CN114696869B - Beidou No. three satellite RDSS service outbound signal capturing method under low signal-to-noise ratio - Google Patents
Beidou No. three satellite RDSS service outbound signal capturing method under low signal-to-noise ratio Download PDFInfo
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- CN114696869B CN114696869B CN202210271885.3A CN202210271885A CN114696869B CN 114696869 B CN114696869 B CN 114696869B CN 202210271885 A CN202210271885 A CN 202210271885A CN 114696869 B CN114696869 B CN 114696869B
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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
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
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- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
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Abstract
The invention provides a Beidou No. three satellite RDSS service outbound signal capturing method under low signal-to-noise ratio, which comprises the following steps of: i, Q obtained by multiplying intermediate frequency signal inputted by intermediate frequency signal with generated local carrier signalA way signal; FFT step: obtaining a baseband complex signal by using I, Q two paths of signals, respectively carrying out fast Fourier transform FFT operation on the baseband complex signal and a local pseudo code sequence, multiplying the baseband complex signal and the local pseudo code sequence, and then carrying out fast Fourier inverse transform on the multiplication result to obtain a sequence correlation value R (n); obtaining a maximum value R (R) and a maximum value occurrence position R from the sequence correlation value R (n); and a detection judgment step: r (R) and a preset judgment threshold are set as V t And comparing, when the capturing is successful, taking the current local carrier frequency as Doppler frequency shift of the captured Beidou No. three satellite RDSS service outbound signal, and taking the position R where R (R) appears as code phase of the captured Beidou No. three satellite RDSS service outbound signal. The invention has the characteristics of high capturing speed and less occupation of hardware resources.
Description
Technical Field
The invention relates to a satellite communication technology, in particular to a technology for capturing a Beidou No. three satellite RDSS service outbound signal.
Background
The Beidou No. three GEO satellite RDSS business outbound signal provides functional services such as positioning report, bidirectional timing, message communication and the like for users on the basis of completely reserving the Beidou No. two RDSS outbound signal. The civil signal of the RDSS service outbound signal of the new system comprises a pilot frequency branch S2C_p and a message branch S2C_d, and the two paths of signals are in UQPSK modulation relation.
The relative motion between the terrestrial receiver and the satellite causes the satellite signal to produce a doppler shift and a code phase shift, resulting in the carrier frequency and code phase of the terrestrial received signal not being coincident with the transmitted signal. In order to provide services for users, the terrestrial receiver needs to achieve reliable acquisition of satellite outbound signals (hereinafter referred to as target signals), i.e., preliminary estimation of carrier frequency offset and code phase offset, and eliminate the influence caused by relative motion. In the satellite signal receiving technology, the capturing result lays a foundation for subsequent tracking and demodulation, and capturing can be rapidly and accurately completed, so that the capturing is an important index of the performance of a receiver.
In order to obtain higher capturing probability, a multi-stay method can be adopted, namely, a multi-detection method is used for improving the overall capturing probability and reducing the false alarm probability. In the methods, the traditional M-taking detection method in N is simple in implementation mode, but has low capture probability when signal-to-noise conditions are poor, and the method has the problem of long residence time, so that the method is not beneficial to being applied to a system with high real-time requirements.
Disclosure of Invention
Aiming at the defects of an M detection method in N, the invention provides a Beidou No. three satellite RDSS service outbound signal capturing method under the condition of low signal-to-noise ratio.
The invention aims to solve the technical problems, and adopts the technical scheme that the method for capturing the Beidou No. three satellite RDSS service outbound signals under the condition of low signal-to-noise ratio comprises the steps of detecting and capturing input intermediate frequency signals by taking cells as units, dividing the intermediate frequency signals to be detected in each cell into N sections, and detecting and capturing the intermediate frequency signals of each cell according to the following steps:
and (3) a down-conversion step: multiplying the nth section intermediate frequency signal in the current cell with the generated local carrier signal, filtering out a high frequency part by a low-pass filtering module, and then enabling the obtained I, Q two paths of signals to enter an FFT step;
FFT step: obtaining a baseband complex signal s (n) by using I, Q two paths of signals, if the local pseudo code sequence is c ' (n), respectively performing fast Fourier transform FFT operation on the baseband complex signal s (n) and a time reversal sequence c ' (-n) of the local pseudo code sequence to obtain FFT (s (n)), and obtaining FFT (c ' (-n)), wherein c ' (-n) is a real sequence, and the FFT (c ' (-n)) is equivalent to FFT according to the property of Fourier transform * (c′(-n)), * Representing the conjugate. FFT (s (n)) and FFT * (c' (-n)) multiplying; finally, performing Inverse Fast Fourier Transform (IFFT) operation on the multiplication result to obtain a sequence correlation value R (n) of the local pseudo code and the baseband signal at each phase offset, where R (n) =ifft (FFT (s (n)). FFT * (c (-n)); spectral amplitudeA maximum value R (R) and a maximum value occurrence position R in the value R (n) enter a detection judgment step;
and a detection judgment step: r (R) and a preset judgment threshold are set as V t Comparing, for example, the current cell is greater than a preset decision threshold of V t If the R (R) count of the current cell reaches the successful judgment count, the acquisition is considered to be successful, if the R (R) count of the current cell is greater than the preset judgment threshold and is V t If the count of R (R) does not reach the successful judgment count and the search times do not reach N, updating n=n+1, and returning to the down-conversion step to continue the next search; if the current cell is greater than the preset decision threshold V t If the R (R) count does not reach the successful judgment count and the search times reach N, discarding the search of the current cell, adjusting the frequency of the generated local carrier wave, and carrying out the detection capture of the next cell; and when the capturing is successful, taking the current local carrier frequency as Doppler frequency shift of the captured Beidou No. three satellite RDSS service outbound signal, and taking the position R where R (R) appears as the code phase of the captured Beidou No. three satellite RDSS service outbound signal.
The invention has the advantages of being beneficial to improving the capturing probability of the Beidou No. three RDSS service outbound signals, and having the characteristics of high capturing speed and less occupation of hardware resources.
Drawings
FIG. 1 is a schematic diagram of a system;
FIG. 2 is a schematic diagram of a down conversion module;
FIG. 3 is a schematic diagram of an FFT module;
fig. 4 is a schematic diagram of a detection decision module.
Detailed Description
The system for realizing the method of the invention is shown in figure 1, and comprises a down-conversion module, a fast Fourier transform FFT module and a detection decision module.
The system detects the input intermediate frequency signals by taking the unit cells as units, and the intermediate frequency signals to be detected in each unit cell are divided into N sections. 1 segment of intermediate frequency signal is detected at a time.
The down-conversion module multiplies the input intermediate frequency signal by the local carrier signal, filters out the high frequency part through the low-pass filtering module, and then inputs the obtained I, Q two paths of signals to the next module.
The FFT module performs FFT operation on the input I, Q two paths of signals and the local pseudo code sequence, and then gives the product of the FFT operation (inverse FFT operation IFFT, and gives the maximum value of the IFFT result and the position where the maximum value appears to the detection decision module.
And the detection judgment module judges whether the capturing is finished or not according to the maximum value output by the FFT module and the position where the maximum value appears. If the acquisition is completed, outputting the acquired code phase and Doppler frequency shift to a subsequent tracking module, otherwise, searching again.
As shown in fig. 2, since the modulation relationship of the civil signal is UQPSK, the doppler shift and the code offset of the two signals are identical, and the intermediate frequency signal s' (n) of the satellite signal entering the acquisition module is defined as:
s′(n)=d(n)c(n)cos(2πfnT s )
wherein d (n) is a data code carried by the signal, c (n) is a spread spectrum code, f is a carrier frequency of the intermediate frequency signal, T s Is the baseband data rate, N is the nth signal sequence, n=1, …, N.
Through a multiplier module, the local carrier signal is multiplied by the intermediate frequency signal after cos mapping and sin mapping, and then the high frequency component is filtered through a low-pass filter, so that signals i (n) and q (n) of I, Q are obtained:
i(n)=d(n)c(n)cos(2πf d nT s )
q(n)=d(n)c(n)sin(2πf d nT s )
wherein f d Representing the difference between the local carrier frequency signal and the intermediate frequency signal, i.e. the doppler shift. The resulting I, Q signal is passed to an FFT module for further processing.
The structure of the FFT module is shown in fig. 3, and includes an FFT computation module, an IFFT computation module, a local pseudo code generation module, a multiplier module, and a maximum signal selector. For convenience of deduction, the obtained I, Q two paths of signals are expressed as baseband complex signals through Euler formula:
if the local pseudocode sequence is defined as c '(n), there is a code phase offset of δ between c (n) and c' (n). Performing fast fourier transform FFT operation on the time-reversed sequences c '(-n) of the baseband complex signal s (n) and the local pseudo code sequence respectively to obtain FFT (s (n)), FFT (c' (-n)), wherein since c '(-n) is a real sequence, FFT (c' (-n)) is equivalent to FFT according to the nature of fourier transform * (c′(-n)), * Representing the conjugate.
Depending on the nature of the fourier transform, the signal frequency domain multiplication is equal to the time domain convolution, i.e
* Representing a time domain convolution operation. It can be seen that if the operation result of the multiplier is subjected to inverse Fourier transform, a time domain correlation function can be obtained
R(n)=s(n)*c′(-n)=IFFT(FFT(s(n))·FFT * (c(-n)))
The value of R (n) represents the magnitude of the local pseudocode summed in relation to the baseband signal at each phase offset. The maximum signal selector gives the maximum value module R (R) of the spectrum amplitude and the maximum amplitude occurrence position R as selection results to the detection decision module.
The detection decision module is shown in fig. 4, and comprises a threshold detection module and a decision module. If the calculation result of the ith detection after the down-conversion and FFT module is V i The decision threshold of the detection decision module is V t Defining a successful detection judgment value as A, a detection initial value as B, counting a variable K, initializing K as B, and searching for N as an upper limit, wherein A, B is an experience value selected according to environment and requirements, and the module comprises the following processing flow:
1. the maximum value of the spectrum amplitude is modulo V i Decision threshold V t Comparing, if V i >V t K is increased by 1, otherwise K is decreased by 1.
2. If K is greater than 0 but less than a, updating i=i+1, and obtainingCalculation result V through down-conversion and FFT module i Returning to the step 1 to continue searching the current signal; if K is equal to A, judging that the capturing is successful; if K is equal to 0, discarding the current cell search, adjusting the frequency of the local carrier generation module, and carrying out the next cell search.
3. If the search times are equal to N, the current cell search is abandoned and the next cell search is performed.
If the acquisition is successful, then the local carrier frequency at this point is the acquired doppler shift, and the acquired code phase is the maximum value modulus R (R) of the spectral amplitude and the maximum amplitude occurrence position R.
Claims (1)
1. The method for capturing the Beidou No. three satellite RDSS service outbound signal under the condition of low signal-to-noise ratio is characterized by comprising the steps of detecting and capturing an input intermediate frequency signal by taking a cell as a unit, dividing the intermediate frequency signal to be detected in each cell into N sections, and detecting and capturing the intermediate frequency signal of each cell according to the following steps:
and (3) a down-conversion step: multiplying the nth section intermediate frequency signal in the current cell with the generated local carrier signal, filtering out a high frequency part by a low-pass filtering module, and then enabling the obtained I, Q two paths of signals to enter an FFT step;
FFT step: obtaining a baseband complex signal s (n) by using I, Q two paths of signals, and if the local pseudo code sequence is c ′ (n) time-reversed sequence c of baseband complex signal s (n) and local pseudo code sequence ′ Respectively performing fast Fourier transform FFT operation to obtain FFT (s (n)), FFT (c) ′ (-n)),FFT * (c ′ (-n))=FFT(c ′ (-n)), * The representation is conjugated and FFT (s (n)) is combined with the FFT * (c' (-n)) multiplying; finally, performing Inverse Fast Fourier Transform (IFFT) operation on the multiplication result to obtain a sequence correlation value R (n) of the local pseudo code and the baseband signal at each phase offset, where R (n) =ifft (FFT (s (n)). FFT * (c (-n)); a maximum value R (R) and a maximum value occurrence position R in the sequence correlation value R (n) enter a detection judgment step;
and a detection judgment step: will beR (R) is V with a preset judgment threshold t Comparing, for example, the current cell is greater than a preset decision threshold of V t If the R (R) count of the current cell reaches the successful judgment count, the acquisition is considered to be successful, if the R (R) count of the current cell is greater than the preset judgment threshold and is V t The R (R) count does not reach the successful judgment count and the searching times do not reach N, and the next searching is continued after the down-conversion step is returned; if the current cell is greater than the preset decision threshold V t If the R (R) count does not reach the successful judgment count and the search times reach N, discarding the search of the current cell, adjusting the frequency of the generated local carrier wave, and carrying out the detection capture of the next cell; and when the capturing is successful, taking the current local carrier frequency as Doppler frequency shift of the captured Beidou No. three satellite RDSS service outbound signal, and taking the position R where R (R) appears as the code phase of the captured Beidou No. three satellite RDSS service outbound signal.
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