Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar. To my father. Acknowledgments First, I would like to express my gratitude to my advisor Professor Dr.-Ing. Hermann Ney, head of the Chair of Computer Science 6 at the RWTH Aachen University. This thesis would not have been possible without his support, advice, and valuable ideas and suggestions. I would also like to thank Professor Dr. José B. Mariño from the Polytechnic University of Catalonia for agreeing to review this thesis and for his precious comments and interest in this work. I would very much like to thank my colleagues at the Chair of Computer Science 6 for motivating discussions, helpful feedback, and for assisting me in proofreading this thesis. I greatly appreciate the very good working atmosphere they helped to create. In particular, I would like to express my deep gratitude to my former and current colleagues Stephan Kanthak, Richard Zens, Gregor Leusch, Björn Hoffmeister, Arne Mauser, Nicola Ueffing, and David Vilar. Many of the results obtained for this thesis would not have been possible without their prior work and the research that we discussed and/or conducted together.