# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 3 Wireless Technologies
### Radio Frequency Identification (RFID)  
The RFID wireless technology is often used for the identification and tracking of objects by using the RF electromagnetic fields, even under NLoS conditions. Its range can be from centimeters up to tens of meters, and it operates in the unlicensed ISM bands. It is an RF technology, and its accuracy and maximum detectable range are affected by the environment and multipath caused from the NLoS conditions [117]. It is a non-contact wireless technology used for data transferring, and the RFID networks consist of readers and tags. RFID tagged objects are uniquely identified; thus, the RFID technology is suitable for inventory and assets' tracking. In particular, the RFID reader sends a signal to either a passive or active tag, which modifies the signal reflected or retransmitted back. The active RFID tags include a transmitter and their own battery, which is used for broadcasting the signal to the reader, whereas the passive tags have no battery. Tags transmit a response of their internal information, such as a unique ID and RSS information. Typically, passive tags are used as a traditional barcode technology replacement and are lighter and less expensive, as compared to active tags, and they provide unlimited operational lifetime [117]. Generally, the RFID technology includes a wide range of applications, such as pet and livestock tracking, inventory management and control, asset, equipment, and personnel tracking, and entrance control. The warehouse tags are usually deployed on the products' cartons and reflect a signal to a handheld reader for identifying the contents of the product. Additionally, in large organizations, the personnel can \"clock in\" and \"clock out\" easily with an RFID key fob or card for accurate time recording, improved payroll accuracy, fairness, and transparency.  
Typically, an RFID-based localization system includes an additional data processing subsystem for processing the data received from the RFID readers and executing the localization algorithms, as shown in Figure 17.  
In particular, the RFID reader interrogates one or more RFID tags for obtaining their unique IDs and retransmits them to the data processing subsystem for further processing. The tags transmit their unique ID signals randomly every 7.5 s on average [117]. By deploying numerous readers acting as transmitters, which receive the RSS information from the same tag, the target location can be estimated at no additional deployment cost [118]. Furthermore, compared to Wi-Fi and BLE technologies, the RFID enables spot localization due to the highly limited range of less than a meter. Although it is a low-cost and easy technology to deploy, it can consist of massive numbers of randomly deployed RFID tags, so message collisions can arise. This can be eliminated by assigning random delays for the various tags' response times, as in the \"LANDMARC\" [117]. Moreover, the ability of RFID to operate in NLoS and rich multipath conditions is evaluated in [119] by deploying the \"PinIt\", a commercial RFID system with UHF reference tags.