Synchronous demodulation method and system of passive RFID tag

Abstract

The invention discloses a synchronous demodulation method for a passive RFID tag, and the method comprises the following steps: carrying out the DC removal and smooth filtering operation of an input signal, and outputting a preprocessing signal; according to the signal peak value distribution, roughly estimating a backscattering link frequency deviation condition of a label signal in the preprocessed signal, and calculating and outputting a roughly estimated oversampling rate; calculating a correlation coefficient between the preprocessed signal and a standard lead code under different oversampling rates, and preliminarily judging a possible position of the lead code in the tag signal; and comparing calculation results of the correlation coefficients under different oversampling rates, judging and outputting an oversampling rate precise estimation value, and completing synchronous detection of the lead code. According to the fine estimation value of the oversampling rate, a corresponding local 0 / 1 bit sequence is used, the preprocessed signal is judged to be 0 / 1 bit output through correlation operation, and demodulation of the input signal is completed. According to the invention, through a mode of first rough estimation and then fine estimation, consumption of hardware computing resources is reduced, and reduction of communication performance is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of passive Internet of Things, and in particular to a synchronous demodulation method of a passive Internet of Things RFID tag with low cost and high robustness. Background Art

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

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

[0004] However, in order to be used on a large scale, the production cost of passive tags has been further compressed, including the passive crystal oscillator components inside them, which makes passive tags more susceptible to factors such as multipath effects, temperature, relative position or physical damage, resulting in phenomena such as crystal oscillation frequency deviation, which in turn reduces the quality of the tag's backscatter signal and compresses the communication distance of the RFID system.

[0005] According to the ISO / IEC 18000-6C international standard, the backscatter signal of the passive tag is amplitude keyed modulated, and its BLF (Backscatter Link Frequency) has a maximum deviation tolerance of ±22% under specific coding and bandwidth. This means that the tag synchronization and demodulation algorithm of the RFID receiver needs to have high robustness to cope with the situation of tag BLF deviation, otherwise it will seriously affect the system's receiving sensitivity and communication distance.

[0006] When faced with passive tag crystal oscillator offset, existing receiver technologies mostly use a multi-path parallel approach to synchronize and demodulate tag signals, and process input signals in parallel in multiple paths with different frequency configurations to maintain the receiving performance when the tag has frequency offset. This method will exponentially increase the hardware computing resource consumption of the receiver, and at the same time, will also increase the hardware cost of the system.

[0007] Therefore, in view of the BLF offset phenomenon of passive tags, designing a tag signal synchronization and demodulation algorithm with low complexity, low cost and high robustness is of vital importance to improving the communication distance of passive IoT systems and promoting the popularization of low-cost applications. Summary of the invention

[0008] The purpose of the present invention is to overcome the defects and shortcomings of the prior art, provide a synchronous demodulation method and system for a passive RFID tag, design an oversampling rate estimation method when a passive tag has a BLF offset, and a Miller coded signal synchronization and demodulation scheme under BLF offset, so as to avoid the degradation of communication performance caused by BLF offset, and can be applied to the international RFID technology standard ISO / IEC 18000-6C.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] A synchronous demodulation method for a passive RFID tag comprises the following steps:

[0011] Perform DC removal and smoothing filtering operations on the input signal and output a pre-processed signal;

[0012] According to the signal peak distribution, roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal, and calculate the oversampling rate of the output rough estimate;

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

[0014] Compare the calculation results of the correlation coefficients under different oversampling rates, identify and output the accurate estimated value of the oversampling rate, and complete the synchronous detection of the preamble code;

[0015] According to the accurate estimation 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 operation, thereby completing the demodulation of the input signal.

[0016] Furthermore, the passive tag backscatter signal is amplitude keyed modulated, using return-to-zero Miller coding, and a subcarrier is added to improve the signal's anti-interference capability;

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

[0018] The input signal is subjected to DC removal and smoothing filtering operations, and the preprocessed signal is output, specifically:

[0019] Sliding window statistics R(n) at 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 get R DC (n) is:

[0020]

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

[0022]

[0023] Where m represents the index variable in the summation process.

[0024] Furthermore, 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 of the rough estimate is calculated and output, specifically:

[0025] Take the parameter as K, then the judgment window length is 2K+1, and execute the following steps in sequence:

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

[0027] S22: Take the modulus value of 2K+1, recorded 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 among them, and record the corresponding maximum value index J as -K, -K+1, -K+2, ..., K+1, K;

[0029] S24: Calculate the sum J of the index J in the first 20·M steps S23 sum , if J sum If it is close to 0, it is considered that the backscatter link frequency of the current tag backscatter signal has not shifted. Otherwise, calculate the rough estimated oversampling rate OSR under the current backscatter link frequency offset. R =2·J sum +OSR std ;

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

[0031] Furthermore, the correlation coefficient between the preprocessed signal and the standard preamble code at different oversampling rates is calculated to preliminarily determine the possible position of the preamble code in the tag signal, specifically:

[0032] Calculate the preamble correlation value and pre-store the standard preamble sequence corresponding to different oversampling rates. When there is no backscatter link frequency offset, the preamble sequence length corresponding to the standard oversampling rate is 10·M·OSR std , the preamble code sequences corresponding to different oversampling rates are calculated by the resampling algorithm and stored;

[0033] When the stored data is called, the input parameter OSR loca As the local oversampling rate, and read the OSR in 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 the positive and negative polarities, and the result is:

[0035]

[0036] The sliding window calculates the correlation value Conv(n) between the input sequence p(n) and the local standard preamble sequence Preamble(n), and obtains:

[0037]

[0038] After the start of the stimulus, the passive tag starts to reply the backscatter signal within a specific time period and starts the communication with the preamble code. The maximum value of Conv(n) is found within the specific time period. maxAnd output, and use Conv max The position where it appears is used as the preamble position, which can indicate the start position of communication.

[0039] Furthermore, 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 required for calculation.

[0040] Furthermore, the calculation results of the correlation coefficients under different oversampling rates are compared, and the accurate estimated value of the oversampling rate is determined and output to complete the synchronous detection of the preamble code, specifically:

[0041] Set the oversampling rate estimation accuracy step OSR , perform precise estimation in N parallel paths, N is an odd number, and according to the rough estimated OSR R , set different oversampling rate parameters OSR for 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, compare the Conv max , with N-channel output Conv max OSR corresponding to the maximum value loca , as the final accurate estimate of the system oversampling rate OSR S The corresponding preamble code position is used as the communication start position for subsequent data demodulation.

[0045] Furthermore, according to the accurate estimation value 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 operation, thereby completing the demodulation of the input signal, specifically:

[0046] The standard 0 / 1 bit sequences corresponding to different oversampling rates are stored in advance. When no backscatter link frequency offset 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 by the resampling algorithm and stored;

[0047] When calculating, the input oversampling rate parameter OSR S As the local oversampling rate, and read the OSR in local storage SThe corresponding 0 / 1 bit sequence is used as the standard 0 / 1 bit sequence p0(n) and p1(n), and the sequence length of p0(n) and p1(n) is M·OSR S .

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

[0049] S51: At time t, calculate the correlation value between the input sequence p(n) and the standard 0 sequence p0(n) within the adjacent 2L+1 time points Recorded as It is expressed as:

[0050]

[0051] In the formula, The superscript 0 represents 0 bit, and the subscript i represents 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 It is expressed as:

[0053]

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

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

[0056] S53: If C max The corresponding superscript is 0, then the 0 of the current data is judged to be 0 bit and output. If C max If the corresponding superscript is 1, the current data is judged to be 1 bit and output;

[0057] S54: If all the signals to be demodulated have been processed, the loop step ends; otherwise, C max The corresponding subscript value is recorded as I, and t is adjusted to t = t + M·OSR S After +I, return to step S51.

[0058] A synchronous demodulation system for a passive RFID tag, using any of the above-mentioned synchronous demodulation methods for a passive RFID tag, comprising a DC removal smoothing module, an oversampling rate rough estimation module, a synchronization module and a matching demodulation module, wherein the oversampling rate rough estimation module is connected to the DC removal smoothing module, the synchronization module is connected to the oversampling rate rough estimation module, and the matching demodulation module is connected to the synchronization module;

[0059] The DC removal and smoothing module is used to remove DC and smooth the input signal and output a preprocessed signal; the oversampling rate rough estimation module is used to roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal according to the signal peak distribution, and calculate and output the rough estimated oversampling rate;

[0060] Multiple correlation modules are set in the synchronization module. The correlation modules are used to calculate the correlation coefficient between the preprocessed signal and the standard preamble code at different oversampling rates, and preliminarily determine the possible position of the tag signal preamble code; the synchronization module is used to compare the calculation results of the correlation coefficient at different oversampling rates, determine and output the oversampling rate precise estimation value, and complete the synchronization detection of the preamble code;

[0061] The matching demodulation module is used to accurately estimate the value of the oversampling rate, use the corresponding local 0 / 1 bit sequence, and determine the pre-processed signal as a 0 / 1 bit output through correlation operation to complete the demodulation of the input signal.

[0062] Compared with the prior art, the present invention can detect and estimate whether the passive tag backscatter signal has frequency offset, and obtain the frequency of the tag backscatter link after frequency offset and the system oversampling rate by estimation. The present invention reduces the hardware computing resource consumption of direct estimation by making a rough estimate first and then a precise estimate.

[0063] The present invention performs DC removal preprocessing on the input signal and determines it as a 1-bit sequence, so that no multiplier is required in the subsequent correlation value calculation, and only basic XOR logic operation is required, which can save a total of 10·M·OSR in hardware implementation. std Plus 2 M OSR S The multiplier resources are used and replaced by corresponding XOR logic units, which significantly reduces the cost of hardware implementation.

[0064] The present invention aims at the tag signal under the backscatter link frequency offset, and uses the local standard signal with different oversampling rates for calculation, so as to meet the preamble synchronization and signal demodulation under the backscatter link frequency offset of ±22% required in ISO / IEC 18000-6C without losing the communication sensitivity performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The figure is a flow chart of a synchronous demodulation method for a passive RFID tag.

[0066] Figure 2 Flowchart of the loop calculation for rough estimation of oversampling ratio.

[0067] Figure 3 Schematic diagram of standard preamble signal of Miller coding under different subcarrier modulation modes.

[0068] Figure 4 Schematic diagram of standard 0 / 1 bit signal of Miller coding under different subcarrier modulation modes.

[0069] Figure 5 Flowchart of the cyclic calculation for matching demodulation.

[0070] Figure 6 The structural framework diagram of the tag synchronous demodulation system with frequency offset in the radio frequency identification system. DETAILED DESCRIPTION

[0071] The synchronous demodulation method and system of the passive RFID tag of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0072] See also Figure 1 The present invention discloses a synchronous demodulation method for a passive RFID tag, comprising the following steps:

[0073] Perform DC removal and smoothing filtering operations on the input signal and output a pre-processed signal;

[0074] According to 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 of the rough estimate is calculated and output;

[0075] Calculate the correlation coefficient between the preprocessed signal and the standard preamble code at different over sampling rates (OSR) to preliminarily determine the possible location of the preamble code in the tag signal;

[0076] Compare the calculation results of the correlation coefficients under different oversampling rates, identify and output the accurate estimated value of the oversampling rate, and complete the synchronous detection of the preamble code;

[0077] According to the accurate estimation 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 operation, thereby completing the demodulation of the input signal.

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

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

[0080] Sliding window statistics R(n) at 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 get R DC (n) is:

[0081]

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

[0083]

[0084] Where m represents the index variable in the summation process.

[0085] Step S2. Rough estimation of oversampling rate: Roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal according to the signal peak distribution, and calculate and output the roughly estimated oversampling rate.

[0086] Taking the parameter as K, the judgment window length is 2K+1, and the logic flow diagram is as follows Figure 2 As shown, the following steps are executed in sequence:

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

[0088] S22: Take the modulus value of 2K+1, recorded 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 among them, and record the corresponding maximum value index J as -K, -K+1, -K+2, ..., K+1, K;

[0090] S24: Calculate the sum J of the index J in the first 20·M steps S23 sum , if J sumIf it is close to 0, it is considered that the backscatter link frequency of the current tag backscatter signal has not shifted. Otherwise, calculate the rough estimated oversampling rate OSR under the current backscatter link frequency offset. R =2·J sum +OSR std ;

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

[0092] Step S3. Correlation value calculation: Calculate the correlation coefficient between the preprocessed signal and the standard preamble code at different oversampling rates to preliminarily determine the possible position of the preamble code in the tag signal. Figure 3 shown.

[0093] Calculate the preamble correlation value and pre-store the standard preamble sequence corresponding to different oversampling rates. According to the ISO / IEC18000-6C protocol, when there is no backscatter link frequency offset, the preamble sequence length corresponding to the standard oversampling rate should be 10·M·OSR std , the preamble sequence corresponding to different oversampling rates is calculated by the resampling algorithm and stored. When the stored data is called, the input parameter OSR loca As the local oversampling rate, and read the OSR in 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, the preprocessed signal S(n) is converted into a 1-bit input sequence p(n) according to the positive and negative polarities, which can greatly reduce the hardware resource consumption of the multiplication calculation in the subsequent correlation operation, and obtain:

[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 hardware implementation only needs to use the basic XOR logic operation to calculate, and the result is:

[0097]

[0098] According to the ISO / IEC 18000-6C protocol, after the start of the stimulus, the passive tag should start to reply to the backscatter signal within a specific time period and start the communication with the preamble code, and find the maximum value of Conv(n) within the specific time period. max And output, and use Conv max The position where it appears is used as the preamble position, which can indicate the start position of communication.

[0099] Step S4. Synchronous determination: Compare the calculation results of the correlation coefficients under different oversampling rates, determine and output the precise estimated value of the oversampling rate, and complete the synchronous detection of the preamble code.

[0100] Set the oversampling rate estimation accuracy step OSR , perform precise estimation in N parallel paths, N is an odd number, and according to the rough estimated OSR R , set different oversampling rate parameters OSR for 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, compare the Conv max , with N-channel output Conv max OSR corresponding to the maximum value loca , as the final accurate estimate of the system oversampling rate OSR S The corresponding preamble code position is used as the communication start position for subsequent data demodulation.

[0104] Step S5. Matching demodulation: Based on the accurate 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 operation to complete the demodulation of the input signal. The schematic diagram of the standard 0 / 1 bit signal of Miller coding under different subcarrier modulation modes is shown in the figure. Figure 4 shown.

[0105] The standard 0 / 1 bit sequences corresponding to different oversampling rates are stored in advance. When no backscatter link frequency offset occurs, the length of the 0 / 1 bit sequence corresponding to the standard oversampling rate is M·OSR std , the 0 / 1 bit sequence corresponding to different oversampling rates is calculated by the resampling algorithm and stored. When calculating, the input oversampling rate parameter OSR S As the local oversampling rate, and read the OSR in local storage SThe corresponding 0 / 1 bit sequence is used as the standard 0 / 1 bit sequence p0(n) and p1(n), and the sequence length of p0(n) and p1(n) is M·OSR S .

[0106] The logic flow diagram is as follows Figure 5 As shown, the following steps are executed in sequence:

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

[0108]

[0109] In the formula, The superscript 0 indicates 0 bit, 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 It is expressed as:

[0111]

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

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

[0114] S53: If C max The corresponding superscript is 0, then the 0 of the current data is judged to be 0 bit and output. If C max If the corresponding superscript is 1, the current data is judged to be 1 bit and output;

[0115] S54: If all the signals to be demodulated have been processed, the loop step ends; otherwise, C max The corresponding subscript value is recorded as I, and t is adjusted to t = t + M·OSR S After +I, return to step S51.

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

[0117] The DC removal and smoothing module is used to remove DC and smooth the input signal and output the preprocessed signal. The oversampling rate rough estimation module is used to roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal according to the signal peak distribution, and calculate and output the rough estimated oversampling rate.

[0118] Multiple correlation modules are set in the synchronization module. The correlation modules are used to calculate the correlation coefficient between the preprocessed signal and the standard preamble at different oversampling rates, and preliminarily determine the possible position of the tag signal preamble. The synchronization module is used to compare the calculation results of the correlation coefficient at different oversampling rates, determine and output the precise estimate of the oversampling rate, and complete the synchronization detection of the preamble.

[0119] The matching demodulation module is used to accurately estimate the value of the oversampling rate, use the corresponding local 0 / 1 bit sequence, and determine the pre-processed signal as a 0 / 1 bit output through correlation operation to complete the demodulation of the input signal.

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

[0121] When the passive tag is affected by the environment, the backscatter link frequency is offset by -20% to 512kHz. At this time, the actual oversampling rate of the receiver is 28.8. If the standard oversampling rate OSR is used, std =24 for tag synchronization and demodulation, which will cause the demodulation sensitivity performance to drop sharply and the communication distance to be reduced exponentially.

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

[0123] The OSR rough estimation module roughly estimates the frequency offset of the backscatter link of the tag signal, taking into account Taking K = 5 can basically cover the 22% frequency deviation required by the standard. Figure 2Determine whether the backscatter link frequency offset occurs and estimate the oversampling rate of the output rough estimate, where J sum is the average value of J obtained by comparing the first 160 calculations. According to the frequency offset of the backscatter link in the embodiment, the final calculated J sum Should be approximately equal to 2.44, the estimated oversampling ratio OSR of the output R =2*2.44+24=28.88.

[0124] Synchronization module, set the oversampling interval step of the precise estimation OSR =0.1, then the output of the previous OSR rough estimation module can be approximated with this accuracy: OSR R =28.88≈28.9. The number of relevant modules for parallel computing is selected as needed. The smaller the interval and the more parallel sub-modules are used, the more accurate the estimation is, but the hardware computing resources consumed are also increased accordingly.

[0125] In this embodiment, N=9 is selected, and there are 9 correlation modules in the synchronization module as submodules. The input signal is converted into a 1-bit sequence under different OSRs, and the correlation coefficient between the input sequence and the standard preamble code corresponding to the OSR is calculated, and the most correlated coefficient value Conv is output. max The synchronization module will provide different input parameters OSR for 9 related modules loca , here OSR loca They are 28.5, 28.6, 28.7, 28.8, 28.9, 29.0, 29.1, 29.3, and 29.4 respectively, comparing 9 different OSRs loca The calculation results of the relevant modules below are Conv max , and select the OSR corresponding to the maximum value loca As the OSR accurate value OSR S Output, in this example it should be OSR S =28.9, and at the same time confirm the leading code position at the time when the maximum value appears, completing the synchronous detection of the tag.

[0126] Matching demodulation module, according to the OSR estimated by synchronization module S = 28.9, using the local 0 / 1 bit sequence corresponding to OSR, according to Figure 3 The process determines the input data as 0 / 1 bit output and completes the demodulation of the input signal. Among them, the local standard preamble or standard 0 / 1 bit sequence corresponding to different OSRs stored inside the synchronization module and the matching demodulation module 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. std, the resampling factor is the target oversampling rate. Since the sequence after conversion is 1-bit, different interpolation schemes and filtering schemes are used in the resampling algorithm, which has little effect on the final sequence generation. The present invention uses linear interpolation and Kaiser window low-pass FIR anti-aliasing filtering.

[0127] At the same time, under Miller8 coding, if there is no backscatter link frequency offset, 10·M·OSR is saved in the relevant calculations of synchronization and demodulation. std +2 M OSR S Here, M=8, OSR S =24, when implemented in hardware, computing resources of 2304 multipliers are required. Even if eight-way time-division multiplexing of the multipliers is considered, 288 multipliers are still required. When the bit width of the multiplier is 16 bits, a total of about 33,408 LUT (lookup table) units are required to implement it using FPGA (field programmable gate array). However, using the 1-bit sequence operation in the present invention, only 1152 LUT units are required, and the comprehensive saving of hardware computing resources reaches 96.5%, which can effectively reduce the cost required for hardware implementation and reduce the corresponding power consumption.

[0128] In summary, the present invention can detect and estimate whether the passive tag backscatter signal has frequency offset, and obtain the frequency of the tag backscatter link after frequency offset and the system oversampling rate by estimation. The present invention reduces the hardware computing resource consumption of direct estimation by making a rough estimate first and then a fine estimate.

[0129] The present invention performs DC removal preprocessing on the input signal and determines it as a 1-bit sequence, so that no multiplier is required in the subsequent correlation value calculation, and only basic XOR logic operation is required, which can save a total of 10·M·OSR in hardware implementation. std Plus 2 M OSR S The multiplier resources are used and replaced by corresponding XOR logic units, which significantly reduces the cost of hardware implementation.

[0130] The present invention aims at the tag signal under the backscatter link frequency offset, and uses the local standard signal with different oversampling rates for calculation, so as to meet the preamble synchronization and signal demodulation under the backscatter link frequency offset of ±22% required in ISO / IEC 18000-6C without losing the communication sensitivity performance.

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

Claims

1. A synchronous demodulation method for a passive RFID tag, characterized in that: The following steps are involved: Perform DC removal and smoothing filtering operations on the input signal and output a pre-processed signal; According to the signal peak distribution, roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal, and calculate the oversampling rate of the output rough estimate; Calculate the correlation coefficient between the preprocessed signal and the standard preamble code at different oversampling rates, and preliminarily determine the possible position of the preamble code in the tag signal; Compare the calculation results of the correlation coefficients under different oversampling rates, identify and output the accurate estimated value of the oversampling rate, and complete the synchronous detection of the preamble code; According to the accurate estimation 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 operation, thereby completing the demodulation of the input signal.

2. The synchronous demodulation method of a passive RFID tag according to claim 1, characterized in that: The passive tag backscatter signal is amplitude keyed modulated, using return-to-zero Miller coding, and adding a subcarrier to improve the signal's anti-interference ability; Let the standard oversampling rate of the system when there is no backscatter link frequency deviation be OSR std , the coding mode of Miller subcarrier modulation is M, and the envelope signal backscattered by the tag received by the system at time n is R(n); The input signal is subjected to DC removal and smoothing filtering operations, and the preprocessed signal is output, specifically: Sliding window statistics R(n) at 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 get R DC (n) is: Then, mean sliding window smoothing is performed to highlight the alternating peak characteristics of subcarrier modulation. The smoothing window length is The preprocessed signal S(n) is obtained as: Where m represents the index variable in the summation process.

3. The synchronous demodulation method of a passive RFID tag according to claim 2, characterized in that: According to the signal peak distribution, the backscatter link frequency offset of the tag signal in the preprocessed signal is roughly estimated, and the oversampling rate of the rough estimate is calculated and output, specifically: Take the parameter as K, then the judgment window length is 2K+1, and execute the following steps in sequence: S21: At time t, take S(tK), S(t-K+1), S(t-K+2), ..., S(t+K-1), S(t+K), a total of 2K+1 sampling points; S22: Take the modulus value of 2K+1, recorded 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 among them, and record the corresponding maximum value index J as -K, -K+1, -K+2, ..., K+1, K; S24: Calculate the sum J of the index J in the first 20·M steps S23 sum , if J sum If it is close to 0, it is considered that the backscatter link frequency of the current tag backscatter signal has not shifted. Otherwise, calculate the rough estimated oversampling rate OSR under the current backscatter link frequency offset. R =2·J sum +OSR std ; S25: Adjustment Order Then, return to step S21.

4. The synchronous demodulation method of a passive RFID tag according to claim 2, characterized in that: Calculate the correlation coefficient between the preprocessed signal and the standard preamble at different oversampling rates, and preliminarily determine the possible position of the preamble in the tag signal, specifically: Calculate the preamble correlation value and pre-store the standard preamble sequence corresponding to different oversampling rates. When there is no backscatter link frequency offset, the preamble sequence length corresponding to the standard oversampling rate is 10·M·OSR std , the preamble code sequences corresponding to different oversampling rates are calculated by the resampling algorithm and stored; When the stored data is called, the input parameter OSR loca As the local oversampling rate, and read the OSR in 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 .

5. The synchronous demodulation method of a passive RFID tag according to claim 4, 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 the positive and negative polarities, and the result is: The sliding window calculates the correlation value Conv(n) between the input sequence p(n) and the local standard preamble sequence Preamble(n), and obtains: After the start of the stimulus, the passive tag starts to reply the backscatter signal within a specific time period and starts the communication with the preamble code. The maximum value of Conv(n) is found within the specific time period. max And output, and use Conv max The position where it appears is used as the preamble position, which can indicate the start position of communication.

6. The synchronous demodulation method of a passive RFID tag according to claim 5, characterized in that: The input sequence p(n) and the local standard preamble sequence Preamble(n) are both 1-bit sequences and only require basic XOR logic operations for calculation.

7. The synchronous demodulation method of a passive RFID tag according to claim 5, characterized in that: Compare the calculation results of the correlation coefficients under different oversampling rates, identify and output the precise estimate of the oversampling rate, and complete the synchronous detection of the preamble code, specifically: Set the oversampling rate estimation accuracy step OSR , perform precise estimation in N parallel paths, N is an odd number, and according to the rough estimated OSR R , set different oversampling rate parameters OSR for N channels loca : OSR loca =OSR R +k·step OSR ; In the formula, And k∈Z; After the N-way correlation values ​​are calculated, compare the Conv max , with N-channel output Conv max OSR corresponding to the maximum value loca , as the final accurate estimate of the system oversampling rate OSR S The corresponding preamble code position is used as the communication start position for subsequent data demodulation.

8. The synchronous demodulation method of a passive RFID tag according to claim 7, characterized in that: According to the accurate estimation 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 operation to complete the demodulation of the input signal, specifically: The standard 0 / 1 bit sequences corresponding to different oversampling rates are stored in advance. When no backscatter link frequency offset 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 by the resampling algorithm and stored; When calculating, the input oversampling rate parameter OSR S As the local oversampling rate, and read the OSR in local storage S The corresponding 0 / 1 bit sequence is used as the standard 0 / 1 bit sequence p0(n) and p1(n), and the sequence length of p0(n) and p1(n) is M·OSR S .

9. The synchronous demodulation method of a passive RFID tag according to claim 8, characterized in that: Execute the following steps in sequence: S51: At time t, calculate the correlation value between the input sequence p(n) and the standard 0 sequence p0(n) within the adjacent 2L+1 time points Recorded as It is expressed as: In the formula, The superscript 0 represents 0 bit, and the subscript i represents 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 It is expressed as: In the formula, The superscript 1 represents 1 bit, and the subscript i represents the adjacent time; S52: Take The maximum value is denoted as C max ; S53: If C max The corresponding superscript is 0, then the 0 of the current data is judged to be 0 bit and output. If C max If the corresponding superscript is 1, the current data is judged to be 1 bit and output; S54: If all the signals to be demodulated have been processed, the loop step ends; otherwise, C max The corresponding subscript value is recorded as I, and t is adjusted to t = t + M·OSR S After +I, return to step S51.

10. A synchronous demodulation system for a passive RFID tag, using the synchronous demodulation method for a passive RFID tag according to any one of claims 1 to 9, characterized in that: It includes a DC removal smoothing module, an oversampling rate rough estimation module, a synchronization module and a matching demodulation module, wherein the oversampling rate rough estimation module is connected to the DC removal smoothing module, the synchronization module is connected to the oversampling rate rough estimation module, and the matching demodulation module is connected to the synchronization module; The DC removal and smoothing module is used to remove DC and smooth the input signal and output a preprocessed signal; The oversampling rate rough estimation module is used to roughly estimate the backscatter link frequency offset of the tag signal in the preprocessed signal according to the signal peak distribution, and calculate and output the rough estimated oversampling rate; Multiple correlation modules are set in the synchronization module. The correlation modules are used to calculate the correlation coefficient between the preprocessed signal and the standard preamble code at different oversampling rates, and preliminarily determine the possible position of the tag signal preamble code; the synchronization module is used to compare the calculation results of the correlation coefficient at different oversampling rates, determine and output the oversampling rate precise estimation value, and complete the synchronization detection of the preamble code; The matching demodulation module is used to accurately estimate the value according to the oversampling rate, use the corresponding local 0 / 1 bit sequence, and judge the pre-processed signal as 0 / 1 bit output through correlation operation to complete the demodulation of the input signal.

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