Positioning Technology of Passive RFID

RFID is a non-contact identification technology that uses radio frequency signals to read and transmit information Stored in RFID electronic tags. It is widely used in logistics tracking, transportation, shopping mall cargo management, and item positioning. According to the specific conditions of the site, the auxiliary RFID electronic tags and readers are evenly deployed as needed. Generally, there are two ways to indicate the distance between the auxiliary RFID tag and the RFID Reader.

The first is to use an RFID reader that can adjust the reading and writing distance by adjusting the energy layer. On which energy layer each auxiliary RFID tag is read by the RFID reader, this energy layer data indicates the auxiliary The distance between the RFID tag and the RFID reader. The smaller the energy layer data, the closer the auxiliary RFID tag is to the RFID reader; the larger the energy layer data, the farther the auxiliary RFID tag is from the RFID reader.

The second is to indicate the distance between the auxiliary RFID tag and the RFID reader according to the delay between when the RFID reader sends a signal and when it reads the RFID tag information. The shorter the delay time, the closer the distance between the auxiliary RFID tag and the RFID reader; the longer the delay time, the farther the distance between the auxiliary RFID tag and the RFID reader.

RFID tags are divided into active and passive. Active tags have a power source, and the signal processing can be more complicated, and the positioning accuracy will be much higher. Ideally, it can cover a range of 100 meters, and the positioning error is about 5 meters. It is mainly completed by triangulation, but this field can also use nodes such as UWB and ZigBee to complete positioning. Since the passive RFID tag has no computing power, all signal processing is limited by the reflected signal received by the RFID reader, so the choice of signal processing algorithms will be much smaller. And because the identification range of the RFID reader is basically within the range of 20 meters, the positioning of passive tags is generally less used.

RFID indoor positioning is to locate tags through RFID readers with known positions, which can be divided into non-ranging methods and ranging methods. The method based on ranging refers to estimating the actual distance between the target RFID device and each RFID tag through various ranging techniques, and then estimating the position of the target device through a geometric method. Commonly used positioning methods based on ranging include: positioning using time of arrival information (divided into TOA, TDOA), positioning based on signal strength information (RSSI), and positioning based on signal angle of arrival (Angle of Arrival, AOA). These technologies are consistent with the technical principles used in UWB and Wi-Fi, but the propagation distance of RFID signals is very short due to energy constraints, generally only a few meters to tens of meters away.

Among them, the non-ranging method refers to collecting scene information in the early stage, and then matching the acquired target with the scene information, so as to locate the target. Typical implementation methods are the reference tag method and the fingerprint positioning method. The commonly used algorithm for the reference tag method is the centroid positioning method. The fingerprint positioning method is basically the same as that used in Wi-Fi positioning, Beacon positioning and other technologies. Arrange some RFID readers in the positioning space. The location of the RFID readers is known. When the target RFID tag enters the scene, multiple RFID readers can read the target RFID tag information at the same time. The location of these RFID readers form a polygon with the connection line, and the centroid of this polygon can be regarded as the position coordinates of the target RFID tag. The implementation steps of the centroid positioning algorithm are simple and easy to operate, but the positioning accuracy is relatively low. It is often used in scenarios where the positioning accuracy is not high and the RFID hardware equipment is limited.

The advantage of the positioning method based on RFID technology lies in its low cost. The cost of active RFID tags is usually tens of yuan, while the cost of passive RFID tags can be several yuan, and the size of the tags is small, usually made into the shape of a sheet, and RFID radio frequency signal has strong penetration and can carry out non-line-of-sight communication. The communication efficiency of the RFID system is very high. Compared with Wi-Fi and Zigbee and other systems that require network access, an RFID reader can complete the reading and writing of hundreds of tags within 1 second. Compared with ZigBee, Bluetooth and Wi-Fi wireless positioning technologies, RFID has lower node cost and faster positioning speed, but its communication capability is weaker, so RFID positioning is especially suitable for simple tagged objects, but does not require a large number of In the case of data communication.

However, the existing positioning system using RFID technology has many shortcomings, such as large positioning error, complex system deployment, and easy to be affected by the environment. For example, the positioning method based on RSSI is limited by the large fluctuation of RSSI itself and the sensitivity to environmental interference. It is difficult to further improve. The positioning method based on TOA and TDOA requires high accuracy of time measurement, but due to the low communication rate of the passive RFID system, it is difficult to observe the precise time. Generally speaking, the application range of RFID positioning technology is narrow, the positioning accuracy is poor, and there are few practical cases.