CN106253947A - Low orbit satellite directly-enlarging system fast Acquisition algorithm based on double-deck time frequency analysis - Google Patents

Abstract

The present invention proposes a kind of low orbit satellite directly-enlarging system fast Acquisition algorithm based on double-deck time frequency analysis, for solving the existing technical problem low based on acquisition accuracy present in the double parallel low orbit satellite DS fast Acquisition algorithm of time-frequency, local pseudo-code is carried out locally opening up folded process in time domain by the present invention, and combines the relevant quickly removal chip fuzziness of FFT；On frequency domain, the docking collection of letters number carries out the quick analysis of spectrum of FFT, and carries out adaptive frequency domain essence compensation according to correlation peak-to-mean ratio, and concrete steps include: the local pseudo-code signal of structure and local pseudo-code open up folded signal；The docking collection of letters number carries out ground floor time frequency analysis；The docking collection of letters number carries out Doppler's coarse compensation；Reception signal after coarse compensation is carried out second layer time frequency analysis.The present invention, while ensureing acquisition speed, has the advantages that acquisition accuracy is high, can be used for the synchronization of direct sequence signal under low orbit satellite height dynamic scene.

Description

Low orbit satellite directly-enlarging system fast Acquisition algorithm based on double-deck time frequency analysis

Technical field

The invention belongs to communication technical field, relate to a kind of low orbit satellite directly-enlarging system fast Acquisition algorithm, be specifically related to A kind of low orbit satellite directly-enlarging system fast Acquisition algorithm based on double-deck time frequency analysis, can be used under low orbit satellite height dynamic scene The synchronization of direct sequence signal.

Background content

Satellite mobile communication uses the direct sequence SS of strong anti-interference performance mostly, have that overlay area is big, communication away from Away from from, the advantage such as communication maneuverability, circuit be reliable and stable, have become as an important development direction of current communication service. Dividing according to location, satellite can be divided into low orbit satellite (Low Earth Orbit, LEO), middle rail satellite (Medium Earth Orbit, MEO) and high rail satellite (Geostationary Earth Orbit, GEO).Its medium and low earth orbit satellites DS system System is compared middle high rail satellite directly-enlarging system and is had networking flexibility, and satellite transit track is low, and the less grade of satellite-ground link propagation delay time is dashed forward Go out advantage.The demand of low orbit satellite, in order to realize the target of Global coverage, will be continuously increased, no by following space-based satellite system With orbit plane many low orbit satellites can network consisting platform, to terrestrial user provide cellular service, in order to ensure The region covered provides service to terrestrial user whenever and wherever possible.

But low orbit satellite directly-enlarging system is while possessing above advantage, have also been introduced new problem.Due to motion speed Degree is fast, and receiver dynamic is higher, between low orbit satellite receiver, all can exist more between low orbit satellite and ground receiver Big carrier doppler skew and regular hour postpone, therefore compared to middle high rail and ground receiver, low orbit satellite DS The signal capture of system will face more challenges.In order to ensure high efficiency and the reliability that low orbit satellite data transmit, at height During high accuracy under dynamic environment, frequency deviation is estimated and compensated is the LEO satellite communication systems key issue that needs to solve.

Short in view of low orbit satellite Covering time, higher to the requirement of real-time of communication, it is therefore necessary to promotion signal Acquisition speed, on the other hand, due to satellite carried process resource-constrained, the complexity therefore captured can not be too high, additionally due to Low orbit satellite flight speed is fast, high dynamically large doppler for fast Acquisition process it is also proposed that the biggest challenge.Therefore, how super In the environment of large doppler, the fast Acquisition process that realizes of high efficient and reliable is the key content of current satellite communication research.

For the above feature existing for low orbit satellite directly-enlarging system signal capture, current industry is under high dynamic environment Main flow synchronization acquistion algorithm utilizes fast Fourier transform (Fast Fourier Transform, FFT) to improve acquisition speed.Its Mainly can be divided into three classes: based on the relevant fast Acquisition algorithm of time domain FFT, fast Acquisition based on frequency domain FFT analysis of spectrum Algorithm and based on the double parallel fast Acquisition algorithm of time-frequency.

Wherein utilize fast Fourier transform by chip related operation process based on the fast Acquisition algorithm that time domain FFT is relevant It is converted into frequency domain quickly to realize, all code-phase under same frequency can be completed by twice FFT process and an IFFT process The search procedure of position, i.e. achieves time-frequency two-dimensional capture dimensionality reduction to the process of frequency domain linear search.Based on frequency domain FFT analysis of spectrum Fast Acquisition algorithm, on the premise of chip is fully aligned, utilizes fast Fourier transform to receiving chip and the phase of local pseudo-code Pass value is converted into frequency domain and carries out spectrum analysis, and i.e. one time FFT can the parallel search process of all frequency domain unit.But both the above The defect of algorithm is the most respectively time domain or frequency domain to be realized parallel search, and the low rail higher at acquisition speed and required precision is defended During star communication, it is necessary to reduce capture operand further.Based on the double parallel fast Acquisition algorithm basis in time domain of time-frequency It is folded that the strong autocorrelation of pseudo-code carries out chip compression exhibition, and then realizes fast time-domain parallel capture；FFT is utilized quickly to mend on frequency domain Repaying and reception chip signal is carried out doppler analysis, and then realize frequency domain parallel capture, the double parallel feature of time-frequency makes such calculate Method has the fastest acquisition speed in acquisition algorithm based on FFT.Liu Xiaoming et al. in March, 2016 in " electronics and information Journal " article " under high dynamic environment, long code spread-spectrum signal catches algorithm soon " delivered proposes to carry out the basis of fft analysis at frequency domain On, it is fast that time domain utilizes code phase compression correlator (Compressed Code Phase Correlator, CCPC) to realize time domain Speed search, i.e. CCPC-FFT algorithm.This algorithm is simultaneously achieved the fast Acquisition of time domain and frequency domain, but the process of code associated compression In will introduce chip fuzziness, need when removing fuzziness each code phase in code phase compression phase interval is carried out by Position search, adds somewhat to the operand of time domain capture, is unfavorable for the fast Acquisition process of time domain.Additionally, code-phase is closed Compression process weakens the autocorrelation of pseudo-code, causes capturing correlation peak and declines, limits the acquisition accuracy of algorithm；Same with this Time, peak value broadening and the intrinsic fence effect of FFT that under high dynamic scene, FFT analysis of spectrum occurs can reduce capture essence further Degree, it is therefore necessary to make improvements for low orbit satellite height dynamic scene.

Summary of the invention

It is an object of the invention to the defect overcoming above-mentioned prior art to exist, propose a kind of based on double-deck time frequency analysis Low orbit satellite directly-enlarging system fast Acquisition algorithm, existing based on the double parallel low orbit satellite DS fast Acquisition of time-frequency for solving The technical problem that present in algorithm, acquisition accuracy is low.

The technical thought of the present invention is: by carrying out locally opening up folded process to local pseudo-code in time domain, and combine FFT phase Close and quickly remove chip fuzziness；Frequency domain carries out FFT spectrum analysis by the docking collection of letters number, and enters according to correlation peak-to-mean ratio Row adaptive frequency domain essence compensates, and obtains the fine estimation of chip time delay and Doppler shift.

According to above-mentioned technical thought, it is achieved the technical scheme that the object of the invention is taked is:

A kind of low orbit satellite fast Acquisition algorithm based on double-deck time frequency analysis, comprises the steps:

(1) the local pseudo-code signal p of structure_c(n) and the local folded signal l of pseudo-code exhibition_c(n), it is achieved step is:

(1a) local original pseudo-code signal p (n) is divided into equal N number of exhibition section of folding, and each exhibition section of folding is compiled Number, obtain N number of exhibition and fold the local pseudo-code truncated signal l of a length of L=M/N_iN (), wherein, M is PN-code capture, the numbering of every section For i (i=1,2 ..., N), corresponding chip position n ∈ { 1,2..., L}；

(1b) N number of local pseudo-code truncated signal l is asked for_iN the sum of correspondence position chip in (), obtains opening up folded compressed code

(1c) compressed code l folded to local original pseudo-code signal p (n) and exhibition_fN () replicates respectively, and duplication obtained Folded compressed code l of P/L local original pseudo-code signal p (n) exhibition_fN joining end to end of (), respectively obtains the local pseudo-of a length of P Code signal p_c(n) and the local folded signal l of pseudo-code exhibition_c(n)；

(2). the docking collection of letters number carries out ground floor time frequency analysis, obtains Doppler's coarse compensation valueWherein realizing step is:

(2a). the docking collection of letters number carries out carrier wave demodulation, intermediate frequency down coversion and filtering sampling, obtains the reception code of a length of P Sheet signal x_c(n)；

(2b). to receiving chip signal x_cN () is circulated chip displacement, obtain chip shift signal x_c' (n)=x_c(n- Q), wherein Q is chip carry digit, and Q ∈ 1,2 ..., N}；

(2c) by chip shift signal x_c' (n) and the local folded signal l of pseudo-code exhibition_cN () is multiplied, obtain the folded coherent signal of time domain exhibition r_c(n)=x ' (n) l_c(n)；

(2d). coherent signal r folded to time domain exhibition_cN () carries out P point quick Fourier conversion, obtain the folded coherent signal X of frequency domain exhibition_c (k)=FFT [r_c(n)], wherein FFT [*] expression carries out fast Fourier transform to signal；

(2e). coherent signal X folded to frequency domain exhibition_cK () carries out delivery and peak value searching, obtain the maximum folded correlation of frequency domain exhibition R_maxAnd the index k of correspondence_max；

(2f) decision threshold is determinedWhereinFor ratio because of Son, N_noiseFor noise power, P_faFor false-alarm probability；

(2g) maximum frequency domain correlation R is judged_maxWhether more than or equal to decision threshold μ, how general if so, calculate reception signal Strangle coarse compensation valueOtherwise, make chip carry digit Q=Q+1, and perform step (2c), wherein f_sRepresent sampling Frequency；

(3) the rough estimate evaluation of Doppler effect correction is utilizedThe docking collection of letters number carries out coarse compensation, obtains the reception after coarse compensation Signal x_ac(n)；

(4) to the reception signal x after coarse compensation_acN () carries out second layer time frequency analysis, obtain chip delay compensation valueWith The accurate offset of DopplerRealizing step is:

(4a) the local pseudo-code signal p to structure_cN () carries out M point quick Fourier conversion, and carry out conjugate transpose, Conjugate transpose P to local pseudo-code frequency-region signal_c(k)=conj [FFT (p_c(n))], wherein signal is carried out by conj [*] expression Conjugate transpose；

(4b). utilize frequency domain correlation peak-to-average force ratioDetermine the adaptable search factor WhereinFor rounding up, α is the invariant that value is relevant to the orbit parameter of satellite, E_pFor transmitter power, A_ccIt is tired Long-pending number of chips；

(4c). utilize adaptable search factor T, calculate Doppler frequency step-size in searchAnd determine hunting zone

(4d). utilize each step Doppler shift amount in the S of hunting zone, respectively to the reception chip signal after coarse compensation x_acN () carries out Doppler and accurately compensates, obtain T and accurately compensate chip sequenceIts In

(4e). respectively T is accurately compensated chip sequence x_tN () carries out M point quick Fourier conversion, obtain T frequency domain Accurately compensate chip sequence X_t(k)=FFT [x_t(n)]；

(4f). T frequency domain is accurately compensated chip sequence X_t(k) and the conjugate transpose P of local frequency domain pseudo-code signal_c(k) phase Take advantage of, and T multiplied result is carried out respectively M point inverse fast fourier transform, obtain T and accurately compensate correlated series r_t(k)= IFFT[X_t(k)P_c(k)], wherein IFFT [*] expression carries out inverse fast fourier transform to signal；

(4g). T is accurately compensated correlated series r_tK () carries out peak value searching, obtain corresponding accurate of maximum related value Compensate the index t of correlated series_maxWith particular location k '_max, and by particular location k '_maxNumber of chips be converted to chip time delay mend Repay valueI.e.Calculate the accurate offset of Doppler simultaneously

(5). utilize step (2) to obtain Doppler's coarse compensation valueThe accurate offset of Doppler obtained with step (4) Calculate Doppler frequency domain offset

$\hat{f}={\hat{f}}_d+{\hat{f}}_d^{\′}=\frac{f_s}{P}(k_{max}+\frac{1}{t_{max}}).$

The present invention compared with prior art, has the advantage that

1. the present invention is during obtaining Doppler frequency domain offset, uses the adaptable search factor accurately to mend Repay, i.e. determine that the adaptable search factor, the recycling adaptable search factor calculate Doppler first with frequency domain correlation peak-to-average force ratio Frequency search step-length also determines hunting zone, finally according to each step Doppler shift amount in hunting zone, mends thick respectively Reception chip signal after repaying carries out Doppler and accurately compensates.FFT peak value broadening that height is dynamically caused by above procedure and FFT Intrinsic fence effect on capture impact reduce further, it is ensured that the accuracy of Doppler effect correction, with existing based on time The double parallel low orbit satellite DS fast Acquisition algorithm of frequency is compared, and effectively improves acquisition accuracy.

2. the present invention is during the offset obtaining chip time delay, uses FFT circular correlation process, quickly eliminates Time-domain chip fuzzy-timing degree present in the folded process of pseudo-code exhibition, it is achieved that the accurate estimation to chip time delay, with existing based on The double parallel low orbit satellite DS fast Acquisition algorithm of time-frequency is compared, and improves the acquisition speed of time domain.

Accompanying drawing explanation

Fig. 1 be the present invention realize FB(flow block)；

Fig. 2 is the operand contrast of the present invention and existing fast Acquisition algorithm；

Fig. 3 is the correlation peak acquisition performance analogous diagram of the present invention；

Fig. 4 is the simulation comparison figure of the present invention and prior art acquisition probability.

Detailed description of the invention

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

With reference to Fig. 1, the present invention comprises the steps:

Step 1, the local pseudo-code signal p of structure_c(n) and the local folded signal l of pseudo-code exhibition_c(n), it is achieved step is:

Step 1a, is divided into local original pseudo-code signal p (n) equal N number of exhibition section of folding, and carries out each exhibition section of folding Numbering, obtains N number of exhibition and folds the local pseudo-code truncated signal l of a length of L=M/N_iN (), wherein, M is PN-code capture, the volume of every section Number be i (i=1,2 ..., N), corresponding chip position n ∈ { 1,2..., L}；

Step 1b, asks for N number of local pseudo-code truncated signal l_iN the sum of correspondence position chip in (), obtains opening up folded compressed code Its l_f(n), its computing formula is:

$l_f\left(n\right)=\underset{i=1}{\overset{N}{\Σ}}{l}_i\left(n\right)=\underset{i=1}{\overset{N-1}{\Σ}}p(n+iL)=p\left(n\right)+p(n+L)+p(n+2L)+...+p(n+(N-1\left)L\right);$

Step 1c, compressed code l folded to local original pseudo-code signal p (n) and exhibition_fN () replicates respectively, and will replicate The P/L arrived folded compressed code l of local original pseudo-code signal p (n) exhibition_fN joining end to end of (), respectively obtains this locality of a length of P Pseudo-code signal p_c(n) and the local folded signal l of pseudo-code exhibition_c(n)；

Step 2, the docking collection of letters number carries out ground floor time frequency analysis, obtains Doppler's coarse compensation valueWherein realize step For:

Step 2a, the docking collection of letters number carries out carrier wave demodulation, intermediate frequency down coversion and filtering sampling, obtains the reception of a length of P Chip signal x_c(n)；

Step 2b, to receiving chip signal x_cN () is circulated chip displacement, obtain chip shift signal x_c' (n)=x_c (n-Q), wherein Q is chip carry digit, and Q ∈ 1,2 ..., N}；

Step 2c, by chip shift signal x_c' (n) and the local folded signal l of pseudo-code exhibition_cN () is multiplied, obtain time domain exhibition folded relevant Signal r_c(n)=x ' (n) l_c(n)；

Step 2d, coherent signal r folded to time domain exhibition_cN () carries out P point quick Fourier conversion, obtain the folded relevant letter of frequency domain exhibition Number X_c(k)=FFT [r_c(n)], wherein FFT [*] expression carries out fast Fourier transform to signal；

The folded coherent signal X of its frequency domain exhibition_c(k) calculate process particularly as follows:

$\begin{array}{c}{X}_c\left(k\right)=FFT\[r_c\left(n\right)\]=FFT\[x^{\′}\left(n\right)l_c\left(n\right)\]\\ =\underset{n=0}{\overset{N-1}{\Σ}}\underset{m=0}{\overset{N-1}{\Σ}}x(n-\mathrm{\τ}-\widehat{\mathrm{\τ}})l_c\left(n\right)e^{2\mathrm{\π}jn\frac{f_d}{f_s}}{e}^{-2\mathrm{\π}jn\frac{k}{N}}=\underset{n=0}{\overset{N-1}{\Σ}}\underset{m=0}{\overset{N-1}{\Σ}}x(n-\mathrm{\τ}-\widehat{\mathrm{\τ}})l_c\left(n\right)e^{-2\mathrm{\π}jn(\frac{k}{N}-\frac{f_d}{f_s})}\end{array}$

Wherein τ is to receive the chip time delay that signal exists.

It can be seen that at frequency domain coherent signal X_cDuring (k) structureAn absolute value minimum will be there is Position, this moment is the position that the frequency deviation of whole frequency domain search is minimum, is i.e. equivalent to be achieved by spectrum analysis many The coarse compensation of Pu Le skew.

Step 2e, coherent signal X folded to frequency domain exhibition_cK () carries out delivery and peak value searching, obtain maximum frequency domain exhibition folded relevant Value R_maxAnd the index k of correspondence_max；

Step 2f, determines decision thresholdWhereinFor ratio The factor, N_noiseFor noise power, P_faFor false-alarm probability；

Step 2g, it is judged that maximum frequency domain correlation R_maxWhether more than or equal to decision threshold μ, if so, calculate reception signal many General Le coarse compensation valueOtherwise, make chip carry digit Q=Q+1, and perform step (2c), wherein f_sExpression is adopted Sample frequency；

Step 3, utilizes the rough estimate evaluation of Doppler effect correctionThe docking collection of letters number carries out coarse compensation, after obtaining coarse compensation Receive signal x_ac(n), its computing formula particularly as follows:

$x_{ac}\left(n\right)=x_c\left(n\right)*\exp (2\mathrm{\π}jn{\hat{f}}_d/f_s)=x_c\left(n\right)*\exp \left(\frac{2{\mathrm{\πjnf}}_s{k}_{max}}{{\mathrm{Pf}}_s}\right);$

Step 4, to the reception signal x after coarse compensation_acN () carries out second layer time frequency analysis, obtain chip delay compensation valueOffset accurate with DopplerRealizing step is:

Step 4a, the local pseudo-code signal p to structure_cN () carries out M point quick Fourier conversion, and carry out conjugate transpose, Obtain the conjugate transpose P of local pseudo-code frequency-region signal_c(k)=conj [FFT (p_c(n))], wherein conj [*] represents to enter signal Row conjugate transpose；

Step 4b, utilizes frequency domain correlation peak-to-average force ratioDetermine the adaptable search factorWhereinFor rounding up, α is the invariant that value is relevant to the orbit parameter of satellite, E_pFor sending out Penetrate acc power, A_ccFor accumulation number of chips；

Step 4c, utilizes adaptable search factor T, calculates Doppler frequency step-size in searchAnd determine hunting zone

Step 4d, utilizes each step Doppler shift amount in the S of hunting zone, believes the reception chip after coarse compensation respectively Number x_acN () carries out Doppler and accurately compensates, obtain T and accurately compensate chip sequence Wherein

Step 4e, accurately compensates chip sequence x respectively to T_tN () carries out M point quick Fourier conversion, obtain T frequency Territory accurately compensates chip sequence X_t(k)=FFT [x_t(n)]；

Step 4f, accurately compensates T frequency domain for chip sequence X_t(k) and the conjugate transpose P of local frequency domain pseudo-code signal_c(k) It is multiplied, and T multiplied result is carried out respectively M point inverse fast fourier transform, obtain T and accurately compensate correlated series r_t(k) =IFFT [X_t(k)P_c(k)], wherein IFFT [*] expression carries out inverse fast fourier transform to signal.Wherein accurately compensate relevant Sequence r_t(k) calculate process particularly as follows:

$\begin{array}{c}{r}_t\left(k\right)=IFFT\[X_t\left(k\right)P_c\left(k\right)\]\\ =\frac{1}{N}\underset{k=0}{\overset{N-1}{\Σ}}\left(X_t\right(k\left)P_c\right(k\left)\right)e^{j2\mathrm{\π}kn/N}\\ =\frac{1}{N}\underset{k=0}{\overset{N-1}{\Σ}}\left(X_t\right(k\left)\underset{m=0}{\overset{N-1}{\Σ}}{p}_c\right(m\left)e^{-j2\mathrm{\π}km/N}\right)e^{j2\mathrm{\π}kn/N}\\ =\underset{m=0}{\overset{N-1}{\Σ}}{x}_t\left(m\right)\[\underset{n=0}{\overset{N-1}{\Σ}}{p}_c(m-n)e^{j2k(m-n)/N}\]e^{-j2\mathrm{\π}kn/N}{e}^{j2\mathrm{\π}kn/N}\\ =\underset{n=0}{\overset{N-1}{\Σ}}{x}_t\left(m\right)p_c(m-n)\end{array};$

(4g). T is accurately compensated correlated series r_tK () carries out peak value searching, obtain corresponding accurate of maximum related value Compensate the index t of correlated series_maxWith particular location k '_max, and by particular location k '_maxNumber of chips be converted to chip time delay mend Repay valueI.e.Calculate the accurate offset of Doppler simultaneously

Step 5, utilizes step (2) to obtain Doppler's coarse compensation valueThe accurate offset of Doppler obtained with step (4)Calculate Doppler frequency domain offset

$\hat{f}={\hat{f}}_d+{\hat{f}}_d^{\′}=\frac{f_s}{P}(k_{max}+\frac{1}{t_{max}}).$

With reference to Fig. 2, contrasting the computational complexity of the present invention with analogous algorithms, wherein Fig. 2 (a) is complex multiplication meter Calculation number of times contrasts, and Fig. 2 (b) is the contrast of complex addition operations number of times.It can be seen that complex multiplication number of times of the present invention and complex addition Number of times is all substantially less than time domain fft algorithm and frequency domain fft algorithm, substantially suitable with CCPC-FFT algorithm, its specific formula for calculation Such as table 1, the Doppler shift search step number during wherein W is time domain fft algorithm, take in the scene that low orbit satellite dynamic is higher 1000-2000.When accumulated time is 10ms, the operation times of complex multiplication of the present invention is the 16.26% of time domain fft algorithm, For the 27.35% of frequency domain fft algorithm, more than CCPC-FFT algorithm 6.55%, and accumulated time is the longest, with CCPC-FFT algorithm Operand closer to.It can be seen that the present invention can effectively save spaceborne calculation resources, promote acquisition speed.

Table 1 present invention contrasts with existing fast Acquisition algorithm computational complexity

Below in conjunction with emulation experiment, the technique effect of the present invention is described further:

1. simulated conditions: the operation system of emulation experiment of the present invention is Intel (R) Core (TM) i5-4590 CPU@ 3.30GHz, 64 Windows operating systems, simulation software uses MATLAB R (2012b), and simulation parameter arranges as follows.

Low orbit satellite transmission system is Direct-Spread system, and test scene is iridium satellite LEO satellite communication systems, pseudorandom Code uses Gold sequence, and cycle M is 1023, and message transmission rate is 10.23kbps, and modulation system is QPSK, and the folded times N of exhibition is 16, adaptable search factor-alpha=0.85 under IRIDIUM, carrier frequency is that 19.5GHz (satellite-ground link)/23.28GHz is (between star Link), false-alarm probability P_faIt is 10^-4；

2. emulation content and interpretation of result:

The correlation peak acquisition performance of the present invention is emulated by emulation 1., its result as it is shown on figure 3, wherein Fig. 3 (a) be The docking collection of letters number carries out a performance map for time frequency analysis, and Fig. 3 (b) is for carrying out the performance of time frequency analysis to the signal that receives after coarse compensation Figure；

The present invention is emulated by emulation 2. with prior art acquisition probability, and its result is as shown in Figure 4；

With reference to Fig. 3, the present invention carried fast Acquisition algorithm is at satellite launch power (Equivalent Isotropic Radiated Power, EIRP) when being 20dBw, the performance of double-deck time frequency analysis, the wherein ground floor time frequency analysis in Fig. 3 (a) The pseudo-code folded technology of exhibition and spectrum analysis based on fast Fourier transform is utilized to achieve the thick capture of Doppler shift, Fig. 3 (b) Middle second layer time frequency analysis is then achieved by time domain fast correlation based on fast Fourier transform and self adaptation fine search Chip offset and the accurate capture of Doppler shift.

With reference to Fig. 4, the acquisition probability travelling with the exhibits on an exhibition tour of the present invention carried fast Acquisition algorithm is folded the increase of hop count N and is promoted, and exhibition is folded Hop count is the most, and its acquisition performance is the most excellent.Compare CCPC-FFT algorithm, the carried fast algorithm of the present invention when the folded times N of exhibition is identical, Having higher acquisition probability, when EIRP is more than 19dBw, acquisition probability is up to 100%；Compare frequency domain FFT acquisition algorithm, this Invention algorithm acquisition performance is declined slightly, but its computational complexity significantly reduces, and acquisition speed promotes substantially.

In sum, two emulation experiments two results obtained show, it is straight that the present invention is capable of low orbit satellite The rapid signal acquisition of enlarging system, on the basis of computational complexity is basically unchanged, improves acquisition probability and acquisition accuracy.

Claims (5)

1. a low orbit satellite directly-enlarging system fast Acquisition algorithm based on double-deck time frequency analysis, comprises the steps:

(1). the local pseudo-code signal p of structure_c(n) and the local folded signal l of pseudo-code exhibition_c(n), it is achieved step is:

(1a). local original pseudo-code signal p (n) is divided into equal N number of exhibition section of folding, and each exhibition section of folding is numbered, Obtain N number of exhibition and fold the local pseudo-code truncated signal l of a length of L=M/N_iN (), wherein, M is PN-code capture, the numbered i of every section (i=1,2 ..., N), corresponding chip position n ∈ { 1,2..., L}；

(1b). ask for N number of local pseudo-code truncated signal l_iN the sum of correspondence position chip in (), obtains opening up folded compressed code

(1c). compressed code l folded to local original pseudo-code signal p (n) and exhibition_fN () replicates respectively, and P/L duplication obtained Folded compressed code l of individual this locality original pseudo-code signal p (n) exhibition_fN joining end to end of (), respectively obtains the local pseudo-code signal of a length of P p_c(n) and the local folded signal l of pseudo-code exhibition_c(n)；

(2). the docking collection of letters number carries out ground floor time frequency analysis, obtains Doppler's coarse compensation valueWherein realizing step is:

(2a). the docking collection of letters number carries out carrier wave demodulation, intermediate frequency down coversion and filtering sampling, obtains the reception chip letter of a length of P Number x_c(n)；

(2b). to receiving chip signal x_cN () is circulated chip displacement, obtain chip shift signal x_c' (n)=x_c(n-Q), Wherein Q is chip carry digit, and Q ∈ 1,2 ..., N}；

(2c). by chip shift signal x_c' (n) and the local folded signal l of pseudo-code exhibition_cN () is multiplied, obtain the folded coherent signal r of time domain exhibition_c (n)=x ' (n) l_c(n)；

(2d). coherent signal r folded to time domain exhibition_cN () carries out P point quick Fourier conversion, obtain the folded coherent signal X of frequency domain exhibition_c(k) =FFT [r_c(n)], wherein FFT [*] expression carries out fast Fourier transform to signal；

(2e). coherent signal X folded to frequency domain exhibition_cK () carries out delivery and peak value searching, obtain maximum frequency domain exhibition folded

Correlation R_maxAnd the index k of correspondence_max；

(2f). determine decision thresholdWhereinFor scale factor, N_noiseFor noise power, P_faFor false-alarm probability；

(2g). judge maximum frequency domain correlation R_maxWhether more than or equal to decision threshold μ, if so, calculate reception signal Doppler thick OffsetOtherwise, make chip carry digit Q=Q+1, and perform step (2c), wherein f_sRepresent sample frequency；

(3). utilize the rough estimate evaluation of Doppler effect correctionThe docking collection of letters number carries out coarse compensation, obtains the reception signal after coarse compensation x_ac(n)；

(4). to the reception signal x after coarse compensation_acN () carries out second layer time frequency analysis, obtain chip delay compensation valueThe most general Strangle accurate offsetRealizing step is:

(4a). the local pseudo-code signal p to structure_cN () carries out M point quick Fourier conversion, and carry out conjugate transpose, obtains this The conjugate transpose P of ground pseudo-code frequency-region signal_c(k)=conj [FFT (p_c(n))], wherein signal is conjugated by conj [*] expression Transposition；

(4b). utilize frequency domain correlation peak-to-average force ratioDetermine the adaptable search factor WhereinFor rounding up, α is the invariant that value is relevant to the orbit parameter of satellite, E_pFor transmitter power, A_ccIt is tired Long-pending number of chips；

(4c). utilize adaptable search factor T, calculate Doppler frequency step-size in searchAnd determine hunting zone

(4d). utilize each step Doppler shift amount in the S of hunting zone, respectively to reception chip signal x after coarse compensation_ac N () carries out Doppler and accurately compensates, obtain T and accurately compensate chip sequenceWherein

(4e). respectively T is accurately compensated chip sequence x_tN () carries out M point quick Fourier conversion, obtain T frequency domain and accurately mend Repay chip sequence X_t(k)=FFT [x_t(n)]；

(4f). T frequency domain is accurately compensated chip sequence X_t(k) and the conjugate transpose P of local frequency domain pseudo-code signal_cK () is multiplied, And T multiplied result is carried out respectively M point inverse fast fourier transform, obtain T and accurately compensate correlated series r_t(k)=IFFT [X_t(k)P_c(k)], wherein IFFT [*] expression carries out inverse fast fourier transform to signal；

(4g). T is accurately compensated correlated series r_tK () carries out peak value searching, obtain maximum related value corresponding accurately compensate phase Close the index t of sequence_maxWith particular location k '_max, and by particular location k '_maxNumber of chips be converted to chip delay compensation value I.e.Calculate the accurate offset of Doppler simultaneously

(5). utilize step (2) to obtain Doppler's coarse compensation valueThe accurate offset of Doppler obtained with step (4)Calculate Doppler frequency domain offset

$\hat{f}={\hat{f}}_d+{\hat{f}}_d^{\′}=\frac{f_s}{P}(k_{max}+\frac{1}{t_{max}}).$

Low orbit satellite fast Acquisition algorithm based on double-deck time frequency analysis the most according to claim 1, it is characterised in that step Suddenly signal l is folded in the N number of local pseudo-code exhibition of asking for described in (1b)_iThe sum of (n) correspondence position chip, its computing formula is:

$l_f\left(n\right)=\underset{i=1}{\overset{N}{\Σ}}{l}_i\left(n\right)=\underset{i=0}{\overset{N-1}{\Σ}}p(n+iL)=p\left(n\right)+p(n+L)+p(n+2L)+...+p(n+(N-1\left)L\right).$

Low orbit satellite fast Acquisition algorithm based on double-deck time frequency analysis the most according to claim 1, it is characterised in that step Suddenly the folded coherent signal X of frequency domain exhibition described in (2d)_c(k)=FFT [r_c(n)], computational methods particularly as follows:

$\begin{array}{c}{X}_c\left(k\right)=FFT\[r_c\left(n\right)\]=FFT\[x^{\′}\left(n\right)l_c\left(n\right)\]\\ =\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}x\left(n-\mathrm{\τ}-\widehat{\mathrm{\τ}}\right)l_c\left(n\right)e^{2\mathrm{\π}jn\frac{f_d}{f_s}}{e}^{-2\mathrm{\π}jn\frac{k}{N}}=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}x\left(n-\mathrm{\τ}-\widehat{\mathrm{\τ}}\right)l_c\left(n\right)e^{-2\mathrm{\π}jn\left(\frac{k}{N}-\frac{f_d}{f_s}\right)}\end{array}$

Wherein τ is the chip time delay existed.

Low orbit satellite fast Acquisition algorithm based on double-deck time frequency analysis the most according to claim 1, it is characterised in that step Suddenly the rough estimate evaluation utilizing Doppler effect correction described in (3)Coarse compensation is carried out, its computing formula to receiving chip signal For:

$x_{ac}\left(n\right)=x_c\left(n\right)*\exp \left(2\mathrm{\π}jn{\hat{f}}_d/f_s\right)=x_c\left(n\right)*\exp \left(\frac{2{\mathrm{\πjnf}}_s{k}_{\max }}{{\mathrm{Pf}}_s}\right).$

Low orbit satellite fast Acquisition algorithm based on double-deck time frequency analysis the most according to claim 1, it is characterised in that step Suddenly T the multiplied result that utilize described in (4f) carries out M point inverse fast fourier transform respectively, and its computing formula is:

$\begin{array}{c}{r}_t\left(k\right)=IFFT\[X_t\left(k\right)P_c\left(k\right)\]\\ =\frac{1}{N}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}\left(X_t\left(k\right)P_c\left(k\right)\right)e^{j2\mathrm{\π}kn/N}\\ =\frac{1}{N}\underset{k=0}{\overset{N-1}{\mathrm{\Σ}}}\left(X_t\left(k\right)\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{p}_c\left(m\right)e^{-j2\mathrm{\π}km/N}\right)e^{j2\mathrm{\π}kn/N}\\ =\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}{x}_t\left(m\right)\[\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{p}_c\left(m-n\right)e^{j2\mathrm{\π}k\left(m-n\right)/N}\]e^{-j2\mathrm{\π}kn/N}{e}^{j2\mathrm{\π}kn/N}\\ =\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}{x}_t\left(m\right)p_c\left(m-n\right)\end{array}.$