| 1. |
- Bergman, Svante
(author)
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Bit loading and precoding for MIMO communication systems
- 2009
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Doctoral thesis (other academic/artistic)abstract
- This thesis considers the joint design of bit loading, precoding and receive filters for a multiple-input multiple-output (MIMO) digital communication system. Both the transmitter and the receiver are assumed to know the channel matrix perfectly. It is well known that, for linear MIMO transceivers, orthogonal transmission (i.e., diagonalization of the channel matrix) is optimal for some criteria such as maximum mutual information. It has been shown that if the receiver uses the linear minimum mean squared error (MMSE) detector, the optimal transmission strategy is to perform bit loading on orthogonal subchannels. In the first part of the thesis, we consider the problem of designing the transceiver in order to minimize the probability of error given maximum likelihood (ML) detection. A joint bit loading and linear precoder design is proposed that outperforms the optimal orthogonal transmission. The design uses lattice invariant operations to transform the channel matrix into a lattice generator matrix with large minimum distance separation at a low price in terms of transmit power. With appropriate approximations, it is shown that this corresponds to selecting lattices with good sphere-packing properties. An algorithm for this power minimization is presented along with a lower bound on the optimization. Apparently, given the optimal ML detector, orthogonal subchannels are (in general) suboptimal. The ML detector may suffer from high computational complexity, which motivates the use of the suboptimal but less complex MMSE detector. An intermediate detector in terms of complexity and performance is the decision feedback (DF) detector. In the second part of the thesis, we consider the problem of joint bit loading and precoding assuming the DF detector. The main result shows that for a DF MIMO transceiver where the bit loading is jointly optimized with the transceiver filters, orthogonal transmission is optimal. As a consequence, inter-symbol interference is eliminated and the DF part of the receiver is actually not required, only the linear part is needed. The proof is based on a relaxation of the discrete set of available bit rates on the individual subchannels to the set of positive real numbers. In practice, the signal constellations are discrete and the optimal relaxed bit loading has to be rounded. It is shown that the loss due to rounding is small, and an upper bound on the maximum loss is derived. Numerical results are presented that confirm the theoretical results and demonstrate that orthogonal transmission and the truly optimal DF design perform almost equally well. An algorithm that makes the filter design problem especially easy to solve is presented. As a byproduct from the work on decision feedback detectors we also present some work on the problem of optimizing a Schur-convex objective under a linearly shifted, or skewed, majorization constraint. Similar to the case with a regular majorization constraint, the solution is found to be the same for the entire class of cost functions. Furthermore, it is shown that the problem is equivalent to identifying the convex hull under a simple polygon defined by the constraint parameters. This leads to an algorithm that produces the exact optimum with linear computational complexity. As applications, two unitary precoder designs for MIMO communication systems that use heterogenous signal constellations and employ DF detection at the receiver are presented.
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| 2. |
- Blum, J., et al.
(author)
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The physics of protoplanetesimal dust agglomerates : I. Mechanical properties and relations to primitive bodies in the solar system
- 2006
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 652:2, s. 1768-1781
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Journal article (peer-reviewed)abstract
- We present laboratory experiments on the formation of macroscopic dust aggregates. The centimeter-sized highly porous bodies are produced by random ballistic deposition from individual micrometer-sized dust particles. We find packing densities between 0.07 and 0.15 for uncompressed samples, dependent on the shape and size distribution of the constituent dust grains. Impacts into these bodies are simulated by uniaxial compression experiments. We find that the maximum compression, equivalent to the highest protoplanetary impact velocities of similar to 50ms(-1), increases the packing density to 0.20-0.33. Tensile strength measurements with our laboratory samples yield values in the range 200-1100 Pa for slightly compressed samples. We review packing densities and tensile strengths found for primitive solar system bodies, e. g., for comets, primitive meteorites, and meteoroids. We find a consistency between packing densities and tensile strengths of our laboratory samples with those from cometary origin.
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| 3. |
- Davidsson, Björn J. R.
(author)
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Comet Knudsen layers
- 2008
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In: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 138:1-4, s. 207-223
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Journal article (peer-reviewed)abstract
- This paper reviews some important results about Knudsen layers obtained in theoretical gas kinetics research in the last few decades, focusing on the weak and strong evaporation problems in two-surface, half-space, and spherical geometries. Furthermore, the application of such results in cometary science is reviewed. In order to illustrate some properties of the half-space evaporation problem for water ice surfaces at temperatures relevant for active comets, a number of numerical Direct Simulation Monte Carlo calculations are presented.
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| 4. |
- Davidsson, Björn J. R., et al.
(author)
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Nucleus properties of Comet 9P/Tempel 1 estimated from non-gravitational force modeling
- 2007
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In: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 187:1, s. 306-320
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Journal article (peer-reviewed)abstract
- The nucleus mass and bulk density of Comet 9P/Tempel 1 have been estimated by utilizing the non-gravitational force modeling technique. Here, the water production rates and non-gravitational perturbations of the orbit are calculated for a large number of model nuclei with different surface ice distribution patterns. By requiring that the empirical water production rate curve is reproduced, a subset of model nuclei are selected, for which masses are calculated by demanding that empirical non-gravitational changes of the orbital period and in the longitude of perihelion (per revolution) are reproduced. We obtain a mass M=5.8(±1.6)×1013 kg, and a bulk density , which compares very well with measurements made by the Deep Impact Science Team. The main goal of the current work is therefore to demonstrate functionality of an indirect method, i.e., mass estimation through non-gravitational force modeling, by comparing such results to ground truth data. Furthermore, the thermal inertia of active areas is estimated as 30–100 MKS, using a comparatively realistic thermophysical model (although a value in the range 100–350 MKS is obtained with a more simple model). An active area fraction of 3% is predicted, and these areas are probably confined to the northern hemisphere, being located close to the cometary equator.
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| 5. |
- Davidsson, Björn J. R., et al.
(author)
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Physical properties of morphological units on Comet 9P/Tempel 1 derived from near-IR Deep Impact spectra
- 2009
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In: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 201:1, s. 335-357
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Journal article (peer-reviewed)abstract
- In this paper we analyze near-infrared thermal emission spectra of the spatially resolved nucleus Of Comet 9P/Tempel 1 obtained by the NASA spacecraft Deep Impact. Maps of spectral reddening. the product X' between the beaming function and directional emissivity, as well as Suit ace temperature are constructed. Thermophysical modeling is used to estimate the degree of small scale surface toughness and thermal inertia by detailed reproduction of the empirical temperature map. Mie and Hapke theories are Used in combination with numerically Calculated beaming functions to analyze the X' trial and place constraints oil composition and grain size of the Surface material. We show that it is absolutely mandatory to include small scale Surface roughness in thermophysical modeling of this object, since the resulting self treating is vital for reproducing the measured temperatures. A small scale self heating parameter in the range 0.6 <= xi <= 0.75 is common, but smoother areas where 0.2 <= xi <= 0.3 are also found. Contrary to models neglecting small scale surface roughness, we find that the thermal inertia of Comet 9P/Tempel 1 generally is high (1000-3000 J m(-1) K-1 s(-1/2)). although it may be substantially lower (40-380 Jm(-2) K-1 s(-1/2)) in specific areas. We obtain a disk-averaged reddening of 3.5% kA(-1), with statistically significant local variations around that value on a +/- 1.0% kA(-1) level. vast regions appear covered by small (similar to 0.1 mu m) highly absorbing grains such as carbon or iron-rich silicates. Other regions appear dominated by somewhat larger (similar to 0.5 mu m) and/or less absorbing grains such as troilite or magnesium-rich silicates. Surface variations in reddening, roughness, thermal inertia, composition and/or grain size are moderately to strongly correlated to the locations of morphological units oil the surface. The existence of morphological units with differing physical properties may be primordial. hence reflecting a diversity in the building block cometesimals, or resulting front evolutionary processes. (c) 2009 Elsevier Inc. All rights reserved.
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| 6. |
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| 7. |
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| 8. |
- Davidsson, Björn
(author)
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The bulk density of cometary nuclei
- 2006
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In: Advances in Geosciences. - : World Scientific Publishing, Singapore. - 9812569839
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Book chapter (peer-reviewed)
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| 9. |
- Davidsson, Henrik, et al.
(author)
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Performance of a multifunctional PV/T hybrid solar window
- 2008
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In: [Host publication title missing]. ; , s. 44-51
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Conference paper (peer-reviewed)abstract
- A PV/T collector have been developed and evaluated at the department of Energy and Building Design at the Technical University of Lund, LTH in Sweden. The PV/T, a “multifunctional solar window” made of PV cells laminated on solar absorbers, is placed in a window behind the glazing. The solar window is built into a single family house, Solgården, in Älvkarleö outside Gävle in the eastern part of Sweden. To reduce the costs of the solar electricity, reflectors have been placed in the construction to focus the radiation onto the solar cells. In this way expensive solar cells can be replaced by considerably cheaper reflector material. The tiltable reflectors give the user a possibility to control the amount of radiation being transmitted into the building. The reflectors can also be used to reduce the thermal losses through the window. A model for electric and hot water production was developed. The simulation program, in Excel, can perform yearly energy simulations where different effects such as shading of the cells or the glazing effects can be included or excluded. The simulation can be run with the reflectors in an active, up right, position or with the reflectors in a passive, horizontal, position. The simulation program was calibrated against a prototype window placed in Lund in the south of Sweden and against the solar window in Solgården. The calculation model serves as a basis for the module written for the simulation program TRNSYS. A “TRNSYS-deck” was built and calibrated for the building Solgården. Yearly simulations of the energy balance for a house with the solar window was compared to simulations where the 16m2 solar window was replaced with an 8m2 normal window. The results show that the annual amount of auxiliary energy is lower with a developed solar window, including low-e coating on the glazing, compared to the normal window case. The developed solar window has considerable lower U-values than the existing solar window.
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| 10. |
- Davidsson, Henrik, et al.
(author)
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Performance of a multifunctional PV/T hybrid solar window
- 2008
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In: EuroSun 2008 Proceedings.
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Conference paper (other academic/artistic)abstract
- A building-integrated multifunctional PV/T collector have been developed and evaluated. The PV/T solar window is constructed of PV cells laminated on solar absorbers and is placed in a window behind the glazing. To reduce the costs of the solar electricity, reflectors have been introduced in the construction to focus radiation onto the solar cells. The tiltable reflectors render a possibility to control the amount of radiation transmitted into the building. The insulated reflectors also reduce the thermal losses through the window. A model for simulation of the electric and hot water production was developed. The model can perform yearly energy simulations where different effects such as shading of the cells or effects of the glazing can be included or excluded. The simulation can be run with the reflectors in an active, up right, position or with the reflectors in a passive, horizontal, position. The simulation program was calibrated against measurements on a prototype solar window placed in Lund in the south of Sweden and against a solar window built into a single family house, Solgården, in Älvkarleö in the middle of Sweden. The results from the simulation shows that the solar window produces about 56% more electric energy per unit cell area compared to a vertical flat PV module.
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