chapter225th 通信系统(communication systems)课件

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,东南大学移动通信国家重点实验室,“通信系统(,Communication Systems)”,课件,Last,Content,s,Chapter 1 Random Process,1.11 Representation of Narrowband Noise in Terms of In-phase and Quadrature Components,1.12 Representation of Narrowband Noise in Terms of Envelope and Phase Components,1.13 Sine Wave Plus Narrowband Noise,Chapter 2 Continuous-Wave Modulation,2.1 Introduction,2.2 Amplitude Modulation,2.3 Linear Modulation Schemes,1,Outline-1,Representation of narrowband noise,The canonical form (,Equ,. 1.100),Properties of the in-phase and,quadrature,components (p. 65-66),Representation using envelop and phase components (,Equ,. 1.105-1.107),Basic concepts of Rayleigh distribution and,Rician,distribution,2,Outline-2,AM,AM signal (,Equ,. 2.2 and Fig. 2.3), and the amplitude sensitivity k,a,Conditions of correct detection (p. 90),Spectrum of AM wave (,Equ,. 2.5 and Fig. 2.4),Transmission bandwidth B,T,= 2W,Virtues and limitations of AM,3,Outline-3,Linear modulation schemes,The general form (Equ. 2.7),DSB,DSB signal (,Equ,. 2.8 and Fig. 2.5),Spectrum of DSB wave (,Equ,. 2.9 and Fig. 2.6),Coherent receiver,Basic knowledge of,costas,receiver,Basic concept of,quadrature,-carrier multiplexing,4,Representation of narrowband noise in terms of in-phase and quadrature component,Representation in terms of in-phase and quadrature component,(1.100),(1.108),5,Representation of narrowband noise in terms of,Envelope and Phase Components,Representation of Narrowband Noise in terms of Envelope and Phase Components,(1.105),(Reyleigh distribution,),6,Properties of narrowband noise,The in-phase and,quadrature,component of narrowband noise have,zero mean,.,If the narrowband noise is Gaussian, the in-phase and,quadrature,components are jointly Gaussian.,If the narrowband noise is stationary, then its in-phase and,quadrature,components are jointly stationary,7,Properties of narrowband noise,The in-phase and,quadrature,component of narrowband noise have the,same variance,as the narrowband noise,n(t,).,Both in-phase and,quadrature,components have the same power spectral density related to spectral density of the narrow,-,band noise,8,Properties of narrowband noise,The cross-spectral density of the in-phase and,quadrature,components of narrowband noise is purely imaginary, as.,If the narrowband noise is Gaussian and its power spectral density is symmetric about the mid-band frequency,fc, then the in-phase component and,quadrature,component are statistically independent,(1.102),9,Sine Wave Plus Narrowband Noise,Mixture of sine wave and narrowband noise,(1.119),(1.120),(1.121),We assume n(t) is Gaussian with zero mean and variance,2,.,Both,n,I,(t,) and,n,Q,(t,) are Gaussian and statistically independent,The mean of,n,I,(t,) is A and that of,n,Q,(t,) is zero.,The variance of both,n,I,(t,) and,n,Q,(t,) is,2,.,10,1.13,Sine Wave Plus Narrowband Noise,Figure 1.23 Normalized,Rician,distribution.,11,2.1 Introduction,Continuous-wave modulation,Amplitude modulation,the amplitude of the sinusoidal carrier wave is varied in accordance with the,baseband signal,AM, DSB,VSB,SSB,Linear modulation,Angle modulation,the angle of the sinusoidal carrier wave is varied in accordance with the baseband signal,FM,PM,12,Figure 2.3,Illustrating the amplitude modulation process,|K,a,m(t)|1,Amplitude Modulation,Envelop detection,Overmodulated,13,Fourier transform of the AM wave,Figure 2.4,(a) Spectrum of baseband signal (b) spectrum of AM wave,14,2.3 Linear Modulation Schemes,h(t),m(t),s,m,(t),cos(2,f,c,t),General mode of linear Modulation,15,2.3 Linear Modulation Schemes,Double sideband-suppressed carrier (DSB-SC) modulation,Envelop detector cannot be used,Coherent detection is required,The frequency of the local oscillator is adjusted to be the same as the carrier frequency,Phase controller,PLL,16,2.3 Linear Modulation Schemes,17,2.3 Linear Modulation Schemes,Coherent detection,(2. 10),Figure 2.7 Coherent detector for demodulating DSB-SC modulated wave,18,This Contents,2.3 Linear Modulation Schemes,2.4 Frequency Translation,2.5 Frequency-Division Multiplexing,2.6 Angle Modulation,2.7 Frequency Modulation,19,Single-sideband modulation (SSB),Single-sideband modulation (SSB),Only upper or lower sideband is transmitted,by frequency discrimination method.,The message spectrum must have energy,gap,centered at the origin. (Eg., voice signal),20,Single-sideband modulation (SSB),21,原理,两个边带包含相同的信息,只需传输一个边带：上边带或下边带,要求,m,(,t,),中无太低频率,方法,滤波法,相移法,-,f,0,H,L,(,f,),特性,上边带,(b),上边带滤波器特性和信号频谱,f,0,0,f,单边带信号的频谱,上边带,S,(,f,),上边带,下边带,H,H,(,f,),特性,H,H,(,f,),特性,(a),滤波前信号频谱,(c),下边带滤波器特性和信号频谱,S,(,f,),S,(,f,),-,f,0,0,f,-,f,0,f,0,f,下边带,f,0,上边带,22,Single-sideband modulation (SSB),H,SSB,(f),m(t),c(t),S,DSB,(t),S,SSB,(t),23,滤波法的技术难点,滤波特性很难做到具有陡峭的截止特性,需要过渡频段,可以采用多级（一般采用两级）,DSB,调制及边带滤波的方法，即先在较低的载频上进行,DSB,调制，目的是增大过渡带的归一化值，以利于滤波器的制作。再在要求的载频上进行第二次调制,当调制信号中含有直流及低频分量时滤波法就不适用了。,24,相移法和SSB信号的时域表示,Single-sideband modulation,Can be derived by Hilbert transform,Hilbert transform = 90-degree phase shift.,90 degree phase shift regardless of frequency.,25,相移法和SSB信号的时域表示,SSB,信号的时域表示式,两式仅正负号不同,26,相移法和SSB信号的时域表示,“”表示上边带信号，“,+”,表示下边带信号,希尔伯特变换,：上式中,A,m,sin,m,t,可以看作是,A,m,cos,m,t,相移,/2,的结果。把这一相移过程称为希尔伯特变换，记为“, ”,，则有,27,相移法和SSB信号的时域表示,一般情况下：,-jsgn,可以看作是希尔伯特滤波器传递函数，即,28,移相法SSB调制器方框图,优点：不需要滤波器具有陡峭的截止特性。,缺点：宽带相移网络难用硬件实现。,Hilbert transform = 90-degree phase shift.,29,Receiver for SSB,Coherent detection,Can use pilot signal for demodulation,Phase distortion,30,SSB信号的性能,SSB,信号的实现比,AM,、,DSB,要复杂,SSB,调制方式在传输信息时，不仅可节省发射功率，而且它所占用的频带宽度比,AM,、,DSB,减少了一半。,它目前已成为短波通信中一种重要的调制方式。,31,Vestigial sideband (VSB) modulation,原理：,残留边带调制是介于,SSB,与,DSB,之间的一种折中方式。,克服了,DSB,信号占用频带宽的缺点，又解决了,SSB,信号实现中的困难。,在这种调制方式中，不像,SSB,那样完全抑制,DSB,信号的一个边带，而是逐渐切割，使其残留,小部分，如下图所示：,32,Vestigial sideband (VSB) modulation,特点,适合包含直流分量和很低频率分量的基带信号。,原理,VSB,仍为线性调制,滤波器的特性应按残留边带调制的要求来进行设计。,33,Vestigial sideband (VSB) modulation,Figure 2.12 Filtering scheme for the generation of VSB modulated wave.,Vestigial sideband (VSB) modulation,34,Vestigial sideband (VSB) modulation,r,(,t,),接收信号,m,(,t,),cos,0,t,H,1,(,f,),基带信,号,m,(,t,),解调,接收机,r,(,t,),的频谱：,滤波输出信号的频谱：,35,Vestigial sideband (VSB) modulation,为了无失真地传输，要求,由于,所以，上式可以写为,上式即产生,VSB,信号的条件。,36,Vestigial sideband (VSB) modulation,37,Vestigial sideband (VSB) modulation,VSB,|H(f,c,)| = 1/2,The sum of the values of the magnitude response |H(f)| at any two frequencies equally displaced above and below f,c,is unity.,the phase reponse arg(H(f) is linear,Figure 2.13,Magnitude response of VSB filter ;,only the positive-frequency portion is shown.,38,Vestigial sideband (VSB) modulation,Vestigial sideband (VSB) modulation,Non-linear AM (carrier included),Non-coherent detection (,envelop detection,: diode, RC),Low cost,Linear modulation (DBS-SC, SSB),Carrier,(,pilot signal (not carrier) can be sent separately),Coherent detection needed,High cost,SSB with carrier,Detection by envelope detector.,Ideal filter or gap in the energy.,39,C,arrier included envelop detection,40,C,arrier included envelop detection,41,Television signals,Figure 2.15 (a)Idealized magnitude spectrum of a transmitted TV signal.(b)Magnitude response of VSB shaping filter in the receiver.,42,Television signals,Television signals,43,2.4 Frequency Translation,Figure 2.16 Block diagram of mixer.,44,2.4 Frequency Translation,Figure 2.17 (,a,) Spectrum of modulated signal,s,1,(,t,) at the mixer input. (,b,) Spectrum of the corresponding signal,s,(,t,) at the output of the product modulator in the mixer.,Up conversion:,Down conversion:,45,CH2,CH1,CH3,原带宽,CH1,CH2,CH3,移频后带宽,MUX,CH1,CH2,CH3,带宽复用,f,适用于模拟信号传输,2.5,Frequency Division Multiplexing,FDM,46,2.5,Frequency Division Multiplexing,Figure 2.18 Block diagram of FDM system.,47,2.5,Frequency Division Multiplexing,Figure 2.19 Illustrating the modulation steps in an FDM system,48,2.5,Frequency Division Multiplexing,49,2.6 Angle Modulation,Angle Modulation,Non linear modulation,FM & PM,The,amplitude,of the carrier wave is maintained,constant.,can provide better discrimination against noise and interference than amplitude modulation,The improvement in performance is achieved at the expense of increased transmission bandwidth,Such a,trade-off,is not possible with AM, regardless of its form.,50,Basic Definitions:,2.6 Angle Modulation,instantaneous,angle,of a modulated sinusoidal carrier .,instantaneous,phase deviation,.,carrier amplitude,instantaneous,frequency,instantaneous,frequency deviation,.,51,Phase modulation,(,PM,),2.6 Angle Modulation,phase sensitivity,52,2.6 Angle Modulation,Frequency modulation,(,FM,),Frequency sensitivity,53,2.6 Angle Modulation,FM, PM,Frequency modulation,Frequency,modulator,Phase,modulator,Phase modulation,54,2.6 Angle Modulation,FM, PM,Figure 2.20 Illustrating the relationship between frequency modulation and phase modulation. (,a,) Scheme for generating an FM wave by using a phase modulator. (,b,) Scheme for generating a PM wave by using a frequency modulator.,55,2.7 Frequency Modulation,FM is nonlinear,Narrowband FM,(,NBFB,),Wideband FM,(,WBFB,),56,Narrowband FM,(,NBFB,),Signal-tone NBFM,(2.27),(2.28),(2.30),frequency deviation,modulation index,57,Narrowband FM,(,NBFB,),Modulation index of FM,(2.33),(2.32),Narrow band FM :, is small compared to one radian,Wideband FM : is large compared to one radian,58,Narrowband frequency modulation,59,Narrowband frequency modulation,60,Amplitude,of NB,FM,& AM,Amplitude,NBFM,NBFM has,varying amplitude,61,Amplitude,of NB,FM,& AM,AMs variation is larger,AM,62,Spectrum,of NB,FM,& AM,Alternative analysis,63,Spectrum of NBFM,64,2.7 Frequency Modulation,Figure 2.22A phasor comparison of narrowband FM and AM waves for sinusoidal modulation. (,a,) Narrowband FM wave. (,b,) AM wave.,65,Demodulation of NBFM,Coherent detection,66,Wideband frequency modulation,(,WBFB,),(2.33),67,Wideband frequency modulation,(,WBFB,),N-th order Bessel function,of the first kind,68,Wideband frequency modulation,(,WBFB,),(2.48),(2.49),69,Bessel function,Bessel function,For small,Figure 2.23 Plots of Bessel functions of the first kind for varying order.,70,Properties of WBFM,Spectrum of FM signal contains a carrier and an infinite set of side frequencies,Transmission BW is very wide,Small ,(,narrowband FM): only J,0,() and J,1,() are not negligible,BW : similar to AM,Average Power of FM is a constant,Power of carrier: determined by J,0,() unlike AM.,Total power,(2.54),71,2.7 Frequency Modulation,Example 2.2,Fixed freq., varying amplitude,Fixed amplitude, varying frequency,72,2.7 Frequency Modulation,Transmission bandwidth of FM signals,Theoretically, infinite,Effectively limited to a finite number of significant side frequencies.,According to previous figure, the components outside,f decreases rapidly,Carsons rule,:,(2.55),73,2.7 Frequency Modulation,Alternative assessment of BW requirement,Max n that satisfies |Jn(,)| 0.01,74,2.7 Frequency Modulation,Consider the,more general,case of an arbitrary modulating,signal m(t) which highest frequency component denoted by,W,then the actual bandwidth of FM is larger than Carsons rules value, but smaller than the value shown in Figure 2.26,.,Carsons rule,75,2.7 Frequency Modulation,Example 2.3,In North America,FM radio,f = 75 kHz, W = 15 kHz,By Carsons rule,From table ( = 75/15 = 5),B,TG,=,16 * 15 = 240 kHz,(From the fig., 3.2*75 = 240),Measured BW is 200kHz,Carsons rule underestimates the transmission bandwidth by 10 percent, whereas the universal curve in Fig. 2.26 overestimates it by 20 percent,76,NFM,Figure 2.27 Block diagram of the indirect method of generating a wideband FM signal.,Generation of FM signals,Direct FM,Carrier frequency is directly varied by voltage controlled oscillator.,Indirect FM,Generate narrow-band FM (by phase modulation) followed by frequency multiplication,Preferred choice,when the carrier frequency is of major concern as in commercial radio broadcasting.,77,Figure 2.28,Block diagram of frequency multiplier,Frequency multiplier,Reason for starting with narrow band FM, for frequency stability.,Use N-th harmonics,Apply band pass filter around n,f,c,78,Frequency multiplier,Ex.,modulation index,modulation index,79,Demodulation of FM Signals,Direct method: Frequency discriminator,The instantaneous output amplitude of the frequency discriminator is directly proportional to the instantaneous frequency of the input FM signal.,Slope circuit followed by envelope detector,Indirect method: Phase-locked loop,80,2.7 Frequency Modulation,Figure 2.29 (,a,) Frequency response of ideal slope circuit. (,b,) Frequency response of the slope circuits complex low-pass equivalent. (,c,) Frequency response of the ideal slope circuit complementary to that of part (,a,).,81,Demodulation of FM Signals,(2.63),(2.26),82,2.7 Frequency Modulation,(2.65),(2.66),83,So the desired response of the slope circuit is :,2.7 Frequency Modulation,(2.67),(2.68),If : ,then we can use an envelope detector to,recover the amplitude variations .the output is :,84,Figure 2.30,Block diagram of frequency discrimination,2.7 Frequency Modulation,(2.69),(2.70),85,2.7 Frequency Modulation,FM Stereo Multiplexing,The transmission has to operate within the allocated FM broadcast channels,It has to be compatible with monophonic radio receivers,Figure 2.31,FM Stereo Multiplexing,86,Outline-1,Linear modulation schemes,Basic concepts of SSB and VSB,Concepts of mixer (Fig. 2.16),Concepts of FDM,Definitions of angle modulation,FM,A nonlinear modulation process,Single-tone FM modulation,Definitions of,f,Basic knowledge of narrowband and wideband FM,87,Outline-2,FM,Transmission bandwidth,Carsons rule (Equ. 2.55),Know the universal curve,Demodulation,Frequency demodulation (a direct method) (Fig. 2.30),Know phase-locked loop (an indirect method),88,Next Contents,2.8 Nonlinear Effects in FM Systems,2.9,Superheterodyne,Receiver,2.10 Noise in CW Modulation Systems,2.11 Noise in Linear Receivers Using Coherent Detection,2.12 Noise in AM Receivers Using Envelope Detection,2.13 Noise in FM Receivers,89,