Ultra-Wideband Wireless Channels - Estimation, Modeling and Material Characterization

Sammanfattning: This licentiate thesis is focused on the characterization of ultra-wideband wireless
channels. The thesis presents results on ultra-wideband communications
as well as on the ultra-wideband characterization of materials.
The communications related work consisted in the measurement and modeling
of outdoor scenarios envisioned for infostation systems. By infostation,
we mean a communication system covering a small area, i.e., ranging up to 20
m, where mobile users can pass by or stop while receiving large amounts of
data in a short period of time. Considering the expected (but perhaps overly
optimistic) 480 Mbps for UWB systems, it should be possible to download a
complete DVD in roughly two minutes, which is something not realizable with
any of the current wireless technologies. Channel models, commonly based on
measurements, can be used to evaluate the performance of such systems. We
therefore, we started by performing measurements at one of the scenarios where
infostation systems can exist in the future, namely, petrol stations. The idealized
model, was one that could correctly describe the continuous evolution of
the channel impulse response for a moving user within the system’s range, and
therefore it was deemed necessary to track the multipath components defining
the impulse responses along a path of several meters. To solve this problem we
designed a novel high-resolution scatterer detection method, which is described
in Paper I, capable of tracking individual multipath components for a moving
user by identifying the originating point scatterers in a two dimensional geometrical
space. The same paper also gives insight on some properties of clusters
of scatterers, such as their direction-selective radiated power.
The scatterer detection method described in Paper I provided us with the
required tools to create the channel model described in Paper II. The proposed
channel model has a geometrical basis, i.e., each realization of the channel is
based on a virtual map containing point scatterers that contribute to the impulse
response by multipath components. Some of the particular characteristics
of the model include non-stationary effects, such as shadowing and cluster’s visibility
regions. At the end of Paper II, in a simple validation step, the output of the channel model showed a good match with the measured impulse responses.
The second part of our work, documented in Paper III, consisted on the dielectric
characterization of soil samples using microwave measurements. This
project was made in cooperation with the Department of Physical Geography
and Ecosystem Analysis at Lund University, which had been developing
research work on methane emissions from the wetlands in Zackenberg, Greenland.
In recent years, a lot of attention has been put into the understanding
of the methane emissions from soils, since methane is a greenhouse gas 20
times stronger than carbon dioxide. However, whereas the methane emissions
from natural soils are well documented, the reason behind this effect is an
open issue. The usage of microwave measurements to monitor soil samples,
aims to address this problem by capturing the sub-surface changes in the soil
during gas emissions. An experiment consisting on the monitoring of a soil
sample was performed, and a good correlation was found between the variations
of the microwave signals and the methane emissions. In addition, the soil
dielectric constant was calculated, and from that, the volumetric fractions of
the soil constituents which provided useful data for the elaboration of models
to describe the gas emission triggering mechanisms.
Based on this laboratory experiment, a complete soil monitoring system
was created and is at the time of writing running at Zackenberg, Greenland.