The thesis is concerned with serial data transmission at 9600 bit/sec
over a voiceband channel, where the main impairments are additive noise
and intersymbol interference, and the latter varles slowly with time.
The thesis includes a brief description of the ionospheric propagation
medium and presents an equivalent baseband model of the HF channel,
suitable for computer simulation of quadrature amplitude modulation
(QAM) systems. A study of 16-point QAM signals transmitted over
voiceband HF channels is then carr-iod out usj-ng the given channel model.
Several cost effective near-maximum-likelihood detection processes have
been developed for HF modems. Each detector is here preceded by an
adaptive linear filter that is adjusted to make the sampled impulse
response of the channel and filter minimum phase. These detectors
require an accurate knowledge of the sampled impulse response of the
channel, if their full potential is to be achieved.
The results of computer-simulation tests on the near-maximum-likelihood
detectors are given, where these tests assume that other receiver
operations such as channel estimation and adaptive linear filtering,
together with element timing synchronisation and Doppler shift
correction, are carried out perfectly.
A recently developed HF channel estimator employing a simple feedforward
transversal-filter and requiring knowledge of the number of skywaves is
next investigated and a starting up procedure is developed for such an
estimator. The technique is then made fully adaptive in the sense that
it continues to operate correctly when the number of skywaves changes.
Results of computer simulation tests are then presented showing the
performance of the above detectors when operating with a channel
estimator and adaptive linear filtering. Finally modem synchronisation is studied and various techniques of
element timing and carrier frequency synchronisation are proposed.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.