We are not able to resolve this OAI Identifier to the repository landing page. If you are the repository manager for this record, please head to the Dashboard and adjust the settings.
PhD ThesisMULTI-access techniques have been adopted widely for communications in
underwater acoustic channels, which present many challenges to the development
of reliable and practical systems. In such an environment, the unpredictable
and complex ocean conditions cause the acoustic waves to be affected by many factors
such as limited bandwidth, large propagation losses, time variations and long latency,
which limit the usefulness of such techniques. Additionally, multiple access interference
(MAI) signals and poor estimation of the unknown channel parameters in the
presence of limited training sequences are two of the major problems that degrade
the performance of such technologies.
In this thesis, two different single-element multi-access schemes, interleave division
multiple access (IDMA) and code division multiple access (CDMA), employing
decision feedback equalization (DFE) and soft Rake-based architectures, are proposed
for multi-user underwater communication applications. By using either multiplexing
pilots or continuous pilots, these adaptive turbo architectures with carrier phase tracking
are jointly optimized based on the minimum mean square error (MMSE) criterion
and adapted iteratively by exchanging soft information in terms of Log-Likelihood
Ratio (LLR) estimates with the single-user’s channel decoders. The soft-Rake receivers
utilize developed channel estimation and the detection is implemented using
parallel interference cancellation (PIC) to remove MAI effects between users.
These architectures are investigated and applied to simulated data and data obtained
from realistic underwater communication trials using off-line processing of
signals acquired during sea-trials in the North Sea. The results of different scenarios
demonstrate the penalty in performance as the fading induces irreducible error rates
that increase with channel delay spread and emphasize the benefits of using coherent
direct adaptive receivers in such reverberant channels. The convergence behaviour of
the detectors is evaluated using EXIT chart analyses and issues such as the adaptation
parameters and their effects on the performance are also investigated. However,
in some cases the receivers with partial knowledge of the interleavers’ patterns or
codes can still achieve performance comparable to those with full knowledge. Furthermore,
the thesis describes implementation issues of these algorithms using digital
signal processors (DSPs), such as computational complexity and provides valuable
guidelines for the design of real time underwater communication systems
Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.