# Resource Allocation for Cognitive LEO Satellite Systems: Facilitating IoT Communications
## 1 Introduction  
Internet-of-Things (IoT) applications, such as financial services, intelligent transportation, and remote sensing, are envisioned to be an important driving force of the smart society [1]. It can be seen, with the popularization of IoT applications, trillions of new devices will be connected to the networks for different application needs, such as smart wearable devices, smart homes, environmental monitoring, and so on [2]. A variety of IoT applications require IoT networks deployed remotely over large areas (e.g., environmental monitoring). However, due to the fact that the infrastructure construction is affected by the geographical environment, terrestrial wireless systems cannot achieve comprehensive coverage, especially in particular environments such as deserts, oceans, and forests [3]. As an extension and supplement of the terrestrial communication system, low earth orbit (LEO), medium earth orbit (MEO), and geostationary earth orbit (GEO) satellites can be used in areas that are not covered by terrestrial networks [4]. For years, industry and academia have focused on the GEO satellites to provide Internet access. But these high-orbiting satellites face significant challenges in providing services. For instance, signals need to span a distance of 36,000 km from Earth to the satellite's return, and time delaysoften exceed 600 ms, which cannot meet many existing service demands [5]. In contrast, the LEO satellite systems between 160 and 2000 km can provide low-latency, high-speed Internet connectivity. Thus, LEO has attracted considerable attention from both academia and industry in recent years [6; 7; 8; 9]. To achieve ubiquitous and massive IoT connectivities, the access enabled by the LEO satellites will be the potential solution for supporting large-scale communications of future smart devices [2; 10]. The LEO satellite communication (SatCom) system has the characteristics of global coverage (including the polar regions), on-demand access, and large capacity, which can effectively solve the shortage of the terrestrial communication system for IoT communications [11]. One promising use case for such consideration is that the LEO satellite can act as a base station to monitor disasters in the forest or mountainous area, such as fires, earthquakes, and mudslides, by collecting the sensing information on the IoT devices [12; 13]. However, there are several challenges to the use of LEO satellites for IoT communications, notably in terms of spectrum shortage issues. On one hand, a growing variety of wireless communication systems lead to an increase in demand for radio spectrum. Nowadays, the available spectrum for satellites has already been allocated for specialized use such as remote sensing, navigation, telecommunications, etc. Thus, it is challenging to allocate a specified spectrum for LEO satellite-enabled IoT communications. On the other hand, unlike human-to-human communications, IoT communications are generally infrequent transmissions and occasionally exchange small amounts of data when transmitting [14; 15; 16]. In this regard, it would be wasteful to launch a dedicated satellite for IoT communications. To deal with the above challenges, cognitive radio (CR) is a potential solution [17; 18; 19]. Specifically, CR allows the secondary users (SUs) to use the same spectrum as the primary users (PUs). In more detail, the PUs can use the spectrum resources without any constraint, while the SUs should use the same spectrum resources without significantly affecting the PU [20]. CR can be further categorized into overlay and underlay modes. In an overlay mode, SUs access the spectrum by sensing the PUs' unused spectrum (spectrum holes), and when PUs appear, they must immediately release the occupied band. Therefore the overlay mode can minimize interference to PUs, while must take precise spectrum sensing for SUs, which consumes a lot of energy. This is very unfriendly to IoT devices, which are usually equipped with limited batteries and are very sensitive to energy consumption [21].