A Low-Power UHF RFID Sensing System Based on Radio Frequency Switches

Abstract

The application belongs to the technical field of wireless communication, and discloses a low-power-consumption UHF RFID sensing system based on a radio frequency switch, which comprises a server, an RFID reader and a sensing node; the sensing node comprises a sensor, a microprocessor, a radio frequency switch module and an RFID tag; the RFID tag comprises an antenna and a chip, and the chip is connected in parallel with the radio frequency switch module; compared with a conventional wireless sensor node, the sensing node in the application eliminates a high-energy-consumption wireless communication module and a complex network communication protocol in the wireless sensor node, and instead communicates with the reader by means of a passive tag, so that the sensing node has extremely low power consumption, the running time of the sensing node is greatly prolonged, and the battery maintenance cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, specifically relating to a low-power UHF RFID (Ultra High Frequency Radio Frequency Identification) sensing system based on radio frequency switches. Background Technology

[0002] Wireless sensor networks are an important component of the Internet of Things (IoT), responsible for sensing and transmitting data from the physical world, and are widely used in fields such as environmental monitoring, smart homes, intelligent transportation, and industrial automation.

[0003] Sensor nodes in wireless sensor networks consist of sensors, microprocessors, and wireless communication modules, possessing sensing, computing, and wireless communication capabilities. These nodes collect environmental data through sensors and then transmit the data using the wireless communication modules. However, wireless sensor networks suffer from high power consumption and short lifespan. The complex wireless network communication protocols and the high-power wireless communication modules consume a significant amount of energy, and sensor nodes are typically battery-powered, leading to frequent battery replacements and expensive maintenance costs.

[0004] UHF RFID technology offers a novel solution to address the aforementioned issues, enabling low-power sensing and extending device operating time. A UHF RFID system comprises a reader and tags. The reader transmits a carrier signal carrying command information. The tag captures this carrier signal to power itself and maintain the normal operation of its internal chip. It then decodes the reader's command and transmits the information carried by the tag back to the reader via backscatter communication. UHF RFID technology boasts advantages such as passive operation, long-range identification, multi-tag identification, ease of deployment, and low cost, and is widely used in retail, logistics management, asset management, security monitoring, and other fields. However, traditional RFID technology is primarily used for positioning and identification. Passive RFID tags can only transmit fixed data, such as ID and EPC (unique code), and cannot transmit variable data; therefore, they lack sensing capabilities. Summary of the Invention

[0005] The purpose of this invention is to provide a low-power UHF RFID sensing system based on radio frequency switches, so as to overcome the shortcomings of traditional UHF RFID technology in lacking sensing capabilities, expand the sensing capabilities of RFID, and realize low-power universal sensing data perception.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A low-power UHF RFID sensing system based on radio frequency switches is disclosed. The system includes a server, an RFID reader, and sensing nodes. Each sensing node comprises a sensor, a microprocessor, a radio frequency switch module, and an RFID tag. The RFID tag includes an antenna and a chip, and the chip is connected in parallel with the radio frequency switch module.

[0008] The sensor is used to collect sensing data and send it to the microprocessor;

[0009] The microprocessor is used to acquire the sensing data collected by the sensor, encapsulate the sensing data into a data packet, and send a control command to the radio frequency switch module to modulate the data packet;

[0010] The radio frequency switch module is configured to switch to a closed state according to a first control command sent by the microprocessor, wherein the RFID tag is unreadable when the radio frequency switch module is in the closed state; and is also configured to switch to an open state according to a second control command sent by the microprocessor, wherein the RFID tag is readable when the radio frequency switch module is in the open state.

[0011] The RFID reader is used to continuously acquire RFID tag information emitted by RFID tags on the sensing node, and send the acquired RFID tag information and the timestamp of the acquisition to the server.

[0012] The server is used to receive RFID tag information and timestamps sent by the RFID reader, calculate the readable and unreadable durations of the RFID tag to obtain modulation data, demodulate to obtain data packets, and decode the data packets to obtain sensing data.

[0013] Several alternative methods are provided below, but they are not intended as additional limitations on the overall solution above. They are merely further additions or optimizations. Provided there are no technical or logical contradictions, each alternative method can be combined individually with respect to the overall solution above, or multiple alternative methods can be combined with each other.

[0014] Preferably, the sensing node further includes a power module for supplying power to the sensor, microprocessor, and radio frequency switch module.

[0015] Preferably, the radio frequency switch module includes two radio frequency switches connected in series with opposite directions, and then connected in parallel to both ends of the chip in the RFID tag. The microprocessor simultaneously controls the two radio frequency switches to close or open.

[0016] Preferably, the microprocessor uses the FM0 encoding scheme when modulating the data packet, as follows:

[0017] When bit 1 is transmitted, the microprocessor controls the RF switch module to generate a state switch and continues to switch to the next state for a time T, where T is the bit time.

[0018] When bit 0 is transmitted, the microprocessor first controls the RF switch module to switch states, and the state remains switched for a period of T / 2. Then, the microprocessor controls the RF switch module to switch states again, and the state remains switched for another period of T / 2. Preferably, the data packet format includes a prefix, sensed data, a check bit, and a trailer.

[0019] Preferably, the calculation of the readable and unreadable durations of the RFID tag to obtain modulation data includes:

[0020] Calculate the latest readable or unreadable duration of the RFID tag, and record it as the current duration to be judged. ;

[0021] The current duration to be judged Input the long-interval normal distribution model and the short-interval normal distribution model respectively;

[0022] like The current duration to be determined If the interval is long, then the current time to be judged is long; otherwise, the time to be judged is long. For short intervals, among which The current duration to be judged Probability density under a long-interval normal distribution model The current duration to be judged Probability density under a short-interval normal distribution model;

[0023] When two consecutive short intervals occur, bit 0 is obtained; when a long interval occurs, bit 1 is obtained. The readable and unreadable durations are calculated repeatedly until the complete modulation data is obtained.

[0024] Preferably, the long-interval normal distribution model and the short-interval normal distribution model are constructed as follows:

[0025] Obtain N datasets containing only long intervals and fit them to obtain a long-interval normal distribution model;

[0026] Obtain N datasets containing only short intervals and fit them to obtain a short-interval normal distribution model.

[0027] Preferably, the method also includes timed updates to the long-interval normal distribution model and the short-interval normal distribution model, wherein the timed updates include:

[0028] If the current duration to be determined If the interval is long, then the current duration to be judged will be... Add the long-interval update dataset, and periodically fit and update the long-interval normal distribution model based on the long-interval update dataset;

[0029] If the current duration to be determined If the interval is short, then the current time to be judged will be... Add the short-interval update dataset and periodically update the short-interval normal distribution model based on the short-interval update dataset.

[0030] This invention provides a low-power UHF RFID sensing system based on radio frequency switches. Utilizing existing commercial UHF RFID technology, it expands the sensing capabilities of RFID, enabling it to sense different types of environmental parameters and supporting the coexistence of multiple sensing nodes, thus achieving low-power, universal sensor data sensing. Compared to traditional wireless sensor nodes, the sensing node in this invention eliminates the high-energy-consuming wireless communication module and complex network communication protocols found in traditional wireless sensor nodes. Instead, it communicates with the reader using passive tags, resulting in extremely low power consumption, significantly extending the operating time of the sensing node and reducing battery maintenance costs. Attached Figure Description

[0031] Figure 1 The equivalent circuit diagram for an existing RFID tag;

[0032] Figure 2 This is an equivalent circuit diagram of the RFID tag regulated by the radio frequency switch of the present invention;

[0033] Figure 3 This is a schematic diagram illustrating the representation of bits 0 and 1 in the existing FM0 encoding scheme;

[0034] Figure 4 This is a schematic diagram of a low-power UHF RFID sensing system based on an radio frequency switch according to the present invention;

[0035] Figure 5 This is a schematic diagram of the data packet structure of the present invention;

[0036] Figure 6 This is a data modulation level state diagram according to an embodiment of the present invention;

[0037] Figure 7 This is a sampling diagram of RFID tag information implementation according to the present invention;

[0038] Figure 8 This is a connection diagram of the radio frequency switch module of the present invention. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] It should be noted that when a component is said to be "connected" to another component, it can be directly connected to the other component or it can be connected to a component in between; when a component is said to be "fixed" to another component, it can be directly fixed to the other component or it can be connected to a component in between.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0042] A UHF RFID system consists of a reader and RFID tags. The reader transmits a carrier signal carrying command information. The RFID tag extracts energy from the carrier signal to power itself, maintain the normal operation of the chip within the RFID tag, and decode the reader's commands. It then returns the tag information to the reader via backscatter communication. The RFID tag includes an antenna and a chip; its simplified equivalent circuit is shown below. Figure 1 As shown. The total impedance of the tag consists of two parts. One is the antenna impedance, expressed as... The other is chip impedance, expressed as .in, and These represent the antenna resistance and the chip resistance, respectively. and These represent the antenna reactance and the chip reactance, respectively. This represents the imaginary unit. The impedance matching degree of the tag affects the chip's received power, and thus the tag's operating performance. When the antenna impedance and chip impedance achieve conjugate matching (i.e., ... , When impedance mismatch occurs, the power transmitted to the chip reaches its maximum value, achieving perfect matching, and the chip receives the maximum energy. However, in cases of impedance mismatch, energy is lost during transmission to the chip, for example, due to signal backscattering. Therefore, the chip receives less power. The power reflection coefficient reflects the impedance matching degree of the tag and is expressed as:

[0043]

[0044] in yes The conjugate of is denoted as . The power reflection coefficient represents the power of the reflected signal, where |*| denotes the modulus of the complex number *. When When conjugate matching occurs, This indicates that there is no signal reflection. Therefore, the power delivered to the chip reaches its maximum, achieving perfect impedance matching. The antenna impedance of a typical commercial RFID tag is... and chip impedance In a perfect match state, the tag can normally capture the carrier signal to power itself and can communicate normally with the reader, allowing the reader to read the tag information normally.

[0045] Because the tag chip cannot be modified or connected, its internal impedance cannot be adjusted. This invention adjusts the impedance of the tag antenna externally, thereby regulating the impedance matching of the entire tag. Variable data is modulated by externally controlling the tag's impedance matching. This invention introduces an RF switch, connected in parallel across the tag chip, such as... Figure 2 As shown. The RF switch is equivalent to a controllable variable resistor with impedance characteristics; therefore, the tag introduces a new impedance. An RF switch has two impedance values: open and closed. Ideally, when the RF switch is open... When the impedance is infinite, the tag is in a perfectly matched state, the tag chip receives sufficient energy, and the tag is in a normal state, at which point it is readable; ideally, when the RF switch is closed... All the current flows through the RF switch, and at this point, the chip impedance has no effect on the overall tag impedance matching. Therefore, the tag reflectance coefficient in this state is:

[0046]

[0047] At this time, the label power reflection coefficient This indicates that the signal is reflected and no energy is transferred to the chip, therefore the tag cannot function and becomes unreadable.

[0048] FM0 coding is a widely used channel coding technique, primarily used in wireless communication to ensure reliable data transmission. It boasts advantages such as strong anti-interference capability, high reliability, and simple coding. It is commonly used in RFID, NFC, and backscatter communication systems. For example... Figure 3As shown, FM0 encoding involves a level transition at each symbol boundary, and bit 0 includes an additional intermediate level transition. The time interval between two adjacent symbol boundaries is divided into a long time interval and a short time interval, where the long time interval is the bit time T, and the short time interval is half the bit time T / 2. When a long time interval occurs, it is decoded as bit 1, and when two consecutive short time intervals occur, it is decoded as bit 0.

[0049] Based on the introduction of radio frequency switches, this embodiment proposes a low-power UHF RFID sensing system based on radio frequency switches, such as... Figure 4 As shown, it includes a server, an RFID reader, and a sensing node. The sensing node includes a sensor, a microprocessor, an RFID switch module, and an RFID tag. The RFID tag contains an antenna and a chip, and the chip is connected in parallel with the RFID switch module.

[0050] Sensors are used to collect perceived data and send it to a microprocessor.

[0051] The microprocessor acquires the sensor data, encapsulates it into data packets, and sends control commands to the RF switch module to modulate the data packets. The microprocessor can be a low-power, low-cost, and low-performance microprocessor, as the node in this invention requires minimal storage and computing resources and needs to be low-cost and low-power. For example, the ultra-low-power chip MSP430F2132 from TI is used.

[0052] The radio frequency (RF) switch module is used to switch to a closed state according to a first control command sent by the microprocessor. When the RF switch module is in the closed state, the impedance of the RFID tag is mismatched, the RF signal is reflected, and the RFID chip cannot obtain sufficient power and therefore cannot work properly, resulting in the RFID tag being unreadable. It is also used to switch to an open state according to a second control command sent by the microprocessor. When the RF switch module is open, the impedance of the RFID tag is perfectly matched, the chip can obtain sufficient power and therefore can work properly, the RFID tag can be read, and the RFID reader can read the RFID tag information normally.

[0053] An RFID reader continuously acquires RFID tag information emitted by RFID tags at sensing nodes and sends the acquired RFID tag information, along with a timestamp of the acquisition, to a server. The RFID tags can be selected from suitable commercial UHF RFID tags, such as those with strong anti-interference capabilities and long communication ranges. Similarly, the RFID reader can be a commercial ultra-high frequency reader compliant with the UHF RFID communication protocol, such as the Impinjee R420 reader.

[0054] The server is used to receive RFID tag information and timestamps sent by RFID readers, calculate the readable and unreadable durations of RFID tags to obtain modulation data, demodulate to obtain data packets, and decode data packets to restore the sensing data.

[0055] The sensor detects environmental parameters such as temperature, humidity, and light intensity, and transmits the information to the microprocessor in the form of analog or digital signals. Commercial sensors are divided into digital sensors and analog sensors; digital sensors generate digital signals, and analog sensors generate analog signals. In practical applications, sensor interfaces can be designed to select different types of sensors and connect them to the sensing node via plug-in connections. For example, this invention uses the DHT11 temperature and humidity sensor to measure temperature and humidity.

[0056] The microprocessor acquires sensed data from the connected sensors. If the data is an analog signal, it converts it into a digital signal using an analog-to-digital converter. The sensed data is then encapsulated into data packets according to a custom data packet format. The data packet format designed in this invention is as follows: Figure 5 As shown, the data includes a prefix, sensor data, a parity bit, and a tail. The prefix is ​​used to wake up and synchronize the receiver, the parity bit is used for error checking of received data packets, and the tail is the end-of-data packet marker. The data packets are converted into a binary bit data stream using FM0 encoding, and then the bits are sent one by one.

[0057] When transmitting bit data, the tag's impedance is mismatched when the microprocessor controls the RF switch module to close, making the tag unreadable (low level). When the RF switch is open, the tag's impedance is perfectly matched, making the tag readable (high level). The microprocessor transmits bit data by switching the tag's high and low levels. Combining this with the previously described FM0 encoding characteristics, when transmitting bit 1, the microprocessor controls the RF switch module to switch the tag state, generating a level transition, which is then timed for a duration of T, where T is the bit time. When transmitting bit 0, the microprocessor first controls the RF switch module to switch the tag state, generating a level transition for a duration of T / 2, and then controls the RF switch module again to switch the tag state, generating another level transition for a duration of another T / 2. For example, when transmitting a data packet bit stream of 001011, the sensing node modulates this bit data stream, resulting in a tag level state such as... Figure 6As shown, the first bit 0 is transmitted by a high-level state lasting T / 2 hours followed by a low-level state lasting T / 2 hours. Then, the second bit 0 is transmitted by a high-level state lasting T / 2 hours followed by a low-level state lasting T / 2 hours. Next, the first bit 1 is transmitted by a high-level state lasting T hours. Then, the third bit 0 is transmitted by a low-level state lasting T hours followed by a high-level state lasting T / 2 hours. Next, the second bit 1 is transmitted by a low-level state lasting T hours. Finally, the third bit 1 is transmitted by a high-level state lasting T hours.

[0058] The server samples RFID tag information from sensing nodes using an RFID reader. RFID tag information can include ID or EPC information. Since each RFID tag's ID or EPC information is unique, different sensing nodes can be distinguished using this information. This allows for demodulation and differentiation of data packets from multiple sensing nodes within the reader's coverage area, thus supporting the coexistence of multiple sensing nodes.

[0059] During the model training phase, long-interval (long time interval) normal distribution models and short-interval (short time interval) normal distribution models are constructed. During the decoding phase, data packets are demodulated based on the normal distribution models, and finally, the data packets are decoded to reconstruct the perceived data.

[0060] Specifically, the server continuously reads RFID tag information from the sensing tags using an RFID reader, and records the timestamps of information acquisition to sample the tag's temporal signals. During the model training phase, the sender first sends N (e.g., 1000) datasets containing only long intervals, where the long interval time is T. Then, it sends N (e.g., 1000) datasets containing only short intervals, where the short interval time is T / 2. The server then constructs both long-interval normal distribution models and short-interval normal distribution models based on these datasets.

[0061] During the decoding phase, the server calculates the duration of continuous readability and continuous unreadableness of the tag based on the time-series data used by the tag, and records this as the current duration to be judged. The current duration to be judged. Input the long-interval normal distribution model and the short-interval normal distribution model respectively; if The current duration to be determined If the interval is long, then the current time to be judged is long; otherwise, the time to be judged is long. For short intervals, among which The current duration to be judged Probability density under a long-interval normal distribution model The current duration to be judged The probability density under the short-interval normal distribution model. When two consecutive short intervals occur, bit 0 is obtained; when a long interval occurs, bit 1 is obtained. The readable and unreadable durations are repeatedly calculated until the complete modulation data is obtained. Figure 7 This is a sampled image of a tag signal with a bitstream of 00101010, where the horizontal axis represents the time of tag information reading and the vertical axis is 1. The time of receiving RFID tag information (i.e., the duration of the high-level signal) and the time of the RFID tag information disappearance are recorded; the difference between these two times serves as a timeframe for judgment. Continue recording the time when new RFID tag information is received. The difference between the time when new RFID tag information is received and the time when the previously recorded RFID tag information disappears will be used as a new timeframe for judgment. By repeatedly recording and calculating the difference between two adjacent recorded values, several long and short intervals are obtained, thereby identifying the binary bit data 00101010.

[0062] The normal distribution model not only accurately identifies long and short intervals but also eliminates time delays caused by hardware devices or signal transmission, resulting in higher accuracy of the ultimately identified binary bit data. Furthermore, to improve the system's adaptability, this invention periodically updates the corresponding normal distribution model. One update method is as follows: if the current time interval to be determined... If the interval is long, then the current duration to be judged will be... Add a long-interval update dataset and periodically fit and update the long-interval normal distribution model based on the long-interval update dataset; if the current time to be judged is... If the interval is short, then the current time to be judged will be... Add the short-interval update dataset and periodically fit and update the short-interval normal distribution model based on the short-interval update dataset.

[0063] Since a single RF switch may not be able to achieve infinite impedance when open, this embodiment's RF switch module includes two RF switches connected in series with opposite directions. The RF switch module is then connected in parallel to both ends of the RFID tag's chip, using conductive adhesive for connection. The microprocessor simultaneously controls the opening and closing of both RF switches. When both RF switches are closed, the RF switch module is in a closed state; when both RF switches are open, the RF switch module is in an open state. Because UHF RFID tags operate in the ultra-high frequency band of 800MHz to 1GHz, a high-frequency RF switch covering this band is used, enabling adjustment of the tag's impedance matching under high-frequency signals.

[0064] The radio frequency switch used in this invention is model ADG902. This chip operates within the UHF RFID frequency band and features ultra-low power consumption and extremely fast switching speed. Ideally, the impedance of the radio frequency switch is 0 when closed and infinite when open. However, in reality, due to manufacturing limitations, existing radio frequency switches cannot completely block high-frequency signals when open. The resistance should not be infinite, as this would result in the tag being in a partially matched state, reducing the energy delivered to the chip and decreasing the tag's communication distance. Experiments showed that when the RF switch ADG902 is open, the resistance measured from the two directions is inconsistent, such as... Figure 8 As shown on the left, Approaching infinity, and The impedance is approximately 1 MΩ, causing partial impedance mismatch in the tag and significantly reducing the tag's communication distance; experimental measurements show the communication distance is much shorter than the normal communication distance. When the RF switch is closed, the resistance in both directions is very low, causing complete impedance mismatch in the tag. Based on this finding, this invention uses two RF switches connected in series with opposite directions, and then connected in parallel to the tag chip, such as... Figure 8 As shown on the right. In this way, when the RF module is disconnected, the resistance in both directions tends to infinity, making the tag perfectly matched, the tag works normally, and the experimentally measured tag communication distance returns to normal.

[0065] In addition, the sensing node also includes a power module. The power module provides a stable power supply to the entire node for normal operation. The power module can be a wireless power source or a wired power source. The wireless power source can be a battery or a renewable energy source such as a solar power module or a wind power module. For mobility, this embodiment uses a button battery because the sensing node in this embodiment has extremely low power consumption, and a button battery can enable it to work for a long time.

[0066] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0067] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A low-power UHF RFID sensing system based on radio frequency switches, characterized in that, The low-power UHF RFID sensing system based on radio frequency switches includes a server, an RFID reader, and sensing nodes. Each sensing node includes a sensor, a microprocessor, a radio frequency switch module, and an RFID tag. The RFID tag contains an antenna and a chip, and the chip is connected in parallel with the radio frequency switch module. The sensor is used to collect sensing data and send it to the microprocessor; The microprocessor is used to acquire the sensing data collected by the sensor, encapsulate the sensing data into a data packet, and send a control command to the radio frequency switch module to modulate the data packet; The radio frequency (RF) switch module is configured to switch to a closed state according to a first control command sent by the microprocessor, in which case the RFID tag is unreadable; and to switch to an open state according to a second control command sent by the microprocessor, in which case the RFID tag is readable. The RF switch module includes two RF switches connected in series with opposite directions, and then connected in parallel to both ends of the chip in the RFID tag. The microprocessor simultaneously controls the two RF switches to close or open. The impedance of the RF switches has two values: open and closed. When the RF switch is open, the impedance of the RF switch is... When the impedance of the RFID tag is infinite, the RFID tag is in a matched state, the RFID tag chip receives power, and the RFID tag can be read; when the RF switch is closed, the impedance of the RF switch... With all current flowing through the RF switch, the chip impedance has no effect on impedance matching of the RFID tag. At this point, the reflection coefficient of the RFID tag is: ； In the formula, The reflectance coefficient of the RFID tag. For chip impedance, Antenna impedance, yes conjugate, Indicates antenna resistance. Indicates antenna reactance. This represents the imaginary unit; in this case, the power reflection coefficient of the RFID tag... This indicates that the signal is reflected and no energy is transferred to the chip, therefore the RFID tag is unreadable; The RFID reader is used to continuously acquire RFID tag information emitted by RFID tags on the sensing node, and send the acquired RFID tag information and the timestamp of the acquisition to the server. The server is used to receive RFID tag information and timestamps sent by the RFID reader, calculate the readable and unreadable durations of the RFID tag to obtain modulation data, demodulate to obtain data packets, and decode the data packets to obtain sensing data.

2. The low-power UHF RFID sensing system based on radio frequency switches according to claim 1, characterized in that, The sensing node also includes a power module for supplying power to the sensor, microprocessor, and radio frequency switch module.

3. The low-power UHF RFID sensing system based on radio frequency switches according to claim 1, characterized in that, The microprocessor uses the FM0 encoding scheme when modulating the data packet, as follows: When bit 1 is transmitted, the microprocessor controls the RF switch module to generate a state switch and continues to switch to the next state for a time T, where T is the bit time. When bit 0 is transmitted, the microprocessor first controls the RF switch module to generate a state switch, and the state is continuously switched for a time T / 2. Then, the microprocessor controls the RF switch module to generate a state switch again, and the state is continuously switched for another time T / 2.

4. The low-power UHF RFID sensing system based on radio frequency switches according to claim 1, characterized in that, The data packet format includes a prefix, sensing data, a check bit, and a tail.

5. The low-power UHF RFID sensing system based on radio frequency switches according to claim 1, characterized in that, The process of calculating the readable and unreadable durations of RFID tags to obtain modulation data includes: Calculate the latest readable or unreadable duration of the RFID tag, and record it as the current duration to be judged. ; The current duration to be judged Input the long-interval normal distribution model and the short-interval normal distribution model respectively; like The current duration to be determined If the interval is long, then the current time to be judged is long; otherwise, the time to be judged is long. For short intervals, among which The current duration to be judged Probability density under a long-interval normal distribution model The current duration to be judged Probability density under a short-interval normal distribution model; When two consecutive short intervals occur, bit 0 is obtained; when a long interval occurs, bit 1 is obtained. The readable and unreadable durations are calculated repeatedly until the complete modulation data is obtained.

6. The low-power UHF RFID sensing system based on radio frequency switches according to claim 5, characterized in that, The construction process of the long-interval normal distribution model and the short-interval normal distribution model is as follows: Obtain N datasets containing only long intervals and fit them to obtain a long-interval normal distribution model; Obtain N datasets containing only short intervals and fit them to obtain a short-interval normal distribution model.

7. The low-power UHF RFID sensing system based on radio frequency switches according to claim 5, characterized in that, It also includes the timed updates of the long-interval normal distribution model and the short-interval normal distribution model, wherein the timed updates include: If the current duration to be determined If the interval is long, then the current duration to be judged will be... Add the long-interval update dataset, and periodically fit and update the long-interval normal distribution model based on the long-interval update dataset; If the current duration to be determined If the interval is short, then the current time to be judged will be... Add the short-interval update dataset and periodically update the short-interval normal distribution model based on the short-interval update dataset.

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