Thesis-1996-Rizk.pdf (40.41 MB)
Download fileDigital data transmission over an HF channel
thesis
posted on 2010-11-18, 12:28 authored by Ziad RizkThe thesis is concerned with detection, estimation techniques and a method of the
adaptive adjustment of the equaliser, for use in a 4800bit/sec synchronous digital
transmission system operating over a voice-band time-varying HF channel. Two main
impairments are additive Gaussian noise and inter-symbol interference (ISI), which can
be very severe at times. All techniques considered here are algorithms or processes that
operate on sequences of sample values. Modern digital modems normally operate in this
way, and the techniques described are of direct application to practical systems, and
could be implemented using the new technology of high speed real-time digital signal
processing (DSP). The performance of the various systems that employ the above
techniques are obtained using the computer simulated model of three types of HF
channels.
The ionospheric propagation medium, the characteristics of HF channel and the
signal distortion introduced by the channel are first described. The thesis then presents a suitable base-band model of the HF channel for computer simulation of quadrature
amplitude modulation systems. A suitable method for the adjustment of the receiver is
described next. This method is suitable both for the adjustment of a conventional
decision feedback equaliser (DFE), and also for the adjustment of a linear feedforward
filter that is employed ahead of a near-maximum likelihood (NML) detector. This
method uses a minimum phase (root-finding) algorithm (MPA) to convert the channel
response from being non-minimum phase to at least approximately minimum phase. The
results of computer simulation tests of this algorithm are then presented over different
types of HF channel models. The results demonstrate the algorithm's capability to make
the channel response minimum (or near-minimum) phase.
Various NML detectors, derived from the Viterbi detector, are discussed. Each
detector is here preceded by an adaptive linear filter that is adjusted adaptively using an
MPA. The performance of these detectors is compared with the conventional DFE,
whose tap-gains are adjusted adaptively using an MPA, and the detector which gives the
best compromise between performance and complexity is selected for combined
receivers. These results are obtained using perfect estimation of the channel
response.
The estimation techniques studied in this thesis include both new and conventional
estiniators, which are based on the least- mean-square (LMS) algorithm or recursive least-square(RLS) algorithm. The estimator provides an estimate of the sampled impulse response (SIR) of the channel, necessary for the NML detector or MPA. The
performances of these estimators are compared using computer simulation tests. The
results also demonstrate that the simpler LMS algorithm with adaptive step size gives a
comparable level of accuracy with the more complex RLS algorithm.
Finally the most promising of the detectors and estimators are connected with an
adaptive equaliser, using an MPA, to form a new combined receiver. The details of the
combined system structure with its computational complexity are given. Extensive
computer simulation tests have been carried out on the different arrangements of the
combined system including DFE, when all the functions of detection, estimation and
MPA are present, in order to find the most cost effective system in terms of performance
and complexity. A considerable reduction in the equipment complexity can be achieved
by allowing a long period between successive adjustment of the adaptive filter and
estimator.
History
School
- Mechanical, Electrical and Manufacturing Engineering
Publisher
© Z. RizkPublication date
1996Notes
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.EThOS Persistent ID
uk.bl.ethos.321243Language
- en