RFID-based two-dimensional indoor positioning algorithm to achieve detailed tutorial

At present, RFID positioning mainly uses LANDARC and its derived methods. The more commonly used method is to place a reference tag every 1 to 2 m on a two-dimensional plane, and more than 4 remote RFID readers are required, resulting in higher hardware costs. This article proposes another method that uses only 4 reference tags and 2 remote RFID readers on a two-dimensional plane to achieve two-dimensional indoor positioning, greatly reducing hardware costs and making up for GPS only for outdoor use. Insufficient positioning.

1.1 One-dimensional positioning

As shown in Figure 1, the four electronic tags #1 to #4 are placed in a fixed position. Assuming that the distances between the reference tags #1 to #4 and the reader are rx1, rx2, rx3, and rx4, the signal strength indicators (RSSIs) received by the reader on the electronic tags #1 to #4 are Sx1, Sx2, and Sx3, respectively. Sx4. Since it is indoors, the RFID reader receives the reflected power Pref and the error power Perr in addition to the linear transmission power of the electronic tag. Therefore, the total power Ptotal from an RFID tag received by the RFID reader can be obtained. Expressed as:

Where: Perr is the error caused by other than reflection; Ptr is the instantaneous power emitted by the electronic tag; Gt, Gr is the antenna gain of the electronic tag and reader; λ is the wavelength of the RF signal.

Figure 1 indoor one-dimensional positioning of remote RFID reading

Since the signal strength index RSSI increases with increasing power, Equation (1) can assume:

However, in the actual measurement, it is difficult to know the values ​​of Ptr, Ptotal, Pref, Perr. In order to measure the value of rx in real time, f(4) in the equation (2) can be approximated by polynomials from the previously known data (rx1, Sx1), (rx2, Sx2), (rx3, Sx3) and (rx4, Sx4). (sx) function. Assumptions:

The coefficients a0, a1, a2, a3 can be obtained from equations (5) and (6).

Actual positioning can be divided into the following steps:

(1) From the electronic tag with fixed position in Fig. 1, four sets of data (rx1, Sx1), (rx2, Sx2), (rx3, Sx3) and (rx4, Sx4) can be obtained.

(2) The coefficients a0, a1, a2, a3 of the polynomial can be calculated from equations (5), (6) and these four sets of data.

(3) The best RSSI value read by the remote reader is an integer between 0 and 256. Available sx value (0

1.2 Two-dimensional positioning

Based on the one-dimensional positioning, the situation of two-dimensional positioning can be further deduced. For the schematic of Fig. 2, two remote RFID readers (X, Y) and electronic tags #1 to #4 are used. Suppose the distance between the electronic tag to be positioned and the remote reader (X, Y) (rX The relationship between (rY) and the RSSI value (sX, sY) is described by equations (7) to (10). The coefficients aX0, aX1, aX2, aX3 and aY0, aY1, aY2, aY3 can be derived from the aforementioned one-dimensional positioning method.

Since the distance between the remote readers (X, Y) in FIG. 2 is fixed and known, the positions of the electronic tags to be positioned on the two-dimensional plane can be known from equations (7) to (10). According to this position method, the position of the electronic tag to be positioned may also be the position below the horizontal axis in FIG. 2 , as shown by the dotted line. In order to distinguish two different positions on the horizontal axis of the electronic tag to be positioned in FIG. 2 , another reader may be arranged. Since the RSSI value of the electronic tag on the horizontal axis read by the reader can be distinguished, the position of the electronic tag above the horizontal axis and below the horizontal axis can be distinguished.

Figure 2 Schematic diagram of two-dimensional positioning

The overall system architecture of the design is shown in Fig. 3. The server executes the RFID positioning program, and at the same time, the Socket communication port is opened, and it is waiting to receive the one-dimensional positioning data from the client. When the Serv-er end itself obtains one-dimensional positioning data and also receives the one-dimensional positioning data of the Client, the final result is displayed on the display screen. The client uses an embedded system (XSCALE architecture PXA-270). The main peripheral devices are a pen drive, a control panel, an RFID reader, and an IP distributor. When the embedded system starts, mount the flash drive to load the specified folder into the memory and execute the set shell file. After the RFID positioning program is executed, the communication port is opened and the RFID module is started. When the RFID positioning program is one-dimensionally positioned After the data is transmitted to the server through the LAN.

Before the RFID positioning system, the reader and reference tag must be placed in a fixed position. The server-side main program starts by reading the set.txt file first to know in advance the card number of the reference tag and the corresponding distance between the server and the reference tag. Then, the embedded system sends a command to the reader to read the RSSI value of the reference electronic tag and store the received RSSI value in an array dedicated to each reference tag. When the RSSI value of the reference tag is read multiple times, the unreasonable RSSI is eliminated according to the variation number, and a reasonable RSSI is retained to be averaged. Then, the RSSI value of the reference tag is used according to the equations (7) to (10) to find the table. Get the distance between the reference tag and the reader. The above procedure can be done only once to achieve the purpose of error correction. Then execute the send command to the card reader and receive the card number and RSSI value of the label to be positioned. After the positioning tag reads Num times, it will be treated as a variation, and the Num value can be adjusted according to the situation. After the server receives the one-dimensional positioning data of the client, it converts the two-dimensional positioning coordinates based on the one-dimensional distance.

The client performs an action similar to that of the server. The difference is that when the subroutine is executed, the main program judges whether the flag bit is 1. If the condition is true, the one-dimensional positioning distance is displayed on the XSCALE-270 display screen, and the client is connected to the Socket port. One-dimensional distance data is transmitted to the server.

The overall system function test was performed in an indoor laboratory. Due to space limitations, the distance experiment (X, Y) coordinates of the positioning were (3 m, 2 m) and (6 m, 4 m), and the server was the origin (0, 0). Each reference tag reads 10 groups of RSSI values ​​and uses the variation number to calculate the distance parameters. The positioning tag reads the 5 groups of RSSI values ​​to make the mutation and then substitutes them into the RSSI value to obtain a one-dimensional estimation distance. The two-dimensional distance coordinates are obtained by converting the one-dimensional distance data obtained by the server and the client, as shown in FIG. 4 .

Figure 4 Two-dimensional distance coordinates

From Fig. 4, it can be observed that the positioning coordinates are within 1 to 30 s, and the (X, Y) coordinates change greatly. According to the characteristics of the electric wave itself, it is known that the electronic tag does not move at a fixed position, but the RSSI value may drift. According to this phenomenon, when the positioning data is obtained, the number of times to read the RSSI value of the reference tag needs to be increased in order to obtain more accurate positioning data.

After increasing the number of times to read the RSSI value of the reference tag, the experimental data is shown in Figure 5 when the distance experiment (X, Y) coordinate is (7 m, 5 m). The positioning results found that the accuracy and stability have been greatly improved, proving that this solution is effective.

Figure 5 Optimized Coordinates

Based on the one-dimensional positioning, this paper proposes RFID two-dimensional positioning technology, which can realize indoor positioning by using only four reference tags and two remote RFID readers on a two-dimensional plane. The indoor positioning system is designed to verify the algorithm. The experimental results show that the accuracy and stability of the two-dimensional positioning have been greatly improved, and the positioning performance has been improved on the basis of reducing the cost of RFID positioning.

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