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The demodulation process is an important aspect in the design of the transceiver. Proper design of the demodulation section can enhance the sensitivity and performance of data detection. Two types of demodulation can be used to despread and recover the data. The matched filter approach simply delays and correlates each delay segment of the signal to produce the demodulated output. This process includes the use of PPM to encode and decode the actual data. Another demodulation process uses a coherent sliding correlator to despread the data. This process requires alignment of the codes in the receiver, which is generally accomplished by a short acquisition code. Tracking loops, such as the early/late gate, align the code for the dispreading process. Carrier recovery loops, such as the squaring loop, Costas loop, and modified Costas loop, provide a means for the demodulator to strip off the carrier. A symbol synchronizer is required to sample the data at the proper time in the eye pattern in order to minimize the effects of ISI. Finally, receivers designed for intercepting transmissions of other transmitters use various means of detection depending on the type of phase modulation utilized.

Chapter Contents:

  • 5.1 Types of Demodulation
  • 5.2 Pulsed Matched Filter
  • 5.3 Digital Matched Filter Correlator
  • 5.4 Pulse Position Modulation
  • 5.5 Code Division Encoding and Decoding
  • 5.6 Coherent versus Differential Digital Modulation and Demodulation
  • 5.6.1 Coherent Demodulation
  • 5.7 Carrier Recovery
  • 5.7.1 Squaring Loop
  • 5.7.2 Costas Loop
  • 5.7.3 Modified Costas Loop and AFC Addition
  • 5.8 Despreading Correlator
  • 5.9 Symbol Synchronizer
  • 5.10 The Eye Pattern
  • 5.11 Digital Processor
  • 5.12 Intersymbol Interference
  • 5.13 Scrambler/Descrambler
  • 5.14 Phase-Shift Detection
  • 5.15 Shannon's Limit
  • 5.16 Summary

Inspec keywords: decoding; correlation methods; demodulation; phase modulation; matched filters; encoding; transceivers

Other keywords: ISI; delay segment correlation; carrier recovery loop; signal; code alignment; transceiver design; sliding correlator; symbol synchronizer; demodulation process; data decode; Costas loop; data detection; data recover; data encode; matched filter; acquisition code; phase modulation; receiver; squaring loop; data despread; PPM; tracking loop; dispreading process

Subjects: Filtering methods in signal processing; Modulation and coding methods

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