Single Sideband (SSB) AM

Size: px

Start display at page:

Download "Single Sideband (SSB) AM"

Lionel Greer
6 years ago
Views:

1 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. Spetral eiieny an be greatly (twie) inreased by transmitting one hal. Sm ( ) Sm ( ) Sm ( ) S ( ) m Sx ( ) S ( ) m m S ( ) Sx ( ) m S ( ) 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 1(12)

2 Generation o SSB: Analysis Leture 7 Hilbert transorm an be eetively used. Start with the message m(t) and show that USB (upper SSB) is given by Sx( ) ( ) ( ) os2 ˆ ( ) x t = A m t π t A m t sin2π t Similarly, LSB an be expressed as ( ) ( ) os2 ˆ ( ) x t = A m t π t A m t sin2π t S m Sm Sx ( ) Sm S m In-phase & quadrature hannels are required to generate SSB. 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 2(12)

3 USB: Frequeny-Domain Viewpoint Time-domain signal Spetra o individual omponents: ( ) ˆ ( ) ( ) ( ) os2 ˆ ( ) x t = A m t π t A m t sin2π t m m m m m t S ( ) S ( ), m t js ( ) js ( ) 1 1 os( ωt) δ( ) δ ( ), sin( ω t) = δ( ) δ ( ) 2 2 j Leture 7 ( ) ( ) Use multipliation property o FT: 1 ( ) os ω ( m( ) m( ) m( ) m( ) ) m t t S S S S 2 m t t S S S S 2 1 ( ) sin ω ( m( ) m( ) m( ) m( ) ) Combine the two expressions above: m m x( t) S ( ) S ( ) Sm Sx ( ) S m 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 3(12)

4 Lower SSB (LSB) Leture 7 Analysis method is the same as or USB. Time-domain signal is Its spetrum is ( ) ( ) os2 ˆ ( ) x t = A m t π t A m t sin2π t m m x( t) S ( ) S ( ) Baseband signal Sm Sm ( ) S m S m LSB Sx ( ) Sm 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 4(12)

5 Example: Sinusoidal Modulating Signal ( ) os Assume that m t = a Ωt Thenx t = aa osωt os ω t aa sin Ωt sin ω t = ( ) ( ) = aa os ω Ω Obviously, this is LSB signal with one spetral omponent only at ω Ω ( ) x 2 S ( ) t Leture 7 Think about it: modulated signal is just a sinusoid! aa How an one transmit a message using a sinusoid? ( F ) 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 5(12)

6 Filtering method: (not pratial in many ases) Using balaned modulators: Hilbert transorm is a linear ilter (phase shiter): j, > H ( ) = j, < Generation o SSB x ( t) (I) (Q) x ( t) Leture 7 J.Proakis, M.Salehi, Communiations Systems Engineering, Prentie Hall, 22 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 6(12)

7 Demodulation o SSB Produt detetor: 1 1 x ( t) os ( 2π t θ ) = A m ( t) osθ A mˆ ( t) sin θ 2 terms 2 2 Ater low-pass ilter, only 1st two terms remain. Coherent demodulation: θ = Leture 7 ( ) ( ) A m( t) x t = A m t os2π t ˆ sin 2π t 1 vout t A m t 2 ( ) = ( ) o ( ) = os ( 2π ) v t t arrier reovery: θ = I not : what to do? 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 7(12)

8 Vestigial-Sideband (VSB) AM Leture 7 SSB an be simpliied by allowing a part o the other sideband to appear. A ilter implementation is easible: Filter requirement: ( ) ( ) onstant, W ( ) ( ) os2 ( ) x t = Am t πt h t A S m S m H 2 ( ) ( ) ( ) H H = linear phase 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 8(12)

9 VSB Spetrum & Filter Response DSB signal Filter response VSB signal spetrum Filter onstrain (real h(t)?) Leture 7 L.W. Couh II, Digital and Analog Communiation Systems, Prentie Hall, Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 9(12)

10 Demodulation o VSB Leture 7 Multiplier (oherent) demodulator: x ( t) v ( t) vl ( t) arrier reovery: θ = A Sv ( ) = m ( 2 ) m ( ) ( ) 4 S S H A S m S m H 4 ( ) ( 2 ) ( ) S A S H H 4 ( ) = ( ) ( ) ( ) l m 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 1(12)

11 Comparison o onventional AM, DSB-SC, SSB and VSB. Conventional AM: simple to modulate and to demodulate, but low power eiieny (33-5% max) and double the bandwidth. DSB-SC: high power eiieny, but more omplex to modulate & demodulate, doubles the bandwidth. SSB: high power eiieny, the same (message) bandwidth, but more diiult to modulate & demodulate. VSB: lower power eiieny & larger bandwidth but easier to implement. 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 11(12)

12 Leture 7 Summary Single sideband (SSB) modulation. USB & LSB. Spetra o SSB signals. Generation & demodulation. Vestigial sideband (VSB) modulation. Spetra o signals. Generation & demodulation. Filter requirement. Comparison o onventional AM, DSB-SC, SSB and VSB. Homework: Reading: Couh, 5.5. Study areully all the examples and make sure you understand them. Attempt some end-o-hapter problems. 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 12(12)

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

Dr. Md. Farhad Hossain Associate Professor Department of EEE, BUET EEE 309 Communiation Theory Semester: January 06 Dr. Md. Farhad Hossain Assoiate Proessor Department o EEE, BUET Email: marhadhossain@eee.buet.a.bd Oie: ECE 33, ECE Building Part 03-3 Single-sideband Suppressed