This thesis is based on the work carried out in 1990 - 1994 at the Laboratory of Telecommunications Technology in the Helsinki University of Technology.

I am obliged to Professor Kauko Rahko, the head of the Laboratory of Telecommunications Technology, for providing me with the opportunity to complete this work and his support and advice.

I am very grateful to my colleague Timoleon Antonelis, M.Sc., who has done many simulations, for enjoyable team-work, and Heikki P. S. Leivo, Lis., who has checked some of my publications and has given me advice. I am also obliged to Tommi Salo, M. Sc., for useful discussions and help as well as the whole staff of the Laboratory of Telecommunications Technology for their help. I am also grateful to our secretary, Ms. Arja Hänninen, for her kind help on various practical problems.

Awarding of a research project from the Association of Electrical Engineers in Finland and the support for participating international conferences from the Academy of Finland are greatefully acknowledged.

Finally, I am deeply indebted to my wife Li Wang for her patience, support and encouragement, and to my parents for encouragement and understanding during these years. I also wish to thank my brother Jun Ma, Dr., for his advice and useful discussions.

Espoo, February 1994