## 3 NTNs for IIoT Applications
### Overview on Spaceborne Platforms for IIoT  
Satellites constitute the main representative of NTNs and refer to spaceborne vehicles in low earth orbit (LEO), medium earth orbit (MEO), geostationary earth orbit (GEO), or in highly elliptical orbit (HEO). Over the past 30 years, satellites have been typified as the unique means of achieving global coverage. More importantly, satellites can potentially promote mMTC [34] and the Internet of Remote Things (IoRT) [35] when terrestrial constraints in terms of connectivity exist over a wide geographical area. A typical example of IoRT is the global sensor network (GSN) for remote environment observation, where massively connected IoT sensor networks are connected via LEO satellites [36]. Aiming at evolving the satellite technology with regard to IoT and realizing the Internet of Space Things (IoST) [37], Iridium Communications Inc., McLean, VA, USA, a major commercial solution provider, has launched Iridium NEXT satellites for Earth and space sensing, supporting payloads of up to 50 kg, called SensorPODs. Besides this, Immarsat, as the leader in global mobile satellite communications services, predominantly used the L-band (1-2 GHz), which supports an extremely low data rate for IoT applications. As IIoT takes hold over the next few years, the network traffic will be significantly increased. Moreover, large data volumes generated from multiple sensors should be aggregated to perform prescriptive and predictive analytics and provide real-time services, such as surveillance and monitoring using closed circuit television (CCTV). Hence, a higher bandwidth technology, notably Ka- and Ku-bands, will facilitate the establishment of a broad  
Figure 3: Simple representation of a space-air-ground integrated network (SAGIN) for IIoT applications that includes space, air, and ground network components and aims at cooperatively expanding radio coverage over large areas and facilitating the collection, as well as the coordination, of multi-dimensional data. range of satellite-based IIoT applications. High throughput satellites (HTS), e.g., ViaSat-1 and EchoStar XVII, operating at the Ka-band, provide more than 100 Gbps of throughput. Although the majority of IIoT scenarios may not require such throughput performance, the significant benefit of Ka/HTS is the dramatic reduction in the transmission cost-per-bit. With the exploitation of the Ku-band, the large number of open Ku-band satellites can be leveraged and ensure lower spectrum cost than L-band. Although using Ka- and Ku-bands seems beneficial, rain substantially affects signal propagation in mm-wave frequencies and a dominant line-of-sight (LoS) signal is required for sufficient radio coverage due to the severe attenuation of the non-line-of-sight (NLoS) links. Since the utilization of the IPv6 is a precondition for the successful deployment of IIoT systems leading to the huge number of simultaneously interconnected nodes, the Digital Video Broadcasting--Return Channel via Satellite--Second Generation (DVB-RCS2) standard supports this protocol [38], whereas the DVB-RCS2+M standard adds mobile and mesh capabilities. The realization of IIoT and machine-to-machine (M2M) communications can be further reinforced owing to the investments in terrestrial segment technologies, i.e., compact, electronically steerable, and/or phased-array transceivers, as well as the cost-effective CubeSat platforms [37, 39] based on micro-, nano-, and pico-satellites, operating as access points. Based on the Northern Sky Research (NSR), the satellite IoT (S-IoT) market will include more than 5.3 million terminals by 2024, whereas the European, Middle Eastern, and African (EMEA) market is expected to reach US$495 million in revenue by 2024. In the context of S-IoT applications, Inmarsat was the first to join the LoRa Alliance as its first satellite member and combine a low-power, low-data-rate, long-range, wide area network (LoRaWAN) on the ground with a satellite mesh in the sky.