Soft Decision Detection Techniques

  • P. G. Farrell
Part of the International Centre for Mechanical Sciences book series (CISM)


It is well known that an optimum method of detection (demodulation and decoding), for a data transmission system with channel (error-correction) coding, is coherent correlation detection (or matched filtering) of the sequence of signal elements corresponding to the block length, in the case of a block code, or to the decoder search length, in the case of a convolutional code (see fig. 1(a)). In practice, unless the block or search length (and therefore the constraint length) is very short, this ideal detector is too complex to realise, because of the difficulty of generating, storing and correlating the large number of analogue signal elements required. Thus most practical detectors consist of an analogue demodulator, possibly coherent, operating on individual signal elements, followed by a purely digital decoder operating on blocks of the digits produced by the “hard” decisions of the demodulator (see fig. 1(b)). However, some of the information which would be lost by only correlating over a signal element can be used to assist and improve the decoding process, and vice-versa. Additional information can be fed forward from the demodulator to improve operation of the decoder, or fed back from the decoder to improve operation of the demodulator (see fig. 1(c)).


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Copyright information

© Springer-Verlag Wien 1979

Authors and Affiliations

  • P. G. Farrell
    • 1
  1. 1.Electronics LaboratoriesThe UniversityCanterbury, KentEngland

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