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Lecture 10Basic Modulation Techniques (VI)

Fall 2008

NCTU EE

Tzu-Hsien Sang

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Outlines

• Linear Modulation

• Angle Modulation

• Interference

• Feedback Demodulators

• Analog Pulse Modulation

• Delta Modulation and PCM

• Multiplexing

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• One unfinished business: the Costas loop. It is very useful for carrier recovery in both analog and digital communication systems.

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Analog Pulse Modulation

• We now study the progression from techniques dealing with pure waveforms ones for “discrete” samples.

• Historically, these methods are the early attempts to achieve modern communications. They are in the twilight zone between analog and digital modulations.

• Today, their spirits can be still found in components such as ADC.

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• Analog pulse modulation: A pulse train is used as the carrier wave. Some characteristic feature of each pulse (e.g., amplitude, duration, or position) is used to represent message samples.

PAM – pulse amplitude

PDM – pulse duration

PPM – pulse position

• Digital Pulse Modulation: Messages are discrete-amplitude (finite levels) samples.

DM – delta modulation

PCM – pulse-code modulation 5

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PAM

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signals. PAM of generation the

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sinc ),()()(

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[)( ),()()(

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SSc

efH(f)fHfMfM

tththtmtm

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• The “conceptual” demodulation scheme:

• The idea of equalizer: Anything that goes through a known distortion can be recovered.

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1/H(f)

LPF

equalizer

mc(t) m(t)

m(t)

PWM• Spectrum: complicated (Fourier-Bessel spectra)

• Demodulation: area of “pulse.” Low-pass filtering (integration)

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PPM

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Delta Modulation

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function impulse delta thefrom come

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)(Δ)(

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SSs dnT)(nTtm

• Slope overload: The message signal m(t) has a slope greater than can be followed by the stair-step approximation ms(t). Assume the step-size = 0 slope (max) = 0/Ts.

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• Solution (may not be perfect): adaptive delta modulation -- adjust the step-size 0 based on xc(t). Idea: If m(t) constant, xc(t) alternates in sign get 0 . If m(t) ( or ) rapidly, xc(t) has the same polarity get 0.

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• Question: How does the receiver knows the time-varying step size?

• Answer: Regenerate the step-size rule. Many so-called “adaptive” schemes, for example, in adaptive video coding, rely on this type of regeneration mechanism.

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PCM

• m(t) samples (analog amplitude) quantized samples binary representation binary modulated waveform (ASK (AM), PSK (PM), FSK (FM) )

• Main advantages of digital communication

– more reliable communication

• Main disadvantages of digital communication

– wide BW (reduced by “compression”)

complicated circuits (cost reduced by VLSI)15

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A rough discussion on BW

Assume the number of quantization levels 2 .

Message bandwidth .

Sampling rate 2 . 2 binary pulses/sec.

1Assume the maximum width of a pulse .

2Transmission band

nq

W

W nW

nW

width . (proportional to )

Now, the recovered message error is mainly due to quantization.

Thus, error bandwidth .

knW nW

q

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