Traffic Offloading in Heterogeneous Mobile Networks: the Centrally Directed and User Collaborated Options

In recent years, we have witnessed the growth of smart devices and an ever-increasing number of users in the cellular system. Mobile applications such as video streaming and online gaming become popular ways of entertainment but also consume excessive network bandwidth. To meet the high bandwidth demand, the vision of future cellular system is to form a converged infrastructure network by integrating alternative radio access technologies such as WiFi, airborne Unmanned Aerial Vehicle (UAV) and satellites into the current cellular system. The converged infrastructure network increases total bandwidth capacity so that cellular traffic can be effectively offloaded to the alternative networks. On the users' side, as smart phones today are commonly equipped with multiple network interfaces, users can toggle between different connections in the heterogeneous network environment, or even concurrently use all the available networks and aggregate their bandwidth.Nevertheless, mobile users may experience intermittent connectivity or fluctuated channel quality when using the infrastructure networks due to the nature of mobile, wireless communications. In this case, it is beneficial to establish an additional device-to-device (D2D) connection to nearby devices opportunistically and use them as temporary network access points. For delay-tolerant data such as emails, short messages, whose delivery is time insensitive, or for popular contents such as video and music chunks, they can even be delivered or shared purely using the D2D transmissions. Therefore, this kind of user collaborated network has the potential to offload cellular traffic as well, as it is ideal for the transmission of delay-tolerant data, and it also saves the redundant downloads of popular contents.This dissertation discusses the methods to realize traffic offloading in a heterogeneous mobile network environment consisting two different but related mobile network scenarios: the infrastructure and opportunistic networks. In the infrastructure network, we developed a centrally directed multipath transmission system to solve the traffic offloading problem. Our system applies the Software-Defined Networking (SDN) paradigm to centralize network control and management. Moreover, it adopts a newly emerged transport layer protocol Multipath TCP (MPTCP) that is run on mobile devices to enable multipath data transmission. MPTCP initiates parallel TCP connections using multiple network interfaces. An SDN controller is designed to manage the MPTCP connections and regulate their bandwidth usage from each connected network. Therefore, when seeing network congestion in the cellular systems, the controller will direct MPTCP traffic to go to the other available networks such as WiFi and satellites. Additionally, our designed system has the capability of real-time topology monitoring and dynamic network reconfigurations leveraging its centralized design. The proposed system is proved to be efficient in both cellular-WiFi and tactical satellite communication networks. In the opportunistic network, while technologies such as MPTCP and WiFi Direct allow mobile users to conveniently collaborate, it is yet a question whether the network itself encourages user participation and whether it prevents non-cooperative behaviors. To this end, we study the cooperative behaviors of mobile users in opportunistic network using the Evolutionary Game Theory (EGT). Through both mathematical proof and numerical simulations, we discover that cooperation is sustainable in the opportunistic network. The emergence of cooperation in the opportunistic network confirms its potential for offloading cellular traffic