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)
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)
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)
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)
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)
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)
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)
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)
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, 21. 8-Feb-16 Leture 7, ELG3175 : Introdution to Communiation Systems S. Loyka 9(12)
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)
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)
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)