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RFID system model
During the operation of the RFID system, the processes occurring in the spatial transmission channel can be summarized into three event models. This article uses the description of these three event models to introduce the typical working mode and workflow of the RFID system.
The RFID tag (RFID label) and the reader (reader-writer) establish a spatial electromagnetic wave transmission channel through their antennas.
The electromagnetic coupling between the RFID tag and the reader can be subdivided into two situations: close-range inductive coupling and long-range electromagnetic coupling. In the inductive coupling mode, the antenna on the reader side is equivalent to the primary coil of the transformer, and the antenna on the RFID tag side is equivalent to the secondary coil of the transformer, so the inductive coupling mode is also called the transformer mode. The coupling medium of the inductive coupling mode is the spatial magnetic field, and the coupling magnetic field forms a closed loop between the primary coil of the reader coil and the secondary coil of the RFID tag coil. The inductive coupling mode is the general coupling principle of the low-frequency, short-range, contactless RFID system. In the electromagnetic coupling mode, the antenna of the reader transmits the read/write RF energy generated by the reader in the form of electromagnetic waves to a directional spatial range, forming the effective reading area of the reader. The RF tag located in the effective reading area of the reader extracts the working power from the electromagnetic field emitted by the reader antenna, and transmits the data information in the tag memory to the reader through the internal circuit of the RF tag and the tag antenna. The difference between electromagnetic coupling and inductive coupling is that in the electromagnetic coupling mode, the reader transmits the RF energy in the form of electromagnetic waves; in the inductive coupling mode, the reader binds the RF energy around the reader inductor coil, and communicates the RF channel between the reader coil and the RF tag coil through the alternating closed coil magnetic field, without radiating electromagnetic energy into space.
During the operation of the RFID system, the process occurring in the spatial transmission channel can be summarized into three event models:
(1) Data exchange is the purpose;
(2) Timing is the way to realize data exchange;
(3) Energy is the basis for the realization of timing.
The following describes the typical working mode and workflow of the RFID system with the description of these three event models.
1. Energy
The reader supplies RF energy to the RFID tag. For passive RFID tags, the energy required for their work is obtained from the RF energy (generally, the RF energy is converted into DC power by rectification method and stored in the capacitor of the tag); for (semi) active RFID tags, the arrival of the RF energy plays a role in waking up the tag to enter the working state. Fully active RFID tags generally do not use the RF energy emitted by the reader, so the reader can transmit a longer communication distance with less energy. The communication mode between the base station and the mobile station in mobile communication can be classified into this mode.
2. Timing
For a two-way system (the reader sends commands and data to the RFID tag, and the RFID tag returns the stored data to the reader), the reader is generally in an active state, that is, after the reader sends an inquiry, the RFID tag responds, which is called the reader-first talk mode. Another case is the RFID tag-first talk mode, that is, after the RFID tag meets the working conditions, it first announces its home. The reader records or further sends some inquiry information based on the RFID tag's self-introduction to form a complete dialogue with the RFID tag to achieve the purpose of the reader identifying the RFID tag.
In the application of RFID system, according to whether a single RFID tag or multiple RFID tags are allowed in the reader's read-write area, the RFID system is called a single-tag identification system, or simply an RFID system, and a multi-tag identification system. When there are multiple tags within the reading range of the reader, for the RFID system with multi-tag reading function, under normal circumstances, the reader is in the active state, that is, the reader speaks first. The reader issues a series of isolation instructions to isolate (make them sleep) multiple RFID tags within the reading range one by one or in batches, and finally retains an active tag to establish collision-free communication with the reader. After the communication is completed, the current active tag is set to the third state (which can be called the dormant state, and can only be released by re-powering on or special commands). The reader further issues a wake-up command to the isolated (sleeping) tag to wake up a batch (or all) of the isolated tags, so that they enter the active state, and then further isolate and select a tag for communication. Repeating this process, the reader can read the information of multiple RFID tags in the reading area, and can also write specified data to multiple tags separately.
To achieve multi-tag reading, there are also applications that use the tag-speak-first approach in real applications. The multi-tag reading and writing problem is a relatively complex problem faced by RFID technology and applications. Currently, there are many practical methods to solve this problem. The evaluation basis of the solution generally considers the following three factors:
(1) the number of tags to be read when reading multiple tags;
(2) the probability distribution of the number of tags identified per unit time;
(3) the joint evaluation of the number of tags and the probability distribution of the number of tags read per unit time.
Theoretical analysis shows that the existing methods have a certain scope of application. It is necessary to give a reasonable evaluation of the multi-tag reading scheme based on the specific application situation and the above three factors, and select the scheme suitable for the specific application. The multi-tag reading scheme involves the protocol coordination between the RFID tag and the reader. Once selected, it is not easy to change.
For an RFID system without multi-tag reading function, when multiple tags appear in the reader's read and write area at the same time, because multiple tags respond to the query command issued by the reader at the same time, the reader will receive conflicting information and cannot read the tag information. In a typical case, no tag information can be read.
3. Data transmission
The function of the RFID system can be attributed to a convenient means of data acquisition, so it is also classified as the automatic data collection ADC (Automatic Data Capture) technology category in foreign countries. Data exchange in the RFID system includes two meanings:
(1) Data exchange from the reader to the RFID tag;
(2) Data exchange from the RFID tag to the reader.
Depending on the specific implementation system and the level of understanding, the meanings of the above two aspects will have different understandings and interpretations. The following is a brief discussion of each.
3.1. Data exchange from the reader to the RFID tag
From the purely technical level of the RFID system implementation process, if we focus on the method of injecting information stored in the RFID tag, the data exchange from the reader to the RFID tag can be divided into two situations, namely wired writing and wireless writing. The specific method to be adopted needs to be determined in combination with factors such as application system requirements, cost, and the difficulty of technical implementation.
In the wired writing mode, the role of the reader is to write data information to the RFID tag (the storage unit in it). The reader is more often called a programmer. According to the design of the RFID tag storage unit and the programming and writing control circuit, the writing can be a one-time writing that cannot be modified, or it can be a situation where wired rewriting is allowed multiple times. Another writing situation is that in most general RFID system applications, each RFID tag is required to have a unique identification. This unique identification is called the ID number of the RFID tag, which is usually fixed in the RFID tag when the tag leaves the factory and cannot be modified by the user. The ID number solidification process can be completed during the production process of the RFID tag chip, or it can be completed during the initialization process after the RFID tag application is specified. Regardless of when it is completed, the ID number is written in a wired (de-touch) manner.
For surface acoustic wave SAW RFID tags and other chipless RFID tags, the tag ID number is generally fixed into the tag memory during the tag manufacturing process.
The wireless writing method is another case of data exchange from the reader to the RFID tag in the RFID system. According to some technical reasons for the implementation of the RFID system, the wireless writing method should generally be avoided as much as possible, especially during the operation of the RFID system. The main reasons for this recommendation are as follows:
(1) RFID systems with wireless writing functions are relatively complex systems. It is a general engineering design principle that a simple system can be used to solve application problems. This implies that simple systems are lower in cost, more reliable, and have lower training and maintenance costs than complex systems.
(2) The energy required to write information to an RFID tag using an integrated circuit chip is much greater than the energy required to read information, which can be estimated to be 10 times greater. This results in the time required for the wireless writing process of an RFID tag being much longer than the time required to read the same amount of data information from it.
(3) After wireless writing, the writing results should generally be checked. The checking process is a reading process, which further increases the time required for the writing process.
(4) The increase in the time required for the writing process is very unfavorable for the application of RFID in identifying high-speed moving objects. This is easy to understand. During the data exchange between the reader and the RFID tag through the space transmission channel, the data is queued up serially one by one, and the speed of the queue is determined when the RFID system is designed. If the RFID tag is regarded as a carrier of data information, the data information is always composed of data bits of a certain length, so it takes a certain amount of time to read or write these data information bits. The higher the speed of the moving object, the less time it takes to pass through the reading area. When there is a requirement for wireless writing, the speed of the object must be limited to ensure that there is enough time for writing information.
(5) The wireless writing process faces security risks to the RFID tag information. Since the writing channel is in a spatially exposed state, this provides an opportunity for a premeditated attacker to rewrite the tag content.
On the other hand, if we focus on whether the reader sends a command to the RFID tag, it can also be divided into two situations, namely, the RFID tag only accepts energy excitation and accepts both energy excitation and reader code commands.
The system in which the RFID tag only accepts energy excitation is a simpler RFID system. This RFID system generally does not have the ability to identify multiple tags. The RFID tag is awakened or powered on by the RF energy excitation within its working frequency band, and at the same time reflects the information stored in the tag. The railway car number recognition system currently in use works in this way.
The system that accepts both energy excitation and reader code commands is a complex RFID system. The RFID tag accepts the reader's instructions for nothing more than two things, namely wireless writing and multi-tag reading.
3.2. Data exchange from RFID tag to reader
The mission of RFID tag is to realize data exchange from tag to reader. Its working modes include:
(1) When the RFID tag receives RF energy from the reader, it is awakened and reflects the data information stored in the tag to the reader;
(2) After the RFID tag is stimulated by the RF energy sent by the reader, it switches to the data transmission state or "sleep/hibernation" state according to the instructions received from the reader.
In terms of working principle, the first working mode is one-way communication, and the second working mode is half-duplex two-way communication.