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ECC1015 Communication System I

Communication System I

Electronics and Communication EngineeringHanyang University

Haewoon Nam

Lecture 9

(ECC1015)

1

ECC1015 Communication System I

Summary of the Previous Lecture

• Single sideband (SSB)– Suppress one of the two sidebands

in the DSB-SC modulated wave

• Comparison of DSB-SCand Single sideband (SSB)

2

)( fM

f0 WW−

fCfCf−

W2W2

0

fCfCf−

W

0

W

fCfCf−

W

0

W

DSB-SC

SSBUpper sideband

Lower sideband

ECC1015 Communication System I

Summary of the Previous Lecture

• Review of DSB-SC– A DSB-SC modulator using the sinusoidal modulating wave

– The resulting DSB-SC modulated wave is

• In upper SSB

3

)2cos()( tfAtm mm π=

)13.3(])(2cos[21])(2cos[

21

)2cos()2cos( )()()(

tffAAtffAA

tftfAAtmtctS

mcmcmcmc

mcmc

DSB

−++=

==

ππ

ππ

)14.3(])(2cos[21)( tffAAtS mcmcUSSB += π

)15.3()2sin()2sin(21)2cos()2cos(

21)( tftfAAtftfAAtS cmmccmmcUSSB ππππ −=

Upper sideband Lower sideband

)2sin()(ˆ21)2cos()(

21)( tfAtmtfAtmtS ccccUSSB ππ −=

Hilbert transform of m(t)

ECC1015 Communication System I

Summary of the Previous Lecture

• A Hilbert transformer is a wide-band phase-shifter whose frequency response is characterized in two parts as follows

– Signum function

– The Hilbert transform of the message signal

4

)22.3()sgn()( fjfH −=

HilbertTransformer

),( fH )(th

)( fX

<>−

=−=0 ),(0 ),(

)()sgn()(ˆffjXffjX

fXfjfX

<−>

=0 ,1

0 ,1)sgn(

ff

f f0

1+

1−

<>−

=0 ),(0 ),(

)(ˆffjMffjM

fM)(

1)(ˆ tmt

tmπ

=

)()(ˆ txt

tx ∗= 1π

tth

1)(

π=

)(tx

)(ˆ fM

f

j+

j−

Phaserotation

ECC1015 Communication System I

Summary of the Previous Lecture

• Cosine and Sine function in frequency domain

• In upper SSB

5

)2sin()(ˆ21)2cos()(

21)( tftmAtftmAtS ccccUSSB ππ −= Hilbert

transformof m(t)

)2cos( tfA cc π

fCfCf− 0

2cA

)2sin( tfA cc π

fCfCf− 0

Phaserotation

90+

90−

)(ˆ fM

f

j+

j−

fCfCf−

W2W2

0

2cA

fCfCf−

W2W2

0

2cA

fCfCf− 0

)( fSUSSB

cA

ECC1015 Communication System I

Summary of the Previous Lecture

• In lower SSB

6

)2sin()(ˆ21)2cos()(

21)( tftmAtftmAtS ccccLSSB ππ += Hilbert

transformof m(t)

)(ˆ fM

f

j+

j−

fCfCf−

W2W2

0

2cA

fCfCf−

W2W2

0

2cA

fCfCf− 0

)( fSLSSB

cA

ECC1015 Communication System I

Single Side Band Modulation

7

ECC1015 Communication System I

Single Side Band Modulation

• Modulators for SSB– Frequency Discrimination Method

• For the design of the band-pass filter to be practically feasible, there must be a certain separation between the two sidebands that is wide enough to accommodate the transition band of the band-pass filter.

8

ECC1015 Communication System I

Single Side Band Modulation

• Modulators for SSB– Phase Discrimination Method

• Wide-band phase-shifter is designed to produce the Hilbert transform in response to the incoming message signal.

• To interfere with the in-phase path so as to eliminate power in one of the two sidebands, depending on whether upper SSB or lower SSB is the requirement.

9

ECC1015 Communication System I

Single Side Band Modulation

• Coherent Detection of SSB– Synchronization of a local oscillator in the receiver with the oscillator

responsible for generating the carrier in the transmitter– The demodulation of SSB is further complicated by the additional

suppression of the upper or lower sideband.• Frequency Translation

– Single sideband modulation is in fact a form of frequency translation• Frequency changing• Mixing• Heterodyning

– Up conversion : the unshaded part of the spectrum in Fig. 3.22(b)

– Down conversion : the shaded part of the spectrum in Fig. 3.22(b)

10

lfff += 12 12 fffl −=

21 fffl −=lfff −= 12

ECC1015 Communication System I

Single Side Band Modulation

11

ECC1015 Communication System I

Vestigial Side Band Modulation

• For the spectrally efficient transmission of wideband signals– Typically, the spectra of wideband signals contain significant low

frequencies, which make it impractical to use SSB modulation.– The spectral characteristics of wideband data befit the use of DSB-SC.

However, DSB-SC requires a transmission bandwidth equal to twice the message bandwidth, which violates the bandwidth conservation requirement.

• Vestigial sideband (VSB) modulation– Instead of completely removing a sideband, a trace of vestige of that

sideband is transmitted, the name “vestigial sideband”– Instead of transmitting the other sideband in full, almost the whole of

this second band is also transmitted.

12

WfB vT +=

ECC1015 Communication System I

Vestigial Side Band Modulation

• Sideband Shaping Filter– The band-pass filter is referred to as a sideband shaping filter– The transmitted vestige compensates for the spectral portion missing

from the other sideband.– The sideband shaping filter must itself satisfy the following condition.

– Two properties of the sideband shaping filter• The transfer function of the sideband shaping filter exhibits odd symmetry

about the carrier frequency• The transfer function Hv(f) is required to satisfy the condition of Eq. (3.26)

only for the frequency interval -W≤f ≤W

13

)26.3(for,1)()( WfWffHffH cc ≤≤−=−++

)27.3(for ),()()( WffffffHffufH cvccvc +<<−−−−=

)28.3(0for ,00for ,1

)(

<>

=ff

fu )29.3()()( fHfH vv −=−

ECC1015 Communication System I

Vestigial Side Band Modulation

14

ECC1015 Communication System I

Vestigial Side Band Modulation

15

ECC1015 Communication System I

Announcement and Assignment

• Reading assignment– Angle Modulation

16