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Firstly I would like to thank my supervisor Professor Pedro Sebastião for his guidance, his suggestions, criticisms, and especially the availability for the frequent review meetings that served to improve the quality of this work. I also thank Professor Américo Correia who, as co-supervisor, also provided a valuable contribution to the successful completion of this phase of my academic life. Many thanks to doctoral student Luís Gonçalves, from whom I was able to gain much experience, relevant suggestions and important data for the various approaches taken throughout the lengthy process of research and development. A sincere thank you to the Instituto de Telecomunicações for the financial support awarded to me for the development of this dissertation. To my family, especially my parents, I would like to thank for the education and for the civic and personal values transmitted to me. I would also like to thank them for the support and stability I always experienced from them, which greatly contributed this life achievement. It is also worth mentioning the herculean effort and selflessness in terms of financial management which involves supporting a loved one in getting his degree, given th