Backscattering method for ZigBee excitation source in uncontrolled environment

Abstract

The present invention relates to the technical field of backscatter communication, and specifically discloses a backscatter method suitable for a ZigBee excitation source in an uncontrolled environment, which specifically includes: a ZigBee transmitter maps binary data into data symbols, uses direct sequence spread spectrum to map each data symbol into a chip sequence, and modulates each chip sequence onto a carrier; generates a tag modulation sequence for tag data, and the tag modulates the carrier in the excitation signal based on the tag modulation sequence to generate a backscatter signal; processes the excitation signal and the backscatter signal to obtain a pre-reflection codeword corresponding to the excitation signal and a post-reflection codeword corresponding to the backscatter signal, and obtains the tag data according to the pre-reflection codeword and the post-reflection codeword. The present invention aims to reduce the limitation of the excitation source while improving the throughput of the system, thereby further improving the universality of the ZigBee backscatter system.

Description

Technical Field

[0001] The invention relates to the technical field of backscatter communication, and in particular to a backscatter method suitable for a ZigBee excitation source in an uncontrolled environment. Background Art

[0002] Through the intelligent interaction of data between network devices, the Internet of Things has brought great convenience to people's lives, which has been significantly demonstrated in application fields such as smart home, smart transportation, smart agriculture, and smart medical care. However, the continuous expansion of network scale and the continuous enhancement of network functions have led to a sharp increase in the power consumption level of network devices. This problem has seriously affected the further realization of the vision of the Internet of Everything.

[0003] In recent years, backscattering technology has attracted widespread attention due to its ultra-low power consumption characteristics and has become one of the important technologies for reducing network power consumption. The backscattering system consists of three main parts, namely the excitation source, the tag and the receiver. The biggest difference from active communication is that in the backscattering system, the tag does not actively generate signals, but transmits data by reflecting the excitation signal, which can significantly reduce the power consumption of the tag. This ultra-low power tag often has a longer working life and reduces the manpower and material costs caused by frequent maintenance. Classic backscattering systems, such as RFID systems, include two parts: readers and tags. The reader transmits a radio frequency signal to the tag, and the tag modulates the tag data onto a carrier and then reflects it to the reader, thereby realizing data transmission. However, the RFID system requires the use of expensive dedicated readers, which limits the large-scale deployment of the system. Therefore, the backscattering system that uses ambient commercial radio as a carrier has become a new focus. In recent years, researchers have implemented backscattering systems on wireless point signals widely deployed in the environment, such as WiFi, Bluetooth, ZigBee, Long Term Evolution (LTE), and Long Range Radio (LoRa), further improving the versatility and universality of backscattering systems.

[0004] Among these popular radio signals, ZigBee has attracted much attention from researchers due to its low power consumption, large number of nodes that can be accommodated in a single network, and strong networking capabilities. However, existing ZigBee backscatter systems either use controlled excitation sources or have low throughput, which seriously limits the large-scale deployment and application of ZigBee backscatter systems.

[0005] Therefore, is it possible to use an uncontrolled ZigBee signal as an excitation source to achieve high-throughput ZigBee backscatter communication? The following observations indicate that this idea is feasible: Existing backscatter systems based on codeword transitions focus on whether the codeword changes after backscattering, without considering the specific codeword generated after backscattering. ZigBee signals consist of 16 different codewords. If each codeword is considered as a state, the original codeword has 16 possible transitions after backscattering. In this way, different tag data can be encoded into each transition, so these 16 transitions correspond to 4 bits of tag data. In this way, each transition can carry 4 bits of tag data. This theoretically allows the throughput of the backscatter system to match the throughput of active ZigBee communication, thereby achieving high-throughput ZigBee backscatter communication. Summary of the invention

[0006] To solve the above technical problems, the present invention provides a backscattering method suitable for ZigBee excitation sources in uncontrolled environments. Chip-level modulation is used on the tag to embed continuous 4-bit tag data into the backscattering codeword. The receiver decodes the tag data by comparing the codewords before and after backscattering.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A backscattering method for a ZigBee excitation source in an uncontrolled environment, specifically comprising:

[0009] Step 1: Send an excitation signal through the ZigBee transmitter;

[0010] Step 2, generate 16 tag modulation sequences for 4-bit tag data: remove the unchanged, equivalent and repeated cases in the 256 code word conversions corresponding to the 16 data symbols in the Zigbee protocol, and obtain 58 non-equivalent code word conversions; use the all-0 and all-1 sequences as two tag modulation sequences, and divide the difference sequences corresponding to the remaining 56 non-equivalent code word conversions into two groups through the first round of grouping, and use symmetry to obtain 7 tag modulation sequences from the first group; then perform a second round of grouping, divide the difference sequences of the first group into 7 second-level groups, and determine the tag modulation sequence corresponding to each second-level group by minimizing the sum of the Hamming distances between the target tag modulation sequence and the difference sequences of the second-level groups; finally obtain 16 tag modulation sequences;

[0011] The tag modulates the carrier in the excitation signal based on the tag modulation sequence, shifts the carrier frequency to a new channel, and generates a backscatter signal;

[0012] Step 3: receive the excitation signal and the backscattered signal respectively through two ZigBee receivers, process the excitation signal and the backscattered signal, and obtain tag data.

[0013] Furthermore, the label modulation sequence is a sequence composed of multiple label modulation operations. When the label modulation operation is '1', the phase difference between two consecutive sampling points in the carrier can be flipped, and when the label modulation operation is '0', the phase difference between two consecutive sampling points in the carrier can be kept unchanged.

[0014] Further, step one specifically includes: the ZigBee transmitter maps binary data into data symbols, maps each data symbol into a chip sequence using direct sequence spread spectrum, modulates each chip sequence onto a carrier using offset quadrature phase shift keying modulation technology with half-sine pulse shaping, and sends it as an excitation signal; wherein, the use of offset quadrature phase shift keying modulation technology with half-sine pulse shaping to modulate each chip sequence onto a carrier specifically includes:

[0015] During the modulation process, the chips with even indexes in the chip sequence modulate the in-phase carrier, and the chips with odd indexes in the chip sequence modulate the quadrature-phase carrier; the duration of each chip is recorded as In order to achieve the time offset between the in-phase carrier and the quadrature-phase carrier, the code corresponding to the quadrature-phase carrier is delayed compared to the code corresponding to the in-phase carrier. .

[0016] Further, step three specifically includes: there are two ZigBee receivers, namely a first ZigBee receiver and a second ZigBee receiver, the first ZigBee receiver receives an excitation signal, the second ZigBee receiver receives a backscattered signal, the excitation signal and the backscattered signal are processed to obtain a pre-reflection codeword corresponding to the excitation signal and a post-reflection codeword corresponding to the backscattered signal, and the tag data is obtained according to the pre-reflection codeword and the post-reflection codeword;

[0017] The processing of the excitation signal and the backscattered signal to obtain a pre-reflection codeword corresponding to the excitation signal and a post-reflection codeword corresponding to the backscattered signal, and obtaining tag data according to the pre-reflection codeword and the post-reflection codeword, specifically includes:

[0018] The excitation signal and the backscatter signal are collectively referred to as ZigBee signals; after receiving the ZigBee signal, the ZigBee receiver uses an analog-to-digital converter to capture the in-phase orthogonal data of the ZigBee signal; the in-phase orthogonal data is sampled to determine whether the phase shift between two consecutive samples is positive, if so, a code chip '1' is output, if not, a code chip '0' is output; the obtained code chips are combined to obtain multiple code chip sequences, and the code chip sequence with the smallest Hamming distance from 16 possible code chip sequences specified in the ZigBee protocol is selected, and every 32 consecutive code chips in the selected code chip sequence are mapped to a data symbol according to a predefined mapping table, and each data symbol corresponds to a backscatter codeword; the backscatter codeword includes the pre-reflection codeword and the post-reflection codeword; the tag data is obtained according to the pre-reflection codeword and the post-reflection codeword.

[0019] Furthermore, in step 2, generating 16 label modulation sequences for the 4-bit label data specifically includes the following steps:

[0020] S21, remove duplication: the 16 tag modulation sequences can convert each data symbol into 16 different data symbols; there are 16 data symbols in the Zigbee protocol, corresponding to 16 code words respectively; therefore, the 16 tag modulation sequences need to realize 256 code word conversions; after removing the code word unchanged, code word equivalent and code word repeated cases from the 256 code word conversions, 58 different non-equivalent code word conversions are obtained;

[0021] S22, using a sequence consisting of all 0s and all 1s as a label modulation sequence respectively, which can realize two different non-equivalent codeword conversions, and the remaining 14 label modulation sequences need to realize the remaining 56 different non-equivalent codeword conversions;

[0022] The first round of grouping: each non-equivalent codeword conversion corresponds to a difference sequence, and the difference sequences corresponding to the 56 non-equivalent codewords are represented by data symbol pairs, and the 56 difference sequences are divided into two groups according to symmetry, namely the first group and the second group, with 28 difference sequences in each group; if the difference sequences of the two groups show symmetry at corresponding positions, then when a label modulation sequence can realize the non-equivalent codeword conversion corresponding to a difference sequence of the first group, the symmetric sequence of the label modulation sequence can realize the non-equivalent codeword conversion corresponding to the difference sequence at the corresponding position of the second group, so that 7 label modulation sequences can be obtained from the first group;

[0023] Second round of grouping: The 28 difference sequences in the first group were evenly divided into 7 second-level groups, each of which contained four difference sequences;

[0024] S23, determine the label modulation sequence; for each second-level group, minimize the sum of the Hamming distances between the target label modulation sequence and the four difference sequences contained in the second-level group, so as to obtain 7 label modulation sequences corresponding to the 7 second-level groups, specifically including: determining the value of each bit of the target label modulation sequence according to the frequency of each bit in the four difference sequences in the second-level group; if the number of bits with the target label modulation sequence value of '0' is equal to the number of bits with the value of '1', then invert any one of the four difference sequences in the second-level group.

[0025] Compared with the prior art, the beneficial technical effects of the present invention are:

[0026] Aiming at the limitations of the existing ZigBee backscatter system, the present invention proposes a novel ZigBee backscatter system after fully exploring the characteristics of ZigBee signals, aiming to reduce the limitation of the excitation source while improving the system throughput, thereby further improving the universality of the ZigBee backscatter system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of the working process of a ZigBee transmitter provided according to an embodiment of the present invention;

[0028] Figure 2 A schematic diagram of the working process of a ZigBee receiver provided according to an embodiment of the present invention;

[0029] Figure 3 A schematic diagram of the conversion relationship between different data symbols provided according to an embodiment of the present invention;

[0030] Figure 4 This is a schematic diagram showing that the difference sequences at corresponding positions in two groups obtained in the first round of grouping according to an embodiment of the present invention show symmetry;

[0031] Figure 5 A schematic diagram of a grouping meeting the conditions provided according to an embodiment of the present invention;

[0032] Figure 6 A schematic diagram of a detailed process of obtaining a target tag modulation sequence according to an embodiment of the present invention;

[0033] Figure 7 A schematic diagram of throughput performance achieved by using different commercial transceiver devices according to an embodiment of the present invention;

[0034] Figure 8 It is a schematic diagram of a backscattering system provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] A preferred embodiment of the present invention is described in detail below with reference to the accompanying drawings.

[0036] Below, the working process of the ZigBee transmitter and receiver and the modulation of the ZigBee backscatter system are first introduced, and then the modulation and demodulation method adopted by the present invention is introduced.

[0037] 1. ZigBee transmitter and receiver working process:

[0038] Figure 1 The workflow of a ZigBee transmitter when transmitting RF signals is shown. First, the binary data from the physical protocol data unit (PPDU) is mapped to data symbols, where consecutive 4 bits of binary data are mapped to one data symbol. ZigBee uses direct sequence spread spectrum (DSSS) to map each data symbol into a 32-chip pseudo-random sequence. These chip sequences are related to each other through cyclic shift and conjugation. Finally, each chip sequence is modulated onto the carrier using offset quadrature phase shift keying (OQPSK) with half-sine pulse shaping. In this modulation process, even-indexed chips modulate the in-phase (I-phase) carrier, while odd-indexed chips modulate the quadrature-phase (Q-phase) carrier. The duration of each chip is (In this embodiment 0.5 microseconds), which results in a ZigBee chip rate of 2Mchip / s and a data transmission rate of 250kbps. In order to achieve the offset between the in-phase carrier and the quadrature carrier, the chip corresponding to the quadrature carrier is delayed relative to the chip corresponding to the in-phase carrier. .

[0039] Figure 2 The working process of the ZigBee receiver is demonstrated. After receiving the signal, the ZigBee receiver uses an analog-to-digital converter (ADC) to capture the in-phase and quadrature data of the signal, and uses offset quadrature phase shift keying (OQPSK) with half-sine pulse shaping for demodulation: if the phase shift between two consecutive samples is positive, the ZigBee receiver outputs chip 1; if the phase shift between two consecutive samples is negative, it outputs chip 0. Multiple chip sequences are obtained, and every 32 consecutive chips are mapped to a data symbol according to a predefined mapping table. Due to the inevitable noise interference during signal transmission, in the process of mapping the chip to the data symbol, ZigBee selects the chip sequence with the smallest Hamming distance from the 16 possible chip sequences specified in the ZigBee protocol.

[0040] 2. Modulation of ZigBee backscatter system:

[0041] Assume that the tag can directly modulate the phase difference of the carrier: the tag modulation operation '1' can reverse the current phase difference, while the tag modulation operation '0' can keep the current phase difference unchanged. In the specific implementation, the phase difference modulation sequence can be converted into the phase modulation sequence according to the following rules: Assume that the phases of two consecutive sampling points of the carrier are and , the original phase difference is When the tag modulation operation is '0', a 180° phase shift is applied to both sampling points, so that the phase difference of the backscattered signal becomes , which means that the phase difference remains unchanged. When the tag modulation operation is '1', only the phase shift is applied to the second sampling point, while the phase of the first sampling point is kept unchanged. Therefore, the phase difference of the backscattered signal becomes , which means the phase difference is reversed.

[0042] 3. Modulation and demodulation method adopted by the present invention:

[0043] 1. Overview:

[0044] In order to achieve the goal, it is necessary to identify 16 tag modulation sequences corresponding to the tag data. There are 16 data symbols in the Zigbee protocol, corresponding to 16 code words. The process of code word conversion is the process of converting one data symbol into another data symbol. For each data symbol, 16 tag modulation sequences can convert it into 16 different data symbols. Therefore, these 16 tag modulation sequences must achieve 256 (i.e. ) codeword transitions. The tag needs to complete the difference between two code chip sequences to achieve the codeword transition. In the worst case, these 256 difference sequences are all different. It is necessary to consider how to achieve multiple codeword transitions using a single tag modulation.

[0045] The present invention observes that the difference sequences corresponding to these codeword conversions are not completely different, but there are repetitions. In addition, the fault tolerance of the ZigBee receiver using minimum Hamming distance decoding makes it possible to use a single tag modulation to achieve multiple codeword conversions. The core process of the present invention is as follows: First, 256 difference sequences are deduplicated. Subsequently, the obtained difference sequences are divided into 16 groups, and the difference sequences of each group are integrated to obtain a modulation sequence that can perform all codeword conversions within the group. This process ultimately results in 16 tag modulation sequences. The following section will describe this process in detail.

[0046] 2. Obtain the label modulation sequence

[0047] 1) Remove repetition. For a given label modulation sequence, a specific codeword conversion should be achieved under the 16 different data symbol conditions of the excitation signal codeword. In the worst case, the difference sequences corresponding to each pair of codeword conversions are completely different. In this case, the Hamming distance between the label modulation sequence and these 16 difference sequences must be minimized to effectively modulate the excitation signal codeword into the target codeword. By observing and analyzing these difference sequences, it can be found that there are repeated difference sequences. The present invention regards codeword conversions with the same difference sequence as equivalent codeword conversions. Therefore, the repeated cases can be removed and only the inequivalent codeword conversions are retained.

[0048] First, when the codeword does not change, these 16 codeword conversions are considered to be equivalent codeword conversions. The difference sequences between these 16 pairs of codewords are all 32-bit all-zero sequences. When the codeword changes, the conversion between data symbol x and data symbol y is also equivalent. Therefore, among the 240 codeword conversions when the codeword changes, half are equivalent, leaving only 120 possible non-equivalent codeword conversions.

[0049] In addition, analyzing the characteristics of data symbols, we can find that Figure 3 As shown, there is a specific conversion relationship between different data symbols. Figure 3 middle and They represent the first and 32nd bit chip values ​​in the chip sequence corresponding to the data symbol. Each symbol has a symmetrical corresponding symbol, whose chip sequence is completely opposite, and the index difference between the two is 8. Based on this feature, the following conclusions can be drawn:

[0050] Represents data symbol 32-bit chip sequence, each bit in the chip sequence represents the data symbol The phase shift over time, Indicates data symbol and data symbols Therefore, combined with Figure 3 The second round of deduplication is performed on the patterns in (1), (2) and (3):

[0051] ; (1)

[0052] ; (2)

[0053] ; (3)

[0054] Represents a 32-bit all-1 sequence, Represents the exclusive OR operation.

[0055] When the codeword transition occurs between two symmetric data symbols, according to equation (1), it can be seen that these 8 codeword transitions are equivalent. Therefore, they are regarded as one type of codeword transition and can be implemented using a 32-bit all-1 modulation sequence. When the codeword transition occurs between asymmetric data symbols, according to equations (2) and (3), it can be found that each codeword transition has an equivalent term. Therefore, of the remaining 112 codeword transitions that occur between asymmetric symbols, half are repeated, resulting in only 56 different codeword transitions. Finally, 58 different non-equivalent codeword transitions were determined, including one case where the codeword remains unchanged and one codeword transition obtained according to equation (1).

[0056] 2) Grouping. In order to achieve 58 different non-equivalent codeword conversions using 16 tag modulation sequences, the characteristics of these tag modulation sequences need to be considered. Each tag modulation sequence can modulate a given excitation codeword into 16 different data symbols. Sequences consisting of all 0s and all 1s can effectively achieve codeword conversion. Therefore, these sequences are selected as 2 of the 16 target tag modulation sequences. These sequences themselves can achieve two different codeword conversions. In this way, the remaining 14 target tag modulation sequences need to achieve the remaining 56 codeword conversions. Each of these tag modulation sequences must be able to achieve multiple codeword conversions to meet the target. In order to minimize the Hamming distance between each tag modulation sequence and take into account the inherent errors in the backscatter code chip sequence, each group of 4 pairs of codeword pairs requires their difference sequences to be as similar as possible. By utilizing these principles, 16 tag modulation sequences can be strategically selected and designed to effectively achieve the 58 different codeword conversions required for high-throughput ZigBee backscatter communication systems.

[0057] Next, we need to consider how these codeword transitions should be grouped to achieve the best results. After analyzing the 56 pairs of difference sequences corresponding to the codeword transitions, we can observe that each difference sequence has a completely opposite symmetric difference sequence. Figure 4 The symmetry between these difference sequences is clearly demonstrated, and these 56 difference sequences can be described using symbol pairs. The difference sequences within these two groups show symmetry at corresponding positions. When a difference sequence can realize the codeword conversion in group 1, its symmetric difference sequence can realize the corresponding codeword conversion in group 2. In view of this feature, attention can be focused only on group 1. Seven label modulation sequences can be obtained from group 1, and their symmetric sequences can be obtained from group 2. According to these criteria, the first round of grouping is completed. Next, the principles of the second round of grouping must be established and the method of obtaining the target label modulation sequence must be determined.

[0058] The second round of grouping task is to divide the 28 difference sequences in group 1 into 7 second-level groups, each containing 4 difference sequences. Considering the characteristics of the target label modulation sequence mentioned above, each label modulation sequence should be universally applicable to all symbols, ensuring that each group covers all data symbols. In addition, each data symbol should be represented by a difference sequence in each of the 7 second-level groups. Figure 5 An example of grouping that meets these criteria is provided. In order to minimize the differences among the four difference sequences in each second-level group, the present invention refers to the IEEE 802.15.4 standard, which adopts a hexadecimal quasi-orthogonal modulation technique. The present invention also analyzes the Hamming distances between these difference sequences. The results show that 80% of the Hamming distances are concentrated around values ​​such as 14, 16, and 18, and a few cases reach relatively large values, such as 24. Therefore, when forming the second-level group, a randomization method that meets the above conditions can be effectively adopted.

[0059] 3) Determine the tag modulation sequence. For each second-level group, the goal is to minimize the sum of the Hamming distances between the target tag modulation sequence and the four difference sequences, thereby reducing the errors in the backscatter codeword. To this end, the value of each bit of the target tag modulation sequence ('0' or '1') is determined based on the frequency of each bit in the four difference sequences within the second-level group. However, if the number of '0' and '1' on a certain bit is equal, such as Figure 6 As shown, the result is undetermined. However, the present invention observes that the number of '0' and '1' contained in each bit is even. Therefore, reversing any one of the four difference sequences can solve this problem.

[0060] The present invention combines two first-level groups, and within each second-level group, data symbols can be interchanged with symmetrical data symbols from another first-level group. This method ensures that when determining the target tag modulation sequence, if the frequency of 0 and 1 on a certain bit is equal, it can be replaced by its symmetrical codeword conversion. Figure 6 As shown in Figure 1, using this method, seven different label modulation sequences can be derived from the seven second-level groups. Finally, by reversing these seven modulation sequences, the codeword conversion in another first-level group can be achieved. Tables 1 and 2 show the codeword conversion in Figure 1. Figure 5 All label modulation sequences and their corresponding codeword conversion tables obtained by the grouping method shown.

[0061] Table 1 Tag modulation sequence corresponding to tag data:

[0062]

[0063] Table 2 Relationship between codeword before reflection, codeword after reflection and tag data:

[0064]

[0065] The present invention realizes for the first time that the system throughput can reach 240kbps when using an uncontrolled environment ZigBee excitation source. Figure 7 As shown, 6 different transceiver combinations are implemented on three different commercial ZigBee devices. The present invention performs well in different implementations, with the highest throughput exceeding 240 kbps and the lowest throughput exceeding 226 kbps.

[0066] Example

[0067] 1. System construction:

[0068] The present invention uses an off-the-shelf field programmable gate array (FPGA) and commercial ZigBee devices to build a verification prototype. The specific details are as follows:

[0069] The prototype of the tag mainly consists of a passive envelope detector, an analog-to-digital converter (ADC), a field programmable gate array (FPGA), and an RF switch. The passive envelope detector is used to remove the high-frequency carrier and obtain the baseband signal for processing by the FPGA. The ADC samples the baseband signal and transmits it to the FPGA. The FPGA implements the user logic and outputs the control signal to the RF switch.

[0070] The ZigBee transmitter and the ZigBee receiver mainly use three different commercial ZigBee devices to provide environmental excitation and receive backscattered signals; the three different commercial ZigBee devices include TI CC1352, TI CC2650 and TICC2530.

[0071] 2. Modulation and demodulation of tag data:

[0072] like Figure 8 As shown, the present invention uses a ZigBee transmitter to continuously send an excitation signal on channel A to provide a carrier for the tag. The tag performs chip-level modulation on the carrier according to the tag modulation sequence in Table 1 to generate a backscattered signal, which is propagated on channel B. The backscattered signal is frequency-shifted to a new channel to avoid interference from the carrier signal. Two ZigBee receivers receive the excitation signal and the backscattered signal respectively, process the excitation signal and the backscattered signal, obtain the pre-reflection codeword corresponding to the excitation signal and the post-reflection codeword corresponding to the backscattered signal, and obtain the tag data according to the pre-reflection codeword, the post-reflection codeword and the corresponding relationship shown in Table 2.

[0073] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention, and any reference numerals in the claims should not be regarded as limiting the claims involved.

[0074] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A backscattering method for ZigBee excitation source in an uncontrolled environment, characterized in that: Specifically include: Step 1: Send an excitation signal through the ZigBee transmitter; Step 2, generate 16 tag modulation sequences for 4-bit tag data: remove the unchanged, equivalent and repeated cases in the 256 code word conversions corresponding to the 16 data symbols in the Zigbee protocol, and obtain 58 non-equivalent code word conversions; use the all-0 and all-1 sequences as two tag modulation sequences, and divide the difference sequences corresponding to the remaining 56 non-equivalent code word conversions into two groups through the first round of grouping, and use symmetry to obtain 7 tag modulation sequences from the first group; then perform a second round of grouping, divide the difference sequences of the first group into 7 second-level groups, and determine the tag modulation sequence corresponding to each second-level group by minimizing the sum of the Hamming distances between the target tag modulation sequence and the difference sequences of the second-level groups; finally obtain 16 tag modulation sequences; The tag modulates the carrier in the excitation signal based on the tag modulation sequence, shifts the carrier frequency to a new channel, and generates a backscatter signal; Step 3: receiving the excitation signal and the backscattered signal respectively through two ZigBee receivers, processing the excitation signal and the backscattered signal to obtain tag data; There are two ZigBee receivers, namely the first ZigBee receiver and the second ZigBee receiver. The first ZigBee receiver receives the excitation signal, and the second ZigBee receiver receives the backscattering signal. The excitation signal and the backscattering signal are processed to obtain a pre-reflection codeword corresponding to the excitation signal and a post-reflection codeword corresponding to the backscattering signal, and the tag data is obtained according to the pre-reflection codeword and the post-reflection codeword. Specifically, the process includes: The excitation signal and the backscatter signal are collectively referred to as ZigBee signals; after receiving the ZigBee signal, the ZigBee receiver uses an analog-to-digital converter to capture the in-phase orthogonal data of the ZigBee signal; the in-phase orthogonal data is sampled to determine whether the phase shift between two consecutive samples is positive, if so, a code chip '1' is output, if not, a code chip '0' is output; the obtained code chips are combined to obtain multiple code chip sequences, and the code chip sequence with the smallest Hamming distance from 16 possible code chip sequences specified in the ZigBee protocol is selected, and every 32 consecutive code chips in the selected code chip sequence are mapped to a data symbol according to a predefined mapping table, and each data symbol corresponds to a backscatter codeword; the backscatter codeword includes the pre-reflection codeword and the post-reflection codeword; the tag data is obtained according to the pre-reflection codeword and the post-reflection codeword.

2. The backscattering method of the ZigBee excitation source suitable for an uncontrolled environment according to claim 1, characterized in that: The tag modulation sequence is a sequence composed of multiple tag modulation operations. When the tag modulation operation is '1', the phase difference between two consecutive sampling points in the carrier can be flipped. When the tag modulation operation is '0', the phase difference between two consecutive sampling points in the carrier can be kept unchanged.

3. The backscattering method of the ZigBee excitation source suitable for an uncontrolled environment according to claim 1, characterized in that: Step 1 specifically includes: the ZigBee transmitter maps binary data into data symbols, maps each data symbol into a chip sequence using direct sequence spread spectrum, modulates each chip sequence onto a carrier using offset quadrature phase shift keying modulation technology with half-sine pulse shaping, and sends it as an excitation signal; wherein, the use of offset quadrature phase shift keying modulation technology with half-sine pulse shaping to modulate each chip sequence onto a carrier specifically includes: During the modulation process, the chips with even indexes in the chip sequence modulate the in-phase carrier, and the chips with odd indexes in the chip sequence modulate the quadrature-phase carrier; the duration of each chip is recorded as In order to achieve the time offset between the in-phase carrier and the quadrature-phase carrier, the code corresponding to the quadrature-phase carrier is delayed compared to the code corresponding to the in-phase carrier. .

4. The backscattering method of the ZigBee excitation source suitable for an uncontrolled environment according to claim 1, characterized in that: In step 2, generating 16 label modulation sequences for 4-bit label data specifically includes the following steps: S21, remove duplication: the 16 tag modulation sequences can convert each data symbol into 16 different data symbols; there are 16 data symbols in the Zigbee protocol, corresponding to 16 code words respectively; therefore, the 16 tag modulation sequences need to realize 256 code word conversions; after removing the code word unchanged, code word equivalent and code word repeated cases from the 256 code word conversions, 58 different non-equivalent code word conversions are obtained; S22, using a sequence consisting of all 0s and all 1s as a label modulation sequence respectively, which can realize two different non-equivalent codeword conversions, and the remaining 14 label modulation sequences need to realize the remaining 56 different non-equivalent codeword conversions; The first round of grouping: each non-equivalent codeword conversion corresponds to a difference sequence, and the difference sequences corresponding to the 56 non-equivalent codewords are represented by data symbol pairs, and the 56 difference sequences are divided into two groups according to symmetry, namely the first group and the second group, with 28 difference sequences in each group; if the difference sequences of the two groups show symmetry at corresponding positions, then when a label modulation sequence can realize the non-equivalent codeword conversion corresponding to a difference sequence of the first group, the symmetric sequence of the label modulation sequence can realize the non-equivalent codeword conversion corresponding to the difference sequence at the corresponding position of the second group, so that 7 label modulation sequences can be obtained from the first group; Second round of grouping: The 28 difference sequences in the first group were evenly divided into 7 second-level groups, each of which contained four difference sequences; S23, determine the label modulation sequence; for each second-level group, minimize the sum of the Hamming distances between the target label modulation sequence and the four difference sequences contained in the second-level group, so as to obtain 7 label modulation sequences corresponding to the 7 second-level groups, specifically including: determining the value of each bit of the target label modulation sequence according to the frequency of each bit in the four difference sequences in the second-level group; if the number of bits with the target label modulation sequence value of '0' is equal to the number of bits with the value of '1', then invert any one of the four difference sequences in the second-level group.

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