Synchronous Demodulation Method and System for Passive RFID Tags

Abstract

This invention discloses a synchronous demodulation method for passive RFID tags, comprising the following steps: performing DC removal and smoothing filtering on the input signal to output a preprocessed signal; roughly estimating the backscattering link frequency offset of the tag signal in the preprocessed signal based on the signal peak distribution, and calculating and outputting a coarsely estimated oversampling rate; calculating the correlation coefficient between the preprocessed signal and the standard preamble under different oversampling rates to preliminarily determine the possible location of the preamble in the tag signal; comparing the calculation results of the correlation coefficient under different oversampling rates, determining and outputting a fine estimate of the oversampling rate to complete the synchronous detection of the preamble; and using the corresponding local 0 / 1 bit sequence, using correlation operations to determine the preprocessed signal as 0 / 1 bits for output, thus completing the demodulation of the input signal. This invention reduces hardware computing resource consumption and avoids communication performance degradation by using a coarse estimation followed by a fine estimation approach.

Description

Technical Field

[0001] This invention relates to the field of passive Internet of Things (IoT) technology, and more particularly to a low-cost, highly robust method for synchronous demodulation of passive IoT RFID tags. Background Technology

[0002] Passive IoT is a cutting-edge technology in the field of 5G-A mMTC (Massive Machine Type Communication). Passive IoT technology achieves seamless integration of data sensing, wireless transmission, and distributed computing by harnessing energy from the environment. Due to its advantages such as low cost, ease of deployment, and maintenance-free operation, it has enormous application potential in social governance, industrial production, and personal consumption, and is expected to achieve a connection scale of hundreds of billions. In particular, there are already several successful 5G-A cellular passive IoT deployments in fields such as warehousing and logistics, power monitoring, traffic supervision, and agricultural monitoring.

[0003] As one of the core technologies in the passive Internet of Things (IoT), passive RFID (Radio Frequency Identification) tags have the characteristic of low cost. Currently, the cost of passive RFID tags can be controlled at 0.2 to 0.3 yuan, providing an economically feasible basis for the large-scale connection and wide-ranging application of passive IoT.

[0004] However, in order to be widely used, the production cost of passive tags has been further compressed, including the passive crystal oscillator components inside them. This makes passive tags more susceptible to factors such as multipath effects, temperature, relative position or physical damage, resulting in phenomena such as crystal oscillator frequency shift. Consequently, the quality of the tag's backscattered signal is reduced, and the communication distance of the RFID system is also compressed.

[0005] According to the ISO / IEC 18000-6C international standard, the backscatter signal of passive tags is amplitude keying modulation. Under specific coding and bandwidth, the maximum offset tolerance of its BLF (Backscatter Link Frequency) is ±22%. This means that the tag synchronization and demodulation algorithm of the RFID receiver needs to have high robustness to cope with the tag BLF offset. Otherwise, it will seriously affect the receiving sensitivity and communication distance of the system.

[0006] Existing receiver technologies mostly employ a multi-parallel approach to synchronize and demodulate tag signals when dealing with passive tag crystal oscillator shifts. This involves processing the input signals in parallel with multiple channels at different frequencies to maintain reception performance when the tag experiences frequency shifts. Such methods significantly increase the hardware computing resources consumed by the receiver and also raise the hardware cost of the system.

[0007] Therefore, designing a tag signal synchronization and demodulation algorithm with low complexity, low cost, and high robustness to address the passive tag BLF offset phenomenon is of great significance for improving the communication distance of passive IoT systems and promoting the popularization of low-cost applications. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects and deficiencies of the prior art and provide a synchronous demodulation method and system for passive RFID tags. It designs an oversampling rate estimation method when passive tags experience BLF offset, as well as a synchronization and demodulation scheme for Miller encoded signals under BLF offset, to avoid communication performance degradation caused by BLF offset. It is applicable to the international RFID technology standard ISO / IEC 18000-6C.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for synchronous demodulation of passive RFID tags includes the following steps:

[0011] The input signal is subjected to DC removal and smoothing filtering operations, and the preprocessed signal is output.

[0012] Based on the signal peak distribution, the backscatter link frequency offset of the tag signal in the preprocessed signal is roughly estimated, and the oversampling rate is calculated and output as a rough estimate.

[0013] Calculate the correlation coefficient between the preprocessed signal and the standard preamble at different oversampling rates to preliminarily determine the possible locations of the preamble in the tag signal;

[0014] By comparing the calculation results of the correlation coefficient under different oversampling rates, the oversampling rate is determined and a precise estimate is output to complete the synchronous detection of the preamble.

[0015] Based on the precise estimate of the oversampling rate, the corresponding local 0 / 1 bit sequence is used to determine the preprocessed signal as a 0 / 1 bit output through correlation operations, thus completing the demodulation of the input signal.

[0016] Furthermore, the passive tag backscatter signal is amplitude keying modulated, using Miller encoding with return-to-zero code, and a subcarrier is added to enhance the signal's anti-interference capability;

[0017] Let the standard oversampling rate of the system when no backscatter link frequency shift occurs be OSR. std The coding mode of Miller subcarrier modulation is M, where M = 2, 4, 8, and the envelope signal received by the system at time n from the backscattered tag is R(n).

[0018] The input signal undergoes DC removal and smoothing filtering to produce a preprocessed output signal, specifically:

[0019] Sliding window statistics R(n) in 2·OSR std The maximum value R within the window length max (n), minimum value R min (n), according to R max (n), R min (n) Remove the DC component of the signal to obtain R DC (n) is:

[0020]

[0021] Then, mean-sliding window smoothing is performed to highlight the alternating peak characteristics of the subcarrier modulation. The smoothing window length is... The preprocessed signal S(n) is obtained as follows:

[0022]

[0023] In the formula, m represents the index variable in the summation process.

[0024] Furthermore, based on the signal peak distribution, the backscattering link frequency offset of the tag signal in the preprocessed signal is roughly estimated, and the coarsely estimated oversampling rate is calculated and output, specifically:

[0025] If the parameter is set to K, then the window length is 2K+1, and the following steps are executed in a loop:

[0026] S21: At time t, take a total of 2K+1 sampling points: S(tK), S(t-K+1), S(t-K+2), ..., S(t+K-1), S(t+K);

[0027] S22: Take the modulus of 2K+1 points, denoted as S. -K S -K+1 S -K+2 ... S K-1 S K ;

[0028] S23: Retrieve S -K S -K+1 S -K+2 ... S K-1 S KThe maximum value in the range is recorded, and the corresponding maximum value index J is recorded as -K, -K+1, -K+2, ..., K+1, K;

[0029] S24: Calculate the sum J of index J in the first 20·M steps of S23. sum If J sum If the value is close to 0, it is considered that the backscatter link frequency of the current tag backscatter signal has not shifted; otherwise, a coarse estimate of the oversampling rate (OSR) under the current backscatter link frequency shift is calculated. R =2·J sum +OSR std ;

[0030] S25: Adjustment Order Then, return to step S21.

[0031] Furthermore, the correlation coefficients between the preprocessed signal and the standard preamble are calculated at different oversampling rates to preliminarily determine the possible locations of the preamble in the tag signal, specifically:

[0032] Calculate the preamble correlation value, pre-store the standard preamble sequences corresponding to different oversampling rates, and when no backscattering link frequency shift occurs, the length of the preamble sequence corresponding to the standard oversampling rate is 10 M·OSR. std The preamble sequences corresponding to different oversampling rates are calculated using a resampling algorithm and then stored.

[0033] When stored data is retrieved, the input parameter OSR will be used. loca As the local oversampling rate, and read the OSR from local storage. loca The corresponding preamble sequence is used as the standard preamble sequence Preamble(n), and the length of Preamble(n) is 10·M·OSR. loca .

[0034] Furthermore, before calculating the correlation value, the preprocessed signal S(n) is converted into a 1-bit input sequence p(n) according to its positive and negative polarities, resulting in:

[0035]

[0036] The correlation value Conv(n) between the input sequence p(n) and the local standard preamble sequence Preamble(n) is calculated using a sliding window, yielding:

[0037]

[0038] After the passive tag is excited, it begins to respond with backscattered signals within a specific time period and initiates communication using a preamble. The maximum value of Conv(n) is then found within this specific time period. maxAnd output, and in Conv max The location where the code appears serves as the preamble position, which indicates the start of communication.

[0039] Furthermore, both the input sequence p(n) and the local standard preamble sequence Preamble(n) are 1-bit sequences, requiring only basic XOR logic operations for computation.

[0040] Furthermore, by comparing the calculated correlation coefficients under different oversampling rates, a precise estimate of the oversampling rate is determined and output to complete the synchronous detection of the preamble. Specifically:

[0041] Setting the precision step of the oversampling rate estimation OSR A parallel N-way fine estimation is performed, where N is an odd number, based on the coarsely estimated OSR. R Set different oversampling rate parameters (OSR) for each of the N channels. loca :

[0042] OSR loca =OSR R +k·step OSR ;

[0043] In the formula, And k∈Z;

[0044] After the N-way correlation values ​​are calculated, the Conv values ​​calculated for each way are compared. max Conv with N outputs max OSR corresponding to the maximum value loca OSR, as the final precise estimate of the system oversampling rate S The corresponding preamble position is used as the communication start position for subsequent data demodulation.

[0045] Furthermore, based on the precise estimate of the oversampling rate, the corresponding local 0 / 1 bit sequence is used to perform correlation operations to determine the preprocessed signal as a 0 / 1 bit output, thus completing the demodulation of the input signal. Specifically:

[0046] Pre-store standard 0 / 1 bit sequences corresponding to different oversampling rates. When no backscattering link frequency shift occurs, the length of the 0 / 1 bit sequence corresponding to the standard oversampling rate is M·OSR. std The 0 / 1 bit sequences corresponding to different oversampling rates are calculated using a resampling algorithm and then stored.

[0047] During the calculation, the input oversampling rate parameter OSR is used. S As the local oversampling rate, and read the OSR from local storage. SThe corresponding 0 / 1 bit sequences are taken as standard 0 / 1 bit sequences p0(n) and p1(n), and the lengths of p0(n) and p1(n) sequences are M·OSR. S .

[0048] Furthermore, the following steps are executed sequentially in a loop:

[0049] S51: At time t, calculate the correlation between the input sequence p(n) and the standard 0 sequence p0(n) over the next 2L+1 time intervals. Recorded as Represented as:

[0050]

[0051] In the formula, The superscript 0 indicates bit 0, and the subscript i indicates the adjacent time; L is a positive integer;

[0052] Calculate the correlation value between the input sequence p(n) and the standard 1 sequence p1(n), denoted as . Represented as:

[0053]

[0054] In the formula, The superscript 1 represents 1 bit, and the subscript i represents adjacent time points;

[0055] S52: Take The maximum value is denoted as C. max ;

[0056] S53: If C max If the superscript is 0, then determine that the 0 in the current data is a 0 bit and output it. If C max If the superscript is 1, then the current data is determined to be 1 bit and output.

[0057] S54: If all signals requiring demodulation have been processed, end the loop step; otherwise, take C. max The corresponding index value is denoted as I, and the adjustment is set as t = t + M·OSR. S After +I, return to step S51.

[0058] A synchronous demodulation system for a passive RFID tag, applying the synchronous demodulation method for a passive RFID tag described above, includes a DC smoothing module, an oversampling rate coarse estimation module, a synchronization module, and a matching demodulation module. The oversampling rate coarse estimation module is connected to the DC smoothing module, the synchronization module is connected to the oversampling rate coarse estimation module, and the matching demodulation module is connected to the synchronization module.

[0059] The DC removal and smoothing module is used to perform DC removal and smoothing filtering on the input signal and output a preprocessed signal; the oversampling rate coarse estimation module is used to roughly estimate the backscattering link frequency offset of the tag signal in the preprocessed signal based on the signal peak distribution and calculate and output a coarsely estimated oversampling rate.

[0060] The synchronization module contains multiple correlation modules. The correlation modules are used to calculate the correlation coefficient between the preprocessed signal and the standard preamble under different oversampling rates, and to preliminarily determine the possible location of the tag signal preamble. The synchronization module is used to compare the calculation results of the correlation coefficient under different oversampling rates, determine and output the precise estimate of the oversampling rate, and complete the synchronous detection of the preamble.

[0061] The matching demodulation module is used to determine the preprocessed signal as 0 / 1 bits and output it through correlation operations based on the oversampling rate estimate and the corresponding local 0 / 1 bit sequence, thus completing the demodulation of the input signal.

[0062] Compared with existing technologies, this invention can detect and estimate whether the backscattered signal of a passive tag has undergone frequency shift, and obtain the frequency of the tag backscattered link after the frequency shift and the system oversampling rate by estimating. This invention reduces the hardware computing resource consumption of direct estimation by using a method of first coarse estimation and then fine estimation.

[0063] This invention performs DC removal preprocessing on the input signal and identifies it as a 1-bit sequence, eliminating the need for multipliers in subsequent correlation value calculations and requiring only basic XOR logic operations. This results in a total saving of 10 M OSR in hardware implementation. std Plus 2·M·OSR S The multiplier resources are replaced with corresponding XOR logic units, which significantly reduces the cost of hardware implementation.

[0064] This invention addresses tag signals with backscatter link frequency offset. By using local standard signals with different oversampling rates for calculation, it can meet the preamble synchronization and signal demodulation requirements of ±22% backscatter link frequency offset as specified in ISO / IEC 18000-6C without sacrificing communication sensitivity performance. Attached Figure Description

[0065] Figure 1 This is a flowchart illustrating the synchronous demodulation method for passive RFID tags.

[0066] Figure 2 The flowchart shows the iterative calculation process for a coarse estimate of the oversampling rate.

[0067] Figure 3 This is a schematic diagram of the standard preamble signal encoded by Miller under different subcarrier modulation modes.

[0068] Figure 4 This is a schematic diagram of the standard 0 / 1 bit signal encoded by Miller under different subcarrier modulation modes.

[0069] Figure 5 The flowchart for the loop calculation of matching demodulation.

[0070] Figure 6 This is a structural framework diagram of a tag synchronization demodulation system for frequency offset in a radio frequency identification (RFID) system. Detailed Implementation

[0071] The synchronous demodulation method and system for passive RFID tags of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0072] Please see Figure 1 This invention discloses a method for synchronous demodulation of passive RFID tags, comprising the following steps:

[0073] The input signal is subjected to DC removal and smoothing filtering operations, and the preprocessed signal is output.

[0074] Based on the signal peak distribution, the backscatter link frequency (BLF) offset of the tag signal in the preprocessed signal is roughly estimated, and the oversampling rate is calculated and output as a rough estimate.

[0075] Calculate the correlation coefficient between the preprocessed signal and the standard preamble under different oversampling ratios (OSR) to preliminarily determine the possible locations of the preamble in the tag signal;

[0076] By comparing the calculation results of the correlation coefficient under different oversampling rates, the oversampling rate is determined and a precise estimate is output to complete the synchronous detection of the preamble.

[0077] Based on the precise estimate of the oversampling rate, the corresponding local 0 / 1 bit sequence is used to determine the preprocessed signal as a 0 / 1 bit output through correlation operations, thus completing the demodulation of the input signal.

[0078] According to the international RFID technical standard ISO / IEC 18000-6C, the backscatter signal of the passive tag is amplitude-keyed, using Miller encoding with return-to-zero code, and a subcarrier is added to enhance the signal's anti-interference capability. The standard oversampling rate of the system when no backscatter link frequency shift occurs is set to OSR. std The coding mode of Miller subcarrier modulation is M, and the envelope signal received by the system at time n from the backscattered tag is R(n).

[0079] Step S1. DC removal and smoothing: Perform DC removal and smoothing filtering on the input signal to output a preprocessed signal.

[0080] Sliding window statistics R(n) in 2·OSR std The maximum value R within the window length max (n), minimum value R min (n), according to R max (n), R min (n) Remove the DC component of the signal to obtain R DC (n) is:

[0081]

[0082] Then, mean-sliding window smoothing is performed to highlight the alternating peak characteristics of the subcarrier modulation, where the smoothing window length is... The preprocessed signal S(n) is obtained as follows:

[0083]

[0084] In the formula, m represents the index variable in the summation process.

[0085] Step S2. Coarse estimation of oversampling rate: Based on the signal peak distribution, roughly estimate the backscattering link frequency offset of the tag signal in the preprocessed signal, and calculate and output the coarsely estimated oversampling rate.

[0086] If the parameter is set to K, then the decision window length is 2K+1. The logic flowchart is as follows. Figure 2 As shown, the following steps are executed in a loop:

[0087] S21: At time t, take a total of 2K+1 sampling points: S(tK), S(t-K+1), S(t-K+2), ..., S(t+K-1), S(t+K);

[0088] S22: Take the modulus of 2K+1 points, denoted as S. -K S -K+1 S -K+2 ... S K-1 S K ;

[0089] S23: Retrieve S -K S -K+1 S -K+2 ... S K-1 S K The maximum value in the range is recorded, and the corresponding maximum value index J is recorded as -K, -K+1, -K+2, ..., K+1, K;

[0090] S24: Calculate the sum J of index J in the first 20·M steps of S23. sum If J sumIf the value is close to 0, it is considered that the backscatter link frequency of the current tag backscatter signal has not shifted; otherwise, a coarse estimate of the oversampling rate (OSR) under the current backscatter link frequency shift is calculated. R =2·J sum +OSR std ;

[0091] S25: Adjustment Order Then, return to step S21.

[0092] Step S3. Correlation Calculation: Calculate the correlation coefficient between the preprocessed signal and the standard preamble under different oversampling rates to preliminarily determine the possible locations of the preamble in the tag signal. A schematic diagram of the Miller-encoded standard preamble signal under different subcarrier modulation modes is shown below. Figure 3 As shown.

[0093] Calculate the preamble correlation value and pre-store the standard preamble sequences corresponding to different oversampling rates. According to the ISO / IEC 18000-6C protocol, when no backscatter link frequency shift occurs, the length of the preamble sequence corresponding to the standard oversampling rate should be 10 M·OSR. std The preamble sequences corresponding to different oversampling rates are calculated using a resampling algorithm and stored. When the stored data is retrieved, the input parameter OSR is used. loca As the local oversampling rate, and read the OSR from local storage. loca The corresponding preamble sequence is used as the standard preamble sequence Preamble(n), and the length of the Preamble(n) sequence should be 10·M·OSR. loca .

[0094] Before calculating the correlation value, converting the preprocessed signal S(n) into a 1-bit input sequence p(n) according to its positive and negative polarities can greatly reduce the hardware resource consumption of multiplication calculations in subsequent correlation operations, resulting in:

[0095]

[0096] The sliding window calculates the correlation value Conv(n) between the input sequence p(n) and the local standard preamble sequence Preamble(n). Since both the input sequence p(n) and the local standard preamble sequence Preamble(n) are 1-bit sequences, the multiplication operation in the hardware implementation only needs to be calculated using basic XOR logic operations, resulting in:

[0097]

[0098] According to the ISO / IEC 18000-6C protocol, passive tags should begin responding to backscattered signals within a specific time period after excitation begins, and initiate communication using a preamble. Within this specific time period, the maximum value of Conv(n) should be found. max And output, and in Conv max The location where the code appears serves as the preamble position, which indicates the start of communication.

[0099] Step S4. Synchronization Judgment: Compare the calculation results of the correlation coefficient under different oversampling rates, determine and output the precise estimate of the oversampling rate, and complete the synchronous detection of the preamble.

[0100] Setting the precision step of the oversampling rate estimation OSR A parallel N-way fine estimation is performed, where N is an odd number, based on the coarsely estimated OSR. R Set different oversampling rate parameters (OSR) for each of the N channels. loca :

[0101] OSR loca =OSR R +k·step OSR ;

[0102] In the formula, And k∈Z;

[0103] After the N-way correlation values ​​are calculated, the Conv values ​​calculated for each way are compared. max Conv with N outputs max OSR corresponding to the maximum value loca OSR, as the final precise estimate of the system oversampling rate S The corresponding preamble position is used as the communication start position for subsequent data demodulation.

[0104] Step S5. Matching Demodulation: Based on the precise estimate of the oversampling rate, using the corresponding local 0 / 1 bit sequence, correlation operations are performed to determine the preprocessed signal as a 0 / 1 bit output, completing the demodulation of the input signal. A schematic diagram of the standard 0 / 1 bit signal of Miller encoding under different subcarrier modulation modes is shown below. Figure 4 As shown.

[0105] Pre-store standard 0 / 1 bit sequences corresponding to different oversampling rates. When no backscattering link frequency shift occurs, the length of the 0 / 1 bit sequence corresponding to the standard oversampling rate is M·OSR. std The 0 / 1 bit sequences corresponding to different oversampling rates are calculated using a resampling algorithm and stored. During calculation, the input oversampling rate parameter OSR is used. S As the local oversampling rate, and read the OSR from local storage. SThe corresponding 0 / 1 bit sequences are taken as standard 0 / 1 bit sequences p0(n) and p1(n), and the lengths of p0(n) and p1(n) sequences are M·OSR. S .

[0106] Logic flow diagram as follows Figure 5 As shown, the following steps are executed in a loop:

[0107] S51: At time t, take L=2, and calculate the correlation between the input sequence p(n) and the standard 0 sequence p0(n) within 5 consecutive time intervals. Recorded as Represented as:

[0108]

[0109] In the formula, The superscript 0 indicates bit 0, and the subscript i indicates the adjacent time.

[0110] Calculate the correlation value between the input sequence p(n) and the standard 1 sequence p1(n), denoted as . Represented as:

[0111]

[0112] In the formula, The superscript 1 represents 1 bit, and the subscript i represents adjacent time points;

[0113] S52: Take The maximum value is denoted as C. max ;

[0114] S53: If C max If the superscript is 0, then determine that the 0 in the current data is a 0 bit and output it. If C max If the superscript is 1, then the current data is determined to be 1 bit and output.

[0115] S54: If all signals requiring demodulation have been processed, end the loop step; otherwise, take C. max The corresponding index value is denoted as I, and the adjustment is set as t = t + M·OSR. S After +I, return to step S51.

[0116] Please see Figure 6 The present invention also discloses a synchronous demodulation system for a passive RFID tag, including a DC smoothing module, an OSR (oversampling rate) coarse estimation module, a synchronization module and a matching demodulation module. The oversampling rate coarse estimation module is connected to the DC smoothing module, the synchronization module is connected to the oversampling rate coarse estimation module, and the matching demodulation module is connected to the synchronization module.

[0117] The DC removal and smoothing module performs DC removal and smoothing filtering on the input signal, outputting a preprocessed signal. The oversampling rate coarse estimation module roughly estimates the backscattering link frequency offset of the tag signal in the preprocessed signal based on the signal peak distribution, and calculates and outputs a coarsely estimated oversampling rate.

[0118] The synchronization module contains multiple correlation modules. These correlation modules calculate the correlation coefficients between the preprocessed signal and the standard preamble at different oversampling rates, initially determining the possible locations of the tag signal preamble. The synchronization module compares the calculated correlation coefficients at different oversampling rates, identifies and outputs a precise estimate of the oversampling rate, and completes the synchronous detection of the preamble.

[0119] The matching demodulation module is used to determine the preprocessed signal as 0 / 1 bits and output it through correlation operations based on the oversampling rate estimate and the corresponding local 0 / 1 bit sequence, thus completing the demodulation of the input signal.

[0120] In this embodiment, considering the tag is configured to use Miller coding and subcarrier modulation with M=8, the standard preamble sequence length is 1920 bits, the standard 0 / 1 bit sequence length is 192 bits, the standard tag backscatter link frequency is 640kHz, the receiver sampling rate is 15.36MHz, and the standard oversampling rate (OSR) is... std It is 24.

[0121] When the passive tag is affected by the environment, the backscatter link frequency shifts by -20% to 512kHz. At this point, the receiver's actual oversampling rate is 28.8. If the standard oversampling rate (OSR) continues to be applied... std Synchronizing and demodulating tags at 24 will cause a sharp drop in demodulation sensitivity and an exponential reduction in communication distance.

[0122] The DC removal and smoothing module performs DC removal and smoothing filtering on the input signal and outputs a preprocessed signal. The DC removal operation involves finding the maximum value R. max (n), minimum value R min (n) The selected window length is 2·OSR std =48, which refers to the first 24 and last 24 sampling points of the current input data. The DC removal operation can remove the carrier power in the tag's reverse signal, and the smoothing operation can filter out some high-frequency noise to highlight the alternating 0 / 1 bits after subcarrier modulation.

[0123] The OSR coarse estimation module roughly estimates the backscatter link frequency offset of the tag signal, taking into account... Setting K=5 is sufficient to basically cover the 22% frequency offset required by the standard. Figure 2Determine whether a backscattering link frequency shift has occurred and estimate the coarse oversampling rate of the output, where J sum The final calculated J is the average of the first 160 calculations and comparisons, based on the backscatter link frequency offset of the embodiment. sum If it is approximately equal to 2.44, then the output estimated oversampling rate (OSR) is... R =2*2.44+24=28.88.

[0124] The synchronization module sets the oversampling interval step for precise estimation. OSR =0.1, then the output of the preceding OSR coarse estimation module can be approximated with this precision: OSR R =28.88≈28.9. Select the number of relevant modules for parallel computing as needed. The smaller the interval and the more parallel sub-modules, the more accurate the estimation will be, but the hardware computing resources consumed will also increase accordingly.

[0125] In this embodiment, N=9 is selected, so there are 9 related modules as sub-modules in the synchronization module. These sub-modules calculate the correlation coefficient between the input signal and the standard preamble of the corresponding OSR under different OSRs, and output the most correlated coefficient value Conv. max The synchronization module will provide different input parameters for the nine related modules (OSR). loca OSR loca The scores were 28.5, 28.6, 28.7, 28.8, 28.9, 29.0, 29.1, 29.3, and 29.4, respectively, comparing nine different OSRs. loca The calculation results Conv from the relevant modules max And determine the OSR corresponding to the maximum value. loca OSR as a precise estimate S The output, in this example, should be OSR. S =28.9, and the position of the preamble is confirmed at the moment when the maximum value appears, thus completing the synchronous detection of the tag.

[0126] Matching demodulation module, based on OSR estimated by synchronization module S =28.9, using the corresponding OSR's local 0 / 1 bit sequence, according to Figure 3 The process involves identifying the input data as 0 / 1 bits for output, thus demodulating the input signal. Within the synchronization and matching demodulation modules, the stored local standard preamble or standard 0 / 1 bit sequences corresponding to different OSRs can be obtained by resampling the standard preamble signal and / or the standard 0 / 1 bit signal and converting it into a 1-bit sequence. The interpolation factor in the resampling is the standard oversampling rate OSR. stdThe resampling factor is the target oversampling rate. Since the converted sequence is a 1-bit sequence, different interpolation and filtering schemes used in the resampling algorithm have little impact on the final sequence generation. This invention uses linear interpolation and Kaiser window low-pass FIR anti-aliasing filtering.

[0127] Meanwhile, under Miller8 coding, if no backscatter link frequency shift occurs, 10 M OSR is saved in the correlation calculations for synchronization and demodulation. std +2·M·OSR S The multiplier, here we take M=8, OSR S =24. In hardware implementation, 2304 multipliers are required. Even considering time-division multiplexing of the multipliers into eight channels, 288 multipliers are still needed. When the multiplier bit width is 16 bits, it requires approximately 33,408 LUT (lookup table) units when implemented using an FPGA (Field Programmable Gate Array). However, using the 1-bit sequence operation in this invention, only 1152 LUT units are needed. The overall hardware computing resources saved reach 96.5%, which can effectively reduce the cost required for hardware implementation and also reduce the corresponding power consumption.

[0128] In summary, this invention can detect and estimate whether a passive tag backscatter signal has undergone frequency shift, and obtain the frequency of the tag backscatter link after the frequency shift and the system oversampling rate by estimation. This invention reduces the hardware computing resource consumption of direct estimation by using a coarse estimation followed by a fine estimation approach.

[0129] This invention performs DC removal preprocessing on the input signal and identifies it as a 1-bit sequence, eliminating the need for multipliers in subsequent correlation value calculations and requiring only basic XOR logic operations. This results in a total saving of 10 M OSR in hardware implementation. std Plus 2·M·OSR S The multiplier resources are replaced with corresponding XOR logic units, which significantly reduces the cost of hardware implementation.

[0130] This invention addresses tag signals with backscatter link frequency offset. By using local standard signals with different oversampling rates for calculation, it can meet the preamble synchronization and signal demodulation requirements of ±22% backscatter link frequency offset as specified in ISO / IEC 18000-6C without sacrificing communication sensitivity performance.

[0131] The above description is a detailed description of the preferred embodiments of the present invention. However, the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modifications made under the technical spirit disclosed in the present invention should fall within the patent scope covered by the present invention.

Claims

1. A method for synchronous demodulation of passive RFID tags, characterized in that, Includes the following steps: The passive tag backscatter signal is amplitude keying modulated, using Miller code return-to-zero, and a subcarrier is added to improve the signal's anti-interference capability; Let the standard oversampling rate of the system when no backscatter link frequency shift occurs be OSR. std The coding mode of Miller subcarrier modulation is M, and the envelope signal received by the system at time n from the backscattered tag is R(n). The input signal undergoes DC removal and smoothing filtering to produce a preprocessed output signal, specifically: Sliding window statistics R(n) in 2·OSR std The maximum value R within the window length max (n), minimum value R min (n), according to R max (n), R min (n) Remove the DC component of the signal to obtain R DC (n) is: Then, mean-sliding window smoothing is performed to highlight the alternating peak characteristics of the subcarrier modulation. The smoothing window length is... The preprocessed signal S(n) is obtained as follows: In the formula, m represents the index variable in the summation process; Based on the signal peak distribution, the backscattering link frequency offset of the tag signal in the preprocessed signal is roughly estimated, and the coarsely estimated oversampling rate is calculated and output, specifically: If the parameter is set to K, then the window length is 2K+1, and the following steps are executed in a loop: S21: At time t, take a total of 2K+1 sampling points: S(tK), S(t-K+1), S(t-K+2), ..., S(t+K-1), S(t+K); S22: Take the modulus of 2K+1 points, denoted as S. -K S -K+1 S -K+2 ... S K-1 S K ; S23: Retrieve S -K S -K+1 S -K+2 ... S K-1 S K The maximum value in the range is recorded, and the corresponding maximum value index J is recorded as -K, -K+1, -K+2, ..., K+1, K; S24: Calculate the sum J of index J in the first 20·M steps of S23. sum If J sum If the value is close to 0, it is assumed that the backscatter link frequency of the current tag's backscatter signal has not shifted; otherwise, a coarse estimate of the oversampling rate (OSR) is calculated based on the current backscatter link frequency shift. R =2·J sum +OSR std ; S25: Adjustment Order Then, return to step S21; Based on the signal peak distribution, the backscatter link frequency offset of the tag signal in the preprocessed signal is roughly estimated, and the oversampling rate is calculated and output as a rough estimate. The correlation coefficients between the preprocessed signal and the standard preamble at different oversampling rates are calculated to preliminarily determine the possible locations of the preamble in the tag signal. Specifically: Calculate the preamble correlation value, pre-store the standard preamble sequences corresponding to different oversampling rates, and when no backscattering link frequency shift occurs, the length of the preamble sequence corresponding to the standard oversampling rate is 10 M·OSR. std The preamble sequences corresponding to different oversampling rates are calculated using a resampling algorithm and then stored. When stored data is retrieved, the input parameter OSR will be used. loca As the local oversampling rate, and read the OSR from local storage. loca The corresponding preamble sequence is used as the standard preamble sequence Preamble(n), and the length of Preamble(n) is 10·M·OSR. loca ; By comparing the calculated correlation coefficients under different oversampling rates, a precise estimate of the oversampling rate is determined and output to complete the synchronous detection of the preamble. Specifically: Setting the precision step of the oversampling rate estimation OSR Parallel N-way fine estimation is performed, where N is an odd number, based on the coarsely estimated OSR. R Set different oversampling rate parameters (OSR) for each of the N channels. loca : OSR loca =OSR R +k·step OSR ; In the formula, And k∈Z; After the N-way correlation values ​​are calculated, the Conv values ​​calculated for each way are compared. max Conv with N outputs max OSR corresponding to the maximum value loca OSR, as the final precise estimate of the system oversampling rate S The corresponding preamble position is used as the communication start position for subsequent data demodulation. Based on the precise estimate of the oversampling rate, the corresponding local 0 / 1 bit sequence is used to determine the preprocessed signal as a 0 / 1 bit output through correlation operations, thus completing the demodulation of the input signal. Specifically: Pre-store standard 0 / 1 bit sequences corresponding to different oversampling rates. When no backscattering link frequency shift occurs, the length of the 0 / 1 bit sequence corresponding to the standard oversampling rate is M·OSR. std The 0 / 1 bit sequences corresponding to different oversampling rates are calculated using a resampling algorithm and then stored. During the calculation, the input oversampling rate parameter OSR is used. S As the local oversampling rate, and read the OSR from local storage. S The corresponding 0 / 1 bit sequences are taken as standard 0 / 1 bit sequences p0(n) and p1(n), and the lengths of p0(n) and p1(n) sequences are M·OSR. S ; The following steps are executed in a loop: S51: At time t, calculate the correlation between the input sequence p(n) and the standard 0 sequence p0(n) over the next 2L+1 time intervals. Recorded as Represented as: In the formula, The superscript 0 indicates bit 0, and the subscript i indicates the adjacent time; L is a positive integer; Calculate the correlation value between the input sequence p(n) and the standard 1 sequence p1(n), denoted as . Represented as: In the formula, The superscript 1 represents 1 bit, and the subscript i represents adjacent time points; S52: Take The maximum value is denoted as C. max ; S53: If C max If the superscript is 0, then determine that the 0 in the current data is a 0 bit and output it. If C max If the superscript is 1, then the current data is determined to be 1 bit and output. S54: If all signals requiring demodulation have been processed, end the loop step; otherwise, take C. max The corresponding index value is denoted as I, and the adjustment is set as t = t + M·OSR. S After +I, return to step S51.

2. The synchronous demodulation method for passive RFID tags according to claim 1, characterized in that, Before calculating the correlation value, the preprocessed signal S(n) is converted into a 1-bit input sequence p(n) according to its positive and negative polarities, resulting in: The correlation value Conv(n) between the input sequence p(n) and the local standard preamble sequence Preamble(n) is calculated using a sliding window, yielding: After the passive tag is excited, it begins to respond with backscattered signals within a specific time period and initiates communication using a preamble. The maximum value of Conv(n) is then found within this specific time period. max And output, and in Conv max The location where the code appears serves as the preamble position, which indicates the start of communication.

3. The synchronous demodulation method for passive RFID tags according to claim 2, characterized in that, The input sequence p(n) and the local standard preamble sequence Preamble(n) are both 1-bit sequences, and only basic XOR logic operations are needed for calculation.

4. A synchronous demodulation system for passive RFID tags, employing the synchronous demodulation method for passive RFID tags as described in any one of claims 1 to 3, characterized in that, It includes a DC smoothing module, an oversampling rate coarse estimation module, a synchronization module, and a matching demodulation module. The oversampling rate coarse estimation module is connected to the DC smoothing module, the synchronization module is connected to the oversampling rate coarse estimation module, and the matching demodulation module is connected to the synchronization module. The DC removal and smoothing module is used to perform DC removal and smoothing filtering on the input signal and output a preprocessed signal. The oversampling rate coarse estimation module is used to roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal based on the signal peak distribution, and calculate and output the coarsely estimated oversampling rate. The synchronization module contains multiple correlation modules. The correlation modules are used to calculate the correlation coefficient between the preprocessed signal and the standard preamble under different oversampling rates, and to preliminarily determine the possible location of the tag signal preamble. The synchronization module is used to compare the calculation results of the correlation coefficient under different oversampling rates, determine and output the precise estimate of the oversampling rate, and complete the synchronous detection of the preamble. The matching demodulation module is used to determine the preprocessed signal as 0 / 1 bits and output it through correlation operations based on the oversampling rate estimate and the corresponding local 0 / 1 bit sequence, thus completing the demodulation of the input signal.

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