Contents I. Introduction
Recently, communication demand has been increasing with higher requirement on the manner of accessing information at any-time, from anywhere and in any situation. Increasingly the services offered are becoming an indispensable part of daily life. A ubiquitous network, therefore, is necessary to satisfy the demand regardless of geographical locations [1]. In such communication network, heterogeneous devices have to be connected to form a universal network. However, in current wireless networks, operators set up and manage their own radio resources and networks separately. This results in high costs and prevents quick deployment of new wireless services, especially in the areas which lack broadband infrastructure like in-building, underground and rural areas [2].
