# SDN-Based Routing for Backhauling in Ultra-Dense Networks
## 1 Introduction  
Fifth-Generation (5G) wireless networks are expected to support a high variety of services and applications, with different quality-of-service requirements and network functions, in a cost-effective manner [1]. Disruptive changes in mobile networks are needed for supporting such an evolution. In particular, access-point densification (i.e., Ultra-Dense Networks, UDN) is one of the key elements of 5G to achieve the desired increase of the network capacity. The transmitter and the receiver are moved closer together, creating a multi-layer heterogeneous architecture where many low-power small cells, such as microcells, picocells, and femtocells are overlaid on high-power macrocells. Small cells can be deployed anywhere by operators and end users [2]. However, small-cell deployment on large scale is challenging. A certain level of centralization is required to manage inter-cell interference, mobility, load balancing and coordinated transmission schemes [3]. The traffic generated by cells (both data and coordination traffic) must be routed through the backhaul network toward the core network (CN) and other cells, often with stringent requirements in terms of capacity, latency, availability, energy, and cost efficiency. The consequent massive traffic forwarding is a great challenge for future 5G backhaul networks that may represent the capacity bottleneck. Moreover, the 5G backhaul network is complex and must have the ability to dynamicallyadapt to changes of the radio access network (RAN) in a flexible, efficient, and timely manner 5G backhaul network is characterized by heterogeneous links: small cells are used in different environments, such as homes, small offices, hotspots, and enterprises, where the available connections to the CN are different (i.e., a mix of wired and wireless links with different characteristics). Finally, in presence of many access points, it is expected that the backhaul network is composed by multi-hop connections, where some small cells act as relays aggregating the incoming traffic and forwarding it. Therefore, there is the need for efficient and flexible routing schemes for capacity-limited backhaul networks, in order to enable a scalable and efficient deployment of UDNs. These schemes must be able to handle the variations of the traffic load at the access points, which fluctuates across space and time. Toward this goal the emerging Software-Defined Network (SDN) paradigm provides an efficient solution. SDN moves the routing decisions from switches to a central controller, thus achieving more flexibility, efficiency, and faster convergence time. The basic idea of SDN is decoupling data (i.e., forwarding) and control planes (i.e., configuration and management) of the network, and facilitating the interaction between the two using an open interface, such as OpenFlow protocol, which specifies how to program the behavior of the network elements by a remote controller [4]. In that way the main intelligence of the network is centralized in a software-based controller which has an abstract view of the physical network topology. The controller also provides application programming interfaces (APIs) to operators that can directly program customized control algorithms depending on their objectives, thus reducing capital and operational expenses. Software-defined networking could effectively facilitate the management of heterogeneity, complexity, and dynamism of the mobile systems [5]. In particular, an SDN-based UDN will be able to dynamically respond to changes of traffic load and interference levels in the small cells, and will facilitate the co-existence of heterogeneous backhaul links. In addition, such an architecture provides the possibility of scaling the network adding/changing some links. This paper proposes and investigates an SDN-approach for an efficient routing in a capacity-limited backhaul network, which carries data and control traffic of a heterogeneous UDN.