A load-free loRa backscatter system and method of implementing the same

Abstract

This invention proposes a loadless LoRa backscattering system and its implementation method, significantly improving energy efficiency and communication range, making it suitable for Internet of Things (IoT) applications. To address the problem that traditional LoRa backscattering systems rely on single-pole double-sideband (DSB) modulation, resulting in significant power loss in the non-target frequency band and severely reduced energy efficiency under dual-path attenuation, this invention proposes an innovative SSB modulation method. Through optimized theoretical analysis and low-overhead hardware design, this method successfully concentrates the reflected power within the target sideband, achieving a 12dB improvement over the traditional DSB system, a 5dB image sideband suppression, and an 8.7dB reduction in carrier power. Experimental results show a 4.2-fold improvement in indoor communication range, a 2.6-fold improvement in outdoor communication range, and a link budget improvement of over 400% compared to traditional systems.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and particularly relates to a loadless LoRa backscattering system and its implementation method. Background Technology

[0002] Backscatter communication technology, with its passive operation and low power consumption, has become a key technology supporting the development of the Internet of Things (IoT). LoRa backscatter technology utilizes the frequency modulation spread spectrum (CSS) characteristics of LoRa signals to achieve long-distance communication without batteries, reducing network maintenance costs. However, traditional LoRa backscatter systems typically use unipolar square wave modulation, which results in low signal spectral efficiency, with approximately 80% of the energy wasted in non-target frequency bands. Especially in dual-path attenuation environments, the system's energy efficiency is severely limited.

[0003] To address these issues, single-sideband (SSB) modulation has been proposed to reduce redundant sideband signals and concentrate energy within the target frequency band, thereby improving spectral efficiency. However, existing SSB modulation techniques suffer from two main problems: a lack of a systematic optimization theoretical framework, failing to effectively optimize energy efficiency and link budget; and non-ideal characteristics of existing load impedance circuits, increasing design complexity and hardware cost. Summary of the Invention

[0004] The purpose of this invention is to provide a no-load LoRa backscattering system and its implementation method. In order to improve the energy efficiency and communication range of traditional LoRa backscattering schemes, the load impedance module is optimized, traditional complex components such as inductors and capacitors are eliminated, and a simplified open / short circuit switch structure is adopted, thereby reducing hardware complexity and improving system energy efficiency.

[0005] To solve the above-mentioned technical problems, the specific technical solution of the present invention is as follows:

[0006] A loadless LoRa backscattering system is provided, which adopts a loadless RF architecture. The loadless RF architecture includes a baseband signal generation module, an RF front-end module, a Hilbert transform module, and a power matching module. The baseband signal generation module is connected to the RF front-end module, and the RF front-end module is connected to the power matching module. The signal output by the power matching module is received by the signal receiving module.

[0007] Furthermore, the baseband signal generation module uses an FPGA to generate a baseband signal B(t) with a phase difference of π / 2. The B(t) signal output port of the FPGA is connected to the in-phase I branch of the IQ demodulation circuit in the RF front-end module. The signal output port is connected to the quadrature Q branch in the IQ demodulation circuit of the RF front-end module;

[0008] The RF front-end module consists of an IQ demodulation circuit and RF switches. The IQ demodulation circuit is composed of a Wilkinson power divider and a delay unit coupled together. Utilizing digital Hilbert transform technology, it receives a LoRa carrier signal from an external excitation source and separates it into two orthogonal RF signals of equal amplitude and 90° phase difference, outputting them to the I and Q paths respectively. The I and Q paths are connected to two RF switches, which operate between open and short circuit states, depending on the baseband signal B(t) and... The switching is achieved under the drive, thereby forming a modulated reflected RF signal at the combined output terminal and realizing the adjustment of the bipolar reflection coefficient; the combined output terminal refers to the node where the outputs of the I-channel and Q-channel RF switches converge, and this node is connected to the input terminal of the matching network.

[0009] The power matching module includes a matching network disposed between the output of the RF front-end module and the antenna. The matching network is used to adjust the input impedance of the reflection path in open-circuit and short-circuit states, so that the trajectory of the system reflection coefficient is closer to the ideal complex plane unit circle, thereby enhancing the energy concentration of the reflected signal in the target frequency band, improving the modulation accuracy and spectrum utilization efficiency, expanding the communication range and reducing system energy consumption.

[0010] The signal receiving module consists of a LoRa gateway and a sensor; the radio frequency signal, adjusted by the matching network, is transmitted to the LoRa gateway for communication with terminals such as mobile phones and computers; the sensor is connected to the FPGA to provide data input for LoRa backscatter communication.

[0011] Furthermore, the delay unit is a λ / 8 microstrip transmission line.

[0012] This invention also discloses a method for implementing LoRa backscattering without load, specifically including the following steps:

[0013] Step 1: Implement two digital logic channels on the FPGA in the baseband signal generation module to output the baseband square wave signal B(t) and... The two baseband square wave signals have a phase difference of π / 2. These two signals are sent out through the I / O port of the FPGA as control signals for the RF switches in the subsequent RF front-end module.

[0014] Step 2: Drive the first ADG902 RF switch with the baseband square wave signal B(t). Drives the second ADG902 RF switch. Each ADG902 RF switch, according to the input digital control level, has an impedance Z in the short-circuit state. L =0 and open-circuit impedance Z L Switching between →∞ generates an ideal bipolar reflection coefficient Γ=±1, eliminating the need for traditional RLC components, simplifying hardware and reducing parasitic losses.

[0015] Step 3: Insert a 45° delay unit into the in-phase branch I of the IQ decomposition circuit in the RF front-end module to ensure that the two control signals output from the FPGA maintain a precise 45° phase difference with the final RF signal.

[0016] Step 4: Send the external LoRa carrier signal into the Wilkinson power divider to separate it into two equal-amplitude, complementary-phase signals, which are used as the RF inputs of the I and Q paths, respectively. The I RF signal passes through the first ADG902 switch, and the Q RF signal passes through the second ADG902 switch.

[0017] Step 5: Connect the combined output terminals of the two ADG902 switches to the matching network. The matching network optimizes the input impedance in the entire open / closed state, so that the actual reflection coefficient trajectory is as close as possible to the ideal state. After the RF signal is processed by the matching network, its reflection coefficient trajectory is effectively compressed and corrected in the complex plane, approaching the ideal unit circle path, thereby enhancing the spectral concentration and modulation accuracy of the signal.

[0018] Step 6: The matched network port transmits the processed RF signal out of the antenna. This RF signal has completed SSB modulation backscattering, carrying sensor data, and is transmitted by the FPGA baseband signal B(t). modulation;

[0019] Step 7: The LoRa gateway deployed in the surrounding signal receiving modules receives the Chirp signal after SSB backscattering, extracts the payload data through the conventional LoRa demodulation process, and realizes long-distance, low-power communication with the sensor node.

[0020] Step 8: Conduct link distance tests in different indoor and outdoor environments to verify the actual communication gain effect and further evaluate the effective communication range and enhanced signal strength of the system.

[0021] The unloaded LoRa backscattering system and its implementation method of the present invention have the following advantages:

[0022] 1. By employing digital Hilbert transform and matching network optimization, the spectral efficiency is significantly improved: theoretically, the target single-sideband power is enhanced by about 12dB; in actual measurements, it achieves 10dB target band gain, 5dB image band suppression, and 8.7dB carrier suppression.

[0023] 2. SSB modulation and matching network optimization: After the high spectrum concentration is improved, the main band power is enhanced, which significantly improves the demodulation threshold coverage distance: the indoor link distance increases by about 4.2 times, and the outdoor open environment by about 2.6 times;

[0024] 3. The IQ demodulation and synthesis architecture is adopted to provide a stable, continuous, and image-free reflection coefficient variation trajectory. The matching network further reduces energy leakage, improves effective reflection power, and increases the link budget: the overall link budget is improved by more than 400%.

[0025] 4. It adopts passive RF switching, with extremely low switching power consumption and uses a loadless architecture to eliminate parasitic power consumption: it only increases power consumption by about 16% (≈50μW) to achieve a significant performance gain, making it extremely suitable for ultra-low power long-distance communication of passive IoT nodes. Attached Figure Description

[0026] Figure 1 This is a schematic diagram illustrating the change of the baseband Chirp signal frequency over time according to the present invention.

[0027] Figure 2 This is a schematic diagram illustrating the transmission of three different chirp signals using Lora CSS modulation in this invention;

[0028] Figure 3 This is a schematic diagram of the SSB backscattering system architecture of the present invention;

[0029] Figure 4 This is an exploded circuit diagram of the IQ circuit on the FR4 substrate of the present invention;

[0030] Figure 5 This is a schematic diagram of the test spectra of SSB and DSB under the same excitation conditions of the present invention;

[0031] Figure 6(a) shows the indoor corridor test environment of the present invention;

[0032] Figure 6(b) shows the test results of the uplink distance in the indoor corridor of the present invention;

[0033] Figure 7(a) shows the test environment of the open suburban site of the present invention;

[0034] Figure 7(b) shows the uplink distance test results for an outdoor open site according to the present invention. Detailed Implementation

[0035] To better understand the purpose, structure, and function of this invention, the following detailed description, in conjunction with the accompanying drawings, provides an unloaded LoRa backscattering system and its implementation method, mainly including the following:

[0036] A loadless LoRa backscattering system is provided, which adopts a loadless RF architecture. The loadless RF architecture includes a baseband signal generation module, an RF front-end module, a Hilbert transform module, and a power matching module. The baseband signal generation module is connected to the RF front-end module, and the RF front-end module is connected to the power matching module. The signal output by the power matching module is received by the signal receiving module.

[0037] The baseband signal generation module uses an FPGA to generate a baseband signal B(t) with a phase difference of π / 2. The B(t) signal output port of the FPGA is connected to the in-phase I branch of the IQ demodulation circuit in the RF front-end module. The signal output port is connected to the quadrature Q branch in the IQ demodulation circuit of the RF front-end module;

[0038] The RF front-end module consists of an IQ demodulation circuit and RF switches. The IQ demodulation circuit is composed of a Wilkinson power divider and a delay unit coupled together. Utilizing digital Hilbert transform technology, it receives a LoRa carrier signal from an external excitation source and separates it into two orthogonal RF signals of equal amplitude and 90° phase difference, outputting them to the I and Q paths respectively. The I and Q paths are connected to two RF switches, which operate between open and short circuit states, depending on the baseband signal B(t) and... Switching is achieved under the drive, thereby forming a modulated reflected radio frequency signal at the combined output terminal and realizing the adjustment of the bipolar reflection coefficient;

[0039] The power matching module includes a matching network disposed between the output of the RF front-end module and the antenna. The matching network is used to adjust the input impedance of the reflection path in open-circuit and short-circuit states, so that the trajectory of the system reflection coefficient is closer to the ideal complex plane unit circle, thereby enhancing the energy concentration of the reflected signal in the target frequency band, improving the modulation accuracy and spectrum utilization efficiency, expanding the communication range and reducing system energy consumption.

[0040] The signal receiving module consists of a LoRa gateway and a sensor; the radio frequency signal, adjusted by the matching network, is transmitted to the LoRa gateway for communication with terminals such as mobile phones and computers; the sensor is connected to the FPGA to provide data input for LoRa backscatter communication.

[0041] The delay unit is a λ / 8 microstrip transmission line.

[0042] This invention also discloses a method for implementing LoRa backscattering without load, specifically including the following steps:

[0043] Step 1: Implement two digital logic channels on the FPGA in the baseband signal generation module to output the baseband square wave signal B(t) and... The two baseband square wave signals have a phase difference of π / 2. These two signals are sent out through the I / O port of the FPGA as control signals for the RF switches in the subsequent RF front-end module.

[0044] Step 2: Drive the first ADG902 RF switch with the baseband square wave signal B(t). Drives the second ADG902 RF switch. Each ADG902 RF switch, according to the input digital control level, has an impedance Z in the short-circuit state. L =0 and open-circuit impedance Z L Switching between →∞ generates an ideal bipolar reflection coefficient Γ=±1, eliminating the need for traditional RLC components, simplifying hardware and reducing parasitic losses.

[0045] Step 3: Insert a 45° delay unit into the in-phase branch I of the IQ decomposition circuit in the RF front-end module to ensure that the two control signals output from the FPGA maintain a precise 45° phase difference with the final RF signal.

[0046] Step 4: Send the external LoRa carrier signal into the Wilkinson power divider to separate it into two equal-amplitude, complementary-phase signals, which are used as the RF inputs of the I and Q paths, respectively. The I RF signal passes through the first ADG902 switch, and the Q RF signal passes through the second ADG902 switch.

[0047] Step 5: Connect the combined output points of the two ADG902 switches to the matching network. The matching network optimizes the input impedance in the entire open / closed state, so that the actual reflection coefficient trajectory is as close as possible to the ideal state. After the RF signal is processed by the matching network, its reflection coefficient trajectory is effectively compressed and corrected in the complex plane, approaching the ideal unit circle path, thereby enhancing the spectral concentration and modulation accuracy of the signal.

[0048] Step 6: The matched network port transmits the processed RF signal out of the antenna. This RF signal has completed SSB modulation backscattering, carrying sensor data, and is transmitted by the FPGA baseband signal B(t). modulation;

[0049] Step 7: The LoRa gateway deployed in the surrounding signal receiving modules receives the Chirp signal after SSB backscattering, extracts the payload data through the conventional LoRa demodulation process, and realizes long-distance, low-power communication with the sensor node.

[0050] Step 8: Conduct link distance tests in different indoor and outdoor environments to verify the actual communication gain effect and further evaluate the effective communication range and enhanced signal strength of the system.

[0051] The modulation method used in LoRa backscatter systems is frequency shift chirp modulation (FSCM), a type of chirp spread spectrum (CSS) modulation. CSS modulation has good anti-interference and anti-fading characteristics. Because chirp signals are not sensitive to frequency, they also have good resistance to multiple frequency shifts. In LoRa communication, chirp modulation uses linear frequency sweep, also known as linear frequency modulation (LFM).

[0052] As attached Figure 1 As shown, chirp is essentially a signal that sweeps through a certain frequency range. Up-chirp refers to a signal that continuously increases within a defined frequency range, while down-chirp refers to a signal that continuously decreases within the same frequency range. A baseband up-chirp signal with a center frequency of 0 can be represented as: Its conjugate can be expressed as: Where e is the natural constant, j is the imaginary unit, k can be any integer, and t is time. This is a baseband downchirp signal with a center frequency of 0. For a chirp signal with bandwidth B, swept within the range of f0 to f1, the expression is:

[0053]

[0054] LoRa employs CSS technology, which uses changes to the initial frequency of the sweep to transmit different information. (See attached image) Figure 2 An example of linear frequency sweep of CSS modulation observed at the RF end is given. This example shows the transmission of three symbols after CSS modulation (where symbol S1 has an initial frequency of f). RF -BW / 2, the initial frequency of symbol S2 is f. RF The symbol S3 can represent a frequency linear scanning waveform (where S3 can represent any initial frequency). This shows that LoRa's CSS modulation, in each symbol transmission time T... s Within this range, the frequency of the modulated signal undergoes a linear frequency sweep change across the entire bandwidth.

[0055] Different symbols correspond to different initial frequencies, and each symbol has a symbol period T. s The frequency linear scan of the entire bandwidth (BW) is completed within the time limit. Therefore, there is a one-to-one correspondence between the starting frequency and the symbol.

[0056] The spreading factor is SF, where SF ∈ [6, 12]. Each symbol carries SF bits, meaning each symbol uses 2^35 bits. SF Each bit is represented by a chip. Each chip represents a unit of data.

[0057] The spreading factor (SF) in LoRa is a trade-off between efficiency and reliability. A smaller SF results in a higher transmission rate but poorer interference immunity. A larger SF results in a slower transmission rate but provides stronger interference immunity.

[0058] The chip period T in each chip c Within this range, the frequency changes linearly, and the range of change is... Transmit 2 in each symbol period SF For each chip, there are T s =2 SF ·T c R c =2 SF ·R s , where R c R is the chip transmission rate. s The symbol transmission rate is 2. The entire bandwidth is divided into 2. SF The part, when the symbol S is divided into 2 SF When there are multiple parts, the starting frequency f for each part is... init for:

[0059]

[0060] R c =BW (2)

[0061] The slope k of the linear increase in frequency:

[0062]

[0063] Increasing bandwidth reduces sensitivity, while increasing the spreading factor (SF) increases sensitivity.

[0064] remember Phase of the signal

[0065]

[0066] Different LoRa symbols are obtained by cyclically shifting corresponding chips from a basic chirp signal. At the LoRa transmitter, each symbol carries modulation information p consisting of SF binary bits, therefore the value of p ranges from 0 to 2. SF -1.

[0067] The baseband LoRa signal can be represented as: +

[0068]

[0069] in, It is a baseband upchirp signal, e j2πΔftIt is a single-frequency signal, and the offset Δf of the signal relative to the reference frequency can be expressed as:

[0070] Δf=f offset -Bu(t-τ m (6)

[0071] Where, τ m It is the deadline for the m-th LoRa symbol.

[0072] f s The sampling rate is used to sample a LoRa symbol, and the number of sampling points for each LoRa symbol is: The number of sampling points on each chip is: Note: The number of sampling points on a LoRa symbol / the number of sampling points on a chip is independent of the center frequency of the LoRa symbol, but the center frequency of the LoRa symbol needs to be considered in order to recover the LoRa signal through sampling (Nyquist sampling theorem).

[0073] Traditional LoRa backscatter communication uses unipolar square wave modulation, where the RF front-end reflects or absorbs the carrier signal. The backscattered double-sideband modulated signal S... DSB (t) can be modeled as:

[0074]

[0075] Where Re{·} extracts the real part of the complex signal, Γ(t)∈{0,1} is the reflection coefficient, approximated as the first-order Fourier series component of a square wave, f c Let f represent the carrier frequency, Δf represent the offset frequency of the backscattered signal, and f t ∈[0, BW] describes the time-varying CSS modulation frequency, which scans linearly over the bandwidth BW. By substituting equation (2) into equation (1), the DSB-modulated LoRa backscatter communication signal can be rewritten as...

[0076]

[0077] The obtained spectrum in f c f c +Δf+f t and f c -Δf-f t The three components are respectively exhibited at the location. Since only 1 / (2π) 2 The reflected energy of the backscattered signal affects the target sideband, leading to inefficient power utilization. To concentrate the backscattered signal power within a single target frequency band, the following loadless SSB-LoRa architecture is proposed.

[0078] First, construct the single-sideband complex reflection coefficient Γ. SSB(t):

[0079]

[0080] To eliminate the DC component, a bipolar reflection coefficient Γ(t) ∈ {-1, 1} is used. The Hilbert transform representing the bipolar reflection coefficient Γ(t) effectively introduces a phase delay of π / 2 into the original signal Γ(t). Combining equations (10) and (7), we have

[0081]

[0082] in Approximately 8 / π 2 The reflected energy is concentrated in the target single sideband after mixing, which is 16 times larger than the DSB signal in equation (9).

[0083] Single-sideband (SSB) modulation schemes are implemented through a combination of digital Hilbert transform and load impedance modulation. (See appendix.) Figure 3 IQ carrier decomposition is achieved by partially coupling a Wilkinson power divider with a λ / 8 transmission line. The orthogonal baseband signal B(t) and Generated by a field-programmable gate array (FPGA), which outputs two square waves with a precise π / 2 phase difference to drive radio frequency (RF) switching elements. This mechanism, acting on the I and Q paths of the RF circuit, will generate Γ(t) and Γ(t) respectively. The reflection coefficient is defined as

[0084]

[0085] Where Z0 = 50Ω represents the characteristic impedance of the radio frequency transmission line, and Z L This represents the load impedance, which determines the system's reflection characteristics. With the default configuration of the RF switch (ADG902), the load impedance Z... L It can switch between 0 (short circuit) and ∞ (open circuit) to produce an ideal bipolar reflection coefficient of Γ = ±1 without the need for additional active or passive components.

[0086] To achieve loadless SSB modulation, on an FR4 substrate (dielectric constant ε) r An IQ decomposition circuit was fabricated on a substrate with a thickness h = 4.6 mm and a thickness h = 1.6 mm, operating at a frequency of 915 MHz. (See attached image.) Figure 4 As shown, the core architecture employs a Wilkinson power divider with optimized structural parameters to achieve balanced amplitude and phase distribution of the carrier signal. A single-sided λ / 8 microstrip transmission line is integrated to introduce a 90° reflection phase shift.

[0087] To characterize modulation performance, a logic level signal is applied to the CTRL pin of the RF switch to dynamically switch between short-circuit and open-circuit states.

[0088] To verify the energy efficiency of SSB modulation in a wireless transmission system, an experimental platform was built with the following parameters: baseband offset frequency Δf = 1MHz, bandwidth BW = 500kHz. Under the same transmit power and channel conditions, the spectral characteristics of SSB and DSB modulation were compared. The reference DSB modulator used for comparison adopted on-off keying (OOK) mode.

[0089] In the appendix Figure 5 In the backscattering spectrum analysis based on the CSS modulation standard, it is shown that the SSB architecture achieves a significant performance improvement. Compared to the traditional DSB implementation, the occupied bandwidth is reduced by 50% (500kHz). Furthermore, the target sideband power is increased by 10dB, the image sideband rejection ratio reaches 5dB, and the carrier rejection ratio reaches 8.7dB. When the I and Q paths are swapped with B(t), the resulting SSB spectrum will be confined to the mirror frequency domain. Under a text configuration with a transmit power TX = 30 dBm and a downlink distance of 1 m, the SSB architecture consistently demonstrates superior average received power compared to the DSB scheme at all measurement locations. Notably, this energy efficiency advantage becomes increasingly pronounced with increasing transmission distance, indicating that the proposed scheme is particularly suitable for long-distance applications.

[0090] The communication link budget is analyzed and modeled using the Friis transmission equations:

[0091]

[0092] Among them, P t and G t P represents the equivalent reflected power of the backscattered tag and the transmit antenna gain, respectively. t and G r Here, λ represents the received power and antenna gain of the LoRa receiver, respectively, λ is the operating wavelength, and R is the transmission distance. Theoretical analysis shows that increasing the target sideband power by 12dB can quadruple the communication range.

[0093] The experimental setup used an unmodified commercial LoRa gateway (SX1276, data rate = 37.5kbps, BW = 600kHz, sensitivity = -111dBm) as the receiver and a single-tone RF source as the transmitter. A bistatic backscatter communication link was established, in which the source TX and the backscatter tag remained stationary, while the receiver RX was moved to different locations.

[0094] In an indoor multipath environment (corridor scenario), as shown in Figure 6(a), with a transmit power TX = 0 dBm and a reference distance of 1 m, it was observed in Figure 6(b) that the proposed SSB system can achieve an indoor uplink distance 4.2 times greater than that of the traditional DSB scheme when the bit error rate (BER) is 10⁻¹.

[0095] In an open suburban environment, as shown in Figure 7(a), with a transmit power of TX = 18 dBm and a reference distance of 1 m, as shown in Figure 7(b), the bit error rate (BER) is 2 × 10⁻⁶. -1 At that time, the uplink distance of the SSB scheme reached 2.6 times that of the traditional DSB scheme.

[0096] This invention proposes a loadless LoRa backscattering system and its implementation method, which significantly optimizes the link budget and spectral efficiency of LoRa backscattering systems. Experimental and theoretical results show that, compared with existing solutions, this system achieves superior communication range and cost efficiency, providing a reliable technical approach for large-scale IoT deployments.

[0097] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A load-free LoRa backscatter system, characterized in that, The system adopts a loadless RF architecture, which includes a baseband signal generation module, an RF front-end module, a Hilbert transform module, and a power matching module. The baseband signal generation module is connected to the RF front-end module, and the RF front-end module is connected to the power matching module. The signal output by the power matching module is received by the signal receiving module. The baseband signal generation module uses an FPGA to generate signals that exist in the network. Phase difference baseband signal and Among them, FPGA The signal output port is connected to the in-phase I branch of the IQ demodulation circuit in the RF front-end module. The signal output port is connected to the quadrature Q branch in the IQ demodulation circuit of the RF front-end module; The RF front-end module consists of an IQ demodulation circuit and RF switches. The IQ demodulation circuit is composed of a Wilkinson power divider and a delay unit coupled together. Utilizing digital Hilbert transform technology, it receives a LoRa carrier signal from an external excitation source and separates it into two orthogonal RF signals of equal amplitude and 90° phase difference, outputting them to the I and Q paths respectively. The I and Q paths are connected to two RF switches, which operate between open and short circuit states, controlling the baseband signal... and Switching is achieved under the drive, thereby forming a modulated reflected radio frequency signal at the combined output terminal and realizing the adjustment of the bipolar reflection coefficient; The merged output terminal refers to the node where the outputs of the I-channel and Q-channel RF switches converge, and this node is connected to the input terminal of the matching network. The power matching module includes a matching network disposed between the output of the RF front-end module and the antenna. The matching network is used to adjust the input impedance of the reflection path in open-circuit and short-circuit states, so that the trajectory of the system reflection coefficient is closer to the ideal complex plane unit circle. The signal receiving module consists of a LoRa gateway and a sensor; the radio frequency signal, adjusted by a matching network, is transmitted to the LoRa gateway; the sensor is connected to the FPGA to provide data input for LoRa backscatter communication.

2. The unloaded LoRa backscattering system according to claim 1, characterized in that, The delay unit is a λ / 8 microstrip transmission line.

3. A method for implementing unloaded LoRa backscattering, used in the unloaded LoRa backscattering system according to any one of claims 1-2, characterized in that, Specifically, the following steps are included: Step 1: Implement two digital logic channels on the FPGA in the baseband signal generation module to output baseband square wave signals respectively. and The phase difference between the two is These two baseband square wave signals are sent out through the FPGA's I / O ports as control signals for the RF switches in the subsequent RF front-end module; Step 2: Convert the baseband square wave signal Drive the first ADG902 RF switch, signal Drives the second ADG902 RF switch. Each ADG902 RF switch, in the short-circuit state, has an impedance that adjusts according to the input digital control level. and open-circuit impedance Switching between these modes generates an ideal bipolar reflection coefficient Γ=±1; Step 3: Insert a 45° delay unit into the in-phase branch I of the IQ decomposition circuit in the RF front-end module to ensure that the two control signals output from the FPGA maintain a precise 45° phase difference with the final RF signal. Step 4: Send the external LoRa carrier signal into the Wilkinson power divider to separate it into two equal-amplitude, complementary-phase signals, which are used as the RF inputs of the I and Q paths, respectively. The I RF signal passes through the first ADG902 switch, and the Q RF signal passes through the second ADG902 switch. Step 5: Connect the combined output terminals of the two ADG902 switches to the matching network. The matching network optimizes the input impedance in the entire open / closed state, so that the actual reflection coefficient trajectory is as close as possible to the ideal state. After the RF signal is processed by the matching network, its reflection coefficient trajectory is effectively compressed and corrected in the complex plane, approximating the ideal unit circle path. Step 6: The matched network port transmits the processed RF signal out of the antenna. This RF signal has undergone SSB modulation backscattering, carrying sensor data, and is transmitted via the FPGA baseband signal. , modulation; Step 7: The LoRa gateway deployed in the surrounding signal receiving modules receives the Chirp signal after SSB backscattering and extracts the payload data through the conventional LoRa demodulation process; Step 8: Conduct link distance tests in different indoor and outdoor environments to verify the actual communication gain effect.

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