Fsk modulation backscatter communication system and method based on active frequency selective surface

Abstract

This invention discloses an FSK modulation backscatter communication system and method based on an active frequency selective surface (ESS), belonging to the field of wireless communication technology. The system includes a signal source, a tag, and a receiver. The signal source transmits a carrier signal, which reaches the tag, is modulated by the tag, backscattered, and then received by the receiver. The tag includes an active frequency selective surface with an inductor and a varactor diode. By changing the bias voltage on the varactor diode, the active frequency selective surface transmits electromagnetic waves at corresponding resonant frequencies and reflects electromagnetic waves at non-resonant frequencies. Finally, different resonant frequencies are encoded to achieve FSK modulation at the tag. Compared to existing FSK modulation methods that use high-frequency switching states, this invention consumes less energy, significantly reduces signal detection difficulty, and improves the accuracy of backscatter communication transmission.

Description

Technical Field

[0001] This invention relates to the field of wireless communication technology, and in particular to an FSK modulation backscatter communication system and method based on an active frequency selective surface. Background Technology

[0002] With the rapid development of the Internet of Things (IoT), its bottlenecks are gradually becoming apparent. On the one hand, with the large-scale deployment of IoT, spectrum resources are becoming increasingly scarce. On the other hand, the explosive growth of IoT devices means that replacing batteries for a large number of devices will incur significant labor costs. Backscatter communication is one of the key technologies for building a green, energy-efficient, low-cost, and flexibly deployable future IoT, and is an important means of realizing "intelligent connectivity of everything." Because its tags transmit their information through the backscattered signal source without actively emitting radio frequency signals, energy consumption can be greatly reduced, as can circuit costs. In today's rapidly developing IoT landscape where the energy consumption requirements for IoT devices are increasingly lower, backscatter communication is expected to become a core technology for IoT development.

[0003] Traditional backscattering technology is mainly used in Radio Frequency Identification (RFID) systems. A typical RFID system consists of a reader and multiple tags, communicating using backscattering technology. However, in traditional backscattering technology, the wireless signal undergoes double-path fading during communication, resulting in significant path loss and a short effective communication distance. To address the short effective communication distance of traditional backscattering technology, the academic community has proposed bistatic backscattering communication systems. Bistatic backscattering involves placing a carrier generator near the tag, which transmits a fixed carrier wave. The tag receives this wave, loads its own information, and reflects it back to the reader. Because the carrier generator is close to the tag, path loss is reduced, extending the communication distance between the tag and the reader. However, most existing modulation methods are based on OOK modulation, and most studies have shown that FSK modulation generally outperforms OOK modulation in backscattering systems. Currently, the main method for implementing FSK modulation typically involves adding an inductor and capacitor slot oscillator to the tag circuit, switching the antenna between open and short-impedance states to backscatter environmental signals at different frequencies to transmit its own information. However, this FSK implementation not only results in higher energy consumption but also a very small frequency modulation range, which is not conducive to demodulation at the receiver. Summary of the Invention

[0004] In order to at least partially solve one of the technical problems existing in the prior art, the purpose of this invention is to provide an FSK modulation backscatter communication system and method based on an active frequency selective surface.

[0005] The technical solution adopted in this invention is:

[0006] An FSK modulation backscatter communication system based on an active frequency selective surface includes a signal source, a tag, and a receiver. The signal source transmits a carrier signal, which reaches the tag, is modulated by the tag, backscattered, and then received by the receiver.

[0007] The tag includes an active frequency selective surface, which is provided with an inductor and a varactor diode. By changing the bias voltage on the varactor diode, the active frequency selective surface can transmit electromagnetic waves at the corresponding resonant frequency and reflect electromagnetic waves at non-resonant frequencies. Finally, different resonant frequencies are encoded accordingly to realize FSK modulation of the tag.

[0008] Furthermore, the active frequency selective surface includes multiple selection units, which, from top to bottom, include a square ring slot-type metal layer, a varactor diode, a dielectric layer, a central cylindrical copper conductor, a grid-type back-feed metal feed layer, and an inductor.

[0009] Furthermore, the varactor diode is an SMV1231 type varactor diode, and the dielectric layer is an FR4 dielectric layer.

[0010] Furthermore, the tag also includes a control circuit. By applying excitation through the control circuit, the transmission coefficient and reflection coefficient of the active frequency selective surface are changed, resulting in a shift of the resonant frequency point. This causes the backscattering effect at the original resonant frequency point to change from transmission to reflection, and the backscattering effect at the new resonant frequency point to change from reflection to transmission.

[0011] The forms of excitation include voltage regulation, current regulation, light regulation, or mechanical regulation.

[0012] Furthermore, the control circuit includes an RF switch, a microcontroller, and a sensor;

[0013] The sensor is used to collect information and send the collected information to the microcontroller;

[0014] The microcontroller is used to generate baseband information, encode the collected information, and drive the radio frequency switch to connect to different voltage application circuits according to the obtained encoded square wave, so as to generate different voltages on the varactor diode, thereby generating modulation information.

[0015] Furthermore, the receiving frequency of the receiving end covers the resonant frequency range of the active frequency selective surface, which is used to receive the backscattered signal and decode it using an energy detection demodulation scheme to obtain the energy level of each frequency point.

[0016] Furthermore, the energy detection demodulation scheme performs decoding by defining an energy detection decision formula:

[0017]

[0018] In the formula, L i For the defined energy detection decision formula, For this frequency point f i The actual energy of the received signal, E iL For this frequency point f i The amount of energy collected in the upward transmission state, For other frequency points f n The actual energy of the received signal, E nH For other frequency points f n The amount of energy collected in the reflection state.

[0019] Furthermore, the energy detection and demodulation scheme includes:

[0020] The signal y′ received by the receiving end after DC blocking filtering m (t) is:

[0021] y′ m (t)=M mod cos(2πF c t-φ CT -φ TR +φ x (t-τ TR )+w(t)

[0022] Where w(t) is additive white Gaussian noise, for energy detection methods, the phase of the received signal does not affect its energy magnitude, so the actual received signal energy... Only with M mod It is proportional to the square of;

[0023] Regarding the frequency characteristics of the active frequency selective surface, let the active frequency selective surface be defined at each frequency point f. i The energy of the received signal on is

[0024] The frequency points f are obtained based on the pilot information. i The amount of energy collected in the reflection state is E. iL The amount of energy collected in the transmission state is E iL Therefore, the energy detection decision expression L is defined. i for:

[0025]

[0026] The events corresponding to the information transmitted at each frequency are:

[0027]

[0028] By comparing the magnitudes of the discriminant expressions, a decision is made regarding the transmitted information, and the event with the smallest discriminant expression is determined to be the transmitted information event.

[0029]

[0030] Furthermore, the mathematical model of the FSK modulated backscatter communication system is as follows:

[0031] Assuming the channel is a flat fading channel, the channel coefficients between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver can be expressed as:

[0032]

[0033]

[0034]

[0035] Among them, a CR a CT a TR These are the channel fading coefficients between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively.

[0036] The phase differences generated during transmission are:

[0037]

[0038]

[0039]

[0040] Where, τ CR τ CT τ TR These are the transmission delays between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively.

[0041] The transmitted carrier at the signal source end is represented as:

[0042] c m (t)=Acos(2πF c t)

[0043] Among them, F c A is the source signal frequency; A is the source signal amplitude.

[0044] The signal received by the tag is:

[0045] c m (t)*h CT (t)=Aa CT cos(2πF c(t-τ CT ))

[0046] =Aa CT cos(2πF c t-φ CT )

[0047] Signal x reflected from the tag end m (t) is:

[0048] x m (t)=Aa CT a x (t)cos(2πF c t-φ CT +φ x (t))

[0049] Among them, a x (t) represents the amplitude of the signal reflected from the tag; at the receiving end, the received signal y m (t) is:

[0050] y m (t)=A[a CR cos(2πF c t-φ CR )+a CT a TR a x (t-τ TR cos(2πF) c t-φ CT -φ TR +φ x (t-τ TR ))]

[0051] +w(t)

[0052] Where w(t) is additive white Gaussian noise; let:

[0053] M dc =Aa CR

[0054] M mod =Aa CT a TR ax(t-τ TR )

[0055] The signal at the receiving end is then rewritten as:

[0056] y m (t)=M dc cos(2πF c t-φ CR )+M mod cos(2πFc t-φ CT -φ TR +φ x (t-τ TR )+w(t)

[0057] In this equation, the first term represents the direct link from the signal source to the receiver, which does not provide any data information. Therefore, the received waveform after using a DC blocking filter is represented as follows:

[0058] y′ m (t)=M mod cos(2πF c t-φ CT -φ TR +φ x (t-τ TR )+w(t)

[0059] Among them, M mod The term is related to the reflection coefficient of the tag. The larger the reflection coefficient of the tag, the higher the scattered energy of the incident wave after reflection. Therefore, by changing the reflection coefficient of the tag, different energy states can be detected by the energy detector at the receiving end to transmit different bit information.

[0060] Another technical solution adopted in this invention is:

[0061] A communication method includes the following steps:

[0062] The signal source sends the carrier signal to the tag.

[0063] The tag terminal changes the bias voltage across the varactor diode on the source frequency selection surface according to the encoding information, so that the active frequency selection surface can transmit electromagnetic waves at the corresponding resonant frequency point and reflect electromagnetic waves at the non-resonant frequency point, thereby realizing FSK modulation at the tag terminal.

[0064] The signal modulated by the tag is backscattered to the receiver, which then uses an energy detection demodulation scheme for decoding.

[0065] The beneficial effects of this invention are as follows: By applying different voltages, the active frequency selective surface (AFSS) reflects energy differently at different frequency points. After receiving the reflected signal, the receiving antenna uses a filtering circuit to demodulate the information by comparing the energy differences of the received signals at corresponding frequency points through energy detection. This invention achieves FSK modulation at the tag end simply by controlling the switch to close, which consumes less energy compared to existing FSK modulation methods that use high-frequency switching. Furthermore, the AFSS resonant frequencies are far apart, resulting in a wider bandwidth, which greatly reduces the difficulty of signal detection and improves the accuracy of backscatter communication transmission. Attached Figure Description

[0066] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following description is provided with accompanying drawings of the relevant technical solutions in the embodiments of the present invention or the prior art. It should be understood that the accompanying drawings described below are only for the purpose of clearly illustrating some embodiments of the technical solutions of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0067] Figure 1 This is a schematic diagram of an AFSS-based backscatter communication system in an embodiment of the present invention;

[0068] Figure 2 This is a schematic diagram of the active frequency selective surface structure designed in the embodiment of the present invention;

[0069] Figure 3 This is a simulation diagram of the active frequency selective surface reflection coefficient (HFSS) designed in the embodiment of the present invention;

[0070] Figure 4 This is a schematic diagram of a specific implementation device of the AFSS-based backscatter communication system in an embodiment of the present invention. Detailed Implementation

[0071] The embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. The step numbers in the following embodiments are set only for ease of explanation, and there is no limitation on the order between the steps. The execution order of each step in the embodiments can be adaptively adjusted according to the understanding of those skilled in the art.

[0072] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0073] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0074] Furthermore, in the description of this invention, unless otherwise stated, "multiple" means two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0075] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0076] To address the existing technical problems, the inventors focused on active frequency selective surfaces (AFSSs). As a type of generalized metasurface, an AFSS refers to a single-layer, multi-layer, or three-dimensional structure composed of basic units arranged periodically according to specific rules. The most important electromagnetic characteristic of an AFSS is its different filtering responses to electromagnetic waves of different frequencies. An AFSS is created by adding active elements to a frequency selective surface, achieving adjustable resonant frequencies. Focusing on the frequency characteristics of AFSSs, we aim to replace the antenna at the backscatter communication end with an AFSS. By controlling the voltage across the AFSS, the tag will exhibit different reflection coefficients at different frequencies, thus achieving FSK modulation. Using this method to achieve FSK modulation in a backscatter communication system not only solves the energy consumption problem of high-frequency switching but also enables FSK modulation over a wider bandwidth, improving communication performance.

[0077] like Figure 1 As shown, this embodiment provides an FSK modulation backscatter communication system based on an active frequency selective surface, including a signal source, a tag, and a receiver. The signal source sends a sinusoidal carrier signal to provide excitation. The carrier signal reaches the tag, is modulated by the tag, backscattered, and then received by the receiver.

[0078] As an optional implementation, the tag uses an Active Frequency Selective Surface (AFSS), and a corresponding control circuit is designed to generate appropriate excitations to change the transmission and reflection coefficients. These excitations can take the form of voltage, current, illumination, and mechanical adjustment. By controlling the excitation, the reflection coefficient can be changed, resulting in a shift in the resonant frequency. This causes the backscattering effect at the original resonant frequency to change from transmission to reflection, and the backscattering effect at the new resonant frequency to change from reflection to transmission. The changes in backscattering effect at different resonant frequencies are encoded accordingly to achieve FSK modulation at the tag.

[0079] As an optional implementation, the receiving antenna receives backscattered signals by covering the resonant frequency range of the active frequency selective surface, and then samples and filters the received signals to analyze the energy levels at each frequency point before demodulating them.

[0080] The energy detection and demodulation scheme based on this system defines an energy detection decision formula. And decode according to their size relationship.

[0081] Regarding the aforementioned backscatter communication system, this embodiment also provides a backscatter communication method. This method, based on an active frequency selective surface (IFS), allows for FSK modulation by replacing the antenna with the FIS and controlling the voltage across its terminals to change the reflection coefficient at the resonant frequency. This method combines the wideband characteristics of the FIS with the advantages of multi-frequency energy detection. Compared to existing backscatter communication methods that achieve FSK modulation by switching switching frequencies, this method has lower energy consumption and higher reliability. The method of this embodiment specifically includes the following steps:

[0082] Step 1: Design an active frequency selective surface. By applying voltage, control the active frequency selective surface to change its transmission coefficient and reflection, thereby shifting its resonant frequency.

[0083] The active frequency selective surface designed in Step 1 is based on a square annular aperture type frequency selective surface design. A frequency selective surface (FSS) is a periodic cell array, mainly composed of patch cells or aperture cells, capable of transmitting or reflecting incident electromagnetic waves; it is also called a special type of spatial filter. Based on the filtering effect of the frequency selective surface on electromagnetic waves, it can be simply divided into four types: bandpass, bandstop, high-pass, and low-pass. This patent design uses aperture cells, making it a bandpass type frequency selective surface. An aperture-type FSS is an FSS array composed of periodically arranged metal apertures, exhibiting transmission characteristics for incident electromagnetic waves at the resonant center frequency. The principle is that when an electromagnetic wave is incident on the frequency selective surface, electrons on the metal surface move over a large range under the influence of the electric field. The induced current formed around the aperture is relatively small, resulting in less electromagnetic wave propagation outward from the frequency selective surface and a smaller transmission coefficient. As the frequency of the incident electromagnetic wave continuously increases, the range of electron movement on the surface gradually decreases, the induced current formed around the aperture gradually increases, and the energy radiated outward gradually increases, thus gradually increasing the transmission coefficient of the frequency selective surface. When the frequency of the incident electromagnetic wave reaches the resonant frequency of the frequency-selective surface, electrons around the aperture move rapidly under the influence of the electric field, generating a large induced current around the aperture. As the electrons move rapidly, they also radiate energy outwards. The energy radiated outwards by the electromagnetic wave passing through the aperture reaches its maximum, and at this point, the transmission coefficient of the frequency-selective surface is at its maximum. When the frequency of the incident electromagnetic wave moves away from the resonant frequency, the range of electron movement around the aperture decreases, the energy radiated outwards by the electromagnetic wave passing through the aperture gradually decreases, and the transmission coefficient of the frequency-selective surface gradually decreases again.

[0084] Step 1 involves adding an inductor and a varactor diode to an existing square annular aperture type frequency selective surface (FSS). By changing the bias voltage of the varactor diode, its resonant frequency is shifted, resulting in different transmission coefficients at different frequencies. Traditional passive FSSs directly design the unit shape according to the required transmission characteristics. The invariance of electromagnetic characteristics remains an unavoidable drawback of passive FSSs. Active FSSs are built upon traditional passive FSSs by adding active devices to the passive FSS structure and controlling the operating state of these devices to change the frequency response characteristics of the FSS unit. A schematic diagram of the active FSS designed in this embodiment is shown below. Figure 2 As shown; see also Figure 2 (a) The active frequency selectable surface, from top to bottom, includes a square ring slot metal layer, an SMV1231 varactor diode, an FR4 dielectric layer, a central cylindrical copper conductor, a grid-type back-feed metal feed layer, and an inductor.

[0085] As a specific implementation, the active frequency selective surface uses an SMV1231 varactor diode. By controlling its bias voltage from 0-3V, its capacitance can vary from 0.97pF to 2.35pF, thereby changing the capacitance value between its gaps and thus altering the resonant frequency. This patented design uses changing the bias voltage to generate different transmission coefficients for the AFSS at different frequency points. Simulation using HFSS electromagnetic simulation software yields the frequency shift effect of the AFSS from 2.15GHz to 3.35GHz. The simulation results are as follows: Figure 3 As shown.

[0086] Step 2: Construct a backscatter communication system based on an active frequency-selective surface, such as... Figure 1 As shown, it includes a signal source end, a tag end, and a receiver end. The tag end uses an active frequency selective surface, and its reflection coefficient can be changed by altering the voltage across its terminals, causing the tag end to produce different backscattering effects at different resonant frequencies.

[0087] Step 3: For the FSK modulation backscatter communication system based on an active frequency selective surface built in Step 2, an active frequency selective surface is used at the tag end, and a corresponding control circuit is designed to change the voltage across the active frequency selective surface, thereby changing the surface's transmission coefficient and reflection coefficient. By controlling the voltage, it can transmit electromagnetic waves at the corresponding resonant frequency and reflect electromagnetic waves at non-resonant frequencies. Finally, different resonant frequencies are encoded accordingly to achieve FSK modulation at the tag end.

[0088] In step 3, the tag end of the backscattering system is replaced by a frequency selective surface (FSS) instead of an antenna. Different voltages are generated across the AFSS by controlling different pressure circuits via an RF switch, altering its transmission and reflection coefficients. This allows electromagnetic waves at the corresponding resonant frequency to be transmitted, thereby reducing the backscattered energy and transmitting the corresponding bit information to achieve FSK modulation. Specifically, the channel frequency is first encoded accordingly, targeting different resonant frequencies f... i =f1,...,f n Corresponding encoding is performed, for example, the modulation information for frequency point f1 is set to "00"; the modulation information for frequency point f2 is set to "01"; the modulation information for frequency point f3 is set to "10"; and the modulation information for frequency point f4 is set to "11". By designing corresponding control voltage circuits, the resonant frequency of AFSS is changed, thereby increasing the transmission coefficient and decreasing the reflection coefficient at the corresponding frequency point of each voltage. This reduces the signal energy received by the receiver at that frequency point, thus realizing FSK modulation at the tag end.

[0089] Step 4: For the FSK modulation backscatter communication system based on the active frequency selective surface built in Step 2, the antenna receiving frequency covers the resonant frequency range of the active frequency selective surface to receive the backscattered signal, and the received signal is sampled and filtered to analyze the energy of each frequency point and then demodulated.

[0090] Specifically, the receiving end demodulates the information transmitted by the tag using an energy detector-based demodulation scheme.

[0091] The mathematical model of an FSK-modulated backscatter communication system based on an active frequency selective surface includes: assuming the channel is a flat fading channel, the channel coefficients between the transmitter and receiver, the transmitter and tag, and the tag and receiver can be expressed as:

[0092]

[0093]

[0094]

[0095] Among them, a CR a CT a TR These are the channel fading coefficients between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively.

[0096] The phase differences generated during transmission are:

[0097]

[0098]

[0099]

[0100] Where, τ CR τ CT τ TR These are the transmission delays between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively.

[0101] The transmitted carrier at the signal source end is represented as:

[0102] c m (t)=Acos(2πF c t)

[0103] Among them, F c A is the source signal frequency; A is the source signal amplitude.

[0104] The signal received by the tag is:

[0105] cm (t)*h CT (t)=Aa CT cos(2πF c (t-τ CT ))

[0106] =Aa CT cos(2πF c t-φ CT )

[0107] Signal x reflected from the tag end m (t) is:

[0108] x m (t)=Aa CT a x (t)cos(2πF c t-φ CT +φ x (t))

[0109] Among them, a x (t) represents the amplitude of the signal reflected from the tag, which is related to the reflection coefficient of the frequency-selective surface. At the receiving end, the received signal y m (t) is:

[0110] y m (t)=A[a CR cos(2πF c t-φ CR )+a CT a TR a x (t-τ TR cos(2πF) c t-φ CT -φ TR +φ x (t-τ TR ))]

[0111] +w(t)

[0112] Here, we consider w(t) to be additive white Gaussian noise. Let:

[0113] y m (t)=A[a CR cos(2πF c t-φ cR )+a Cr a IR sa x (t-τ IR cos(2πF) c t-φ CT -φ IR +φx (t-τ IR ))]

[0114] +w(t),

[0115] M dc =Aa CR

[0116] M mod =Aa CT a TR ax(t-τ TR )

[0117] The signal at the receiving end is then rewritten as:

[0118] y m (t)=M dc cos(2πF c t-φ CR )+M mod cos(2πF c t-φ CT -φ TR +φ x (t-τ TR )+w(t)

[0119] In this equation, the first term represents the direct link from the signal source to the receiver, which does not provide any data information. Therefore, the received waveform after using a DC blocking filter is represented as follows:

[0120] y′ m (t)=M mod cos(2πF c t-φ CT -φ TR +φ x (t-τ TR )+w(t)

[0121] Among them, M mod The term is related to the reflection coefficient of the tag. The larger the reflection coefficient of the tag, the higher the scattered energy of the incident wave after reflection. Therefore, by changing the reflection coefficient of the tag, different energy states can be detected by the energy detector at the receiving end to transmit different bit information.

[0122] The specific steps of the FSK demodulation scheme for a backscattering system are as follows:

[0123] The signal y′ received by the receiving end after DC blocking filtering m (t) is:

[0124] y′ m (t)=M mod cos(2πF c t-φCT -φ TR +φ x (t-τ TR )+w(t)

[0125] Where w(t) is additive white Gaussian noise, for energy detection methods, the phase of the received signal does not affect its energy magnitude, so the actual received signal energy... Only with M mod It is proportional to the square of the value. Regarding the frequency characteristics of the designed active frequency selective surface, let its value at each frequency point f... i The energy of the received signal on is The frequency points f are obtained based on the pilot information. i The amount of energy collected in the reflection state is E. iH The amount of energy collected in the transmission state is E iL Therefore, we define the energy detection decision expression L. i for:

[0126]

[0127] The events corresponding to the information transmitted at each frequency are:

[0128]

[0129] We determine the transmitted information by comparing the magnitudes of the discriminant expressions, identifying the event with the smallest discriminant expression as the transmitted information event.

[0130]

[0131] As an optional implementation, the PC-side visual control of the FSK backscattering system based on an active frequency selective surface is achieved through the CVI development platform.

[0132] To facilitate design and use in practical applications, a visual window software was developed using CVI to control the backscatter communication system. The specific method includes setting CVI button callback functions and driving the transmitting and receiving modules through serial communication.

[0133] The above technical solution will be explained in detail below with reference to specific embodiments.

[0134] See Figure 4 This example provides a design method for a backscatter communication system based on an active frequency selective surface (AFSS). First, an AFSS is designed using HFSS software. Then, a backscatter communication system is designed based on the frequency characteristics of the AFSS. Finally, CVI software is written to visualize and control this backscatter communication system. The specific steps include:

[0135] S1. Design an active frequency selective surface (AFSS) using HFSS software. (See [link]) Figure 2 (a) Specifically, it includes a square ring-shaped slotted metal layer, an SMV1231 varactor diode, an FR4 dielectric layer, a central cylindrical copper conductor, a grid-type back-feed metal feed layer, and an inductor. See also Figure 2 (b) The design unit consists of 8*8=64 elements, using square elements with strong angular stability. The AFSS reflection coefficient obtained through simulation is as follows: Figure 3 As shown.

[0136] S2. The AFSS is fabricated using existing mature PCB patch antenna fabrication technology. The dielectric layer is FR4 with a thickness of 1.6mm. For the ideal RLC boundary designed in the simulation, an SMV1231 varactor diode is used for soldering during fabrication, and corresponding wires and feed ports are provided.

[0137] S3. Construct a backscatter communication system transmitter, using a broadband antenna. See [link / reference] Figure 4 Specifically, this is implemented by controlling an MSP430 microcontroller via serial communication, configuring a CC2500 RF chip, and transmitting a 2.15GHz-3.35GHz sinusoidal carrier wave. CVI software is used to control the transmit port, including transmitting power and setting debugging information.

[0138] S4. Construct a backscatter communication system tag terminal. The tag terminal uses an AFSS (Automatic Backscattering System). Design a control circuit, specifically using an RF switch to control different voltages across the AFSS terminals, changing its transmission and reflection coefficients. This allows electromagnetic waves at the corresponding resonant frequency to be transmitted, thereby reducing backscattered energy and transmitting corresponding bit information to achieve FSK modulation. The baseband information is generated by an STM32 microcontroller. Temperature and humidity information collected by an AHT10 sensor is encoded, and the STM32 generates an encoded square wave to drive the RF switch, which is connected to different voltage application circuits to generate different voltages, thus producing modulation information.

[0139] S5. Build a backscatter communication system receiver. The receiver uses a wideband receiving antenna and a corresponding frequency filtering circuit. By analyzing the received energy at the corresponding frequency through envelope detection, the information transmitted by the tag is demodulated.

[0140] S6. In this example, the receiver demodulation uses energy detection, which demodulates information by comparing the amount of energy reflected from the tag at a preset frequency.

[0141] S7. To control the transmission and reception of the backscatter communication system, a control program is written using CVI software to make it visually operational and to detect received signals and information.

[0142] In summary, compared with the prior art, the present invention has the following advantages and beneficial effects:

[0143] (1) This invention innovatively combines AFSS with backscatter communication technology to provide a new backscatter communication tag-end modulation method.

[0144] (2) The AFSS resonant frequency points designed in this invention are far apart and occupy a wide bandwidth, which can greatly reduce the difficulty of signal detection and improve the accuracy of backscatter communication transmission.

[0145] (3) The present invention can realize FSK modulation at the tag end by controlling the switch, without the need for high-frequency switching of the switch state, which greatly saves energy at the tag end.

[0146] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," or "some embodiments" indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of the present invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0147] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

[0148] The above is a detailed description of the preferred embodiments of the present invention. However, the present invention is not limited to the above embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A backscatter communication system based on an active frequency selective surface (FSK) modulation, characterized in that, It includes a signal source, a tag, and a receiver. The signal source sends a carrier signal, which arrives at the tag, is modulated by the tag, backscattered, and then received by the receiver. The tag end includes an active frequency selective surface, on which an inductor and a varactor diode are provided. By changing the bias voltage on the varactor diode, the active frequency selective surface can transmit electromagnetic waves at the corresponding resonant frequency point and reflect electromagnetic waves at non-resonant frequency points. Finally, different resonant frequency points are encoded accordingly to realize FSK modulation of the tag end. The mathematical model of the FSK modulated backscatter communication system is as follows: Assuming the channel is a flat fading channel, the channel coefficients between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver are expressed as follows: in, , , These are the channel fading coefficients between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively. The phase differences generated during transmission are: in, , , These are the transmission delays between the signal source and receiver, between the signal source and the tag, and between the tag and the receiver, respectively. The transmitted carrier at the signal source end is represented as: in, The source signal frequency; The amplitude of the source signal; The signal received by the tag is: Signal reflected from the tag end for: in, This represents the amplitude of the signal reflected from the tag; at the receiving end, it represents the received signal. for: Among them, considering Let be additive white Gaussian noise; The signal at the receiving end is then rewritten as: In this equation, the first term represents the direct link from the signal source to the receiver, which does not provide any data information. Therefore, the received waveform after using a DC blocking filter is represented as follows: in, The term is related to the reflection coefficient of the tag. The larger the reflection coefficient of the tag, the higher the scattered energy of the incident wave after reflection. Therefore, by changing the reflection coefficient of the tag, different energy states can be detected by the energy detector at the receiving end to transmit different bit information.

2. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 1, characterized in that, The active frequency selective surface includes multiple selection units, which, from top to bottom, include a square ring slot-type metal layer, a varactor diode, a dielectric layer, a central cylindrical copper conductor, a grid-type back-feed metal feed layer, and an inductor.

3. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 2, characterized in that, The varactor diode is an SMV1231 type varactor diode, and the dielectric layer is an FR4 dielectric layer.

4. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 1, characterized in that, The tag also includes a control circuit. By applying excitation through the control circuit, the transmission coefficient and reflection coefficient of the active frequency selective surface are changed, resulting in a shift of the resonant frequency point. This causes the backscattering effect at the original resonant frequency point to change from transmission to reflection, and the backscattering effect at the new resonant frequency point to change from reflection to transmission. The forms of excitation include voltage regulation, current regulation, light regulation, or mechanical regulation.

5. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 4, characterized in that, The control circuit includes an RF switch, a microcontroller, and a sensor; The sensor is used to collect information and send the collected information to the microcontroller; The microcontroller is used to generate baseband information, encode the collected information, and drive the radio frequency switch to connect to different voltage application circuits according to the obtained encoded square wave, so as to generate different voltages on the varactor diode, thereby generating modulation information.

6. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 1, characterized in that, The receiving frequency of the receiving end covers the resonant frequency range of the active frequency selective surface, and is used to receive backscattered signals. An energy detection demodulation scheme is used for decoding to obtain the energy level of each frequency point.

7. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 6, characterized in that, The energy detection demodulation scheme decodes by defining an energy detection decision formula: In the formula, For the defined energy detection decision formula, For this frequency point The actual energy of the received signal on the surface. For this frequency point The amount of energy collected in the upward transmission state, For other frequency points The actual energy of the received signal on the surface. For other frequency points The amount of energy collected in the reflection state.

8. The FSK modulation backscatter communication system based on an active frequency selective surface according to claim 6, characterized in that, The energy detection and demodulation scheme includes: The signal received by the receiver after passing through a DC blocking filter for: in, For additive white Gaussian noise, in energy detection methods, the phase of the received signal does not affect its energy magnitude; therefore, the actual received signal energy is... Only with It is proportional to the square of; Regarding the frequency characteristics of the active frequency selective surface, the active frequency selective surface is configured at various frequency points. The energy of the received signal on is ; Each frequency point is obtained based on the pilot information. The amount of energy collected in the reflection state is The amount of energy collected in the transmission state is Therefore, the energy detection decision expression is defined. for: The events corresponding to the information transmitted at each frequency are: By comparing the magnitudes of the discriminant expressions, a decision is made regarding the transmitted information, and the event with the smallest discriminant expression is determined to be the transmitted information event.

9. A communication method applied to the FSK modulated backscatter communication system according to any one of claims 1-8, characterized in that, Includes the following steps: The signal source sends the carrier signal to the tag. The tag terminal changes the bias voltage across the varactor diode on the source frequency selection surface according to the encoding information, so that the active frequency selection surface can transmit electromagnetic waves at the corresponding resonant frequency point and reflect electromagnetic waves at the non-resonant frequency point, thereby realizing FSK modulation at the tag terminal. The signal modulated by the tag is backscattered to the receiver, which then uses an energy detection demodulation scheme for decoding.

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