1. Introduction

The rapid growing data traffic brings more and more pressure to the wireless networks, which is predicted to increase by over 10,000 times in the next 20 years. In order to improve the system capacity and satisfy the increasing demands, ultra-dense networks (HetUDNs), massive MIMO, and new spectrum of millimeter wave band are considered as promising methods in future terrestrial networks, especially for the fifth generation (5G). Meanwhile, mobile service coverage is also crucial for the development and economics. However, currently, there are still large number of population without coverage of mobile service. By 2015, there are about 71% rural population that are not yet covered. Furthermore, for the less developed countries, the population that have access to mobile broadband service is only 12%. In addition to the issue of coverage, future wireless networks also need to guarantee the service continuity for emerging services such as Machine-to-Machine and Internet of Things. Many emerging scenarios, such as Unmanned Aerial Vehicle (UAV) control, pipeline monitoring, or global container tracking, require the aid of wireless networks. However, existing networks are severely hindering these services from achieving their true potentials. Considering the deficiency in existing wireless networks nowadays, ubiquitous on-demand coverage is the key to ensure service continuity in future communication paradigm.