Within this thesis, the satellite broadcast scenario of geostationary satellites is reviewed. The densely crowded geostationary arc in the common broadcast frequencies may create significant interference from adjacent satellites (ASI). The possible use of multiple-input receivers and of interference processing techniques is analyzed in this specific context. In addition the synchronization problem is studied under interference limited conditions for broadcast as well as broadband satellite systems.We address fixed satellite broadcast reception with the goal of decreasing the aperture of the receiving antenna. The front-end antenna size is commonly defined by the presence of interference from adjacent satellites. A small antenna aperture leads to interference from neighboring satellites utilizing the same frequency bands. We propose a multi-input reception system with subsequent joint detection which provides reliable communication in the presence of multiple interfering signals. An iterative least square technique is adopted combining spatial and temporal processing. This approach achieves robustness against pointing errors and against changing interference scenarios. Different temporal interference processing methods are evaluated, including Minimum Mean Square Error (MMSE) based iterative soft-decision interference cancellation as well as Iterative Least Square with Projection (ILSP) based approaches, which include spatial and temporal iterations. Furthermore the potential of an additional convolutional channel decoding step in the interference cancellation mechanism is verified.Also, we demonstrate how to accurately synchronize the signals as part of the detection procedure. The technique is evaluated in a realistic simulation study representing the conditions encountered in typical broadcast scenarios.In a second part of the thesis the problem of synchronization is reviewed in the context of interference limited scenarios for broadband satellite return channels. Spectral efficiency is of great concern in the return channel of satellite based broadband systems. In recent work the feasibility of increased efficiency by reducing channel spacing below the Symbol Rate was demonstrated using joint detection and decoding for a synchronized system. We extend this work by addressing the critical synchronization problem in the presence of adjacent channel interference (ACI) which limits performance as carrier spacing is reduced.A pilot sequence aided joint synchronization scheme for a multi-frequency time division multiple access (MF-TDMA) system is proposed. Based on a maximum likelihood (ML) criterion, the channel parameters, including frequency, time and phase are jointly estimated for the channel of interest and the adjacent channels. The impact of ACI on the synchronization and detection performance is investigated. It is shown that joint channel parameter estimation outperforms single carrier synchronization with reasonable additional computational complexity in the receiver. Based on the proposed synchronization scheme in conjunction with an appropriate joint detection mechanism the carrier spacing can be reduced significantly compared to current systems providing a substantial increase in spectral efficiency / QC 20100727

This thesis deals with three estimation problems motivated by spatial signal processing using arrays of sensors. All three problems are approached using tools from estimation theory, including asymptotical analysis of performance and Cramér-Rao lower bound; Monte Carlo methods are used to evaluate small sample performance. The first part of this thesis treats direction of arrival estimation for narrowband signals. Most algorithms require the noise covariance matrix to be known or to possess a known structure. In many cases, the noise covariance is estimated from a separate batch of signal-free samples; in a non-stationary environment this sample set can be small. By deriving the Cramér-Rao bound in a form that can be compared to well-known results, we investigate the combined effects of finite sample sizes, both in the estimated noise covariance matrix and in the data with signals present. Under the same data model, we derive the asymptotical covariance of weighted subspace fitting, where the signal-free samples are used for whitening. The obtained expression suggests optimal weights that improve performance compared to the standard choice and that result in an asymptotically efficient estimate. In addition, we propose a new, asymptotically efficient, method based on the likelihood function. If the array is uniform and linear, then an iterative search can be avoided. We propose two such algorithms, based on the two general, iterative, algorithms discussed. We also treat the detection problem, and provide results that are useful in a joint detection and estimation algorithm based on the proposed estimators. Parameter estimation for the reduced rank linear regression is the second estimation problem treated in the thesis. It appears in, for example, system identification and signal processing for communications. We propose a new method based on instrumental variable principles and we analyze its asymptotical performance. The new method is asymptotically efficient if the noise is temporally white, and outperforms previously suggested algorithms when the noise is temporally correlated. As part of the estimation algorithm, the closest low rank approximation of a matrix, as measured under a weighted norm, has to be calculated. This problem lacks solution in the general case. We propose two new methods that can be computed in fixed time; both methods are approximate but asymptotically optimal as part of the estimation procedure in question. We also propose a new algorithm for the related rank detection problem. The third problem is that of estimating the covariance matrix of a multivariate stochastic process. In some applications, the structure of the problem suggests that the underlying, true, covariance matrix is the Kronecker product of two matrix factors. The covariance matrix of the channel realizations in multiple input multiple output (MIMO) communications systems can, under certain assumptions, have such Kronecker product structure. Moreover, the factor matrices can sometimes, in turn, be assumed to possess additional structure. We propose two asymptotically efficient estimators for the case where the channel realizations can be assumed known. Both estimators can be computed in fixed time; they differ in their small sample performance and in their ability to incorporate extra structure in the Kronecker factors. In a practical MIMO system, the channel realizations have to be estimated from training data. If the amount of training data is limited, then it is better to treat the training data, rather than the channel estimates, as inputs to the channel covariance estimator. We derive and analyze an estimator based on this new data model. This estimate can be computed in fixed time and the estimator is also able to optimally use extra structure in the factor matrices / QC 20100820

In this thesis a sampling time domain measurement system primarily intended for measurements on radio frequency power amplifiers is discussed. The need for such a measurement system is established. Impairments due to non-ideal measurement instruments are discussed as well as methods to compensate for these impairments. Techniques to improve upon the raw measurement performance of the measurement instruments with regard to bandwidth, dynamic range, linear and nonlinear distortion are discussed. | A method to simultaneously find the phase and amplitude ripple of a vector signal generator and a vector signal analyzer is presented. The method is verified with extensive measurements. Two techniques, frequency stitching and Zhu’s generalized sampling theorem, to extend the effective measurement bandwidth of the measurement system is discussed and evaluated with measurements. They are both found to be able to extend the effective bandwidth for measurements of output signals of nonlinear power amplifiers with more than five times. The measurement system is used for sampled input – output measurements of power amplifiers and the obtained data are fitted to different behavioral power amplifier models including memory. Some different behavioral models are evaluated and compared for different kinds of power amplifiers. A neural network model and extensions to the well-known parallel Hammerstein model are specifically discussed. The parallel Hammerstein model are also used together with frequency stitching and Zhu’s generalized sampling theorem. A general hardware and software structure of a versatile measurement system based on virtual instruments for measurements on power amplifiers is discussed in some detail. Special attention is given to the software architecture and to the concepts of hardware and software reusability. An automated, fast, accurate and production-friendly method for two-tone power and frequency sweep measurements, including measurement of the phase of the intermodulation products in addition to the amplitude, is also presented. / QC 20100823

Spatial reuse TDMA has been proposed as an access scheme for multi-hop radio networks where real-time service guarantees are important. The idea is to allow several radio terminals to use the same time slot when possible. A time slot can be shared when the radio units are geographically separated such that small interference is obtained. The transmission rights of the different users are described with a schedule. In this thesis we will study various aspects of STDMA scheduling. A common thread in these various aspects is the use of an interference-based network model, as opposed to a traditional graph-based network model. While an interference-based network model is more complex than a graph-based model, it is also much more realistic in describing the wireless medium. An important contribution of this thesis is a comparison of network models where we show that the limited information of a graph model leads to significant loss of throughput as compared to an interference-based model, when performing STDMA scheduling. The first part ot this thesis is a study of assignment strategies for centralized scheduling. Traditionally, transmission rights have been given to nodes or to links, i.e., transmitter/receiver pairs. We compare these two approaches and show that both have undesirable properties in certain cases. Furthermore, we propose a novel assignment strategy, achieving the advantages of both methods. Next we investigate the effect of a limited frame length on STDMA schedules. We first show that the required frame length is larger for link assignment than for node assignment. Further, we propose a novel assignment strategy, the joint node and link assignment, that has as low frame length requirements as node assignment but with the capacity of link assignment. In the last part of this thesis we describe a novel interfence-based distributed STDMA algorithm and investigate its properties, specifically its overhead requirement. In addition we show that this algorithm can generate as good schedules as a centralized algorithm can. / QC 20101015

Spectrum regulation is tricky and until recently the methods used for almost a century has sufficed. But as wireless communication has increased the demands on spectrum has increased. The regulators have responded by relaxing the current regulatory framework as well as opening up more bands for license exempt or unlicensed operation. In unlicensed spectrum users can be expected to act greedily and possibly also break etiquette rules. Using game theory we find that in most cases a user benefits form acting greedily and this decrease total system capacity. It is possible to deter a user from cheating by applying punishment to the user. This function should preferably be incorporated in the access network. We also study the case of networks competing in unlicensed spectrum and find that the most successful network is the one with lowest quality guarantees and with the most dense access network. In the case studied here the greedy behavior of the networks increases the spectrum utilization. We also evaluate a number of cases where two networks that cooperate in unlicensed spectrum. Isolation between the networks is the key factor to achieve better performance than splitting the spectrum. The evaluations are carried out using numerical experiments and game theory. Game theory ia a powerful tool for modelling coexistence problems in unlicensed spectrum, but the systems are too complex to allow a fully analytical treatment. / QC 20101012

Die Tagungsreihe zur Hochschuldidaktik der Informatik HDI wird vom Fachbereich Informatik und Ausbildung / Didaktik der Informatik (IAD) in der Gesellschaft für Informatik e. V. (GI) organisiert. Sie dient den Lehrenden der Informatik in Studiengängen an Hochschulen als Forum der Information und des Austauschs über neue didaktische Ansätze und bildungspolitische Themen im Bereich der Hochschulausbildung aus der fachlichen Perspektive der Informatik. Diese fünfte HDI 2012 wurde an der Universität Hamburg organisiert. Für sie wurde das spezielle Motto „Informatik für eine nachhaltige Zukunft“ gewählt, um insbesondere Fragen der Bildungsrelevanz informatischer Inhalte, der Kompetenzen für Studierende informatisch geprägter Studiengänge und der Rolle der Informatik in der Hochschulentwicklung zu diskutieren.

Internetbasierte Informatiksysteme beeinflussen in steigendem Maße Situationen in unterschiedlichen Lebensbereichen. Kompetenzen zur Verwendung von Internetanwendungen und -diensten müssen explizit erworben werden, weil damit ein notwendiger Einblick in nicht beobachtbare Abläufe und nicht offen sichtbare Strukturen verbunden ist. Bisher gibt es Vorschläge für die Gestaltung schulischer Lehr-Lernprozesse zu ausgewählten Teilaspekten des Internets. Es fehlt eine systematische Analyse des Bildungsbedarfs und ein daraus resultierendes Unterrichtsmodell. In dieser Arbeit wird ein Gesamtkonzept für den Informatikunterricht in der Sekundarstufe II vorgestellt, das zu zielgerichteter und verantwortungsvoller Anwendung des Internets beiträgt. Die vorliegende Arbeit umfasst den Prozess von der Analyse erforderlicher Kompetenzen bis zur Realisierung von Lehr-Lernprozessen im Informatikunterricht in der Sekundarstufe II. Es werden der Beitrag der Informatik zu identifizierten Kompetenzen untersucht und Bildungsanforderungen bestimmt. Bildungsempfehlungen und Forschungsergebnisse zu erfolgreichen Unterrichtseinheiten werden im Hinblick auf die Bildungsziele analysiert. Der Informatikunterricht unterstützt die Kompetenzentwicklung zu internetbasierten digitalen Medien. Es wird die Entwicklung eines Unterrichtsmodells zu Internetworking beschrieben. Dazu wird der Ansatz der Didaktischen Systeme untersucht, weiter entwickelt und auf den Bereich Internetworking übertragen. Der theoretische Ansatz wird dazu in vier Unterrichtsprojekten zu Internetworking in der Praxis realisiert. Beziehungen zwischen Fachkonzepten zu Internetworking werden untersucht und durch Wissensstrukturen zur Planung von Unterrichtsprojekten eingesetzt und in der Praxis erprobt. Die Beschreibung von Lernaktivitäten erfolgt auf der Basis von Aufgabenklassen, die das notwendige Wissen zur Bearbeitung einer Aufgabenstellung repräsentieren. Auf der Grundlage des Ablaufs der Aufgabenbearbeitung werden Eigenschaften von Aufgaben beschrieben und zu deren Gestaltung nutzbar gemacht. Bisher nicht durchführbare Tätigkeiten im Unterricht werden durch die Entwicklung der Lernsoftware Filius ermöglicht. Die Reduktion der komplexen Wirklichkeit durch Simulation realer internetbasierter Informatiksysteme und die Auswahl geeigneter Sichten auf den Untersuchungsgegenstand werden mit Ergebnissen der Informatikdidaktik begründet. Unterrichtsprojekte zu den Zielen werden durchgeführt, um Lehr-Lernprozesse zu erkunden und das entwickelte Didaktische System zu erproben. Ausgehend von der theoretischen Fundierung erfolgt die praktische Realisierung von Lehr-Lernprozessen. Zur Erprobung im Informatikunterricht der Sekundarstufe II in Nordrhein-Westfalen werden Minimalziele aufgrund der Lehrvorgaben bestimmt. Die methodische Gestaltung in der Erprobung erfolgt unter Berücksichtigung der Vorgaben für den Informatikunterricht und allgemeinen Anforderungen der Fachdidaktik. Handlungsorientierte Unterrichtsmittel werden ausgewählt und in der Praxis zur Untersuchung der Lehr-Lernprozesse verwendet. Im Unterricht identifizierte Lernschwierigkeiten führen zur Modifikation der Wissensstrukturen und werden im Entwicklungsprozess von Filius berücksichtigt. Die Erkenntnisse aus Unterrichtsprojekten werden genutzt, um zu bestimmen, zu welchen Aufgabenklassen weitere Aufgaben erforderlich sind und inwieweit das aus den identifizierten Merkmalen abgeleitete Vorgehen zur Entwicklung niveaubestimmender Aufgaben genutzt werden kann. Die Erprobungen bestätigen die Tragfähigkeit des Didaktischen Systems Internetworking und leisten mit der Implementierung in der Praxis einen Beitrag zur Untersuchung von Kompetenzentwicklung im Informatikunterricht. Mit dem Didaktischen System Internetworking wird ein theoretisch fundiertes und empirisch erprobtes Unterrichtsmodell zur Entwicklung von Kompetenzen zur Einrichtung und Anwendung internetbasierter Informatiksysteme beschrieben. / Internet-based informatics systems increasingly influence real-life situations. Competencies for Internet applications and services have to be explicitly attained because not observable processes and invisible structures have to be considered. There are examples of successful learning processes for selected parts of the Internet. A systematic analysis of educational requirements and a derived didactic concept for general education is missing. An overall didactic concept for informatics in secondary education is presented in this work, which contributes to goal-oriented and responsible application of the Internet. This work comprises the process from analysis of necessary competencies to realisation of learning processes in classes in secondary education. The contribution of informatics to identified competencies is examined and educational requirements are determined. Recommendations for informatics education and research results of successfully introduced classes are analysed with regard to the learning objectives. Informatics courses support the development of competencies related to Internet-based digital media. The development of a didactic concept concerning Internetworking is described. The approach of Didactic Systems is examined, elaborated, and transferred to Internetworking. The theory-based approach is realised in practice during four classroom projects. Relations between concepts about Internetworking are analysed, applied to plan classroom projects, and evaluated by means of knowledge networks. Learning activities are described on the basis of exercise classes, which represent the necessary knowledge to solve an exercise. Properties of exercises are described and utilised. Learning activities that were not realizable up to this point are facilitated by the developed learning software Filius. The reduction of complexity of real informatics systems by simulation and the selection of appropriate views of the object of examination are based on results of didactics of informatics. Classroom projects are performed to explore learning processes and to evaluate the Didactic System Internetworking. Learning processes are put into practice based on the theoretic approach. Minimal objectives for the classroom projects are determined from educational guidelines of informatics in secondary education at North Rhine-Westfalia. The methodical design of the learning processes is based on the guidelines and general requirements of didactics of informatics. Activity-oriented learning material is selected and introduced into practice to examine learning processes. Identified learning difficulties result in modification of the knowledge structures and are considered during the development of Filius. Further findings allow conclusions regarding the necessity of exercises of identified exercise classes and regarding the applicability of determined exercise properties to design assignments to define the outcome of learning processes. The classroom projects confirm the viability of the Didactic System Internetworking and contribute to further analysis of the development of competencies in informatics education. The Didactic System Internetworking provides a theory-based empirically approved didactic concept for the development of competencies to establish and to use Internet-based informatics systems.

Mit der 4. Tagung zur Hochschuldidaktik Informatik wird eine Reihe fortgesetzt, die ihren Anfang 1998 in Stuttgart unter der Überschrift „Informatik und Ausbildung“ genommen hat. Seither dienen diese Tagungen den Lehrenden im Bereich der Hochschulinformatik als Forum der Information und des Diskurses über aktuelle didaktische und bildungspolitische Entwicklungen im Bereich der Informatikausbildung. Aktuell zählen dazu insbesondere Fragen der Bildungsrelevanz informatischer Inhalte und der Herausforderung durch eine stärkere Kompetenzorientierung in der Informatik. Die eingereichten Beiträge zur HDI 2010 in Paderborn veranschaulichen unterschiedliche Bemühungen, sich mit relevanten Problemen der Informatikdidaktik an Hochschulen in Deutschland (und z. T. auch im Ausland) auseinanderzusetzen. Aus der Breite des Spektrums der Einreichungen ergaben sich zugleich Probleme bei der Begutachtung. Letztlich konnten von den zahlreichen Einreichungen nur drei die Gutachter so überzeugen, dass sie uneingeschränkt in ihrer Langfassung akzeptiert wurden. Neun weitere Einreichungen waren trotz Kritik überwiegend positiv begutachtet worden, so dass wir diese als Kurzfassung bzw. Diskussionspapier in die Tagung aufgenommen haben.

Informatics as a school subject has been virtually absent from bilingual education programs in German secondary schools. Most bilingual programs in German secondary education started out by focusing on subjects from the field of social sciences. Teachers and bilingual curriculum experts alike have been regarding those as the most suitable subjects for bilingual instruction – largely due to the intercultural perspective that a bilingual approach provides. And though one cannot deny the gain that ensues from an intercultural perspective on subjects such as history or geography, this benefit is certainly not limited to social science subjects. In consequence, bilingual curriculum designers have already begun to include other subjects such as physics or chemistry in bilingual school programs. It only seems a small step to extend this to informatics. This paper will start out by addressing potential benefits of adding informatics to the range of subjects taught as part of English-language bilingual programs in German secondary education. In a second step it will sketch out a methodological (= didactical) model for teaching informatics to German learners through English. It will then provide two items of hands-on and tested teaching material in accordance with this model. The discussion will conclude with a brief outlook on the chances and prerequisites of firmly establishing informatics as part of bilingual school curricula in Germany.

In this paper we report on our experiments in teaching computer science concepts with a mix of tangible and abstract object manipulations. The goal we set ourselves was to let pupils discover the challenges one has to meet to automatically manipulate formatted text. We worked with a group of 25 secondary school pupils (9-10th grade), and they were actually able to “invent” the concept of mark-up language. From this experiment we distilled a set of activities which will be replicated in other classes (6th grade) under the guidance of maths teachers.