Lecture 10 Basic Modulation Techniques (VI)

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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

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PAM
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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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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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• 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)

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