EE140 Introduction to Communication Systems Lecture 7

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1 3/4/08 EE40 Introdution to Communiation Systems Leture 7 Instrutor: Prof. Xiliang Luo ShanghaiTeh University, Spring 08 Arhiteture of a (Digital) Communiation System Transmitter Soure A/D onverter Soure enoder Channel enoder Modulator Absent if soure is digital Noise Channel User D/A onverter Soure deoder Channel deoder Detetor Reeiver

2 3/4/08 Contents Analog Modulation Amplitude modulation DSB SSB VSB Pulse modulation Angle modulation (phase/frequeny) 3 Examples of Analog Modulation 4

3 3/4/08 What is modulation? Modulation Transform a message into another signal to failitate transmission over a ommuniation hannel Generate a arrier signal at the transmitter Modify some harateristis of the arrier with the information to be transmitted Detet the modifiations at the reeiver Why modulation? Frequeny translation Frequeny-division multiplexing Noise performane improvement 5 Analog Modulation Charateristis that an be modified in the arrier C(t ) A(t )osπf (t )t θ(t ) Amplitude Frequeny Phase Amplitude modulation Angle modulation AM PM 6 3

4 3/4/08 Amplitude Modulation Double-sideband suppressed-arrier AM (DSB-SC) S(t ) Baseband signal (modulating wave): m(t ) Carrier wave C(t ) Modulated wave Aos πft θ 0 m(t )C(t ) Am(t )os πft θ 0 7 DSB-SC Spetrum S(f ) A M(f f ) M(f f ) 8 4

5 3/4/08 Demodulation of DSB-SC Signals Phase-oherent demodulation r(t ) s(t )Aos A πf t m(t )A os m(t ) m(t ) os 4πf t πf t s(t) v(t) vo(t) Produt modulator os(πf t) Low-pass filter Loal osillator PLL (phase-loked loop) 9 Demodulation of DSB-SC Signals DSB-SC demodulation: graphi interpretation 0 5

6 3/4/08 Question: not Phase Coherent? Phase error r(t ) s(t )os( πf t θ ) Am(t ) os πf t os( πf t θ ) Aosθm(t ) Aos( 4πft θ )m(t ) Unertainty in the amplitude r (t ) Am(t ) A os 4 πf t m(t ) Another Way to Generate DSB-SC Signals A signal spetrum an be translated an amount ω by multiplying the signal with any periodi wave form whose fundamental frequeny is ω 6

7 3/4/08 Double-sideband, Large-arrier (DSB-LC) DSB-LC signal (onventional AM signal) = DSB-SC signal + a arrier term s(t ) m(t )os ω t Aos ω t 3 DSB-LC signal Graphi Interpretation Time domain Frequeny domain 4 7

3/4/08 DSB-LC Properties The role played by parameter A If A is larger enough, say A minm(t ), the envelope of the modulated waveform will be proportional to m(t ).

8 3/4/08 DSB-LC Properties The role played by parameter A If A is larger enough, say A minm(t ), the envelope of the modulated waveform will be proportional to m(t ). The DC response of the signal m(t ) has been lost in the demodulation as a result of the addition of the arrier. 5 Modulation Index Modulation index m is a dimensionless sale fator and used to represent the relative magnitudes of the sideband and arrier portion of the modulated signal. peak DSB - SC amplitude max m peak arrier amplitude A m(t ) 0 When m<, and maxm(t ) m(t ) m(t ) minm(t ) we also have (max. magnitude) - (min. magnitude) m (max. magnitude) (min. magnitude) 6 8

3/4/08 Modulation Index (ont d) 7 Demodulation of

easy. Phase and frequeny synhronizations are

RC iruit an perform low pass filtering Condition:

9 3/4/08 Modulation Index (ont d) 7 Demodulation of DSB-LC Signals Coherent demod: possible, but not easy. Phase and frequeny synhronizations are required; Nonoherent demod: envelope detetion The RC iruit an perform low pass filtering Condition: A min m(t ) or m Corret RC RC too large The simpliity of envelop detetor has made Conventional AM a pratial hoie for AM-radio broadasting RC too small 8 9

10 3/4/08 Transmission Effiieny Transmission (modulation) effiieny: Ps μ P t m (t ) A m (t ) If m(t ) maos ωt, we have m (t ) ma. The transmission effiieny m η m Beause m, the transmission effiieny of a DSB-LC system is at best 33.3%. That is, at least 67% of the total power is expended in the arrier and wasted as the arrier term does not ontain any information. For omparison, the transmission effiieny of a DSB-SC system is 00%. 9 Single-sideband (SSB) Modulation DSB modulation results in a doubling of the bandwidth of a given signal. Eah pair of sidebands (i.e. upper or lower) ontains the omplete information of the original signal. The original signal an be reovered again from either the upper or lower pair of sidebands by an appropriate frequeny translation. singlesideband modulation 0 0

11 3/4/08 SSB Advantage: SSB modulation is effiient beause it requires no more bandwidth than that of the original signal and only half that of the orresponding DSB signal. Generation of SSB-SC Signals Generation of SSB-SC signals Method : filtering generate a DSB-SC signal; filter out one pair of sidebands (upper or lower). Requirement of method : does not ontain signifiant low-frequeny omponents; the sideband filter is usually built at a ditated frequeny.

12 3/4/08 Generation of SSB-SC Signals (Cont d) Method : phase-shift generate the quadrature funtion mˆ (t ) by shifting the phase of m(t ) by 90 degrees at eah frequeny omponent. Upper (SSB+) sideband and lower (SSB-) sideband are given by φ SSB Requirement: phase shifted by exatly 90 degrees. used in low-freq SSB generation and in digital ( t ) m(t )os ωt mˆ (t )sin ωt 3 SSB-SC Demodulation Synhronous detetion Reeived SSB-SC signal: s(t ) m(t )os ωt mˆ (t )sin ωt Loal generated arrier signal: C(t ) os[( ω Δω)t θ ] s(t )C(t ) [ m(t )os m(t ) {os[( Δω)t θ ] os[( ω mˆ ω t mˆ (t )sin ω t ] os[( ω (t ) {sin[( Δω)t θ ] sin[( ω Δω)t θ ] Δω)t θ ]} Δω)t θ ]} Through a low-pass filter (LPF), the output is given by: m e (t ) If no error m(t ) os[( Δ ω)t θ ] m e (t ) m(t ) mˆ (t ) sin[( Δω)t θ ] 4

13 3/4/08 SSB-SC Demodulation (Cont d) Frequeny domain graphi interpretation 5 Single-sideband, Large-arrier (SSB-LC) SSB-LC signal: SSB-SC signal + a arrier term In time domain: s(t ) Aos ω t m(t )os ω t mˆ (t )sin ω t The frequeny response at DC is NOT desired. Synhronous detetion: Envelope detetion, not straightforward r(t ) [ A m(t )] [ mˆ (t )] A m(t ) A A m(t ) A Requirement of envelope detetion: the arrier is muh larger than the SSB-SC envelope, i.e. A m (t ) mˆ m(t ) m (t ) mˆ (t ) A A A (t ) 6 3

14 3/4/08 Good: Save spetrum Save energy Bad: Complex implementation Comments on SSB 7 Vestigial-sideband (VSB) Modulation The generation of SSB signals may be quite diffiult when the modulating signal bandwidth is wide or where one annot disregard the low-frequeny omponents. In Vestigial-sideband (VSB) modulation, a portion of one sideband is transmitted. VSB is a ompromise between SSB and DSB. Generation of VSB-SC signals: in frequeny domain S VSBSC ( ω ) M( ω ω ) M( ω ω ) H ( ω) where filter H V ( ω) passes some of the lower (or upper) sideband and most of the upper (or lower) sideband. V 8 4

15 3/4/08 VSB Modulation (Cont d) Frequeny domain graphi interpretation 9 DSB-SC: Comparison of AM Tehniques more power effiient. Seldom used DSB-LC (AM): simple envelop detetor Example: AM radio broadast SSB: requires minimum transmitter power and bandwidth. Suitable for point-topoint and over long distanes VSB: bandwidth requirement between SSB and DSBSC. Example: TV transmission 30 5

16 3/4/08 Performane of AM Demod Synhronous detetion vs envelope detetion In synhronous detetion, the output signal and noise always remain additive and the urve-slope is a onstant, independent of input SNR. The nonlinear behavior of envelope detetion delines the SNR performane wheninput noise inreases. 3 Contents Analog Modulation Amplitude modulation Pulse modulation Angle modulation (phase/frequeny) 3 6

17 3/4/08 Analog Pulse Modulation Modulating Signal Pulse-Amplitude Modulation (PAM) Pulse-Width Modulation (PWM) Pulse-Position Modulation (PPM) 33 Analog Pulse Modulation (ont d) PAM: onstant-width, uniformly spaed pulses whose amplitude is proportional to the values of at the sampling instants. PWM: onstant-amplitude pulses whose width is proportional to the values of the input at the sampling instants. PPM: onstant-width, onstant-amplitude pulses whose position is proportional to the values of the input at the sampling instants. 34 7

18 3/4/08 Remarks Analog Pulse Modulation (ont d) PWM is a popular hoie where the remote proportional ontrol of a position or a position rate is desired. Disadvantages of PWM inlude the neessity for detetion of both pulse edges and a relatively large guard time is needed. Only the trailing edges of the PWM waveforms ontain the modulating information. PPM onveys only the timing marks of the trailing edges. PAM and PWM are self-loking (the waveform presents lok timing), while the use of PPM requires a method of regenerating lok timing. PWM and PPM are nonlinear, Fourier analysis annot be used diretly. 35 Pulse-ode Modulation (PCM) Quantization: the sampled analog signal is quantized into a number of disrete levels. Digitization: assign a digit to eah level (one-to-one mapping) so that the waveform is redued to a set of digits at the suessive sample times. Code: the digits are expressed in a oded form. Binary ode (i.e. a ode using only two possible pulse levels) is the most popular hoie. (8 levels) (3-bit ode) Representations of PCM ode 36 8

19 3/4/08 PCM (ont d) Advantages of PCM systems In long-distane ommuniations, PCM signals an be ompletely regenerated (noise-free) at intermediate repeater stations beause all the information is ontained in the ode. The effets of noise do not aumulate and only the transmission noise between adjaent repeaters need be onerned. Modulating and demodulating iruitry is all digital, thus affording high reliability and stability. Signals may be stored and time saled effiiently. Effiient odes an be utilized to redue unneessary repetition (redundany) in message. (soure oding) Appropriate oding an redue the effets of noise and interferene. (hannel oding) 37 Thanks for your kind attention! Questions? 38 9

ENSC327 Communications Systems 4. Double Sideband Modulation. Jie Liang School of Engineering Science Simon Fraser University

ENSC327 Communications Systems 4. Double Sideband Modulation. Jie Liang School of Engineering Science Simon Fraser University ENSC327 Communiations Systems 4. Double Sideband Modulation Jie Liang Shool of Engineering Siene Simon Fraser University 1 Outline DSB: Modulator Spetrum Coherent Demodulator: Three methods Quadrature-arrier