CN103632117B - Active RFID (Radio Frequency Identification) positioning method based on direct sequence spread spectrum technology - Google Patents

Abstract

The invention discloses an RFID (Radio Frequency Identification) positioning method based on a direct sequence spread spectrum technology. The method mainly comprises the steps: a fixed tag awakes an appointed mobile tag through broadcasting the positioning information; the mobile tag replies the position information by a direct spread spectrum sequence; the distance information is computed by the fixed tag and is transmitted to a reader; a host computer collects a plurality of pieces of distance information from the reader and calculates the position of the mobile tag. The communication between the position tag and the mobile tag is completed by using the direct spread spectrum sequence, and the position of the tag is obtained by calculation; thus when the measurement distance is longer, the measurement precision can be still higher.

Description

Active RFID Location Method Based on Direct Sequence Spread Spectrum Technology

technical field

The invention belongs to the technical field of wireless communication and relates to an active RFID positioning method, in particular to the design of an active RFID positioning method based on direct sequence spread spectrum technology.

Background technique

The existing radio frequency positioning short-distance technologies include radar positioning, wireless direction finding, signal strength RSSI positioning, TOA (radio frequency signal transmission time) and TDAO (radio frequency signal transmission time difference) and other positioning methods. The radar search and positioning system uses radar to send out signals in different directions, and then determines the position of the searched target through the analysis of the returned signal of the searched target. A typical example of this method is to use military radar to search for the enemy. However, this method is obviously not suitable for general positioning applications due to the expensive equipment and bulky size. The wireless direction finding and positioning system first needs to install a simple wireless signal generating device on the object to be searched, and transmit the signal outward intermittently. The search personnel need to use the antenna and monitoring equipment to determine the general direction of the signal source. And according to the strength of the signal, to judge the approximate position of the searched target, although this method is simple, but the accuracy is poor.

Due to the advancement of technology, many micro-power transceivers made of single chips today have the function of signal strength indication (RSSI). Therefore, using the received signal strength (RSSI) to determine the position of a moving target has also become a method of positioning.

It is not practical to use the method of measuring the radio frequency signal from searching for the target to the time of transmission (TOA) positioning of the listening receiver, because the time synchronization between the signal source and the signal receiver is very high, so it is not practical; using two different signals ( Such as radio frequency signal and ultrasonic signal), the method of transmission time difference (TDOA) positioning from the signal source to the signal receiver, because two different signals need to be used, it also needs to use two devices for transmitting and receiving different signals, such as ultrasonic generator And receiving equipment, which increases the cost. At the same time, the accuracy of this positioning method is directly related to the accuracy of time difference measurement, which increases the difficulty of its use, which also hinders its application.

RFID (Radio Frequency Identification) is a wireless communication technology that can identify specific targets and read and write related data through radio signals without establishing mechanical or optical contact between the identification system and specific targets. Radio signals transmit data from a tag attached to an item through an electromagnetic field tuned to a radio frequency to automatically identify and track the item. Some tags can get energy from the electromagnetic field emitted by the identifier during identification, and do not need batteries; there are also tags that have their own power supply and can actively emit radio waves (electromagnetic fields tuned to radio frequencies). However, the disadvantages of the RFID positioning method in the prior art are that it is susceptible to signal interference, and the measurement accuracy is not high if the distance is relatively long when locating a moving target.

The current ranging method based on RSSI generally adopts the following logarithmic loss model:

In the formula, d is the distance between the transmitter and the receiver; d ₀ is the reference distance; η is the channel attenuation index; the value generally ranges from 2 to 4; X _σ is a Gaussian random noise variable with zero mean and mean square error σ; PL(d ₀ ) is the signal strength at a distance d ₀ from the transmitter; PL(d) is the signal strength at a distance d from the transmitter; PL(d ₀ ) can be obtained empirically or defined from hardware specifications. From this equation, the distance d can be calculated from the signal strength PL(d). There are also attenuation factor models that include building type effects as well as changes due to obstructions. Although these methods are very simple and do not require additional hardware, the accuracy is also very poor due to the large error in the measurement of RSSI signal strength and many influencing factors, especially in short-range and short-distance.

Contents of the invention

The purpose of the present invention is to provide an RFID positioning method based on direct sequence spread spectrum technology in order to solve the shortcomings of the RFID positioning method in the prior art.

The technical solution of the present invention is: an RFID positioning method based on direct sequence spread spectrum technology, specifically comprising:

S1. Set the position of the fixed label, and send the position information and ID information of the fixed label to the host computer through the reader, and store it by the host computer;

S2. The reader sends a positioning command to the fixed tag, and the fixed tag receives the positioning command;

S3. The fixed tag performs direct spread spectrum communication with the mobile tag within the coverage area, and the mobile tag replies its own ID information to the fixed tag through a direct spread spectrum sequence;

S4. The fixed tag calculates the distance from the mobile tag, and sends the distance information and the ID information of the fixed tag to the reader. The reader receives all the positioning information sent by the fixed tag and uploads it to the upper computer, which is based on the stored fixed tag. The ID information of the mobile tag is compared with the received positioning information to obtain the location information of the mobile tag.

Further, both the fixed tag and the mobile tag are in the periodic sleep state by default, and wake up from the periodic sleep at regular intervals to listen for positioning instructions within the range.

Further, the positioning information sent by the mobile tag to the fixed tag will go through the following processes in the mobile tag: information encoding, signal format generation, direct spread spectrum modulation, baseband filtering, shaping, quadrature up-conversion, passband filtering , the signal is amplified and finally sent to the fixed tag through the antenna.

Further, the fixed tag calculates the distance between the fixed tag and the mobile tag by obtaining the phase difference between the pseudo-codes of the uplink sequence and the downlink sequence through fine synchronization.

Further, the communication process of the direct sequence spread spectrum between the fixed tag and the mobile tag and the process of calculating the distance between the fixed tag and the mobile tag are as follows: firstly, the fixed tag frames the positioning instruction, and then spreads the spectrum, After orthogonal up-conversion, it is modulated onto the carrier and transmitted by the antenna, and the spread spectrum signal is broadcast in the area covered by the fixed tag;

The mobile tag receives the positioning command, after demodulation and despreading, obtains the positioning command, the pseudo code generator generates a copy of the pseudo code sequence, synchronized with the locked uplink sequence, and the copied pseudo code sequence is modulated to the downlink carrier as a downlink signal, through the direct sequence Sent to the corresponding fixed tag by means of spread spectrum;

When the fixed tag obtains the positioning information replied by the mobile tag, it compares the phase of this pseudo-code sequence with the initially sent uplink pseudo-code sequence through a comparator, calculates the time difference, subtracts the fixed delay, and obtains the total delay, and calculates Distance between fixed and moving labels.

Furthermore, the pseudo-code selects the m sequence as the spread-spectrum pseudo-code in the spread spectrum process of the sequence, and the format of the signal is made up of frame header, identification bit, data segment and CRC check, wherein the frame header is used for signal tracking, identification The bit is used as the arrival time timing identification, the data segment contains the ID of the position tag and the mobile tag, which is used to distinguish different tags, and the CRC check is used for error checking.

The beneficial effects of the present invention are: the RFID positioning method based on the direct sequence spread spectrum technology of the present invention completes the communication between the position tag and the mobile tag by using the direct spread spectrum sequence, and calculates the distance between the two, and sends the related The distance information is calculated and sent to the reader, and the reader sends multiple distance information to the computer, and the computer can realize the positioning of the electronic tag in one-dimensional and multi-dimensional spaces; because the spreading code information is coherent, that is, one information A complete pseudo-code cycle is filled in the code, so the characteristic phase of the spreading code can be used as the bit synchronization signal of the information code, so the accuracy of ranging depends on the accuracy of the pseudo-code tracking at the receiving end, and the unambiguous distance is determined by the information frame The length is determined, and it can be used for long-distance ranging. The precise position can be obtained by means of spread spectrum, and then the tag position can be obtained through calculation. This can solve the problem of relatively high measurement accuracy when the measurement distance is long.

Description of drawings

Fig. 1 is the block flow diagram of the RFID positioning method based on the direct sequence spread spectrum technology of the embodiment of the present invention;

Fig. 2 is the positioning environment in the RFID positioning method based on the direct sequence spread spectrum technology of the embodiment of the present invention;

Fig. 3 is the process that the positioning information goes through in the mobile tag in the RFID positioning method based on the direct sequence spread spectrum technology according to the embodiment of the present invention;

Fig. 4 is a schematic diagram of positioning between a mobile tag and a fixed tag in an RFID positioning method based on direct sequence spread spectrum technology according to an embodiment of the present invention.

detailed description

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 1, it is a block flow diagram of an RFID positioning method based on direct sequence spread spectrum technology in an embodiment of the present invention, specifically including:

S1. Set the position of the fixed label, and send the position information and ID information of the fixed label to the host computer through the reader, and store it by the host computer;

S2. The reader sends a positioning command to the fixed tag, and the fixed tag receives the positioning command;

S3. The fixed tag performs direct spread spectrum communication with the mobile tag within the coverage area, and the mobile tag replies its own ID information to the fixed tag through a direct spread spectrum sequence;

S4. The fixed tag calculates the distance from the mobile tag, and sends the distance information and the ID information of the fixed tag to the reader. The reader receives all the positioning information sent by the fixed tag and uploads it to the upper computer, which is based on the stored fixed tag. The ID information of the mobile tag is compared with the received positioning information to obtain the location information of the mobile tag.

In order for those skilled in the art to understand and implement the present invention, the following will be described in detail in conjunction with specific examples, as shown in Figure 2 is the positioning environment in the RFID positioning method based on direct sequence spread spectrum technology in the embodiment of the present invention:

In the technical solution of the present invention, the mobile tag is a relatively simple single-chip signal receiver and transmitter. Its transmission power can be controlled by software or hardware. It is generally powered by a battery and carried with the chip. After starting to work, in the default state, it will enter the periodic sleep state, and it will periodically wake up every once in a while to listen for positioning instructions from fixed tag broadcasts. If there is positioning instruction information from the fixed tag, then the mobile tag will send the related information of the tag to the fixed tag in a short period of time through direct spread spectrum.

The structure of a fixed tag is roughly the same as that of a mobile tag. Its location information and its own related heart are fixed and stored in the host computer, and similarly, it also sleeps periodically, and wakes up every once in a while to listen for There is a positioning command from the reader, once there is, the positioning command will be broadcast through the spread spectrum sequence in the covered area, and wait to receive the position information returned by the mobile tag, calculate the relative distance between the two, and finally use the calculated The distance information is sent to the reader.

Generally, the reader is a signal receiver and transmitter connected to the upper computer, which can communicate with the fixed tag or directly with the mobile tag. When positioning, the reader first sends a positioning command to the fixed tag and accepts it. The distance information sent back by the location tag, after receiving the distance information, sends the relevant distance to the host computer.

The upper computer is also a computer in the general sense, which stores the ID information and location information of each fixed tag, and also stores the ID information of each mobile tag, which is used to identify different tag information, so when it accepts information from readers After the sensor collects the distance information transmitted from different fixed tags, it can calculate the real-time position of the mobile tag.

The following will introduce in detail through the specific working process. First, the reader sends a positioning command to the fixed tag. Before receiving the positioning command from the reader, the fixed tag goes to sleep periodically and wakes up periodically. Listen to the signal from the reader, once it receives the positioning command from the reader, it will enter the working state, and send a specific positioning command to the coverage area, and the mobile tag will wake up after receiving the specific wake-up command. After a specific mobile tag is awakened, it will reply a position information to the corresponding fixed tag. The fixed tag and the mobile tag communicate with direct sequence spread spectrum. The fixed tag calculates the phase difference between the uplink sequence and the downlink sequence pseudocode. The distance between the two, and immediately send the calculated distance to the reader, the reader recognizes the distance information sent from different fixed tags, and sends the distance information to the host computer, and the host computer uses the fixed location information of the fixed tag and the corresponding distance information to calculate the position of the mobile tag in real time.

The communication process of the direct sequence spread spectrum between the fixed tag and the mobile tag and the process of calculating the distance between the fixed tag and the mobile tag are as follows: firstly, the fixed tag frames the positioning instruction, and then spreads the spectrum, and after the quadrature Frequency conversion, modulated to the carrier and transmitted by the antenna, the spread spectrum signal is broadcast in the area covered by the fixed tag; the mobile tag receives the positioning command, after demodulation and despreading, obtains the positioning command, and the pseudo code generator generates a pseudo code The code sequence is synchronized with the locked uplink sequence, and the pseudocode sequence is copied as a downlink signal and modulated to the downlink carrier, and sent to the corresponding fixed tag through direct sequence spread spectrum; the fixed tag obtains the positioning information replied by the mobile tag, and compares the The device compares the phase of this pseudo-code sequence with the initially sent uplink pseudo-code sequence, calculates the time difference, subtracts the fixed delay, obtains the total delay, and calculates the distance between the fixed tag and the mobile tag. And the pseudocode of the sequence in the spread spectrum process selects the m sequence as the spread spectrum pseudocode, and the format of the signal is made up of frame header, identification bit, data segment and CRC check, wherein said frame header is used for signal tracking, and the identification bit is used as The arrival time timing mark, the data segment contains the ID of the position tag and the mobile tag, which is used to distinguish different tags, and the CRC check is used for error checking.

In the process of sending location information from a mobile tag to a fixed tag, the location information has to go through roughly: information encoding, signal format generation, direct spread spectrum modulation, baseband filtering, shaping, quadrature up-conversion, passband filtering, signal amplification, Finally, it is sent to a fixed tag through the antenna, and the process is shown in Figure 3.

In the process of obtaining the spread spectrum signal, because the distance between the fixed tag and the mobile tag in the present invention will not exceed one chip in the embodiment of the pseudo code, so it is not necessary to capture this link during the synchronization process. To ensure that the phase is smaller than the width of one symbol, it is only necessary to perform fine synchronization to obtain the phase difference between the uplink and downlink pseudo-codes, and fine synchronization is also tracking. The tracking problem can be well solved by the delay locked loop. The final phase difference is obtained by the pseudo-code phase comparator of the position tag, and then the distance between the mobile tag and the position tag can be calculated. The specific process is shown in the figure 4.

Those skilled in the art will appreciate that the embodiments described herein are to help readers understand the principles of the present invention, and it should be understood that the protection scope of the invention is not limited to such specific statements and embodiments. All possible equivalent replacements or changes made according to the above descriptions are deemed to belong to the protection scope of the claims of the present invention.

Claims (3)

1. The RFID positioning method based on direct sequence spread spectrum technology, is characterized in that, specifically comprises: S1. Set the position of the fixed label, and send the position information and ID information of the fixed label to the host computer through the reader, and store it by the host computer; S2. The reader sends a positioning command to the fixed tag, and the fixed tag receives the positioning command; S3. The fixed tag performs direct spread spectrum communication with the mobile tag within the coverage area, and the mobile tag replies its own ID information to the fixed tag through a direct spread spectrum sequence; S4. The fixed tag calculates the distance from the mobile tag, and sends the distance information and the ID information of the fixed tag to the reader. The reader receives all the positioning information sent by the fixed tag and uploads it to the upper computer, which is based on the stored fixed tag. The ID information of the mobile tag is compared with the received positioning information to obtain the location information of the mobile tag; The fixed tag calculates the distance between the fixed tag and the mobile tag by obtaining the phase difference between the uplink sequence and the downlink sequence pseudocode through fine synchronization; The communication process of the direct sequence spread spectrum between the fixed tag and the mobile tag and the process of calculating the distance between the fixed tag and the mobile tag are as follows: firstly, the fixed tag frames the positioning instruction, and then spreads the spectrum, and after positive up-conversion , modulated onto the carrier and transmitted by the antenna, the spread spectrum signal is broadcast in the area covered by the fixed tag; The mobile tag receives the positioning command, after demodulation and despreading, obtains the positioning command, the pseudo code generator generates a copy of the pseudo code sequence, synchronized with the locked uplink sequence, and the copied pseudo code sequence is modulated to the downlink carrier as a downlink signal, through the direct sequence Sent to the corresponding fixed tag by means of spread spectrum; When the fixed tag obtains the positioning information replied by the mobile tag, it compares the phase of this pseudo-code sequence with the initially sent uplink pseudo-code sequence through a comparator, calculates the time difference, subtracts the fixed delay, and obtains the total delay, and calculates The distance between the fixed label and the mobile label; In the process of spreading, the pseudo code selects the m sequence as the spreading pseudo code, and the format of the signal is composed of frame header, identification bit, data segment and CRC check, wherein the frame header is used for signal tracking, and the identification bit is used as the arrival time Timing identification, the data segment contains the ID of the position tag and the mobile tag, which is used to distinguish different tags, and the CRC check is used for error checking. 2. The method according to claim 1, wherein the fixed tag and the mobile tag are in a periodic sleep state by default, and wake up from the periodic sleep at regular intervals to listen to Whether there is a positioning instruction. 3. The method according to claim 1, wherein the positioning information sent by the mobile tag to the fixed tag undergoes the following processes in the mobile tag: information encoding, generation of signal format, direct spread spectrum modulation, baseband Filtering, shaping, quadrature up-conversion, passband filtering, signal amplification, and finally sent to a fixed tag through the antenna.