CN113595586B - Direct sequence spread spectrum signal capturing and tracking method based on MD-PMF-FFT - Google Patents
Direct sequence spread spectrum signal capturing and tracking method based on MD-PMF-FFT Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
Abstract
The invention belongs to the technical field of satellite communication, and discloses a direct sequence spread spectrum signal capturing and tracking method based on fast Fourier transform and partial matched filtering of mobile data, which is suitable for a spread spectrum signal capturing and tracking method under the condition of large carrier Doppler frequency offset; and continuously moving the reading starting point of the buffered sampling data in smaller steps through the size comparison of the FFT result to finish the tracking of the spread spectrum signal; meanwhile, carrier capture can be realized through FFT processing, and carrier tracking can be accurately realized through a Costas loop after carrier capture is finished.
Description
Technical Field
The invention relates to the technical field of satellite communication, in particular to a direct sequence spread spectrum signal capturing and tracking method based on Fast Fourier Transform and partial Matched filtering (namely MD-PMF-FFT, moving Data-Part of the Matched Filter-Fast Fourier Transform) of mobile Data, which is suitable for a spread spectrum signal capturing and tracking method under the condition of large carrier Doppler frequency offset.
Background
Compared with the conventional communication mode, the spread spectrum communication technology has the characteristics of excellent noise resistance, interference resistance and information concealment, is widely applied to the military and civil fields, and develops rapidly. Spread spectrum communication techniques require the receiver to use the same pseudo-random code as the sender in implementing the de-spreading process and recover the information code in the original information.
Spread spectrum communication systems are divided into direct sequence spread spectrum systems, time-modulated spread spectrum systems, frequency hopping spread spectrum systems, and the like, wherein the direct sequence spread spectrum systems multiply pseudo-random codes and information codes to generate spread spectrum signals for transmission. The realization mode is simple, and the application is wider. In the information transmission process, the anti-interference performance and the concealment performance of the information are important indexes for measuring the quality of an information transmission mode. Particularly in the field of satellite communication, the stability and security of communication are increasingly emphasized. The pseudo-random code period used in spread spectrum communication is generally longer, so that the bandwidth of a spread spectrum signal is much larger than that of an original information code, the power spectral density is lower, and the interference of other wireless signals is reduced. Meanwhile, under the condition of unknown pseudo random code of a sender, the original information code is difficult to solve. Thus, the purposes of improving the information interference resistance and concealment are achieved.
The core idea of the traditional despreading method is that in the time domain, the received signal and the local pseudo code are subjected to correlation processing through an integral accumulation method to realize pseudo code acquisition. However, such methods have limitations, that is, when the method of moving the local pseudo code realizes pseudo code acquisition, the correlation of the code is affected during data switching, and when the doppler frequency offset exceeds the rate of the information code, the correlation processing in the time domain cannot obtain a peak value, and thus acquisition and tracking of a spread spectrum signal cannot be realized.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to provide a direct sequence spread spectrum signal capturing and tracking method based on MD-PMF-FFT, by which under the condition that the Doppler frequency of a signal exceeds the information code rate, the reading starting point of cache sampling data is moved according to the processing result of fast Fourier transform, and the capture of a spread spectrum signal is rapidly realized; and continuously moving the reading starting point of the buffered sampling data in smaller steps through the size comparison of the FFT result to complete the tracking of the spread spectrum signal; meanwhile, carrier capture can be realized through FFT processing, and carrier tracking can be accurately realized through a Costas loop after carrier capture is finished.
In order to achieve the purpose, the invention is realized by adopting the following technical scheme.
The direct sequence spread spectrum signal acquisition and tracking method based on the MD-PMF-FFT comprises the following steps:
step 1, receiving a direct sequence spread spectrum signal, and performing digital down-conversion processing on the direct sequence spread spectrum signal to obtain a spread spectrum signal with a carrier wave removed;
step 2, a local pseudo code generating module is formed by utilizing a shift register, local pseudo codes are generated by the local pseudo code generating module, and a lag pseudo code branch, an instant pseudo code branch and an advance pseudo code branch are sequentially obtained by delaying the local pseudo codes by half code elements each time;
multiplying the late pseudo code branch, the immediate pseudo code branch and the early pseudo code branch with the carrier wave-removed spread spectrum signal respectively, and performing integral accumulation to obtain a late pseudo code capture signal, an immediate pseudo code capture signal and an early pseudo code capture signal respectively;
step 3, respectively carrying out complex FFT processing on the lag pseudo code capturing signal, the instant pseudo code capturing signal and the advance pseudo code capturing signal, and respectively and correspondingly obtaining a complex FFT result of the lag pseudo code capturing signal, a complex FFT result of the instant pseudo code capturing signal and a complex FFT result of the advance pseudo code capturing signal;
judging whether the pseudo code is successfully captured or not according to the complex FFT result of the instant pseudo code capture signal, if the pseudo code capture fails, moving the mark for reading the input data by half code element, and repeating the step 2; if the pseudo code is successfully captured, performing step 4;
step 4, after the pseudo code is successfully captured, utilizing the maximum value D of the complex FFT result of the advanced pseudo code captured signal max And a maximum value F of complex FFT results of said late pseudo-code acquisition signal max Carrying out pseudo code tracking; meanwhile, the maximum value of the real FFT result of the real-time pseudo code capturing signal is used for carrying out carrier capturing;
and 5, after the carrier wave capturing is successful, utilizing the instant pseudo code capturing signal to track the carrier wave.
The technical scheme of the invention has the characteristics and further improvements that:
(1) In step 1, the direct sequence spread spectrum signal is obtained by converting an information code and a pseudo code from an unsigned number to a signed number, multiplying the information code and the pseudo code, and finally carrying out carrier modulation on a result after multiplication operation.
(2) In step 1, the digital down-conversion processing is as follows:
the local numerical control oscillator generates an original carrier signal through a frequency control word, and the spread spectrum signal with the carrier removed is obtained through limited-length single-bit impulse response low-pass filtering after the direct sequence spread spectrum signal is multiplied by an I path and a Q path in the original carrier signal.
(3) In step 2, the stage number of the shift register is a, and the period of the generated pseudo code is 2 a -1。
(4) In step 2, the half code element is M/2 sampling points; where M is the number of samples of one symbol.
(5) In step 3, the method for judging whether the pseudo code is successfully captured comprises the following steps: maximum value G of complex FFT result of signal captured by the instantaneous pseudo code max Average value G of complex FFT results of the instantaneous pseudo code capture signal multiplied by K mean By comparison, if G max ≥KG mean If yes, the pseudo code is successfully captured; if G is max <KG mean If so, indicating that the pseudo code acquisition fails; wherein K is more than or equal to 8 and less than or equal to 16.
(6) In step 3, the number of points of the complex FFT is 2n.
(7) In step 3, the flag for reading the input data is a read enable flag of the module for storing the input data, and data reading is started only when the flag meets the condition.
(8) In step 4, the method for tracking the pseudo code comprises the following steps:
comparing the maximum value D of the complex FFT results of the advanced pseudo code acquisition signal max And the maximum value F of the complex FFT result of the delayed pseudo code acquisition signal max (ii) a If D is max >F max Moving the data to a sampling point along the advance direction; if D is max <F max Moving the data to a sampling point in the direction behind the delay; if D is max ＝F max If so, the pseudo code tracking is successful, and the synchronization of the spread spectrum signal is completed.
(9) In step 4, the carrier capture method includes:
multiplying the signals of the I path and the Q path of the instant pseudo code capture signal, then performing real FFT processing, and obtaining the frequency corresponding to the maximum value by using the maximum value of the real FFT processing, thereby obtaining a frequency traction control word;
then adding the default fixed frequency control word and the frequency traction control word and inputting the sum to a numerically controlled oscillator to generate traction carrier waves; if the frequencies of the traction carrier and the spread spectrum signal carrier are the same, the carrier capture is successful.
(10) In step 5, the carrier tracking method includes:
respectively carrying out integral accumulation processing on the signals of the path I and the path Q of the instant pseudo code capture signal, and respectively processing the processing results by an arc tangent phase discriminator and a loop filter to obtain a loop filtering frequency control word;
adding the loop filtering frequency control word with the fixed frequency control word and the frequency traction control word in the step 4 to obtain a numerical control oscillator control word; the numerical control oscillator control word is used for controlling a local numerical control oscillator to generate a local carrier signal; after digital down-conversion processing is carried out on the local carrier signal and the spread spectrum signal of the updated and read starting point, the local carrier signal and the spread spectrum signal are multiplied by an instant pseudo code branch and are subjected to integral accumulation processing, and two paths of signals I, Q of the carrier tracking signal can be obtained; whether the carrier tracking link is successful or not can be judged by utilizing the I, Q two paths of signals.
(11) In step 5, the method for judging whether the carrier tracking link is successful comprises the following steps:
the I, Q signals of the carrier tracking signals are respectively squared to obtain I 2 、Q 2 A signal;
to I 2 And 2Q 2 Thirty size comparisons are made for signals, if more than twenty I 2 >2Q 2 If the carrier tracking is successful, the carrier tracking is considered to be successful; otherwise, if the carrier tracking fails, the carrier tracking step is continuously circulated until the carrier tracking is successful.
Compared with the prior art, the invention has the beneficial effects that:
the invention indirectly moves the data by moving the starting point of the read data, thereby avoiding the influence on the code correlation when the data is switched. Compared with the traditional time domain correlation synchronous pseudo code processing process, the speed is higher by utilizing the FFT processing result to move the data reading starting point. Meanwhile, the problem that pseudo codes cannot be synchronized in time domain correlation processing in a scene with large Doppler frequency offset is solved.
Drawings
The invention is described in further detail below with reference to the figures and specific embodiments.
FIG. 1 is a schematic flow chart of a direct sequence spread spectrum signal acquisition and tracking method based on MD-PMF-FFT;
FIG. 2 is a schematic flow chart of pseudo code capture for mobile data according to the present invention;
FIG. 3 is a schematic flow chart of pseudo code tracking of mobile data according to the present invention;
FIG. 4 is a schematic diagram of the present invention for carrier acquisition and tracking by FFT pulling and Costas loop;
fig. 5 shows a carrier signal, a PN code, and a carrier signal modulated by the PN code in sequence from top to bottom;
fig. 6 shows three signals from top to bottom, namely a pseudo code synchronization signal, a carrier synchronization signal and a I, Q synchronization success signal in the process of a pseudo code synchronization to a carrier synchronization to a synchronization success; wherein a synchronization signal of 1 indicates a successful synchronization.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by those skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention.
Referring to fig. 1, the direct sequence spread spectrum signal acquisition and tracking method based on the MD-PMF-FFT comprises the following steps:
step 1, receiving a direct sequence spread spectrum signal, and performing Digital Down Conversion (DDC) processing on the direct sequence spread spectrum signal to obtain a spread spectrum signal with a carrier wave removed.
Specifically, the input signal is a spread spectrum signal obtained by performing two steps of high-frequency carrier modulation and pseudo code modulation on an information code, the spread spectrum signal acquisition and tracking system generates two paths of signals with the same frequency but with the phase difference of 1/4 cycle through a local numerical control oscillator, and two paths of cos signals and sin signals are multiplied by I, Q of the input signal respectively. As is apparent from the sum and difference formula, the multiplied result can be regarded as a signal with a frequency of 2 times the carrier and a low-frequency signal with a frequency close to zero frequency in the frequency domain. The signals pass through an FIR low-pass filter at the same time, so that the signals with the frequency of 2 times of the carrier waves can be filtered, and the low-frequency signals with the frequency close to zero frequency are reserved, so that the digital down-conversion processing of the input signals is completed, and the spread spectrum signals with the carrier waves removed are obtained.
Step 2, a local pseudo code generating module is formed by utilizing a shift register, local pseudo codes are generated by the local pseudo code generating module, and a lag pseudo code branch, an instant pseudo code branch and an advance pseudo code branch are sequentially obtained by delaying the local pseudo codes by half code elements each time; wherein, the stage number of the shift register is a, the period of the generated pseudo code is 2 a -1; each symbol samples M points.
Specifically, referring to fig. 2, the data is indirectly moved by moving the initial reading mark of the cache data, so as to achieve the pseudo code capture. First, data with two pseudo code lengths, i.e. 2 x (2) a -1) M dots. And does not update the cached data until the arrival of the pseudo-code capture flag. The pseudo code branch generated by the local pseudo code generator is used as an advanced pseudo code branch, the pseudo code with half of the code element length is delayed as an instant pseudo code branch, and the pseudo code with half of the pseudo code length is delayed as a delayed pseudo code branch.
And multiplying the late pseudo code branch, the immediate pseudo code branch and the early pseudo code branch with the carrier wave-removed spread spectrum signal respectively, and performing integral accumulation to obtain a late pseudo code capture signal, an immediate pseudo code capture signal and an early pseudo code capture signal respectively and correspondingly.
And 3, respectively carrying out complex FFT processing on the lag pseudo code capturing signal, the instant pseudo code capturing signal and the advance pseudo code capturing signal, and respectively and correspondingly obtaining a complex FFT result of the lag pseudo code capturing signal, a complex FFT result of the instant pseudo code capturing signal and a complex FFT result of the advance pseudo code capturing signal.
Specifically, the real-time pseudo code capture signal is subjected to complex FFT processing, and the maximum value of each complex FFT processing result is reserved. Repeating twice the period of the pseudo code in this way, and enabling the maximum value G of the maximum value of FFT results of twice the period of the pseudo code max And average value G mean And storing the data for size comparison. Wherein, the number of points of the complex FFT is 2n.
Judging whether the pseudo code is successfully captured according to the complex FFT result of the instant pseudo code capture signal, which specifically comprises the following steps:
maximum value G of complex FFT result of instant pseudo code capture signal max Average value G of complex FFT results of K times instantaneous pseudo code capture signal mean And (3) comparison: if G is max <KG mean If the pseudo code capture fails, the step 2 is repeated by moving the initial mark of the read cache data by half the length of the code element; if G is max ≥KG mean If the pseudo code capturing is successful, pulling up the pseudo code capturing success flag and updating the cached input signal data in real time, and turning to step 4, after the pseudo code capturing is realized, the code deviation between the data and the local instant pseudo code branch can be ensured within half code element, namely M/2 sampling data points. Wherein K is more than or equal to 8 and less than or equal to 16. The mark for reading the input data is a read enabling mark of the module for storing the input data, and the data reading is started only when the mark meets the condition.
Step 4, after the pseudo code is successfully captured, utilizing the maximum value D of the complex FFT result of the advanced pseudo code captured signal max And the maximum value F of the complex FFT result of the delayed pseudo code acquisition signal max Carrying out pseudo code tracking; meanwhile, the maximum value of the real FFT result of the real-time pseudo code capturing signal is used for carrying out carrier capturing;
multiplying the signals of the I path and the Q path of the instant pseudo code capture signal, then performing real FFT processing, and obtaining the frequency corresponding to the maximum value by using the maximum value of the real FFT processing, thereby obtaining a frequency traction control word; then adding the default fixed frequency control word and the frequency traction control word and inputting the sum to a numerically controlled oscillator to generate traction carrier waves; if the frequencies of the traction carrier and the spread spectrum signal carrier are the same, the carrier capture is successful.
Specifically, referring to fig. 3, the data is indirectly moved by moving the initial reading mark of the cache data, so as to realize the pseudo code tracking. And after the pseudo code capturing is realized, a pseudo code tracking link is switched to. Maximum value D of complex FFT result of signal captured by calculating advanced pseudo code max Maximum value F of complex FFT result of sum lag pseudo code acquisition signal max And comparing the twoThe maximum value is used to adjust the position of the start point of the read data, which is as follows:
if D is max >F max Moving the initial point position of the read data to a sampling point along the advance direction; if D is max <F max Moving the starting point position of the read data to a sampling point in the direction of the delay back; if D is max ＝F max And if so, indicating that the pseudo code tracking is successful, so that the acquisition and tracking of the input spread spectrum signal are realized, namely the input spread spectrum signal and the local pseudo code are synchronously completed.
Referring to fig. 4, after the acquisition of the pseudo code is completed, the system starts to perform carrier acquisition at the same time as the pseudo code tracking starts. And (3) respectively performing J-point integral accumulation processing on the I, Q signals of the instant pseudo code branch in the step (2), and performing F-point real FFT processing to obtain the frequency point position corresponding to the signal frequency. However, the real FFT processing cannot determine whether the signal frequency is positive or negative, so the result of the complex FFT in the pseudo code acquisition step is used to determine whether the frequency is positive or negative. And converting the frequency point position into a frequency control word and inputting the frequency control word into a local numerically controlled oscillator. If the frequency difference between the carrier wave (i.e. the pulling carrier wave) output by the local numerically-controlled oscillator and the carrier frequency of the spread spectrum signal is within K times of the FFT resolution, namely the frequency of the pulling carrier wave is the same as that of the carrier wave of the spread spectrum signal, the capture is considered to be successful, wherein (K is more than or equal to 1 and less than or equal to 2). At which point carrier tracking begins to start. If the acquisition fails, the step 5 is continuously executed until the carrier acquisition is successful.
And 5, after the carrier wave is successfully captured, utilizing the instant pseudo code capturing signal to track the carrier wave.
Specifically, referring to fig. 4, step 5 is to perform carrier tracking work by the system after carrier acquisition is successful. 2n-1 integral accumulation processing is carried out on I, Q two paths of signals of the instant pseudo code capturing signal, the processing result is input into an arc tangent phase discriminator to obtain angle value information of original information, then the angle value information is input into a loop filter, the result output by the loop filter is converted into a loop filtering frequency control word, and the loop filtering frequency control word is sent to a local numerical control oscillator. The invention adopts a second-order loop filter, and the coefficient of the loop filter directly influences the performance of a Costas loop in a carrier tracking link.
The digital controlled oscillator is a module for controlling the frequency of the output signal by a frequency control word, and the module can generate a sin signal and a cos signal which have the same frequency corresponding to the control word but are different in phase by 1/4 period, and input the sin signal and the cos signal into the digital down-conversion module in the local carrier input step 1. Specifically, adding the loop filtering frequency control word with the fixed frequency control word and the frequency traction control word in the step 4 to obtain a numerically controlled oscillator control word; the numerical control oscillator control word is used for controlling a local numerical control oscillator to generate a local carrier signal; after digital down-conversion processing is carried out on the local carrier signal and the spread spectrum signal of the updated and read starting point, the local carrier signal and the spread spectrum signal are multiplied by an instant pseudo code branch and are subjected to integral accumulation processing, and two paths of signals I, Q of the carrier tracking signal can be obtained; the method can judge whether the carrier tracking link is successful or not by utilizing the I, Q signals of the carrier tracking signal, and comprises the following specific judgment method: the two paths of signals I, Q of the carrier tracking signals are respectively squared to obtain I 2 、Q 2 A signal; to Q 2 And 2Q 2 Thirty size comparisons are made for signals, and if more than twenty times Q are made 2 >2Q 2 If the carrier tracking is successful, the carrier tracking is considered to be successful; otherwise, if the carrier tracking fails, the carrier tracking step is continuously circulated until the carrier tracking is successful.
Simulation experiment
The effect of the present invention will be further described with reference to Simulink simulation experiments.
(1) Simulation conditions
The system clock, namely the sampling frequency is set to be 60MHz, the number of stages of the shift registers of the sending end and the receiving end for generating the pseudo code is 10, the period of the pseudo code is 1023, and the number of points of complex FFT processing is 1024 during capturing the pseudo code. In carrier acquisition, the number of real FFT points is 16384, the pseudo code rate is 5M, and the number of sampling points of each code element is 12.
(2) Emulated content
The pseudo code offset of an analog input spread spectrum signal is 5 code elements, the carrier frequency offset is set to be 100KHz, a simulation system is established by using Simulink, and whether the spread spectrum signal method can realize acquisition and tracking of the spread spectrum signal is tested.
(3) Analysis of simulation results
Fig. 5 shows a carrier signal generated in the Simulink system, a PN code, and a carrier signal modulated by the PN code, and tests whether the modulated carrier signal can be used as an input signal to realize the acquisition and tracking of a spread spectrum signal quickly and stably. Fig. 6 shows two signal waveforms of the local digitally controlled oscillator I, Q in the pseudo code synchronization and carrier synchronization processes. It can be seen that the I, Q signals gradually tend to be stable, and the synchronous mark signal is observed to be pulled high and stable in a short time, which verifies that the method can realize capture and tracking of the signal with larger Doppler frequency offset in a short time.
Although the present invention has been described in detail in this specification with reference to specific embodiments and illustrative embodiments, it will be apparent to those skilled in the art that modifications and improvements can be made thereto based on the present invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (8)
1. The direct sequence spread spectrum signal acquisition and tracking method based on the MD-PMF-FFT is characterized by comprising the following steps of:
step 1, receiving a direct sequence spread spectrum signal, and performing digital down-conversion processing on the direct sequence spread spectrum signal to obtain a spread spectrum signal with a carrier wave removed;
step 2, a local pseudo code generating module is formed by utilizing a shift register, local pseudo codes are generated by the local pseudo code generating module, and a lag pseudo code branch, an instant pseudo code branch and an advance pseudo code branch are sequentially obtained by delaying the local pseudo codes by half code elements each time;
multiplying the late pseudo code branch, the immediate pseudo code branch and the early pseudo code branch with the carrier wave-removed spread spectrum signal respectively, and performing integral accumulation to obtain a late pseudo code capture signal, an immediate pseudo code capture signal and an early pseudo code capture signal respectively;
step 3, respectively carrying out complex FFT processing on the lag pseudo code capturing signal, the instant pseudo code capturing signal and the advance pseudo code capturing signal, and respectively and correspondingly obtaining a complex FFT result of the lag pseudo code capturing signal, a complex FFT result of the instant pseudo code capturing signal and a complex FFT result of the advance pseudo code capturing signal;
judging whether the pseudo code is successfully captured or not according to the complex FFT result of the instant pseudo code capturing signal, if the pseudo code is failed to be captured, moving the mark for reading the input data by half code element, and repeating the step 2; if the pseudo code is successfully captured, performing step 4;
step 4, after the pseudo code is successfully captured, utilizing the maximum value D of the complex FFT result of the advanced pseudo code captured signal max And a maximum value F of complex FFT results of said late pseudo-code acquisition signal max Carrying out pseudo code tracking; meanwhile, the maximum value of the real FFT result of the real-time pseudo code capturing signal is used for carrying out carrier capturing;
the carrier capturing method comprises the following steps:
multiplying the signals of the I path and the Q path of the instant pseudo code capture signal, then performing real FFT processing, and obtaining the frequency corresponding to the maximum value by using the maximum value of the real FFT processing, thereby obtaining a frequency traction control word;
then adding the default fixed frequency control word and the frequency traction control word and inputting the sum to a numerically controlled oscillator to generate traction carrier waves; if the frequencies of the traction carrier and the spread spectrum signal carrier are the same, the carrier capture is successful;
and 5, after the carrier wave capturing is successful, utilizing the instant pseudo code capturing signal to track the carrier wave.
2. The method according to claim 1, wherein in step 1, the direct sequence spread spectrum signal is obtained by converting an information code and a pseudo code from an unsigned number to a signed number, multiplying the unsigned number and the pseudo code, and finally performing carrier modulation on a result of the multiplication.
3. The method for direct sequence spread spectrum signal acquisition and tracking based on MD-PMF-FFT according to claim 1, wherein in step 1, the digital down-conversion process is:
the local numerical control oscillator generates an original carrier signal through a frequency control word, and the spread spectrum signal with the carrier removed is obtained through limited-length single-bit impulse response low-pass filtering after the direct sequence spread spectrum signal is multiplied by an I path and a Q path in the original carrier signal.
4. The MD-PMF-FFT (MD-PMF-FFT) -based direct sequence spread spectrum signal acquisition and tracking method of claim 1, wherein in step 2, the number of stages of the shift register is a, and the period of the generated pseudo code is 2 a -1; the half code element is M/2 sampling points; where M is the number of samples of one symbol.
5. The method for acquiring and tracking the direct sequence spread spectrum signal based on the MD-PMF-FFT as claimed in claim 1, wherein in step 3, the method for determining whether the pseudo code is successfully acquired is: maximum value G of complex FFT result of signal captured by the instantaneous pseudo code max Average value G of complex FFT results of the instantaneous pseudo code capture signal multiplied by K mean By comparison, if G max ≥KG mean If yes, the pseudo code is successfully captured; if G is max ＜KG mean If so, indicating that the pseudo code acquisition fails; wherein K is more than or equal to 8 and less than or equal to 16.
6. The method for acquiring and tracking the direct sequence spread spectrum signal based on the MD-PMF-FFT according to claim 1, wherein in step 4, the method for tracking the pseudo code is:
comparing the maximum value D of the complex FFT results of the advanced pseudo code acquisition signal max And the maximum value F of the complex FFT result of the delayed pseudo code acquisition signal max (ii) a If D is max ＞F max Moving the data to a sampling point along the advance direction; if D is max ＜F max Moving the data to a sampling point in the direction behind the delay; if D is max ＝F max If so, the pseudo code tracking is successful, and the synchronization of the spread spectrum signal is completed.
7. The method for acquiring and tracking the direct sequence spread spectrum signal based on the MD-PMF-FFT according to claim 1, wherein in step 5, the carrier tracking method is as follows:
respectively carrying out integral accumulation processing on the signals of the path I and the path Q of the instant pseudo code capture signal, and respectively processing the processing results by an arc tangent phase discriminator and a loop filter to obtain a loop filtering frequency control word;
adding the loop filter frequency control word with the fixed frequency control word and the frequency traction control word to obtain a numerical control oscillator control word; the numerical control oscillator control word is used for controlling a local numerical control oscillator to generate a local carrier signal; after digital down-conversion processing is carried out on the local carrier signal and the spread spectrum signal of the updated and read starting point, the local carrier signal and the spread spectrum signal are multiplied by an instant pseudo code branch and are subjected to integral accumulation processing, and two paths of signals I, Q of the carrier tracking signal can be obtained; the I, Q signals are used to judge whether the carrier tracking link is successful.
8. The method for capturing and tracking the direct sequence spread spectrum signal based on the MD-PMF-FFT as claimed in claim 7, wherein in step 5, the method for determining whether the carrier tracking procedure is successful is:
the I, Q signals of the carrier tracking signals are respectively squared to obtain I 2 、Q 2 A signal;
to I 2 And 2Q 2 Thirty size comparisons are made for signals, if more than twenty I 2 >2Q 2 If the carrier tracking is successful, the carrier tracking is considered to be successful; otherwise, if the carrier tracking fails, the carrier tracking step is continuously circulated until the carrier tracking is successful.
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