9 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
A Combined Equaliser and Decoder for Maximum Likelihood Decoding of Convolutional Codes in the presence of ISI. Incorporation into GSM 3GPP Standard
The dissertation describes a new approach in combining the equalising and decoding operations in wireless telecommunications, namely MS decoder. It provides performance results (SNR) and carries out simulations based on GSM 3GPP standard
A likelihood ratio analysis of digital phase modulation
Bibliography: p. 180-188.Although the likelihood ratio forms the theoretical basis for maximum likelihood (ML) detection in coherent digital communication systems, it has not been applied directly to the problem of designing good trellis-coded modulation (TOM) schemes. The remarkably simple optimal receiver of minimum shift keying (MSK) has been shown to result from the mathematical simplification of its likelihood ratio into a single term. The log-likelihood ratio then becomes a linear sum of metrics which can be implemented as a so-called simplified receiver, comprising only a few adders and delay elements. This thesis project investigated the possible existence of coded modulation schemes with similarly simplifying likelihood ratios, which would have almost trivially simple receivers compared to the Viterbi decoders which are typically required for maximum likelihood sequence estimation (MLSE). A useful notation, called the likelihood transform, was presented to aid the analysis of likelihood ratios. The work concentrated initially on computer-aided searches, first for trellis codes which may give rise to simplifying likelihood ratios for continuous phase modulation (CPM), and then for mathematical identities which may aid in the simplification of generic likelihood ratios for equal-energy modulation. The first search yielded no simplified receivers, and all the identities produced by the second search had structures similar to the likelihood ratio of MSK. These observations prompted a formal proof of the non-existence of simplified receivers which use information from more than two symbols in their observation period. This result strictly bounds the error performance that is possible with a simplified receiver. It was also proved that simplified receivers are only optimal for modulation schemes which use no more than two pairs of antipodal signals, and that only binary modulation schemes can have simplified receivers which use information from all the symbols in their observation period
Proceedings of the Second International Mobile Satellite Conference (IMSC 1990)
Presented here are the proceedings of the Second International Mobile Satellite Conference (IMSC), held June 17-20, 1990 in Ottawa, Canada. Topics covered include future mobile satellite communications concepts, aeronautical applications, modulation and coding, propagation and experimental systems, mobile terminal equipment, network architecture and control, regulatory and policy considerations, vehicle antennas, and speech compression
Synchronization in digital communication systems: performance bounds and practical algorithms
Communication channels often transfer signals from different transmitters. To avoid interference the available frequency spectrum is divided into non-overlapping frequency bands (bandpass channels) and each transmitter is assigned to a different bandpass channel. The transmission of a signal over a bandpass channel requires a shift of its frequency-content to a frequency range that is compatible with the designated frequency band (modulation). At the receiver, the modulated signal is demodulated (frequency shifted back to the original frequency band) in order to recover the original signal. The modulation/demodulation process requires the presence of a locally generated sinusoidal signal at both the transmitter and the receiver. To enable a reliable information transfer, it is imperative that these two sinusoids are accurately synchronized.
Recently, several powerful channel codes have been developed which enable reliable communication at a very low signal-to-noise ratio (SNR). A by-product of these developments is that synchronization must now be performed at a SNR that is lower than ever before. Of course, this imposes high requirements on the synchronizer design.
This doctoral thesis investigates to what extent (performance bounds) and in what way (practical algorithms) the structure that the channel code enforces upon the transmitted signal can be exploited to improve the synchronization accuracy at low SNR
Broadband wireless communication systems: Channel modeling and system performance analysis
Wideband channel modeling, which can accurately describe the most important
characteristics of wideband mobile fading channels, is essential for the design,
evaluation, and optimization of broadband wireless communication systems. In the
field of wideband channel modeling, the tradeoff between the prediction accuracy
and simulation efficiency has to be taken into account. On one hand, channel models
should be as accurate as possible. On the other hand, channel models are supposed
to be simple and easy to put into use. There are several commonly used approaches
to channel modeling, e.g., measurement-based channel modeling and deterministic
channel modeling. Both methods are efficient in capturing the fading behavior
of real-world wireless channels. However, the resulting channel models are only
valid for the specific environments as those where the measurements were carried
out or the ray-tracing scenario was considered. Moreover, these methods are quite
time consuming with high computational cost. Alternatively, the geometry-based
stochastic channel modeling approach can be employed to model wideband mobile
fading channels. The most attractive feature of this method is that the derived
channel models are able to predict fading behavior for various propagation environments,
and meanwhile they can be easily implemented. Thus, the dissertation
will complete the wideband channel modeling task by adopt the geometry-based
stochastic approach.
In the dissertation, several geometry-based channel models are proposed for
both outdoor and indoor propagation scenarios. The significance of the work lies in
the fact that it develops channel models under more realistic propagation conditions
which have seldom been considered, such as for non-isotropic scattering environxi
ments and mobile-to-mobile (M2M) fading channels. In addition, the proposed
channel models remove the scarcity that proper geometry-based channel models
are missing for indoor environments. The most important statistical properties
of the developed channel models including their temporal autocorrelation function
(ACF), the two-dimensional (2D) space cross-correlation function (CCF), and the
frequency correlation function (FCF) are analyzed. Furthermore, efficient channel
simulators with low realization expenditure are obtained. Finally, the validity of the
proposed channel models is demonstrated by comparing their analytical channel
statistics with the empirical ones measured from real world channels.
Besides the work in the field of wideband channel modeling, another part of
the dissertation is dedicated to investigate the performance of SISO1 orthogonal
frequency division multiplexing (OFDM) broadband communication systems and
space-time (ST) coded MIMO2 OFDM broadband communication systems. This
work provides a deep insight into the performance of a broadband mobile radio
communication system over realistic wideband fading channels. Analytical expressions
are derived for bit error probability (BEP) or symbol error rate (SER) of systems.
In order to confirm the correctness of the theoretical results as well as to
show the usefulness of the wideband channel models in the testing and analysis of
a broadband communication system, SISO OFDM systems and space-time coded
MIMO OFDM systems are simulated in the dissertation.
In order to improve the reliability of digital transmission over broadband wireless
radio channels, a differential super-orthogonal space-time trellis code (SOSTTC)
is designed for noncoherent communications, where neither the transmitter nor the
receiver needs the channel state information (CSI) for decoding. In addition, a new
decoding algorithm is proposed. The new algorithm has exactly the same decoding
performance as the traditional one. However, it is superior from the standpoint of
overall computing complexity