| 1. |
- Andersson, Anders, 1957-
(författare)
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Numerical Conformal mappings for regions Bounded by Smooth Curves
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Inom många tillämpningar används konforma avbildningar för att transformera tvådimensionella områden till områden med enklare utseende. Ett exempel på ett sådant område är en kanal av varierande tjocklek begränsad av en kontinuerligt deriverbar kurva. I de tillämpningar som har motiverat detta arbete, är det viktigt att dessa egenskaper bevaras i det område en approximativ konform avbildning producerar, men det är också viktigt att begränsningskurvans riktning kan kontrolleras, särkilt i kanalens båda ändar. Denna avhandling behandlar tre olika metoder för att numeriskt konstruera konforma avbildningar mellan ett enkelt standardområde, företrädesvis det övre halvplanet eller enhetscirkeln, och ett område begränsat av en kontinuerligt deriverbar kurva, där begränsningskurvans riktning kan kontrolleras, exakt eller approximativt. Den första metoden är en utveckling av en idé, först beskriven av Peter Henrici, där en modifierad Schwarz-Christoffel-avbildning avbildar det övre halvplanet konformt på en polygon med rundade hörn. Med utgångspunkt i denna idé skapas en algoritm för att konstruera avbildningar på godtyckliga områden med släta randkurvor. Den andra metoden bygger också den på Schwarz-Christoffel-avbildningen, och utnyttjar det faktum att om enhetscirkeln eller halvplanet avbildas på en polygon kommer ett område Q i det inre av dessa, som till exempel en cirkel med centrum i origo och radie mindre än 1, eller ett område i övre halvplanet begränsat av två strålar, att avbildas på ett område R i det inre av polygonen begränsat av en slät kurva. Vi utvecklar en metod för att hitta ett polygonalt område P, utanför det Omega som man önskar att skapa en avbildning för, sådant att den Schwarz-Christoffel-avbildning som avbildar enhetscirkeln eller halvplanet på P, avbildar Q på Omega. I båda dessa fall används tangentpolygoner för att numeriskt bestämma den önskade avbildningen. Slutligen beskrivs en metod där en av Don Marshalls så kallade zipper-algoritmer används för att skapa en avbildning mellan det övre halvplanet och en godtycklig kanal, begränsad av släta kurvor, som i båda ändar går mot oändligheten som räta parallella linjer.
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| 2. |
- Andersson, Mattias
(författare)
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Coding for the Wiretap Channel
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We consider code design for Wyner’s wiretap channel. Optimal coding schemes for this channel require an overall code that is capacity achieving for the main channel, partitioned into smaller subcodes, all of which are capacity achieving for the wiretapper’s channel. To accomplish this we introduce two edge type low density parity check (LDPC) ensembles for the wiretap channel. For the scenario when the main channel is error free and the wiretapper’s channel is a binary erasure channel (BEC) we find secrecy capacity achieving code sequences based on standard LDPC code sequences for the BEC. However, this construction does not work when there are also erasures on the main channel. For this case we develop a method based on linear programming to optimize two edge type degree distributions. Using this method we find code ensembles that perform close to the secrecy capacity of the binary erasure wiretap channel (BEC- WT). We generalize a method of M ́easson, Montanari, and Urbanke in order to compute the conditional entropy of the message at the wire- tapper. This conditional entropy is a measure of how much information is leaked to the wiretapper. We apply this method to relatively simple ensembles and find that they show very good secrecy performance. Based on the work of Kudekar, Richardson, and Urbanke, which showed that regular spatially coupled codes are capacity achieving for the BEC, we construct a regular two edge type spatially coupled ensem- ble. We show that this ensemble achieves the whole capacity-equivocation region for the BEC-WT. We also find a coding scheme using Arıkans polar codes. These codes achieve the whole capacity-equivocation region for any symmetric binary input wiretap channel where the wiretapper’s channel is degraded with respect to the main channel.
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| 3. |
- ANDERSSON, MICHAEL, 1988
(författare)
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Microwave characterisation of electrodes and field effect transistors based on graphene
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The isolation of the two-dimensional material graphene, a single hexagonal sheet of carbon atoms, is believed to trigger a revolution in electronics. Theory predicts unprecedented carrier velocities in ideal graphene, from which ultrahigh speed graphene field effect transistors (GFETs) are envisioned. In this thesis, the prospects of GFETs for microwave receivers are investigated with the emphasis on low noise amplifiers (LNAs). A microwave amplifier at 1 GHz with 10 dB small-signal gain and 6.4 dB noise figure was realised using a mechanically exfoliated graphene flake on a SiO2 substrate. Comparable GFET performance was demonstrated with large-area graphene grown by chemical vapour deposition (CVD) on copper and transferred to SiO2. From a device level noise characterisation, the CVD GFET minimum noise figure (Fmin) in the frequency range 2-8 GHz was measured to be 2.5-5 dB and estimated by de-embedding parasitics to be 1-4 dB for the intrinsic device. However, the GFET noise is sensitive to impedance mismatch as the noise resistance is high. In addition, subharmonic resistive GFET mixers utilising the symmetry of electron and hole conduction in graphene were assessed. Conversion loss (CL) and noise figure were approximately equal and ≥20 dB and the input third order intercept point (IIP3) was ≤3.9 dBm at a local oscillator power of 2 dBm, less linear then fundamental resistive mixers. Finally, the properties of graphene and metal-graphene contacts were investigated by parameter extraction based on measurements at both DC and microwave frequencies. Using a palladium based contact, a contact resistance as low as
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| 4. |
- Andersson, Peter, 1981-
(författare)
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Optimization of Equipment for Tomographic Measurements of Void Distributions using Fast Neutrons
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis describes a novel nondestructive measuring technique for determiningspatial distributions of two-phase water flows. In Boiling Water Reactors, which compose themajority of the world's commercial nuclear reactors, this so called void distribution is of importance for safe operation.The presented measurement technique relies on fast neutron transmission tomography using portable neutron generators. Varying hardware options for such an instrument based on this technique and a prototype instrument, which is under construction, are described. The main design parameters are detailed and motivated from a performance point of view. A Paretomultiple objective optimization of the count rate and image unsharpness is presented. The resulting instrument design comprises an array of plastic scintillators for neutron detection. Such detector elements allow for spectroscopic data acquisition and subsequent reduction of background events at low energy by means of introducing an energy threshold in the analysis.The thesis includes two papers: In paper I, the recoil proton energy deposition distribution resulting from the interaction of the incoming neutrons is investigated for thin plastic scintillator elements. It is shown that the recoil proton losses have a large effect on the pulse height distribution and the intrinsic neutron detection efficiency is calculated for varying energy thresholds.In paper II the performance of the planned FANTOM device is investigated using the particle transport code MCNP5. An axially symmetric phantom void distribution is modeled and there construction is compared with the correct solution. According to the solutions, the phantom model can be reconstructed with 10 equal size ring-shaped picture elements, with a precision of better than 5 void percent units using a deuterium-tritium neutron generator with a yield of 3 · 107 neutrons per second and a measurement time of 13 h. However, it should be noted that commercial neutron generators with a factor of 103 higher yields exist and that the measurement time could decrease to less than a minute if such a neutron generator would beutilized.
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| 5. |
- Sederlin, Michael, 1989-
(författare)
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Methods for Bathymetry Informed Planning of Archipelago Transport Systems
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Archipelagos are rural regions characterized by their distinct geo-graphic layouts with many separated island communities. Water bodies hinder access to community services and make a well-functioning ferry-based public transport system the only feasible means of mass transportation. Both between islands and to connect to the mainland. Maritime environments provide great routing flexibility. However, outdated or imprecise nautical charts can hinder the safe use of such possibilities in commercial traffic. This thesis develops methods for strategic planning of archipelago transportation systems, enabling the valuation of novel fairways. Contributions are made to the fields of applied optimization and statistical modeling of spatial data considering the distinct features of archipelago environments. To improve the inference of fairways, a data fusion model for combining traditional bathymetric data and ship trajectory data from the automatic identification system (AIS) for depth inference is developed. It utilizes probabilistic modeling and survival analysis for spatial interpolation of the two qualitatively different sources of information. To handle the large amounts of automatic identification system (AIS) data and the non-stationarity cause by holes (islands) in the model region, a discretized mesh approach is used. In the second step an optimization model for joint network design and scheduling of ferry public transport, with the capability of evaluating currently unavailable fairways with opening costs. In this study, characteristic features of archipelago ferry networks are identified. These features are then expressed using a modified vehicle routing model with multiple trips. Results show that factors such as traffic requirements, operating costs and fleet composition has a large impact on the value of a fairway. In summary, the thesis develops methods for inferring novel fairways using the alternate source of information provided by ship trajectory data. To evaluate such fairways, an optimization model for assessing their scheduling value in ferry based public transport systems is developed.
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| 6. |
- Sjölund, Jens, 1987-
(författare)
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MRI based radiotherapy planning and pulse sequence optimization
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radiotherapy plays an increasingly important role in cancer treatment, and medical imaging plays an increasingly important role in radiotherapy. Magnetic resonance imaging (MRI) is poised to be a major component in the development towards more effective radiotherapy treatments with fewer side effects. This thesis attempts to contribute in realizing this potential.Radiotherapy planning requires simulation of radiation transport. The necessary physical properties are typically derived from CT images, but in some cases only MR images are available. In such a case, a crude but common approach is to approximate all tissue properties as equivalent to those of water. In this thesis we propose two methods to improve upon this approximation. The first uses a machine learning algorithm to automatically identify bone tissue in MR. The second, which we refer to as atlas-based regression, can be used to generate a realistic, patient-specific, pseudo-CT directly from anatomical MR images. Atlas-based regression uses deformable registration to estimate a pseudo-CT of a new patient based on a database of aligned MR and CT pairs.Cancerous tissue has a dierent structure from normal tissue. This affects molecular diusion, which can be measured using MRI. The prototypical diusion encoding sequence has recently been challenged with the introduction of more general waveforms. To take full advantage of their capabilities it is, however, imperative to respect the constraints imposed by the hardware while at the same time maximizing the diffusion encoding strength. In this thesis we formulate this as a constrained optimization problem that is easily adaptable to various hardware constraints.
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