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 capture method for satellite navigation system

Technical field

The present invention relates to radio spread spectrum communication and technical field of satellite communication, relate in particular to the phase acquisition method of the pseudo-random code spread spectrum communication that the cycle is long, code check is higher.

Background technology

No matter, in civilian or military channels, Direct Sequence Spread Spectrum Communication is all widely used.In order to catch the code phase of input signal at receiver end, all unknown codes phase places that detect pseudorandom (pseudo noise) code are necessary.In addition the unknown Doppler shift of being introduced by channel also needs to be eliminated.Therefore,, in order to detect PN code phase and the Doppler shift of the direct spread spectrum sequence signals of input, it is the problem of a 2D signal search that PN code is caught.And for some military communications and GLONASS (Global Navigation Satellite System), what they used is long PN code, the cycle is longer, code check is higher, and this just becomes and has more challenge.

In worldwide navigation Positioning System (GPS), C/A code and P code, two kinds of pseudo-random codes have obtained application.Conventionally, P code catches the relevant information that need to depend on corresponding C/A code.Yet because long P code is anti-interference, anti-intercepting and capturing and anti-deception aspect have superior performance, the Direct Acquisition Methods of seeking P code seems meaningful especially.Because the Direct Acquisition of P code needs a large amount of Parallel correlation receivers, so his calculated amount is quite large.So far, the method for many P code fast Acquisitions is suggested.Yet it is not practicable being equipped with a large amount of parallel receivers.In fact, can reduce the method that correlation receiver quantity can shorten again capture time is to enjoy expectation always.

Frequency-region signal treatment technology is widely applied, and processes and compares with time domain, and it has more convenient, the feature that means are various.Van Nee, Coenen and Davenport in 1991, first propose to use FFT realize C/A code fast Acquisition [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], after this multiple scheme that realizes circular correlation based on FFT is suggested.Wherein, zero padding method is a kind of effective catching method, and it is by using neatly the Fast Fourier Transform (FFT) can a plurality of code phases of parallel detection.The related data of zero padding method refers to [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 zero padding method has promoted parallel capture ability, do not make full use of the result of quick inversefouriertransform.

Summary of the invention

In order to address the above problem, strengthen the parallel capture ability of code phase, the invention provides a kind of PN code capture method for satellite navigation system, comprise the steps:

(a) local baseband signal is sampled and be complex signal by result treatment; Wherein the Doppler frequency deviation of local baseband signal can be 0Hz, and sampling rate can be PN bit rate;

(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 $\stackrel{\→}{r^{\′}}=[c_{0+\mathrm{\τ}},c_{1+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}},c_{N+\mathrm{\τ}},...,c_{\frac{3N}{2}-1+\mathrm{\τ}}],$ Be divided into $\stackrel{\→}{r_1}=[c_{0+\mathrm{\τ}},c_{1+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}}]$ (a, b, c, d) two parts; By inciting somebody to action folding (folding) arrives front N/2 point on, form new local signal vector $\stackrel{\→}{r}=[c_{0+\mathrm{\τ}}+c_{N+\mathrm{\τ}},c_{1+\mathrm{\τ}}+c_{N+1+\mathrm{\τ}},...,c_{\frac{N}{2}-1+\mathrm{\τ}}+c_{\frac{3N}{2}-1+\mathrm{\τ}},c_{\frac{N}{2}+\mathrm{\τ}},...,c_{N-1+\mathrm{\τ}}],$ Right do Fast Fourier Transform (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 (being the peak value of energy measuring) has surpassed default thresholding, rough detection success, enters tracking phase (tracking); Otherwise if do not have peak value 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 may there is blur level while being peaked position, now can be by twice relevant peak value fuzzy problem that solves of time domain coupling.

Local code in the inventive method (local code) signal refers to, Navsat can use pseudo-random code (P code) to carry out spread spectrum communication, the cycle of p code is one week (very long), generally can only intercept lock-on signal and (suppose to have removed other frequency and navigation data information, being simple P code) wherein one section carries out relevant matches operation (autocorrelation performance of application pseudo-random code), and this local code signal just refers to the P code in whole cycle.

In step of the present invention (c), (a, b, c, d) represents local code (being P code), and τ represents the initial code phase positions of position, | τ | < 1, N=1024, here the effect of playing vector explanation, without concrete meaning.

Because the present invention is based on Fast Fourier Transform (FFT), can be by regulating the problem of counting to adapt to phase acquisition under low signal-to-noise ratio of Fast Fourier Transform (FFT); N is above larger, can adapt to the environment that signal to noise ratio (S/N ratio) is lower; Generally the value of N is relevant with hardware, is not infinitely-great.

Default thresholding in the present invention, is to arrange according to false-alarm probability default in signal to noise ratio (S/N ratio) and reality, and default thresholding is relevant with noise item.

The inventive method takes full advantage of the operation result of Fast Fourier Transform (FFT), can obtain the enhancing of the parallel capture ability of twice, aspect mean acquisition time, embodies superiority.Because the parallel capture ability to code phase is stronger, the present invention is particularly useful for the code capture systems that initial code phase positions uncertainty is higher.

Accompanying drawing explanation

Fig. 1 is the inventive method process flow diagram.

Fig. 2 is the circular correlation key diagram of PN code capture method Fast Fourier Transform (FFT) result of the present invention.

Fig. 3 is the mean acquisition time performance map of the embodiment of the present invention.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in further detail.

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

(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, then process by complex conjugate;

(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 $\stackrel{\&RightArrow;}{r_2}=[c_{N+\mathrm{\&tau;}},...,c_{\frac{3N}{2}-1+\mathrm{\&tau;}}]$ Fold into $\stackrel{\&RightArrow;}{r_1}=[c_{0+\mathrm{\&tau;}},c_{1+\mathrm{\&tau;}},...,c_{N-1+\mathrm{\&tau;}}]$ 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 (peak value) has surpassed default thresholding, rough detection success, enters tracking phase; Otherwise if do not have peak value 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 largest correlation result, just can access the correct code phase that receives signal; In N result of inversefouriertransform, first may there is blur level while being peaked position, now can be by twice relevant peak value fuzzy problem that solves of traditional time domain coupling.

The sampling rate of above-described embodiment step (a) can also be PN bit rate.

The Doppler frequency deviation of the local baseband signal of above-described embodiment step (a) can be 0Hz.

The required object to be processed of main technical schemes of the present invention, the model of communication system relating to is as follows: satellite-signal is first down to low frequency through frequency mixer, after low-pass filtering, then samples.The equivalent low pass complex signal obtaining can be expressed as

s _l＝s _I,l+js _Q,l, (l＝0,1,2,...)

Wherein

A represents signal amplitude, c _{l+ ι}represent pseudo-random code, its value be+1 ,-1}, d _{l+ ι}represent BPSK data message, its value in set of signals+1 ,-1}, f _drepresent unknown carrier shift, Δ t represents sampling interval, represent random carrier phase, n _i,lrepresent that average is that 0 variance is 2 σ ²white Gaussian noise.

As shown in Figure 2, each fan-shaped of annulus represents the local code signal of out of phase, and signal cycle is relevant to receiving, and the circular correlation here realizes by the cyclic convolution of Fourier transform.Here get N=8, (a, b, c, d) receives signal, (a ₁, b ₁..., h ₁, a ₂, b ₂, c ₂, d ₂) be local code signal, local signal is carried out to partially folded processing, be about to (a ₂, b ₂, c ₂, d ₂) fold into (a ₁, b ₁, c ₁, d ₁) upper, zero padding method is through after inversefouriertransform, only retained the result of the complete dependence that front N/2 orders, so rear N/2 point is owing to being that the result that part is relevant has been rejected; Due to the application of partially folded technology, the inventive method is through after Fourier transform and inversefouriertransform, and the result of output is all the value that receives the complete dependence of signal sampling point and local signal code, does not have the problem that part is relevant.

As shown in Figure 3, in figure, horizontal ordinate is signal to noise ratio snr, and ordinate is mean acquisition time (T _d), in figure, 2 curves from top to bottom represent respectively:

The performance of zero padding method (ZP(zero padding), T_ZP represents the mean acquisition time of zero padding method);

The performance of the inventive method (T_MZP represents the mean acquisition time of the inventive method); N gets 1024, and non-coherent integration times 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.