CN110018500B - A Beidou Satellite Signal Acquisition Method Based on Circumferential Shift - Google Patents

Abstract

The application discloses a Beidou satellite signal capturing method based on circumferential shift, which comprises the following steps of: s1, multiplying the input digital intermediate frequency signal in each integration time with the in-phase component and the quadrature component of a local carrier generator respectively, carrying out carrier stripping to obtain a baseband complex signal sequence X (n), and carrying out fast Fourier transform FFT to obtain a spectrum sequence X (k); s2: the ranging code C (n) is obtained after the local NH code secondary modulation, and C is obtained after fast Fourier transform FFT and complex conjugation ^* (k) The method comprises the steps of carrying out a first treatment on the surface of the S3, each time the sequence X (k) is shifted circumferentially one time, the shift l bits are denoted as sequence X (k-l) and are compared with C ^* (k) Multiplying; s4, performing Inverse Fast Fourier Transform (IFFT) on the result in S3 to obtain Doppler frequency shift f corresponding to the l shift _d Is related to the result r of the correlation _l (m); s5, entering S1, reading the digital intermediate frequency signal in the next integration time, obtaining a correlation matrix of the next adjacent integration time according to the steps, carrying out conjugate multiplication on the correlation matrices of the two adjacent integration times, and accumulating to obtain a differential coherent integration result.

Description

A Beidou Satellite Signal Acquisition Method Based on Circumferential Shift

technical field

The invention relates to the technical field of satellite communication, in particular to a Beidou satellite signal acquisition method based on circular displacement.

Background technique

With the implementation of my country's Beidou satellite navigation system, the application of satellite communication technology is becoming more and more important. The capture of satellite signals is the first step in satellite digital signal processing. The purpose is to obtain the two parameters of rough carrier Doppler frequency shift and ranging code phase of all visible Beidou satellites, so as to provide more accurate conditions for the follow-up tracking module.

The acquisition of carrier Doppler frequency and ranging code phase is a two-dimensional search process. Traditional satellite signal acquisition algorithms mainly include serial search algorithm, parallel frequency domain search algorithm and parallel code phase acquisition algorithm. The serial search acquisition algorithm is the earliest traditional method proposed by the satellite navigation system to solve the acquisition problem. The method is to conduct serial searches on the ranging code phase and the Doppler frequency shift respectively. Since the step length of the code phase search is usually half a chip, and a symbol period is longer, the serial search efficiency is low. The Doppler frequency search step is determined according to the coherent integration time, which is usually the reciprocal of the coherent integration time. Most traditional hardware receivers use serial search algorithms. The acquisition method of parallel code phase search is proposed to solve the problem of slow serial search speed. This method is based on Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) to convert the correlation operation of satellite signals in the time domain to the multiplication operation in the frequency domain, which reduces the time used for capture and realizes real-time processing. Foundation.

Parallel code phase acquisition algorithm is a parallel processing method, and its schematic diagram is shown in Figure 1. The input digital intermediate frequency signal is multiplied by the in-phase and quadrature components of the local carrier generator, and the carrier is stripped to obtain the I-channel and Q-channel signals respectively. After fast Fourier transform, the obtained complex signal is multiplied by the complex conjugate of the local ranging code Fourier transform, and then the obtained result is subjected to inverse fast Fourier transform (IFFT), and the correlation value is obtained by modulo. When the peak value exceeds the preset threshold, it indicates that the capture is successful. Two parameter values of the rough ranging code phase and Doppler frequency shift of the input signal are obtained therefrom. If there is no obvious peak, you need to adjust the frequency to make the local carrier generator generate the sine-cosine signal of the next frequency point, and repeat the above operation until all possible frequency units are searched. It can be drawn through simulation that the signal whose integration time is 1ms can be captured at -15dB using the parallel code phase search algorithm. However, it is difficult to capture the Doppler frequency and ranging code phase of the satellite signal in a weak signal environment.

Due to the high-speed movement of space satellites, the carrier frequency of satellite signals is shifted by Doppler frequency, which makes the acquisition of satellite signals complicated and difficult. At the same time, there is NH (Neumann-Hoffman) code modulation effect on the Beidou satellite B1 frequency point signal. Without removing the effect of NH code phase jump, the correlation integration time cannot exceed 1ms. Therefore, the gain of the signal cannot be increased by extending the integration time under low signal-to-noise ratio, and the capture sensitivity is low. At the same time, due to the excessive number of FFT operations in the parallel code phase algorithm, the related calculations are too large, so the speed and sensitivity still need to be improved.

Contents of the invention

According to the problems existing in the prior art, the present invention discloses a Beidou satellite signal acquisition method based on circular displacement, which specifically adopts the following steps:

S1: Multiply the digital intermediate frequency signal of each input integration time with the in-phase and quadrature components of the local carrier generator and perform carrier stripping to obtain the baseband complex signal sequence x(n), and then fast Fourier transform it Transform FFT to obtain its spectrum sequence X(k);

S2: The ranging code c(n) is obtained after secondary modulation by the NH code locally, and C ^* (k) is obtained through fast Fourier transform FFT and complex conjugation;

S3: each time the sequence X(k) is shifted once, and the shift is expressed as a sequence X(kl), which is multiplied by C ^* (k);

S4: the result in S3 is carried out fast Fourier transform IFFT and obtains the correlation result r _l (m) corresponding to the Doppler frequency shift f _d of this l shift, repeats S3 until l=N-1, merges all r _l (m) Obtain the correlation matrix Y(m,l), where m is the corresponding ranging code phase, and l is the number of circular shift bits. The correlation matrix can also be expressed as Y(τ,f _d ), where τ is the code phase delay, determined by τ=m/f _s , and f _d is the Doppler frequency shift, determined by f _d =l·f _s /N , where f _s is the sampling frequency;

S5: Enter S1, read the digital intermediate frequency signal in the next integration time and obtain the correlation matrix of the next adjacent integration time according to the above steps, carry out conjugate multiplication and accumulation of the correlation matrices of two adjacent integration times, and obtain Differential coherent integration results Judging whether the maximum correlation value in the obtained matrix Z(τ, f _d ) is greater than the capture threshold, if it exceeds the capture threshold, the current satellite is visible, record and save the corresponding Doppler frequency value and code phase delay value; if the matrix Z(τ , f _d ) where the maximum correlation value is less than the capture threshold, it is concluded that the capture fails.

In the above steps, the cyclic circular shift operation of the input frequency domain signal in S2 is equivalent to the fast Fourier transform after the frequency shift of the time domain signal, where the circular shift is as shown in the formula:

Among them, X(k) is the FFT of x(n), N is the data length of x(n), n=0,1,...N-1, l is the number of digits shifted by the circle, and the number input The correlation r _l (m) between the signal x(n) and the local ranging code c(n) is as follows:

Wherein, N is the number of digits of x(n), m=0,1,...N-1, l is the number of digits of the circular circular shift, and a Doppler is completed by taking the modulus of the correlation sequence result r _l (m) Intra-unit ranging code related process.

Due to the adoption of the above technical solution, the present invention provides a Beidou satellite signal acquisition method based on circular shift. This method is to replace the parallel code phase by performing circular circular shift on the frequency spectrum sequence after the carrier stripping of the input intermediate frequency signal. In the capture algorithm, multiple carrier stripping and fast Fourier transform (FFT) operations are performed on the input signal to reduce the amount of calculation; at the same time, the idea of differential integration is used to conjugate and multiply the coherent results within each integration time and then accumulate to enhance the signal. noise ratio, improving capture sensitivity.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments described in this application. Those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is the flowchart of the method of the present invention.

Fig. 2 is an effect diagram of the differential coherent integration results using the present invention under different conditions.

Figure 3 is an effect diagram using the non-coherent integration method.

Detailed ways

In order to make the technical solutions and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the embodiments of the present invention:

As shown in Figure 1, a Beidou satellite signal acquisition method based on circular shifting, the actually received satellite signal passes through the RF front end and is converted into a digital intermediate frequency signal by A/D. The details of the Beidou satellite signal acquisition algorithm process are as follows:

S1: Multiply the digital intermediate frequency signal of each input integration time with the in-phase and quadrature components of the local carrier generator and perform carrier stripping to obtain the baseband complex signal sequence x(n), and then fast Fourier transform it Transform FFT to obtain its spectrum sequence X(k);

S2: The ranging code c(n) is obtained after secondary modulation by the NH code locally, and C ^* (k) is obtained through fast Fourier transform FFT and complex conjugation;

S3: each time the sequence X(k) is shifted once, and the shift is expressed as a sequence X(kl), which is multiplied by C ^* (k);

S4: the result in S3 is carried out fast Fourier transform IFFT and obtains the correlation result r _l (m) corresponding to the Doppler frequency shift f _d of this l shift, repeats S3 until l=N-1, merges all r _l (m) Obtain the correlation matrix Y(m,l), where m is the corresponding ranging code phase, and l is the number of circular shift bits. The correlation matrix can also be expressed as Y(τ,f _d ), where τ is the code phase delay, determined by τ=m/f _s , and f _d is the Doppler frequency shift, determined by f _d =l·f _s /N , where f _s is the sampling frequency;

S5: Enter S1, read the digital intermediate frequency signal in the next integration time and obtain the correlation matrix of the next adjacent integration time according to the above steps, carry out conjugate multiplication and accumulation of the correlation matrices of two adjacent integration times, and obtain Differential coherent integration results Judging whether the maximum correlation value in the obtained matrix Z(τ, f _d ) is greater than the capture threshold, if it exceeds the capture threshold, the current satellite is visible, record and save the corresponding Doppler frequency value and code phase delay value; if the matrix Z(τ , f _d ) where the maximum correlation value is less than the capture threshold, it is concluded that the capture fails.

In the above steps, the cyclic circular shift operation of the input frequency domain signal is equivalent to the fast Fourier transform after frequency shifting of the time domain signal, where the circular shift is shown in the formula:

Among them, X(k) is the FFT of x(n), N is the data length of x(n), n=0,1,...N-1, l is the number of digits shifted by the circle, and the number input The correlation r _l (m) between the signal x(n) and the local ranging code c(n) is as follows:

Wherein, N is the number of digits of x(n), m=0,1,...N-1, l is the number of digits of the circular circular shift, and a Doppler is completed by taking the modulus of the correlation sequence result r _l (m) Intra-unit ranging code related process.

Therefore, when searching for satellite signals, it is only necessary to perform an FFT operation on the input signal, and all possible Doppler frequency units can be obtained through the operation of circular circular shift. Compared with the traditional parallel code phase acquisition algorithm, the number of FFTs is reduced, thereby improving the search speed.

In the traditional GPS satellite signal acquisition algorithm, the methods of coherent integration and non-coherent integration are generally used to improve the signal gain. Coherent integration can improve the signal-to-noise ratio by increasing the data integration time, but it will be affected by the increase in the amount of calculations caused by navigation data jumps and longer integration times. Non-coherent integration can not be limited by the jump of navigation data, but it introduces square loss, and with the increase of integration time, the square loss becomes larger and larger. Therefore, the paper adopts the differential coherent integration algorithm, and its basic idea is to multiply the correlation integral matrix conjugates at multiple integration times, and then accumulate them. Its mathematical model is as follows:

In the formula, Y _{i and} Y _i+1 are two adjacent coherent integration matrices, which can be expressed as the sum of the useful signal V _i and the noise signal N _i . The results of the useful signal matrix at adjacent moments are correlated, while the random noise is non-correlated At the same time, it has the characteristics of Gaussian noise, and the influence of noise can be reduced by superposition. Therefore, by multiplying the conjugate of the coherent integration matrix, the signal-to-noise ratio can be enhanced, and the square loss caused by the non-coherent integration can be suppressed.

The simulation experiment of the capture algorithm is carried out using the MATLAB platform. According to the simulation of the ranging code structure characteristics of the actual Beidou satellite signal, a 20ms analog intermediate frequency signal is generated. The data in each integration time is operated according to the algorithm block diagram. It is assumed that the frequency of the intermediate frequency signal is 4.092MHz, the sampling rate is 20.46MHz, and the ranging code rate is 2.046MHz. The navigation message rate is 50b/s, the Doppler frequency range is -10KHz～10KHz, and the integration time is 1ms. The results of the two capture algorithms are shown in Figure 2 and Figure 3 when the signal-to-noise ratio is -32dB. By comparing Figure 2 and Figure 3, it can be seen that by processing the 20ms simulation data, when the signal-to-noise ratio is -32dB, the difference algorithm can get an obvious peak value, and the peak value is the input signal in the pseudo-random code phase and Doppler The maximum value of the integral obtained during the frequency shift two-dimensional search is the same as the signal parameters simulated in the simulation. The non-coherent algorithm has no obvious peak, and cannot determine the Doppler frequency shift and the phase value of the ranging code in the analog signal. For the simulation data of 20ms, the algorithm operation is carried out on each integral time. When parallel code phase search is used for satellite signal data, if the Doppler frequency range is -10KHz to 10KHz, the frequency search interval is 250Hz, and the integration time is 1ms, then 20*82 FFT operations and 20* 81 IFFT operations. The improved circular shift algorithm requires 20*2 FFT operations and 20*81 IFFT operations. Wherein, an FFT operation of N points requires (Nlg ₂ N)/2 complex multiplication operations and Nlg ₂ N complex addition operations. Table 1 shows the comparison of the calculation amount of the algorithm. It can be seen that the total calculation amount of the capture algorithm is significantly reduced.

Table 1 Algorithm operation load comparison

As mentioned above, it is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any equivalent replacement or change of the inventive concepts thereof shall fall within the protection scope of the present invention.

Claims (2)

1. A Beidou satellite signal acquisition method based on circular displacement, is characterized in that comprising the following steps: S1: Multiply the digital intermediate frequency signal of each input integration time with the in-phase and quadrature components of the local carrier generator and perform carrier stripping to obtain the baseband complex signal sequence x(n), and then fast Fourier transform it Transform FFT to obtain its spectrum sequence X(k); S2: The ranging code c(n) is obtained after secondary modulation by the NH code locally, and C ^* (k) is obtained through fast Fourier transform FFT and complex conjugation; S3: each time the sequence X(k) is shifted once, and the shift is expressed as a sequence X(kl), which is multiplied by C ^* (k); S4: the result in S3 is carried out fast Fourier transform IFFT and obtains the correlation result r _l (m) corresponding to the Doppler frequency shift f _d of this l shift, repeats S3 until l=N-1, merges all r _l (m) Get the correlation matrix Y(m,l), where m is the corresponding ranging code phase, l is the number of circular shifts, the correlation matrix can be converted into Y(τ,f _d ) where τ is the code phase delay, Determined by τ=m/f _s , f _d is the Doppler frequency shift, determined by f _d =l·f _s /N, where f _s is the sampling frequency; S5: Enter S1, read the digital intermediate frequency signal in the next integration time and obtain the correlation matrix of the next adjacent integration time according to the above steps, carry out conjugate multiplication and accumulation of the correlation matrices of two adjacent integration times, and obtain Differential coherent integration results Judging whether the maximum correlation value in the obtained matrix Z(τ, f _d ) is greater than the capture threshold, if it exceeds the capture threshold, the current satellite is visible, record and save the corresponding Doppler frequency value and code phase delay value; if the matrix Z(τ , f _d ) where the maximum correlation value is less than the capture threshold, it is concluded that the capture fails. 2. A kind of Beidou satellite signal acquisition method based on circular shift according to claim 1, further characterized in that: wherein the circular circular shift operation of the input frequency domain signal is equivalent to that of its time domain signal after frequency shifting Fast Fourier Transform, where the circular shift is given by: Among them, X(k) is the FFT of x(n), N is the data length of x(n), n=0,1,...N-1, l is the number of digits shifted by the circle, and the number input The correlation r _l (m) between the signal x(n) and the local ranging code c(n) is as follows: Wherein, N is the number of digits of x(n), m=0,1,...N-1, l is the number of digits of the circular circular shift, and a Doppler is completed by taking the modulus of the correlation sequence result r _l (m) Intra-unit ranging code related process.