## 2 Integration of Wireless Connectivity into the Industrial Domain  
To provide IIoT services and massive machine type communications (mMTC) [27] supporting millions of IoT devices with intermittent activity and transmission of small data packets, advanced information, network, and communication technologies should be adopted by traditional industries. The convergence between industrial networks and traditional networks is also known as IT/OT convergence, where the operational technology (OT) refers to highly reliable and secure industrial networks and products, and information technology (IT) refers to the Internet and the end-to-end (E2E) flow control [28]. Similar to the IoT, the notion of IIoT relies on the availability of radio connections among the devices within an industrial environment. In massive IIoT, the data transmission is infrequent and not persistent, whereas the latency requirements are not tight. However, in mission-critical IIoT applications, system designers should consider a number of factors regarding the ubiquitous connectivity of the vast number of devices to guarantee the end-user experience and prevent failures and severe consequences in a systematic, secure, and cost-effective manner. More specifically, critical IIoT goes beyond massive IIoT and enhanced mobile broadband (eMBB), which aims at maximizing the data throughput without ensuring extremely low latency radio access and high E2E reliability [29]. Additionally, safeguarding the operation of IIoT against cyber-physical attacks is requisite. Figure 2 depicts the main requirements of critical IIoT applications.  
Figure 1: Key enablers for globally accelerating the adoption of non-terrestrial network (NTN)-based Industrial Internet of Things (IIoT).  
Although the majority of the devices in an industrial environment can be interconnected through homegrown wired technologies, e.g., Fieldbus technology [30], the most progressive manufacturers have started to integrate wireless Internet protocol (IP)-enabled solutions for industrial sensing and control systems. The introduction of short- and long-range wireless communications for locally and remotely connected IIoT entities, respectively, enhances flexibility, improves security, and significantly reduces the operational cost for deployment and maintenance, while driving new revenue through IIoT-based solutions. For localized IIoT scenarios, short-range networks based on wireless industrial standards, including ISA100.11a and WirelessHART, are the most convenient, since they take advantage of the unlicensed spectrum and maximize the battery life [31]. Other wireless standards and protocols providing limited range and low-power consumption, such as wireless fidelity (Wi-Fi) HaLow based on Institute of Electrical and Electronics Engineers (IEEE) 802.11 ah, IPv6 over low-power wireless personal area networks (6 LoWPAN), ZigBee, Bluetooth low energy (BLE), and near field communication (NFC), may suffice depending on the application requirements. Besides the above, exploiting current and future conventional cellular networks and low-power wide area network (LPWAN) 3 rd Generation Partnership Project (3 GPP) standards, e.g., Extended Coverage-Global System for Mobile Communications for the IoT (EC-GSM-IoT), Narrowband-IoT (NB-IoT), and Long-Term Evolution for Machines (LTE-M), as well as non-3 GPP standards [6], e.g., Long Range (LoRa) and SigFox, could satisfy the needs for long-range communication of IIoT applications in the near term. Currently, terrestrial wireless networks offer a limited radio coverage of approximately 20% across the territories of China and U.S. [32].