A backscatter communication method and system

Abstract

The present invention belongs to the field of wireless communication technology, and provides a backscatter communication method and system. The method includes: combining baseband data to be transmitted into data frames; obtaining clock frequency information according to the FSCM signal in the data frames, converting the clock frequency information into a continuous frequency transformation signal by a DAC, controlling a VCO to generate a clock signal with a corresponding frequency, returning the clock signal to an FPGA, and driving a ROM array to output control information in sequence; an RF switch outputting a reflection signal according to the control information; receiving the reflection signal and detecting the data frames; performing oversampling and low-pass filtering; performing downsampling; performing de-chirping on the FSCM signal in the data frames, then performing fast Fourier transform, and making a decision according to the peak value on the Fourier transform result; extracting valid baseband data after synchronization; performing fast Fourier transform on the extracted valid baseband data, and making a decision according to the peak value on the Fourier transform result to recover the baseband symbol information and complete demodulation.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technologies, and particularly to a backscatter communication method and system. Background Art

[0002] The development of the Internet of Things has had a profound impact on people's work, life, and production. Through Internet of Things technologies, we can achieve mutual communication between various intelligent devices, and realize intelligent and automated production, management, and services. Wireless communication technology is an important foundation for the realization of the Internet of Things. Traditional active wireless communication technologies have exposed many defects in the application scenarios of the Internet of Things. For example, they consume a large amount of power and require frequent battery replacement, which greatly increases the manual maintenance cost. Moreover, active wireless communication requires more complex devices, increasing the device cost. In addition, since active wireless communication communicates by emitting and receiving electromagnetic waves, it must also occupy scarce spectrum resources. Summary of the Invention

[0003] In view of this, embodiments of the present invention provide a backscatter communication method and system to solve or partially solve the above problems.

[0004] In a first aspect, embodiments of the present invention provide a backscatter communication method, including the following steps:

[0005] Step S1: Select four different frequency initial offsets to form four different FSCM symbols, and the FSCM symbols are used to constitute an FSCM signal;

[0006] Step S2.1: Combine the baseband data to be transmitted into a data frame according to a specific frame structure.

[0007] Step S2.2: The FPGA obtains clock frequency information according to the FSCM signal in the data frame, converts the clock frequency information into a continuous frequency transformation signal by the DAC, the frequency transformation signal controls the VCO to generate a clock signal with a corresponding frequency, and returns the clock signal to the FPGA to drive the ROM array inside the FPGA to output control information in sequence;

[0008] Step S2.3: The radio frequency switch outputs a reflected signal according to the control information, and the reflected signal contains the data frame;

[0009] Step S3.1: The receiver receives the reflected signal, performs energy detection on the received signal, and detects the data frame in the received signal according to the signal strength;

[0010] Step S3.2: Oversample the data frame, and perform low-pass filtering on the oversampled data frame;

[0011] Step S3.3: Downsample the data frame to reduce the sampling rate to the bandwidth B, where B is the symbol bandwidth of FSCM;

[0012] Step S3.4: Dechirp the FSCM signal in the data frame, then perform a fast Fourier transform, and make a decision based on the peak value in the Fourier transform result;

[0013] Step S3.5: Perform frame synchronization on the synchronization header of the data frame to find the start flag and extract the valid baseband data;

[0014] Step S3.6: Perform a fast Fourier transform on the extracted valid baseband data, make a decision based on the peak value in the Fourier transform result, and recover the baseband symbol information to complete demodulation.

[0015] According to a specific implementation manner of the embodiment of the present invention, the four different symbols respectively use K = 0, K = round[0.25(2 SF -1)], K = round[0.5(2 SF -1)], K = round[0.75(2 sF -1)] as modulation symbols. Therefore, each symbol can transmit log 2 4 = 2 bit of information, where round[] is the rounding function.

[0016] According to a specific implementation manner of the embodiment of the present invention, the specific frame structure consists of three parts, namely the synchronization header, the start flag, and the baseband data.

[0017] According to a specific implementation manner of the embodiment of the present invention, in step S3.3, the sampling time is T = 1 / B, and the downsampled FSCM signal s(n, K) is:

[0018]

[0019] where N s is the total number of sampling points obtained by sampling the FSCM signal, and N 0 is the number of sampling points when corresponding to T 0 Since e -j2πn = 1, s(n, K) can be simplified to the following formula:

[0020]

[0021] According to a specific implementation manner of the embodiment of the present invention, step S3.4 is specifically: multiply the downsampled FSCM signal by the conjugate of the reference signal, where the reference signal b(n) is a discrete-time sequence obtained by sampling a chirp signal without cyclic shift at a sampling rate of B;

[0022]

[0023] Multiply the received complete FSCM signal by the conjugate of b(n) to obtain d(n):

[0024]

[0025] The obtained d(n) becomes a single-frequency discrete-time sequence, and perform a fast Fourier transform operation on it to obtain:

[0026]

[0027] When a peak with an amplitude of N appears in the Kth FFT bin s the symbol whose modulation information is K is determined.

[0028] In a second aspect, an embodiment of the present invention provides a backscatter communication system, including: a single-tone signal source, a backscatter tag, and a receiver. The single-tone signal source circularly transmits a sine excitation signal. The backscatter tag is responsible for CSS backscatter modulation of the data to be transmitted, and the receiver is responsible for receiving and demodulating the tag reflection signal. This system can achieve long-distance backscatter communication at the hundred-meter level.

[0029] According to a specific implementation manner of an embodiment of the present invention, the backscatter tag includes: an FPGA, a DAC, a VCO, a radio frequency switch chip, and an impedance matching network. The FPGA is used to convert baseband data and accordingly give the DAC control signal, generate an analog voltage to drive the VCO, so as to generate a clock signal with a specific frequency; the FPGA is also used to complete the output of the radio frequency switch control signal stored in the ROM according to the clock signal input from the VCO to the FPGA, control the radio frequency switch chip to switch between multiple impedance networks, and generate a reflected wave frequency offset; the DAC is used to convert the digital signal given by the FPGA into an analog signal, so as to control the VCO to output the clock signal; the VCO is used to convert the magnitude of the input voltage into the magnitude of the output clock signal frequency, and the radio frequency switch chip generates a reflected signal under the control of the FPGA. The backscatter tag is used to execute all the contents of steps S2.1-S2.3 in the foregoing embodiment.

[0030] A backscatter communication method and system provided by an embodiment of the present invention provide a user with a communication method and system with low cost and ultra-low power consumption, and solve the current dilemmas faced by the Internet of Things. Description of the Drawings

[0031] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0032] Figure 1 Shows a schematic diagram of a backscatter communication system model;

[0033] Figure 2 Shows a flowchart of the steps of a backscatter communication method;

[0034] Figure 3 Shows a schematic diagram of the frame structure;

[0035] Figure 4 Shows a state transition diagram of the receiver frame synchronization and demodulation algorithm;

[0036] Figure 5 Shows a schematic diagram of a backscatter tag structure;

[0037] Figure 6 Shows the MATLAB simulation system block diagram of the backscatter device and the backscatter mechanism, as well as the images of each output signal;

[0038] Figure 7 Shows the bit error rate signal-to-noise ratio curve of the CSS backscatter communication system under different environmental signal conditions. Specific Embodiments

[0039] The following will describe in detail the embodiments of the technical solutions of the present invention with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, so they are only examples and cannot be used to limit the protection scope of the present invention.

[0040] It should be noted that unless otherwise specified, the technical terms or scientific terms used in this application should have the ordinary meaning understood by those skilled in the art to which the present invention belongs.

[0041] To more conveniently represent the FSCM modulation principle mathematically, the following frequency modulation parameters are defined:

[0042] 1. Bandwidth B: Bandwidth B = fmin max , where f max is the maximum value of the chirp frequency; f min is the minimum value of the chirp frequency.

[0043] 2. Spreading factor SF: Divide the bandwidth B into 2 SF parts, and the frequency change amount of each part Δf = B / 2 SFTheoretically, 2 SF cyclically shifted FSCM symbols can be obtained.

[0044] 3. Symbol period T s : It is stipulated that the symbol period T s = 2 SF / B.

[0045] 4. Frequency modulation slope μ: The rate of change of the frequency of the FSCM signal with time, μ = B / T s .

[0046] Initial offset frequency f 0 : f 0 = K·B / 2 SF + f min .

[0047] The backscatter communication system structure based on chirp spread spectrum modulation of the present invention includes:

[0048] The model of the system is as Figure 1 shown. The backscatter communication system based on chirp spread spectrum modulation consists of three parts, namely an environmental single-tone signal source, a backscatter tag TAG, and a receiver. In this system, (1) the single-tone signal source cyclically transmits a sinusoidal excitation signal; (2) the backscatter tag is responsible for chirp spread spectrum backscatter modulation of the data to be transmitted; (3) the receiver is responsible for receiving and demodulating the tag reflection signal.

[0049] In the above system, in order to modulate information into the environmental single-tone signal, the backscatter tag uses the complex impedance design on the tag to frequency-shift the incident signal, and adopts the method of frequency shift chirp modulation (FSCM) to backscatter modulate the incident single-tone signal into an FSCM signal.

[0050] The FSCM signal is based on the chirp signal, and cyclically shifted with different offsets in the time domain to obtain a series of symbols with different starting frequencies. Assuming that the set spreading factor is SF, then there can be at most 2 SF different frequency offsets, that is, each FSCM signal can generate 2 SF different symbols, and finally the amount of information carried by each FSCM signal is SF bit. Let the frequency function of the unmodulated FSCM time-domain signal be f(t), then there is:

[0051] f(t) = μt + f min ; (1)

[0052] The frequency function after cyclic shift can be expressed as the following formula:

[0053]

[0054] where the frequency modulation slope μ = B / T s = B 2 / 2 SF , the frequency of the FSCM signal after cyclic shift linearly changes from f 0 to f max (the time ranges from t = 0 to t = T 0 ), and then linearly changes from f min to f 0 (the time ranges from t = 0 to t = T s ).

[0055] Assume that the number K (K = 0 to 2 SF -1) is to be modulated into the FSCM signal, then the initial frequency offset of the FSCM signal can be set such that At this time, the frequency function of the modulation information K can be expressed as the following formula:

[0056]

[0057] The instantaneous phase φ(t, K) can be derived from the instantaneous frequency f(t, K):

[0058]

[0059] According to the instantaneous phase of the FSCM signal, the time-domain expression of the FSCM signal with modulation information K can be obtained:

[0060]

[0061] Figure 2 is the step flowchart of a backscatter communication method provided by an embodiment of the present invention. Refer to Figure 2 , this method includes the following steps:

[0062] Step S1: Select four different initial frequency offsets to form four different FSCM symbols, and the FSCM symbols are used to constitute the FSCM signal;

[0063] In the present invention, four different initial frequency offsets are used to form four different FSCM symbols. These four different symbols respectively use K = 0, K = round[0.25(2 SF -1)], K = round[0.5(2 SF -1)], K = round[0.75(2 SF -1)] as modulation code elements. Therefore, each symbol can transmit log 2 4 = 2 bit of information, where round[] is the rounding function.

[0064] Step S2: The backscatter tag modulates the baseband data into a reflected signal. Specifically:

[0065] Step S2.1: Combine the baseband data to be transmitted into data frames according to a specific frame structure;

[0066] The specific frame structure consists of three parts, namely the synchronization header, start flag, and data.

[0067] Through the design of the frame structure, the receiver can judge the presence or absence of the backscatter signal and perform symbol alignment. The schematic diagram of the frame structure is as Figure 3 shown.

[0068] The entire frame structure is composed of several FSCM symbols. Figure 3 In [the figure], the first row represents the symbol category (Symbol 1 - Symbol 4), the second row shows the curve of the corresponding symbol's frequency changing with time, and the third row explains the components of the frame structure. Among them, the synchronization header is composed of 8 Symbol 1s. The role of the synchronization header is to assist the receiver in judging the presence or absence of the backscatter signal. If there is, time synchronization is performed using the properties of the CSS signal; the role of the start header is to mark the end of the synchronization header and the start of the valid data. The start flag is composed of two Symbol 3s; immediately following the start flag is the valid data. The number of symbols of the valid data is fixed. After the receiver demodulates a fixed number of valid data symbols, it jumps to the next frame to re-perform time synchronization, avoiding the error caused by the first time synchronization from affecting the demodulation of the next frame of data.

[0069] Step S2.2: The FPGA obtains the clock frequency information according to the FSCM signal in the data frame, converts the clock frequency information into a continuous frequency change signal by the DAC. The frequency change signal controls the VCO to generate a clock signal with the corresponding frequency, and returns the clock signal to the FPGA to drive the ROM array inside the FPGA to output control information in sequence;

[0070] The FSCM signal needs to form a continuous frequency change, which requires the tag to be able to output a clock signal with a continuously changing frequency to drive the Rom in the FPGA, thereby realizing continuously variable control of the RF switch.

[0071] Step S2.3: The RF switch outputs a reflected signal according to the control information, and the reflected signal contains the data frame;

[0072] One end of the RF switch is connected to 8 impedances, and the other end is connected to the Rom in the FPGA. The 8 impedances are complex impedance samplings on the constant reflection coefficient circles such as the Smith chart, and 8 evenly distributed discrete points on the complex plane are taken as the 8 states of the reflection coefficient. The specific values of these 8 reflection coefficients are 1, , -1, -j, The control information of the ROM inside the FPGA switches the RF switch in a continuously varying frequency manner, sequentially switching 8 impedances. The resulting reflection coefficient is the FSCM signal. The incident single-tone signal and this FSCM signal are mixed at the antenna and then reflected back into the environment to achieve FSCM backscatter modulation.

[0073] Step S3: The receiver receives the reflected signal and obtains baseband data. Specifically:

[0074] Step S3.1: The receiver receives the reflected signal and performs energy detection on the received signal to detect the data frame in the received signal according to the signal strength;

[0075] For signal detection, the received signal is composed of discontinuous frequency-modulated continuous wave signal frames. First, energy detection needs to be performed on the received signal, and the data frame in the received signal is detected according to the received signal strength (Received Signal Strength Indicator, RSSI).

[0076] Step S3.2: Oversample the data frame and perform low-pass filtering on the oversampled data frame;

[0077] To reduce the noise power as much as possible, a certain degree of oversampling is performed at the receiver, and the oversampled signal is low-pass filtered to eliminate out-of-band noise.

[0078] Step S3.3: Downsample the data frame to reduce the sampling rate to the bandwidth B, where B is a set value;

[0079] In step S3.3, the sampling time is T = 1 / B, and the FSCM signal s(n, K) after downsampling can be obtained as follows:

[0080]

[0081]

[0082] Among them, N s is the total number of sampling points obtained by sampling the FSCM signal, N 0 is the number of sampling points when corresponding to T 0 Since e -j2πn = 1, s(n, K) can be simplified to the following formula:

[0083]

[0084] It can be seen that by reducing the sampling rate of the received signal to the bandwidth B of the FSCM signal and using the periodicity of the discrete-time complex exponential signal with respect to frequency, the 0 - T 0 part of the data frame and T0 ~T s is exactly the same as the discrete-time sequence

[0085] Step S3.4: Dechirp the FSCM signal in the data frame, then perform a fast Fourier transform, and make a decision based on the peak value in the Fourier transform result;

[0086] Specifically: Multiply the downsampled FSCM signal by the conjugate of the reference signal. The reference signal b(n) is a discrete-time sequence obtained by sampling a chirp signal without cyclic shift at a sampling rate of B:

[0087]

[0088] Multiply the received complete FSCM signal by the conjugate of b(n) to obtain d(n):

[0089]

[0090] The obtained d(n) becomes a single-frequency discrete-time sequence, and perform a fast Fourier transform operation on it to obtain:

[0091]

[0092] When a peak with an amplitude of N appears in the Kth FFT bin s it is determined that the modulation information is the symbol with K

[0093] Step S3.5: Perform frame synchronization on the synchronization header of the data frame, find the start flag, and extract the valid baseband data;

[0094] As Figure 4 shown, in the first step, when the communication system is working, the receiver is in a continuous working state, while the backscatter device may work at any time and reflect the signal modulating the valid data. This requires judging the existence of the signal through the synchronization header in the reflected signal frame structure. Since the start time of the backscatter device is random, the received synchronization header symbols without time synchronization may demodulate any one of the numbers from K = 0 to 2 SF If 7 consecutive identical symbols are detected, it is determined that a frame of signal has arrived. This is because the synchronization header has 8 symbols of 1, and due to the uncertainty of the receiver's start sampling time, the first symbol may be lost. Therefore, as long as 7 consecutive identical symbols are detected, it can be determined that a frame has arrived.

[0095] Second, after the receiver detects seven consecutive identical symbols, it determines the existence of the reflected signal, calculates the time offset based on K demodulated from symbol 1, and further calculates the number of sampled points of the offset. When the receiver receives seven identical symbols, it discards several sampled points and directly aligns them to the eighth symbol. At this time, there are two cases. If the first symbol in the frame structure is missing, the eighth symbol should be the first symbol in the start flag; if the first symbol in the frame structure is not missing, the eighth symbol should be the last symbol in the sync header. In either case, after receiving the seven symbols, the receiver has achieved synchronization with the symbols in the frame. Next, the receiver only needs to wait to receive the symbols in the two start flags. After receiving these two symbols, the several symbols following immediately are the valid data carried by this frame, and the valid data can be demodulated in sequence.

[0096] Step S3.6: Perform a fast Fourier transform on the extracted valid baseband data, make a decision based on the peak value in the Fourier transform result, and restore the baseband symbol information to complete demodulation.

[0097] It should be noted that arranging each module in a flow layout is only one embodiment of the present invention, and other arrangements can also be adopted. The present invention does not limit this.

[0098] The embodiments of the present invention have the following technical effects:

[0099] First, the power consumption of modulating the reflected signal by this backscatter communication method is extremely low, and the power consumption of the backscatter tag is only at the uW level;

[0100] Second, by adopting this backscatter communication method, the transmission distance of the reflected signal is far, and the spread spectrum signal of the FSCM signal improves the anti-interference ability of the system.

[0101] As Figure 1 shown, a backscatter communication system includes: an environmental signal source, a backscatter tag TAG, and a receiver. The environmental single-tone signal source circularly transmits a sine excitation signal. The backscatter tag is responsible for CSS backscatter modulation of the data to be transmitted, and the receiver is responsible for receiving and demodulating the reflected signal of the tag. This system can achieve backscatter communication at a long distance of up to hundreds of meters.

[0102] As Figure 5 shown, the backscatter tag includes: a field programmable gate array chip (FPGA), a digital-to-analog converter chip (DAC), a voltage controlled oscillator chip (VCO), and a radio frequency switch chip. The functions of each component are as follows:

[0103] (1) FPGA: As the main control chip of the entire backscatter device, the FPGA mainly has the following functions: First, it drives the digital-to-analog conversion chip, gives the DAC control signal, generates an analog voltage to drive the VCO, and thus generates a clock signal with a specific frequency; Second, according to the clock signal input from the VCO to the FPGA, it outputs the radio frequency switch control signal stored in the ROM, controls the radio frequency switch to switch between multiple impedance networks, and generates a reflected wave frequency offset; Third, it drives and acquires data from the temperature sensor integrated in the backscatter device, and converts the temperature data into baseband data in a specific format.

[0104] (2) Digital-to-Analog Converter (DAC): Converts the digital signal given by the FPGA into an analog signal, thereby controlling the voltage-controlled oscillator to output a clock signal with a specific frequency. The main parameters of the DAC include resolution, sampling rate, etc. In this application, the DAC used in the backscatter tag has a resolution of 14 bits and a sampling rate as high as 140 MSPS.

[0105] (3) Voltage-Controlled Oscillator (VCO): The VCO used in this application is composed of pure analog devices. It can convert the magnitude of the input voltage into the magnitude of the output clock signal frequency. Its voltage-frequency transfer characteristic has a highly linear feature, can complete the voltage-frequency conversion with less distortion, and the response speed can meet the requirements.

[0106] (4) Radio Frequency Switch Chip: The radio frequency switch chip and the impedance network connected to it are the core of realizing backscattering. The radio frequency switch used in this application is an SP8T switch, that is, it has a common channel, which can be connected to 8 different channels. This means that there are 8 different impedance states connected to the radio frequency switch respectively. The harmonic of the reflected wave generated by it is smaller, and the energy is concentrated on the fundamental frequency, effectively improving the signal-to-noise ratio of the reflected wave.

[0107] The embodiments of the present invention have the following technical effects:

[0108] First, the backscatter tag is only composed of 4 components, with low cost and simple structure;

[0109] Second, the power consumption of the backscatter tag is extremely low, only at the uW level.

[0110] In this embodiment, the backscatter communication method described in the present invention is simulated, and the bit error rate signal-to-noise ratio curves of the communication system under different conditions are made. The entire MATLAB simulation system mainly includes a backscatter device part and a receiver part.

[0111] The main simulation content of the backscatter device part is the generation of reflected waves. By controlling the reflection coefficient and then multiplying the reflection coefficient by the environmental signal to simulate the reflection effect of the antenna, the corresponding reflected waves are modulated. It mainly includes generating random baseband data, generating the time-domain waveforms of 4 symbols, generating the reflection coefficients corresponding to the four symbols, framing the symbols, multiplying the generated reflection coefficients by the generated environmental signal to obtain the time-domain signal of the reflected wave, and finally adding the environmental signal and the reflected wave signal to obtain the signal mixed with the direct link and the reflected link. Additive white Gaussian noise is added to the communication link to evaluate the communication performance of the communication system.

[0112] Figure 6 The simulation system block diagram of the backscatter device part and the signal images of some nodes are given. The first picture on the left is the time-domain image of the FSCM signal; the three pictures on the right from top to bottom are the short-time Fourier transform of the reflection coefficient, the short-time Fourier transform of the reflected wave, and the short-time Fourier transform of the signal mixed with the direct link and the reflected link respectively. In the figure, the frequency variation law of the STFT of the reflected wave is the same as that of the reflection coefficient over time, indicating that the reflected wave can be indirectly modulated by controlling the reflection coefficient of the antenna.

[0113] Figure 7 The bit error rate vs. signal-to-noise ratio curves of the CSS backscatter communication system under different environmental signal conditions are shown. The curves in the figure from left to right are the environmental signal being a single-tone signal, the environmental signal being a BPSK signal with a bandwidth of 1 MHz, the environmental signal being a BPSK signal with a bandwidth of 2 MHz, the environmental signal being a BPSK signal with a bandwidth of 4 MHz, and the environmental signal being a BPSK signal with a bandwidth of 8 MHz.

[0114] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A backscatter communication method, characterized in that, it includes the following steps: Step S1: Select four different initial frequency offsets to form four different FSCM symbols, and the FSCM symbols are used to constitute an FSCM signal; the four different symbols respectively use K = 0, K = round[0.25(2 SF -1)], K = round[0.5(2 SF -1)], K = round[0.75(2 SF -1)] as modulation symbols; Step S2.1: Combine the baseband data to be transmitted into data frames according to a specific frame structure; Step S2.2: The FPGA obtains clock frequency information according to the FSCM signal in the data frame, converts the clock frequency information into a continuous frequency conversion signal by the DAC, the frequency conversion signal controls the VCO to generate a clock signal with a corresponding frequency, and returns the clock signal to the FPGA to drive the ROM array inside the FPGA to output control information in sequence; Step S2.3: The radio frequency switch outputs a reflection signal according to the control information, and the data frame is included in the reflection signal; Step S3.1: The receiver receives the reflection signal, performs energy detection on the received signal, and detects the data frame in the received signal according to the signal strength; Step S3.2: Oversample the data frame, and perform low-pass filtering on the oversampled data frame; Step S3.3: Downsample the data frame, and reduce the sampling rate to the bandwidth B, where B is the FSCM symbol bandwidth; Step S3.4: Dechirp the FSCM signal in the data frame, then perform a fast Fourier transform, and make a decision according to the peak on the Fourier transform result; specifically: multiply the downsampled FSCM signal by the conjugate of the reference signal, and the reference signal b(n) is a discrete time sequence obtained by sampling a chirp signal without cyclic shift at a sampling rate of B: Multiply the received complete FSCM signal by the conjugate of b(n) to get d(n): The obtained d(n) becomes a single-frequency discrete time sequence, and perform a fast Fourier transform operation on it to get: Ns is the total number of sampling points obtained by sampling the FSCM signal. When a peak with an amplitude of Ns appears in the FFT bin at k = K, it is determined that the modulation information is a symbol of K; Step S3.5: Perform frame synchronization on the synchronization header of the data frame, find the start flag, and extract the valid baseband data; Step S3.6: Perform a fast Fourier transform on the extracted valid baseband data, make a decision according to the peak on the Fourier transform result, and recover the baseband symbol information to complete demodulation.

2. The backscatter communication method according to claim 1, characterized in that, in the step S2.1: The specific frame structure consists of three parts, namely the synchronization header, the start flag, and the baseband data.

3. The backscatter communication method according to claim 1, characterized in that, in the step S3.3: The sampling time is T = 1 / B, and the downsampled FSCM signal s(n,K) can be obtained as: Among them, N s is the total number of sampling points obtained by sampling the FSCM signal, N 0 is the number of sampling points when corresponding to T 0 , since e -j2πn = 1, s(n, K) can be simplified to the following formula:

4. A backscatter communication system, characterized in that, adopts the backscatter communication method according to any one of claims 1-3, and includes: a single-tone signal source, a backscatter tag, and a receiver. The single-tone signal source circularly sends a sine excitation signal, the backscatter tag is responsible for CSS backscatter modulation of the data to be transmitted, and the receiver is responsible for receiving and demodulating the tag reflection signal.

5. The backscatter communication system according to claim 4, characterized in that, the backscatter tag includes: an FPGA, a DAC, a VCO, and a radio frequency switch chip. The FPGA is used to convert baseband data and accordingly give a DAC control signal, generate an analog voltage to drive the VCO, so as to generate a clock signal; the FPGA is further used to, according to the clock signal input from the VCO to the FPGA, complete the output of the radio frequency switch control signal stored in the ROM, control the radio frequency switch chip to switch between multiple impedance networks, and generate a reflected wave frequency offset; the DAC is used to convert the digital signal given by the FPGA into an analog signal, so as to control the VCO to output the clock signal; the VCO is used to convert the magnitude of the input voltage into the magnitude of the output clock signal frequency, and the radio frequency switch chip generates a reflected signal under the control of the FPGA.

Images