

8 5.1 Amplitude Modulation (modulation percentage for AM signal ) • Definition %positive modulation= (Amax-Ac )/ Ac ×100=max[m(t)] ×100 %negative modulation= (Ac -Amin) / Ac×100=-min[m(t)] ×100 • %overall modulation= (Amax- Amin)/ (2Ac )×100 ={max[m(t)]-min[m(t)]}/2 ×100 • where Amax is the maximum value of Ac [1+m(t)], and Ac is the level of the AM envelope in the absence of modulation. • notice:The percentage of modulation can be over100 %(Amin will have a negative value).If the transmitter uses a two-quadrant multiplier that produces a zero output when Ac [1+m(t)] is negative,the output signal will be:

5.1 Amplitude Modulation Ac[1+m(t)]cos@t if mt)> 1) if mt)<-1 Which is a distorted aM signal which is call as over modulated signal. Its bandwidth is much wider than that of the undistorted AM signal e. if the percentage of negative modulation is less than 100%0, an envelope detector may be used to recover the modulation without distortion; if the percentage of negative modulation is over 100%o,undistorted modulation can still be recovered provided the product detector is used A product detector is superior to an envelope detector when the input signal-to-noise ratio is small

9 5.1 Amplitude Modulation • Which is a distorted AM signal which is call as over modulated signal . Its bandwidth is much wider than that of the undistorted AM signal.     +  = 0 i f m(t) -1 [1 ( )]cos i f m(t) -1 ( ) A m t t s t c c • if the percentage of negative modulation is less than 100%,an envelope detector may be used to recover the modulation without distortion;if the percentage of negative modulation is over 100%,undistorted modulation can still be recovered provided the product detector is used. • A product detector is superior to an envelope detector when the input signal-to-noise ratio is small

5.1 Amplitude Modulation because the total average normalized power of a band pass waveform v(t)is given by =(()CO=R20)=2(ao0 Where "normalized?" implies that the load is equivalent to l ohm. The normalized average power of the am SIgnal IS. ()=1/2(g()2)=1/242 4[1+m() 1/2A(1+2m(t)+m2( 1242+4(m0)+124(m(0) If the modulation contains no dc level, then (m(t)) O,and the normalized power of the am signal is S 1/2A+1/2A2(m (5-9 10 discre te carrierpower side band power

10 5.1 Amplitude Modulation • because the total average normalized power of a band￾pass waveform v(t) is given by 2 2 ( ) 2 1 P v (t) P ( f )df R (0) g t v = =  v = v =  − • Where “normalized” implies that the load is equivalent to 1 ohm. The normalized average power of the AM signal is: 1/ 2 ( ) 1/ 2 ( ) 1/ 2 1 2 ( ) ( ) ( ) 1/ 2 ( ) 1/ 2 [1 ( )] 2 2 2 2 2 2 2 2 2 A A m t A m t A m t m t s t g t A m t c c c c c = + + = + + = = + • If the modulation contains no dc level,then 〈m(t) 〉 =0,and the normalized power of the AM signal is 1/ 2 1/ 2 ( ) (5 -9) 2 carrier power 2 ( ) 2 2    sideband power t c discrete c s t = A + A m