<p>The overall objective of the current study is to develop a quantitative understanding of the effects of spatial variability in physical and geochemical properties of crystalline rock on the migration of radionuclides along a single fracture in bedrock. A stochastic model was developed to describe the transport of solutes in fractured rock. The model describes the migration of radionuclides along a one-dimensional path and includes the transversal diffusion into the rock matrix and sorption kinetics. By using a Lagrangian method of description we can extend the model to the description of a two-dimensional transport problem in single fractures. </p><p>This study presents the first analysis of the impact of heterogeneous mass transfer on the transport of radionuclides in rock fractures, where most of the relevant rock properties such as aperture, porosity, effective diffusivity, sorption capacity and maximum diffusion depth are defined as being spatially random. The stochastic analysis performed here reflects the uncertainty in our knowledge of the properties associated with a discrete sampling technique in site investigations.</p><p>Geostatistics of the main parameters was determined experimentally on a large number of rock samples taken from the Swedish crystalline basement. The knowledge of the covariance functions of the main rock properties is then used as a basis for a stochastic analysis. By combining the small perturbation approach with the spectral method the problem could be solved in terms of closed form solutions for the central temporal moments of the residence time probability density function. </p><p>In order to be able to distinguish between the effects of various mechanisms from the effects of heterogeneity on the migration of radionuclides, it was necessary to perform independent studies of the effect of the variation of the dispersion coefficient on the aspect ratio of a rectangular flow section and the effect of adsorption kinetics on the migration. </p><p>Finally, the effect of the heterogeneous rock properties on the solute transport observed in a limited number of migration experiments corresponds fairly well to the theoretical effect expected on the basis of the experimentally determined auto-covariance functions. </p>

In this thesis the interplay between hydrodynamic transport and specific adhesion is theoretically investigated. An important biological motivation for this work is the rolling adhesion of white blood cells experimentally investigated in flow chambers. There, specific adhesion is mediated by weak bonds between complementary molecular building blocks which are either located on the cell surface (receptors) or attached to the bottom plate of the flow chamber (ligands). The model system under consideration is a hard sphere covered with receptors moving above a planar ligand-bearing wall. The motion of the sphere is influenced by a simple shear flow, deterministic forces, and Brownian motion. An algorithm is given that allows to numerically simulate this motion as well as the formation and rupture of bonds between receptors and ligands. The presented algorithm spatially resolves receptors and ligands. This opens up the perspective to apply the results also to flow chamber experiments done with patterned substrates based on modern nanotechnological developments. In the first part the influence of flow rate, as well as of the number and geometry of receptors and ligands, on the probability for initial binding is studied. This is done by determining the mean time that elapses until the first encounter between a receptor and a ligand occurs. It turns out that besides the number of receptors, especially the height by which the receptors are elevated above the surface of the sphere plays an important role. These findings are in good agreement with observations of actual biological systems like white blood cells or malaria-infected red blood cells. Then, the influence of bonds which have formed between receptors and ligands, but easily rupture in response to force, on the motion of the sphere is studied. It is demonstrated that different states of motion-for example rolling-can be distinguished. The appearance of these states depending on important model parameters is then systematically investigated. Furthermore, it is shown by which bond property the ability of cells to stably roll in a large range of applied flow rates is increased. Finally, the model is applied to another biological process, the transport of spherical cargo particles by molecular motors. In analogy to the so far described systems molecular motors can be considered as bonds that are able to actively move. In this part of the thesis the mean distance the cargo particles are transported is determined. / In der vorliegenden Arbeit wird das Zusammenspiel zwischen hydrodynamischem Transport und spezifischer Adhäsion theoretisch untersucht. Eine wichtige biologische Motivation für diese Arbeit ist die rollende Adhäsion weißer Blutkörperchen, die experimentell in Flusskammern untersucht wird. Die spezifische Adhäsion wird durch schwache Bindungen zwischen komplementären molekularen Bausteinen vermittelt, die sich einerseits auf der Zelloberfläche, Rezeptoren genannt, andererseits auf der unteren begrenzenden Platte der Flusskammer, Liganden genannt, befinden. Das untersuchte Modellsystem besteht aus einer festen Kugel, die mit Rezeptoren bedeckt ist und sich unter dem Einfluss einer einfachen Scherströmung, deterministischer Kräfte und der Brownschen Molekularbewegung oberhalb einer mit Liganden bedeckten Wand bewegt. Es wird ein Algorithmus angegeben, mit dessen Hilfe diese Bewegung sowie das Entstehen und Reißen von Bindungen zwischen Rezeptoren und Liganden numerisch simuliert werden kann. In der numerischen Modellierung werden die Positionen von Rezeptoren und Liganden räumlich aufgelöst, wodurch sich die Möglichkeit ergibt, die Ergebnisse auch mit Flusskammerexperimenten, in denen moderne nanotechnologisch strukturierte Substrate verwendet werden, zu vergleichen. Als Erstes wird der Einfluss von Strömungsrate sowie Zahl und Form der Rezeptoren bzw. Liganden auf die Wahrscheinlichkeit, mit der es zu einer Bindung kommen kann, untersucht. Hierfür wird die mittlere Zeit bestimmt, die vergeht bis zum ersten Mal ein Rezeptor mit einem Liganden in Kontakt kommt. Dabei stellt sich heraus, dass neben der Anzahl der Rezeptoren auf der Kugel insbesondere der Abstand, welchen die Rezeptoren von der Oberfläche haben, eine große Rolle spielt. Dieses Ergebnis ist in sehr guter Übereinstimmung mit tatsächlichen biologischen Systemen wie etwa weißen Blutkörperchen oder mit Malaria infizierten roten Blutkörperchen. Als Nächstes wird betrachtet, welchen Einfluss Bindungen haben, die sich zwischen Rezeptoren und Liganden bilden, aber unter Kraft auch leicht wieder reißen. Dabei zeigt sich, dass verschiedene Bewegungstypen auftreten, beispielsweise Rollen, deren Erscheinen in Abhängigkeit wichtiger Modellparameter dann systematisch untersucht wird. Weiter wird der Frage nachgegangen, welche Eigenschaften von Bindungen dazu führen können, dass Zellen in einem großen Bereich von Strömungsraten ein stabiles Rollverhalten zeigen. Abschließend wird das Modell auf einen etwas anderen biologischen Prozess angewendet, nämlich den Transport kugelförmiger Lastpartikeln durch molekulare Motoren. In Analogie zu den bisher beschriebene Systemen können diese molekularen Motoren als sich aktiv bewegende Bindungen betrachtet werden. In diesem Teil der Arbeit wird ermittelt, wie weit die Lastpartikel im Mittel transportiert werden.

The overall objective of the current study is to develop a quantitative understanding of the effects of spatial variability in physical and geochemical properties of crystalline rock on the migration of radionuclides along a single fracture in bedrock. A stochastic model was developed to describe the transport of solutes in fractured rock. The model describes the migration of radionuclides along a one-dimensional path and includes the transversal diffusion into the rock matrix and sorption kinetics. By using a Lagrangian method of description we can extend the model to the description of a two-dimensional transport problem in single fractures. This study presents the first analysis of the impact of heterogeneous mass transfer on the transport of radionuclides in rock fractures, where most of the relevant rock properties such as aperture, porosity, effective diffusivity, sorption capacity and maximum diffusion depth are defined as being spatially random. The stochastic analysis performed here reflects the uncertainty in our knowledge of the properties associated with a discrete sampling technique in site investigations. Geostatistics of the main parameters was determined experimentally on a large number of rock samples taken from the Swedish crystalline basement. The knowledge of the covariance functions of the main rock properties is then used as a basis for a stochastic analysis. By combining the small perturbation approach with the spectral method the problem could be solved in terms of closed form solutions for the central temporal moments of the residence time probability density function. In order to be able to distinguish between the effects of various mechanisms from the effects of heterogeneity on the migration of radionuclides, it was necessary to perform independent studies of the effect of the variation of the dispersion coefficient on the aspect ratio of a rectangular flow section and the effect of adsorption kinetics on the migration. Finally, the effect of the heterogeneous rock properties on the solute transport observed in a limited number of migration experiments corresponds fairly well to the theoretical effect expected on the basis of the experimentally determined auto-covariance functions.

Shipment Consolidation (SCL) is a logistics strategy that combines two or more orders or shipments so that a larger quantity can be dispatched on the same vehicle to the same market region. This dissertation aims to emphasize the importance and substantial cost saving opportunities that come with SCL in a logistics supply chain, by offering new models or by improving on the current body of literature. Our research revolves around "three main axes" in SCL: Single-Item Shipment Consolidation (SISCL), Multi-Item Shipment Consolidation (MISCL), and Pricing and Shipment Consolidation. We investigate those topics by employing various Operations Research concepts or techniques such as renewal theory, dynamic optimization, and simulation. In SISCL, we focus on analytical models, when the orders arrive randomly. First, we examine the conditions under which an SCL program enables positive savings. Then, in addition to the current SCL policies used in practice and studied in the literature, i.e. Quantity-Policy (Q-P), Time-Policy (T-P) and Hybrid Policy (H-P), we introduce a new one that we call the Controlled Dispatch Policy (CD-P). Moreover, we provide a cost-based comparison of those policies. We show that the Q-P yields the lowest cost per order amongst the others, yet with the highest randomness in dispatch times. On the other hand, we also show that, between the service-level dependent policies (i.e. the CD-P, H-P and T-P), H-P provides the lowest cost per order, while CD-P turns out to be more flexible and responsive to dispatch times, again with a lower cost than the T-P. In MISCL, we construct dispatch decision rules. We employ a myopic analysis, and show that it is optimal, when costs and the order-arrival processes are dependent on the type of items. In a dynamic setting, we apply the concept of time-varying probability to integrate the dispatching and load planning decisions. For the most common dispatch objectives such as cost per order, cost per unit time or cost per unit weight, we use simulation and observe that the variabilities in both cost and the optimal consolidation cycle are smaller for the objective of cost per unit weight. Finally on our third axis, we study the joint optimization of pricing and time-based SCL policy. We do this for a price- and time-sensitive logistics market, both for common carriage (transport by a public, for-hire trucking company) and private carriage (employing one's own fleet of trucks). The main motivation for introducing pricing in SCL decisions stems from the fact that transportation is a service, and naturally demand is affected by price. Suitable pricing decisions may influence the order-arrival rates, enabling extra savings. Those savings emanate from two sources: Scale economies (in private carriage) or discount economies (in common carriage) that come with SCL, and additional revenue generated by employing an appropriate pricing scheme. Throughout the dissertation, we offer numerical examples and as many managerial insights as possible. Suggestions for future research are offered.

Shipment Consolidation (SCL) is a logistics strategy that combines two or more orders or shipments so that a larger quantity can be dispatched on the same vehicle to the same market region. This dissertation aims to emphasize the importance and substantial cost saving opportunities that come with SCL in a logistics supply chain, by offering new models or by improving on the current body of literature. Our research revolves around "three main axes" in SCL: Single-Item Shipment Consolidation (SISCL), Multi-Item Shipment Consolidation (MISCL), and Pricing and Shipment Consolidation. We investigate those topics by employing various Operations Research concepts or techniques such as renewal theory, dynamic optimization, and simulation. In SISCL, we focus on analytical models, when the orders arrive randomly. First, we examine the conditions under which an SCL program enables positive savings. Then, in addition to the current SCL policies used in practice and studied in the literature, i.e. Quantity-Policy (Q-P), Time-Policy (T-P) and Hybrid Policy (H-P), we introduce a new one that we call the Controlled Dispatch Policy (CD-P). Moreover, we provide a cost-based comparison of those policies. We show that the Q-P yields the lowest cost per order amongst the others, yet with the highest randomness in dispatch times. On the other hand, we also show that, between the service-level dependent policies (i.e. the CD-P, H-P and T-P), H-P provides the lowest cost per order, while CD-P turns out to be more flexible and responsive to dispatch times, again with a lower cost than the T-P. In MISCL, we construct dispatch decision rules. We employ a myopic analysis, and show that it is optimal, when costs and the order-arrival processes are dependent on the type of items. In a dynamic setting, we apply the concept of time-varying probability to integrate the dispatching and load planning decisions. For the most common dispatch objectives such as cost per order, cost per unit time or cost per unit weight, we use simulation and observe that the variabilities in both cost and the optimal consolidation cycle are smaller for the objective of cost per unit weight. Finally on our third axis, we study the joint optimization of pricing and time-based SCL policy. We do this for a price- and time-sensitive logistics market, both for common carriage (transport by a public, for-hire trucking company) and private carriage (employing one's own fleet of trucks). The main motivation for introducing pricing in SCL decisions stems from the fact that transportation is a service, and naturally demand is affected by price. Suitable pricing decisions may influence the order-arrival rates, enabling extra savings. Those savings emanate from two sources: Scale economies (in private carriage) or discount economies (in common carriage) that come with SCL, and additional revenue generated by employing an appropriate pricing scheme. Throughout the dissertation, we offer numerical examples and as many managerial insights as possible. Suggestions for future research are offered.

This thesis attempts to resolve the contradictory relationship between the ecological merits of wood construction and the significant material intensity of recent free form timber surface structures. The building industry is now adept in the design and construction of freeform surface architecture, however new challenges have been introduced with the environmentally conscious desire to build these structures in wood. Lacking the formal versatility of steel and concrete, wood introduces a great deal of difficulty in the realization of complex form at an architectural scale. Powerful digital design and fabrication tools have recently made it possible to model, analyze and construct these buildings, but at the cost of heavy structural solutions that involve energy intensive fabrication processes and significant material waste. This approach contradicts the ecological benefits of wood, and raises the question of whether it is possible to achieve free and expressive form in timber surface architecture while maintaining an economy of means and material. This question is addressed through the development of a generative design tool for the creation of material conscious free form timber surface architecture. The formation of the tool is informed by the field of computational morphogenesis, which draws from the natural growth processes of biological structures in the virtual synthesis of form. The tool is conceived as a morphogenetic material system, which consists of a generative algorithm that integrates material, structure and form in a single computational process. Specific material saving techniques deployed in the algorithm draw from existing research in timber shell design and material optimization. Established methods in the use of geodesic lines for the structural patterning of wood shells and stress driven material distribution make up the core concepts deployed in the algorithm. The material system is developed, refined and tested through the design and construction of an experimental free form timber lattice.

The presented work wants to contribute a new solution based on the analysis of stochastic signals. On the basis of the Dresden Live-Camera-System which is providing real-time information about the traffic state on 31 focal points (March 2005) one is able to use a wide range of image types under different and severe conditions. The method is based on the analysis of stochastic signals derived from the live-camra images. These signals are analyzed with cross-correlation, amplitude- and frequency filters. The resulting data enable the distinction of stable and non-stable traffic flow and the automated identification of traffic congestion. Moreover fundamental diagrams are calculated. / Die vorgelegte Arbeit beinhaltet ein neuartiges Verfahren zur automatischen Ermittlung des Verkehrszustandes aus Live-Kamera-Bildern. Die Datengrundlage dafür liefert das im Rahmen des BMBF-Leitprojektes intermobil Region Dresden geschaffenen Live-Kamerasystem mit 31 Standorten (Stand März 2005). Das entwickelte Verfahren basiert auf der Analyse stochastischer Signale, die aus den Kamerabildern ermittelt werden. Die methodischen Grundlagen des Verfahrens sind Korrelationsanalyse sowie Amplituden- und Frequenzfilterung. Die Rahmenbedingungen für den praktischen Einsatz sind durch die Variabilität von Auflösung und Blickwinkel an den unterschiedlichen Kamerastandorten geprägt. Die ermittelten Messwerte ermöglichen eine Unterscheidung der Verkehrszustände &amp;quot;flüssiger Verkehr&amp;quot;, &amp;quot;zähflüssiger Verkehr&amp;quot; und &amp;quot;Stop&amp;amp;Go&amp;quot; und werden am Fundamentaldiagramm interpretiert.

Im Rahmen der Arbeit wurden die minutengenauen Daten des Regelleistungsbedarfs (Summe aus Sekundärregelleistung und Minutenreserve) der Monate April bis Dezember des Jahres 2009 einer Regelzone einer Zeitreihenanalyse unterzogen und in Komponenten gemäß dem klassischen Komponentenmodell zerlegt. Diese sind die Trendkomponente, ermittelt durch einen gleitenden Durchschnitt mit der Länge einer Stunde, weiterhin zwei periodische Komponenten mit der Periodenlänge einer Stunde sowie der Periodenlänge eines Tages und die Restkomponente, welche mit einem ARIMA(2,1,5)-Prozess modelliert wurde. In der Zukunft sollte das erstellte Modell des Regelleistungsbedarfs durch Hinzunahme einer jahreszeitlichen Komponente noch verbessert werden. Dies war im Rahmen der Arbeit nicht möglich, da keine Daten über einen Zeitraum von mehreren Jahren vorhanden waren. Zusätzlich kann geprüft werden, inwiefern mit dem Komponentenmodell Prognosen durchführbar sind. Dafür sollte die Trendkomponente anders gewählt werden, da sich der hier gewählte Weg zu sehr an den Daten orientiert. Der zweite Teil der Aufgabenstellung dieser Arbeit bestand im Identifizieren inhaltlicher Komponenten, also möglicher Zusammenhänge zwischen dem Regelleistungsbedarf und verschiedenen denkbaren Ursachen. Als potentielle Ursachen wurden der Lastverlauf sowie die Windenergieeinspeisung untersucht. Zwischen der Zeitreihe des Lastverlaufs und der des Regelleistungsbedarfs bestand eine leichte positive Korrelation, zwischen der Zeitreihe der Windenergieeinspeisung und der des Regelleistungsbedarfs eine geringe negative Korrelation.

Στην απαρχή της καινούριας χιλιετίας, έχει γίνει πλέον ξεκάθαρος ο σπουδαίος ρόλος και η χρησιμότητα των ασυρμάτων τηλεπικοινωνιών και των εφαρμογών τους, η εξέλιξη των οποίων επικεντρώνεται σε μεγάλο βαθμό στις Ασύρματες Προσωπικές Τηλεπικοινωνίες (ΑΠΤ). Εξαιτίας της μεγάλης δημοτικότητας που έχουν τα συστήματα αυτά, υπάρχει συνεχής ανάγκη για αύξηση της χωρητικότητας και αναβάθμιση των παρεχόμενων υπηρεσιών ϕωνής και δεδομένων. Μια πολύ σημαντική περιοχή έρευνας για τις ΑΠΤ, που έχει προκύψει από τις παραπάνω απαιτήσεις, είναι η μελέτη και η αντιμετώπιση των προβλημάτων που δημιουργούνται στην επίδοση των δικτύων αυτών από διάφορα ϕαινόμενα που επιδρούν στο ασύρματο κανάλι. Σε αυτήν την ερευνητική περιοχή εστιάζεται και η παρούσα διδακτορική διατριβή, που έχει ως στόχο τη ϑεωρητική μελέτη της επίδοσης ασύρματων ψηφιακών τηλεπικοινωνιακών συστημάτων τα οποία υπόκεινται σε διαλείψεις. Για την καταπολέμηση των αρνητικών συνεπειών των διαλείψεων έχει υιοθετηθεί η τεχνική του διαφορισμού στο δέκτη. Σύμφωνα με αυτήν, ο δέκτης διαθέτει περισσότερες από μια κεραίες λήψης με αποτέλεσμα να λαμβάνει πολλαπλά αντίγραφα του εκπεμπόμενου σήματος, τα οποία συνδυάζει κατάλληλα ανάλογα με τις απαιτήσεις του διαχειριστή του δικτύου. Η διεθνής έρευνα για τη χρήση δεκτών διαφορισμού σε κανάλια Διαλείψεων Μικρής Κλίμακας (ΔΜΙΚ) είναι εκτενής και αφορά κυρίως κανάλια που βασίζονται στις κατανομές Rayleigh, Rice και Nakagamim. Δύο άλλες σημαντικές κατανομές για τη μοντελοποίηση των ΔΜΙΚ είναι η Weibull και, η γενίκευση της, η Γενικευμένη-Γάμα (ΓG), οι οποίες σε σύντομο χρονικό διάστημα έχουν κερδίσει εκτεταμένο επιστημονικό ενδιαφέρον στην περιοχή των ασύρματων κινητών τηλεπικοινωνιών. ΄Ομως, παρά το ενδιαφέρον αυτό, η μελέτη της επίδοσης δεκτών διαφορισμού σε κανάλια που μοντελοποιούνται με τις κατανομές αυτές, δεν έχει λάβει τις διαστάσεις που έχουν λάβει οι αντίστοιχες έρευνες για τις άλλες κατανομές. Αυτό οφείλεται σε μεγάλο βαθμό στη μεγάλη πολυπλοκότητα, από μαϑηματικής άποψης, που παρουσιάζει η στατιστική αναπαράσταση του σήματος της εξόδου πολλών τύπων δεκτών διαφορισμού που σε συνδυασμό με τη σχετικά σύνθετη μαθηματική έκφραση των κατανομών Weibull και ΓG, δημιουργεί δυσεπίλυτα προβλήματα. Στην παρούσα διατριβή, προσπερνώντας αυτές τις δυσκολίες, αρχικά μελετήθηκε η απόδοση μιας ειδικής κατηγορίας δεκτών διαφορισμού, του δέκτη Συνδυασμού Γενικευμένης Επιλογής (ΣΓΕ), ο οποίος λειτουργεί σε Ανεξάρτητα αλλά Μη-Ομοιόμορφα Κατανεμημένα (ΑΜΟΚ) κανάλια διαλείψεων Weibull. Σε αυτήν τη μελέτη εξετάζεται η περίπτωση όπου επιλέγονται οι δύο κλάδοι με το υψηλότερο λόγο σήματος προς ϑόρυβο, Signal to Noise Ratio (SNR), από τους L διαθέσιμους, ΣΓΕ(2, L). Κατά συνέπεια, με την παρουσίαση μιας καινούριας ιδιότητας για το γινόμενο των ροπών ταξινομημένων Τυχαίων Μεταβλητών (ΤΜ) Weibull, γίνεται δυνατή η εξαγωγή εκφράσεων κλειστής μορφής για τις ροπές του SNR στην έξοδο των δεκτών ΣΓΕ(2, L). Χρησιμοποιώντας αυτές τις εκφράσεις δίνονται, σε κλειστή μορφή επίσης, σημαντικά κριτήρια επίδοσης όπως είναι ο Μέσος Λόγος Σήματος προς Θόρυβο (ΜΛΣΘ) και η Ποσότητα των Διαλείψεων (ΠΔ). Επιπλέον, με την εφαρμογή της ϑεωρίας των προσεγγιστών Pad´e και της προσέγγισης μέσω της Ροπο-Γεννήτριας Συνάρτησης (ΡΓΣ), γίνεται δυνατή η μελέτη της Πιθανότητας Διακοπής της Επικοινωνίας (ΠΔΕ) και της Μέσης Πιθανότητας Σφάλματος Bit (ΜΠΣΒ) του συστήματος. Στη συνέχεια, υποθέτοντας Ανεξάρτητα και Ομοιόμορφα Κατανεμημένα (ΑΟΚ) κανάλια διαλείψεων ΓG, παρουσιάστηκε σε κλειστή μορφή έκφραση για τις ροπές του SNR στην έξοδο των δεκτών ΣΓΕ(2, L), ενώ η ΡΓΣ και η Αθροιστική Συνάρτηση Κατανομής (ΑΣΚ) εξήχθησαν υπό τη μορφή απειροσειρών. Χρησιμοποιώντας αυτές τις εκφράσεις απλοποιείται σημαντικά η μελέτη των κριτηρίων επίδοσης της ΠΔΕ και της ΜΠΣΒ. Επιπλέον, παρουσιάστηκε η χωρητικότητα του καναλιού ΓG με τη χρήση ή όχι δεκτών διαφορισμού, με βάση πολύ γνωστές τεχνικές προσαρμογής της εκπομπής. Μια άλλη πολύ σημαντική ερευνητική περιοχή που αφορά τα κανάλια διαλείψεων και επηρεάζει σημαντικά την επίδοση των ασύρματων ψηφιακών τηλεπικοινωνιακών συστημάτων που λειτουργούν σε αυτά, είναι το σύνθετο περιβάλλον διάδοσης που δημιουργείται όταν οι ΔΜΙΚ συνυπάρχουν με Διαλείψεις Μεγάλης Κλίμακας (ΔΜΕΚ). Στην παρούσα διδακτορική διατριβή, αυτό το περιβάλλον διάδοσης μοντελοποιείται με τις σύνθετες κατανομές K και γενικευμένη-K (KG). Αυτές οι κατανομές είναι αρκετά γενικές, αλλά ταυτόχρονα και μαθηματικά ευέλικτες, και έχει διαπιστωθεί ότι είναι κατάλληλες για τη μοντελοποίηση των συνθηκών του καναλιού που προκύπτει όταν οι ΔΜΙΚ συμβαίνουν ταυτόχρονα με ΔΜΕΚ. Στην περίπτωση της κατανομής KG, εξάγονται σημαντικά στατιστικά χαρακτηριστικά του SNR στην έξοδο των δεκτών διαφορισμού Συνδυασμού Μεγίστου Λόγου (ΣΜΛ), Συνδυασμού ΄Ισης Απολαβής (ΣΙΑ), Συνδυασμού Μεταγωγής και Παραμονής (ΣΜΠ) και του δέκτη Διαφορισμού Επιλογής (ΔΕ). Χρησιμοποιώντας αυτές τις εκφράσεις και υποθέτωντας ΑΜΟΚ συνθήκες διαλείψεων του καναλιού KG, εξάγονται σε κλειστή μορφή τα κριτήρια επίδοσης ΜΛΣΘ, ΠΔΕ και ΠΔ. Επιπλέον, ακολουθώντας την προσέγγιση που βασίζεται στη ΡΓΣ για τους δέκτες ΣΜΛ και ΣΜΠ και τους προσεγγιστές Pad´e για τους δέκτες ΣΙΑ και ΔΕ μελετάται η ΜΠΣΒ. Για την περίπτωση της κατανομής K, υποθέτωντας επιπλέον συσχετισμένες ΤΜ, παρουσιάζεται και μελετάται η συσχετισμένη (δι-μεταβαλλόμενη) κατανομή K, η οποία χαρακτηρίζεται από μη ομοιόμορφους παράγοντες μορφοποίησης και κλίμακας. Η σύνθετη αυτή κατανομή είναι βολική για τη μοντελοποίηση του συσχετισμένου περιβάλλοντος διαλείψεων/σκίασης, όταν η συσχέτιση μεταξύ των πλατών των σημάτων και της ισχύος τους είναι διαφορετική. Για αυτήν την κατανομή, εξάγονται γενικές εκφράσεις με απειροσειρές για τη Συνάρτηση Πυκνότητας Πιθανότητας (ΣΠΠ), την ΑΣΚ και τις από κοινού ροπές. Υποθέτοντας ομοιόμορφους παράγοντες κλίμακας και ασυσχέτιστη ισχύ σημάτων, παρέχονται απλούστερες εκφράσεις για τη ΣΠΠ, την ΑΣΚ και τη χαρακτηριστική συνάρτηση, ενώ οι από κοινού ροπές εξάγονται σε κλειστή μορφή. Επιπλέον, βασιζόμενοι σε αυτές τις ϑεωρητικές εκφράσεις της δι-μεταβαλλόμενης κατανομής K, παρουσιάζεται η ανάλυση της επίδοσης σημαντικών δεκτών διαφορισμού, όπως είναι οι δέκτες ΣΜΛ, ΣΙΑ και ΔΕ οι οποίοι λειτουργούν σε συσχετισμένα κανάλια διαλείψεων K. Για το δέκτη ΔΕ μελετάται η ΠΔΕ, ενώ για τους δέκτες ΣΜΛ και ΣΙΑ η ΜΠΣΒ. Γενικά η προτεινόμενη ανάλυση συνοδεύεται από αριθμητικά αποτελέσματα, τα οποία επιδεικνύουν ξεκάθαρα τη χρησιμότητα και την ευελιξία της ϑεωρητικής προσέγγισης. Τα αριθμητικά αυτά αποτελέσματα περιλαμβάνουν σύγκριση της επίδοσης διαφορετικών δεκτών διαφορισμού, συνθηκών καναλιού και τεχνικών διαμόρφωσης. / At the beginning of the new millennium, it becomes totaly clear the important role and usefulness of the wireless communication systems and their applications, which continue to evolute in several areas including Personal Communication Services (PCS). In this context there is a continuing demand for increased capacity and integration of the provided services, such as voice, data and other types of traffic, over radio channels. One of the key technical problem areas that has emerged out is the requirement for determining the fading effects on network performance. This also the research area of the current Philosophy diploma, which aims to the theoretical study of the performance analysis of wireless communication systems, which are subject to fading. The current study has employed diversity reception as a fading mitigation technique. According to this technique, the receiver employs more than one antennae, in order to receive multiple copies of the transmitted signal. Hence, these copies are appropriate combined in order to satisfy network administrator demands. The technical literature concerning diversity reception techniques over fading channels is quite extensive. However, it deals mainly with fading channels, which are based on Rayleigh, Rice, and Nakagami-m distributions. Additionally, important distributions for modeling small scale fading are also the Weibull and, its generalization, the generalized-gamma (ΓG), which have recently gained an increased scientific interest in the wireless mobile communication area. However, the performance analysis of diversity receivers operating over fading channels, modeled by these distributions, has not been thoroughly investigated. Reasons for that include the composite statistical representation of several diversity receivers output in conjunction with the quite complicated mathematical expressions of the Weibull and/or ΓG distributions. In this thesis, by overcoming these difficulties the performance of a class of Generalized-Selection Combining (GSC) receivers operating over independent but non identically distributed Weibull fading channels is studied. The case where the two branches with the largest instantaneous Signal-to-Noise Ratio (SNR) are selected, from a total of L available GSC(2, L), is considered. By introducing a novel property for the product of moments of ordered Weibull Random Variables (RV), convenient closed form expressions for the moments of the GSC(2, L) output SNR are derived. Using these expressions, important performance criteria, such as average output SNR and amount of fading (AF ), are obtained in closed form. Furthermore, employing the Pad´e approximants theory and the Moment-Generating Function (MGF)-based approach, outage and bit-error rate performances are studied. Additionally, by considering independent and identical distributed (iid) ΓG fading channels, the moments of the output SNR of the GSC(2, L) receivers were derived, also, in closed form, while infinite series representations were obtained for the MGF and the Cumulative Distribution Function (CDF). Using these expressions, a simplified study of the Outage Probability (OP) and Average Bit Error Probability (ABEP) performance criteria can be performed. Moreover, the capacity of the ΓG fading channel, using different adaptation policies, is also studied, with and without diversity reception. Another important research issue concerning the performance analysis of wireless digital communications systems operating over fading channels, is the composite propagation environment arising in cases where fading is superimposed on shadowing. In the current thesis this propagation environment is modeled by the K and generalized-K (KG) composite distributions. These distributions are quite general and mathematical versatile, and have been found to be appropriate for accurately modeling the channel conditions when fading occurs simultaneously with shadowing. For the KG distribution, expressions for important statistical metrics of the output SNR for the Selection Diversity (SD) receiver and Maximal Ratio Combining (MRC), Equal Gain Combining (EGC), and Switch and Stay Combining (SSC) diversity receivers are derived. Using these expressions and by considering independent but not necessarily identical distributed fading channel conditions, performance criteria, such as average output SNR, AF and OP are obtained in closed form. Moreover, following the MGFbased approach for MRC and SSC receivers, and the Pad´e approximants theory for SD and EGC receivers, the ABEP is also studied. For the case of the K distribution, assuming correlated RVs, the correlated (bivariate) K distribution, with not necessarily identical shaping and scaling parameters, is introduced and studied. This composite distribution is convenient for modeling multipath/shadowing correlated fading environments when the correlations between the signal envelopes and their powers are different. For this distribution, generic infinite series representations are derived for the Probability Density Function (PDF), the CDF and the joint moments. Assuming identical shaping parameters and uncorrelated signal powers, simpler expressions for the PDF, CDF and the Characteristic Function (CF) are provided, while the joint moments are derived in closed form. Capitalizing on these theoretical expressions for the statistical characteristics of the correlated K distribution, the performance analysis of various diversity reception techniques, such as MRC, EGC and SD, over bivariate K fading channels is presented. For the SD, the outage probability is studied, while for the MRC and EGC the ABEP is obtained. In general the proposed analysis is accompanied by several numerical evaluated results, clearly demonstrating the usefulness and flexibility of the theoretical approach. These numerical results include performance comparisons of several diversity receiver structures, employing various modulation formats and different channel conditions.

This thesis attempts to resolve the contradictory relationship between the ecological merits of wood construction and the significant material intensity of recent free form timber surface structures. The building industry is now adept in the design and construction of freeform surface architecture, however new challenges have been introduced with the environmentally conscious desire to build these structures in wood. Lacking the formal versatility of steel and concrete, wood introduces a great deal of difficulty in the realization of complex form at an architectural scale. Powerful digital design and fabrication tools have recently made it possible to model, analyze and construct these buildings, but at the cost of heavy structural solutions that involve energy intensive fabrication processes and significant material waste. This approach contradicts the ecological benefits of wood, and raises the question of whether it is possible to achieve free and expressive form in timber surface architecture while maintaining an economy of means and material. This question is addressed through the development of a generative design tool for the creation of material conscious free form timber surface architecture. The formation of the tool is informed by the field of computational morphogenesis, which draws from the natural growth processes of biological structures in the virtual synthesis of form. The tool is conceived as a morphogenetic material system, which consists of a generative algorithm that integrates material, structure and form in a single computational process. Specific material saving techniques deployed in the algorithm draw from existing research in timber shell design and material optimization. Established methods in the use of geodesic lines for the structural patterning of wood shells and stress driven material distribution make up the core concepts deployed in the algorithm. The material system is developed, refined and tested through the design and construction of an experimental free form timber lattice.