Analog Communications

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Edwin Wiggins
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1 1 Analog Communiations Amplitude Modulation (AM) Frequeny Modulation (FM)

2 Radio broadasting 30-300M Hz SOURCE Soure Transmitter Transmitted signal

2 2 Radio broadasting M Hz SOURCE Soure Transmitter Transmitted signal Channel Reeived signal Reeiver User Analog baseband signal Bandpass hannel

3 Analog signal Soure Analog Modulation t Analog Communiations A-D Conversion Bit sequene Digital Baseband Modulation 3 Digital Bandpass Modulation Bandpass Channel Baseband Channel Bandpass Channel Analog Demodulation User D-A Conversion Digital Baseband Demodulation Digital Bandpass Demodulation

4 Analog Communiations 4 SOURCE Soure Transmitted signal Transmitter Channel Reeived signal Reeiver Reeived info. User Analog baseband signal Bandpass hannel Ignore noise S(f) S T (f) H(f) ˆ( ) S f -B s 0 B s f? 1 0 f 1 2 B h f f 1 2 B h f -B s 0 B s f For reliable ommuniations, i.e., S ˆ( f) S( f), all frequeny omponents of the transmitted signal should pass through the hannel, whih requires: Frequeny omponents of transmitted signal should be entered at f ; The hannel bandwidth B h should be no smaller than the bandwidth of transmitted signal B m.

5 Analog Modulation 5 Modulation property of Fourier Transform: S(f) s()os(2 t f t) 1 [ ( ) ( )] 2 S f f S f f arrier S AM (f) 0 f -f 0 f Amplitude Modulation (AM) s () t As()os(2 t f t) AM f Phase Modulation (PM) s () t Aos(2 f ts()) t Frequeny Modulation (FM) sfm () t Aos(2 ( ftk s( ) d)) PM t

6 6 Analog Modulation SOURCE Soure Modulation (AM or FM or PM ) Modulated signal Channel Modulated signal Bandpass hannel Demodulation Reeived info. User Analog baseband signal Ignore noise Bandwidth effiieny is an important performane metri, whih is defined as: Information Signal Bandwidth Required Channel Bandwidth B B h s Information Signal Bandwidth B Modulated Signal Bandwidth B s m Required hannel bandwidth B h = Modulated signal bandwidth B m A higher indiates a better spetral utilization.

7 7 Leture 3. Analog Communiations Part I. Amplitude Modulation (AM) AM-DSB-SC AM-DSB-C AM-SSB AM-VSB

8 AM-DSB-SC -- Modulation 8 Time Domain: s () t As()os(2 t f t) AM DSBSC s(t) s () AM DSBSC t

9 AM-DSB-SC -- Modulation 9 Frequeny Domain: S ( f) [ S( f f ) S( f f )] A AM DSB SC 2 S(f) Lower sideband Upper sideband -B s B s 0 f S ( ) AM DSBSC f -f -B s -f -f +B s 0 f -B s f f +B s f AM-DSB-SC: Amplitude Modulation-Double SideBand-Suppressed Carrier

10 Bandwidth Effiieny of AM-DSB-SC 10 Bandwidth Effiieny : Information Signal Bandwidth Required Channel Bandwidth B B h s Information Signal Bandwidth B Modulated Signal Bandwidth B m s With AM-DSB-SC : S ( ) AM DSBSC f -f -B s -f -f +B s 0 f -B s f f +B s f Bs AM DSBSC 50% 2B s B m =2B s

11 AM-DSB-SC -- Demodulation 11 Time Domain: s () t s() t AM DSBSC? sam DSB SC()os(2 t ft) As( t)os(2 ft)os(2 ft) 0.5 As( t) 0.5 Ast ( )os(2 2 ft) Frequeny Domain: S ( f) S( f) AM DSBSC? 1 2 S AM DSBSC ( f f ) S AM DSBSC ( f f ) [ S( f 2 f ) S( f 2 f )] S( f ) A A 4 2 [ S( f 2 f ) S( f)] [ S( f) S( f 2 f )] A A S AM DSBSC ( f f) S AM DSBSC ( f f ) -2f -f 0 f 2f f

12 Modulator and Demodulator of AM-DSB-SC 12 Modulator s(t) s AM-DSB-SC (t) Aos(2f t) Demodulator What if there is a frequeny error? s AM-DSB-SC (t) bandpass filtering os(2f t) lowpass filtering s demod (t) Coherent Demodulation: the demodulator requires a referene signal whih has exatly the same frequeny and phase as the arrier signal.

13 13 Frequeny Error of Coherent Demodulator Consider that the referene signal has a small frequeny error, f. wt () Ast ()os(2 ft)os(2 ( f f)) t 0.5 As( t) os(2 ft) os(2 (2 f f) t) After lowpass filtering, we have 0.5 As( t)os(2 ft) os(2 ft) 1 when f 0 os(2 ft) hanges with t when f 0 The performane of AM-DSB-SC is sensitive to the frequeny error of the referene signal.

14 Pros and Cons of AM-DSB-SC 14 Straightforward Sensitive to frequeny and phase error of the referene signal (oherent demodulation) Bandwidth ineffiient ( AM-DSB-SC =50%)

15 15 AM-DSB-C

16 Envelope and Envelope Detetor 16 Envelope Consider a signal s(t)os2f t. If s(t) varies slowly in omparison with the arrier os2f t, the envelope of s(t)os2f t is s(t). The envelope st () st ( ) if st ( ) 0. Envelope Detetor: + s(t)os2f - t + - y(t)= s(t) How to apply the Envelope Detetor to AM systems?

17 AM-DSB-C -- Modulation 17 Time Domain: s(t) samdsb C() t A s() t os(2 ft) s AM-DSB-C (t) s(t) + 0 is a d offset to ensure for any time t. st () 0

18 AM-DSB-C -- Modulation 18 Frequeny Domain: sam DSBC () t A s() t os(2 ft) As()os(2 t ft) Aos(2 ft) S ( f) [ S( f f ) S( f f )] [ ( f f ) ( f f )] A A AM DSB C 2 2 S ( ) AM DSBC f -f -B s -f -f +B s 0 f -B s f f +B s f AM-DSB-C: Amplitude Modulation-Double SideBand-Carrier

19 Bandwidth Effiieny of AM-DSB-C 19 Bandwidth Effiieny : Information Signal Bandwidth Required Channel Bandwidth B B h s Information Signal Bandwidth B Modulated Signal Bandwidth B m s With AM-DSB-C : S ( ) AM DSBC f -f -B s -f -f +B s 0 f -B s f f +B s f B m =2B s Bs AM DSBC 50% 2B s AMDSBSC

20 AM-DSB-C -- Demodulation 20 Time Domain: s () t s() t AM DSBC? Envelope Detetor: s AM DSBC () t A st () os(2 ft) yt () A st () (() st 0) A(() st ) Non-oherent demodulation (no need to generate a referene signal) Apply s AM-DSB-C (t) to an envelope detetor. Simple Remove the d offset. Robust Any prie to pay?

21 More about AM-DSB-C 21 Define the power effiieny of an AM-DSB-C system as: power of information signal s( t) power of modulating signal s( t) 2 Ps P s 50% inreases as the d offset dereases. to ensure st () 0, 2 P s Define the modulation index of an AM-DSB-C system as: m max[ s( t) ] min[ s( t) ] max[ s( t) ] min[ s( t) ] max st ( ) min st ( ) max st ( ) min st ( ) 2 m inreases as the d offset dereases. Can m be arbitrarily large?

22 Modulation Index m of AM-DSB-C m max[

22 22 Modulation Index m of AM-DSB-C m max[ s( t) ] min[ s( t) ] max[ s( t) ] min[ s( t) ] max st ( ) min st ( ) max st ( ) min st ( ) 2 m < 1 when min(s(t)+) > 0 m = 1 when min(s(t)+) = 0 m > 1 when min(s(t)+) < 0 m should not exeed 1 to avoid over-modulation.

23 Pros and Cons of AM-DSB-C 23 Simple and robust reeiver design (non-oherent demodulation) Commerial radio broadasting Cost of power effiieny (<50%) Bandwidth ineffiient ( AM-DSB-C = AM-DSB-SC =50%)

24 24 AM-SSB and AM-VSB

25 How to Improve Bandwidth Effiieny? 25 Double sideband (DSB) 0 S AM-DSB-SC (f) f -B s f f +B s f Upper sideband arries the same information as the lower sideband! 0 f -B s f f 0 f f +B s f Lower sideband Upper sideband Use a bandpass filter to selet the desired sideband, and only transmit the desired sideband.

26 26 AM-SSB Amplitude Modulation-Single SideBand (AM-SSB) Modulation (Frequeny Disrimination Method) 0 s(t) Aos(2f t) f DSB f f +B Bandpass filter with a very sharp rolloff f s AM-SSB (t) Demodulation (oherent) r(t) lowpass filtering s demod (t) f f +B s os(2f t)

27 Pros and Cons of AM-SSB 27 Bandwidth effiient ( AM-SSB =100%) Mobile ommuniations, military ommuniations, High requirement on filtering (sharp rolloff) Sensitive to frequeny and phase error of the referene signal (oherent demodulation)

28 DSB SSB VSB -f -B s AM-VSB S AM-DSB-SC (f) -f 0 S AM-SSB (f) -f +B s f -B s f +B s -f -B s -f 0 Amplitude Modulation-Vestigial SideBand (AM-VSB) 1 SAM VSB ( f ) [ S( f f) S( f f)] H s ( f ) 2 f f f +B s f f 28 -f -B s -f 0 f f +B s f Allow a small portion (or vestige) of the lower sideband,, along with the upper sideband.

29 Spetrum of AM-VSB Signal 29 1 modulation: SAM VSB ( f) [ S( f f ) S( f f)] Hs ( f) S( f f) Hs( f) S( f f) Hs( f) 2 2 Demodulation: 1 Sdemod ( f) [ SAM VSB( f f) SAM VSB( f f)] [ S ( f 2 f ) S ( f )] Hs ( f f ) [ S ( f ) S ( f 2 f )] Hs ( f f ) S( f)[ Hs ( f f) Hs( f f)] S( f 2 f) Hs( f f) S( f 2 f) Hs( f f) After lowpass filtering: S ( f ) S( f )[ H ( f f ) H ( f f )] demod s s H ( f f ) H ( f f ) k 0 f B s s

30 Spetrum of AM-VSB Signal 30 Baseband signal S( f) Modulated Signal -f -B s 0 B s f S ( ) AM VSB f f De-modulation -f -B s -f -f + 0 f - f f +B s 1 Sdemod ( f ) [ S AMVSB( f f) S AMVSB( f f)] 2 f -2f -B s -2f -2f + -f -B s 0 B s f 2f - 2f 2f +B s f

31 Tradeoff between Complexity and Bandwidth Effiieny 31 S ( ) AM VSB f -f 0 f s B f m B s 50% AM VSB Bs B 100% s 0< s The bandwidth effiieny dereases as the vestige inreases. The larger vestige, the lower reeiver omplexity.

32 Summary of AM 32 AM-DSB-SC Power effiient Coherent demodulation Bandwidth ineffiient =50% AM-DSB-C Commerial radio broadasting Power ineffiient Non-oherent demodulation (simple and robust) Bandwidth ineffiient =50% AM-SSB Mobile and military ommuniations Power effiient Coherent demodulation Bandwidth effiient =100% AM-VSB Publi television systems Power effiient Coherent demodulation Tradeoff between bandwidth and omplexity Bs 50% 100% B s

EE140 Introduction to Communication Systems Lecture 7

EE140 Introduction to Communication Systems Lecture 7 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