3 research outputs found
Techniques of detection, estimation and coding for fading channels
The thesis describes techniques of detection, coding and estimation, for use in
high speed serial modems operating over fading channels such as HF radio and land mobile
radio links. The performance of the various systems that employ the above techniques are
obtained via computer simulation tests.
A review of the characteristics of HF radio channels is first presented, leading
to the development of an appropriate channel model which imposes Rayleigh fading on the
transmitted signal. Detection processes for a 4.8 kbit/s HF radio modem are then
discussed, the emphasis, here, being on variants of the maximum likelihood detector that is
implemented by the Viterbi algorithm. The performance of these detectors are compared
with that of a nonlinear equalizer operating under the same conditions, and the detector
which offers the best compromise between performance and complexity is chosen for
further tests.
Forward error correction, in the form of trellis coded modulation, is next
introduced. An appropriate 8-PSK coded modulation scheme is discussed, and its
operation over the above mentioned HF radio modem is evaluated. Performance
comparisons are made of the coded and uncoded systems.
Channel estimation techniques for fast fading channels akin to cellular land
mobile radio links, are next discussed. A suitable model for a fast fading channel is
developed, and some novel estimators are tested over this channel. Computer simulation
tests are also used to study the feasibility of the simultaneous transmission of two 4-level
QAM signals occupying the same frequency band, when each of these signals are
transmitted at 24 kbit/s over two independently fading channels, to a single receiver. A
novel combined detector/estimator is developed for this purpose.
Finally, the performance of the complete 4.8 kbit/s HF radio modem is
obtained, when all the functions of detection, estimation and prefiltering are present, where
the prefilter and associated processor use a recently developed technique for the adjustment
of its tap gains and for the estimation of the minimum phase sampled impulse response
Digital data transmission over mobile radio channels
The aim of this work is to study data transmission over a microwave
digital mobile radio channel at 900 MHz, where the channel is subjected
to multipath fading. Besides the fading, the other impairments assumed
here are additive noise, co-channel interference and adjacent channel
interference. Two modulation techniques are investigated in this work,
namely Quadrature-Amplitude-Modulation (QAM) and Quadrature-Phase-Shift-
Keying (QPSK). The channel is characterised digitally, assuming multipath
Rayleigh fading in the presence of noise. The detection process
studied here are near-maximum likelihood schemes: non-linear equalisation
methods are also considered in detail.
The thesis is also concerned with carrier synchronisation and channel
estimation under conditions of Rayleigh fading. Since the carrier
syncn,honisation is a most important requirement in mobile radio, a
Digital Phase Locked Loop (DPLL) technique has been designed and investigated
in the form of a feedback digital synchronisation system. Two
types of channel estimation technique, namely feedforward and feedback
estimators, are also investigated in this work. The feedback estimator
is modified by the addition of a digital control system, in order to
reduce its delay, and to cope with rapidly fading signals. Successful
carrier synchronisation is demonstrated by the use of space diversity.
The study was completed using models of the component parts of the system,
and by the use of extensive computer simulations to analyse the system
under various operating conditions
Detection processes for digital satellite modems
The aim of this study is to devise detectors for digital satellite
modems, that have tolerances to additive white Gaussian noise which
are as close as possible to that for optimal detection, at a fraction
of the equipment complexity required for optimal detection. Computer
simulation tests and theoretical analyses are used to compare the
proposed detectors. [Continues.