# Review of Indoor Positioning: Radio Wave Technology
## 1 Introduction
Radio frequency technologies based on an indoor positioning system (IPS) are widely used. It is based on the signal strength technologies, especially for wireless communication devices, and uses narrow-band with spread-spectrum signals. Radio waves are generated by the electromagnetic field of sources or devices. Useful localization technologies in IPS are radio frequency technologies involving wireless communication technologies, which define the physical and medium access control (MAC) layer of open system interconnection (OSI) model--Bluetooth, radio frequency identification (RFID), ZigBee, ultra-wide band (UWB), wireless local area network (WLAN), and mobile network. Two groups of indoor localization systems are used, namely active localization and passive localization [22]. RFID [23, 24, 25, 26, 27, 28], UWB [29, 30, 31], Bluetooth [32, 33, 34], ZigBee [35, 36, 37, 38], IR [15, 17, 39], ultrasonic [40, 41, 42], hybrid systems and the standard of the Institution of Electrical and Electronic Engineering (IEEE) 802.11 WLANs [42] are included in active IPS, in which the person is attached to the tag or device to enable locating the position in a dynamic indoor localization framework. If the person does not carry any tag or device alongside with the trajectory in the location area, then it is defined as a passive indoor localization. Device-free passive localization such as that in [43, 44, 45], UWB [46, 47, 48], physical contact, computer vision and differential air are categorized as passive indoor localization. Furthermore, the location determination scheme mainly involves distance estimations and position calculations. Distance estimation is usually mentioned as ranging, and is based on different traditional parameters such as the received signal strength (RSS), angle of arrival (AOA), TOA, and TDOA of beacon signal changes between the target node and the beacon nodes. The accuracy of positioning is reliant on the signal parameters, particularly on the wireless technology used, as it defines the value of the approximation of the signal parameters [31]. Therefore, several researchers have studied other parameters including round trip time (RTT), direction of arrival (DOA), channel state information (CSI), reference signal received power (RSRP), and reference signal received quality (RSRQ). On the other hand, a number of positioning methods have been investigated, such as triangulation, trilateration, proximity methods, and the advanced positioning algorithms, such as fingerprinting or pattern recognition, PDR, machining learning, and deep learning, all to precisely locate people or devices in localization systems. In addition, hybrid algorithms were also proposed for indoor localization approaches since each method has its own advantages and limitations [49]. For existing works, an author conducted a survey of support technologies for network localization systems [50]. Indoor localization parameters and technologies were reviewed in [4; 51]. The author overviewed smartphones based on indoor positioning schemes [52]. Indoor positioning based on Wi-Fi fingerprint localization was compared in [53]. The survey described parameter comparisons relying on distinct indoor technologies. The localization techniques for indoor scenarios are TOA or time of flight (TOF), TDOA, round trip time of flight (RTOF), AOA, phase of arrival (POA), received signal strength indicator (RSSI), and CSI. Then, preferable positioning approaches or positioning techniques have been developed in IPS, which are proximity estimation, triangulation estimation, trilateration estimation and pattern recognition or fingerprinting. There are many techniques for accurate indoor positioning based on signal strength measurements. The technologies for indoor location and tracking have led to calculating the user's location which is based on an entire range localization such as building, room or limited spaces. Some issues are currently encountered as the main problem of indoor location because of the multipath signal and NLoS of signal in a given area.