Active RFID system technology and reliable data transmission

Introduction
In modern life, more and more electronic technologies are used to complete information query. The most common technologies are barcode technology and RFID (radio frequency identification) technology. Barcode technology is widely used, but its limitations are also very prominent, such as: the content of the barcode label cannot be modified, the barcode label must be clean and not worn, the reader should maintain an appropriate angle when reading/writing the barcode, and the reader and the barcode must be visible. RFID is the abbreviation of radio frequency identification. It is a non-contact automatic identification technology that uses radio technology to establish communication between the reader and the electronic tag to achieve contactless information transmission and achieve identification through the transmitted information. RFID technology has now been widely used in data collection and processing in application fields such as production, logistics, transportation, medical care, and tracking. The advantages of RFID technology are more prominent, such as: tag information can be modified, it can be read at a long distance in harsh environments, and multiple tags can be processed at the same time.
According to different implementation methods, RFID technology can be divided into passive RFID technology and active RFID technology. The research and application of passive RFID technology are relatively mature. All the power required for the operation of the electronic tag in the system is obtained by converting the electromagnetic waves sent by the received reader. The transmission power of the reader is generally large. The electronic tags in the active RFID technology system have batteries that can provide power for all devices to work; the transmission power requirements of the reader are lower, and the effective reading distance is also increased compared to the former, which is very useful in tracking and identifying high-value goods.

l Design of active RFID system
1.1 System structure
The active RFID system consists of three parts: active electronic tags (tags), readers (readers) and computers; active electronic tags are composed of coupling elements and chips, each tag has a unique electronic code (EPC) and stores electronic data in an agreed format; the reader is a device that reads the electronic tag information, which can be designed as a handheld or fixed type; the computer (host computer) is used for data management and data processing, and its system structure is shown in Figure 1.

In practical applications, the reader sends out a radio frequency signal of a certain frequency, and the electronic tag attached to the surface of the object to be identified performs the corresponding action after receiving the radio frequency signal. Usually the reader is connected to a computer, and the information sent back by the electronic tag is read and decoded by the reader and sent to the computer for further processing, thereby achieving the purpose of automatic identification.
1.2 Hardware Design
RFID systems can be divided into different types according to different standards. According to the working frequency used by the RFID system, it can be divided into four categories: low frequency (LF, 30-300 kHz), high frequency (HF, 3-30 MHz), ultra-high frequency (UHF, 300-968 MHz) and microwave (UWF, 2.4-5.8 GHz). According to the literature and the analysis and comparison of the advantages and disadvantages of RFID systems in different frequency bands, combined with the requirements of the subject, it can automatically monitor tags within a range of 50 m and can read and write data. The RFID system with a microwave frequency band of 2.4 GHz is selected for research. The 2.4 GHz frequency has the characteristics of small attenuation, long transmission distance, fast data transmission, high data throughput, and strong ability to identify multiple targets. At the same time, the 2.4 GHz ISM band width exceeds 83 MHz, with 125 channels, which can meet the needs of multi-frequency and frequency hopping, and increase the anti-interference ability of wireless communication.
The active electronic tag and reader main chip use the CC2510 of Texas Instruments. The device contains a high-performance, low-power 8051 microcontroller (MCU) and UHF RF transceiver, integrates 32 KB in-system programmable FLASH and embedded 4 KB SRAM, and has functions such as power amplifier (PA), low noise amplifier (LNA), modem (MODEM), QLP package, small size (6 mm×6 mm), supports popular frequency hopping technology, and can programmatically control the data transmission rate. The fastest data transmission rate can reach 500 Kb/s. Using CC2510 in the system has the advantages of low power consumption, low cost, simple and reliable peripheral circuits, etc. In addition, CC2510 can set the frequency hopping communication mode, transmission power and storage program, which can realize frequency change, read and write distance control and multiple security protocols to apply to multiple security levels.
The active electronic tag is composed of CC2510 wireless transceiver and control module and antenna. It exchanges data with the reader through electromagnetic waves. The reader is composed of CC2510 wireless transceiver and control module, antenna, USB interface conversion module and interface circuit. The reader is connected to the host computer through the USB interface to receive the data sent by the active electronic tag. 1.3 Software Design
In the entire active RFID system, the software plays a control role and is the soul of coordinating the various parts of the hardware. It mainly involves the human-machine interface of the PC and the RFID system software design, the communication software design between the PC and the reader, and the communication software design between the reader RF and the active electronic tag RF.
The PC software mainly manages the RFID system database. It is compiled in C" and the database uses SQL Setver 2005. The PC operates the reader through the interface circuit. This part is compiled in VB. The data communication between the reader and the tag is compiled in a mixed way of standard C language and assembly. There are many descriptions of the communication software design between PC and single-chip microcomputer in single-chip microcomputer, which will not be repeated here. This article focuses on the design and research of reliable communication between the reader and active electronic tag.

2 Reliable communication research
In active RFID system, reliable communication is the most important link, which almost determines the performance of the entire system. Here, reliable data communication is studied from four aspects: frame identification replacement algorithm, data frame structure, anti-collision processing and retransmission mechanism.
2.1 Frame identification replacement algorithm
In the communication protocol, Ox76 (0x represents hexadecimal counting) is used as the frame identifier. In order to ensure the uniqueness of the frame identifier, a replacement algorithm is used for Ox76 in the frame content, so that Ox76 no longer appears in the frame content, which solves the synchronization problem of the receiving end and improves the reliability of reception. If the data sent is 0x76, it is replaced by two bytes of Ox77 and Ox77; if the data sent is Ox77, it is replaced by two bytes of Ox77 and 0x78. The algorithm flow is shown in Figure 2.

2.2 Data frame structure
The reader/writer operates the tag as read/write. The electronic tag device has built-in MCU and FLASH, which can store relatively complex programs and control the tag operation by the program. In order to enhance the simplicity and efficiency of the active RFID system structure, the instruction format is standardized, and the design of the data frame format also adopts simple and fixed specifications to improve communication efficiency.
During communication, the data frame format is as follows:

The boot area contains the preamble and synchronization word. In the case of good channel characteristics, a 16-bit preamble and a 16-bit synchronization word can be set to improve the recognition speed. The check area is checked by the CRC algorithm. The boot area and the check area are automatically added by the CC2510 hardware and automatically removed by the hardware during reception. In this design, the address area is used for electronic tag recognition, and the commands in the command area are used to complete the data query function or complete the generation of tag information. The data area is used for data payload storage. In the sending mode, the data in the address area, command area and data area are sent to the buffer area in the RAM for corresponding packaging operations. The CC2510 adds a 4-byte preamble and synchronization word, adds CRC check and sends it out. In the receiving mode, the packet processing support will decompose the data packet, that is, first perform synchronization word detection, then detect the address, perform data length matching, calculate and check CRC, and finally submit the operation command and data payload to the upper layer for processing, thereby completing one sending and receiving interaction. 2.3 Anti-collision processing
The focus of the active RFID system implementation is the implementation of the anti-collision algorithm. At present. The implementation methods of this type of algorithm include space division multiple access (SDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) and time division multiple access (TD-MA). This design adopts the method based on carrier sense multiple access (CSMA) in the ETSI 302 208 standard. CSMA is a distributed media access control protocol. Within the coverage range of the reader, each active electronic tag can independently decide to send and receive data frames.
Before sending a data frame, each active electronic tag must first perform carrier sensing. Only when the medium is idle can it be allowed to send frames. Compared with FDMA and TDMA, it can make better use of resources. Because this communication method always detects whether there is a carrier with the same frequency in the air before sending data. If there is a carrier with the same frequency, data will not be sent; if there is no carrier with the same frequency in the air, it means that the current space resources are not occupied and data can be sent. In this way, not only the utilization efficiency of space resources is improved, but also the reliability of communication is improved.
CSMA is implemented by using the function of CC2510 to support automatic clearing of channel access (CCA) before transmission. After the electronic tag is initialized, the program enters the main loop program. The electronic tag starts carrier sensing. When CCA is not 1, it means that there is no identical carrier data in the air, and then it sends the corresponding data. Each electronic tag sends in a competitive manner. The CSMA sending flow chart is shown in Figure 3.

2.4 Retransmission mechanism
The retransmission mechanism mainly adopts the ACK (acknowledge) method, that is, the sender sets a cache and a corresponding retransmission timer for each data packet sent. If an ACK control packet for this data packet is received from the destination node before the timer times out, it is considered that this data packet has been successfully transmitted. At this time, the cache and timing of the data packet are canceled, otherwise, the data packet will be retransmitted and the timer will be reset. For each data packet, the receiver needs to feedback ACK.
The retransmission mechanism is mainly implemented by the following function functions. The Init() function is used to initialize the device, set DMA, clock, etc.; the Send() function is used to send data packets; the ackTimeolJt() function is used to retransmit data packets if ACK is not received within the specified time; the waitForAck() function is used to wait for ACK. Within the specified time, the ACK is marked as T, otherwise it is marked as F; the aekReceived() function is used to receive ACK and cancel retransmission; the Receive() function is used to receive data packets in the specified format; the dataCheck() function is used to detect the integrity and reliability of the data; the sendAck() function is used to send ACK feedback to the sender. The retransmission mechanism program flow chart is shown in Figure 4. 3 Test results
In the actual environment, the active RFID system reader and active electronic tag were tested for read/write, with the modulation mode of MSK, data transmission rate of 60 Kb/s, and filter bandwidth of 540 kHz. 10 active electronic tags were placed at different distances for 200 B transceiver operation. The reading results of the reader are shown in Tables 1 and 2.

From the above data, we can see that:
(1) The system recognition effect is good within the visual distance of 25 m;
(2) After adding the reliable communication mechanism, the protocol recognition effect between the tag and the reader is good within the visual communication distance of 60 m; as the communication distance increases. At 70 m, the CRC check errors in the normal communication signal between the tag and the reader increase, but most tags can still be identified;
(3) When encountering obstacles, the communication distance decreases rapidly, and the packet loss rate and the number of CRC check errors increase.

4 Conclusion
Compared with passive RFID technology, active RFID technology has obvious technical advantages, with the characteristics of long communication distance, large amount of transmitted data, and low transmission power. The reliable transmission of data in the active RFID system is the most important part. This paper proposes a solution for the reliable transmission of the active RFID system, which better handles this problem.
In the future, active RFID technology will not only be widely used in various industries, but will also be combined with common computer technologies such as sensor networks (wSN), which will have a profound impact on the information society.