通信原理英文版課件：Ch2 Continuous-Wave Modulation part1

1、ReviewCentral limit theoremGaussian processDefinitionsFive properties1Review (Contd)NoiseShot noise: Poisson distributionThermal noise: Gaussian distributionWhite noise: An idealized form of noise in communication systemsTwo examples:Ideal low-pass filtered white noiseCorrelation of white noise with

2、 a sinusoidal wave2Review (Contd)Bandpass signalsNarrowband signals3Review (contd)Narrowband noiseIn-phase and quadrature componentsEnvelop and phase4Review (contd)Rayleigh distributionRician distribution5Review (contd)Concept of flat-fading channels6Ch. 2 Continuous-Wave Modulation2.1 Introduction2

3、.2 Amplitude Modulation2.3 Linear Modulation Schemes 2.4 Frequency Translation2.5 Frequency-Division Multiplexing2.6 Angle Modulation2.7 Frequency Modulation2.8 Nonlinear Effects in FM Systems2.9 Superheterodyne Receiver2.10 Noise in CW Modulation Systems2.11 Noise in Linear Receivers Using Coherent

4、 Detection2.12 Noise in AM Receivers Using Envelope Detection 2.13 Noise in FM Receivers 2.14 Computer Experiments: Phase Locked Loop72.1 IntroductionSome basic conceptsBaseband signals: information-bearing signalsModulation: the process by which some characteristic of a carrier is varied in accorda

5、nce with a modulating wave (signal)Continuous-Wave (CW) modulation: the carrier is a sinusoidal waveThe baseband signal modulating waveResult of the modulation process modulated waveModulation and demodulation8Two Families of CW Modulation SystemsAmplitude modulationAngle modulationPhase modulationF

6、requency modulation9Two Families of CW Modulation Systems (Contd)Fig. 2.2 Illustrating AM and FM signals produced by a signal tone.(a) Carrier wave(b) Sinusoidal modulating signal(c) Amplitude-modulated signal(d) Frequency-modulated signal102.2 Amplitude ModulationSinusoidal carrier waveCarrier ampl

7、itude (volt)Carrier frequencyAmplitude-modulated waveAmplitude sensitivity(Constant, volt-1)Modulating signal (volt)11Amplitude Modulation (Contd) Fig. 2.3 Illustrating the amplitude modulation process. (a) m(t); (b) AM wave for |kam(t)| 1 for some t.The envelope of s(t) has the same shape as m(t) p

8、rovided that:(1) |kam(t)| WKam(t)100%: percentage modulation|kam(t)| 1: overmodulation12Amplitude Modulation (Contd)Fig. 2.4 (a) Spectrum of baseband signal; (b) Spectrum of AM wave.Fourier transform of AM modulated signal13Amplitude Modulation (Contd)Spectrum of an AM modulated signal consists of 2

9、 delta functions at fc and 2 versions of baseband spectrum shifted by fcThe negative frequencies become visibleUpper sideband and lower sidebandThe transmission bandwidth is twice the message bandwidth14Virtues and LimitationsVirtuesSimple in both transmitter and receiver designLimitationsWasteful o

10、f power (due to the existence of carrier)Wasteful of bandwidth (due to the symmetry of upper and lower sidebands)152.3 Linear Modulation SchemesDefinition of linear modulationThree types of linear modulationDouble sideband-suppressed carrier (DSB-SC)Single sideband (SSB)Vestigial sideband (VSB)sI(t)

11、 and sQ(t) are linearly related to the message signal m(t).16DSB-SC Modulation Fig. 2.5 (a) Block diagram of product modulation; (b) Baseband signal; (c) DSB-SC modulation.17DSB-SC Modulation (Contd)Fourier transform of DSB-SC modulated signalFig. 2.6 (a) Spectrum of baseband signal; (b) Spectrum of

12、 DSB-SC modulated wave.18DSB-SC Modulation (Contd)Fig. 2.7 Coherent detector for demodulating DSB-SC modulated wave.19Costas ReceiverAn synchronization methodConsists of 2 coherent detectors with 90 phase difference in the local oscillatorsPhase error of the Voltage-Controlled Oscillator (VCO) is co

13、mpensated by output of the phase discriminator, which varies with the phase error20Costas Receiver (Contd)Fig. 2.9 Costas receiver.21Quadrature-Carrier MultiplexingFig. 2.10 Quadrature-carrier multiplexing system. (a) Transmitter; (b) Receiver.(QAM)22SSB ModulationOnly the upper or lower sideband is

14、 transmittedTwo stagesGenerating DSB-SC modulated wavePass one of the sideband and suppress the otherRequires highly selective filters23SSB Modulation (Contd)Fig. 2.11 (a) Spectrum of a message signal m(t) with an energy gap of width 2fa centered on the origin; (b) Spectrum of corresponding SSB sign

15、al containing the upper sideband.A spectrum gap around the origin is desired for SSB modulation.24SSB Modulation (Contd)Synchronization methodsTransmitting an additional low power pilot carrierUsing highly stable oscillators phase error is inevitable, introducing a phase distortion25VSB ModulationOn

16、e of sideband is partially suppressed and a vestige of the other sideband is transmitted to compensate for that suppression.Transmission bandwidth of VSB is between those of DSB and SSB.Implementation complexity of VSB is also between those of DSB and SSB.26VSB Modulation (Contd)Fig. 2.12 Filtering

17、scheme for the generation of VSB modulated wave.27VSB Modulation (Contd)Fig. 2.13 Magnitude response of VSB filter; only the positive-frequency portion is shown.28VSB Modulation (Contd)Fig. 2.14 Frequency response of a filter for producing the quadrature component of the VSB modulated wave.29Televis

18、ion SignalsFig. 2.15 (a) Idealized magnitude spectrum of a transmitted TV signal; (b) Magnitude response of VSB shaping filter in the receiver.Envelope detection302.4 Frequency TranslationFig. 2.16 Block diagram of mixer.f1f2Up conversion: f2 = f1 + flDown conversion: f2 = f1 -flFrequency translatio

19、nFrequency changingMixingHeterodyning31Frequency Translation (Contd)Figure 2.17 (a) Spectrum of modulated signal s1(t) at the mixer input. (b) Spectrum of the corresponding signal s(t) at the output of the product modulator in the mixer.Mixing is a linear operation.322.5 Frequency-Division Multiplex

20、ingMultiplexingDefinition: A signal processing operation that combines a number of independent signals into a composite signal for transmission over a common channel.ClassificationsFrequency-division multiplexing (FDM)Time-division multiplexing (TDM)Code-division multiplexing (CDM)Others33FDM (Contd

21、)Figure 2.18 Block diagram of FDM system.34FDM (Contd)Figure 2.19 Illustrating the modulation steps in an FDM system.352.6 Angle ModulationAngle modulationAngle of the carrier wave is varied with the baseband signalProvides a practical means of exchanging channel bandwidth for improved noise perform

22、anceIncludes two closely related methods: phase modulation (PM) and frequency modulation (FM)Zero-crossing spacing is not constant of a PM or FM signalEnvelope of a PM or FM signal is constant36Angle Modulation (Contd)Phase modulationAngle-modulated wave:Frequency modulationis the instantaneous freq

23、uency372.7 Frequency ModulationA nonlinear modulation processTo analysis the spectrum of an FM signal, consider a special case: the single-tone modulationNarrowband FM signalWideband FM signal38Single-Tone Modulationis the frequency deviation, which is proportional to the amplitude of the modulation

24、 signal and is independent of the modulation frequency.is the phase deviation (or modulation index) and is measured in radians.Narrowband FM: 1Wideband FM: is large39Narrowband FMFigure 2.21 Block diagram of a method for generating a narrowband FM signal.40Narrowband FM (Contd)A narrowband FM signal

25、 requires the same bandwidth as the AM signal.The narrowband FM signalThe AM signal41Narrowband FM (Contd)Figure 2.22 A phasor comparison of narrowband FM and AM waves for sinusoidal modulation. (a) Narrowband FM wave. (b) AM wave.42Wideband FM43Wideband FM (Contd)Figure 2.23 Plots of Bessel functio

26、ns of the first kind for varying order.For small values of ,44Wideband FM (Contd)About the spectrum of an FM signal:Contains a carrier component at fc and an infinite set of side frequencies at fc nfm;For small values of (the narrowband FM), only component at fc and fc nfm are significant;The amplit

27、ude of the carrier component varies with according to J0().45Example 2.2Figure 2.24 Discrete amplitude spectra of an FM signal, normalized with respect to the carrier amplitude, for the case of sinusoidal modulation of fixed frequency and varying amplitude. Only the spectra for positive frequencies

28、are shown.46Example 2.2 (Contd)Figure 2.25 Discrete amplitude spectra of an FM signal, normalized with respect to the carrier amplitude, for the case of sinusoidal modulation of varying frequency and fixed amplitude. Only the spectra for positive frequencies are shown.When , the bandwidth of an FM s

29、ignal 2f47Transmission Bandwidth of FM SignalsThe Carsons ruleThe universal curveFigure 2.26 Universal curve for evaluating the 1 percent bandwidth of an FM wave.48Generation of FM SignalsDirect FMIndirect FMFigure 2.27 Block diagram of the indirect method of generating a wideband FM signal.49Generation of FM Signals (C