CN103941269A - PN code capturing method used for satellite navigation system - Google Patents

Abstract

The invention discloses a PN code capturing method used for a satellite navigation system. The method achieves the facts that under the environment that the initial phases of chips are unknown, a rapid search for all chip phase information is achieved, and code phases are captured correctly. The method comprises the steps that a section of a received signal is intercepted and supplemented with the same length of zeros, a local signal is segmented, partial folding processing is conducted on the local signal, and related matching operation is conducted on the received signal and the local signal; the code phases are captured in parallel by means of circular correlation calculation of fast Fourier transformation; energy detection is conducted on a related result corresponding to each code phase, wherein when an energy detection value is greater than a preset threshold value, it is determined that capturing is successful, and otherwise, the local signal is shifted horizontally, and the operation is repeated until capturing is successful. According to the method, the calculation capability of fast Fourier transformation can be fully used, and the parallel capturing capability of a traditional zero-padding method is enhanced.

Description

PN Code Acquisition Method for Satellite Navigation System

technical field

The invention relates to the technical fields of wireless spread spectrum communication and satellite communication, in particular to a code phase acquisition method for pseudorandom code spread spectrum communication with longer period and higher code rate.

Background technique

Direct sequence spread spectrum communication has been widely used in both civilian and military communication systems. In order to capture the code phase of the input signal at the receiver, it is necessary to detect all unknown code phases of the pseudo‐noise code. In addition, the unknown Doppler shift introduced by the channel also needs to be eliminated. Therefore, PN code acquisition is a two-dimensional signal search problem in order to detect the PN code phase and Doppler shift of the incoming direct spread spectrum sequence signal. And for some military communications and global navigation satellite systems, they use long PN codes with longer periods and higher code rates, which becomes more challenging.

In the global navigation and positioning system (GPS), two pseudo-random codes, C/A code and P code, have been applied. Usually, the capture of the P code needs to depend on the relevant information of the corresponding C/A code. However, because long P codes have superior performance in anti-interference, anti-interception and anti-spoofing, it is particularly meaningful to seek the direct acquisition method of P codes. Because the direct acquisition of the P code requires a large number of parallel correlation receivers, the amount of calculation is quite large. So far, many methods for fast acquisition of P-codes have been proposed. However, it is not practical to have a large number of parallel receivers. In fact, methods that reduce both the number of correlated receivers and the acquisition time are highly desirable.

Frequency-domain signal processing technology has been widely used. Compared with time-domain processing, it has the characteristics of more convenience and various methods. Van Nee, Coenen and Davenport first proposed to use FFT to realize the fast acquisition of C/A code in 1991 [D.J.R.Van Nee and A.J.R.M.Coenen.New Fast GPS Code‐Acquisition Technique Using FFT.Electronic Letters,1991,vol.27,NO.2 :158‐160], and then a variety of schemes based on FFT to achieve circular correlation were proposed. Among them, the zero padding method is an effective acquisition method, which can detect multiple code phases in parallel by using the fast Fourier transform flexibly. See [H.Li, X.Cui, M.Lu, and Z.Feng, Generalized zero‐padding scheme for direct GPS P‐code acquisition, IEEE Trans.Wireless Commun., vol.8, no .6, pp.2866‐2871, June.2009.]. Although the zero-padding method improves the parallel capture capability, it does not make full use of the results of the fast inverse Fourier transform.

Contents of the invention

In order to solve the above problems, enhance the parallel acquisition capability of code phase, the invention provides a kind of PN code acquisition method for satellite navigation system, comprising the steps:

(a) Sampling the local baseband signal and processing the result into a complex signal; where the Doppler frequency offset of the local baseband signal can be 0Hz, and the sampling rate can be the PN code rate;

(b) Select N/2 points of the baseband signal after sampling, fill in zeros and extend to N points, then perform N-point fast Fourier transform, and then perform complex conjugate processing;

(c) Select 3N/2 points from the local code signal to get the vector $\stackrel{\&Right\; Arrow;}{r^{\′}}=[c_{0+\mathrm{\τ}},c_{1+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}},c_{N+\mathrm{\τ}},...,c_{\frac{3N}{2}-1+\mathrm{\τ}}],$ divide it into $\stackrel{\&Right\; Arrow;}{r_1}=[c_{0+\mathrm{\τ}},c_{1+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}}]$ and (a,b,c,d) two parts; by adding folding to On the first N/2 points of , a new local signal vector is formed $\stackrel{\&Right\; Arrow;}{r}=[c_{0+\mathrm{\τ}}+c_{N+\mathrm{\τ}},c_{1+\mathrm{\τ}}+c_{N+1+\mathrm{\τ}},...,c_{\frac{N}{2}-1+\mathrm{\τ}}+c_{\frac{3N}{2}-1+\mathrm{\τ}},c_{\frac{N}{2}+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}}],$ right Do fast Fourier transform;

(d) multiplying the result of the fast Fourier transform of step (b) by step (c), and then inverse Fourier transforming the result;

(e) save all N point elements of the result of step (d);

(f) If the maximum correlation result (that is, the peak value of the energy detection) exceeds the preset threshold, the rough detection is successful and enters the tracking stage (tracking); otherwise, if no peak value exceeds the preset threshold or the tracking fails, the N-point local code signal, go to step (a) and repeat the above steps;

(g) When the tracking is successful, the correct code phase of the received signal can be obtained according to the position of the maximum correlation result in step (f); when the first point among the N results of the inverse Fourier transform is the position of the maximum value, There may be ambiguity, and at this time, the peak ambiguity problem can be solved by two time-domain matching correlations.

The local code (local code) signal in the method of the present invention means that navigation satellites can use pseudo-random code (P code) to carry out spread spectrum communication, and the period of p code is one week (very long), generally can only intercept capture signal (assuming Remove other frequency and navigation data information, just a section of the pure P code) to perform correlation matching operations (applying the autocorrelation characteristics of the pseudo-random code), the local code signal refers to the entire period of the P code.

In step (c) of the present invention, (a, b, c, d) represent the local code (i.e. P code), τ represents the initial code phase of the position, |τ|<1, N=1024, where It serves as a vector illustration and has no specific meaning.

Since the present invention is based on Fast Fourier Transform, the number of points of Fast Fourier Transform can be adjusted to adapt to the problem of code phase capture under low SNR; the larger N in the above, can adapt to lower SNR Environment; in general, the value of N is related to the hardware and is not infinite.

The preset threshold in the present invention is set according to the signal-to-noise ratio and the preset false alarm probability in practice, and the preset threshold is related to the noise item.

The method of the invention makes full use of the operation result of the fast Fourier transform, can obtain twice the enhancement of the parallel capture ability, and shows superiority in the average capture time. Due to the strong parallel acquisition capability of the code phase, the present invention is especially suitable for a code acquisition system with high initial code phase uncertainty.

Description of drawings

Fig. 1 is a flow chart of the method of the present invention.

Fig. 2 is an explanatory diagram of circular correlation of fast Fourier transform results of the PN code acquisition method of the present invention.

Fig. 3 is a graph of average capture time performance of an embodiment of the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

As shown in Figure 1, the flow process of the inventive method is as follows:

(a) sampling the local baseband signal and processing the result as a complex signal;

(b) Select N/2 points of the baseband signal after sampling and extend it to N points with zero padding, then perform N-point fast Fourier transform, and then process through complex conjugate;

(c) Select 3N/2 points from the local code signal to get the vector $\stackrel{\&Right\; Arrow;}{r^{\&prime;}}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}},c_{N+\mathrm{\&tau;}},...,c_{\frac{3N}{2}-1+\mathrm{\&tau;}}],$ divide it into $\stackrel{\&Right\; Arrow;}{r_1}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}]$ and (a,b,c,d) two parts; by adding $\stackrel{\&Right\; Arrow;}{r_2}=[c_{N+\mathrm{\&tau;}},...,c_{\frac{3N}{2}-1+\mathrm{\&tau;}}]$ fold to $\stackrel{\&Right\; Arrow;}{r_1}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}]$ On the first N/2 points of , a new local signal vector is formed $\stackrel{\&Right\; Arrow;}{r}=[c_{0+\mathrm{\&tau;}}+c_{N+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}}+c_{N+1+\mathrm{\&tau;}},...,c_{\frac{N}{2}-1+\mathrm{\&tau;}}+c_{\frac{3N}{2}-1+\mathrm{\&tau;}},c_{\frac{N}{2}+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}],$ right Do fast Fourier transform;

(d) multiplying the result of the fast Fourier transform of step (b) by step (c), and then inverse Fourier transforming the result;

(e) save all N point elements of the result of step (d);

(f) If the maximum correlation result (peak value) exceeds the preset threshold, the rough detection is successful and enters the tracking stage; otherwise, if no peak value exceeds the preset threshold or the tracking fails, shift the N-point local code signal and go to step ( a) Repeat the above steps;

(g) When the tracking is successful, the correct code phase of the received signal can be obtained according to the position of the maximum correlation result; when the first point of the N results of the inverse Fourier transform is the position of the maximum value, there may be ambiguity. The peak ambiguity problem can be solved by two traditional time-domain matching correlations.

The sampling rate in step (a) of the above embodiment may also be a PN code rate.

The Doppler frequency offset of the local baseband signal in step (a) of the above embodiment may be 0 Hz.

The object to be processed by the main technical solution of the present invention, that is, the model of the communication system involved is as follows: the satellite signal is first reduced to low frequency through a mixer, and then sampled after low-pass filtering. The obtained equivalent low-pass complex signal can be expressed as

s _l ＝s _I,l +js _Q,l , (l＝0,1,2,...)

in

A represents the signal amplitude, c _l+ι represents a pseudo-random code, and its value is {+1,-1}, d _l+ι represents BPSK data information, and its value is in the signal set {+1,-1}, f _D represents the unknown carrier offset, Δt represents the sampling interval, Indicates the random carrier phase, and n _I,l indicates Gaussian white noise with mean value 0 and variance 2σ ² .

As shown in Figure 2, each sector of the ring represents a local code signal of a different phase, which is circularly correlated with the received signal, and the circular correlation here is realized by circular convolution of Fourier transform. Here N=8, (a,b,c,d) is the received signal, (a ₁ ,b ₁ ,...,h ₁ ,a ₂ ,b ₂ ,c ₂ ,d ₂ ) is the local code signal, Perform partial folding processing on the local signal, that is, fold (a ₂ , b ₂ , c ₂ , d ₂ ) onto (a ₁ , b ₁ , c ₁ , d ₁ ), and after the zero-padding method undergoes inverse Fourier transform, Only the complete correlation results of the first N/2 points are kept, and the rear N/2 points are discarded because they are partial correlation results; due to the application of partial folding technology, the method of the present invention undergoes Fourier transform and inverse Fourier After the transformation, the output results are all completely correlated values between the sampling point of the received signal and the local signal code, and there is no partial correlation problem.

As shown in Figure 3, the abscissa in the figure is the signal-to-noise ratio (SNR), and the ordinate is the average capture time (T _D ). The two curves in the figure are respectively represented from top to bottom:

The performance of the zero-padding method (ZP (zero-padding), T_ZP represents the average capture time of the zero-padding method);

The performance of the method of the present invention (T_MZP represents the average capture time of the method of the present invention); N is taken as 1024, and the number of times of non-coherent integration is 1.

Claims (3)

1. for a PN code capture method for satellite navigation system, it is characterized in that, comprise the steps:

(a) local baseband signal is sampled and be complex signal by result treatment;

(b) choose sample N/2 point the zero padding of baseband signal afterwards and extend to N point, then carry out the conversion of N point quick Fourier, remake complex conjugate and process;

(c) from local code signal, choose 3N/2 point, obtain vector $\stackrel{\&RightArrow;}{r^{\&prime;}}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}},c_{N+\mathrm{\&tau;}},...,c_{\frac{3N}{2}-1+\mathrm{\&tau;}}],$ Be divided into $\stackrel{\&RightArrow;}{r_1}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}]$ (a, b, c, d) two parts; By inciting somebody to action fold into front N/2 point on, form new local signal vector $\stackrel{\&RightArrow;}{r}=[c_{0+\mathrm{\&tau;}}+c_{N+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}}+c_{N+1+\mathrm{\&tau;}},...,c_{\frac{N}{2}-1+\mathrm{\&tau;}}+c_{\frac{3N}{2}-1+\mathrm{\&tau;}},c_{\frac{N}{2}+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}],$ Right do Fast Fourier Transform (FFT);

(d) step (b) and the result of the Fast Fourier Transform (FFT) of step (c) are multiplied each other, then its result is made to inversefouriertransform;

(e) preserve all N point elements of step (d) result;

(f), if largest correlation result has surpassed default thresholding, rough detection success, enters tracking phase; Otherwise if do not have largest correlation result surpass default thresholding or follow the tracks of unsuccessfully, translation N point local code signal, goes to step (a) and repeats above-mentioned steps;

(g), when following the tracks of successfully, according to the position of the largest correlation result in step (f), just can access the correct code phase that receives signal; In N result of inversefouriertransform first while being peaked position, if there is blur level, carry out twice time domain coupling relevant.

2. method according to claim 1, is characterized in that in step (a), sampling rate is PN bit rate.

3. method according to claim 1 and 2, the Doppler frequency deviation that it is characterized in that local baseband signal in step (a) is 0Hz.