A long-distance backscatter communication modulation and demodulation system and method

Abstract

The invention discloses a long-distance backscatter communication modulation and demodulation method and method, and relates to the technical field of wireless communication; the method comprises the following steps: S10, cyclically sending an excitation signal through a frequency modulated continuous wave signal source, the excitation signal being a frequency modulated continuous wave signal; S20, a backscatter tag receives the frequency modulated continuous wave signal sent by the frequency modulated continuous wave signal source, and frequency-shift modulates the frequency modulated continuous wave signal, including the steps of generating an environmental frequency modulated continuous wave signal, generating a tag baseband signal, and generating a reflected signal; S30, receiving a received signal through a receiver, the received signal consisting of a straight link signal and a reflected signal; and demodulating the received signal, eliminating the straight link signal, and demodulating the reflected signal; the beneficial effect of the invention is that it can correctly demodulate when the reflected signal power is lower than the noise power, thereby realizing long-distance communication.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technology, and more specifically, to a long-distance backscatter communication modulation and demodulation method and system. Background Art

[0002] As one of the underlying technologies for building a future society of intelligent interconnection of all things, the Internet of Things promotes the improvement of operating efficiency, the reduction of production costs and the conservation of resources and energy consumption through the perception, connection and intelligent management of the physical world. In the future, the intelligent interconnection of all things at the level of hundreds of billions will be built on the basis of low-carbon energy supply and low-power operation of equipment. Existing Internet of Things technologies still cannot meet the needs of many Internet of Things applications, mainly including: (1) Harsh and complex communication environment: For application scenarios such as ultra-high voltage power stations, cold zone monitoring, industrial production lines, and train track monitoring, the Internet of Things faces extremely complex environments such as high voltage, high temperature, high cold, high radiation, and high-speed movement. Existing Internet of Things terminals based on conventional power supplies are difficult to replace batteries or even fail to work; (2) Requirements for extremely small terminal form factors: For application scenarios such as commodity circulation traceability and smart wearables, Internet of Things terminals need to have extremely small sizes for easy use; (3) A sufficiently large communication range to ensure low-cost wide-area coverage. Therefore, it is urgent to develop new Internet of Things communication technologies with extremely low power consumption, extremely small size and extremely low cost to better serve economic and social development.

[0003] Environmental backscatter communication technology is one of the key technologies that is expected to solve the above problems due to its advantages of ultra-low power consumption, simple terminal structure and spectrum sharing. Passive backscatter communication can obtain energy through ubiquitous wireless signals. Its sensors do not require the support of batteries and expensive RF components. It has the advantages of no battery, low cost, easy deployment and easy maintenance, but it also has the disadvantages of short communication distance and difficult interconnection. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a long-distance backscatter communication modulation and demodulation method and system.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a long-distance backscatter communication modulation and demodulation method, the improvement of which is that the method comprises the following steps:

[0006] S10, cyclically sending an excitation signal through a frequency modulated continuous wave signal source, including the step of generating an environmental frequency modulated continuous wave signal, wherein the excitation signal is a frequency modulated continuous wave signal with a linearly increasing frequency;

[0007] S20, the backscatter tag receives the frequency modulated continuous wave signal sent by the frequency modulated continuous wave signal source, and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating a tag baseband signal and generating a reflected signal;

[0008] S30, receiving a received signal through a receiver, where the received signal consists of a straight link signal and a reflected signal; and demodulating the received signal to eliminate the straight link signal and demodulate the reflected signal.

[0009] Further, the path for the FMCW signal source to send the FMCW signal to the backscatter tag is called path g, the path for the backscatter tag to send the reflected signal to the receiver is called path b, and the path for the FMCW signal source to send the straight link signal to the receiver is called path f;

[0010] In step S20, the step of generating an ambient frequency modulated continuous wave signal includes:

[0011] The signal sent by the frequency modulated continuous wave signal source is s(t), which is a spread spectrum signal with a linearly increasing frequency. Its frequency is expressed as: f(t) = f 0 +kt; where f 0 is the initial frequency of the FM continuous wave, then the phase of the FM continuous wave signal is:

[0012]

[0013] The frequency modulated continuous wave signal s(t) is expressed as:

[0014]

[0015] The transmitting power of the FM continuous wave signal source is P t , then the received signal reaching the receiver through path g is:

[0016]

[0017] Among them, L f is the free space propagation loss of path f, expressed in dB, L f The calculation method is:

[0018] L=32.45+20logD+20logF.

[0019] Furthermore, in step S20, the step of generating a tag baseband signal includes:

[0020] The tag baseband signal m(t) consists of the square wave signal x(t) and the baseband signal a(t), expressed as:

[0021] m(t)=a(t)x(t);

[0022] The baseband signal a(t) is the binary baseband data of the backscatter tag, and the square wave signal x(t) is used to frequency shift the frequency modulated continuous wave signal using the first harmonic of the square wave.

[0023] Perform Fourier series transform on the square wave signal x(t):

[0024]

[0025] Since the square wave signal x(t) is an odd signal, the DC component a 0 and cosine component a n are all 0, and the sine component b n Only has values ​​at odd harmonics:

[0026]

[0027] Furthermore, the generation of the square wave signal x(t) is achieved by repeatedly transforming the two impedance states of "matching" and "mismatching" on the backscattering tag.

[0028] Furthermore, in step S20, the step of generating a reflection signal includes:

[0029] The tag baseband signal m(t) backscatters the FMCW signal, and the reflected modulated signal s after the tag impedance frequency shift modulation is b (t) is expressed as:

[0030]

[0031] |Γ tag | is the reflection coefficient of the tag, L g and L b is the reflection path loss from the FMCW signal source to the backscatter tag and from the backscatter tag to the receiver, θ g and θ b is the phase delay caused by the reflection path delay, then the received signal at the receiver is expressed as:

[0032] y(t)=s f (t)+s b (t)+n(t);

[0033] Among them, s f (t) is the direct link signal, s b (t) is the tag reflection signal, n(t) is Gaussian white noise with mean 0 and variance σ 2 The normal distribution of .

[0034] Further, in step S30, demodulating the received signal includes:

[0035] S301, energy detection, performing energy detection on the received signal, and detecting the effective signal part in the received signal according to the signal strength;

[0036] S302, bandpass filtering, using a bandpass filter to eliminate the straight-link signal, and the bandpass filter simultaneously extracts the reflection signals of the upper and lower sidebands to obtain the reflection signal from the tag;

[0037] S303, upper sideband frequency shift, using a complex exponential signal to move the reflected signal of the upper sideband to zero frequency, and using a low-pass filter to filter out the lower sideband signal;

[0038] S304, frequency shifting the lower sideband, moving the reflected signal of the lower sideband to zero frequency, and filtering out the upper sideband signal using a low-pass filter;

[0039] S305, equal gain combining, using a complex exponential signal to combine the obtained upper and lower sideband reflection signals with equal gain, and then downsampling the reflection signal at a sampling rate BW;

[0040] S306, symbol synchronization, dividing the signal into separate symbols according to a preset symbol period;

[0041] S307, fast Fourier transform, performing an operation on each downsampled code element and a conjugate code element with a linearly reduced frequency generated locally by the receiver, and then performing a fast Fourier transform on the result obtained by the correlation operation;

[0042] S308, codeword judgment, if the maximum value of the fast Fourier transform result of the codeword is at "0", the reflected signal symbol is judged to be bit "1", if the fast Fourier transform peak value of a symbol is not at "0", the reflected signal symbol is judged to be bit "0", thereby completing the demodulation of the reflected signal.

[0043] Furthermore, the BW is the signal bandwidth of the frequency modulated continuous wave.

[0044] The present invention also provides a long-distance backscatter communication modulation and demodulation system, the improvement of which is that it includes a frequency modulated continuous wave signal source, a backscatter tag and a receiver;

[0045] The frequency modulated continuous wave signal source cyclically sends an excitation signal, and the excitation signal is a frequency modulated continuous wave signal with a linearly increasing frequency;

[0046] The backscatter tag receives a frequency modulated continuous wave signal sent by a frequency modulated continuous wave signal source and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating an environmental frequency modulated continuous wave signal, generating a tag baseband signal, and generating a reflected signal;

[0047] The receiver receives a received signal, which is composed of a straight link signal and a reflected signal; and demodulates the received signal, eliminates the straight link signal, and demodulates the reflected signal.

[0048] The beneficial effects of the present invention are as follows: the modulation method adopted by the backscatter tag in the present invention is to frequency shift the incident frequency modulated continuous wave signal to an adjacent frequency band. The frequency shift modulation method can not only eliminate the interference of the straight-link signal, but also obtain the spread spectrum gain brought by the frequency modulated continuous wave signal, and can correctly demodulate when the reflected signal power is lower than the noise power, thereby realizing long-distance communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a structural diagram of a long-distance backscatter communication modulation and demodulation system in the present invention.

[0050] Figure 2 It is a schematic diagram of the frequency modulated continuous wave signal source in the present invention cyclically sending the excitation signal.

[0051] Figure 3 It is the time domain and amplitude-frequency diagram of the square wave signal x(t) in the present invention.

[0052] Figure 4 It is the spectrum diagram of the straight link signal in the present invention.

[0053] Figure 5 It is the spectrum diagram of the reflected signal in the present invention.

[0054] Figure 6 It is a schematic diagram of the process of demodulating the received signal in the present invention.

[0055] Figure 7 It is a schematic diagram of the reflected signal power at different positions of the backscattering tag in the present invention.

[0056] Figure 8 It is a schematic diagram of the bit error rate of the reflected signal of the backscattering tag at different positions in the present invention. DETAILED DESCRIPTION

[0057] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0058] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by technicians in this field without creative work are all within the scope of protection of the present invention. In addition, all the connection / connection relationships involved in the patent do not refer to the direct connection of components, but refer to the formation of a better connection structure by adding or reducing connection accessories according to the specific implementation situation. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0059] Reference Figure 1 As shown, the present invention provides a long-distance backscatter communication modulation and demodulation system, which includes a frequency modulated continuous wave signal source, a backscatter tag and a receiver; wherein the frequency modulated continuous wave signal source cyclically sends an excitation signal, which is a frequency modulated continuous wave signal; the backscatter tag receives the frequency modulated continuous wave signal sent by the frequency modulated continuous wave signal source, and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating an environmental frequency modulated continuous wave signal, generating a tag baseband signal and generating a reflected signal, and the specific steps will be described in detail below; the receiver realizes the reception of the received signal, which is composed of a straight link signal and a reflected signal; and demodulates the received signal, eliminates the straight link signal, and demodulates the reflected signal.

[0060] In this scheme, a special signal source device is deployed to transmit a frequency modulated continuous wave as a stable and reliable excitation signal; the modulation method adopted by the tag is to shift the incident frequency modulated continuous wave signal to an adjacent frequency band. The frequency shift modulation method can not only eliminate the interference of the direct link signal, but also obtain the spread spectrum gain brought by the frequency modulated continuous wave signal, and can correctly demodulate when the reflected signal power is lower than the noise power, thereby realizing long-distance communication. Therefore, a long-distance backscatter communication modulation and demodulation system proposed by the present invention can expand the communication distance, enhance interconnection, and realize a low-power, low-cost, easy-to-maintain, and widely-accessed passive Internet of Things, which has broad application prospects in the fields of smart homes, logistics tracking, environmental monitoring, wearable devices, etc.

[0061] In this embodiment, a long-distance backscatter communication modulation and demodulation method is also provided. The method is based on the above-mentioned system implementation. The method proposes a method for performing frequency shift modulation (Frequency Shift Key, FSK) on a frequency modulated continuous wave to achieve long-distance backscatter communication for the IoT application scenario of long-distance communication. Figure 1 As shown, the method comprises the following steps:

[0062] S10, cyclically sending an excitation signal through a frequency modulated continuous wave signal source, including the step of generating an environmental frequency modulated continuous wave signal, wherein the excitation signal is a frequency modulated continuous wave signal with a linearly increasing frequency;

[0063] In this embodiment, Figure 2 As shown, it is a schematic diagram of a frequency modulated continuous wave signal source cyclically sending an FMCW (Frequency Modulated Continuous Wave) excitation signal.

[0064] In this embodiment, combined with Figure 1As shown, the path for the FM continuous wave signal source to send the FM continuous wave signal to the backscatter tag is called path g, the path for the backscatter tag to send the reflected signal to the receiver is called path b, and the FM continuous wave signal source to send the straight link signal to the receiver is called path f; in step S10, the step of generating an environmental FM continuous wave signal includes:

[0065] The signal sent by the frequency modulated continuous wave signal source is s(t), which is a spread spectrum signal with a linearly increasing frequency. Its frequency is expressed as: f(t) = f 0 +kt; where f 0 is the initial frequency of the FM continuous wave, then the phase of the FM continuous wave signal is:

[0066]

[0067] The frequency modulated continuous wave signal s(t) is expressed as:

[0068]

[0069] The transmitting power of the FM continuous wave signal source is P t , then the received signal reaching the receiver through path g is:

[0070]

[0071] Among them, L f is the free space propagation loss of path f, in dB; L f The calculation method is:

[0072] L=32.45+20logD+20logF.

[0073] Furthermore, in step S20, the step of generating a tag baseband signal includes:

[0074] The tag baseband signal m(t) consists of the square wave signal x(t) and the baseband signal a(t), expressed as:

[0075] m(t)=a(t)x(t);

[0076] Among them, the baseband signal a(t) is the binary baseband data of the backscatter tag, and the square wave signal x(t) is used to frequency shift the frequency modulated continuous wave signal using the first harmonic of the square wave; Figure 3 As shown, the time domain and amplitude-frequency diagram of the square wave signal x(t) are shown; the generation of the square wave signal x(t) is achieved by repeatedly transforming the two impedance states of "matching" and "mismatching" on the backscattering tag;

[0077] Perform Fourier series transform on the square wave signal x(t):

[0078]

[0079]

[0080] Since the square wave signal x(t) is an odd signal, the DC component a 0 and cosine component a n are all 0, and the sine component b n Only has values ​​at odd harmonics:

[0081]

[0082] S20, the backscatter tag receives the frequency modulated continuous wave signal sent by the frequency modulated continuous wave signal source, and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating a tag baseband signal and generating a reflected signal;

[0083] In this embodiment, the step of generating a reflection signal includes:

[0084] The tag baseband signal m(t) backscatters the FMCW signal, and the reflected modulated signal s after the tag impedance frequency shift modulation is b (t) is expressed as:

[0085]

[0086] |Γ tag | is the reflection coefficient of the tag, L g and L b is the reflection path loss from the FMCW signal source to the backscatter tag and from the backscatter tag to the receiver, θ g and θ b is the phase delay caused by the reflection path delay, then the received signal at the receiver is expressed as:

[0087] y(t)=s f (t)+s b (t)+n(t);

[0088] Among them, s f (t) is the direct link signal, s b (t) is the tag reflection signal, n(t) is Gaussian white noise with mean 0 and variance σ 2 The normal distribution of .

[0089] In this embodiment, the spectrum diagram of the direct link signal is as follows: Figure 4 As shown, the spectrum of the reflected signal is as follows Figure 5 As shown, the tag reflects the signal s b (t) and the direct link signal s f(t) is located in a different frequency band, thus avoiding direct link interference. At the same time, the reflected signal is a broadband linear frequency modulation spread spectrum signal, which can obtain spread spectrum gain to achieve low-sensitivity detection, which greatly improves the communication distance of the tag backscattering.

[0090] S30, receiving a received signal through a receiver, where the received signal consists of a straight link signal and a reflected signal; and demodulating the received signal to eliminate the straight link signal and demodulate the reflected signal.

[0091] In this embodiment, combined with Figure 6 As shown, in step S30, demodulating the received signal includes:

[0092] S301, energy detection, energy detection is performed on the received signal, and the effective signal part in the received signal is detected according to the signal strength; in this scheme, since the received signal is composed of discontinuous frequency-modulated continuous wave signal frames, the reflected signal is superimposed on the ambient frequency-modulated continuous wave signal in the time domain; first, energy detection is performed on the received signal, and the effective signal part in the received signal is detected according to the received signal strength (Received Signal Strength Indicator, RSSI);

[0093] S302, bandpass filtering, using a bandpass filter to eliminate the straight-link signal, and the bandpass filter simultaneously extracts the reflection signals of the upper and lower sidebands to obtain the reflection signals of the upper and lower sidebands;

[0094] S303, upper sideband frequency shift, using a complex exponential signal to move the reflected signal of the upper sideband to zero frequency, and using a low-pass filter to filter out the lower sideband signal;

[0095] S304, frequency shifting the lower sideband, moving the reflected signal of the lower sideband to zero frequency, and filtering out the upper sideband signal using a low-pass filter;

[0096] S305, equal gain merging, using a complex exponential signal to perform equal gain merging on the acquired upper and lower sideband reflection signals, and then downsampling the reflection signals at a sampling rate BW; wherein BW is the signal bandwidth of the frequency modulated continuous wave;

[0097] S306: symbol synchronization: dividing the signal into separate symbols according to a preset symbol period.

[0098] S307, fast Fourier transform, performing an operation on each downsampled symbol and a conjugate symbol with a linearly reduced frequency generated locally by the receiver, and then performing a fast Fourier transform on the result of the operation;

[0099] S307, codeword judgment, if the maximum value of the fast Fourier transform result of each codeword is at "0", the reflected signal symbol is judged to be bit "1", if the fast Fourier transform peak value of a symbol is not at "0", the reflected signal symbol is judged to be bit "0", thereby completing the demodulation of the reflected signal.

[0100] In addition, the present invention also provides an embodiment, in which a long-distance backscatter communication modulation and demodulation system is mainly composed of three parts: a frequency modulated continuous wave signal source, a backscatter tag and a receiver. The frequency modulated continuous wave signal source cyclically sends a frequency modulated continuous wave signal with a bandwidth of 500KHz and a fixed period of 0.256ms; the backscatter tag is responsible for the backscatter modulation transmission of the data to be transmitted, and the tag adopts a frequency shift modulation method; the receiver is responsible for receiving and demodulating the tag reflection signal. In this scheme, a verification platform is constructed by a frequency modulated continuous wave signal source, a backscatter tag and a receiver, wherein the receiver adopts NI's USRP2952R; the system operating frequency is 440MHz, and the structure of the backscatter tag is composed of an impedance board and an FPGA control board. The frequency modulated continuous wave signal source sends an excitation signal, and the maximum transmission power is 20dBm. The experimental parameters of the verification platform are as follows: the frequency of the frequency modulated continuous wave signal source is 440MHz, the sampling rate of the receiver is 10M / s, the signal source transmission power is 20dBm, and the distance between the signal source and the receiver is 200m.

[0101] The verification platform is operated with a fixed distance of 200m between the FM continuous wave signal source and the receiver, and the backscattering tag moves between the FM continuous wave signal source and the receiver, proving the feasibility and correctness of the long-distance frequency shift modulation scheme based on FM continuous wave. When the tag sends a bit "1", the FM continuous wave signal is frequency shifted; when the tag sends a bit "0", the FM continuous wave signal is not frequency shifted. The experimental results of the verification platform are shown in Figure 1. Figure 7 , Figure 8 As shown, Figure 7 Indicates the reflected signal power at different positions of the tag, Figure 8 Indicates the bit error rate of the reflected signal of the tag at different positions.

[0102] The present invention discloses a frequency modulated continuous wave based long-distance backscatter communication modulation and demodulation system, which can achieve a communication range of 3 km and a maximum communication rate of 15.625 Kbps. The core advantages of this system are long-distance communication and uW-level power consumption of tags, which is one of the development trends of 6G passive Internet of Things in the future.

[0103] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A long-distance backscatter communication modulation and demodulation method, characterized in that: The method comprises the following steps: S10, cyclically sending an excitation signal through a frequency modulated continuous wave signal source, including the step of generating an environmental frequency modulated continuous wave signal, wherein the excitation signal is a frequency modulated continuous wave signal with a linearly increasing frequency; The path for the FMCW signal source to send the FMCW signal to the backscatter tag is called path g, the path for the backscatter tag to send the reflected signal to the receiver is called path b, and the path for the FMCW signal source to send the straight link signal to the receiver is called path f; In step S10, the step of generating an ambient frequency modulated continuous wave signal includes: The signal sent by the FM continuous wave signal source is s(t), which is a spread spectrum signal with a linearly increasing frequency. Its frequency is expressed as: f(t) = f0 + kt; where f0 is the initial frequency of the FM continuous wave, k is the rate at which the frequency changes with time, and t is time. The phase of the FM continuous wave signal is: The frequency modulated continuous wave signal s(t) is expressed as: The transmitting power of the FM continuous wave signal source is P t , then the received signal reaching the receiver through path g is: Among them, L f is the free space propagation loss of path f, in dB; θ f The phase delay introduced for propagation; L f The calculation method is: L = 32.45 + 20logD + 20logF; Where D is the transmission distance of the electromagnetic wave, and F is the frequency of the electromagnetic wave; S20, the backscatter tag receives the frequency modulated continuous wave signal sent by the frequency modulated continuous wave signal source, and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating a tag baseband signal and generating a reflected signal; In step S20, the step of generating a tag baseband signal includes: The tag baseband signal m(t) consists of the square wave signal x(t) and the baseband signal a(t), expressed as: m(t)=a(t)x(t) m(t)=a(t)x(t); The baseband signal a(t) is the binary baseband data of the backscatter tag, and the square wave signal x(t) is used to frequency shift the frequency modulated continuous wave signal using the first harmonic of the square wave. Perform Fourier series transform on the square wave signal x(t): Among them, ω0 is the frequency of the first harmonic; since the square wave signal x(t) is an odd signal, the DC component a0 and the cosine component a n are all 0, and the sine component b n Only has values ​​at odd harmonics: Where A is the amplitude of the square wave signal x(t); In step S20, the step of generating a reflection signal includes: The tag baseband signal m(t) backscatters the FMCW signal, and the reflected modulated signal s after the tag impedance frequency shift modulation is b (t) is expressed as: |Γ tag | is the reflection coefficient of the tag, L g and L b is the reflection path loss from the FMCW signal source to the backscatter tag and from the backscatter tag to the receiver, θ g and θ b is the phase delay caused by the reflection path delay, then the received signal at the receiver is expressed as: y(t)=s f (t)+s b (t)+n(t); Among them, s f (t) is the direct link signal, s b (t) is the tag reflection signal, n(t) is Gaussian white noise with mean 0 and variance σ 2 Normal distribution of S30, receiving a received signal by a receiver, where the received signal consists of a straight link signal and a reflected signal; and demodulating the received signal to eliminate the straight link signal and demodulate the reflected signal; In step S30, demodulating the received signal includes: S301, energy detection, performing energy detection on the received signal, and detecting the effective signal part in the received signal according to the signal strength; S302, bandpass filtering, using a bandpass filter to eliminate the straight-link signal, and the bandpass filter simultaneously extracts the reflection signals of the upper and lower sidebands to obtain the reflection signal from the tag; S303, upper sideband frequency shift, using a complex exponential signal to move the reflected signal of the upper sideband to zero frequency, and using a low-pass filter to filter out the lower sideband signal; S304, frequency shifting the lower sideband, moving the reflected signal of the lower sideband to zero frequency, and filtering out the upper sideband signal using a low-pass filter; S305, equal gain combining, using a complex exponential signal to combine the obtained upper and lower sideband reflection signals with equal gain, and then downsampling the reflection signal at a sampling rate BW; S306, symbol synchronization, dividing the signal into separate symbols according to a preset symbol period; S307, fast Fourier transform, performing an operation on each downsampled symbol and a conjugate symbol with a linearly reduced frequency generated locally by the receiver, and then performing a fast Fourier transform on the result of the operation; S308, codeword judgment: if the maximum value of the fast Fourier transform result of the codeword is at "0", the reflected signal symbol is judged to be bit "1". If the fast Fourier transform peak value of a symbol is not at "0", the reflected signal symbol is judged to be bit "0", thereby completing the demodulation of the reflected signal.

2. A long-distance backscatter communication modulation and demodulation method according to claim 1, characterized in that: The square wave signal x(t) is generated by repeatedly transforming the two impedance states of "match" and "mismatch" on the backscatter tag.

3. A long-distance backscatter communication modulation and demodulation method according to claim 1, characterized in that: The BW is the signal bandwidth of the frequency modulated continuous wave.

4. A long-distance backscatter communication modulation and demodulation system, characterized in that: The system implements a long-distance backscatter communication modulation and demodulation method as described in any one of claims 1-3, and the system includes a frequency modulated continuous wave signal source, a backscatter tag and a receiver; The frequency modulated continuous wave signal source cyclically sends an excitation signal, and the excitation signal is a frequency modulated continuous wave signal with a linearly increasing frequency; The backscatter tag receives a frequency modulated continuous wave signal sent by a frequency modulated continuous wave signal source and performs frequency shift modulation on the frequency modulated continuous wave signal, including the steps of generating an environmental frequency modulated continuous wave signal, generating a tag baseband signal, and generating a reflected signal; The receiver receives a received signal, which is composed of a straight link signal and a reflected signal; and demodulates the received signal, eliminates the straight link signal, and demodulates the reflected signal.

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