# Energy-Efficient Controller Placement in Software-Defined Satellite-Terrestrial Integrated Network
## 1 Introduction  
With the increasing demand for data traffic, the terrestrial network has been unable to satisfy people's demand, especially in rural areas [1]. Limited by the coverage of terrestrial networks, it is difficult to provide ubiquitous wireless access with reliability only relying on ground communication systems. The emerging applications, such as virtual reality (VR) streaming, the Internet of Things (IoT), remote sensing, and navigation, requires a high-throughput bandwidth and low latency response, new concepts, and technology for the next-generation system are being developed to accommodate diverse applications in various scenarios [2]. Low earth orbit (LEO) satellite constellation, e.g., Starlink [3] and OneWeb [4], has attracted intensive attention for its wide coverage and low latency. The wide coverage of the satellite constellation makes communication convenient, even in the ocean and mountains. The integration of satellite networks and terrestrial networks brings lots of benefits to future communication systems. As a promising architecture, the satellite-terrestrial integrated network (STIN) achieves resilient seamless coverage in future networks [5]. The typical architecture of STIN is shown in Figure 1, which includes the space segment, aerial platform, and ground segment. Thereare Geostationary Earth Orbit (GEO) and LEO satellites in the space segment. Three GEO satellites are connected by inter-satellite links (ISLs), which achieve global coverage in most areas. The GEO satellites often serve as the space backbone network. The deployment of LEO satellites enlarges the coverage area of the satellite backbone network. The aerial platforms are divided into high-altitude platforms (HAPs) and low-altitude platforms (LAPs). Airships and balloons, forming HAPs, have long-distance flight capabilities to provide stable access for users on both ground and sea. Various aircrafts, such as unmanned aerial vehicles (UAVs) and fighter jets, form an LAP. The high mobility and low latency in LAPs can provide wireless communications for emergency scenarios. The ground segment connects to the space segment via satellite gateways, which provides Internet interfaces for mobile devices. User equipment such as personal laptops and phones can access the Internet through wireless access points. However, the natural heterogeneity of the STIN poses a challenge for global network management [6]. The paradigm of software-defined networking (SDN) separates the data and control planes, making the upgrading and maintenance of the network easier [7]. The SDN-enabled STIN adopts distributed control by multiple controllers, which solves the heterogeneous management caused by various hardware equipment and can meet the failure tolerance and latency requirements [8]. Therefore, the controller placement problem (CPP) becomes a critical problem for improving the flexibility of the STIN. The research on CPP has been already discussed in SDN-enabled terrestrial networks for years, and the number and location of the controllers determine the efficiency of resource scheduling [9]. Despite its non-negligible value in STIN model design, the CPP in the satellite segment, especially in the LEO mega-constellation, has not received as much attention yet. Figure 1: The typical architecture of the STIN. Moreover, network traffic around the world has witnessed a rising trend. According to the statistics, the energy consumption from communication technology will account for 51% of the total electricity consumption by 2030 [10]. As the satellite load is limited and the onboard resources are scarce, it is expected that satellites can provide as much operation time as possible. The rapid growth of network traffic makes energy consumption faster. Furthermore, the population distribution also results in uneven flows on Earth. Even if the power supply is sufficient, the unplanned use of energy leads to the rapid aging of satellites.