Principles of Communications Lecture 3: Analog Modulation Techniques (1) Chih-Wei Liu 劉志尉 National Chiao Tung University

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1 Priniples of ouniaions Leure 3: nalog Modulaion Tehniques 1 hih-wei Liu 劉志尉 Naional hiao Tung Universiy wliu@wins.ee.nu.edu.w

2 Oulines Linear Modulaion ngle Modulaion Inerferene Feedbak Deodulaors nalog Pulse Modulaion Dela Modulaion and PM Muliplexing oun.-le3

3 Types of Modulaion x nalog odulaion and Digial odulaion odulaed arrier proess o ranslae he inforaion daa o a new speral loaion depending on he inended frequeny for ransission. Modulaion, hisorially, is done on he RF ransission syse. Thus, he onversion fro essage signals o RF signals is alled odulaion. nalog odulaion: oninuous-wave odulaion and pulse odulaion sapled daa oninuous-wave odulaion: linear odulaion M and angle odulaion FM oun.-le3 wliu@wins.ee.nu.edu.w 3

4 oun.-le3 4 Linear Modulaion General for: : 1-o-1 orrespondene o he essage osω : arrier ω is fixed DSB Double-Sideband Suppressed arrier S x ω os = 1 1 X os f f M f f M f x = = ω

5 DSB-S Lower sideband LSB Upper sideband USB oun.-le3 5

6 DSB-S oheren Synhronous Deodulaor Deeor: The reeiver knows exaly he phase and frequeny of he arrier in he reeived signal. d = x osω = [ osω] osω = osω desired par High freq. noise Message is reovered! oun.-le3 wliu@wins.ee.nu.edu.w 6

7 Wha if he reeiver referene is no oheren? -- phase error ours θ, unknown, rando, ie-varying, x d osω θ d = = osω osθ os ω θ osω θ y D = osθ, 1 osθ 1 I is ie-varying!! oun.-le3 wliu@wins.ee.nu.edu.w 7

8 arrier Reovery arrier reovery: Regenerae he arrier f and θ a he reeiver sie Exaple: Square irui x r Narrowband BPF a f f osω osω x r = os ω = 1 D 1 osω arrier xf! oun.-le3 wliu@wins.ee.nu.edu.w 8

9 arrier Reover How o exra he arrier? I beoes learer when we exaine i in he frequeny doain. FT of x r : FT Narrow BPF -f 0 f f oun.-le3 wliu@wins.ee.nu.edu.w 9

10 Rearks The speru of DSB signal does no onain a disree speral oponen a he arrier frequeny unless has a D oponen. DSB syses wih no arrier frequeny oponen presen are ofen referred o as suppressed arrier S syses. If he arrier frequeny is ransied along wih DSB signal, he deodulaion proess an be raher siplified. lernaively, le s see he following apliude odulaion M shee. oun.-le3 wliu@wins.ee.nu.edu.w 10

11 pliude Modulaion D bias is added o prior o he odulaion proess The resul is ha a arrier oponen is presen in he ransied signal Definiion x = [ ] osω = [1 a n ]osω oun.-le3 wliu@wins.ee.nu.edu.w 11

12 M pliude Modulaion M: DSB wih arrier x a = = [1 a n = in in n, ]osω n : he noralized essage : he original essage : he D bias a: he odulaion index had beer be less han 1 Noralized essage 1 n >=0 oun.-le3 wliu@wins.ee.nu.edu.w 1

13 Envelope Deeion The odulaion index is defined suh ha if a=1, he iniu value of [1a n ] is zero a <1, i resuls in [1a n ] >0 for all In M, all he inforaion is jus he envelop. The envelop deeion is a siple and sraighforward ehnique oun.-le3 wliu@wins.ee.nu.edu.w 13

14 M Over-odulaion: odulaion index a > 1 D bias: he shifed level of he zero-value essage ax x 1/ in n in -1 d bias x 1/ in d bias n 1/a 1/a oun.-le3 wliu@wins.ee.nu.edu.w 14

15 M 0 -f 0 f oun.-le3 wliu@wins.ee.nu.edu.w 15

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17 M Deodulaion oheren deeion: preise bu requires arrier reovery irui. Inoheren deeion, envelope deeion: siple reeiver LPF bu requires suffiien arrier power a < 1 and f >> W. In heory, f >W is suffiien, bu a good LPF is needed. Ipulse response of R irui: 1 h = e R u R oun.-le3 wliu@wins.ee.nu.edu.w 17

18 Rearks The ie onsan R of he envelop deeor is an iporan design paraeer. The appropriae R ie onsan is relaed o he arrier frequeny f and o he bandwidh W of he original signal 1/f << R << 1/W, beween hen and us be well separaed fro boh oun.-le3 wliu@wins.ee.nu.edu.w 18

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20 oun.-le3 0 Power Effiieny of M Suppose ha has zero ean, hen he oal power onained in he M odulaor oupu is The power effiieny : he power raio of he inpu inforaion o he ransied signal ] [ 1 ] [ 1 os ] [ 1 ] [ 1 os ] [ x = = = = ω ω 100% 1 100% Effiieny = a a E n n denoes he ie average value

21 Power Effiieny Exaple E Effiieny n = in, 100% = a 1 a n n 100% If he signal has syerial value, i.e. in = ax, hen n 1 and hene n 1. If a 1, he axiu effiieny is 50%, e.g. he square wave-ype For a sine wave, n =1/, for a=1, he effiieny is 33.3% If we allow a>1, Effiieny an exeed 50%, a, he effiieny=100% Bu, he envelope deeor is preluded. oun.-le3 wliu@wins.ee.nu.edu.w 1

22 Rearks The ain advanage of M: oheren referene is no neessary for deodulaion as long as a 1 The disadvanage of M: The power effiieny The D value of he essage signal anno be auraely reovered. ixed wih arrier oun.-le3 wliu@wins.ee.nu.edu.w

23 Single Sideband SSB Modulaion Why SSB? In DSB, eiher he USB and he LSB have equal apliude and odd phase syery abou he arrier frequeny Send only half signal USB & LSB syeri; Good power effiieny; Good bandwidh uilizaion Basis of ore advaned odulaions Mehods o generae SSB signals Mehod 1: Sideband BPF filering Easy o undersand, bu diffiul o ipleen. Mehod : Phase-shif odulaion oun.-le3 wliu@wins.ee.nu.edu.w 3

24 Sideband Filering Sideband filering n ideal passband filer is neessary The very low frequeny oponen will be enapulaed oun.-le3 wliu@wins.ee.nu.edu.w 4

25 SSB Modulaion oun.-le3 5

26 SSB Signal Generaion DSB signal: XDSB f = M f f M f f 1 LPF: HL f = [sgn f f sgn f f] X f = X f H f DSB L 1 = [ M f fsgn f f M f fsgn f f] 4 1 [ M f fsgn f f M f fsgn f f] 4 = [ M f f M f f ] par- 4 [ M f f sgn f f M f f sgn f f ] par-b 4 oun.-le3 wliu@wins.ee.nu.edu.w 6

27 SSB Signal Generaion Par- FT of DSB signal: osω ˆ I j f M f 1 Par-B: Le { sgn } ˆ -jsgnf Define Hilber Transfor: Thus, Mˆ f = jsgn f M f Mˆ f f ˆ e π j f 1 {par-b} [ je j π f ˆ je j π f I = ˆ ] 4 1 jπ f jπ f = ˆ [ j e e ] = ˆ sinω oun.-le3 wliu@wins.ee.nu.edu.w 7

28 Phase-shif SSB Modulaor Lower-Side Band: x os ˆ = ω sinω x = osω ˆ sin ω x = osω ˆ sin ω oun.-le3 wliu@wins.ee.nu.edu.w 8

29 Phase-shif SSB Modulaor Upper-Side Band: x os ˆ = ω sinω oun.-le3 wliu@wins.ee.nu.edu.w 9