## 2 Localization in IIoTs
### Communication Technologies for Indoor Positioning System  
In this following subsection, the most common existing wireless communication technologies that are widely applied for indoor localization system [48; 49; 50], such as RFID, Wi-Fi, Bluetooth, ZigBee, and UWB, are briefly presented and discussed. * **Wireless Fidelity, (Wi-Fi):** Wi-Fi, a widely used wireless networking technology, operates on the IEEE 802.11 standard and uses radio frequency bands of 2.5 GHz for IEEE 802.11\(b\) and 5 GHz for IEEE 802.11\(a\)[49]. Many smart devices, such as smartphones, tablets, and audio players, are Wi-Fi enabled, making Wi-Fi-based IPSs more practical and cost effective. In a typical large indoor area, such as office buildings, universities, and malls, the widespread distribution of Wi-Fi hotspots provides a complete building coverage. Wi-Fi-based localization systems are typically based on fingerprinting the radio signal strength indicator (RSSI) and have an accuracy range of 1-10 m [50]. Wi-Fi offers a reception range of about 100 m, and its low infrastructure cost makes it a practical option for IPS. Its reasonable accuracy, availability, large coverage, high data rate, and widespread support in many devices make Wi-Fi a suitable choice for IPSs. * **Bluetooth Low Energy, (BLE):** Bluetooth communication technology operates in the radio frequency range from 2.402 GHz to 2.480 GHz [51]. It is designed for short-range communication between devices and has become a competitive technology in IPSs due to its characteristics of cost effectiveness, very low power consumption, long battery life, high security, and communication efficiency [48; 52]. Bluetooth-based localization solutions typically use the RSSI-based range-estimate technique. The latest version of Bluetooth, known as BLE, has a data rate of 24 Mbps, and the signal range coverage can reach 70-100 m with the high power efficiency, making it ideal for use in public-space areas, such as airports or shopping centers [53]. * **ZigBee:** ZigBee is a wireless communication protocol based on the IEEE 802.15.4 standard that is designed for personal-area networks that are cost effective, have low data rate, and are energy efficient. It operates on different frequency bands, including  
Figure 1: Key benefits of indoor positioning system (IPS). 868 MHz in Europe, 915 MHz in the USA, and 2.4 GHz in other regions. It can be easily applied for IPSs with a coverage range of up to 100 m [54], which is ideal for most indoor environments, including buildings and underground structures. The energy-efficient feature of ZigBee makes it a suitable choice for IPSs in terms of low power consumption. * **Radio Frequency Identification, (RFID):** RFID is a key technology enabling the real-time monitoring of objects. It involves data transfer and storage, and operates on backscattering communication, which consists of a RFID reader, RFID tags, and data processing system [55; 56]. The RFID reader emits frequency pulses that are received by the RFID tags, and the data are processed with the help of a chip embedded in the tags. The RFID tag contains three different types, which are active, passive and semi-active. Active RFID tags, which have an internal battery, are used in various applications and operate in ultra-high frequency ranges with a coverage range of up to 100 m [57; 58; 59]. RSSI information between the RFID tags and the reader is used to estimate the range and localization, but this information is easily affected by multi-path, noise, and changing channel conditions in indoor environments. Factors such as node density, antenna type, and frequency used can also impact the accuracy of the system. As a result, active RFID technology may not provide sub-meter-level precise accuracy of the positioning system, but it is still popular due to its low cost, ease of implementation, miniaturize size, and low power consumption [60; 61].