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Demodulation is the process of recovering the transmitter's symbol stream from the received signal. There are entire books devoted to demodulation, so a single chapter in this book has no hope of covering the topic adequately. Our goal is describe, in broad strokes, the nature of the received signal and some introductory methods we might use to compensate for the corruption of the signal by the propagation environment, oscillator inaccuracies, and other effects. We will examine only continuous quadrature amplitude modulation (QAM)-type signals in this chapter. The propagation path through which the signal travels exhibits a variable time delay, multipath, and signal power loss. These parameters will change with time. The oscillators in the receiver and transmitter exhibit frequency drift that causes the received signal to appear at an incorrect frequency and baud rate. The time delay of the propagation path and the oscillator inaccuracies cause the phase of the received signal to be indeterminate and varying. At the receiver, we wish to measure the precise phase and amplitude of a constellation point. That is our received symbol. The (considerable) task of the demodulator is to remove the ambiguities (amplitude, frequency, and phase) of the arrived signal and allow the symbol decision to proceed with accuracy.

Chapter Contents:

  • 12.1 Introduction
  • 12.2 A Transmitter Model
  • 12.3 The Pulse-Shaping Filter
  • 12.4 A 16QAM Modulator
  • 12.5 A Receiver Model
  • 12.6 Estimation of Carrier Frequency
  • 12.7 Estimation of Baud Rate
  • 12.8 Constellation Impairments
  • 12.9 Bibliography
  • 12.10 Problems

Inspec keywords: quadrature amplitude modulation; radio transmitters; demodulation; oscillators

Other keywords: QAM-type signals; propagation environment; oscillator inaccuracies; constellation point; continuous quadrature amplitude modulation; received signal; demodulation; propagation path; transmitter symbol stream

Subjects: Modulation and coding methods; Oscillators

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