Six years ago, I wanted to give myself some challenges and decided to come to Spain for PhD study. The beginning was hard, either in the sense of language or research. I have to say that I am really lucky that I have met lots of good people who have helped me a lot. I would like to give my sincere thanks to all my colleagues from CeDInt, the classmates and professors from UPM, the colleagues from ETH Zurich and my friends who have been always supporting me. First, I would like to give my special thank to my supervisor Asun, for providing me the opportunity to work at CeDInt and all the help during the preparation of this thesis. Second, I would like to give my thanks to all the people of CeDInt, I remember all the help that you have given me, six years have been quite a long time, or better to say, eight years, since my first time here. Thanks for