Chirp interval modulation backscatter communication method and system based on variable-length scrambling codes

Abstract

The invention discloses a chirp interval modulation backscatter communication method and a chirp interval modulation backscatter communication system based on variable-length scrambling codes, which mainly solve the problems of high power consumption of backscatter labels and high system deployment cost in the prior art. According to the scheme, a sending end sends an environment LoRa signal; the backscattering label modulates information to be sent to an environment LoRa signal in a time interval modulation mode, and modulates a variable-length scrambling code to an anchor symbol; a receiving end performs frequency shift compensation on the backscattering signal by adopting a frequency shift compensation method, demodulates the backscattering signal by combining a sliding window with a variable-length scrambling code, intercepts a backscattering signal sample through the sliding window, and performs de-spreading and fast Fourier transform (FFT) on the signal in the window; the position of an anchor symbol is determined by detecting the maximum value of an FFT peak value, information sent by the backscattering label is recovered from the position of the anchor symbol, and data demodulation is completed. On the premise of keeping demodulation reliability, the power consumption of the tag is effectively reduced, the deployment cost of a backscatter communication system is reduced, and the method can be used for ultra-low power consumption Internet of Things communication.

Description

Technical Field

[0001] This invention belongs to the field of communication technology, and specifically relates to a chirp-spaced modulation backscatter communication method and system, which can be used for low-power Internet of Things (IoT) communication in smart homes, smart cities, smart agriculture, smart factories, and telemedicine. Background Technology

[0002] In recent years, with the rapid development of IoT technology, its application scenarios have covered many fields such as smart homes, smart cities, smart agriculture, and telemedicine. Against this backdrop, the demand for wireless communication technology in various industries continues to grow. To ensure the sustainability of IoT devices in remote deployment and long-term operation, and to further reduce the energy consumption and maintenance costs of devices, the development of wireless communication technologies that combine long-distance communication capabilities with low power consumption is particularly important. As a typical low-power communication solution, backscatter technology has attracted widespread attention in the field of wireless communication in recent years. Among many low-power wide-area network (LPWAN) technologies, long-range radio LoRa is considered an ideal choice for IoT applications due to its long-distance communication capabilities and excellent energy efficiency. Its core advantage stems from the linear frequency modulation spread spectrum (CSS) technology used in the physical layer. This technology not only effectively combats channel fading through spreading gain, but also relies on high processing gain to enable the system to reliably demodulate under extremely low signal-to-noise ratio (SNR) conditions, thereby significantly improving the communication link budget and coverage. With its excellent anti-interference performance, robustness in low SNR environments, and simple, low-complexity demodulation implementation, LoRa has been widely used in large-scale, low-power IoT communication scenarios.

[0003] Based on the aforementioned advantages, existing research incorporates the physical layer characteristics of LoRa into backscattering systems to further expand its application potential in ultra-low power scenarios. While traditional backscattering boasts extremely low power consumption, it suffers from limitations in anti-interference capabilities, communication distance, and demodulation in weak signal environments. To improve the reliability and coverage of backscattering links, existing LoRa backscattering systems typically use ambient LoRa signals, dedicated LoRa excitation signals, or single-frequency signals as excitation sources, enabling backscattering nodes to inherit the processing gain and frequency spread characteristics brought by LoRa modulation. LoRa backscattering systems developed based on this approach can utilize LoRa signals for data modulation and transmission without requiring active RF transmission, achieving a balance between ultra-low power consumption and long-distance communication. Therefore, LoRa backscattering is gradually becoming an important direction in low-power IoT research, demonstrating broad application prospects in large-scale deployments, long-life nodes, and difficult-to-maintain scenarios.

[0004] Patent document CN202510824865.8 discloses a "chirp delay modulation backscatter communication method and system combined with pseudo-random sequence detection". The scheme includes: a transmitter periodically sending an up-sweep frequency signal as an excitation signal; a backscatter tag determining the delay time based on the data value to be transmitted, completing one modulation cycle within two consecutive excitation signal periods, and modulating a pseudo-random sequence onto the backscatter signal; and a receiver using a sliding window combined with a pseudo-random sequence to demodulate the backscatter signal, with the receiver performing despreading processing and fast Fourier transform on the signal samples within the window to determine the symbol positions to achieve data demodulation. This method uses a dedicated signal source as the excitation signal, thus occupying additional spectrum resources and increasing deployment costs. Furthermore, because it uses a fixed-length pseudo-random sequence, the backscatter device needs a high frequency to switch when modulating the pseudo-random sequence onto the reflected signal, increasing tag power consumption and subsequent maintenance costs, thus hindering the large-scale deployment of IoT devices. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by proposing a chirp-spaced modulation backscatter communication method and system based on variable-length scrambling codes, so as to reduce the power consumption of backscatter tags and the deployment cost of backscatter communication systems.

[0006] The technical approach to achieving the objective of this invention is as follows: by adopting a variable-length scrambling strategy, the scrambling code length used during modulation is shortened, the switching frequency of the backscatter tag is reduced, and its power consumption is effectively reduced; by using the ambient LoRa signal as the excitation signal, the deployment cost of the backscatter communication system is reduced.

[0007] Based on the above ideas, the technical solution of the present invention includes the following:

[0008] 1. A chirp-spaced modulation backscatter communication method based on variable-length scrambling codes, characterized in that,

[0009] include:

[0010] (1) The transmitting end sends the environmental LoRa signal as the excitation signal;

[0011] (2) The backscatter tag reflects and absorbs the excitation signal by controlling the opening and closing of the radio frequency switch, and controls the switch to modulate the reflected signal with a variable length scrambling code, thus completing the modulation of the excitation signal;

[0012] (3) The receiving end uses a demodulation algorithm combining a sliding window and a variable length scrambling code to complete the demodulation of the modulated signal.

[0013] Furthermore, in (2), the backscatter tag reflects and absorbs the excitation signal by controlling the opening and closing of the radio frequency switch, which is achieved by:

[0014] 2a) Deploy backscatter tags between the transmitter and receiver;

[0015] 2b) The backscatter tag disables the RF switch, keeping it in a continuous absorption state;

[0016] 2c) The number of time slots corresponding to the decimal information to be transmitted by the backscatter tag is used as the time interval. When the duration of the backscatter tag in the absorption state reaches the set time interval, the backscatter tag turns on the radio frequency switch to put it in the reflection state, so as to generate a reflected signal called the anchor symbol, and the reflection state is maintained for one chirp signal cycle.

[0017] 2d) After the tag maintains the reflective state for one chirp signal cycle, the RF switch is turned off, so that the backscatter tag is in the absorption state. During this period, the tag does not reflect the signal until two consecutive chirp signal cycles end.

[0018] Furthermore, in step (2), the control switch modulates the variable-length scrambling code onto the reflected signal. When the backscattering tag is in a reflective state, the control switch switches the reflection coefficient of the backscattering tag between positive and negative 1 according to the preset scrambling code sequence.

[0019] 2. A chirp-spaced modulation backscatter communication system based on variable-length scrambling codes, comprising a transmitter, a backscatter tag, and a receiver, characterized in that:

[0020] The backscatter tag includes a time interval modulation module and a variable length scrambling code modulation module;

[0021] The time interval modulation module is used to modulate information onto the incident ambient LoRa signal, and according to the decimal information to be transmitted, the number of time slots corresponding to the decimal information is used as the time interval to control the radio frequency switch to make the backscatter tag enter the reflection state after the corresponding time interval to maintain one chirp signal cycle.

[0022] The variable length scrambling code modulation module is used to control the switching of the radio frequency switch according to the preset scrambling code sequence when the backscattering tag is in the reflective state, so that the reflection coefficient of the backscattering tag switches between positive and negative 1, so as to modulate the variable length scrambling code onto the reflected signal.

[0023] The receiver includes a frequency shift compensation module, a sliding window module, a despreading module, an FFT module, and a data decoding module;

[0024] This frequency shift compensation module is used to eliminate frequency discontinuities within the reflected signal;

[0025] This sliding window module is used to extract signal samples with a length of one chirp signal period to form multiple windows for detecting reflected signals;

[0026] This despreading module is used to despread the signal samples within each window;

[0027] This FFT module is used to perform a fast Fourier transform on the despread signal, obtain the FFT peak value in the frequency domain, and record the FFT peak value of the signal within each window to obtain the FFT peak value sequence.

[0028] This data decoding module is used to decode time intervals.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] Firstly, it saves tag power consumption.

[0031] The backscatter tag of this invention employs a variable-length scrambling strategy. When the backscatter tag is in a reflective state, according to a preset scrambling sequence, a control switch switches the reflection coefficient of the backscatter tag between positive and negative 1, modulating the variable-length scrambling code onto the reflected signal. Since the length of the scrambling code determines the switching frequency of the radio frequency switch, the shorter the scrambling code, the lower the switching frequency of the radio frequency switch, and the lower the power consumption of the tag. Therefore, the variable-length scrambling strategy of this invention can shorten the length of the scrambling code and reduce the power consumption of the backscatter tag.

[0032] Secondly, it saves on the deployment costs of backscatter communication systems.

[0033] The receiver of this invention uses a frequency shift compensation method to eliminate the frequency discontinuity in the reflected signal caused by using the ambient LoRa signal as the excitation source. Therefore, it does not need to deploy a dedicated upsweep signal as the excitation source, but directly uses the ambient LoRa signal as the excitation source, thus saving spectrum resources and deployment costs of the backscatter communication system. Attached Figure Description

[0034] Figure 1 This is a flowchart of the chirp-spaced modulation backscatter communication method based on variable-length scrambling codes in this invention;

[0035] Figure 2 This is a schematic diagram of the backscatter tag modulation method in the present invention;

[0036] Figure 3 This is a schematic diagram of the frequency shift compensation principle in the method of this invention;

[0037] Figure 4This is a schematic diagram of the sliding window anchor symbol detection in the method of the present invention;

[0038] Figure 5 This is a block diagram of the chirp-spaced modulation backscatter communication system module based on variable-length scrambling codes according to the present invention;

[0039] Figure 6 The simulation results of sliding window FFT using existing delay modulation methods with fixed-length scrambling codes are shown in the figure.

[0040] Figure 7 This is a simulation result diagram of the sliding window FFT when shortening the scrambling code according to the present invention;

[0041] Figure 8 The figure shows the simulation results of the symbol error rate under different m-length sequence conditions of this invention. Detailed Implementation

[0042] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the present invention and not all of it. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative effort should all fall within the protection scope of the present invention.

[0043] Example 1: Chirp-spaced modulation backscatter communication method based on variable-length scrambling code.

[0044] Reference Figure 1 The implementation steps of this example include the following:

[0045] Step 1: The transmitting end sends an environmental LoRa signal.

[0046] The environmental LoRa signal consists of continuous chirp signals, which are arranged sequentially in time. Each chirp signal has the same bandwidth and spreading factor, with the bandwidth BW set to 500kHz and the spreading factor SF set to 10.

[0047] Step 2: The backscatter tag modulates the incident environmental LoRa signal.

[0048] The backscatter tag is a passive low-power communication device that does not generate radio frequency carrier signals. Instead, it modulates the incident environmental LoRa signal by controlling its own radio frequency switch to change the load impedance connected to the antenna port, and then sends the modulated signal to the receiver by reflection.

[0049] The implementation of this step includes the following:

[0050] 2.1) Deploy backscatter tags between the transmitter and receiver so that the backscatter tags can receive the ambient LoRa signal from the transmitter and reflect the modulated signal back to the receiver;

[0051] 2.2) Under the above deployment conditions, using two consecutive chirp signals in the environmental LoRa signal as a modulation unit, the backscatter tag performs a time interval modulation operation on the modulation unit.

[0052] Reference Figure 2 The modulation operation in this step is as follows:

[0053] 2.2.1) The backscatter tag turns off its own RF switch, matching the antenna port impedance with the connected load impedance, thus putting the backscatter tag into an absorption state, and controlling the absorption state to last for a certain period of time:

[0054] The time interval consists of several time slots, where a time slot is a basic time unit after time discretization, and its length is determined by the sampling rate. In this embodiment, the sampling rate is set to the bandwidth BW of the chirp signal; therefore, the duration of one time slot is... ;

[0055] Based on this, this embodiment uses a time interval encoding method based on the number of time slots, that is, the number of time slots during which the backscatter tag is in the absorption state is used to represent the decimal data to be transmitted. Taking one backscatter modulation as an example, let the decimal data to be modulated be... ,in ,and ;

[0056] Using the aforementioned decimal data Corresponding to Calculate the time interval for each consecutive time slot. for:

[0057] ;

[0058] 2.2.2) After the backscatter tag maintains its absorption state for the aforementioned time interval, the backscatter tag turns on its RF switch, causing the antenna port impedance to be mismatched with the connected load impedance, and begins to reflect LoRa signals into the environment, maintaining this reflection for one chirp signal cycle. The signal reflected within that period is defined as the anchor symbol;

[0059] 2.2.3) After reflecting the LoRa signal from the incident environment and continuing for one chirp signal cycle, the backscatter tag turns off the RF switch, returning it to the absorption state, and maintains this absorption state until the end of two consecutive chirp signal cycles.

[0060] 2.3) When the backscatter tag is in a reflective state, the radio frequency switch is controlled according to the preset scrambling code sequence to switch the reflection coefficient of the backscatter tag between positive and negative 1, thereby modulating the variable length scrambling code onto the anchor symbol.

[0061] In this embodiment, an m-sequence is used as the scrambling sequence, but not limited to. When the spreading factor is 10, the lengths of the m-sequences that can be selected include 1024, 512, 256, 128, 64, 32, 16, 8 and 4. By adjusting the length of the selected m-sequence, the scrambling length can be flexibly configured.

[0062] Step 3: The receiving end demodulates the received signal.

[0063] 3.1) The receiver first performs standard LoRa demodulation on the received ambient LoRa signal to obtain the starting frequency corresponding to the two consecutive chirp signals. and ;

[0064] 3.2) The receiver uses the demodulated starting frequency and Frequency offset compensation was performed on the two chirp signals respectively, and the compensation results are as follows: Figure 3 As shown, from Figure 3 It is evident that the starting frequencies of the two chirp signals are reduced to zero, thus eliminating the frequency discontinuity within the anchor symbol;

[0065] 3.3) The receiver uses a sliding window method to detect the anchor symbol on the backscattered signal after frequency shift compensation processing. The length of the sliding window is set to one chirp signal period, and the step size of the sliding window is set to one time slot. Each time the sliding window moves one step along the time axis, it acquires a backscattered signal sample with a length of one chirp signal period.

[0066] 3.4) As the sliding window gradually slides along the time axis, the receiver performs despreading processing on the backscattered signal samples acquired within the current sliding window at each sliding position. The specific process includes:

[0067] 3.4.1) Perform a dot product operation between the standard downchirp signal generated locally by the receiver and the m-sequence used in the modulation process to obtain the processed downchirp signal;

[0068] 3.4.2) Perform a dot product operation between the signal sample within the sliding window and the processed downchirp signal to obtain the despread signal;

[0069] 3.5) Perform a Fast Fourier Transform (FFT) on the despread signal within the sliding window and record the FFT peak value.

[0070] 3.6) Record the different FFT peak values ​​corresponding to different positions of the sliding window, and obtain the peak value recording results as follows: Figure 4 As shown;

[0071] 3.7) Detect the maximum value among the FFT peaks of the sliding window, and determine the position of the sliding window corresponding to the maximum FFT peak value as the position of the anchor symbol in the backscattered signal. This position is the starting sampling point index of the anchor symbol.

[0072] 3.8) The number of time slots is obtained by subtracting the starting sampling point index of the anchor symbol from the starting sampling point index of the decoded backscatter signal. This number of time slots is the backscatter tag modulation decimal information, thereby completing the demodulation of the backscatter signal.

[0073] It should be noted that the step numbers in the above embodiments and the serial numbers in the claims are only for the purpose of clearly and completely describing the implementation scheme of the present invention and for ease of understanding, and their order is not limited.

[0074] Example 2: Chirp-spaced modulation backscatter communication system based on variable-length scrambling code.

[0075] Reference Figure 5 This embodiment includes a transmitter, a backscatter tag 2, and a receiver 3, wherein: the backscatter tag 2 includes a time interval modulation module 21 and a variable length scrambling code modulation module 22; the receiver includes a frequency shift compensation module 31, a sliding window module 32, a despreading module 33, an FFT module 34, and a data demodulation module 35.

[0076] The working principle of the entire system is as follows:

[0077] The transmitter transmits LoRa signals from the environment;

[0078] The backscatter tag 2 is used to modulate the incident ambient LoRa signal. The time interval modulation module 21 modulates the information to be transmitted by the backscatter tag onto the incident ambient LoRa signal. Specifically, it first turns off the RF switch, putting the backscatter tag in an absorption state. Based on the decimal information to be transmitted by the backscatter tag, it uses the number of time slots corresponding to the decimal information as the time interval, and then controls the backscatter tag to maintain the absorption state for the specified time interval. After the backscatter tag maintains the absorption state for the specified time interval, it turns on the RF switch, putting the backscatter tag into a reflection state and maintaining it for one chirp signal cycle. The reflected signal within this cycle is defined as the anchor symbol. After maintaining the reflection state for one chirp signal cycle, it controls the backscatter tag to return to the absorption state and maintain this state for two consecutive chirps. After the signal period ends, the time interval modulated signal is transmitted to the variable length scrambling module 22. The variable length scrambling module 22 is used to perform variable length scrambling modulation on the signal processed by the time interval modulation module 21. That is, when the backscatter tag is in the reflection state, the radio frequency switch is controlled according to the preset scrambling sequence to switch the reflection coefficient of the backscatter tag between positive and negative 1, thereby modulating the variable length scrambling code onto the anchor symbol. Finally, the modulated signal is backscattered to the receiver 3.

[0079] The receiver 3 is used to demodulate the signal reflected by the backscatter tag 2, wherein:

[0080] The frequency shift compensation module 31 is used to perform frequency shift compensation on the received backscatter signal. That is, firstly, the ambient LoRa signal is demodulated to obtain the starting frequency corresponding to two consecutive chirp signals. Based on the demodulated starting frequency, the received backscatter signal is frequency shifted to make the starting frequency of the chirp signal return to zero. Then, the signal after frequency shift compensation is transmitted to the sliding window module 32.

[0081] The sliding window module 32 is used to perform sliding window processing on the backscattered signal after it has been processed by the frequency shift compensation module 31. The length of the sliding window is set to one chirp signal period, and the step size of the sliding window is set to one time slot, so as to obtain the backscattered signal sample within the sliding window and transmit the signal sample within the sliding window to the despreading module 33.

[0082] The despreading module 33 is used to despread the backscattered signal samples transmitted by the sliding window module. Specifically, it first performs a dot product operation between the standard downchirp signal and the scrambling sequence used in the modulation process to obtain the processed downchirp signal. Then, it performs a dot product operation between the backscattered signal samples in the sliding window and the processed downchirp signal to obtain the corresponding despread signal. Finally, it transmits the despread signal to the FFT module 34.

[0083] FFT module 34 is used to perform Fast Fourier Transform (FFT) on the signal despread by despread module 33, record the FFT peak sequence corresponding to the sliding window, and transmit the recorded peak sequence to data demodulation module 35.

[0084] The data demodulation module 35 is used to select the maximum value of the FFT peak from the FFT peak sequence corresponding to the sliding window recorded by the FFT module 34, and determine the position of the sliding window corresponding to the maximum value as the position of the anchor symbol in the backscatter signal. The difference between this position and the position of the decoded backscatter signal is used to obtain the corresponding number of time slots, thereby recovering the decimal data sent by the backscatter tag and completing the data demodulation.

[0085] It should be noted that the above functional modules can be implemented, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented in software, they can be implemented, in whole or in part, as program instruction products. A program instruction product includes one or a set of program instructions. When the program instructions are loaded and executed on a computer, the described process or function is generated, in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The program instructions can be stored in a computer-readable and writable storage medium, or transferred from one computer's readable and writable storage medium to another.

[0086] In this embodiment, the direct coupling or communication connection between the modules can be achieved through indirect coupling or communication connection via interfaces, devices, or modules. The functional modules and sub-modules in this embodiment can dynamically reside within a single processing unit, or each module can exist physically independently, or two or more modules can dynamically reside within a single processing unit. When these dynamic components are implemented as software functional modules and sold or used as independent products, they can also be stored in a computer-readable and writable storage medium. This storage medium can be a memory, disk, or optical disc, etc.

[0087] The effects of this invention can be achieved through the following simulations:

[0088] I. Simulation Conditions

[0089] The simulation software used is MATLAB.

[0090] Parameter settings: chirp signal bandwidth BW is 500kHz, spreading factor SF is 10, and sampling rate is 500kHz.

[0091] II. Simulation Content

[0092] Simulation 1: Under the above conditions, the environmental LoRa signal is modulated using a fixed-length scrambling code from existing delay modulation methods. At the receiver, a sliding window combined with a fixed-length scrambling code is used to despread and perform FFT on the signal samples within the window. The FFT peak values ​​corresponding to different positions of the sliding window are recorded. The results are as follows: Figure 6 As shown.

[0093] Simulation 2: The variable-length scrambling code of this invention is used to modulate the environmental LoRa signal, shortening the scrambling code length to half that of the fixed-length scrambling code. At the receiving end, a sliding window combined with the variable-length scrambling code is used to despread and perform FFT on the signal samples within the window, and the different FFT peak values ​​corresponding to different positions of the sliding window are recorded. The results are as follows. Figure 7 As shown.

[0094] contrast Figure 6 and Figure 7 The peak recording results show that although both methods have the same sharp peak, the scrambling code length used in this invention is only half that of the existing delay modulation method. The shorter the scrambling code length, the lower the frequency of radio frequency switching during backscatter tag modulation, and the lower the power consumption of the backscatter tag. Therefore, compared with the existing delay modulation method, this invention can effectively reduce the power consumption of the tag without sacrificing system reliability.

[0095] Simulation 3: In this invention, scrambling codes of different lengths are selected to modulate the environmental LoRa signal. Under different signal-to-noise ratio conditions, the receiver demodulates the modulated signal and compares the demodulated symbol sequence with the original transmitted symbol sequence. The curve of demodulation error rate as a function of signal-to-noise ratio is calculated, as shown in Figure 8.

[0096] from Figure 8 It can be seen that when the m-sequence length is 1024 and 512, the corresponding symbol error rate curves with signal-to-noise ratio are basically the same, indicating that when the scrambling code length is shortened by half, the reliability of the backscatter communication system is almost unaffected. When the m-sequence length is further shortened, the symbol error rate curve with signal-to-noise ratio shifts to the right as a whole, that is, the demodulation reliability decreases. However, at the same time, the further reduction of the scrambling code length reduces the RF switching frequency of the backscatter tag, which helps to further reduce the tag power consumption.

[0097] Simulation results show that the present invention can achieve a trade-off between the power consumption of backscattered tags and the reliability of the communication system.

Claims

1. A chirp-spaced modulation backscatter communication method based on variable-length scrambling codes, characterized in that, include: (1) The transmitting end sends the environmental LoRa signal as the excitation signal; (2) The backscatter tag reflects and absorbs the excitation signal by controlling the opening and closing of the radio frequency switch, and controls the switch to modulate the reflected signal with a variable length scrambling code, thus completing the modulation of the excitation signal; (3) The receiving end uses a demodulation algorithm combining a sliding window and a variable length scrambling code to complete the demodulation of the modulated signal.

2. The method according to claim 1, characterized in that, The backscatter tag in (2) reflects and absorbs the excitation signal by controlling the opening and closing of the radio frequency switch, which includes: (2a) Deploy backscatter tags between the transmitter and receiver; (2b) The backscatter tag turns off the radio frequency switch, keeping it in an absorption state. (2c) The number of time slots corresponding to the decimal information to be transmitted by the backscatter tag is used as the time interval. When the duration of the backscatter tag in the absorption state reaches the set time interval, the backscatter tag turns on the radio frequency switch to put it in the reflection state, so as to generate a reflected signal called the anchor symbol, and the reflection state is maintained for one chirp signal cycle. (2d) After the tag maintains the reflective state for one chirp signal cycle, the radio frequency switch is turned off, so that the backscatter tag is in the absorption state. During this period, the tag does not reflect the signal until the two consecutive chirp signal cycles end.

3. The method according to claim 1, characterized in that, In step (2), the control switch modulates the variable-length scrambling code onto the reflected signal. When the backscattering tag is in a reflective state, the control switch switches the reflection coefficient of the backscattering tag between positive and negative 1 according to the preset scrambling code sequence.

4. The method according to claim 1, characterized in that, In step (3), the receiver employs a demodulation algorithm combining a sliding window and a variable-length scrambling code to demodulate the backscattered signal. This demodulation includes: (3a) The receiver demodulates the received ambient LoRa signal to obtain the starting frequency corresponding to each chirp signal; (3b) Based on the starting frequency obtained by demodulation above, frequency shift compensation is performed on the backscattered signal of the tag; (3c) Within each symbol period, a sliding window is used to slide over the frequency-shift compensated signal with a fixed step size. The length of the sliding window is one chirp signal period, and the step size of the window is one time slot. (3d) Perform a dot product between the standard downchirp signal and the same m-sequence used during modulation to obtain the downchirp signal used for despreading, and perform a dot product between the signal samples in each sliding window and the downchirp signal used for despreading to obtain the despread signal; (3e) Perform Fast Fourier Transform (FFT) on the despread signal in each window and record the peak value of FFT in each window; (3f) Determine the position of the anchor symbol in the backscattered signal based on the FFT peak size corresponding to each sliding window; (3g) Based on the determined anchor symbol position, obtain the starting sampling point index of the anchor symbol, and determine the time interval accordingly to complete the information decoding.

5. The method according to claim 5, characterized in that, In step (3f), the position of the anchor symbol in the backscattered signal is determined based on the FFT peak size corresponding to each sliding window. This is achieved by selecting the maximum peak value from the FFT peak sequence, and the position of the sliding window corresponding to the maximum peak value is the position of the anchor symbol in the backscattered signal.

6. The method according to claim 5, characterized in that, In step (3g), the time interval is determined based on the starting sampling point index of the anchor symbol. This is achieved by subtracting the starting sampling point index of the anchor symbol from the starting sampling point index of the decoded backscatter symbol to obtain the number of time slots. This number of time slots is the time interval.

7. A chirp-spaced modulation backscatter communication system based on variable-length scrambling codes, comprising a transmitter, a backscatter tag, and a receiver, characterized in that: The backscatter tag includes a time interval modulation module and a variable length scrambling code modulation module; The time interval modulation module is used to modulate information onto the incident ambient LoRa signal, and according to the decimal information to be transmitted, the number of time slots corresponding to the decimal information is used as the time interval to control the radio frequency switch to make the backscatter tag enter the reflection state after the corresponding time interval to maintain one chirp signal cycle. The variable length scrambling code modulation module is used to control the switching of the radio frequency switch according to the preset scrambling code sequence when the backscattering tag is in the reflective state, so that the reflection coefficient of the backscattering tag switches between positive and negative 1, so as to modulate the variable length scrambling code onto the reflected signal. The receiver includes a frequency shift compensation module, a sliding window module, a despreading module, an FFT module, and a data demodulation module; This frequency shift compensation module is used to eliminate frequency discontinuities within the reflected signal; This sliding window module is used to extract signal samples with a length of one chirp signal period to form multiple windows for detecting reflected signals; This despreading module is used to despread the signal samples within each window; This FFT module is used to perform a fast Fourier transform on the despread signal, obtain the FFT peak value in the frequency domain, and record the FFT peak value of the signal within each window to obtain the FFT peak value sequence. This data demodulation module is used to demodulate the time interval.

8. The system according to claim 7, characterized in that, The frequency shift compensation module eliminates the frequency discontinuity within the reflected signal by first demodulating the starting frequency corresponding to each chirp signal in the ambient LoRa signal, and then performing frequency shift compensation on the received backscattered signal based on the starting frequency to eliminate the frequency discontinuity within the reflected signal.

9. The system according to claim 7, characterized in that, The despreading module despreads the signal samples within each window by multiplying the locally generated downchirp signal with the same variable-length scrambling code used during modulation to obtain the processed downchirp signal. Then, the signal samples within each window are multiplied with the processed downchirp signal to complete the signal despreading.

10. The system according to claim 7, characterized in that, The data decoding module decodes the time interval by selecting the maximum peak value from the FFT peak sequence, taking the position of the sliding window corresponding to the maximum peak value as the position of the reflected signal in the backscatter symbol, taking this position as the starting sampling point index of the reflected signal, and subtracting this index from the starting sampling point index of the decoded backscatter symbol to obtain the number of time slots, which is the time interval.

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