京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18801号 / 理博第4059号 / 新制||理||1584(附属図書館) / 31752 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 岩田 知孝, 教授 平原 和朗, 准教授 久家 慶子 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Being one of the fastest growing fields of engineering, wireless has gained the attention of researchers and commercial businesses all over the world. Extensive research is underway to improve the performance of existing systems and to introduce cutting edge wireless technologies that can make high speed wireless communications possible. The first part of this dissertation deals with discrete channel models that are used for simulating error traces produced by wireless channels. Most of the time, wireless channels have memory and we rely on discrete time Markov models to simulate them. The primary advantage of using these models is rapid experimentation and prototyping. Efficient estimation of the parameters of a Markov model (including its number of states) is important to reproducing and/or forecasting channel statistics accurately. Although the parameter estimation of Markov processes has been studied extensively, its order estimation problem has been addressed only recently. In this report, we investigate the existing order estimation techniques for Markov chains and hidden Markov models. Performance comparison with semi-hidden Markov models is also discussed. Error source modeling in slow and fast fading conditions is also considered in great detail. Cognitive Radio is an emerging technology in wireless communications that can improve the utilization of radio spectrum by incorporating some intelligence in its design. It can adapt with the environment and can change its particular transmission or reception parameters to execute its tasks without interfering with the licensed users. One problem that CR network usually faces is the difficulty in detecting and classifying its low power signal that is present in the environment. Most of the time traditional energy detection techniques fail to detect these signals because of their low SNRs. In the second part of this thesis, we address this problem by using higher order statistics of incoming signals and classifying them by using the pattern recognition capabilities of HMMs combined with cased-based learning approach. This dissertation also deals with dynamic spectrum allocation in cognitive radio using HMMs. CR networks that are capable of using frequency bands assigned to licensed users, apart from utilizing unlicensed bands such as UNII radio band or ISM band, are also called Licensed Band Cognitive Radios. In our novel work, the dynamic spectrum management or dynamic frequency allocation is performed by the help of HMM predictions. This work is based on the idea that if Markov models can accurately model spectrum usage patterns of different licensed users, then it should also correctly predict the spectrum holes and use these frequencies for its data transmission. Simulations have shown that HMMs prediction results are quite accurate and can help in avoiding CR interference with the primary licensed users and vice versa. At the same time, this helps in sending its data over these channels more reliably. / Ph. D.

Les sensations en provenance de notre corps se combinent pour donner lieu à des perceptions extrêmement variées, pouvant aller de la brûlure douloureuse au toucher agréable. Ces deux types d'informations dites nociceptives et non nociceptive sont traitées au sein du système nerveux somatosensoriel. Dans ce travail de thèse, nous avons modélisé et caractérisé l'activité électrique du cortex operculo-insulaire au sein des réseaux somatosensoriels non-douloureux et nociceptif, grâce à des enregistrements non-invasifs chez l'Homme. La validité du modèle en réponse à un stimulus nociceptif a été évaluée par comparaison avec des enregistrements intra-corticaux réalisés chez des patients épileptiques. Nous avons ensuite utilisé ce modèle pour déterminer si la stimulation corticale non invasive classiquement utilisée pour soulager les douleurs neuropathiques (stimulation magnétique du cortex moteur) permettait de modifier les réponses nociceptives chez des participants sains. Nous avons montré que cette intervention n'est pas plus efficace qu'une stimulation factice (placebo) sur le plan du blocage nociceptif. Finalement, nous avons tenté de stimuler directement le cortex operculo-insulaire, par trois méthodes différentes : par stimulation électrique locale, intracrânienne et par stimulations non-invasives magnétique (rTMS) et électrique (tDCS). Dans l'ensemble, les travaux présentés ici montrent comment une approche non-invasive chez l'Homme permet de caractériser et de moduler l'activité du cortex operculo-insulaire, qui pourrait être une cible intéressante pour le traitement des douleurs réfractaires / The somatosensory system participates in both non-nociceptive and nociceptive information Processing. In this thesis work, we model and characterize the electrical activity of the operculo-insular cortex within non-painful and nociceptive networks, using non-invasive electrophysiological recordings in humans. Validity of the modeled response to a nociceptive stimulus was evaluated by comparing it to intra-cranial recordings in epileptic patients, revealing excellent concordance. We went on to use this model to determine whether a technique of non-invasive cortical stimulation currently used to relieve neuropathic pain (motor cortex magnetic stimulation) was able to modulate acute nociceptive processing in healthy participants. We show that this intervention is not more efficacious than placebo stimulation in blocking nociception. This raises questions regarding the mechanisms of action of this technique in patients, which might implicate a modulation of pain perception at a higher level of processing. Finally, we attempted to stimulate the operculo-insular cortex directly, using three different methods. Low-frequency intra-cortical stimulation in epileptic, transcranial magnetic stimulation (TMS) of the same region in healthy participants and multipolar transcranial electrical stimulation (tDCS).Altogether, the studies presented here show how a non-invasive approach in humans allows characterising and modulating the activity of the operculo-insular cortex. While this region might be an interesting target for future treatment of drug-resistant pain, its stimulation in patients would require further investigation of parameters and procedures

Vessels constructed in electrically conductive materials give rise to frequency-dependent, induced magnetic fields when waves of water cause them to roll in the Earth's magnetic field. These fields, typically referred to as roll-induced magnetic vortex fields, are a component of the ship's overall signature, where signature refers to measurable quantities which can reveal or identify objects. It is crucial for military platforms to keep the signature low and thereby increase the possibilities of operation. For magnetic signatures, this is done through strategic design and construction of the platform or by using magnetic silencing systems. The signature is then decreased to minimize the risk of detection from naval mines and marine detection systems.  This report covers the initial research on the subject of an inverse source model for roll induced magnetic fields. By limiting the analysis to two basic objects and applying a time variant magnetic field to them, we induce a magnetic field which we then model. The inverse modeling is done using magnetic dipoles as sources which are placed around the area of the object. The parameters of the model are then found by applying a least squares algorithm coupled with Tikhonov regularization. The focus of this report is the configuration of this setup in terms of measurements and sources, as well as finding a proper regularization parameter. Since the applied magnetic field is dependent on the roll frequency, also the inverse model depends on a frequency parameter in addition to the geometry and material of the object. The objects here studied are of two simple geometries, a rectangular block and a hollow cylinder. Both objects are constructed in an aluminum alloy with well known material parameters. Measurement data is gathered using a numerical solver utilizing the finite element method for solving the partial differential equations. The numerical measurement data is compared to physical measurements as well. The physical measurement data is gathered by placing the objects in a Helmholtz-cage which is used to apply a homogeneous time variant magnetic field upon them. The project was carried out at the Swedish Defence Research Agency (FOI) at the department of underwater research.