The thesis is concerned with the detection of digital data transmitted
over voice frequency channels such as telephone circuits and HF
radio links, where the main impairment is additive noise and intersymbol
interference, and the latter may be time-invariant or may vary slowly with
time. The characteristics of these channels are briefly reviewed and a
survey of the most important known detection techniques is presented.
The thesis includes also a detailed study of quadrature amplitude
modulated (QAM) signals transmitted over voice-channels, both, when the
transmission path has time-invariant characteristics and when it introduces
Rayleigh fading into the transmitted data signal. Based on this study,
baseband models of QAM systems are suggested for use when these are to be
computer simulated. A systematic study of channel models is carried out
The transmission and detection of baseband signals over telephone
circuits in the presence of frequency offset is investigated and a baseband
signal generated by means of Hilbert transform pairs is suggested for this
purpose. It is shown that this signal exhibits theoretical and experimental
equivalence to a QAM signal.
Several near-maximum likelihood detection techniques have been
developed for the detection of digital data signals serially transmitted
at 19200 bit/s over telephone lines and at 9600 bit/s over HF radio links.
The performance of the detection systems has been evaluated by computer
simulation and is given in terms of their tolerance to additive white
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.