1. Principles of RFID TechnologyRadio Frequency Identification (RFID) technology is a simple-to-operate technology suitable for the field of automatic control. It utilizes the transmission characteristics of inductive and electromagnetic coupling or radar reflection to achieve automatic identification of objects. Radio frequency (RF) is an electromagnetic wave with a specific wavelength, whose frequency is described in kHz, MHz, GHz, ranging from low frequency to microwave.
2. Basic Structure of RFID Indoor Positioning System
This system typically consists of electronic tags, RFID readers, middleware, and a computer database, as shown in Figure 2. RFID tags and readers exchange data through the transmission channel of spatial electromagnetic waves established by antennas. In positioning system applications, the RFID reader is placed on the moving object to be located, and the electronic tags are embedded in the operating environment. The electronic tags store position identification information, while the reader connects to the information database via wired or wireless means.
RFID Indoor Positioning System Structure
3. Typical RFID Indoor Positioning System: LANDMARKThe LANDMARK system is a typical indoor positioning system using RFID. It analyzes and calculates the RSSI signal strength of reference tags and unknown tags, and uses the "nearest neighbor" algorithm and empirical formulas to calculate the coordinates of the tags to be located.
LANDMARK system positioning accuracy: average 1m.
Disadvantages: The LANDMARK system has several shortcomings. First, the positioning accuracy is determined by the positions of the reference tags, which affect positioning. Second, to improve accuracy, the system requires a higher density of reference tags, but this can cause greater interference and affect signal strength. Third, because the Euclidean distance between reference tags and unknown tags must be calculated via formulas, the computational load is high.
4. Applicability of RFID Technology in Different Frequency BandsCommon RFID frequency bands include: low frequency (LF), high frequency (HF), ultra-high frequency (UHF), and microwave. For indoor positioning systems, a comparison of radio frequency signals in different bands is shown in Table 1.
Table 1: Comparison of RF Signal Characteristics in Different RFID Frequency Bands
Based on the above comparison, this paper proposes that building a wireless positioning network using 2.45 GHz microwave signals is feasible and effective. 2.45 GHz is gaining increasing attention and application, with growing global commonality, better bandwidth than other bands, faster transmission rates, and smaller antennas and products, making them more portable and convenient to use.
5. Two Types of RFID Wireless Indoor Positioning Systems Based on the 2.45 GHz Band
Wireless Local Area Network (WLAN) Technology (Wi-Fi/IEEE 802.11b):Wireless Ethernet based on the IEEE 802.11b standard has successfully entered human social life, widely applied in campuses, workplaces, and public places. Users can easily obtain wireless signals using mobile phones, laptops, or PDAs. WLAN technology can also be easily applied to indoor positioning systems. AP access points or wireless network cards in the WLAN can conveniently measure wireless signal strength, enabling positioning by matching signal strength. The location fingerprint method is a common WLAN indoor positioning technique, exemplified by the RADAR prototype system developed by Microsoft.
The RSSI-based RADAR indoor positioning system operates in two processes: first, offline collection of location information and signal strength from several fixed AP points within the system coverage area, transmitted via wired network to a data center to form a location fingerprint database; then, real-time signal strength measured from the target is analyzed and matched using the nearest neighbor method to determine its location.
Accuracy: 2~3m. Disadvantages: The data collection workload is heavy, and to achieve high accuracy, the location calculation and setup of fixed AP points are relatively cumbersome.
ZigBee/IEEE 802.15.4:
ZigBee technology is used for short-range wireless communication, primarily for Wireless Personal Area Networks (PAN). The network system exhibits characteristics such as short range, low power consumption, and low cost, which meet the requirements of indoor positioning systems. Such systems consist of reference nodes and mobile nodes arranged in sensor networks. Reference nodes are static nodes that send location information and RSSI values to mobile target nodes, which then write the data into a positioning module to calculate their own positions. The system often adopts distributed node settings to reduce network data workload and communication latency.
Accuracy: within 2m, average 1m. Disadvantages: Network stability still needs improvement and is susceptible to environmental interference.
**Summary:** RFID technology is suitable for indoor positioning systems, being convenient to implement, offering satisfactory positioning accuracy, and featuring low cost. Based on the study of the typical LANDMARK system, future improvements can target its shortcomings, using 2.45 GHz to build wireless networks for positioning to enhance efficiency and accuracy. Among the technologies, ZigBee is particularly suitable for indoor positioning systems due to its simple structure, easy deployment, excellent performance, very low power consumption, and convenient positioning effect that meets application requirements. Further research and development on the design improvement of ZigBee-based indoor positioning systems is needed, making it a good case and application for studying RFID technology.