# Smart Beamforming for Direct LEO Satellite Access of Future IoT
## 1 Introduction  
The communications market has experienced a steady growth in the last years, owing to the large number of devices connected to the Internet. The network that supports the connection of these devices leads to a system of interrelated machines, objects, or people, which gives rise to the well-known concept of Internet of things (IoT). In the coming era, it is estimated that there will be billions of devices connected globally. Industry organizations, such as GSMA, expect IoT connections to reach 25 billions by 2025, which is slightly more than double the 12 billion connections of 2019 [1]. In this context, non-terrestrial networks (NTN) emerge as a key technology to extend and complement terrestrial networks, which is crucial to deliver services any time and anywhere. Services can be provisioned in areas that are not covered by conventional infrastructures, such as fiber, cables, or radio frequency links. For instance, remote rural regions, deserts, and maritime areas, to mention a few. According to 3 GPP, NTN include satellites located at different orbits as well as high altitude platforms [2]. In regards to the architecture, one of the possible applications of satellite communications is in the backhaul segment. In this deployment, terminals are indirectly connected to the satellite through a terrestrial node that aggregates the traffic. A more challenging architecture scenario considers the satellite in the radio access network (RAN), so that terrestrial infrastructures in the user link are not needed. This architecture allows directaccess between the user equipment (UE) and the satellite. Establishing a direct link between the devices and satellite poses several challenges. In fact, typical satellite channel impairments, such as large path losses, delays, and Doppler shifts have to be assessed on radio protocols [3; 4; 5]. The orbit, frequency, and on-board digital processing usually dictate the severity of the adverse effects. Considering the large propagation loss and the long delay involved in geostationary Earth orbit (GEO) satellite communications, it follows that the most suitable option to allow direct access is to rely on non-GEO (NGEO), especially low Earth orbit (LEO), satellite communication systems. It is important to clarify that in regards to the delay, LEO is advantageous over GEO only if the LEO gateway coverage is global or if routing is allowed between LEO satellites by means of inter-satellite links (ISLs). Otherwise, LEO satellites need to adopt store and forward protocols, which significantly increase the delay. In such a situation, the network does not support real-time IoT services. It is worth emphasizing that there are two possible options of satellite payload implementations [2]. On the one hand, a transparent or bent-pipe satellite acts as a radio frequency repeater. In such a case, the satellite can be regarded as a flying relay node. On the other hand, the option considered in this paper is the regenerative transponder. With regenerative architecture, the satellite is able to generate/detect the signal to/from the devices. The broadcast capabilities and the large footprint that characterize LEO satellite constellations can be harnessed to fit the needs of massive machine-type communications (mMTC), also referred to as massive IoT. In the satellite mMTC context, there are several challenges that deserve some attention. The main issue stems from the large population of devices that are located within the coverage of a single satellite. Indeed, the geographical area covered by a satellite is significantly higher than the size of typical terrestrial cells [6]. Hence, at a given instant, the number of concurrent users requesting access to the network through the satellite could yield congestion, unless some measures are taken. The problem of providing massive connectivity is addressed by employing multibeam technology and the non-orthogonal multiple access (NOMA) principle, where multiple users can overlap in the time-frequency plane.