Dr. Md. Farhad Hossain Associate Professor Department of EEE, BUET

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1 EEE 309 Communiation Theory Semester: January 06 Dr. Md. Farhad Hossain Assoiate Proessor Department o EEE, BUET marhadhossain@eee.buet.a.bd Oie: ECE 33, ECE Building

2 Part 03-3 Single-sideband Suppressed arrier (SSB-SC) Modulation

3 SSB-SC: Priniple () Either USB or LSB is transmitted Required BW: B Hz Spetral eiieny (SE) is improved by 00% Transmitter Side: (Frequeny Domain) M( ) -B -B

4 SSB-SC: Priniple () Reeiver Side (Frequeny Domain): Can be deteted oherently - Ater the multipliation by the arrier - 4

5 SSB: Time Domain Representation () Hilbert Transorm: x h ( t) = H{ x( t) } = π ( τ ) x dτ = x t τ ( t) * πt ( ) = jx ( ) sgn( ) H ( ) X ( ) X h = πt j sgn ( ) Thus, H ( ) = j sgn( ) = j j =. e =. e jπ jπ, > 0, < 0 H () θ h () Thus, a Hilbert transormer is an ideal phase shiter that shits the phase o every spetral omponent by -π/ Diiult to ahieve suh sharp hange in phase response 5

6 SSB: Time Domain Representation () SSB: M () -B B M + () B M + ( ) = M ( ) u( ) = M ( ) [ + sgn( )] = [ M ( ) + jm ( )] h M _ () -B M ( ) = M ( ) u( ) = M ( ) [ sgn( )] = [ M ( ) jm ( )] h ( ) = jm ( ) ( ) Note : M h sgn 6

7 SSB: Time Domain Representation (3) M - (+ ) M + (- ) USB - M + (+ ) M - (- ) ϕ USB - ( ) M ( ) + M ( + ) = + = [ M ( ) + M ( + )] [ M ( ) M ( + )] j h h LSB Hene, Similarly, 7

8 Generation o SSB-SC: Generation o SSB-SC. Phase-shit method: requires ideal Hilbert phase shiter. Seletive iltering method (most ommonly used): First, DSB-SC signal is generated and then passed through a band pass ilter or seleting the desired band. Requires ideal ilter or null around DC. 3. Weaver method: Uses two stages o modulation (SSB using seletive iltering) irst using a smaller arrier requeny and then again using a higher arrier requeny. Thus ahieves a gap o or seond stage seletive iltering. Q. Study the relative advantages/disadvantages o these methods. 8

9 Demodulation using a oherent detetor: ϕ = SSB m t SSB-SC: Detetion ( t) osω [ ( ) ( ) ] t = m t osωt mh t sinωt ( ) + [ m( t) os ω t m ( t) sin ω t] h SSB SC signal with arrierω osω t An LPF will suppress unwanted SSB terms and produe m(t) Any o the synhronous DSB-SC demodulators an be used or demodulating SSB-SC signal Q. SSB-WC an be deteted using envelope detetor. How? Any pre-ondition? 9

10 Comparison between AM, DSB-SC and SSB-SC AM (DSB-WC) DSB-SC SSB-SC Modulation Simple Simple Costly and Complex (diiult to generate) Demodulation Both envelope detetion and oherent detetion Power Eiieny Max 33 % => Ineiient Bandwidth (BW) Requirement Twie the signal BW => Ineiient Coherent detetion => Costly and omplex Better => Eiient Twie the signal BW => Ineiient Coherent detetion => Costly and omplex (Envelope detetion possible or SSB-WC) Better => Eiient Equal to the signal BW => Eiient 0

11 Single-Sideband With Carrier (SSB+C) Modulation

12 SSB+C SSB+C Signal: Envelope detetion: Envelope: From Taylor series expansion, Not power eiient as signiiantly higher amplitude is required or arrier

13 Quadrature Amplitude Modulation (QAM) 3

14 QAM: Priniple SSB signals are diiult to generate QAM is an attrative alternative to SSB Two base band signals, eah o bandwidth B Hz, are sent over the same band o bandwidth B Hz (Modulation: DSB SC) The two arriers are o the same requeny with a phase dierene o π/ QAM is also known as quadrature multiplexing (QM) Synhronous/ Coherent detetor 4

15 QAM: Detetion () In-phase (I) Channel x = Quadrature (Q) Channel x = ( t) = ϕ QAM ( t) osωt = [ m ( t) osωt + m( t) sinωt] m ( t) + m ( t) os ω t + m ( t) sin ω t ( t) = ϕ QAM ( t) sinωt = [ m ( t) osωt + m( t) sinωt] ( t) m ( t) os ω t + m ( t) sin ω t m osω t sinω t 5

16 QAM: Detetion () Impat o loss o synhronization Loss o power intererene Output o the I-hannel Q. Derive the output o the quadrature (Q) hannel. 6

17 Vestigial Sideband (VSB) Modulation 7

18 VSB (): Priniple SSB signals are diiult to generate and DSB requires twie the signal bandwidth VSB is a ompromise between DSB and SSB VSB inherits the advantages o DSB and SSB, but avoids their disadvantages at a small ost Bandwidth o VSB is little (typially 5%) greater than SSB VSB is also known as asymmetri sideband system 8

19 VSB(): Generation and Detetion BPF H i () LPF H o () H i () = Vestigial shaping ilter Synhronous detetion SSB signal generation: ϕ e M ( ) = [ M ( ) + M ( )] H ( ) VSB + Coherent detetion: ( t) = ϕ ( t) osω t [ ϕ ( + ) + ϕ ( )] VSB i VSB ( ) = [ ϕ ( + ) + ϕ ( )] H ( ) VSB VSB o VSB H o ( ) =, H i ( + ) + H ( ) i B 9

20 VSB (3): Example The arrier requeny is 0 khz. Baseband signal bandwidth is 6 khz. H i () is shown i ig (a). Determine H 0 (). H i ( ) Solution: H i ( + ) + H ( ) i ( ) H o 0

21 VSB Appliation: Broadast Television Video signal: large bandwidth (4.5 MHz) DSB requires 9 MHz ontains signiiant low-requeny omponent SSB is not easible The demodulation o the TV signal must be simple and ost eetive envelope detetor is preerred So, VSB modulation with the arrier is hosen or TV broadast DSB Spetrum Transmitted Spetrum

22 End o AM

Single Sideband (SSB) AM

Single Sideband (SSB) AM Leture 7 Why SSB-AM? Spetral eiieny is o great importane. Conventional & DSB-SC oupy twie the message bandwidth. All the inormation is ontained in either hal the other is redundant.