Digital data transmission over voice channels
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 here. 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 Gaussian noise.