ECC1015 Communication System I

Communication System I

Electronics and Communication EngineeringHanyang University

Haewoon Nam

Lecture 10

(ECC1015)

1

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

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)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

• Cosine and Sine function in frequency domain

• In upper SSB

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)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

4

)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

Summary of the Previous Lecture

• 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.

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ECC1015 Communication System I

Summary of the Previous Lecture

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ECC1015 Communication System I

Summary of the Previous Lecture

• Vestigial side band modulation

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ECC1015 Communication System I

Basic Definitions of Angle Modulation

• Angle modulation– The angle of the carrier wave is varied according to the information-

bearing signal– Angle modulation is a nonlinear process– Whereas the transmission bandwidth of an amplitude-modulated wave

is of limited extent, the transmission bandwidth of an angle-modulated wave may an infinite extent, at least in theory.

– The amplitude of the carrier wave is maintained constant, additive noise would affect the performance of angle modulation to a lesser extent than amplitude modulation

• Two types of angle modulation– Phase modulation– Frequency modulation

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ECC1015 Communication System I

Basic Definitions of Angle Modulation

• Angle-modulated wave

• Phase modulation (PM): form of angle modulation in which the instantaneous angle is varied linearly with the message signal

• Frequency modulation (FM): form of angle modulation in which the instantaneous frequency is varied linearly with the message signal

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)3.4()(2)( tmktft pci += πθ[ ] )4.4()(2cos)( tmktfAts pcc += π

)5.4()()( tmkftf fci +=

)7.4()(22cos)(0

+=

t

fcc dmktfAts ττππ

).()()()( 64222 00 ττππττπθ dmktfdftt

fc

t

ii +==

)1.4()](cos[)( tAts ic θ=

ECC1015 Communication System I

Basic Definitions of Angle Modulation

10[Ref] Introduction to Analog and Digital Communications,

by Haykin and Moher, Wiley

ECC1015 Communication System I

Amplitude Mod. and Angle Mod.

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Amplitude Modulation

Phase Modulation

Frequency Modulation

Carrier Signal

Message Signal

[Ref] Introduction to Analog and Digital Communications,by Haykin and Moher, Wiley

ECC1015 Communication System I

Amplitude Mod. and Angle Mod.

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a. Carrier signalb. Message signalc. Amplitude modulationd. Frequency modulatione. Phase modulation

ECC1015 Communication System I

Properties of Angle Modulated Waves

• Property 1 : Constancy of transmitted power– The amplitude of PM and FM waves is maintained at a constant value equal to

the carrier amplitude for all time.– The average transmitted power of angle-modulated waves is a constant

• Property 2 : Nonlinearity of the modulation process– Its nonlinear character

• Property 3 : Irregularity of zero-crossings– Zero-crossings are defined as the instants of time at which a waveform changes

its amplitude from a positive to negative value or the other way around.

• Property 4 : Visualization difficulty of message waveform– The difficulty in visualizing the message waveform in angle-modulated waves is

also attributed to the nonlinear character of angle-modulated waves.

• Property 5 : Tradeoff of increased transmission bandwidth for improved noise performance

– The transmission of a message signal by modulating the angle of a sinusoidal carrier wave is less sensitive to the presence of additive noise

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ECC1015 Communication System I

Relationship Between PM and FM

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ECC1015 Communication System I

Relationship Between PM and FM

15[Ref] Introduction to Analog and Digital Communications,

by Haykin and Moher, Wiley

ECC1015 Communication System I

Announcement and Assignment

• Reading assignment– Angle Modulation

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