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The demodulation process is part of the receiver process that takes the downconverted signal and retrieves or recovers the data information that was sent. This demodulation process requires three basic functions to retrieve the sent data: Recover the carrier, since the digital modulation results in a suppressed carrier and the carrier is recovered to remove it from the incoming signal, remove spread spectrum coding, if using spread spectrum techniques to mitigate jammers; generally done using a despreading matched filter correlator or a sliding correlator, align and synchronize the sample point for sampling the data stream at the optimal signal-to-noise ratio (SNR) point, which requires lining up the bits with the sample time using a bit synchronizer or over sampling the return. The process detects the digital data that was sent from the transmitter with a minimum bit error rate (BER). This was performed in the past using analog circuitry, such as mixers and filters to remove the carrier frequency and spread spectrum codes, but today, the process is incorporated in the digital circuitry using application-specific integrated circuits, field-programmable gate arrays, and digital signal processing (DSP)-integrated circuits.

Chapter Contents:

  • 5.1 Carrier recovery for suppressed carrier removal
  • 5.1.1 Squaring loop
  • 5.1.2 Costas loop
  • 5.1.3 Modified or hard-limited Costas loop and automatic frequency control addition
  • 5.1.4 Decision-directed Costas loop
  • 5.2 Demodulation process to remove spread spectrum code
  • 5.2.1 Matched filter correlator
  • Pulse position modulation
  • 5.2.2 Sliding correlator
  • 5.3 The eye pattern
  • 5.4 Intersymbol interference
  • 5.5 Symbol synchronizer
  • 5.6 Digital processor
  • 5.7 Scrambler/descrambler
  • 5.8 Shannon's limit
  • 5.9 Phase-shift detection
  • 5.10 Summary
  • 5.11 Problems
  • Further reading

Inspec keywords: matched filters; signal detection; error statistics; demodulation

Other keywords: spread spectrum coding; DSP; SNR point; field-programmable gate arrays; digital modulation; data information recovery; optimal signal-to-noise ratio point; digital signal processing-integrated circuits; application-specific integrated circuits; jammer mitigation; analog circuitry; bit error rate; data stream sampling; receiver process; carrier frequency; demodulation process; BER; sample point synchronization; digital circuitry; bit synchronizer; matched filter correlator; digital data detection; sliding correlator; downconverted signal

Subjects: Signal detection; Other topics in statistics; Filtering methods in signal processing; Modulation and coding methods

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