I would like to express my sincere thanks to my advisors, Prof. Ai-Chun Pang. Without her supervision and perspicacious advice, I can not complete this thesis. Thanks also to the colleagues in Personal Communications Services and Wireless VoIP Laboratory. I also would like to express my thanks to Dr. Jin-Cheng Huang and my dear friends Yung-Feng and Kenji for their encouragements and suggestions. Finally, I am grateful to my dear parents and brother for their unfailing love and firmly support.