Detailed explanation of the application scheme of WLAN+RFID combined system,NFC Label

　　Detailed explanation of the application scheme of WLAN+RFID combined system

　　Wireless local area network (WLAN) can reduce the cost of network infrastructure, enabling users to enjoy the application of mobile network and obtain high-efficiency, high-quality and low-cost services.

　　Introduction

　　Wireless local area network (WLAN) can reduce the cost of network infrastructure, enabling users to enjoy the application of mobile network and obtain high-efficiency, high-quality and low-cost services. Mobility enables users to freely change positions while using devices such as handheld computers or data collectors, which greatly facilitates personnel who need to constantly move their positions during work. These devices have real-time access to data stored in a central database. Radio frequency identification (RFID) is a non-contact automatic identification technology. Its basic principle is to use radio frequency signals and space coupling transmission characteristics to realize automatic identification of identified objects. It has the characteristics of large data storage, readable and writable, strong penetrating power, long recognition distance, fast recognition speed, long service life and good environmental adaptability. In addition, it is the only automatic identification technology that can realize the simultaneous reading of multiple tags. The research in this paper is composed of WLAN and RFID, which is called the combined system of WLAN+RFID (hereinafter referred to as combined system). It is built on the basis of wireless local area network and radio frequency identification technology, using the wireless transmission of WLAN and the characteristics of non-contact automatic identification of RFID, to provide users with faster and more flexible identification services.

　　Detailed explanation of the application scheme of WLAN+RFID combined system

　　1 Application of WLAN+RFID combined system

　　RFID equipment consists of electronic tags (referred to as tags, Tag) and readers (read heads, Reader). In practical applications, the label is attached to the surface or interior of the identified item. When the identified item passes through the readable range, the reader automatically takes out the agreed identification information in the label in a non-contact manner, thereby realizing automatic identification. Items or the ability to automatically collect item flag information. Handheld Reader is one of the readers, which can be used by staff to quickly identify the goods they are looking for from the pile of goods.

　　Taking the WaveLAN series products of LucentTechnologies as an example, WLAN equipment includes: wireless access bridge (Access Point, AP), products include Orinoco AP-1000 and Orinoco Wave POINT-II, wireless network card OrinocoPCCard, antenna, network Bridge setting software OrinocoWaveManagerAP v6.00, wireless network card driver OrinocoClientv6.0 and roaming agreement Wave AROUND.

　　In the large outdoor open-air cargo yard of port terminals, stations or logistics centers, due to the large space and a wide variety of materials, traditional manual operations consume a lot of manpower and time, and are prone to errors. The establishment of a combined system to identify the goods and wireless access to the central database makes the whole process of goods receipt, delivery and inventory very convenient and fast, and the error rate is greatly reduced. Clerks and forklifts need to move as they check, store, register, pick, find and transport goods. Install multiple APs on the roof of the cargo yard building or related places, so that the WLAN wireless signal covers the entire cargo yard. Handsets can roam within the coverage area and connect to the yard's inventory system via WLAN, thus solving the problem of operator mobility.

　　When receiving the goods, the clerk approaches the goods with the handset. Within the range of reading, the data of the tag can be displayed on the LCD screen of the handheld and entered into the central database through WLAN. Staff can also enter other relevant data on the small keyboard of the handset, and these data also enter the central database through WLAN. Next, the system feeds back the storage location of the goods and relevant data back to the handheld, and displays it on the display. Clerks and forklift drivers deliver goods to storage locations as instructed. When shipping, the staff arrives near the storage location of the goods according to the shipping list provided by the inventory management system. As long as the label of the goods enters the reading range of the handheld machine, the handheld machine will identify the goods to be found from the pile of goods, and display the data of the batch of goods on the display screen. Counting goods is the same as receiving and shipping them. The inventory management system keeps track of the entire process of receiving, shipping, and counting, enabling accurate inventory reports to be generated in real time and easily. A read head is installed at the exit of the freight yard to identify the outgoing goods. When the truck is about to pass the exit, the read head identifies the tags of all goods. If all the goods are on the shipping list, the reading head will issue a voice prompt, and automatically open the gate of the channel through the control system to release the goods. At the same time, the inventory management system database is automatically updated with data. On the other hand, if there are goods that are not allowed to be released, including hidden in the packing boxes of goods that are allowed to be shipped out, the reading head will automatically identify them, close the gate of the passage, and issue a prompt. Therefore, the application combination system can also play the role of anti-theft monitoring for goods.

　　2 RFID and its data communication

　　2.1 RFID readers and electronic tags

　　The reader is composed of modules such as control system, communication interface, microstrip antenna and power supply. The handheld device is a kind of reader suitable for user's hand-held use, and its working principle is exactly the same as that of other readers and reading heads. In addition to the 4 modules that the general reader has, it also has an LCD screen and a keyboard.

　　The power supply voltage of the handheld is powered by a rechargeable battery, 6 V or 9 V DC; the operating system can use WinCE or other operating systems; data storage 32MB flash memory, 32 MB memory: the antenna is a built-in antenna or a probe detector; the communication interface can be Select interfaces such as RS232 and 802.11 interfaces. The handheld can also add a barcode scanning module to make it have both RFID identification and barcode scanning functions.

　　The electronic tag is composed of modules such as data storage, data processing, communication interface, microstrip antenna and power supply. The tag writes the ID code and information about the object. Tags are divided into passive tags and active tags according to different power supply forms. The power supply of the passive tag is obtained from the radio frequency signal sent by the reader, so the reader must have a higher transmit power and a shorter identification distance. The current low-voltage and low-power power supply technology can solve the disadvantage that passive tags require high transmit power of the reader. Active tags rely on their own miniature batteries for power supply, so the requirements for the transmitter power of the reader are low, and the recognition distance of the system is long. Compared with active tags, passive tags have the advantages of low cost, no maintenance, high reliability and long life. In a combined system, passive tags and active tags can be applied.

　　2.2 RFID data communication

　　RFID data communication is carried out by radio frequency transmission between the reader and the tag. The information transfer between the reader and the tag should conform to the selected communication protocol. Tag First Talk (TTF) and Reader First Talk (RTF) are two anti-collision protocols of RFID. TTF means that the tag can actively send its own ID code without the instruction of the reader. RTF means that the tag can only send after receiving the instruction of the reader. Compared with the two, TTF has the characteristics of fast and stable recognition, and can adapt to the situation of high-speed transmission and dynamic changes in the number of labels. For passive tags, it needs to convert the energy of the radio frequency signal of the reader into the power required by itself, so RTF is generally used. Active tags do not require power from the reader, so RTF or TTF can be used.

　　A great feature of RFID is that multiple tags can be read at the same time, that is, a reader can dock multiple tags. The reader transmits signals to each tag at the same time by radio broadcasting, and each tag receives these signals at the same time; each tag transmits signals to the reader in a way of multiple access. When using TTF, all tags randomly and repeatedly send their own ID codes, and different tags are read by readers at different times. When RTF is used, the reader first issues an isolation command to a batch of tags within the reading range, leaving only one tag in an active state, and establishes a conflict-free communication connection with it; after the communication is completed, the tag is instructed to enter a dormant state. Then complete the reading of each label according to the above.

　　3 Working frequency selection of combined system

　　Part 15 (Amendment) of the FCC's Radio Spectrum Regulations allows wireless networking products to operate in the ISM band without approval. ISM includes 3 frequency bands: 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.85 GHz. Currently, WLAN in the 902MHz band is deployed and operated in the United States, it is not applicable globally. 2.4 GHz is the only unlicensed frequency band available globally. The world's dominant WLAN products now operate in the 2.4 GHz band.

　　The frequency bands for wireless communication between tags and readers are: low frequency below 135KHz, high frequency 13.56 MHz, ultra-high frequency 869 MHz, 902-928 MHz, and microwave 2.4 GHz and 5.8 GHz. The communication distance of low frequency and high frequency is close, and the data transmission rate is slow. High frequency, ultra-high frequency and microwave have long communication distance and fast data transmission rate.

　　At present, there are two technical standards for RFID: the EPC standard in Europe and the United States and the UID standard in Japan. The wireless frequency bands used by the two are different. The EPC standard adopts UHF 902-928 MHz, and the UID standard adopts 2.45 GHz and 13.56 MHz.

　　In the combined system, WLAN and RFID must coexist, and the radio frequency signals of the two do not interfere with each other. Frequency division is the best and easiest option. Therefore, the WLAN in the combined system selects the 2.4 GHz band, and the RFID selects the 902 MHz band. They are all open frequency bands that can be used freely, but the transmit power is limited by regulations.

　　4 Modulation techniques used in the combined system

　　So far, DSSS and FHSS have been compared with the existing product parameters, DSSS has a better advantage in applications requiring the best reliability, while FHSS has an advantage in applications requiring low cost. Because DSSS uses the full frequency band to transmit data, the speed is fast, and it has the performance advantages of anti-interference and anti-noise ability, anti-fading ability, strong concealment and confidentiality, and does not interfere with the system of the same frequency.

　　Because DSSS has the above advantages, the WLAN in the combined system adopts DSSS. The high data transmission rate of DSSS can meet the data transmission rate requirements of large cargo yards; good anti-jamming performance adapts to the more complex propagation environment in the cargo yard, such as multipath interference caused by signal fading caused by moving objects; DSSS The transmission distance is relatively long, and the number of APs can be set in the same area of the freight yard.

　　The WLAN in the combined system adopts the IEEE802.11b standard. 802.11b is an extension of 802.11, which stipulates the use of the 2.4GHz frequency band. It adds two new rates to the physical layer of the 802.11 protocol: 5.5 Mbps and 11 Mbps. In order to achieve this goal, DSSS was selected as the only physical layer transmission technology for this technology, because FHSS cannot increase the speed any more without violating FCC principles. The modulation mode of DSSS is DBPSK at 1 Mbps, DQPSK at 2 Mbps, and CCK at 5.5 Mbps and 11 Mbps, all using QPSK as the modulation technique. The radio frequency transmission of RFID can use inductive coupling, also known as load modulation, which is commonly used in low-frequency and high-frequency identification systems in short distances. Radio frequency transmission can also use electromagnetic backscatter coupling, also known as backscatter modulation, which is commonly used in long-distance ultra-high frequency (such as 902 MHz) and microwave (such as 2.4 GHz) identification systems. In the combined system, RFID adopts backscatter modulation, and the operating frequency is the 902MHz frequency band.

　　The working principle of backscatter modulation is: when the passive tag sends data back to the read head, the tag controls the antenna switch according to the data signal to be sent back, so that the antenna of the tag is in a state of impedance mismatch or impedance matching, so the tag reflects The energy of the read-back reader is correspondingly large and small, which represent "0" and "1" in the binary signal, respectively.

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