An improved RFID reader frequency estimation method and estimation module

Abstract

This invention discloses an improved frequency estimation method and module for RFID readers. The method includes: using a first correlator group with multiple first parallel correlators to perform correlation processing on I and Q signals respectively, and using the frequency offset of the correlator corresponding to the maximum correlation value as a coarse estimated frequency offset; using a second correlator group to further perform correlation processing on the I and Q signals; calculating the power of the correlation values ​​output by each correlator, adding and combining the I and Q power, and then performing serial peak detection to obtain the correlator frequency offset corresponding to the peak value, thereby calculating the most approximate estimated value of the received signal frequency. This invention proposes a frequency estimation method based on correlation operations, which differs from the traditional method using correlator arrays. This invention employs a two-stage correlator group to perform correlation operations on the preamble and synchronization frame header respectively to obtain the frequency offset, and then calculates the precise frequency estimate.

Description

Technical Field

[0001] This invention relates to the field of electronic technology, and specifically to an improved RFID reader frequency estimation method and corresponding estimation module. Background Technology

[0002] RFID is a contactless radio frequency identification technology that uses electromagnetic wave induction and radio waves for contactless communication and data exchange to identify objects. Currently, RFID technology is widely used in logistics, tracking, monitoring, access control, and other fields. Due to the advantages of passive tags, such as low cost, small size, long lifespan, and ease of production, they have become the mainstream application in RFID systems. However, passive systems suffer from problems such as unreliable tag power supply, weak return signals, and susceptibility to spatial interference, placing higher demands on readers.

[0003] Because passive tags lack an internal clock calibration circuit, the link frequency returned by the tag will deviate from the data frequency set by the reader by -22% to +22%. Additionally, the data frame header used for synchronization in the received signal is very short, and the synchronization header appears only once in each received data frame. If the header cannot be parsed, communication will fail.

[0004] Considering the unfavorable factors analyzed above, RFID systems still need further improvement and refinement in order to be better applied. Summary of the Invention

[0005] This invention mainly proposes an effective solution to the problem of received signal frequency deviation.

[0006] Specifically, the present invention provides an improved RFID reader frequency estimation method, characterized in that the method includes:

[0007] Step 1: Receive RFID radio frequency signals and convert them into I and Q signals;

[0008] Step 2: The I and Q signals are correlated using a first correlator group with multiple first parallel correlators respectively. The multiple first parallel correlators have a preset frequency offset from each other and the multiple first parallel correlators have the same local code.

[0009] Step 3: Compare the correlation results obtained by each correlator, and take the frequency offset of the correlator corresponding to the maximum correlation value as the coarse estimate of the frequency offset;

[0010] Step 4: Perform correlation processing on the I and Q signals using a second correlator group with multiple second parallel correlators. The second parallel correlators have a second frequency offset from each other, and the frequency offset of the second correlator group is set to cover the coarsely estimated frequency offset.

[0011] Step 5: Calculate the power of the correlation values ​​output by each second parallel correlator, add and combine the I and Q power, perform serial peak detection, obtain the correlator frequency offset corresponding to the peak value, and then calculate the most approximate estimate of the received signal frequency.

[0012] In a preferred implementation, the first correlator group covers at least -22% to 22% of the frequency offset of the target RFID tag's transmission frequency.

[0013] In another preferred implementation, the first correlator group includes 8-14 correlators; the second correlator group includes 3-6 correlators.

[0014] In another preferred implementation, the method further includes counting each symbol in the received data sequentially. When the serial peak value output by the correlator is retrieved in step 5, the corresponding data count value is used as the position of the synchronization frame header, and this position information is used as the initial position for reading data from the storage unit.

[0015] On the other hand, the present invention provides an improved RFID reader frequency estimation module, characterized in that the RFID reader frequency estimation module includes: a first correlator group having multiple first correlators, a second correlator group having multiple second correlators, a comparison module, a power calculation module, and a peak detection module.

[0016] The plurality of first correlators are used to perform correlation processing on the I and Q signals received by the RFID reader, and the plurality of first parallel correlators have a preset frequency offset from each other, and the plurality of first parallel correlators have the same local code.

[0017] The comparison module is used to compare the correlation results obtained by each correlator and use the frequency offset of the correlator corresponding to the maximum correlation value as the coarse estimated frequency offset.

[0018] The plurality of second parallel correlators are used to perform correlation processing on the I and Q signals. The second parallel correlators have a second frequency offset from each other, and the frequency offset of the second correlator group is set to cover the coarsely estimated frequency offset.

[0019] The power calculation module is used to calculate the power of the correlation values ​​output by each second parallel correlator, and to add and combine the I and Q power.

[0020] The peak detection module is used to perform serial peak detection and obtain the correlator frequency offset corresponding to the peak value in order to obtain the most approximate estimate of the received signal frequency.

[0021] In another preferred implementation, the first correlator group covers at least -22% to 22% of the frequency offset of the target RFID tag's transmission frequency.

[0022] In another preferred implementation, the first correlator group includes 8-14 correlators; the second correlator group includes 3-6 correlators.

[0023] In another preferred implementation, an offset counter is also included. The offset counter is used to count each symbol in the received data sequentially. When the serial peak value output by the correlator is retrieved, the corresponding data count value is used as the position of the synchronization frame header. This position information is used as the initial position for reading data from the storage unit.

[0024] In another preferred implementation, a storage unit is also included for storing the received data.

[0025] Technical effect

[0026] This invention proposes a frequency estimation method based on correlation operations. Unlike the traditional method of frequency estimation using correlator arrays, this paper uses a two-stage correlator group to perform correlation operations on the preamble and the synchronization frame header to obtain the frequency offset, and then calculates the accurate frequency estimate.

[0027] First, this invention uses a multi-channel (12-channel) parallel correlator as the first-stage correlator group for coarse frequency estimation. Each correlator's local symbol consists of 2 symbol preamble symbols, and its frequency offset estimation accuracy is 4%. The frequency offset is obtained by taking the maximum correlation value. Then, based on the frequency offset, a 4-channel parallel correlator is used as the second-stage correlator group to perform approximate frequency estimation of the received signal. Each correlator's local symbol consists of 6 symbol synchronization frame header symbols, and its frequency offset estimation accuracy reaches 1% of the base frequency (preset RFID signal frequency). An approximate frequency estimate is calculated based on the frequency offset. At the same time, during the second-stage correlation power peak detection of the synchronization frame header, the position information of the synchronization frame header in the received data is extracted. Therefore, the starting position of the data after removing the synchronization frame header in the received data can be obtained. This provides accurate frequency estimates and received data start identification information for subsequent data demodulation, and under the same signal-to-noise ratio, it more effectively improves the demodulation accuracy, thereby optimizing the reader's receiving performance. Attached Figure Description

[0028] Figure 1This is a block diagram of the improved frequency estimation structure based on the method of the present invention;

[0029] Figure 2 This is a schematic diagram of the architecture of the frequency coarse estimation module based on the method of the present invention;

[0030] Figure 3 This is a schematic diagram of the architecture of the frequency approximation estimation module based on the method of the present invention;

[0031] Figure 4 To test the received data waveform (time-domain signal characteristics of the received code data frame at a signal-to-noise ratio of 6dB, with frequency offset);

[0032] Figure 5 The waveform diagram of the test code data is shown (time domain signal characteristics of the test received code data frame at a signal-to-noise ratio of 6dB).

[0033] Figure 6 This is the time-domain waveform for power peak detection. Detailed Implementation

[0034] The RFID reader synchronization method of the present invention will be described in detail below with reference to the accompanying drawings.

[0035] The improved frequency estimation method for the RFID reader in this embodiment adopts the following... Figure 1 The architecture design shown is as follows. This frequency estimation design includes two parts: coarse frequency estimation and approximate frequency estimation.

[0036] 1. Frequency coarse estimation module

[0037] The frequency coarse estimation module samples 12 sets of correlators to perform a coarse frequency estimation of the received signal (including the synchronization frame header and data information), such as... Figure 2 As shown, the module architecture consists of a preamble correlator group module, a maximum value extraction module, and a coarse frequency estimate acquisition module.

[0038] By definition of the correlation function, correlation calculation can be expressed as the sum of the product of two correlation functions over a given time interval, as shown below:

[0039]

[0040] Equation (1) is the cross-correlation function of signals x(n) and y(n). This equation represents r xy The correlation calculation value at time m is equal to the product of the corresponding sequences of x(n) and y(n) after shifting y(n) left by m sampling points while keeping x(n) unchanged. In this invention, x(n) represents the local code element within the correlation, and y(n) is the input signal, i.e., the received signal.

[0041] Because the signals returned by RFID tags have frequency deviations, this embodiment uses 12 parallel correlators to perform correlation processing on the received signals to achieve preamble acquisition, in order to obtain better frequency estimation accuracy. The frequency deviation range covered by the correlator group is -22% to 22%, and there is a 4% frequency difference between any two correlators relative to the fundamental frequency. Each correlator in this embodiment uses the same local symbol, i.e., the preamble of the input signal, but each correlator uses a different frequency. Miller2 is selected as the embodiment. Each symbol of the Miller2 subcarrier preamble represents two fundamental frequencies, and each fundamental frequency is oversampled 20 times. When the fundamental frequency deviation is -22%, the number of sampling points of the correlator is:

[0042]

[0043] When the fundamental frequency deviation is 22%, the number of sampling points of the correlator is:

[0044]

[0045] Based on the above calculations, the number of sampling points for correlators corresponding to different frequency offsets can be obtained. The frequency offset and the number of sampling points for each fundamental frequency in the correlator group are shown in Table 1. This embodiment uses two symbols as the local code of the correlator. Taking the minimum number of sampling points as an example, the local code of the correlator corresponding to a frequency offset of 22% for Miller2 is: Miller2_preamble=66'b01_0101_0101_0101_1111_1111_1111_1111_1101_0101_0101_0101_0111_1111_1111_1111_1111_1111_1111.

[0046] In this embodiment, each correlator is designed as a complex correlator, meaning that each correlator performs correlation operations on the I and Q data simultaneously. The correlation results calculated by each correlator are input into the maximum value module, which compares each correlation result, selects the maximum value, and uses the correlator frequency offset corresponding to the maximum correlation value as the frequency offset output by the frequency coarse estimation module.

[0047] 2. Frequency approximation estimation module

[0048] Because the data frame header (synchronization header data) usable for synchronization in the received signal is very short, containing only 6 symbols, this embodiment uses a 4-channel parallel correlator as a second-stage correlator group to approximate the frequency offset estimation in order to increase the success rate of capturing the synchronization header. Figure 3 As shown.

[0049] This module employs a correlator group consisting of four approximate correlators to correlate the received I and Q data. The reference base frequency for this approximate correlator group is the frequency offset output from the coarse frequency estimation (for example, assuming the frequency offset obtained from the coarse frequency estimation is 22%, the second group of correlators uses this frequency offset as a basis for offset settings, such as 22%, 21%, 20%, and 19% relative to the base frequency, respectively, covering the range covered by the corresponding correlators in the first correlator group, but with finer differentials). Based on this, four more groups of correlators are used to set the frequency offset. The frequency offset setting method for each correlator is the same as the coarse frequency estimation setting method described above. As shown in Table 2, the frequency estimation value can achieve a frequency deviation of 1% relative to the base frequency, further improving the frequency estimation accuracy. In this embodiment, Miller2 is used as a reference example, with the synchronization frame header symbol 010111. Each correlator uses a 6-symbol synchronization frame header as its local symbol. Assuming the frequency offset of the coarse frequency estimate is 18%, according to Table 2, the number of fundamental sampling points corresponding to the approximate correlator 2 is 17.09, and the number of local symbols of the correlator is 6*2*17.09=205.8. Similarly, the number of local symbols of each correlator can be calculated, and the local symbol information can be converted with reference to the synchronization frame header symbols.

[0050] To increase the accuracy of synchronization frame header detection, the power of the correlation values ​​output by each correlator in the second group of correlators is calculated (by squaring the correlation value), and the power of the I and Q channels is added together. Then, serial peak detection is performed to obtain the correlator frequency offset corresponding to the peak value, and then the most approximate estimate of the received signal frequency is calculated.

[0051] Simultaneously, an offset counter is set in the frequency approximation estimation module to record the deviation between the received data (including preamble and data information) and the peak point of the correlation result. When the receiver starts receiving data frames, the offset counter starts counting for each received symbol. When the correlation power peak of the correlator group is found, the accumulated value of the received data, i.e., the accumulated value of the offset counter, is the position information of the synchronization frame header (the correlation peak is largest only when all six symbols are synchronized, determining that these six symbols are the synchronization frame header). The next position information (of the six symbols) is used as the initial position for reading data in the storage unit. From this moment on, the data in the data buffer unit is the received data excluding the synchronization frame header. The next position of the offset counter at this time is the true data start point. Therefore, the improved synchronization method in this embodiment can obtain more accurate frequency estimates and synchronization frame header position information, providing frequency and effective data information for the subsequent data decoding module.

[0052] Simulation tests show that the improved frequency estimation method based on a two-stage correlator array can be applied in channel environments with a signal-to-noise ratio (SNR) greater than 6 dB. In this embodiment, a test received data with an SNR of 6 dB is introduced. This test data consists of the preamble, synchronization frame header, and random data from the protocol, sampled at a 20x oversampling rate. Figure 4 As shown. Since the received test data is the sampled data after ADC sampling and down-conversion, the received signal is mapped before code data processing. When the received data is greater than 0, the sampled data is mapped to 01; when the received data is less than 0, the sampled data is mapped to 11. The waveform of the code data after mapping is shown below. Figure 5 As shown. Secondary frequency estimation is performed on the received code data, where... Figure 6 The time-domain waveform of power peak detection when the frequency is approximated.

[0053] Table 1 Correlator Assembly Parameters

[0054] 1 22％ 16.39 2 18％ 16.94 3 14％ 17.54 4 10％ 18.18 5 6％ 18.86 6 2％ 19.6 7 -2％ 20.4 8 -6％ 21.27 9 -10％ 22.22 10 -14％ 23.25 11 -18％ 24.3 12 -22％ 25.64

[0055] Table 2 Approximate Correlator Parameters

[0056]

[0057]

[0058] Although the principles of the present invention have been described in detail above with reference to preferred embodiments, those skilled in the art should understand that the above embodiments are merely illustrative explanations of the implementation of the present invention and are not intended to limit the scope of the present invention. The details in the embodiments do not constitute a limitation on the scope of the present invention. Any obvious changes, such as equivalent transformations or simple substitutions, based on the technical solutions of the present invention without departing from the spirit and scope of the present invention fall within the protection scope of the present invention.

Claims

1. An improved method for estimating the frequency of an RFID reader, characterized in that, The method includes: Step 1, receiving RFID radio frequency signals and converting them into I and Q signals; Step 2, using a first correlator group with multiple first parallel correlators to perform correlation processing on the I and Q signals respectively, wherein the multiple first parallel correlators have a preset frequency offset from each other, and the multiple first parallel correlators have the same local code element, and each correlator's local code element is 2 symbols. The preamble symbol has a frequency offset estimation accuracy of 4%. Step 3: Compare the correlation results obtained by each correlator, and take the frequency offset of the correlator corresponding to the maximum correlation value as the coarse estimated frequency offset. Step 4: Use a second correlator group with multiple second parallel correlators to perform correlation processing on the I and Q signals. The second parallel correlators have a second frequency offset from each other, and the frequency offset of the second correlator group is set to cover the coarse estimated frequency offset. Step 5: Calculate the power of the correlation value output by each second parallel correlator, add and merge the I and Q power, and then perform serial peak detection to obtain the correlator frequency offset corresponding to the peak value. Then, calculate the most approximate estimated value of the received signal frequency. This also includes counting each symbol in the received data in sequence. When the serial peak value output by the second parallel correlator is found in step 5, the count value of the corresponding received data is taken as the position of the synchronization frame header, and the next position is taken as the initial position for reading data in the storage unit.

2. The RFID reader frequency estimation method according to claim 1, characterized in that, The first correlator group covers at least -22% to 22% of the frequency offset of the target RFID tag's transmission frequency.

3. The RFID reader frequency estimation method according to claim 1, characterized in that, The first correlator group includes 8-14 correlators; the second correlator group includes 3-6 correlators.

4. An improved RFID reader frequency estimation module, characterized in that, The RFID reader frequency estimation module includes: a first correlator group with multiple first parallel correlators, a second correlator group with multiple second parallel correlators, a comparison module, a power calculation module, and a peak detection module. The multiple first parallel correlators are used to perform correlation processing on the I and Q signals received by the RFID reader, and the multiple first parallel correlators have a preset frequency offset from each other, and the multiple first parallel correlators have the same local symbol. The comparison module is used to compare the correlation results obtained by each correlator, and use the frequency offset of the correlator corresponding to the maximum correlation value as a coarsely estimated frequency offset. The multiple second parallel correlators are used to perform correlation processing on the I and Q signals, and the second parallel... The correlators have a second frequency offset from each other, and the frequency offset of the second correlator group is set to cover the coarsely estimated frequency offset; the power calculation module is used to calculate the power of the correlation values ​​output by each second parallel correlator and add and combine the I and Q power; the peak detection module is used to perform serial peak detection, obtain the correlator frequency offset corresponding to the peak value, so as to obtain the most approximate estimate of the received signal frequency, and also includes an offset counter, which is used to count each symbol in the received data sequentially. When the serial peak value output by the correlator is retrieved, the count value of the corresponding received data is used as the position of the synchronization frame header, and the next position is used as the initial position for reading data in the storage unit.

5. The RFID reader frequency estimation module according to claim 4, characterized in that, The first correlator group covers at least -22% to 22% of the frequency offset of the target RFID tag's transmission frequency.

6. The RFID reader frequency estimation module according to claim 4, characterized in that, The first correlator group includes 8-14 correlators; the second correlator group includes 3-6 correlators.

7. The RFID reader frequency estimation module according to claim 4, characterized in that, It also includes a storage unit for storing the received data.

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