## 2 Enabling Satellites Technologies and Related Works  
The reference scenario is the use of space systems/data for IoT applications, particularly in support of transport management and the development of intelligent transport systems. In this scenario, the use of satellites for M2M/IoT applications enables reaching remote and widely scattered geographical areas (maritime, aerial, railway, vehicular) to connect devices with high mobility characteristics. Satellite M2M/IoT communications, either alone  
Figure 1: 5 GT System architecture mapped to reference standards [3; 4; 5]. or as a complement to other terrestrial channels, can offer advantageous services for a range of applications:  
* Addressing a massive number of devices simultaneously, for alarm management, triggering wake-up from deep sleep mode, and enabling other activities (e.g., software updates);
* Backup for terrestrial connectivity;
* Greater network control and independence from cellular operators;
* More stable and controlled Quality of Service (QoS), when low latency is not required;
* Very precise energy usage by end nodes;
* Ultra-low power transmissions are required due to regulatory constraints, i.e., RF emissions lower than some terrestrial solutions;
* Integration with GNSS data transmission applications;
* New services for platforms already connected via satellite (e.g., maritime platforms). The types of IoT communication via satellite are of two kinds, either 'Direct Access' or 'Backhaul of Aggregated Sensors' (see Figure 2). For each type, several solutions exist as the state of the art, some still at the prototype level, but also commercially evolved solutions [6]. For example, for direct access, some LEO satellite constellations dedicated exclusively to M2M/IoT equipment are already in orbit. Referring only to actual launches to date, there are 18 companies [7] that have put into orbit constellations of nano- and microsatellites for M2M/IoT applications. Among them, the main international initiatives by numbers and prospects are Swarm Technologies (US), Iridium (US), Orbcomm (US), Astrocast (CH), Satielot (ES) and Lacuna Space (UK). To date, the first four companies offer commercial services, while the latter two do not yet offer paid services. Most of them are still completing their launch programs. Direct-access communication protocols are based on several approaches, with some still under development. One approach is to repurpose standard protocols for terrestrial LPWA networks to satellite channel characteristics. For example, the possibility of using LoRa with low-orbit satellites was first analyzed [8,9] and, thus, demonstrated on some occasions (e.g., in 2018 during a conference held in Amsterdam, NORSAT-2, which normally transmits AIS information in the VHF bands, was modified to transmit LoRa messages [10]). These approaches are behind the industrial initiatives of the British firm \"Lacuna Space\" [11] and the German firm \"Sat4M2M\" [12], which, to date, do not yet offer a commercial service but are gathering interest and pre-orders from IoT solution operators. Another approach in the same scope is to adapt standard NB-IoT protocols to satellite networks [13]. Mobile telecommunications standard 3 GPP release 17 (March 2022) has  
Figure 2: IoT via satellite topologies. brought a new set of capabilities, integrating 5G New Radio with satellite-based systems, effectively delivering narrowband IoT (NB-IoT and eMTC support) direct communications between satellites and cellular-enabled devices [14]. Presently, a commercial 3 GPP R17 NTN RAN already exists [15], offering a service that allows NB-IoT cellular devices to connect directly over existing GEO satellites. Parallelly, the use of the LEO constellation for implementing direct 3 GPP-compliant NB-IoT communication is still gaining efforts from both applied research [16, 17, 18] and the cellular industry [19].