# Trends in LPWAN Technologies for LEO Satellite Constellations in the NewSpace Context
## 3 Architectures and Protocols for IoT Communications via Satellites
### Lpwan
#### 3.2.1 NB-IoT  
NB-IoT is an LPWAN communication technology created to enhance the performance of the LTE standard [47]. Its main objective is to support low-cost, low-power devices while improving the coverage. This is achieved using a narrower bandwidth, a single transmission subcarrier (3.75 or 15 kHz). This makes connecting low-transmission-rate devices easier and enhances the signal quality [44].  
To expand the reach of NB-IoT and connect devices outside terrestrial networks, the 3 GPP Release 17 framework presents the possibility of adapting it to be compatible with non-terrestrial networks (NTNs). Additionally, Release 17 has incorporated improvements that cover extending coverage, increasing service reliability, and utilizing NTN multicast and broadcast capabilities to enable and improve the scalability [48]. It focuses on the bent-pipe architecture, as it facilitates NB-IoT's integration into existing satellite-based NTNs with a transparent payload. As defined by the 3 GPP, a transparent payload satellite is considered an advanced repeater capable of performing frequency shifts between the feeder link frequency band and the serving link frequency band, as opposed to a basic repeater [49].  
The bent-pipe architecture is a technique widely used in satellite communications. In this mode, the signal received by the satellite is amplified, filtered, and retransmitted in real time to the ground station without any additional processing on the satellite. This architecture connects IoT devices in remote or hard-to-reach areas in satellite communications with the NB-IoT. Its architecture is implemented through a transponder, which receives signals from IoT devices operating in the NB-IoT frequency band and retransmits them to the corresponding ground station.  
The NB-IoT architecture is built upon the existing cellular infrastructure, but it can be extended to connect devices beyond the range of terrestrial networks. This is achieved by applying NB-IoT adaptation to support NTN, like satellite communications. Release 17 focuses on the \"bent-pipe\" architecture, which enables NB-IoT's integration into existing satellite-based NTNs with a transparent payload. This is performed by integrating NB-IoT into the operator's network core, as shown in Figure 4. It is the central part of the network infrastructure responsible for managing and controlling device communications. This architecture is implemented through a transponder that connects IoT devices in remote or hard-to-reach areas [48].  
In practical terms, as shown in Figure 4, the communication process in IoT devices takes place in the following steps: firstly, the IoT devices located in remote or hard-to-reach areas transmit their data through the NB-IoT protocol to the satellite close to them. The satellite then acts as a relay and forwards the data to the ground station. This ground station is a crucial part of the NB-IoT infrastructure owned by the operator. Once the ground station receives the datum, it is integrated into the NB-IoT network and operator cloud and forwarded to the end-user's business applications.  
Sateliot has designed IoT satellite hardware for NB-IoT, emphasizing several crucial aspects. Firstly, they employed a multi-beam approach to ensure broad coverage and an increased capacity. This approach requires using multiple radiofrequency fronts and antennas pointed in different directions or more complex antennas with beamforming capabilities. Secondly, the link design, including the link budget and beam arrangement, was analyzed with specific considerations for antenna gain, polarization, temperature, and user equipment noise. The design also addressed other essential adaptations for operation in low-density satellite constellations, such as discontinuous service operation, feeder connectivity, time and frequency synchronization, time relationships, and mobility in idle and connected modes [50].