| 1. |
- Almers, Peter, et al.
(författare)
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Antenna subset selection in measured indoor channels
- 2007
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Ingår i: IET Microwaves, Antennas & Propagation. - : Institution of Engineering and Technology (IET). - 1751-8725. ; 1, s. 1092-1100
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Tidskriftsartikel (refereegranskat)abstract
- Antenna subset selection can greatly reduce the implementation complexity of multiple input multiple output (MIMO) systems while retaining most of their benefits. This paper investigates the diversity gain and capacity of such systems in wireless personal area networks. Considered scenarios include both the communication between access point to a laptop, and between two handheld devices. We analyse the performance of different antenna selection algorithms and signal combining methods in measured dual-polarised narrowband and wideband propagation channels. We find that line-of-sight and non-line-of-sight situations have fairly similar behaviour. Different polarisations result in similar signal-to-noise ratio gains when the multiple antennas are used for diversity, but result in noticeably different capacities in spatial-multiplexing systems. We also find that radiofrequency (RF) preprocessing of the signals is less effective for handheld handsets with non-uniform antenna arrangements than for uniform linear arrays. For communications between handheld devices, simple selection (of one out of four antennas) shows extremely high performance gains compared to no-selection. Finally, we compare bulk selection (same antenna subset is used for all frequency sub-channels) to per-tone selection (different antenna subsets can be used for each frequency sub-channel) for wideband channels. Bulk selection together with RF preprocessing performs almost as well as per-tone selection for some scenarios.
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| 2. |
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| 3. |
- Pirk, Norbert, et al.
(författare)
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Methane emission bursts from permafrost environments during autumn freeze-in: New insights from ground-penetrating radar
- 2015
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Ingår i: Geophysical Research Letters. - 1944-8007. ; 42:16, s. 6732-6738
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Tidskriftsartikel (refereegranskat)abstract
- Large amounts of methane (CH4) are known to be emitted from permafrost environments during the autumn freeze-in, but the specific soil conditions leading up to these bursts are unclear. Therefore, we used an ultrawide band ground-penetrating radar in Northeast Greenland in autumn 2009 to estimate the volumetric composition inside the soil through dielectric characterization from 200 to 3200 MHz. Our results suggest a compression of the gas reservoir during the phase transition of soil water, which is accompanied by a peak in surface CH4 emissions. About 1 week thereafter, there seems to be a decompression event, consistent with ground cracking which allows the gas reservoir to expand again. This coincides with the largest CH4 emission, exceeding the summer maximum by a factor of 4. We argue that these complementary measurement techniques are needed to come to an understanding of tundra CH4 bursts connected to soil freezing.
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| 4. |
- Santos, Telmo, et al.
(författare)
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Dielectric Characterization of Soil Samples by Microwave Measurements
- 2009
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Northern high-latitude wetlands are well known to seasonally emit methane gas into the atmosphere, and therefore contribute to greenhouse effects. While these gas emissions are well documented, their causes are not well understood. The method described in this work can be used to analyze the changes happening in the soil during gas emissions, and therefore help the understanding of the sub-surface gas dynamics. We have monitored a sample of peat soil through an artificial freezing and thawing cycle, using both a gas detector to measure the methane flux at the soil surface and a vector network analyzer to measure the transmission of microwaves through the soil. It was observed that the results from the two measurement approaches had a very good match under specific microwave signal conditions. In addition, from the microwave measured data, the dielectric properties of the soil and the volumetric fractions of its constituents were also calculated based on a dielectric mixing model.
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| 5. |
- Santos, Telmo, et al.
(författare)
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Modeling the ultra-wideband outdoor channel - measurements and parameter extraction method
- 2010
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Ingår i: IEEE Transactions on Wireless Communications. - 1536-1276. ; 9:1, s. 282-290
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents results from one of the few existing outdoor measurement campaigns for UWB. We specifically focus on scenarios applicable for "infostations," where large amounts of data can be downloaded to a user within a limited amount of time. We describe the measurement setup, and present a novel high-resolution algorithm that allows the extraction of the scatterer's positions. Measurement data is extracted using eight meter uniform linear virtual array where incoming front waves are spherical, and thus allowing for high-precision location of the scatterers. Insight is given on how these components can be tracked in the impulse response for a spatially varying terminal. We then cluster the detected components, and investigate how the angular power variations of a given scatterer are correlated with the power variations of the other scatterers belonging to the same cluster. This results in the definition of the clusters' angular radiation pattern. Further sample measurements show how obstacles obstruct the line-of-sight component; a phenomenon that we describe mathematically by "shadowing regions," and compare these measurements with the theoretical results predicted by diffraction theory.
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| 6. |
- Santos, Telmo, et al.
(författare)
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Modeling the Ultra-Wideband Outdoor Channel - Model Specification and Validation
- 2010
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Ingår i: IEEE Transactions on Wireless Communications. - 1536-1276. ; 9:6, s. 1987-1997
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we establish a geometry-based stochastic ultra-wideband channel model for outdoor infostations scenarios. We statistically describe the location and strength of clusters of scatterers, as well as the shape and width of visibility and shadowing regions. We also separately model diffuse radiation (which cannot be explained by the scatterer components), and show that its fading statistics can be best described by a Weibull distribution with a delay dependent beta-parameter. A step-by-step implementation recipe demonstrates that the overall model can be implemented with little programming effort. Finally, we validate our model comparing with secondary channel parameters as rms delay spread, and show that the model can accurately describe the measured channel properties having a general applicability for these class of scenarios.
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| 7. |
- Santos, Telmo, et al.
(författare)
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Scatterer detection by successive cancellation for UWB - method and experimental verification
- 2008
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Ingår i: [Host publication title missing]. - 1550-2252. - 9781424416448 ; , s. 445-449
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Konferensbidrag (refereegranskat)abstract
- We present a new high delay resolution method to detect ultra-wideband (UWB) scatterers when using frequency domain measurements. Our approach makes use of the impulse response envelope amplitudes and delays measured over a distance that is larger than the region of stationarity, and detects the 2D coordinates of the channel scatterers, assuming that only single-scattering (single-interaction) processes occur. The identification methodology is based on multiple application of interference cancellation: at every step, we detect the strongest scatterer from an array of measurements, save its information, cancel it from the channel and search for the next strongest scatterer. To precisely define the strength of each scatterer, we present a method to define its birth and death locations along the measurement array. Finally, we verify the method by applying it to measurement results in an outdoor environment; the scatterer locations identified from the measurements show excellent agreement with the physically present objects like walls and columns.
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| 8. |
- Santos, Telmo
(författare)
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Ultra-Wideband Wireless Channels - Estimation, Modeling and Material Characterization
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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