I would like to express my sincere thanks and appreciation to my supervisor Dr. Guoan Bi for his invaluable guidance, support and suggestions. His knowledge, suggestions, and discussions help me to become a capable researcher. His encouragement also helps me to overcome the difficulties encountered in my research. I also want to thank my senior graduate fellow Dr. Xiaolei Lv for his generous help, technical and personal suggestions. Without his guidance, insightful discussions and continuous encouragements, I could not have achieved today's results. I am very grateful to the members of our team in NTU. It is a pleasure to collaborate with my team mates, Dr.