Spreading code capturing method based on dynamic threshold
By combining dynamic threshold and PMF-FFT algorithm, fast spreading code acquisition of hypersonic vehicles under Doppler frequency offset was achieved, which solved the problems of long acquisition time and low success rate of communication signals and improved communication reliability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-24
- Publication Date
- 2026-04-10
AI Technical Summary
Direct spread spectrum communication for hypersonic vehicles is difficult to achieve fast and accurate spread spectrum code acquisition under Doppler frequency offset, resulting in communication signal acquisition failure or inaccurate information.
A dynamic threshold-based spreading code acquisition method is adopted, which combines a triple matched filter plus Fourier transform (PMF-FFT) to perform a two-dimensional search of the phase-carrier frequency of the spreading code sequence. Through Doppler frequency offset elimination and dynamic threshold adjustment, fast acquisition is achieved.
It greatly shortens the acquisition time, improves the success rate of spreading code acquisition, avoids false locking, and adapts to environments with large dynamic Doppler frequency offset and low signal-to-noise ratio.
Smart Images
Figure CN121841394A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of digital communication, and relates to a spread spectrum code acquisition method based on a dynamic threshold. BACKGROUND
[0002] Direct spread spectrum communication (DSSS) technology can effectively reduce the power spectral density of a signal, and is beneficial to the concealment and low probability of interception of communication. However, the high flight speed of a spacecraft brings a large carrier Doppler and Doppler rate to communication. This characteristic brings great difficulty to the acquisition and tracking of a direct spread spectrum signal. Meanwhile, if the spread spectrum signal cannot be acquired for a long time, the spacecraft will be out of the communication range of a ground station. Therefore, it is particularly important for a hypersonic vehicle to design a fast acquisition method of a direct spread spectrum signal that can adapt to a large Doppler, and it is of great significance to the establishment of a space-ground communication link.
[0003] Currently, common spread spectrum code acquisition methods include serial acquisition and parallel acquisition. The serial acquisition method is to search for only one spread spectrum code sequence phase and one frequency interval each time, then adjust the carrier frequency under the same code phase, and search again. Although the serial search requires less hardware resources, its biggest disadvantage is that a large amount of time is occupied, and when the acquisition is completed, the received signal has often exceeded several chip periods. The time-domain parallel acquisition method refers to searching for a complete spread spectrum code sequence phase at a time, and changing the carrier Doppler each time. In one search of the time-domain parallel acquisition algorithm, three FFT operations are required in total. The first is to perform FFT operation on the local spread spectrum code sequence, the second is to perform FFT operation on the received signal, and the third is to perform IFFT operation on the product of the results of the two FFT operations. This method requires more computing resources, and since the Doppler frequency offset needs to be searched serially, this method is more suitable for occasions with a low dynamic range of Doppler. SUMMARY
[0004] On the basis of studying the frequency-domain parallel acquisition of direct spread spectrum communication of a hypersonic vehicle, the application designs an acquisition scheme of a direct spread spectrum signal that can adapt to a large dynamic Doppler frequency offset. The application proposes a method of two-dimensional search of spread spectrum code sequence phase-carrier frequency based on a dynamic decision threshold and a three-time matched filter plus Fourier transform (PMF-FFT) method. The method solves the problems of the reduction of a correlation peak value caused by a large Doppler frequency offset, and the inaccuracy of acquisition information or acquisition failure caused by chip drift during spread spectrum code acquisition.
[0005] To achieve the above object, the application adopts the following technical scheme: A spread spectrum code acquisition method based on a dynamic threshold, comprising the following steps: S1, the capture flow control module realizes the timing control and data flow control of the capture flow, and completes the capture flow; the capture flow is divided into three stages: the initial capture stage, the recapture stage and the confirmation capture stage; S2, the Doppler frequency offset elimination module eliminates the Doppler frequency offset of the received BPSK direct spread spectrum modulation signal corresponding to the quadrature baseband data din_I, din_Q, the capture frequency offset of the initial capture stage and the recapture stage is set to 0, and the capture frequency offset of the confirmation capture stage is the actual value acq_freq. S3, after the Doppler frequency offset elimination module processing, the obtained in-phase and quadrature data ddc_I, ddc_Q are buffered in the received signal storage module, and are used as the reference received data for spread spectrum code capture; S4, the local spread spectrum code adjustment module loads the local spread spectrum code at the beginning of each stage of the capture flow, and continuously changes the phase of the local spread spectrum code along with the capture flow, to obtain the local spread spectrum code sequence formed after phase shifting; S5, the received signal Data_I, Data_Q stored in step S3 and the local spread spectrum code Pn (n) adjusted in phase in step S4 are used as input data to enter the PMF-FFT module to perform PMF-FFT operation, and each time the local spread spectrum code phase is adjusted, a set of in-phase and quadrature signals FFT_I, FFT_Q in the frequency domain and the index FFT_index corresponding to the FFT algorithm are output. S6, the peak search module calculates the maximum value fft_max_da in each group of data and the corresponding index fft_max_Index according to the data FFT_I, FFT_Q output by the PMF-FFT module. S7, the capture judgment module judges whether the maximum value fft_max_da of each group of data meets the decision threshold, if the threshold is met, it is considered that the received signal spread spectrum code is captured; if the threshold is not met, the threshold value is changed until it is met; if the threshold is changed for 5 times and still cannot be met, it is considered that the received signal spread spectrum code is not captured, and the capture flow is restarted from S2.
[0006] The beneficial effects of the present application compared with the prior art are: (1) The method first uses three PMF-FFT algorithms, reduces the number of FFT operation points, and improves the capture time of single capture. The two-dimensional search of the spread spectrum code sequence phase-carrier frequency is realized, which greatly shortens the capture time and the success rate of spread spectrum code capture.
[0007] (2) This method is the first to use a capture decision strategy with 5 dynamic threshold adjustments, which achieves a high capture success rate under large dynamic Doppler frequency offset and low signal-to-noise ratio, while also avoiding the received signal from entering the spread spectrum code "false lock" state. Attached Figure Description
[0008] Figure 1 This is the overall flowchart of this method.
[0009] Figure 2 To capture the flowchart of the process control module.
[0010] Figure 3 This is a flowchart of the Doppler frequency offset cancellation module.
[0011] Figure 4 This is a flowchart of the received signal storage module.
[0012] Figure 5 This is cached data that has been completely stored.
[0013] Figure 6 Flowchart for adjusting the local spreading code module.
[0014] Figure 7 This is the local spreading code sequence formed after phase shifting.
[0015] Figure 8 This is a flowchart of the PMF-FFT algorithm module.
[0016] Figure 9 This is a schematic diagram of matching and partial correlation operations.
[0017] Figure 10 This is a flowchart of the peak search module.
[0018] Figure 11 To capture the flowchart of the decision-making module.
[0019] Figure 12 The results show the Doppler frequency offset and spreading code sequence phase in three PMF-FFT two-dimensional searches. Detailed Implementation
[0020] The method of this invention first adjusts the initial frequency offset and phase of the signal, then performs a coarse estimation of the code rate based on the signal bandwidth distribution, and performs joint processing of demodulation and fine code rate estimation on the signal based on the coarse estimation result, thereby obtaining a demodulated sequence and completing an accurate estimation of the signal code rate.
[0021] The present invention will be further described below with reference to specific embodiments.
[0022] like Figure 1 As shown, the spreading code acquisition method based on dynamic threshold of the present invention specifically includes the following steps: S1, the capture flow control module realizes the timing control and data flow control of the capture flow, and completes the capture flow; it is the core module of the scheme and also plays a control role for other modules. According to the capture flow, it is divided into three stages, namely, the initial capture stage, the recapture stage and the confirmation capture stage. As shown in the figure, it specifically includes the following steps: Figure 2 S1.1, according to the capture flow, it is divided into three stages, namely, the initial capture stage, the recapture stage and the confirmation capture stage.
[0023] S1.2, in the initial capture stage, after completing the initial loading of the local spreading code, the received signal is stored in the cache.
[0024] S1.3, the received signal and the local spreading code jointly complete the PMF-FFT algorithm. The capture of the spreading code is equivalent to the local spreading code sequence, which is correlated with the received signal.
[0025] S1.4, after the peak value calculation module calculates the peak value, if the peak value does not meet the threshold value required by the spreading code capture, the local spreading code sequence is shifted by 1, and the PMF-FFT operation is performed again in S1.3, until a total of 1022 shifts are still not captured, then it is determined that the capture fails, and the capture flow control module returns to step S2 to restart the capture flow. If the peak value meets the threshold value required by the spreading code capture, it enters the recapture stage of S1.5.
[0026] S1.5, in the recapture stage, after the initial capture is successful, the current captured spreading code phase is recorded, and the current spreading code sequence phase is taken as the starting point and shifted back by 16 phases.
[0027] S1.6, the local spreading code and the newly cached received signal are subjected to the PMF-FFT algorithm.
[0028] S1.7, the peak value calculation module calculates the peak value of the PMF-FFT operation output data; If the peak value does not meet the threshold value required by the spreading code capture, the local spreading code sequence is shifted by 1, and the PMF-FFT operation is performed again, until a total of 31 shifts are still not captured, then it is determined that the capture fails, and it returns to step S1.2 to restart the capture flow; if the peak value meets the threshold value required by the spreading code capture, it enters S1.8 to start the confirmation capture stage.
[0029] S1.8, after the recapture stage capture is successful, not only the current captured spreading code phase is recorded, but also the Doppler frequency shift acq_flag of the current received signal is obtained.
[0030] S1.9, in the confirmation capture stage, according to the Doppler frequency shift recorded in the recapture stage, up / down conversion processing is first performed to move the digital spectrum to the baseband.
[0031] S1.10, PMF-FFT algorithm is performed on the local spreading code under the current phase and the newly buffered received signal.
[0032] S1.11, the peak calculation module calculates the peak value of the PMF-FFT operation output data. If the correlation peak does not meet the threshold value of the spreading code capture requirement, it is considered that the capture fails, and the capture process is restarted in step S1.2; if the peak value meets the threshold value of the spreading code capture requirement, it is considered that the spreading code capture is successful.
[0033] S2, the Doppler frequency offset elimination module eliminates the Doppler frequency offset of the received BPSK direct spread spectrum modulation signal corresponding to the quadrature baseband data din_I, din_Q, the capture frequency offset in the initial capture stage and the recapture stage is set to 0, and the capture frequency offset in the confirmation capture stage is the actual value acq_freq. As shown in Figure 3 , the specific implementation steps are as follows: S2.1, according to the captured Doppler frequency offset acq_freq, the operation process is described in detail in the following S7, using a local direct digital synthesis (DDS) module, a local carrier 、 ; the capture frequency offset acq_freq in the initial capture stage and the recapture stage is 0, and the acq_freq in the confirmation capture stage is the actual captured Doppler frequency f dop， Reference S7.10; S2.2, according to the quadrature down-conversion principle, the received spread spectrum signal with large Doppler frequency offset is processed. 、
[0034] The in-phase and quadrature signals after frequency offset elimination are:
[0035] .
[0036] S3, after the Doppler frequency offset elimination module, the obtained in-phase and quadrature data ddc_I, ddc_Q are buffered in the received signal storage module for a certain amount of data as the reference received data for spreading code capture; as shown in Figure 4 , the specific implementation steps are as follows: S3.1, ddc_I, ddc_Q pass through a CIC (CIC) to obtain the decimated filtered in-phase and quadrature data, cic_I, cic_Q.
[0037] S3.2, in the initial capture phase, recapture phase, confirm capture phase, a part of the uplink data cic_I, cic_Q is stored again; under the control of write enable control, write address control, the uplink data is written into the address 1~1023 of the buffer, and the stored data is recorded as D1_1, D1_2, D1_3,..., D1_1023 respectively.
[0038] S3.3, in the same way, 7 groups of uplink data are stored again; the second group of stored data is recorded as D2_1, D2_2, D2_3,..., D2_1023, and so on, the eighth group of stored data is recorded as D8_1, D8_2, D8_3,..., D8_1023. The completely stored buffer data is as shown in the following table. Figure 5
[0039] S4, the local spreading code adjustment module completes the loading of the local spreading code at the beginning of each stage of the capture process, and continuously changes the phase of the local spreading code with the capture process to obtain the local spreading code sequence formed after phase shifting. As shown in the following table, the specific implementation steps are as follows: Figure 6 S4.1, the local spreading code sequence is pre-stored in the read-only memory (ROM).
[0040] S4.2, in the initial stage of the initial capture phase, recapture phase, confirm capture phase, under the control of read enable, read address control signal, the local spreading code in the ROM is read out, and under the control of shift enable control signal, it is written into 1023-bit shift register, completing the writing of the initial phase spreading code.
[0041] S4.3, under the control of the capture process control module, each time the phase shifting instruction is given, the 1023-bit shift register is shifted back by 1, generating a new local spreading code sequence.
[0042] S4.4, in this way, the capture process can execute a maximum of 1022 phase shifting instructions each time, and the local spreading code sequence formed after each phase shifting is as shown in the following table. Figure 7
[0043] S5, the received signal Data_I, Data_Q stored in step S3 and the local spreading code Pn(n) adjusted in phase in step S4 are used as input data together to enter the PMF-FFT module to perform PMF-FFT operation, and each time the local spreading code phase is adjusted, a group of in-phase and quadrature-phase signals in the frequency domain FFT_I, FFT_Q and the corresponding index FFT_index of the FFT algorithm are output. As shown in the following table, the specific implementation steps are as follows: Figure 8 S5.1, after adding 0 to the current phase local spreading code sequence generated in step S4.4, evenly divide it into 256 sub-matched filters, and use 4 code symbols of the current phase local spreading code sequence in each sub-matched filter to match the buffered received signal Data_in.
[0044] S5.2, the matched local spreading code and the buffered received data Data_in are subjected to partial correlation operation through a partial matched filter group, and then the output 256-point correlation result is subjected to FFT operation.
[0045] S5.3, after the FFT operation, a set of in-phase and quadrature data FFT_I, FFT_Q and their corresponding indexes FFT_index are output.
[0046] Preferably, in steps S5.1-S5.2, the implementation mode of matching and partial correlation operation is as shown in Figure 9 : for the first group of sub-matched filters PMF-1, PN1, PN2, PN3 and PN4 represent the first 4 phases of the local spreading code sequence, D1, D2, D3 and D4 represent the first 4 uplink data in the buffer, and the sum of the correlation operation of each is taken as the output of the filter , and the rest of the groups of sub-matched filters are the same.
[0047] S6, the peak search module calculates the maximum value fft_max_da in each group of data and the corresponding index fft_max_Index according to the data FFT_I, FFT_Q output by the PMF-FFT module. As shown in Figure 10 , the specific implementation steps are as follows: S6.1, square operation is performed on the in-phase and quadrature data FFT_I(n), FFT_Q(n) output by each group of FFT calculation to obtain each group of FFT_I(n) 2 , FFT_Q(n) 2 .
[0048] S6.2, add FFT_I(n) 2 , FFT_Q(n) 2 to obtain the modulus value of each group of data: Sum(n) = FFT_I(n) 2 +FFT_Q(n) 2 , n=1~256.
[0049] S6.3, after 8 times of coherent accumulation, send it to the maximum value retrieval module.
[0050] S6.4, the maximum value retrieval module outputs the peak data fft_max_da of each group of data, the corresponding index fft_max_index, and the cumulative sum fft_max_acc within the group of data.
[0051] S7, the capture judgment module judges whether the current group maximum value meets the decision threshold according to the maximum value fft_max_da of each group of data. If the threshold is met, it is considered that the current spread spectrum code of the received signal is captured. If the threshold is not met, the threshold value is changed until it is met. If the threshold is changed 5 times and still cannot be met, it is considered that the current spread spectrum code of the received signal is not captured, and the process jumps to S2 to start the capture process again. As shown in the figure, the specific implementation steps are as follows: Figure 11 S7.1, fixed threshold decision stage: after receiving the peak data fft_max_da, it is judged whether it is greater than the fixed threshold value TH_fix. The fixed threshold value TH_fix is set according to the current link environment, and the method is set to 50.
[0052] S7.2, if it is greater than the fixed threshold, the dynamic threshold decision stage is executed; if it is not greater than the fixed threshold, it is considered that the capture fails, and the capture process control module returns to S2 to start a round of capture.
[0053] S7.3, dynamic threshold decision stage: five decision thresholds are formulated according to the current link environment. The five thresholds are sorted from high to low as TH1, TH2, TH3, TH4, and TH5. Multiply the cumulative sum fft_max_acc within the group of data obtained by the peak value search module to obtain the decision values Max_TH_1, Max_TH_2, Max_TH_3, Max_TH_4, and Max_TH_5 of the five decisions. In the method of the present application: TH1=200, TH2=150, TH3=100, TH4=75, and TH5=50.
[0054] S7.4, S7.4, five thresholds are formulated as follows:
[0055]
[0056]
[0057]
[0058] .
[0059] S7.5, first compare the peak data max da and the highest threshold Max TH 1, if the peak data max da is greater than the threshold Max TH 1, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.6 is executed.
[0060] S7.6, compare the peak data max da and the second high threshold Max TH 2, if the peak data max da is greater than the threshold Max TH 2, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.7 is executed.
[0061] S7.7, compare the peak data max da and the third high threshold Max TH 3, if the peak data max da is greater than the threshold Max TH 3, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.8 is executed.
[0062] S7.8, compare the peak data max da and the fourth high threshold Max TH 4, if the peak data max da is greater than the threshold Max TH 4, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.9 is executed.
[0063] S7.9, compare the peak data max da and the fourth high threshold Max TH 5, if the peak data max da is greater than the threshold Max TH 5, it can be determined that the acquisition is successful; otherwise, it is considered that this stage acquisition fails; the acquisition flow control module will jump to S2, and a new round of spread spectrum code acquisition process is started.
[0064] S7.10, after the acquisition is successful, the current spread spectrum code phase is the received signal spread spectrum code phase; in the recapture stage, after the spread spectrum code phase is acquired, the frequency offset acquired can be calculated according to the corresponding subscript fft max Index.
[0065] f dop =Rc*fft_max_Index / 1024, wherein Rc is the rate of the spread spectrum code.
Claims
1. A dynamic threshold based spread spectrum code acquisition method, comprising: It comprises the following steps: S1, the capture flow control module realizes the timing control and data flow control of the capture flow, and completes the capture flow; the capture flow is divided into three stages: the initial capture stage, the recapture stage, and the confirmation capture stage; S2, the Doppler frequency offset elimination module eliminates the Doppler frequency offset of the received BPSK direct spread spectrum modulation signal corresponding to the quadrature baseband data din_I, din_Q, the capture frequency offset of the initial capture stage and the recapture stage is set to 0, and the capture frequency offset of the confirmation capture stage is the actual value acq_freq; S3, after the Doppler frequency offset elimination module processing, the obtained in-phase and quadrature data ddc_I, ddc_Q are cached in the received signal storage module for a certain amount of data as the reference received data for the spread spectrum code capture; S4, the local spread spectrum code adjustment module loads the local spread spectrum code at the beginning of each stage of the capture flow, and continuously changes the phase of the local spread spectrum code to obtain the local spread spectrum code sequence formed after phase shifting; S5, the received signal Data_I, Data_Q stored in step S3 and the local spread spectrum code Pn(n) adjusted in phase in step S4 are input into the PMF-FFT module as input data to perform PMF-FFT operation, and a set of in-phase and quadrature signals FFT_I, FFT_Q in the frequency domain and the index FFT_index corresponding to the FFT algorithm are output after each phase adjustment of the local spread spectrum code; S6, the peak search module calculates the maximum value fft_max_da in each group of data and the corresponding index fft_max_Index according to the data FFT_I, FFT_Q output by the PMF-FFT module; S7, the capture judgment module judges whether the maximum value fft_max_da of each group of data meets the decision threshold, if the threshold is met, it is considered that the received signal spread spectrum code is captured; if the threshold is not met, the threshold value is changed until it is met; if the threshold is changed for 5 times and still cannot be met, it is considered that the received signal spread spectrum code is not captured, and the capture flow is restarted from S2.
2. The method of claim 1, wherein The capture flow control module of step S1 comprises the following steps: S1.1, the capture flow is divided into three stages: the initial capture stage, the recapture stage, and the confirmation capture stage; S1.2, in the initial capture stage, after completing the initial loading of the local spread spectrum code, the received signal is stored in the cache; S1.3, the received signal and the local spread spectrum code jointly complete the PMF-FFT operation; S1.4, the peak calculation module calculates the peak value of the PMF-FFT operation output data; If the peak value does not meet the threshold value required for spread spectrum code capture, the phase of the local spread spectrum code is shifted by 1, and the PMF-FFT operation is performed again in S1.3, until the phase is shifted by 1022 times and the spread spectrum code is still not captured, it is determined that the capture fails, and the capture flow control module returns to step S2 to restart the capture flow; If the peak value meets the threshold value required for spread spectrum code capture, it enters the recapture stage of S1.5; S1.5, in the recapture stage, after the initial capture is successful, the current captured spread spectrum code phase is recorded, and the current local spread spectrum code phase is taken as the starting point, and 16 phases are moved backward; S1.6, the local spread spectrum code is subjected to PMF-FFT operation with the newly buffered received signal; S1.7, the peak value calculation module calculates the peak value of the PMF-FFT operation output data; If the peak value does not satisfy the threshold value required by the spread spectrum code capture, the local spread spectrum code phase is moved forward by 1, and the PMF-FFT operation is performed again, until the capture is not captured after 31 times of moving forward, and it is determined that the capture fails, and the capture process is restarted in step S1.2; If the peak value satisfies the threshold value required by the spread spectrum code capture, step S1.8 is entered, and the confirmation capture stage is started; S1.8, after the capture in the recapture stage is successful, the current captured spread spectrum code phase and the Doppler frequency offset acq_flag of the current received signal are recorded; S1.9, in the confirmation capture stage, according to the Doppler frequency offset recorded in the recapture stage, up / down conversion processing is first performed to move the digital spectrum to the baseband; S1.10, the local spread spectrum code at the current phase is subjected to PMF-FFT operation with the newly buffered received signal; S1.11, the peak value calculation module calculates the peak value of the PMF-FFT operation output data; If the correlation peak value does not satisfy the threshold value required by the spread spectrum code capture, it is considered that the capture fails, and the capture process is restarted in step S1.2; If the peak value satisfies the threshold value required by the spread spectrum code capture, it is considered that the spread spectrum code capture is successful.
3. The method of claim 1, wherein the step of determining the threshold is performed by: determining a first threshold value based on a first threshold value table; determining a second threshold value based on a second threshold value table; and determining the threshold value based on the first threshold value and the second threshold value. The Doppler frequency offset elimination module in step S2 is specifically implemented as follows: S2.1, using a local direct digital synthesizer module, generate a local carrier based on the captured Doppler frequency offset acq_freq , ; The acquisition frequency acq_freq=0 in the initial acquisition stage, the reacquisition stage, and the confirmation acquisition stage is the actual captured Doppler frequency f dop ; S2.2, according to the principle of quadrature down-conversion, the received spread spectrum signal with large Doppler frequency offset is processed to eliminate the Doppler frequency offset; , ; The in-phase and quadrature signals after frequency offset elimination are respectively: 。 4. The method of claim 1, wherein the step of determining the threshold is performed by: determining a first threshold value based on a first threshold value table; determining a second threshold value based on a second threshold value table; and determining the threshold value based on the first threshold value and the second threshold value. The received signal storage module in step S3 is specifically implemented as follows: S3.1, ddc_I and ddc_Q pass through the integration comb filter to obtain the in-phase and quadrature data after decimation filtering, cic_I and cic_Q; S3.2, in the initial capture stage, the recapture stage and the confirmation capture stage, a part of the uplink data cic_I and cic_Q will be stored again; under the control of write enable control and write address control, the uplink data is written into the addresses 1-1023 of the buffer, and the stored data is respectively denoted as D1_1, D1_2, D1_3,..., D1_1023; S3.3, in the same way, 7 groups of uplink data are stored; the second group of data stored is denoted as D2_1, D2_2, D2_3,..., D2_1023, and so on, and the eighth group of data stored is denoted as D8_1, D8_2, D8_3,..., D8_1023.
5. The method of claim 1, wherein The local spread spectrum code adjustment module in step S4 is specifically implemented as follows: S4.1, the local spread spectrum code sequence is pre-stored in the read-only memory; S4.2, in the initial stage of the initial capture stage, the recapture stage and the confirmation capture stage, the local spread spectrum code in the ROM is read out under the control of the read enable and read address control signal, and is written into the 1023-bit shift register under the control of the shift enable control signal, so that the initial phase spread spectrum code is written. S4.3, under the control of the acquisition flow control module, each time the phase shift instruction, 1023-bit shift register is shifted back 1 time, to generate a new local spreading code sequence; S4.4, in this way, each time the acquisition flow can execute 1022 phase shift instructions, each time the phase shift forms a local spreading code sequence.
6. The method of claim 1, wherein The PMF-FFT module in step S5 is implemented as follows: S5.1, after the current phase local spreading code sequence generated in step S4.4 is supplemented with 1 0, it is uniformly divided into 256 sub-match filters, and 4 symbols of the current phase local spreading code sequence are used in each sub-match filter to match the buffered received signal Data_in; S5.2, the matched local spreading code and the buffered received data Data_in are subjected to partial correlation operation through a partial match filter bank, and then the output 256-point correlation result is subjected to FFT operation; S5.3, after the FFT operation, a set of in-phase and quadrature data FFT_I, FFT_Q and their corresponding index FFT_index are output.
7. The method of claim 6, wherein the step of determining the threshold is performed by: determining a first threshold value based on a first threshold value table; determining a second threshold value based on a second threshold value table; and determining the threshold value based on the first threshold value and the second threshold value. In steps S5.1-S5.2, the matching and partial correlation operation is implemented as follows: For the first group of sub-match filters PMF-1, PN1, PN2, PN3, PN4 represent the first 4 phases of the local spreading code sequence, D1, D2, D3, D4 represent the first 4 uplink data in the buffer, which are summed after correlation operation respectively as the output of the filter The same applies to the remaining groups of sub-match filters.
8. The method of claim 1, wherein The peak value search module in step S6 is implemented as follows: S6.1, square operation on the in-phase, quadrature data FFT_I(n), FFT_Q(n) output by each group of FFT calculation, to obtain each group of FFT_I(n) 2 , FFT_Q(n) 2 ; S6.2, FFT_I(n) 2 FFT_Q(n) 2 Add them together to get the modulus of each set of data: Sum(n) = FFT_I(n) 2 + FFT_Q(n) 2 , n = 1 ~ 256; S6.3, after 8 times of coherent accumulation, it is sent to the maximum value retrieval module; S6.4, the maximum value retrieval module outputs the peak value data fft_max_da of each set of data, the corresponding index fft_max_index, and the accumulation sum fft_max_acc in the set of data.
9. The method of claim 1, wherein The acquisition judgment module in step S7 is implemented as follows: S7.1, fixed threshold decision stage: after receiving the peak value data fft_max_da, it is judged whether it is greater than the fixed threshold value TH_fix, which is set according to the current link environment; S7.2, if it is greater than the fixed threshold, the dynamic threshold decision stage is executed; if it is not greater than the fixed threshold, it is considered that the acquisition fails, and the acquisition flow control module returns to S2 to start a new round of acquisition; S7.3, dynamic threshold decision stage: according to the current link environment, 5 decision thresholds are formulated, which are sorted from high to low as TH1, TH2, TH3, TH4, TH5, and multiplied by the accumulation sum fft_max_acc in the set of data obtained by the peak value search module in step S6 to obtain 5 decision values Max_TH_1, Max_TH_2, Max_TH_3, Max_TH_4, Max_TH_5; S7.4, the 5 thresholds are formulated as follows: ； S7.5, the dynamic threshold decision stage first compares the peak value data max_da and the highest threshold Max_TH_1, if the peak value data max_da is greater than the threshold Max_TH_1, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.6 is executed; S7.6, compare the peak data max da and the second high threshold Max TH 2, if the peak data max da is greater than the threshold Max TH 2, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.7 is executed; S7.7, compare the peak data max da and the third high threshold Max TH 3, if the peak data max da is greater than the threshold Max TH 3, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.8 is executed; S7.8, compare the peak data max da and the fourth high threshold Max TH 4, if the peak data max da is greater than the threshold Max TH 4, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails, and S7.9 is executed; S7.9, compare the peak data max da and the fourth high threshold Max TH 5, if the peak data max da is greater than the threshold Max TH 5, it can be determined that the acquisition is successful; otherwise, it is considered that this acquisition fails; the acquisition flow control module will jump to S2, and a new round of spread spectrum code acquisition process is started; S7.10, after the acquisition is successful, the current spread spectrum code phase is the received signal spread spectrum code phase; in the recapture stage, after the spread spectrum code phase is acquired, the frequency offset acquired can be calculated according to the corresponding subscript fft max index. f dop = Rc * fft_max_index / 1024, where Rc is the rate of the spreading code.