Parallel backscatter communication method and system based on LoRa

Abstract

The invention discloses a LoRa-based parallel backscatter communication method and system. The method comprises the following steps: transmitting a single-tone radio frequency source signal as a signal excitation source; designing a backscattering label, and performing backscattering modulation processing on the environmental perception data in combination with the signal excitation source to obtain a modulated standard LoRa data packet; and the receiver presets the center frequency of a communication link, and performs frequency band distinguishing and demodulation processing on the modulated standard LoRa data packet to complete parallel backscatter communication of the LoRa data. According to the invention, while a parallel backscattering LoRa communication system compatible with commercial LoRa equipment is realized, the throughput of the system is improved, and frequency division multiple access parallel transmission is realized. The LoRa-based parallel backscatter communication method and system can be widely applied to the technical field of parallel backscatter communication.

Description

Technical Field

[0001] The present invention relates to the technical field of parallel backscatter communication, and in particular to a LoRa-based parallel backscatter communication method and system. Background Art

[0002] Backscatter technology is a low-power wireless communication method that uses the reflection and modulation of electromagnetic waves to transmit information. Existing LoRa backscatter parallel transmission schemes such as the LoRa cyclic shift keying scheme and the frequency offset keying scheme. In the former, each tag generates chirp symbols with the same initial frequency of a certain "0""1" sequence according to the transmitted data instead of the standard LoRa data packet format, and a specially designed receiver is required. In the latter, each tag offsets the frequency of the unknown environmental LoRa data packet to the allocated frequency band and performs data processing operations such as symbol splicing interference cancellation at the receiver, but does not consider the compatibility with the standard LoRa protocol. The special form of data packets generated by them cannot be detected or demodulated by existing inexpensive commercial LoRa receivers, so expensive special-designed receivers are required, which hinders coexistence with the standard LoRaWAN network. In addition, existing schemes greatly reduce the already low throughput of LoRa. The basic unit of LoRa, the chirp signal, often lasts for several microseconds to distribute energy over the entire bandwidth of the signal and transmit data over a relatively long distance. Each chirp carries 7-12 bits of data. This linear spread spectrum modulation method obtains resistance to noise interference and a relatively long transmission distance at the cost of data rate. Existing parallel transmission schemes use a variant of on-off keying modulation, where each chirp symbol carries one bit of data based on its presence or absence. Compared with each chirp in the standard LoRa protocol carrying up to 12 bits of data, this is a significant reduction, thus greatly reducing the already low throughput of LoRa. Summary of the Invention

[0003] In order to solve the above technical problems, the purpose of the present invention is to provide a LoRa-based parallel backscatter communication method and system, which can realize a parallel backscatter LoRa communication system compatible with commercial LoRa devices, while improving the throughput of the system and realizing frequency division multiple access parallel transmission.

[0004] The first technical solution adopted by the present invention is: a LoRa-based parallel backscatter communication method, including the following steps:

[0005] Transmit a single-tone radio frequency source signal as a signal excitation source;

[0006] Design a backscatter tag, and combine the signal excitation source to perform backscatter modulation processing on environmental perception data to obtain a modulated standard LoRa data packet;

[0007] The receiver preset the center frequency of the communication link, and then distinguished the frequency bands and demodulated the modulated standard LoRa data packet to complete the parallel backscatter communication of LoRa data.

[0008] Further, the step of designing the backscatter tag and performing backscatter modulation processing on the environmental perception data in combination with the signal excitation source to obtain the modulated standard LoRa data packet specifically includes:

[0009] Design the backscatter tag and excite it through the signal excitation source;

[0010] Based on the LoRa parameters, convert the environmental perception data to obtain a standard LoRa data packet with a digital chirp information sequence;

[0011] Perform frequency synthesis processing according to the digital chirp information sequence to generate a clock control drive signal;

[0012] Perform mapping processing on the clock control drive signal to obtain a number of control bit signals;

[0013] Based on a number of control bit signals, perform modulation processing on the standard LoRa data packet with a digital chirp information sequence through the switch network harmonic elimination method to obtain the modulated standard LoRa data packet.

[0014] Further, the backscatter tag specifically includes a digital baseband circuit and a radio frequency front-end circuit, and the output end of the digital baseband circuit is connected to the input end of the radio frequency front-end circuit, where:

[0015] The digital baseband circuit includes a data packet processor, a DDS frequency synthesizer, and a Gray counter, and the data packet processor, the DDS frequency synthesizer, and the Gray counter are connected in sequence;

[0016] The radio frequency front-end circuit includes a switch network module and a number of reflection circuits, and the output end of the switch network module is respectively connected to the input ends of the number of reflection circuits.

[0017] Further, the step of based on the LoRa parameters, converting the environmental perception data to obtain a standard LoRa data packet with a digital chirp information sequence specifically includes:

[0018] Pre-define LoRa parameters, the LoRa parameters include a preamble, a header, a payload, and a cyclic redundancy check, the preamble includes a variable preamble, a sync word, and a frame start delimiter, and the header includes a payload length, a coding rate, a cyclic redundancy check indicator, and a header checksum;

[0019] Generate a standard LoRa data packet with a digital chirp information sequence in the form of a digital chirp information sequence based on predefined LoRa parameters and a synchronization word.

[0020] Further, the step of performing frequency synthesis processing according to the digital chirp information sequence to generate a clock control drive signal specifically includes:

[0021] Obtain a reference clock and a control word signal according to the digital chirp information sequence, perform a phase increment calculation to obtain a current phase increment;

[0022] Introduce an N-bit register as a phase accumulator. At each clock cycle, add the current phase increment to the current phase value to obtain the value of the phase accumulator;

[0023] Introduce a bit truncation operation, directly select the most significant bit of the phase accumulator for judgment. When the phase accumulator is greater than a preset threshold, the output is 1, otherwise the output is 0, and generate a clock control drive signal.

[0024] Further, the step of modulating the standard LoRa data packet with a digital chirp information sequence by the switch network harmonic cancellation method based on several control bit signals to obtain a modulated standard LoRa data packet specifically includes:

[0025] Based on an SP8T switch network, design the reflection coefficient of the switch network as a piecewise function according to the impedance value;

[0026] Based on several control bit signals, in combination with the piecewise function, use a frequency synthesizer to synthesize a clock eight times the required frequency shift frequency, and modulate the standard LoRa data packet with a digital chirp information sequence to obtain a modulated standard LoRa data packet.

[0027] Further, the expression of the piecewise function of the reflection coefficient of the switch network is specifically as follows:

[0028]

[0029] In the above formula, Γ(t) represents the piecewise function of the reflection coefficient, T represents the period of the frequency shift frequency, a represents the number of reflection circuits, k represents a natural number, represents the reflection coefficient.

[0030] The second technical solution adopted by the present invention is: A LoRa-based parallel backscatter communication system, including:

[0031] A single-tone RF source for transmitting a single-tone RF source signal as a signal excitation source;

[0032] Backscatter tag, used to design a backscatter tag, which combines a signal excitation source to perform backscatter modulation processing on environmental perception data to obtain a modulated standard LoRa data packet;

[0033] Commercial LoRa receiver, which is used for the receiver to preset the center frequency of the communication link in advance, and then perform frequency band discrimination and demodulation processing on the modulated standard LoRa data packet to complete the parallel backscatter communication of LoRa data.

[0034] The beneficial effects of the method and system of the present invention are as follows: The present invention uses a single-tone RF source signal as a signal excitation source, and then designs a backscatter tag. By combining the signal excitation source, backscatter modulation processing is performed on environmental perception data to obtain a modulated standard LoRa data packet. Through the special design of the backscatter tag, multiple backscatter tags in the parallel LoRa backscatter communication system can generate standard LoRa data packets distributed in their respective corresponding sub-channels with low power consumption. While realizing a parallel backscatter LoRa communication system compatible with commercial LoRa devices, the throughput of the system is improved. Finally, the receiver presets the center frequency of the communication link in advance, and then performs frequency band discrimination and demodulation processing on the modulated standard LoRa data packet to complete the parallel backscatter communication of LoRa data, and assigns an independent sub-channel to each tag to achieve frequency division multiple access parallel transmission. Description of the Drawings

[0035] Figure 1 is the step flow chart of a LoRa-based parallel backscatter communication method of the present invention;

[0036] Figure 2 is the structural block diagram of a LoRa-based parallel backscatter communication system of the present invention;

[0037] Figure 3 is the schematic diagram of a backscatter LoRa communication system compatible with commercial LoRa devices provided by a specific embodiment of the present invention;

[0038] Figure 4 is the schematic diagram of the overall structure of a parallel backscatter LoRa communication system provided by a specific embodiment of the present invention;

[0039] Figure 5 is the schematic diagram of the design framework of a backscatter tag provided by a specific embodiment of the present invention;

[0040] Figure 6 is the schematic diagram of the structure of a LoRa data packet provided by a specific embodiment of the present invention;

[0041] Figure 7 is the schematic diagram of the structure of a traditional DDS provided by a specific embodiment of the present invention;

[0042] Figure 8 It is a schematic structural diagram of an improved DDS provided by a specific embodiment of the present invention;

[0043] Figure 9 It is a schematic diagram of a channel allocation scheme provided by a specific embodiment of the present invention;

[0044] Figure 10 It is a schematic diagram of an outdoor evaluation deployment provided by a specific embodiment of the present invention;

[0045] Figure 11 It is a schematic diagram of an indoor evaluation deployment provided by a specific embodiment of the present invention;

[0046] Figure 12 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the outdoor transmission distance of = 0.1m;

[0047] Figure 13 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the outdoor transmission distance of = 1m;

[0048] Figure 14 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the outdoor transmission distance of = 5m;

[0049] Figure 15 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the indoor transmission distance of = 0.1m;

[0050] Figure 16 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the indoor transmission distance of = 1m;

[0051] Figure 17 It is d provided by a specific embodiment of the present invention st Schematic diagram of the test results of the indoor transmission distance of = 5m;

[0052] Figure 18 It is a schematic diagram of the results of the throughput of multi-tag parallel transmission provided by a specific embodiment of the present invention. Detailed implementation manners

[0053] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. For the step numbers in the following embodiments, they are only set for the convenience of explanation and illustration, and no limitation is imposed on the order between the steps. The execution order of each step in the embodiments can be adjusted adaptively according to the understanding of those skilled in the art.

[0054] First of all, it should be noted that backscatter technology is a low-power wireless communication method that uses the reflection and modulation of electromagnetic waves to transmit information. When an electromagnetic wave encounters the interface of media with different impedance characteristics during propagation, it will be reflected back with a certain amplitude and phase mutation according to the impedance of the two ends of the media. This mutation information described as a complex number is also called the reflection coefficient. Using this characteristic, the backscatter device does not need to actively generate signals, but dynamically changes the impedance characteristics of the circuit connected to the antenna through a switching circuit, and reflects back the electromagnetic waves emitted by other radio frequency signal sources. This change in impedance can encode data according to a predetermined modulation method. By utilizing the reflection and modulation of radio frequency signals, backscatter technology avoids the need for devices to actively emit signals, eliminates expensive and energy-consuming active radio frequency devices, and has the advantages of ultra-low power consumption and low manufacturing cost. Therefore, it is widely regarded as a key technology for realizing the interconnection of all things in the next-generation Internet of Things. However, backscatter signals often undergo two-stage signal attenuation from the radio frequency source to the backscatter device and from the backscatter device to the receiver, resulting in very low backscatter signal intensity and being often overwhelmed by noise, which greatly limits the communication range of the backscatter system.

[0055] In the prior art, there are problems that the prior art cannot be compatible with inexpensive commercial LoRa receivers and greatly reduces the already low throughput of LoRa. Based on this, in the embodiments of the present invention, a single-tone electromagnetic wave of a specified frequency is emitted by a radio frequency source as the system carrier wave, and multiple backscatter tags modulate the data to be sent into independent LoRa data packets on different frequency bands using this electromagnetic wave. At the receiving end, the center frequency of the communication link of the LoRa receiver is adjusted with the pre-designed LoRa parameters to detect and demodulate the LoRa data packets from the corresponding backscatter tags. An improved pure digital direct digital frequency synthesizer (DDS) is proposed, which realizes accurate frequency synthesis and standard chirp signal synthesis with low power consumption by omitting the unnecessary digital-to-analog converter (DAC) and replacing the N-bit memory with a simple most significant bit truncation operation. The present invention further uses a digital baseband including this frequency synthesizer and a radio frequency front end to backscatter standard LoRa data packets to be compatible with commercial LoRa receivers. The present invention uses hardware such as FPGA and self-made PCB to implement the communication system and conducts actual tests. The test results show that the present invention can be received by existing standard commercial LoRa receivers respectively when multiple tags perform parallel transmission, and while realizing LoRa backscatter parallel transmission, it also takes into account the compatibility with the standard LoRa protocol. In addition, the embodiments of the present invention also use the digital baseband of the FPGA to generate standard LoRa data packets compliant with the LoRa protocol, which can not only be demodulated by existing commercial LoRa devices, but also maintain the same throughput as the standard LoRa. Through experimental verification, compared with the existing best parallel transmission scheme, the data rate of a single tag of the present invention is increased by 8-9 times, greatly improving the data rate of the LoRa parallel backscatter communication system. In addition, the present invention also uses the design of the switch network radio frequency front end to reduce the interference between multiple tags during parallel transmission.

[0056] First, as Figure 4 shown, the parallel backscatter LoRa communication system designed in the embodiments of the present invention includes a single-tone radio frequency source, backscatter tags and a commercial LoRa receiver. The single-tone radio frequency source emits an electromagnetic wave of a specific frequency as the excitation signal source of the system, which can be implemented by a commercial LoRa device, a signal generator or a USRP. Multiple backscatter tags can modulate the single-tone radio frequency source signal into standard LoRa data packets occupying different channels through a specific signal reflection strategy, so as to transmit data in parallel. In Figure 4 it, we use lines of different colors to represent LoRa signals on different channels generated by different tags. At the receiving end, since the backscatter tags generate standard LoRa data packets compliant with the protocol, existing inexpensive commercial LoRa receivers can easily distinguish and demodulate the data packets from the corresponding backscatter tags by adjusting to the appropriate communication link center frequency.

[0057] Reference Figure 1 , the present invention provides a LoRa-based parallel backscatter communication method, which includes the following steps:

[0058] S100. Transmit a single-tone RF source signal as a signal excitation source;

[0059] S200. Design a backscatter tag, and perform backscatter modulation processing on environmental perception data in combination with the signal excitation source to obtain a modulated standard LoRa data packet;

[0060] First of all, it should be noted that the key point of the entire LoRa parallel transmission system lies in the design of the backscatter tag. The backscatter tag consists of two main parts, namely a digital baseband and a RF front-end. When the backscatter tag needs to transmit data, the data packet processor generates a series of digital chirp information representing the standard LoRa data packet according to the transmission data and LoRa parameters. Then, this chirp information is transmitted to the frequency synthesizer, and the frequency synthesizer generates an appropriate frequency to drive the subsequent Gray counter. The Gray counter maps the input clock signal to three control bits, and these control bits are used to switch the switch network of the RF front-end. The switch network sequentially turns on eight reflection circuits, and each circuit is configured with a predetermined impedance value. By switching these reflection circuits at the input frequency, the backscatter tag modulates the incoming single-tone signal into a standard LoRa data packet. The implementation method of the present invention includes the following parts. One is the data packet processor: converting the transmission data into a standard LoRa data packet represented by a digital chirp information sequence; the second is the frequency synthesizer: using an improved pure digital DDS to achieve frequency synthesis under low power consumption and further generate standard chirp signals; the third is the RF front-end: adopting switch network harmonic cancellation technology to reduce interference between tags; the fourth is parallel transmission based on frequency band allocation.

[0061] S210. Design a backscatter tag and excite it through a signal excitation source;

[0062] Specifically, as Figure 5 shown, the backscatter tag specifically includes a digital baseband circuit and a RF front-end circuit. The output end of the digital baseband circuit is connected to the input end of the RF front-end circuit. Among them, the digital baseband circuit includes a data packet processor, a DDS frequency synthesizer, and a Gray counter, and the data packet processor, the DDS frequency synthesizer, and the Gray counter are sequentially connected; the RF front-end circuit includes a switch network module and several reflection circuits, and the output end of the switch network module is respectively connected to the input ends of several reflection circuits.

[0063] S220. Based on LoRa parameters, perform conversion processing on environmental perception data to obtain a standard LoRa data packet with a digital chirp information sequence;

[0064] Specifically, pre-define LoRa parameters, where the LoRa parameters include a preamble, a header, a payload, and a cyclic redundancy check. The preamble includes a variable preamble, a sync word, and a frame start delimiter. The header includes a payload length, a coding rate, a cyclic redundancy check indicator, and a header checksum. Generate a standard LoRa data packet with a digital chirp information sequence based on the pre-defined LoRa parameters and the sync word in the form of a digital chirp information sequence.

[0065] In this embodiment, to ensure compatibility with existing commercial LoRa receivers, it is necessary to generate a standard LoRa data packet that conforms to the LoRa protocol. However, the physical layer details of LoRa are proprietary and not publicly disclosed. To address this limitation, the embodiment of the present invention analyzed the structure of the LoRa data packet with reference to gr-lora-sdr, which is an open-source implementation of software-defined radio LoRa using a general software radio peripheral on the GNU Radio platform. The structure of the LoRa data packet is as Figure 6 shown. The data packet starts with a preamble consisting of a series of basic up-chirps with an initial frequency offset of 0, with a length ranging from 6 to 65535 up-chirp symbols. Next is a sync word (Sync) with a length of two symbols. The LoRa receiver will discard data packets with mismatched sync words, so the sync word can quickly identify the LoRa network. To indicate the end of the preamble, a frame start delimiter (SFD) follows, including 2.25 basic down-chirp symbols. The subsequent data packet contains an optional 20-bit header that provides necessary data packet information, such as payload length, coding rate, cyclic redundancy check (CRC) indicator, and checksum. The subsequent payload can be up to 255 bytes, and an optional 16-bit CRC is appended to ensure data integrity. In the digital data packet processor, the present invention generates a standard LoRa data packet that conforms to the above LoRa data packet structure based on the pre-defined LoRa parameters and the sync word. These data packets are in the form of a digital chirp information sequence, and each chirp information includes basic chirp information such as chirp initial frequency value, chirp type, and quarter chirp indicator, which serves as the input for the next frequency synthesizer.

[0066] S230. Perform frequency synthesis processing according to the digital chirp information sequence to generate a clock control drive signal;

[0067] Specifically, according to the digital chirp information sequence, a reference clock and a control word signal are obtained, and phase increment calculation is performed to obtain the current phase increment. An N-bit register is introduced as an accumulator for the phase. In each clock cycle, the current phase increment is added to the current phase value to obtain the value of the phase accumulator. A bit truncation operation is introduced, and the most significant bit of the phase accumulator is directly selected for judgment. When the phase accumulator is greater than a preset threshold, the output is 1, otherwise the output is 0, generating a clock control drive signal.

[0068] In this embodiment, synthesizing a standard chirp signal is crucial for ensuring that the signal can be demodulated by a commercial LoRa receiver. The key challenge in generating a standard chirp signal is that the frequency complexity grows exponentially with the spreading factor. As mentioned before, a chirp signal with a spreading factor of SF traverses 2 SF linearly varying frequencies over time, with a very small frequency interval on the order of hertz and an extremely short time interval on the order of microseconds. Since the RF source emits a single-frequency signal, the frequency difference between the source signal and the target chirp signal is a frequency sequence consisting of 2 SF frequencies. To generate a chirp signal, the backscatter device must generate 2 SF frequencies with microsecond-level timing accuracy and hertz-level frequency resolution. Some LoRa backscatters use a hybrid digital-analog method, using a DAC and a voltage-controlled oscillator (VCO) to generate the required clock. However, the output clock accuracy of the VCO is easily affected by environmental factors such as temperature, which may cause frequency and amplitude drift, thus affecting system performance. To accurately synthesize frequencies, we propose an improved pure digital DDS to achieve low-power frequency synthesis without using a VCO. Traditional DDS is a hybrid digital-analog system that can generate arbitrary waveform signals at a specified frequency. Compared with the VCO method, the digital part of DDS provides a faster response time and is not affected by environmental drift. As Figure 7As shown, a traditional DDS consists of three main functional blocks: a phase accumulator, a waveform look-up table, and a DAC. The phase accumulator consists of a phase register and an adder, while the waveform look-up table uses an N-bit read-only memory (ROM). Before operation, the required signal waveform is sampled and stored in the waveform look-up table, which associates the address of the signal waveform amplitude with its phase, thus enabling precise signal reconstruction. The frequency of the DDS output signal is determined by two inputs, namely the frequency control word and the reference clock. The frequency control word determines the phase increment, while the reference clock determines the operating speed of the DDS. At each rising edge of the reference clock, the phase accumulator adds the N-bit values of the phase register and the frequency control word to calculate the address of the look-up table. This address corresponds to a specific phase angle, and the look-up table outputs the signal amplitude corresponding to the phase angle to the DAC. Then, the DAC converts the input digital value into a proportional analog voltage. A larger frequency control word results in a larger phase increment, causing the phase accumulator to quickly traverse the look-up table and generate a high-frequency waveform. Conversely, a smaller frequency control word requires more clock cycles to traverse the look-up table, resulting in a low-frequency waveform. To generate a fixed-frequency waveform, a fixed frequency control word is added to the phase accumulator at each clock cycle. When the frequency control word is set to 1, the phase accumulator requires 2 N reference clock cycles to traverse the entire look-up table, thus generating the waveform with the lowest frequency, which is also defined as the basic frequency resolution Δf = f c / 2 N , where f c is the frequency of the reference clock and N is the number of bits in the frequency control word and the phase accumulator. Given an input frequency control word B, the frequency of the output signal is f out = B·Δf. To reduce the power consumption of the frequency synthesizer, we modified the traditional DDS to a pure digital design. Specifically, we noticed that our frequency synthesizer does not need to generate analog signals such as sine waves. Instead, our requirement is only to generate a digital square-wave clock signal with the required frequency to drive the subsequent connected radio frequency front-end circuit. Therefore, it becomes unnecessary to sample the amplitude of complex analog signals into the look-up table, and only the most significant bit of the phase accumulator needs to be simply intercepted to output "0" or "1". In addition, the conversion from digital to analog voltage is no longer required, so we can omit the DAC. As Figure 8 shown, we removed the DAC and replaced the look-up table with a simple bit-interception operation to implement a low-power frequency synthesizer.

[0069] S240. Map the clock control drive signal to obtain several control bit signals;

[0070] S250. Based on several control bit signals, modulate the standard LoRa packet with a digital chirp information sequence through the switching network harmonic elimination method to obtain the modulated standard LoRa packet.

[0071] Specifically, based on the SP8T switching network, the reflection coefficient of the switching network is designed as a piecewise function according to the impedance value; based on several control bit signals, combined with the piecewise function, use a frequency synthesizer to synthesize a clock eight times the required frequency shift frequency, and modulate the standard LoRa packet with a digital chirp information sequence to obtain the modulated standard LoRa packet.

[0072] In this embodiment, existing backscatter tags typically use square waves to achieve frequency shift. However, the square wave is very different from the ideal frequency shift signal, and the harmonics it generates may cause interference with other LoRa devices on overlapping channels, thereby reducing the signal-to-noise ratio (SNR) of the backscatter signal. To achieve an accurate and harmonic-free frequency shift Δf, the backscatter tag must generate an ideal complex exponential wave e j2πΔft As the reflection coefficient, it rotates counterclockwise along the unit circle in the complex plane at a frequency of Δf. However, it is challenging to implement this reflection coefficient for low-power, resource-constrained backscatter tags. We use the SP8T switching network as the RF front-end to approximate the ideal reflection coefficient. The RF switch in the embodiment of the present invention connects the antenna to a reflection network composed of eight reflection circuits, and only one circuit is connected to the antenna at a time. When the backscatter tag has data to transmit, the switch sequentially switches the connection between the antenna and the eight reflection circuits. Each circuit applies a reflection coefficient Γ(t) determined by its impedance to the incident signal. These circuit impedances are carefully configured so that their reflection coefficients are respectively Corresponding to eight symmetric samples on the ideal signal circle with a phase interval. Therefore, by sequentially switching these eight circuits at a frequency eight times the required frequency shift, we can represent the reflection coefficient of the entire switching network as a piecewise function:

[0073]

[0074] where a ∈ {0, 1, …, 7}, T is the period of the frequency shift frequency, and k is an arbitrary natural number. Obviously, Γ(t) is a periodic function and can be decomposed into a sum of exponential signals where f0 = 1 / T is the fundamental frequency shift frequency. The Fourier series coefficient c k can be calculated by the following formula:

[0075]

[0076] When k is in the form of 8n + 3, 8n + 5, or 8n + 7, the coefficient c kis equal to zero, thus eliminating all corresponding harmonics. It is worth noting that a square wave is actually a special case of a switching network, which only alternates between two discrete samples: 1 and -1. Coarse-grained approximation of a square wave will thus generate more harmonics. To approximate the ideal signal, we use a frequency synthesizer to synthesize a clock that is eight times the desired frequency shift frequency to trigger a Gray counter, which outputs three control bits to switch the switching network between eight reflection circuits. This design enables us to cancel the third, fifth, and seventh harmonics, thereby reducing inter-tag interference and enhancing the SNR of the backscattered signal.

[0077] S300. The receiver pre-sets the center frequency of the communication link, and then differentiates and demodulates the modulated standard LoRa data packet by frequency band to complete the parallel backscatter communication of LoRa data.

[0078] Specifically, the embodiment of the present invention realizes frequency division multiple access parallel transmission by allocating an independent sub-channel to each tag. To ensure stable and efficient parallel transmission, the interval between adjacent sub-channels must be carefully considered. Reducing the sub-channel interval can save spectrum usage and support simultaneous transmission of more tags, while increasing the sub-channel interval can further reduce inter-tag interference. Considering the trade-off of the sub-channel interval, the channel allocation in the present invention is further referred to the LoRaWAN regional parameters. Specifically, the present invention uses a part of the 902 - 928 MHz ISM band in the United States as our communication channel. As Figure 9 shown, for the 125 kHz bandwidth LoRa signal to be synthesized in the present invention, we divide it into 50 sub-channels, starting from 918 MHz and ending at 927.8 MHz, and each sub-channel occupies a 200 kHz bandwidth.

[0079] To implement the frequency division scheme with a 200 kHz interval in practice, the backscatter tag designed in the embodiment of the present invention precisely controls the frequency shift of the reflected signal by regulating the frequency control word in the frequency synthesizer. According to the fact that the clock frequency generated by the frequency synthesizer is directly proportional to the frequency control word, for the backscatter tag assigned to a specific sub-channel, we add a fixed offset to the frequency control word to shift the LoRa data packet generated by it to the corresponding channel. By applying these fixed offsets, the backscatter tags of the present invention generate standard LoRa data packets in their designated sub-channels, thereby realizing multi-tag parallel transmission with a channel interval of 200 kHz.

[0080] In summary, the embodiment of the present invention has the following improvement points compared with the prior art:

[0081] 1) Parallel Backscatter LoRa Communication System Proposed for Compatibility with Low-Cost Commercial LoRa Devices and Throughput Improvement: The goal of the embodiments of the present invention is to enable multiple backscatter tags in a parallel LoRa backscatter communication system to generate standard LoRa data packets distributed in their respective corresponding sub-channels with low power consumption through the special design of the backscatter tags, while realizing a parallel backscatter LoRa communication system compatible with commercial LoRa devices and improving the throughput of the system. Through experimental verification, the solution designed by the present invention can not only be compatible with commercial LoRa devices, but also outperform the existing solutions in terms of the throughput of the backscatter tags.

[0082] 2) Pure Digital DDS Frequency Synthesis Scheme Proposed for Low-Power and High-Precision Clock Signal Synthesis: Before traditional DDS is used, the target signal is sampled first and the waveform amplitude is stored in a look-up table, and at the same time, a DAC is used to convert the digital amplitude into an analog voltage value to achieve accurate waveform restoration. The present invention uses a digital clock square wave signal to drive the subsequent RF front-end circuit without the need for complex waveform signals. Therefore, the present invention digitalizes DDS completely, uses the most significant bit truncation operation to replace the waveform look-up table and omits the digital-to-analog conversion process, discarding the energy-consuming DAC, and realizes the synthesis of a low-power and high-precision clock signal. The experimental results show that the pure digital DDS can meet the system requirements and generate the required frequency clock with the required high precision and fast response speed.

[0083] 3) Frequency Division Multiple Access Parallel Transmission Method for Interference Cancellation between Tags Proposed for Multi-Tag Parallel Transmission: Based on the above-mentioned pure digital DDS frequency synthesis scheme, the present invention can shift the data packet to the specified channel by assigning specific offset values to the frequency synthesizers of each backscatter tag, and for the problem of interference between multi-tag parallel transmissions, the present invention uses a switching network as the RF front-end to realize frequency division multiple access parallel transmission with harmonic cancellation. The experimental results prove that the parallel transmission method of the present invention has the characteristics of simple implementation and small interference between tags.

[0084] Therefore, compared with the prior art, the embodiments of the present invention have the following advantages. The parallel LoRa backscatter transmission system that can effectively avoid data packet collisions is crucial for supporting reliable large-scale Internet of Things applications. However, the special form of data packets generated by the best existing parallel LoRa backscatter systems reduces the already low throughput of LoRa and cannot be demodulated by existing inexpensive commercial LoRa receivers, requiring expensive special-designed receivers and being incompatible with existing LoRaWan networks. The present invention realizes precise frequency synthesis with low power consumption through pure digital DDS, uses a switched network RF front-end that eliminates harmonics to reduce interference between tags, and then realizes sub-channel allocation by assigning corresponding fixed offsets to the frequency control words of the pure digital DDS for each tag, enabling the system to achieve high-throughput parallel transmission while being compatible with inexpensive commercial LoRa receivers. Through experimental verification, compared with two parallel LoRa backscatter systems using LoRa cyclic shift keying modulation and frequency shift keying modulation, the parallel LoRa backscatter system solution proposed in the present invention that uses pure digital DDS for precise frequency synthesis and frequency division multiple access can not only be compatible with commercial LoRa receivers, but also significantly improve the throughput of backscatter tags.

[0085] Furthermore, through physical experiment simulation verification, the embodiments of the present invention are further described. Among them, the single-tag long-distance transmission effect and the overall throughput of multi-tag parallel transmission in outdoor and indoor scenarios of the present invention are evaluated. The evaluation system of the present invention includes a radio frequency source, a backscatter tag, and a LoRa receiver. Among them, the radio frequency source consists of a USRP connected to a 4dBi strip antenna, and the LoRa receiver is an existing commercial device with a Semtech SX1262 radio chip. The backscatter tag includes a digital baseband and a radio frequency front-end. The digital baseband is implemented on a DE1-SOC development board equipped with an Altera Cyclone V FPGA. We use the 50MHz default reference clock of this development board to implement a 24-bit pure digital DDS with hertz-level frequency synthesis and microsecond-level response time and a data packet processor. The radio frequency front-end uses a custom PCB with three ADG904 radio frequency switches to switch a 4dBi strip antenna on eight reflection circuits to eliminate harmonics. To minimize the phase error introduced by the transmission line, the wiring of these circuits is carefully matched to ensure that the eight reflection coefficients are evenly distributed on the ideal complex unit circle.

[0086] In the physical experiment, we set the parameters for generating LoRa packets as SF = 10, BW = 125 kHz, 4 / 5 coding rate, and synchronization words {8, 16}. The RF source transmits a single-frequency signal with a frequency of 915 MHz at a transmission power of 17 dBm. The backscatter tag of the present invention frequency-shifts the source signal by 3 MHz to generate a LoRa packet centered at a frequency of 918 MHz. We define the distance from the RF source to the tag as d st , and the distance from the tag to the receiver as d tr . At each measurement position, the backscatter tag generates 500 LoRa packets carrying 32-bit data. We measure the bit error rate (BER), received signal strength indicator (RSSI), and throughput at these positions. It should be noted that the RSSI data read from a commercial LoRa receiver is a reference value and has a certain offset from the true signal power. If no packet is received, we cannot obtain RSSI data from the receiver. In this case, we calibrate the RSSI to a value of -140 dBm to indicate beyond the data transmission range. After experimental verification, compared with two parallel LoRa backscatter systems using LoRa cyclic shift keying modulation and frequency shift keying modulation, the parallel LoRa backscatter system solution proposed in the present invention using pure digital DDS for precise frequency synthesis and frequency division multiple access can not only be compatible with commercial LoRa receivers, but also significantly improve the throughput of the backscatter tag.

[0087] As Figure 10 shown, long-distance transmission indoors and outdoors compatible with commercial LoRa devices: In the outdoor scenario, we deploy the RF source and the backscatter tag at fixed positions on the side of a straight road, and move the LoRa receiver along the road. The distance d tr between the tag and the receiver is gradually increased from 100 meters to 1 kilometer, and the BER and RSSI of the backscatter signal are measured at regular intervals. In the indoor scenario, as Figure 11 shown, we place the RF source and the backscatter tag in an open hall outside a room in an office building, and place the LoRa receiver at both ends of four adjacent enclosed rooms to ensure that the backscatter signal can only be received after penetrating a certain number of walls. The distances between these four walls and the backscatter tag are 1, 11, 29, and 47 meters respectively. By changing the position of the LoRa receiver, we measure the BER and RSSI of the backscatter signal passing through multiple wall obstacles. Initially, the RF source is placed very close to the tag, i.e., d st = 0.1. To evaluate the influence of the distance d st from the source to the tag and the spreading factor SF on the transmission distance, we conduct repeated experiments at SF = 9, with d st being 1 m and 5 m.

[0088] In the outdoor test results, as Figure 12 shown, when d st = 0.1m and the spreading factor SF = 10, the present invention supports a transmission range of 800m and can still receive data packets at 1000m, while when the spreading factor SF = 9, it supports a transmission range of 700m. The present invention exhibits a lower BER and a longer transmission range when SF = 10 compared to SF = 9, mainly because a higher spreading factor introduces a longer signal period and more signal energy, enabling more robust signal recovery in a noisy environment. As Figure 13 shown, when d st is increased to 1m, the transmission ranges of the present invention are reduced to 600m and 500m respectively when SF = 10 and SF = 9. As Figure 14 shown, when d st is further increased to 5 meters, when d tr is 400m, the BER rises to 0.4% under the spreading factor SF = 10, while the BER rises to 2% under the spreading factor SF = 9. In addition, the LoRa receiver can decode data packets with a minimum received signal strength of -113dBm at SF = 9 and -116dBm at SF = 10 respectively, which reveals the basic trade-off between noise immunity and data rate in LoRa. Specifically, a higher spreading factor in LoRa improves signal reliability and increases the transmission range, but at the same time reduces the throughput. In the indoor test results, the RSSI drops sharply after passing through the wall, highlighting the challenge of chirp signal transmission across walls in complex indoor scenarios. As Figure 15 shown, when d st = 0.1m, under the spreading factors SF = 9 and SF = 10, the backscattered signals successfully penetrate four concrete walls and are decoded by a commercial LoRa receiver. As Figure 16 shown, when d st is increased to 1 meter, the chirp signal of SF = 10 can still penetrate four walls, while the backscattered signal of SF = 9 can penetrate three walls. As Figure 17 shown, when d st is set to 5 meters, both spreading factors can only penetrate two walls and achieve transmission ranges of 29 meters and 12 meters respectively. Our test results show that the proposed parallel LoRa backscattering system scheme using pure digital DDS for precise frequency synthesis and frequency division multiple access can achieve reliable indoor and outdoor long-distance communication compatible with commercial LoRa receivers.

[0089] Finally, we also deployed a tag with randomly assigned sub-channels and gradually increased the number of tags to 10. Each new tag was assigned a unique random sub-channel. Whenever a LoRa packet was received, its throughput was calculated in accordance with standard LoRa. The throughput was measured as 980 bps at SF = 10 and 1760 bps at SF = 9. To ensure reliability, we repeated the test experiment ten times and took the average of the measurement results. As Figure 18 shown, the total throughput of the present invention increases approximately linearly with the number of tags. When 10 tags are deployed, the total throughput reaches 9.61 kbps at SF = 10 and 17.11 kbps at SF = 9, approaching the ideal maximum throughputs of 9.8 kbps and 17.6 kbps respectively. In contrast, an existing technology achieved a throughput of 11.27 kbps in the case of 101 tags, while the trace-driven simulation of another existing technology showed a total throughput of 68.36 kbps for 700 parallel tags, equivalent to a single-tag throughput of 97.7 bps. The test results show that the single-tag throughput of the present invention is approximately 8 - 9 times higher than that of existing solutions.

[0090] Referring to Figure 2 , a LoRa-based parallel backscatter communication system includes:

[0091] A first module 201 for transmitting a single-tone radio frequency source signal as a signal excitation source;

[0092] A second module 202 for designing a backscatter tag, combining the signal excitation source to perform backscatter modulation processing on environmental perception data to obtain a modulated standard LoRa packet;

[0093] A third module 203 for the receiver to preset the center frequency of the communication link and then perform frequency band discrimination and demodulation processing on the modulated standard LoRa packet to complete the parallel backscatter communication of LoRa data.

[0094] As Figure 3As shown in the figure, the embodiment of the present invention uses a parallel backscatter LoRa communication system compatible with commercial devices to collect and transmit node sensing data. The backscatter LoRa communication system compatible with commercial devices consists of a single-tone radio frequency signal source, multiple backscatter tags, and a standard LoRa gateway. Each backscatter tag first collects various sensing data according to the requirements of the scenario application and caches it, and then waits for the opportunity to transmit the cached data back to the LoRa gateway. When the gateway needs to collect the sensing data of each node, the signal source transmits a single-tone carrier as the excitation source for the backscatter tags. Multiple backscatter tags simultaneously use the single-tone carrier to backscatter modulate their cached data into standard LoRa data packets occupying different channels. The standard LoRa gateway at the receiving end only needs to adjust the center frequency of the receiving link to detect and demodulate the LoRa data packets of the corresponding tags. The embodiment of the present invention realizes the operation of the parallel backscatter LoRa communication system through a special design of the backscatter tags. First, the backscatter tag synthesizes the transmission data into the form of a standard LoRa data packet using a digital baseband including a digital DDS and generates a corresponding clock sequence. Then, the backscatter tag drives the radio frequency front end composed of a switching network with the generated clock sequence to generate a standard LoRa signal with harmonic cancellation.

[0095] The content in the above method embodiments is applicable to the system embodiments of the present invention. The functions specifically implemented by the system embodiments of the present invention are the same as those in the above method embodiments, and the beneficial effects achieved are also the same as those in the above method embodiments.

[0096] The above is a specific description of the preferred embodiment of the present invention. However, the present invention is not limited to the described embodiment. Those skilled in the art can make various equivalent deformations or substitutions without departing from the spirit of the present invention. These equivalent deformations or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A LoRa-based parallel backscatter communication method, characterized in that It includes the following steps: Transmit a single-tone RF source signal as a signal excitation source; Design a backscatter tag, combine the signal excitation source to perform backscatter modulation processing on environmental perception data, and obtain a modulated standard LoRa data packet; The receiver presets the center frequency of the communication link in advance, and then performs frequency band discrimination and demodulation processing on the modulated standard LoRa data packet to complete the parallel backscatter communication of LoRa data.

2. The method for parallel backscatter communication based on LoRa according to claim 1, wherein The step of designing a backscatter tag, combining the signal excitation source to perform backscatter modulation processing on environmental perception data, and obtaining a modulated standard LoRa data packet specifically includes: Design a backscatter tag and excite it through a signal excitation source; Based on LoRa parameters, perform conversion processing on environmental perception data to obtain a standard LoRa data packet with a digital chirp information sequence; Perform frequency synthesis processing according to the digital chirp information sequence to generate a clock control drive signal; Perform mapping processing on the clock control drive signal to obtain a number of control bit signals; Based on a number of control bit signals, perform modulation processing on the standard LoRa data packet with a digital chirp information sequence through the switch network harmonic cancellation method to obtain a modulated standard LoRa data packet.

3. The method for parallel backscatter communication based on LoRa according to claim 2, wherein The backscatter tag specifically includes a digital baseband circuit and a radio frequency front-end circuit. The output end of the digital baseband circuit is connected to the input end of the radio frequency front-end circuit, where: The digital baseband circuit includes a data packet processor, a DDS frequency synthesizer, and a Gray counter, and the data packet processor, the DDS frequency synthesizer, and the Gray counter are connected in sequence; The radio frequency front-end circuit includes a switch network module and a number of reflection circuits, and the output end of the switch network module is respectively connected to the input ends of the number of reflection circuits.

4. The method for parallel backscatter communication based on LoRa according to claim 3, characterized in that, The step of performing conversion processing on environmental perception data based on LoRa parameters to obtain a standard LoRa data packet with a digital chirp information sequence specifically includes: Pre-define LoRa parameters, where the LoRa parameters include a preamble, a header, a payload, and a cyclic redundancy check. The preamble includes a variable preamble, a synchronization word, and a frame start delimiter. The header includes a payload length, a coding rate, a cyclic redundancy check indicator, and a header checksum; Generate a standard LoRa data packet with a digital chirp information sequence based on the pre-defined LoRa parameters and the synchronization word in the form of a digital chirp information sequence.

5. The method for parallel backscatter communication based on LoRa according to claim 4, characterized in that The step of performing frequency synthesis processing according to the digital chirp information sequence to generate a clock control drive signal specifically includes: According to the digital chirp information sequence, obtain a reference clock and a control word signal, perform phase increment calculation to obtain a current phase increment; Introduce an N-bit register as a phase accumulator. At each clock cycle, add the current phase increment to the current phase value to obtain the value of the phase accumulator; Introduce a bit truncation operation, directly select the most significant bit of the phase accumulator for judgment. When the phase accumulator is greater than a preset threshold, the output is 1, otherwise the output is 0, and a clock control drive signal is generated.

6. The method for parallel backscatter communication based on LoRa according to claim 5, characterized in that, The step of modulating a standard LoRa packet with a digital chirp information sequence by a switching network harmonic elimination method based on several control bit signals to obtain a modulated standard LoRa packet specifically includes: Based on an SP8T switching network, the reflection coefficient of the switching network is designed as a piecewise function according to the impedance value; Based on several control bit signals, combined with the piecewise function, a frequency synthesizer is used to synthesize a clock that is eight times the required frequency shift frequency, and a standard LoRa packet with a digital chirp information sequence is modulated to obtain a modulated standard LoRa packet.

7. The method for parallel backscatter communication based on LoRa according to claim 6, characterized in that, The expression of the piecewise function of the reflection coefficient of the switching network is specifically as follows: In the above formula, Γ(t) represents the piecewise function of the reflection coefficient, T represents the period of the frequency shift frequency, a represents the number of reflection circuits, and k represents a natural number. represents the reflection coefficient.

8. A LoRa-based parallel backscatter communication system, characterized in that, It includes the following modules: A single-tone RF source, which is used to transmit a single-tone RF source signal as a signal excitation source; A backscatter tag, which is used to design a backscatter tag and perform backscatter modulation processing on environmental perception data in combination with the signal excitation source to obtain a modulated standard LoRa packet; A commercial LoRa receiver, which is used to set the center frequency of the communication link in advance by the receiver, and then perform frequency band discrimination and demodulation processing on the modulated standard LoRa packet to complete the parallel backscatter communication of LoRa data.

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