First and foremost, praises and thanks to God, the Almighty, for His abundant grace and showers of blessings along this journey.I would like to express my deepest gratitude to my supervisor, Dr. Na Jin Cheon, for his guidance in the past four years. His kindness, sincerity, vision, and motivation have deeply inspired me. It has been a priceless experience working with him. I really appreciate him, both professionally and personally. I am also grateful to my family and church members for their love, support, and sacrifices. Without their prayers, this challenging endeavor would not have been possible. Finally, my thanks go to all the people who have supported me directly and indirectly throughout this research work. viiTABLE OF CONTENTS Statement of Originality .