| Summary: | In recent years, ultra-wideband (UWB) communications has gained tremendous
popularity in both research community and industry. The large bandwidth
of UWB systems raises new wireless channel effects and consequently
unique advantages as well as challenges to be dealt with, compared to conventional
wireless systems. One of these advantages is the ability to resolve
dense multipath components and use Rake combining at the receiver in order
to significantly reduce the negative effects of fading. However, implementing
a Rake receiver with a sufficiently large number of fingers to make use
of this advantage is an evident challenge for most UWB devices with limited
signal processing capabilities. A possible approach to overcome this problem
is to move computational complexity from the receiver to the more powerful
transmitter, which is the main focus of the present work.
In this thesis, we propose two novel pre-equalization schemes for multiple-
input single-output (MISO) direct-sequence ultra-wideband (DS-UWB) systems
with pre-Rake combining and symbol-by-symbol detection. The first
pre-equalization filter (PEF) scheme employs one PEF per transmit antenna,
whereas in the second, simplified PEF (S-PEF) scheme all transmit antennas
share the same PEF. For both schemes the optimum finite impulse response
(FIR) and infinite impulse response (IIR) PEFs are calculated based on the
minimum mean squared error (MMSE) criterion. We show that in contrast to
previously proposed schemes for DS-UWB, both our proposed PEF schemes
efficiently exploit the channel shortening properties of the pre-Rake filter.
In particular, our proposed PEF schemes operate at the symbol level. We also
show that under certain conditions the S-PEF scheme achieves the same performance
as the more complex PEF scheme. Finally, we demonstrate that a
single-input multiple-output (SIMO) DS-UWB system with post-Rake combining
and MMSE post-equalization is the dual system to the considered MISO
DS–UWB system with pre-Rake combining and MMSE pre-equalization. This
uplink-downlink duality can be exploited for efficient calculation of the PEFs
and for complexity reduction.
Our simulation results show that the proposed PEF schemes achieve significant
performance gains over pre-Rake combining without equalization even if only
short PEFs are employed, and this is the case even for long UWB channel
impulse responses. === Applied Science, Faculty of === Electrical and Computer Engineering, Department of === Graduate
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