A transmission method for an LTE backscatter system based on subcarrier cyclic shift

Abstract

The application discloses a transmission method of an LTE backscatter system based on subcarrier cyclic shift, and belongs to the technical field of wireless communication and backscatter communication. The application comprises the following steps: an LTE signal source generates an environmental carrier; tag data is mapped into a cyclic shift index, a time domain linear phase sequence is generated, and the whole frequency domain subcarrier is cyclically shifted; a single receiver receives a backscatter frequency band signal, an effective subcarrier and an empty subcarrier mask are constructed, a comprehensive score is calculated for all candidate cyclic shifts, the highest score shift is directly output or an adaptive candidate set is generated, the cyclic shift of each symbol is determined and is mapped into a tag bit; inverse cyclic shift compensation is performed on the detected shift, and environmental LTE data is output. The application realizes single-receiver, high-throughput, high-reliability tag communication under the premise of maintaining the integrity of the LTE signal structure and the compatibility of the standard communication process, and is suitable for various LTE bandwidths, modulation modes and channel environments.

Description

Technical Field

[0001] This invention relates to the fields of wireless communication and backscatter communication technology, and specifically to a transmission method for an LTE backscatter system based on subcarrier cyclic shifting. Background Technology

[0002] Environmental backscatter communication reflects and modulates existing wireless signals in the environment by changing the load impedance of the tag antenna, so that the tag does not need to actively generate a high-power carrier, thus having the advantages of low power consumption, low cost and easy large-scale deployment.

[0003] LTE (Long Term Evolution) networks are characterized by wide coverage, relatively continuous downlink signals, well-developed infrastructure, and abundant operating frequency bands, making them suitable for wide-area low-power backscatter communication. LTE downlink signals employ OFDM (Orthogonal Frequency-Division Multiplexing) modulation. A radio frame is 10 ms long, consisting of 10 1ms subframes. Under normal cyclic prefix configuration, each subframe contains two time slots, each containing seven OFDM symbols. The Physical Downlink Shared Channel (PDSCH) carries the primary downlink user data, thus providing abundant modulated time resources for tags.

[0004] However, existing LTE backscattering schemes typically struggle to simultaneously meet the requirements of high throughput, LTE signal structure compatibility, and single-receiver demodulation. Firstly, there are codeword-based modulation methods. While these methods preserve the LTE waveform structure, each LTE symbol can only carry a fixed and limited amount of tag data. Representative systems employing this type of modulation include SyncLTE and CABLTE. SyncLTE requires two synchronous receivers to acquire the original signal and the backscattered signal respectively. CABLTE achieves single-receiver demodulation by enumerating candidates and utilizing LTE's own cyclic redundancy check (CRC). However, this method requires forcibly disabling retransmission during LTE decoding, affecting the transmission of the original LTE data, reducing compatibility with standard LTE procedures, and also has a large search space and high computational complexity. Secondly, there are R2Scatter systems employing finer-grained time-domain modulation. This system shifts half of the time-domain signal to another frequency band and determines the start position of the shift through signal matching to encode tag data. This encoding method can improve tag rate but destroys the complete OFDM symbol structure, causing spectral leakage, and still requires two receivers for decoding.

[0005] The structure of LTE signals also presents two unique problems. First, only a portion of the physical downlink shared channel (PHS) symbols contain a known reference signal (RS), approximately 31.2%. The vast majority of the remaining PHS symbols carry unknown LTE data, making it impossible for a single receiver to directly use the original content as a reference. If the tag only modulates the reference signal, it will waste a significant amount of PHS resources and limit throughput. Second, the normal cyclic prefix in LTE accounts for only about 7% to 8% of the effective symbol length, resulting in fewer repeated sampling points available for correlation detection. Directly using cyclic shift identification methods based on cyclic prefixes can easily lead to a high bit error rate.

[0006] Furthermore, under conditions of low signal-to-noise ratio, multipath fading, or higher-order physical downlink shared channel modulation, the energy difference between effective subcarriers and unused subcarriers weakens, causing multiple cyclic shift candidates to achieve similar scores. Using the highest-scoring candidate indefinitely results in insufficient reliability; exhaustively searching all possible shifts and their combinations leads to unacceptable time complexity and storage overhead. Therefore, a novel method is needed that preserves the LTE OFDM structure, utilizes all physical downlink shared channel symbols, supports single-receiver demodulation, and performs demodulation with lower search complexity under low signal quality conditions. Summary of the Invention

[0007] To address the problem that existing LTE backscatter systems struggle to simultaneously achieve tag transmission throughput, LTE signal compatibility, and receiver complexity, this invention aims to provide an LTE backscatter system transmission method that can simultaneously meet the three conditions of high throughput, LTE signal structure compatibility, and single-receiver demodulation.

[0008] In this invention, the tag maps tag data to a cyclic shift index. A linear phase sequence corresponding to the index is applied to the OFDM symbols in the time domain, causing a cyclic shift of the entire frequency domain subcarrier. The receiver estimates the cyclic shift based on the energy distribution of the effective subcarriers, DC subcarriers, and empty subcarriers. At high confidence levels, the highest-scoring result is directly received; at low confidence levels, an adaptive candidate set is constructed, and reference signal symbols are used as anchor points for group verification and priority combination search to recover the tag data. After tag demodulation, the receiver performs inverse compensation on the detected cyclic shift and continues with the standard LTE decoding process to recover the environmental LTE data.

[0009] Through the above scheme, the present invention increases the number of tag data that each OFDM symbol can carry without destroying the LTE signal structure and OFDM orthogonality, and without disabling the LTE retransmission mechanism, thereby achieving tag communication with a single receiver, high throughput and high reliability. At the same time, the receiver can restore the original LTE resource grid by reverse cyclic shifting after tag demodulation, thereby simultaneously completing the decoding of tag data and environmental LTE data.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A transmission method for an LTE backscatter system based on subcarrier cyclic shifting, comprising: The LTE signal source generates an OFDM-modulated downlink signal as an ambient carrier; The tag maps tag data to a cyclic shift index, generates a linear phase sequence based on the cyclic shift index, and continuously applies the time-domain linear phase sequence to the cyclic prefix and valid symbols of the specified OFDM symbols in the ambient carrier to generate a backscatter signal. At the same time, a fixed frequency offset is superimposed to shift the backscatter signal to the target frequency band. A single receiver receives the backscattered signal of the target frequency band, constructs an effective subcarrier mask and an empty subcarrier mask based on LTE resource configuration, calculates a comprehensive score reflecting the energy distribution of the effective subcarrier area and the empty subcarrier area for all candidate cyclic shift indices, selects to directly output the cyclic shift index corresponding to the highest comprehensive score based on the confidence level of the comprehensive score, or generates an adaptive candidate set, and uses OFDM symbols containing reference signals as anchor points to perform group verification and priority combination search to determine the cyclic shift index; The determined cyclic shift index is mapped to tag data; the cyclic shift index is compensated by inverse cyclic shift, and standard LTE decoding is performed to output LTE data in the ambient carrier.

[0011] In one embodiment, the label maps label data to a cyclic shift index, specifically including: Tags can be used for each group Each tag data is mapped to a circular shift index. ,in, ,and The mapping uses Gray code encoding, so that the bit codewords corresponding to adjacent cyclic shift indices differ by only one bit; This represents the total number of circular shift indices.

[0012] In one embodiment, the step of generating a linear phase sequence based on a cyclic shift index and continuously applying the time-domain linear phase sequence to the cyclic prefix and valid symbols of the specified OFDM symbols in the ambient carrier to generate a backscatter signal specifically includes: Let the cyclic shift index be d, and the time-domain sampling point index be n, n=0,1,…,N-1. The total number of cyclic shift indices generates a linear phase sequence. , The imaginary unit is used; the label multiplies the time-domain effective symbol x[n] of the specified OFDM symbol in the ambient carrier with the linear phase sequence point by point to obtain the modulated baseband signal. , This is the backscattered signal. Frequency domain representation X[k] is the frequency domain subcarrier of the OFDM symbol before modulation. This is the frequency domain subcarrier index.

[0013] In one embodiment, the superimposed fixed frequency offset shifts the backscattered signal to the target frequency band, specifically including: Let the fixed frequency offset be... Sampling rate Then the complete modulation sequence , It is a linear phase modulation. This is for frequency band shifting.

[0014] In one embodiment, the calculation of the comprehensive score reflecting the energy distribution of the effective subcarrier region and the empty subcarrier region specifically includes: For each candidate cyclic shift index d, calculate the average energy of the effective subcarrier region. and average energy of empty subcarrier region ,in, These are the k1th effective subcarrier and the k2th empty subcarrier, respectively. For the index of the effective subcarrier, The index of the empty subcarrier. The set of effective subcarriers, The set of empty subcarriers; construct the energy ratio between the effective subcarrier region and the empty subcarrier region. Where ε is a positive number used to avoid numerical instability when the energy of empty subcarrier regions approaches zero; all subcarrier energies are sorted in descending order, and the top L subcarriers with the highest energy are selected to form a set. Calculate the consistency of energy ranking Overall score , , For weights.

[0015] In one embodiment, the step of selecting the cyclic shift index corresponding to the highest comprehensive score directly based on the confidence level of the comprehensive score, or generating an adaptive candidate set, specifically includes: Sort all candidate circular shift indices in descending order of their overall scores: ; and These are the highest and second-highest overall scores, This is the overall score ranked Nth in descending order; Indicates the total number of circular shift indices; Calculate confidence level ,in Let be the standard deviation of the overall score of all candidate circular shift indices, and ε be a positive number to prevent the denominator from being zero; when When the highest overall score is reached, the circular shift index corresponding to it is directly output; when... At that time, an adaptive candidate set is generated, in which To pre-set the reliability threshold.

[0016] In one embodiment, generating the adaptive candidate set specifically includes: Robust normalization of the composite score is performed using the median function `median()` and the median absolute deviation (MAD) to obtain the normalized score. ;Will Mapped to candidate probabilities using the softmax function , where η represents a hyperparameter used to control the degree of concentration of the probability distribution; Candidate cyclic shift index The normalized score; the candidate cyclic shift indices are sorted in descending order of candidate probability, and those that meet the requirements are selected. The minimum variable K is composed of the cyclic shift indices of the K candidates with the highest candidate probabilities, forming an adaptive candidate set, where ρ is the target probability coverage.

[0017] In one embodiment, the group verification specifically includes: dividing the OFDM symbols without reference signals into multiple groups based on the original position of the OFDM symbols with reference signals in the LTE frame, with each group containing an anchor symbol with reference signals and several symbols without reference signals following the anchor symbol; For anchor symbols containing reference signals, inverse compensation is performed on the candidate cyclic shift index d, and the reference signal vector is extracted. The normalized similarity is calculated with the local known reference signal r, and the candidate cyclic shift index with the highest normalized similarity is used as the anchor point for shifting. Within the group, there is no reference signal symbol carrying the tag data. The bit string of the tag data is concatenated to generate a check code, which is carried by the anchor symbol containing the reference signal within the group. The receiving end recovers the check code based on the anchor shift. And recalculate the check code for each candidate cyclic shift index combination of OFDM symbols without reference signals within the group, only if the recalculated check code matches... If they match, the candidate cyclic shift index combination of each OFDM symbol without a reference signal in the group is considered to have been successfully verified.

[0018] In one embodiment, the preferred combination search specifically includes: assuming a group contains Q no-reference symbols, and the ordered candidate list for the j-th no-reference symbol is... , yes The top-ranked candidate circular shift index. yes The number of candidate circular shift indices; For each unreferenced signal symbol, select one candidate cyclic shift index from the ordered candidate list to form a combination, which is then ranked using a rank vector. express, This represents the rank of the j-th unreferenced signal symbol. Represents the number of unreferenced signal symbols; defines the combination cost. : ; A smaller combination cost indicates a higher probability that the circular shift index of each candidate in the combination is the correct combination; using a min-heap... Output combinations in ascending order: With the initial ranking vector Initialize a min-heap, pop the ranking vector with the minimum combined cost, and perform the grouping verification. If the verification fails, select one dimension from the current ranking vector, increment its ranking by 1, and generate an adjacency vector. Calculate the combination cost, add it to the min-heap, and repeat until a successfully verified combination is found or the min-heap is empty; Let be a j-dimensional unit vector.

[0019] In one embodiment, the step of performing inverse cyclic shift compensation on the cyclic shift index and performing standard LTE decoding to output LTE data in the ambient carrier specifically includes: The receiver performs an inverse cyclic shift on the OFDM symbol based on the cyclic shift index d detected for each OFDM symbol to restore the original LTE resource grid. The restored LTE resource grid is then fed into the standard LTE channel estimation, equalization, soft demodulation, channel decoding and cyclic redundancy check process to output LTE data in the ambient carrier.

[0020] Compared with the prior art, the beneficial technical effects of the present invention are: 1. Improve tag transmission capability and decoding reliability while reducing system deployment complexity. This invention maps tag data to a cyclic shift index, allowing each OFDM symbol to be encoded. Compared to existing codeword conversion methods that can only encode 1-2 bits per symbol, this invention significantly improves the information carrying capacity of a single symbol by increasing the coding capacity logarithmically with the number of Fast Fourier Transform points. At the demodulation end, this invention utilizes the characteristic of cyclic shifting to maintain the overall structure of effective and empty subcarriers in the OFDM frequency domain, and performs blind detection based on energy distribution scoring. Compared to methods relying on time-domain waveform matching, frequency-domain energy detection has stronger anti-interference capabilities against noise and multipath interference. This invention requires only a single receiver to complete tag demodulation and environmental LTE data recovery, eliminating the need for a second receiver to provide a reference signal or synchronously capture the original signal, resulting in a simpler system architecture and lower deployment costs.

[0021] 2. Maintaining the integrity of the LTE signal structure and compatibility with standard communication procedures. This invention applies a linear phase sequence in the time domain to achieve frequency domain cyclic shifting, shifting the frequency domain subcarriers as a whole while keeping the structural relationships of effective subcarriers, empty subcarriers, and DC subcarriers unchanged. It does not disrupt the orthogonality of OFDM symbols and does not generate spectral leakage. After the receiver completes tag shift detection, the original LTE resource grid can be completely restored by performing an inverse cyclic shift on the frequency domain receive vector. No modifications are needed to the synchronization process, control channel resolution, channel estimation and equalization, soft demodulation, decoding, or Hybrid Automatic Repeat Request (HARQ). It can be directly fed into the standard LTE receive link and can synchronously output environmental LTE data, ensuring that existing LTE communication links are unaffected.

[0022] 3. Possesses adaptive decoding capability and environmental universality. This invention divides demodulation into two paths—high-confidence direct reception and low-confidence candidate verification—through confidence assessment. The adaptive candidate set dynamically adjusts the number of candidates based on the concentration of the scoring probability distribution, ensuring a balance between decoding complexity and reliability under different signal qualities. Energy distribution scoring only requires knowledge of the positions of effective and empty subcarriers, without needing to know the specific modulation scheme and data content of the physical downlink shared channel. Therefore, it can adapt to different modulation orders such as Quadrature Phase Shift Keying (QPSK), 16th-order Quadrature Amplitude Modulation (16QAM), and 64th-order Quadrature Amplitude Modulation (64QAM). Reference signal-assisted group verification utilizes the native LTE reference signal structure, does not depend on specific subframe configurations, can adapt to different bandwidths and transmission modes, and is suitable for multipath fading channels and different Doppler frequency conditions, exhibiting good scenario universality. Attached Figure Description

[0023] Figure 1 This is a system architecture diagram of the present invention.

[0024] Figure 2 This is a flowchart of the overall tag modulation process of the present invention.

[0025] Figure 3 This is a diagram of the LTE signal frame structure of the present invention.

[0026] Figure 4 This is a diagram showing the energy structure of effective subcarriers and empty subcarriers before and after the cyclic shift of the present invention.

[0027] Figure 5 This is a schematic diagram of the decoding path selection based on confidence estimation according to the present invention.

[0028] Figure 6 This is a schematic diagram of the reference signal auxiliary group verification structure of the present invention.

[0029] Figure 7 This is a complete label demodulation flowchart of the present invention.

[0030] Figure 8 This is a comparison chart of the bit error rate of tag data between the present invention and existing systems.

[0031] Figure 9 This is a comparison chart of the tag data throughput of the present invention and existing systems.

[0032] Figure 10 This is a comparison chart of the bit error rate of LTE data decoding under different modulation schemes according to the present invention.

[0033] Figure 11 This is a comparison chart of the throughput of LTE data transmission under different modulation methods according to the present invention.

[0034] Figure 12 This is a comparison chart of the bit error rate of LTE data decoding under different bandwidths and modulation methods according to the present invention.

[0035] Figure 13 This is a comparison chart of the throughput of LTE data transmission under different bandwidths and modulation methods according to the present invention.

[0036] Figure 14 This is a comparison chart of the throughput of tag data transmission and the bit error rate of decoding under different bandwidths according to the present invention.

[0037] Figure 15 This is a comparison chart of the bit error rate of tag data decoding under different modulation methods of the present invention.

[0038] Figure 16 This is a comparison chart of the bit error rate of tag data decoding under different channel models according to the present invention.

[0039] Figure 17 This is a comparison chart of the bit error rate of tag data decoding at different Doppler frequencies according to the present invention.

[0040] Figure 18 This is an overall flowchart of the method of the present invention. Detailed Implementation

[0041] A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0042] like Figure 18 As shown, an LTE backscattering system transmission method based on subcarrier cyclic shifting according to the present invention includes the following steps: S1, the LTE signal source generates an OFDM symbol modulated downlink signal as an ambient carrier; S2, a time-domain linear phase sequence is continuously applied to the cyclic prefix and effective symbols of the OFDM symbols specified in the background carrier to generate a backscatter signal, and a fixed frequency offset is superimposed to shift the backscatter signal to the target frequency band. S3: A single receiver receives the backscattered signal of the target frequency band, constructs an effective subcarrier mask and an empty subcarrier mask based on LTE resource configuration, calculates a comprehensive score reflecting the energy distribution of the effective subcarrier area and the empty subcarrier area for all candidate cyclic shift indices, selects to directly output the cyclic shift index corresponding to the highest comprehensive score based on the confidence level of the comprehensive score, or generates an adaptive candidate set, and uses OFDM symbols containing reference signals as anchor points to perform group verification and priority combination search to determine the cyclic shift index; S4. Map the determined cyclic shift index to tag data; perform inverse cyclic shift compensation on the cyclic shift index, and perform standard LTE decoding to output LTE data in the ambient carrier.

[0043] The following sections will first introduce the basic structure and transmission process of the LTE backscatter system, then introduce the tag modulation method based on subcarrier cyclic shift, and finally introduce the single-receiver data demodulation method based on subcarrier energy distribution and reference signal-assisted optimization.

[0044] 1. System composition.

[0045] like Figure 1As shown, the system of this invention includes an LTE signal source, a tag, and a receiver. The main function of the LTE signal source is to generate a downlink LTE signal, providing an ambient carrier for the tag. The device generating the LTE signal can be an LTE base station, a software-defined radio transmitter, or other devices capable of generating LTE downlink OFDM waveforms. The tag applies linear phase modulation to a specified OFDM symbol in the carrier signal, generating a subcarrier cyclic shift corresponding to the tag data, and shifting the backscattered signal to the target frequency band. The receiver performs tag data demodulation and ambient LTE data recovery, and outputs the corresponding data. This operation can be performed by an LTE user equipment with physical layer data access capabilities or a software-defined radio receiver.

[0046] 2. Tag modulation based on subcarrier cyclic shift.

[0047] like Figure 2 As shown, the entire tag modulation process includes three steps: mapping tag data to a cyclic shift index, using linear phase modulation to achieve the corresponding subcarrier shift, and converting the backscattered signal to another frequency band to avoid self-interference.

[0048] (1) Mapping of tag data to circular shift index: Suppose the number of Fast Fourier Transform points for a given LTE OFDM symbol to be modulated is . The total number of frequency domain subcarriers and the total number of cyclic shift indices are also... Because frequency domain vectors are shared Different integer cyclic shifts, labels can be used for each group Each bit is mapped to a cyclic shift index. ,in ,and: ; This invention uses Gray code to establish the correspondence between tag data and cyclic shift indices, ensuring that codewords corresponding to adjacent shift indices differ by only one bit. When noise causes the estimation result to fall into adjacent shifts, the number of errors in the corresponding tag data can be reduced. For example, when the data to be transmitted by the tag is '0000011', according to the encoding principle of Gray code, the cyclic shift index... .

[0049] (2) Time-domain linear phase modulation: like Figure 3As shown, the LTE frame structure includes various physical signals and channels, such as a control region, synchronization signal, reference signal, and physical downlink shared channel (OFDM). The control region is mainly located at the beginning of each subframe and carries the control information required for resource scheduling, transmission configuration, and data decoding. The synchronization signal is used to identify and synchronize LTE signals, maintaining the standard control information parsing and resource scheduling process. The reference signal is used for channel estimation and equalization. To ensure compatibility with the standard LTE reception process, the tag does not perform data correlation modulation on the control region and synchronization signal, but only modulates the selected physical downlink shared channel OFDM symbols. Therefore, the receiver can still use the standard LTE decoding process to obtain the signal control information and perform signal synchronization. Figure 3 An LTE frame is 10 ms long and contains 10 subframes, each 1 ms long. Each subframe contains two 0.5ms time slots. Under normal cyclic prefix configuration, each time slot contains 7 OFDM symbols, so each subframe contains a total of 14 OFDM symbols. These symbols have different functions: there are control regions responsible for resource scheduling, synchronization signals responsible for signal synchronization, physical downlink shared channel symbols carrying data, and reference signals for channel estimation. Figure 3 The structures of subframes 0, 5, and 9 are shown. The structures of the other unshown subframes are the same as those of subframe 9.

[0050] Let the frequency domain subcarrier of the selected physical downlink shared channel OFDM symbol be... , Its effective time-domain symbol is The label is based on the circular shift index. Generate linear phase sequence : ; Where n is the index value of the time-domain sampling point, The tag multiplies the incident LTE time-domain signal with the linear phase sequence sample by sample to obtain the modulated backscattered baseband signal. : ; Based on the cyclic shift property of the Discrete Fourier Transform, the frequency domain expression of the modulated signal is: .

[0051] Therefore, the tag does not need to change the original complex symbol carried by each subcarrier; all frequency-domain subcarriers can be cyclically shifted according to the same shift index simply through time-domain linear phase modulation. Cyclic shifting preserves the OFDM frequency-domain structure, with the effective subcarrier, DC subcarrier, and the empty subcarriers on both sides moving as a whole. This facilitates detection by the receiver based on structural characteristics without knowing the original payload of the physical downlink shared channel.

[0052] Preferably, the tag continuously applies the same linear phase pattern to the cyclic prefix (CP) and the valid OFDM samples, ensuring that the modulated cyclic prefix maintains a copy relationship with the end of the valid symbol. This preserves standard cyclic prefix removal and Fast Fourier Transform processing, retains the orthogonal structure of OFDM, and utilizes the cyclic prefix's tolerance to multipath delays and moderate timing errors.

[0053] (3) Reverse dispersion frequency band shifting: To reduce co-channel interference between strong direct LTE signals and weak backscattered signals, the tag is superimposed with a fixed frequency offset in addition to the aforementioned data-correlated linear phase sequence. The entire backscattered signal is shifted to the preset target frequency band. The complete modulation sequence can be represented as: ; in The sampling rate is denoted by . The first term is used to generate a data-related subcarrier cyclic shift within the original LTE bandwidth, while the second term is used to shift the modulated complete spectrum to the backscattering operating band. These two functions are independent of each other. Band shifting can be achieved through switching mixers, periodic impedance switching, digitally controlled oscillator-driven RF switches, or other equivalent methods.

[0054] 3. Shift detection and tag decoding based on subcarrier energy distribution.

[0055] (1) LTE synchronization, cyclic prefix removal and fast Fourier transform: The receiver first follows the standard LTE reception procedure, using synchronization symbols to complete cell search, frame synchronization, symbol timing, and frequency offset correction; then it down-converts the reverse-scattered frequency band, removes the cyclic prefix, and performs... Point Fast Fourier Transform (FFT) is used to obtain the frequency domain receive vector of each physical downlink shared channel OFDM symbol to be demodulated. The additional processing module of this invention is located between the standard LTE Fast Fourier Transform and the subsequent physical downlink shared channel decoding, and does not replace the existing LTE synchronization and channel decoding module.

[0056] (2) Construction of effective subcarrier mask and empty subcarrier mask: Based on the LTE bandwidth and resource mapping configuration, the set of effective subcarriers when no tag shift occurs is determined in advance. and sets of empty subcarriers and DC subcarriers For candidate circular shift indices Construct the corresponding shift mask: ; ; The correct candidate cyclic shift index should make the main energy in the received signal equal to... Overlapping, and It mainly contains noise and a small amount of spectral leakage. For example... Figure 4 As shown, since cyclic shift only changes the subcarrier position without destroying the overall structure of the effective subcarrier and the empty subcarrier, this criterion can be used for all physical downlink shared channel OFDM symbols, including physical downlink shared channel OFDM symbols with and without reference signals.

[0057] (3) Energy ratio score between effective subcarrier region and empty subcarrier region: Circular shift index for each candidate Calculate the average received energy for the effective subcarrier region and the empty subcarrier region after shifting, respectively: ; .

[0058] Further construct the active-to-null energy ratio (ANR): ; in, It is a small positive number used to avoid numerical instability when the energy in the empty subcarrier region is close to zero. The larger the value, the more consistent the candidate mask is with the distribution of effective subcarriers and empty subcarriers of the received signal.

[0059] (4) Consistency score of energy ranking: To mitigate the impact of deep fading, anomalous noise, or high-order modulation amplitude variations in individual subcarriers, the receiver further utilizes the positional relationships of high-energy subcarriers. Let... All subcarrier energy Sort by high to low, and take the first few. A set of high-energy subcarriers Preferably, Take the number of effective subcarriers Candidate cyclic shift indices The Energy-Rank Consistency (ERC) score is: ; This score reflects the proportion of high-energy subcarriers falling into the candidate active region and can complement the average energy ratio. Even if the radio channel changes the absolute amplitude of each subcarrier, as long as the majority of effective subcarriers still have higher energy than the empty subcarriers, It can still provide an effective distinction.

[0060] (5) Overall energy distribution score and initial results: The receiving end respectively to and Normalization is performed on all candidate cyclic shift indices to obtain scores with the same dimensions, and then the subcarrier energy distribution (ED) scores are synthesized according to weights: ; In a preferred embodiment The weights can also be adaptively adjusted based on bandwidth, physical downlink shared channel modulation scheme, signal-to-noise ratio, or historical statistical results.

[0061] The cyclic shift index of the highest-scoring candidate is the initial cyclic shift estimate: ; Since the cyclic shift indices of different candidates only correspond to the cyclic shifts of the effective subcarrier and the empty subcarrier mask, the energy summation can be performed using a sliding update without having to re-traverse all cyclic shift indices for each candidate. Subcarriers, thus reducing the scoring computation complexity from Reduced to approximately The sorting process is the main overhead.

[0062] 4. A method for enhancing decoding reliability based on reference signal assistance.

[0063] (1) Confidence assessment: like Figure 5 As shown, the receiver uses a confidence index to measure the distinguishability between the circular shift index of the highest-scoring candidate and the circular shift indexes of other candidates. All scores are sorted in descending order as follows: ,in and These are the highest and second-highest scores, respectively. Define the normalized score interval: ; in, The standard deviation of the scores for all candidate circular shift indices. When Not less than the preset threshold When the circular shift index of the highest-scoring candidate is considered sufficiently distinguishable from the circular shift indices of other candidates, the circular shift index of the highest-scoring candidate is directly accepted without entering the candidate search; when Less than Then, the subsequent adaptive candidate set generation and reference signal-assisted recovery steps are performed. The threshold can be calibrated offline to ensure that the target bit error rate of the directly received symbols is lower than a predetermined value of 1‰. According to experimental results, for bandwidths of 1.4 MHz, 3 MHz, and 5 MHz, The preferred values ​​are 0.5, 0.5 and 0.15, respectively.

[0064] (2) Adaptive candidate set generation based on probability distribution: For low-confidence symbols, the receiver does not consistently use the same number of candidate cyclic shift indices. Instead, it converts the scores into probabilities and retains the smallest candidate set that achieves the target cumulative coverage. First, it robustly normalizes the scores using the median function `median()` and the Median Absolute Deviation (MAD): ; The candidate probabilities are then obtained by mapping using the softmax function: ; in The concentration of the probability distribution can be adjusted according to the bandwidth and the size of the candidate space. Preferably, the values ​​are 9, 12, and 15 at bandwidths of 1.4 MHz, 3 MHz, and 5 MHz, respectively.

[0065] Arrange the candidate circular shift indices according to Arrange them in descending order, and select the smallest K that satisfies the following formula: ; in For the first The probability of a large candidate set. The target probability coverage is preferably 0.995. When the signal quality is good, the scoring probability distribution is relatively concentrated, requiring only a few candidate cyclic shift indices to achieve the target cumulative coverage. When the signal quality is poor, the probability distribution tends to flatten, and the candidate set automatically increases to improve the probability of correct shifts being retained. Therefore, this module achieves an adaptive balance between reliability and subsequent search complexity.

[0066] (3) Reference signal group verification: Under low signal-to-noise ratio (SNR) conditions, although the candidate cyclic shift index with the highest score may not be the correct result, the correct shift usually still ranks high among the candidate cyclic shift indices. Furthermore, physical downlink shared channel (PHS) OFDM symbols containing reference signals have known complex reference values, allowing for matching using both amplitude and phase, resulting in higher discrimination capability than methods relying solely on subcarrier energy distribution. Therefore, this invention utilizes symbols containing reference signals (i.e., OFDM symbols containing reference signals in the PHS) as anchor points to perform group verification of candidate cyclic shift index combinations for adjacent symbols without reference signals (i.e., OFDM symbols without reference signals in the PHS).

[0067] Reference signal anchor point shift extraction. For symbols containing a reference signal, the receiver performs a cyclic shift index for each candidate. Perform inverse cyclic shift compensation and extract the compensated reference signal vector. The normalized similarity is calculated between the signal and the known reference signal sequence r. The anchor point shift is selected as follows: ; Because the known reference signal provides both amplitude and phase constraints, the anchor point shift is generally more reliable than simple energy distribution scoring. The tag codewords derived from the anchor point carry verification information for the corresponding group.

[0068] Packet verification based on the distribution of native LTE reference signals. For example... Figure 6 As shown, the receiver divides the no-reference-signal symbols into multiple groups based on the native location of the physical downlink shared channel (OFDM) symbols carrying reference signals in the LTE frame. Each group... Includes an anchor symbol containing a reference signal. and several subsequent sets of referenceless signal data symbols Different subframes contain different numbers and locations of physical downlink shared channels. A preferred grouping method is as follows: Subframe 0: ; Subframe 5: ; Other subframes: .

[0069] The numbers represent the OFDM symbol numbers within a subframe. Actual packets can be dynamically generated based on LTE bandwidth, control area length, reference signal port, transmission mode, and resource mapping, and are not limited to the fixed numbers mentioned above.

[0070] At the tag encoding end, no reference signal symbol is used within the group to carry the tag's payload; the corresponding bit strings are concatenated to generate the block check code. The reference signal anchor symbol carries the check code, which is preferably a cyclic redundancy check (CRC) check. Here, it's a packet CRC check set on the tag side, not the native LTE physical downlink shared channel CRC check. The receiver recovers the packet check code based on the tag codeword corresponding to the anchor shift. And recalculate the checksum for the tag data corresponding to the candidate cyclic shift index combinations without reference signal symbols within the group; only if the calculation result is consistent with... When the match is consistent, the combination is considered valid.

[0071] If there is a total Each no-reference signal symbol needs to be verified, and each symbol can retain a maximum of [number missing]. With 10 candidate cyclic shift indices, the number of combinations for joint search of the entire subframe can reach 100,000. After the above grouping, each group contains at most 3 no-reference signal symbols, and the search complexity can be expressed as: The upper bound of the number of candidates is Time not exceeding Therefore, the search size changes from growing exponentially with the total number of uncertain symbols to growing approximately linearly with the number of groups. This represents the number of groups in the candidate set to be validated within a given group. This represents the number of combinations to be searched without grouping.

[0072] Prioritize combinational search. To avoid exhaustively searching the Cartesian product of the candidate list in each group, this invention prioritizes checking the most likely combinations based on the cyclic shift index ranking of the candidates. Suppose a group contains... The first no-reference signal symbol, the... The ordered candidate list of symbols is ,in It is ranked as The candidate cyclic shift terms, It is the first The number of candidate sets for each symbol. A combination is used with a ranking vector. Represent and define the combination cost: ; The lower the cost, the closer the cyclic shift index of each candidate in the combination is to the top of its respective list, and the higher the probability of it becoming the correct combination.

[0073] Prioritize combined search by Sorted min-heaps specifically include: D1, with Initialize the min-heap and the visited set; D2: Pop the ranking vector with the minimum cost from the min-heap, construct the corresponding shift combination, and perform group verification. D3. If the verification passes, output the combination and terminate the search for this group. D4. If the validation fails, increment the rank of each dimension by 1 and generate an adjacency vector. ,in for A unit vector of dimension; D5, for adjacent vectors that have not crossed the boundary of the corresponding candidate list and have not been visited, calculate the cost and add them to the min-heap; D6. Repeat steps D2 to D5 until a valid combination is found or the min-heap is empty.

[0074] This search order prioritizes combinations where all candidate circular shift indices rank highly, typically finding the correct result with fewer validations and avoiding redundant expansion by accessing already visited sets.

[0075] 5. Complete tag demodulation and environmental LTE data recovery.

[0076] The complete receiving process is as follows: Figure 7 As shown, the following steps may be included: It receives backscattered signals and performs LTE frame synchronization, cyclic prefix removal, and fast Fourier transform. Based on LTE resource configuration, establish effective subcarrier and empty subcarrier / DC subcarrier masks, and calculate energy distribution scores for all possible cyclic shifts; Calculate the score confidence; for high-confidence OFDM symbols, directly apply Top-1 cyclic shift, and for low-confidence OFDM symbols, generate an adaptive candidate set; Perform reference signal similarity matching on symbols containing reference signals to recover anchor point shift and block check codes; Symbols without reference signals are grouped according to the reference signal position, and candidate combination verification is performed within each group using a priority combination search. All recovered cyclic shift indices are mapped to tag data using inverse Gray code or the corresponding codebook; Perform a reverse cyclic shift on each physical downlink shared channel OFDM symbol based on the detected shift to restore the original LTE resource grid; The restored resource grid is fed into the standard LTE channel estimation, equalization, soft demodulation, channel decoding and cyclic redundancy check process, and outputs environmental LTE data.

[0077] The aforementioned tag demodulation module, as an additional processing module in the standard LTE receive chain, does not require modification of LTE synchronization, control channel resolution, physical downlink shared channel standard decoding, or hybrid automatic repeat request.

[0078] First, this invention demonstrates its advantages in tag transmission capability, LTE compatibility, and the number of receivers by making qualitative and quantitative comparisons with existing LTE backscatter systems. Second, this invention verifies its compatibility with standard LTE decoding procedures through environmental LTE data recovery results. Finally, this invention illustrates its applicability and robustness through experimental results under different LTE configurations and channel conditions.

[0079] Table 1 presents a qualitative comparison between the method of this invention (SCScatter) and existing LTE backscatter systems (SyncLTE, CABLTE, and R2Scatter systems). As can be seen from Table 1, the SyncLTE system can only encode 1-2 tag data per symbol and requires two receivers. While the CABLTE system eliminates the dependence on two receivers, its encoding capability is not improved, and it requires disabling LTE retransmission during decoding, which reduces compatibility with LTE transmission to some extent. Although the R2Scatter system improves tag encoding capability, its local time-domain frequency shifting operation destroys the complete OFDM symbol structure and causes spectral leakage, while still relying on two receivers for demodulation. In contrast, the method of this invention improves encoding capability while maintaining compatibility with LTE signals and can utilize a single receiver for decoding.

[0080] Table 1 Qualitative comparison between the present invention and existing systems

[0081] Figure 8 and 9 The comparison of the bit error rate (BER) and throughput of tag data decoding of different systems under different signal-to-noise ratios (SNR) is presented.

[0082] Compared to the SyncLTE system, the method of this invention (SCScatter) has a higher bit error rate. This is mainly because the SyncLTE system uses two receivers to capture the raw signal and the backscattered signal respectively, thereby obtaining additional reference information for tag demodulation. In contrast, the method of this invention (SCScatter) requires only a single receiver, making it more practical and easier to deploy. Furthermore, the method of this invention (SCScatter) encodes more tag data within each LTE symbol, achieving a theoretical throughput of 44.1 kbps, approximately 2.4 times that of the SyncLTE system (18.4 kbps).

[0083] Compared to the CABLTE system, the method of this invention (SCScatter) achieves a lower bit error rate and higher tag throughput. At a signal-to-noise ratio of 5 dB, the method of this invention (SCScatter) reduces the bit error rate (BER) from 11.44% to 5.65% and increases the throughput from 16.3 kbps to 41.6 kbps.

[0084] Compared to the R2Scatter system, the method of this invention (SCScatter) has a significantly lower BER. At a signal-to-noise ratio of 5 dB, the BER of the method of this invention (SCScatter) is only 5.65%, while the BER of the R2Scatter system is as high as 33.49%. This is because the R2Scatter system relies on time-domain waveform matching and is sensitive to noise; while the method of this invention (SCScatter) utilizes the frequency-domain structure maintained by cyclic shift modulation, which has stronger robustness under noisy conditions. Furthermore, the method of this invention (SCScatter) can achieve this performance using only a single receiver, while the R2Scatter system requires two receivers. The tag throughput of the method of this invention (SCScatter) is also approximately 1.16 times that of the R2Scatter system.

[0085] The subcarrier cyclic shift modulation method used in this invention maintains compatibility with both LTE and OFDM structures. After compensating for the shift caused by tag modulation, the standard LTE data decoding process can continue, thereby recovering the environmental LTE data. After the tag shift is identified, the receiver reconstructs the original resource grid through inverse cyclic shift. Figure 10 The paper demonstrates the bit error rate (BER) of LTE data decoding at 1.4 MHz with different modulation schemes, including QPSK, 16QAM, and 64QAM, on the physical downlink shared channel. When using QPSK modulation, the proposed method (SCScatter) achieves highly reliable data recovery, with the BER approaching zero at a signal-to-noise ratio (SNR) above 15 dB. As the modulation order increases, the BER of the recovered data also increases because higher-order modulations have closer constellation point spacing, making LTE decoding more sensitive to noise. Nevertheless, when the SNR exceeds 15 dB, the BER of the recovered data using 16QAM (Quadrature Amplitude Modulation) remains below 1%; and when the SNR exceeds 20 dB, the BER of the recovered data using 64QAM (64-ary Quadrature Amplitude Modulation) is also below 1%. Figure 11This invention demonstrates the LTE data transmission throughput at 1.4 MHz with different modulation schemes for the physical downlink shared channel, including QPSK, 16QAM, and 64QAM. The recovery environment data throughput for QPSK, 16QAM, and 64QAM reaches 1.25 Mbps, 2.5 Mbps, and 3.75 Mbps, respectively. The invention also evaluates the environment data recovery performance under different LTE bandwidths. Figure 12 The results show the bit error rate (BER) of LTE data decoding under different bandwidths (1.4 MHz, 3 MHz, 5 MHz) and different modulation schemes such as QPSK, 16QAM and 64QAM for the physical downlink shared channel when SNR=15 dB. The BER is similar under different bandwidths, and only increases slightly with the increase of bandwidth. Figure 13 The results demonstrate the LTE data transmission throughput at SNR=15 under different bandwidths (1.4 MHz, 3 MHz, 5 MHz) and with different modulation schemes (QPSK, 16QAM, and 64QAM) on the physical downlink shared channel. The environmental data throughput significantly increases with the increase of available LTE bandwidth. Specifically, under the 5 MHz and 64QAM configurations, the method of the present invention (SCScatter) can achieve a throughput of up to 15.6 Mbps. Overall, these results show that the method of the present invention (SCScatter) can reliably recover environmental LTE data under different modulation schemes and bandwidths while maintaining a high environmental data throughput.

[0086] The method of this invention has good applicability and robustness. First, this invention was tested with different LTE configurations, and the results show that the method can be used with different LTE bandwidths such as 1.4 MHz, 3 MHz, and 5 MHz, as well as QPSK, 16QAM, and 64QAM modulation schemes.

[0087] LTE bandwidth. This invention first evaluates the performance of the method (SCScatter) under different LTE bandwidths. As shown in Table 2, with the increase of bandwidth, the number of Fast Fourier Transform (FFT) points increases, and the number of available cyclic shifts also increases accordingly, enabling the tag to encode 7, 8, and 9 bits in each modulation symbol at bandwidths of 1.4 MHz, 3 MHz, and 5 MHz, respectively. Figure 14 The throughput and bit error rate of tag data transmission at different bandwidths (1.4 MHz, 3 MHz, 5 MHz) are shown. Tag throughput increases with increasing bandwidth, reaching approximately 56.7 kbps at 5 MHz. Meanwhile, the bit error rate only increases slightly with increasing bandwidth.

[0088] Table 2. Number of subcarriers and Fast Fourier Transform points under different bandwidths in this invention.

[0089] Modulation scheme. This invention further evaluates the bit error rate of the method (SCScatter) under different physical downlink shared channel modulation schemes. Figure 15 The diagram demonstrates the bit error rate (BER) of tag data decoding at 1.4 MHz when different modulation schemes (QPSK, 16QAM, and 64QAM) are used in the physical downlink shared channel. The BER increases slightly with increasing modulation order because some effective subcarriers in higher-order modulation have lower amplitudes and are more easily confused with empty subcarriers. Nevertheless, the performance differences between QPSK, 16QAM, and 64QAM remain limited.

[0090] Subsequently, the performance of three multipath channel models (EPA, EVA, and ETU) and under different Doppler frequency conditions was further tested. The experimental results show that the present invention can maintain stable tag demodulation performance in various channel environments.

[0091] Channel Models. This invention considers three channel models: Extended Pedestrian A (EPA), Extended Vehicular A (EVA), and Extended Typical Urban (ETU). These represent a simple pedestrian environment with small delay spread, a vehicular scenario with more pronounced multipath propagation, and a complex urban environment with larger delay spread and stronger multipath effects, respectively. Figure 16 The bit error rate of tag data decoding is shown under different channel models (EPA, EVA, ETU). The bit error rate increases slightly from EPA to EVA and ETU, but the overall performance difference is limited.

[0092] Doppler frequencies. This invention further evaluates the impact of mobility by setting Doppler frequencies to 5 Hz, 30 Hz, and 70 Hz, representing low-mobility walking scenarios, medium-mobility scenarios, and relatively high-mobility vehicle scenarios, respectively. Figure 17 The bit error rate of tag data decoding is shown at different Doppler frequencies (5 Hz, 30 Hz, 70 Hz). Figure 17 The results show that the bit error rate is basically the same under these settings, and there is no significant deterioration as the Doppler frequency increases.

[0093] The following embodiments are used to illustrate the working process of the present invention and do not limit the scope of protection of the present invention. The bandwidth, number of Fast Fourier Transform points, threshold, weight, check code length, packet position and hardware devices in each step can be adjusted according to the actual LTE configuration and deployment conditions.

[0094] Example 1: 1.4 MHz Single-Receiver LTE Backscatter System (1) System parameters and setup: This embodiment employs frequency division duplex LTE downlink signaling, single transmit single receive, and transmission mode 1. The LTE bandwidth is set to 1.4 MHz, corresponding to 6 resource blocks, 72 effective subcarriers, and a 128-point Fast Fourier Transform. The physical downlink shared channel uses Quadrature Phase Shift Keying (QPSK) modulation with a normal cyclic prefix. Each tag modulation symbol uses one of 128 cyclic shifts, thus representing 7 tag data.

[0095] LTE signal sources can be implemented using commercial LTE base stations or software-defined radio. The receiver needs to be able to obtain complex baseband sampling of the backscattered radio band and perform standard LTE synchronization, fast Fourier transform, and physical downlink shared channel decoding. This invention uses a Software-Defined Radio (SDR) USRP B210 to build an LTE transceiver prototype, utilizing the MATLAB LTE Toolbox and the Communications Toolbox support package for USRP radio to control the USRP's transmission and reception of LTE signals. Tag data and LTE data demodulation processes are all implemented in MATLAB. The tag includes a passive envelope detector, a high-speed comparator TLV3501, and an FPGA. The passive envelope detector consists only of diodes, capacitors, and resistors, used to extract the carrier envelope. A Xilinx ZYNQ development board EBAZ4205 (a low-power FPGA) is used to process the comparator's input signal and output a square wave control signal. An ADG 902 RF switch is used to backscatter LTE signals from the surrounding environment.

[0096] (2) Tag encoding and transmission: Step A1: Divide the tag data to be sent into 7-bit groups and map them to a cyclic shift index using a Gray code table. .

[0097] Step A2: For a no-reference signal symbol that directly carries the effective load, generate... For reference signal symbols that serve as group anchors, the cyclic redundancy check of the corresponding group's load without reference signal is mapped to an anchor shift.

[0098] Step A3: Select the physical downlink shared channel OFDM symbol. Starting from the beginning of the cyclic prefix, continuously apply a linear phase sequence to the entire cyclic prefix and the effective OFDM symbol. The content of the cyclic prefix is ​​the same as the phase applied to the tail of the OFDM symbol. Maintain a fixed impedance state for the control region, synchronization symbol, and resources that do not want to be modulated to avoid affecting LTE synchronization and control information.

[0099] Step A4: Superimpose a fixed frequency offset to shift the backscattered signal to a target frequency band that is separate from the direct LTE signal, and transmit it through antenna reflection.

[0100] (3) Initial demodulation at the receiving end: Step B1: The receiver performs downconversion, sampling, LTE synchronization, cyclic prefix removal, and 128-point fast Fourier transform in the reverse scattering frequency band.

[0101] Step B2: Generate masks for 72 valid subcarriers and empty subcarrier masks including DC and side guard subcarriers based on the 1.4 MHz LTE configuration.

[0102] Step B3: Calculate for d = 0 to 127 ,use generate .

[0103] Step B4: Calculate the normalized scoring interval The preferred threshold can be set to 0.5; when It directly receives the Top-1 shift.

[0104] Step B5: When At that time, MAD normalized scoring is used, and settings can be configured. Select the minimum number of candidate cyclic shift indices that achieves the cumulative probability coverage. .

[0105] (4) Reference signal-assisted recovery and LTE data recovery: Step C1: Perform an inverse shift on all or high-ranking candidates of the cyclic shift index containing the reference signal symbol, calculate the normalized correlation between the recovered reference signal and the known reference signal, and determine the anchor point shift.

[0106] Step C2: Establish based on subframe type Figure 6 The shown group is used to extract the group's cyclic redundancy check from the tag codeword corresponding to the anchor point shift.

[0107] Step C3: Use the cyclic shift index of the first candidate in each candidate list of no reference signal symbols to form an initial candidate combination (corresponding to the initial ranking vector), and then use a min-heap to search for candidate combinations in ascending order of combination cost.

[0108] Step C4: Perform inverse Gray code mapping on the recovered shift and output the tag data.

[0109] Step C5: Reverse cyclic shift for each LTE physical downlink shared channel OFDM symbol Each subcarrier is used to restore the original resource grid, and then standard LTE channel estimation, equalization, physical downlink shared channel soft demodulation, channel decoding and cyclic redundancy check are performed to output the original LTE data.

[0110] Example 2, different LTE bandwidths and modulation schemes: When the LTE bandwidth is 3 MHz or 5 MHz, the number of Fast Fourier Transform points is adjusted to 256 or 512 respectively, and each modulation symbol of the tag is mapped to 8 bits or 9 bits respectively. The receiver regenerates the effective subcarrier and empty subcarrier masks based on the number of resource blocks. Candidate probability mapping parameters. These values ​​can be set to 12 and 15 respectively; the confidence threshold can be determined through offline statistics, for example, 0.5 for 3 MHz and 0.15 for 5 MHz. The physical downlink shared channel can use QPSK, 16QAM, or 64QAM. The energy distribution scoring does not require knowledge of the specific payload value, only the location of the effective subcarrier.

[0111] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0112] It should be understood that although the steps in the flowcharts of the accompanying drawings are shown sequentially as indicated by the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some of the steps in the flowcharts of the accompanying drawings may include multiple steps or stages, which are not necessarily completed at the same time, but may be executed at different times, and the execution order of these steps or stages is not necessarily sequential, but may be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0113] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0114] It will be apparent 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 invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0115] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A transmission method for an LTE backscatter system based on subcarrier cyclic shifting, characterized in that, include: The LTE signal source generates an OFDM-modulated downlink signal as an ambient carrier; The tag maps tag data to a cyclic shift index, generates a linear phase sequence based on the cyclic shift index, and continuously applies the time-domain linear phase sequence to the cyclic prefix and valid symbols of the specified OFDM symbols in the ambient carrier to generate a backscatter signal. At the same time, a fixed frequency offset is superimposed to shift the backscatter signal to the target frequency band. A single receiver receives the backscattered signal of the target frequency band, constructs an effective subcarrier mask and an empty subcarrier mask based on LTE resource configuration, calculates a comprehensive score reflecting the energy distribution of the effective subcarrier area and the empty subcarrier area for all candidate cyclic shift indices, selects to directly output the cyclic shift index corresponding to the highest comprehensive score based on the confidence level of the comprehensive score, or generates an adaptive candidate set, and uses OFDM symbols containing reference signals as anchor points to perform group verification and priority combination search to determine the cyclic shift index; The determined cyclic shift index is mapped to tag data; the cyclic shift index is compensated by inverse cyclic shift, and standard LTE decoding is performed to output LTE data in the ambient carrier.

2. The LTE backscatter system transmission method based on subcarrier cyclic shifting according to claim 1, characterized in that, The label maps label data to a circular shift index, specifically including: Tags can be used for each group Each tag data is mapped to a circular shift index. ,in, ,and The mapping uses Gray code encoding, so that the bit codewords corresponding to adjacent cyclic shift indices differ by only one bit; This represents the total number of circular shift indices.

3. The LTE backscatter system transmission method based on subcarrier cyclic shifting according to claim 1, characterized in that, The step of generating a linear phase sequence based on a cyclic shift index and continuously applying the time-domain linear phase sequence to the cyclic prefix and valid symbols of the specified OFDM symbols in the ambient carrier to generate a backscatter signal specifically includes: Let the cyclic shift index be d, and the time-domain sampling point index be n, n=0,1,…,N-1. The total number of cyclic shift indices generates a linear phase sequence. , The imaginary unit is used; the label multiplies the time-domain effective symbol x[n] of the specified OFDM symbol in the ambient carrier with the linear phase sequence point by point to obtain the modulated baseband signal. , This is the backscattered signal. Frequency domain representation X[k] is the frequency domain subcarrier of the OFDM symbol before modulation. This is the frequency domain subcarrier index.

4. The LTE backscatter system transmission method based on subcarrier cyclic shift according to claim 3, characterized in that, The superimposed fixed frequency offset shifts the backscattered signal to the target frequency band, specifically including: Let the fixed frequency offset be... Sampling rate Then the complete modulation sequence , It is a linear phase modulation. This is for frequency band shifting.

5. The LTE backscatter system transmission method based on subcarrier cyclic shifting according to claim 1, characterized in that, The calculation reflects a comprehensive score that reflects the energy distribution of the effective subcarrier region and the empty subcarrier region, specifically including: For each candidate cyclic shift index d, calculate the average energy of the effective subcarrier region. and average energy of empty subcarrier region ,in, These are the k1th effective subcarrier and the k2th empty subcarrier, respectively. For the index of the effective subcarrier, The index of the empty subcarrier. The set of effective subcarriers, The set of empty subcarriers; construct the energy ratio between the effective subcarrier region and the empty subcarrier region. Where ε is a positive number used to avoid numerical instability when the energy of empty subcarrier regions approaches zero; all subcarrier energies are sorted in descending order, and the top L subcarriers with the highest energy are selected to form a set. Calculate the consistency of energy ranking Overall score , , For weights.

6. The LTE backscatter system transmission method based on subcarrier cyclic shifting according to claim 1, characterized in that, The step of selecting the cyclic shift index corresponding to the highest comprehensive score based on the confidence level of the comprehensive score, or generating an adaptive candidate set, specifically includes: Sort all candidate circular shift indices in descending order of their overall scores: ； and These are the highest and second-highest overall scores, respectively. This is the overall score ranked Nth in descending order; Indicates the total number of circular shift indices; Calculate confidence level ,in Let be the standard deviation of the overall score of all candidate circular shift indices, and ε be a positive number to prevent the denominator from being zero; when When the highest overall score is reached, the circular shift index corresponding to it is directly output; when... At that time, an adaptive candidate set is generated, in which To pre-set the reliability threshold.

7. The LTE backscatter system transmission method based on subcarrier cyclic shift according to claim 6, characterized in that, The generation of the adaptive candidate set specifically includes: Robust normalization of the composite score is performed using the median function `median()` and the median absolute deviation (MAD) to obtain the normalized score. ;Will Mapped to candidate probabilities using the softmax function , where η represents a hyperparameter used to control the degree of concentration of the probability distribution; Candidate cyclic shift index The normalized score; the candidate cyclic shift indices are sorted in descending order of candidate probability, and those that meet the requirements are selected. The minimum variable K is composed of the cyclic shift indices of the K candidates with the highest candidate probabilities, forming an adaptive candidate set, where ρ is the target probability coverage.

8. The LTE backscatter system transmission method based on subcarrier cyclic shifting according to claim 1, characterized in that, The group verification specifically includes: dividing the OFDM symbols without reference signals into multiple groups based on the original position of the OFDM symbols with reference signals in the LTE frame, with each group containing an anchor symbol with reference signals and several symbols without reference signals following the anchor symbol; For anchor symbols containing reference signals, inverse compensation is performed on the candidate cyclic shift index d, and the reference signal vector is extracted. The normalized similarity is calculated with the local known reference signal r, and the candidate cyclic shift index with the highest normalized similarity is used as the anchor point for shifting. Within the group, there is no reference signal symbol carrying the tag data. The bit string of the tag data is concatenated to generate a check code, which is carried by the anchor symbol containing the reference signal within the group. The receiving end recovers the check code based on the anchor shift. And recalculate the check code for each candidate cyclic shift index combination of OFDM symbols without reference signals within the group, only if the recalculated check code matches... If they match, the candidate cyclic shift index combination of each OFDM symbol without a reference signal in the group is considered to have been successfully verified.

9. The LTE backscatter system transmission method based on subcarrier cyclic shift according to claim 8, characterized in that, The priority combination search specifically includes: assuming a group contains Q unreferenced symbols, and the ordered candidate list for the j-th unreferenced symbol is... , yes The top-ranked candidate circular shift index. yes The number of candidate circular shift indices; For each unreferenced signal symbol, select one candidate cyclic shift index from the ordered candidate list to form a combination, which is then ranked using a rank vector. express, This represents the rank of the j-th unreferenced signal symbol. Represents the number of unreferenced signal symbols; defines the combination cost. : ； A smaller combination cost indicates a higher probability that the circular shift index of each candidate in the combination is the correct combination; using a min-heap... Output combinations in ascending order: With the initial ranking vector Initialize a min-heap, pop the ranking vector with the minimum combined cost, and perform the grouping verification. If the verification fails, select one dimension from the current ranking vector, increment its ranking by 1, and generate an adjacency vector. Calculate the combination cost, add it to the min-heap, and repeat until a successfully verified combination is found or the min-heap is empty; Let be a j-dimensional unit vector.

10. The LTE backscattering system transmission method based on subcarrier cyclic shift according to claim 1, characterized in that, The process of performing inverse cyclic shift compensation on the cyclic shift index and executing standard LTE decoding to output LTE data in the ambient carrier specifically includes: The receiver performs an inverse cyclic shift on the OFDM symbol based on the cyclic shift index d detected for each OFDM symbol to restore the original LTE resource grid. The restored LTE resource grid is then fed into the standard LTE channel estimation, equalization, soft demodulation, channel decoding and cyclic redundancy check process to output LTE data in the ambient carrier.

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