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With the increasing demand for mobile communications, allowing access to specific location information has shown widespread importance in pervasive computing and applications. In the outdoor environment, the Global Positioning System (GPS) or Beidouxing positioning system based on Global Navigation Satellite System (GNSS) can already meet certain outdoor positioning requirements. However, these technologies cannot be used well in indoor positioning systems, and some alternative technologies must be adopted. In order to achieve this goal, the technology based on IEEE802.11 wireless local area network (Wi-Fi) provides a cost-effective solution. The indoor positioning problem must consider the transmission channel characteristics shown in the indoor environment. Due to the influence of walls and obstacles, a series of problems such as multipath attenuation, absorption and shadowing will be brought. Therefore, measurement techniques based on geometric angles, such as Angle of Arrival (AOA), Phase of Arrival (POA), Time of Arrival (TOA), or Time Difference of Arrival (TDOA), cannot be well applied in indoor positioning systems. In recent years, with the continuous improvement and promotion of radio frequency identification technology (Radio Frequently Identification, RFID), and with its unique advantages such as light weight, low power consumption and strong identification capabilities, it has gradually been used in various occasions, such as identity identification. , Engineering control and positioning tracking. The research focus of this paper is to combine the research of Wi-Fi wireless transmission technology with RFID identification technology, and realize the solution of Wi-Fi system positioning through RFID tags. According to the preliminary search, there are not many researches on Wi-Fi-based RFID positioning technology in China. The work of this article will discuss the positioning scheme of Wi-Fi-based RFID positioning system.
1 The hardware structure of the positioning system
Radio frequency identification technology (RFID) is an emerging automatic identification technology since the 1990s. It mainly uses the unique ID number corresponding to the tag to identify the marker. Similar to sensor technology, RFID technology is considered a supporting technology for The Internet of Things. Some people believe that the former is only identification, without the ability to process, while the latter can process the perceived objects. Compared with traditional magnetic cards and IC cards, the biggest advantage of radio frequency cards is non-contact. Therefore, the identification work does not require manual intervention and can work in various harsh environments. RFID technology can identify high-speed moving objects and can identify multiple tags at the same time, and the operation is quick and convenient.
RFID is a simple wireless system with only two basic components. The system is used to control, detect and track objects. The system consists of a reader and multiple tags.
RFID technology uses wireless radio frequency to carry out non-contact two-way transmission of data between the reader and the tag, which has achieved the purpose of target identification and data exchange. The most basic RFID system consists of three parts: an electronic tag (Tag), a reader (Reader) and a miniature antenna (Antenna) that transmits radio frequency signals between the tag and the reader.
The positioning test platform we will study below is mainly composed of multiple Wi-Fi access points that support the function of RFID readers and Wi-Fi terminals with built-in RFID tags.
2 Positioning plan
Based on Wi-Fi RFID positioning program, we will consider the following two solutions: (1) a composite positioning scheme based on signal strength and time difference of arrival; (2) a positioning scheme based on position distance and angle.
2.1 Compound positioning scheme based on signal strength and time difference of arrival
Before locating indoor Wi-Fi terminals, first plan the indoor terminal distribution map according to the actual indoor situation and store it in the information processing database; then set the AP access point according to the indoor distribution situation, and we need to set a fixed position reference tag , As a measurement reference point to help with position calibration, the number of access points to be set depends on the specific indoor situation.
When the Wi-Fi terminal enters the room, the RFID reader in the AP access point is awakened, and the reader sends a wireless wake-up signal to wake up the RFID tag in the Wi-Fi terminal. After the tag receives the wake-up signal, it wakes up from sleep mode, and then compares whether its ID number is consistent with the ID number in the received signal. The tag that does not match the ID number enters the sleep mode again, and the tag that matches the ID enters the receiving mode and receives The signal and positioning solution calculates its position data, and sends the position data to the processor for processing.literature[4]The RFID-based parking lot wireless positioning scheme has been studied in China, and we have proposed a Wi-Fi-based RFID positioning scheme. The readers in multiple Wi-Fi AP access points respectively read the Wi-Fi to be located. The RFID tag in the Fi terminal integrates the obtained location data, and finally obtains the location information of the RFID tag. Figure 2 shows the structured block diagram based on the positioning algorithm.
The positioning algorithm based on signal strength uses a reference tag with a known location and the tag of the Wi-Fi terminal to be located to receive the positioning signal of the same AP access point. After comparing the measured position of the reference tag with the known value of the point, the correction value of the positioning data of the reader is obtained, and then this correction value is sent to the tag to be positioned in time to correct the real-time position information measured by the tag S, and upload this information to the reader. During this process, only one reader is always working, and the other readers are in a dormant state. After one reader completes the location information collection, the other readers complete the same process. In this positioning mode, at least Need to collect location information three times. Assuming that the signal strength formula is related to the distance between the reader and the tag, a simple signal propagation model can be expressed by equation (1):
P(r )= (P )-10alog(r/0r)(1)
In the formula, P(r) is the signal power received by the tag; r is the distance between the tag and the reader; r0 is the reference distance relative to the reader; P(r)0 is the signal power of the reference point, the parameter a is the rate at which the path loss increases as the distance r increases.It needs to be pointed out that the signal strength of a certain location calculated by this model is often overestimated and has little practicality. The more practical signal strength model is in the literature.[6~7]There is a more detailed introduction.
Adopting the time difference of arrival (TDOA) algorithm is to measure the time difference between the positioning signals of the same tag received by different readers, and to calculate the distance difference between the tag and different readers. The distance difference D between the tag and any two readers is a fixed value, and the tag must be positioned on the hyperbola with two readers as the focus. When N readers are involved in distance measurement at the same time (N≥3), The intersection area between multiple hyperbolas is an estimate of the label position. TDOA only measures the arrival time difference of the same tag positioning signal received by each reader, and each reader participating in the positioning does not require strict synchronization in time. Assuming that the arrival times of the i-th reader and the j-th reader are TAi and TAj respectively, the time difference between the signal reaching the i-th and j-th reader is TAji = TAi-TAj, and their distance difference Rji = C*TAji. Use the coordinate value calculated based on the signal strength positioning algorithm and some prior knowledge (such as indoor radius) to distinguish the specific location of the Wi-Fi terminal from its two solutions.
2.2 Positioning scheme based on position distance and angle
literature[5]In, P.Munishwar et al. proposed a robot positioning solution based on RFID. Based on this work, we proposed a terminal positioning solution that can be used in Wi-Fi systems. The difference between this solution and the positioning solution based on signal strength is that it introduces the calculation of the direction angle, and realizes the positioning of the Wi-Fi terminal through the distance and angle information. One or more sensors need to be set up in the Wi-Fi system to track the movement of the terminal to be located. These sensors can be queried through the serial interface and return the movement distance and angle information of the terminal since the last query.
The RFID tag built into the Wi-Fi terminal allows the terminal to be located to determine its absolute position (x, y), and the position uncertainty is equal to the maximum tag sensing radius of the RFID reader.
The system runs the algorithm periodically (for example, 50ms) to obtain and process the data from the sensors and RFID readers.
Regularly recalibrate the terminal position and angle data through RFID technology. Each time the positioning algorithm runs, it reads the change in distance and angle difference after two queries of the sensor, and accumulates the changes to the known position and angle respectively. , And reset the sensor. The sensor can judge its moving direction from the coordinates (x1, y1) and (x2, y2) measured twice. If the terminal is moving in a straight line, then △θRFID is equal to 0, and finally according to the position and angle information of the terminal, we can locate the terminal.
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