CN109412644A - A kind of doppler frequency estimation method of direct expansion msk signal - Google Patents
A kind of doppler frequency estimation method of direct expansion msk signal Download PDFInfo
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- CN109412644A CN109412644A CN201811064884.1A CN201811064884A CN109412644A CN 109412644 A CN109412644 A CN 109412644A CN 201811064884 A CN201811064884 A CN 201811064884A CN 109412644 A CN109412644 A CN 109412644A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- 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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- 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
- H04B1/70754—Setting of search window, i.e. range of code offsets to be searched
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Abstract
The invention discloses a kind of doppler frequency estimation methods of direct expansion msk signal, direct expansion msk signal is configured to a kind of approximate DS-BPSK signal signal form, then in large doppler low signal-to-noise ratio, Doppler shift value is optimized using the triangle fitting of cross-ambiguity function.Specifically: direct expansion msk signal, which will be received, using intermediate frequency matched filter is converted to approximate DS-BPSK signal signal；Pairing approximation DS-BPSK signal signal carries out code phase search using the pseudo-code parallel capture algorithm based on FFT；Doppler shift value is optimized by the triangle fitting of cross-ambiguity function, obtains accurate Doppler-frequency estimation value, and improve acquisition probability using the amplitude of frequency essence estimation front and back, frequency information.The present invention can effectively improve Doppler shift estimated accuracy.
Description
Technical field
The invention belongs to direct sequence signal simultaneous techniques field, a kind of specifically Doppler frequency of direct expansion msk signal
Estimation method.
Background technique
Currently, spread spectrum system mostly uses BPSK, QPSK modulation system, but both modulation systems can not be suitable for existing
Serious non-linear distortion, the application field of Doppler frequency shift and multipath fading.Direct expansion MSK communication system has spread spectrum system simultaneously
System low intercepting and capturing, the random address ability of multi-user, strong anti-interference performance the advantages that and msk signal constant-envelope, frequency spectrum benefit
With the advantages that rate is high, energy is concentrated, side lobe attenuation is fast, out-of-band radiation power is low, insensitive to non-linear distortion, in tactics number
It is widely applied according to fields such as chain, civil aviation Ground-to-Air Data Link, missile guidance instruction transmission, satellite communications.So
Direct expansion msk signal still has a good application prospect in the field that DS-BPSK signal/QPSK signal can not be applicable in.
G.J.R.Povey et al. proposes to combine (PMF-FFT) with FFT based on numerical portion matched filter first
Capturing Models, although this method alleviates influence of the Doppler shift to acquiring pseudo code performance to a certain extent, and real
The two dimension capture of pseudo-code phase and carrier wave frequency deviation is showed, but this method is mainly adapted to mpsk signal, and Doppler's frequency
Inclined capture range is smaller, still not applicable under high dynamic, low signal-to-noise ratio environment.
Summary of the invention
The purpose of the present invention is to provide a kind of doppler frequency estimation method of direct expansion msk signal, solve high dynamic,
The problem of the pseudo-code phase of spread-spectrum signal and Doppler shift two dimension capture inaccuracy under low signal-to-noise ratio environment.
The technical solution for realizing the aim of the invention is as follows: a kind of doppler frequency estimation method of direct expansion msk signal,
The following steps are included:
Step 1 will receive direct expansion msk signal using intermediate frequency matched filter and be converted to approximate DS-BPSK signal signal；
Step 2, pairing approximation DS-BPSK signal signal carry out code phase search using the pseudo-code parallel capture algorithm based on FFT,
Local pseudo-code is realized in frequency domain and receives the circumference related operation of pseudo-code, obtains the correlation on all code phases；
Step 3 detects the irrelevant integral output of frequency domain, is more than CFAR detection threshold value to non-coherent integration
As a result, carrying out Doppler shift value f by the fitting of triangle to cross-ambiguity functiondOptimization, obtains accurate Doppler and estimates
Evaluation.
Preferably, direct expansion msk signal, the impulse of filter h (t) are received using intermediate frequency matched filter h (t) in step 1
Response are as follows:
Wherein, Tc=1/RcFor spreading code period, RcFor spread-spectrum code rate；fcFor carrier frequency.
Preferably, approximation DS-BPSK signal signal expression obtained in step 1 are as follows:
Wherein, A is signal amplitude,For initial phase, TsFor the sampling interval for receiving signal, d () is transmission symbol,
C () is PN code, and τ is the code phase error received between signal and local pseudo-code, fdFor Doppler shift.
Preferably, in step 2, signal result relevant to local PN code is received are as follows:
Wherein,For the copy of local PN code,For code phase difference estimated value,
For the Doppler-frequency estimation value of k-th of search unit, K is the unit sum of frequency search, fspFor frequency search stepping, N is
The total sample number of correlated process.
Preferably, the result that the irrelevant integral of k-th of frequency search unit exports in step 3 are as follows:
Wherein,TN=NTs, L is noncoherent integration length, Rc() is PN code
Normalized autocorrelation functions.
Preferably, in step 3, it is assumed that the non-coherent integration results S in k-th of frequency search areakMore than CFAR detection
Threshold value, save this more than threshold value point and side frequency search in same position two points, according to the suitable of amplitude size
These three points are defined as P by sequencemax, PmidAnd Pmin, frequency and amplitude are respectively defined as fmax, fmidAnd fminAnd Amax, Amid
And Amin, the accurate vertex position of cross-ambiguity function is found using triangle fitting.
Preferably, accurate Doppler's estimated value are as follows:
Wherein,And α ∈ [- 0.5,0].
Compared with prior art, the present invention its remarkable advantage are as follows: 1) present invention passes through intermediate frequency matched filter for direct expansion
Msk signal is converted to DS-BPSK signal signal, reduces the synchronous difficulty of receiver；2) present invention is using to cross-ambiguity function
Triangle fitting, Doppler frequency value is accurately estimated；3) present invention effectively improves the acquisition probability of receiver.
Present invention is further described in detail with reference to the accompanying drawing.
Detailed description of the invention
Fig. 1 is the pseudo-code parallel capture functional block diagram based on FFT.
Fig. 2 is the pseudo-code parallel capturing method flow chart based on FFT.
Fig. 3 is triangle fitting scheme schematic diagram when signal is correctly detected with false-alarm, and (a) is triangle when correctly detecting
Fitting scheme schematic diagram, (b) be occur false-alarm when triangle fitting scheme schematic diagram.
Fig. 4 is a kind of flow chart of the doppler frequency estimation method of direct expansion msk signal.
Specific embodiment
As shown in figure 4, a kind of doppler frequency estimation method of direct expansion msk signal, first converts direct expansion MSK signal
At approximate DS-BPSK signal signal, i.e., at the intermediate frequency matched filter for being h (t) through impulse response by intermediate frequency direct expansion msk signal
Reason:
Wherein, Tc=1/RcFor spreading code chip period, RcFor spread-spectrum code rate；fcFor carrier frequency.
According to impulse response, the frequency response of matched filter can be acquired are as follows:
The filter coefficient of matched filter is acquired with convex optimized algorithm.By intermediate frequency direct expansion msk signal through this matched filtering
Device can must export result are as follows:
Wherein, A is signal amplitude,For initial phase, TsFor the sampling interval for receiving signal, d () is transmission symbol,
C () is PN code, and τ is the code phase error received between signal and local pseudo-code, fdFor Doppler shift.
As shown in Figure 1, the parallel capture algorithm basic thought based on FFT is the circumference correlation of two discrete signal time domains
It is equivalent to the conjugate multiplication of its frequency-region signal, therefore calculates using FFT and IFFT and can realize local pseudo-code in frequency domain and receive
The circumference related operation of pseudo-code calculates the correlation that can be calculated on all code phases by 3 FFT.Receive signal with
The relevant result of local PN code is
Wherein,For the copy of local PN code,For code phase difference estimated value,
For the Doppler-frequency estimation value of k-th of search unit, K is the unit sum of frequency search, fspFor frequency search stepping, N is
The total sample number of correlated process.
The irrelevant integral output of frequency domain is detected, the knot of the irrelevant integral output of k-th of frequency search unit
Fruit are as follows:
Wherein,TN=NTs, L is noncoherent integration length, Rc() is PN code
Normalized autocorrelation functions.
Assuming that the non-coherent integration results S in k-th of frequency search areak'More than constant false alarm (constant false
Alarmrate, CFAR) detection threshold value, save in point and the side frequency search more than threshold value two of same position
Point.According to the sequence of amplitude size, these three points are defined as Pmax, PmidAnd Pmin, frequency and amplitude are respectively defined as
fmax, fmidAnd fminAnd Amax, AmidAnd Amin, cross-ambiguity function (cross-ambiguity is found using triangle fitting
Function, CAF) accurate vertex position.Fit solution is as shown in Figure 2.Firstly, assessment PmaxAnd PminLinear equation.Its
It is secondary, it assesses in PmidThe linear equation of upper fitting, the linear coefficient of the equation are opposite with coefficient symbols derived in the first step.Most
Afterwards, according to the feature of isosceles triangle, vertex of the vertex of CAF as the isosceles triangle fitted in two lines is obtained,
It is defined as point Pv, frequency fvAnd amplitude AvIt indicates are as follows:
As shown in Figure 3a, if signal is correctly detected and fmin> fmax> fmid(fmid> fmax> fminAnalysis situation
It is similar), then the range value of three points may be expressed as:
Amax=A | sin c (Δ fd,k'TN)|Rc(Δτ)
Amid=A | sin c (Δ fd,k'-1TN)|Rc(Δτ) (8)
Amin=A | sin c (Δ fd,k'+1TN)|Rc(Δτ)
By formula (8), fvWith AvIt is expressed as
Av=Amax+(Amid-Amin)/2 (10)
Compare Pmax、PvFrequency and amplitude, can obtain
Av≥Amax (12)
Residual frequency after the estimation of frequency essence are as follows:
Wherein,For frequency essence estimation before residual frequency andIn order to guarantee
Pmax、PmidAnd PminIn the main lobe of CAF and three's amplitude size can be distinguished obviously, and frequency search stepping f is arrangedsp=2/
(3TN).Frequency estimation after accurate estimation are as follows:
Wherein,And α ∈ [- 0.5,0].Therefore, the range of Δ f is
(8×10-5)·fsp≤Δf≤0.5·fsp (15)
By formula (15) it is found that Doppler-frequency estimation precision greatly improves after triangle fitting.
On the contrary, if when false-alarm occurs, as shown in Figure 3b, 3 points of frequency and amplitude is closed without correctly detecting signal
System will change.Formula (11) (12) is no longer applicable in but can be used to exclude false-alarm.
In frequency search, the false-alarm probability of neighbouring correct frequency search unit is higher than other search units.
Therefore, the maximum false-alarm probability of intermediate cam fitting of the present invention are as follows:
Wherein, PdIt is the detection probability of non-coherent integration,For non-coherent integration
False-alarm probability, VtIt is detection threshold.Due to Pd< 1, Pfa< 1 can be obtainedTherefore, in the requirement of identical false-alarm probability
Under, CFAR detection threshold can be arranged lower, improve detection probability.According to this feature, this method can not only be mentioned
The estimated accuracy of high-doppler frequency, and acquisition probability can be improved.
The range of residual frequency before the estimation of frequency essence is [0, fsp/ 2], residual frequency range shorter after the estimation of frequency essence
It is [(8 × 10-5)·fsp,0.5·fsp], it is seen that Doppler-frequency estimation precision greatly improves after triangle fitting.
Claims (7)
1. a kind of doppler frequency estimation method of direct expansion msk signal, which comprises the following steps:
Step 1 will receive direct expansion msk signal using intermediate frequency matched filter and be converted to approximate DS-BPSK signal signal；
Step 2, pairing approximation DS-BPSK signal signal carry out code phase search using the pseudo-code parallel capture algorithm based on FFT, in frequency
Domain realizes local pseudo-code and receives the circumference related operation of pseudo-code, obtains the correlation on all code phases；
Step 3 detects the irrelevant integral output of frequency domain, is more than the knot of CFAR detection threshold value to non-coherent integration
Fruit carries out Doppler shift value f by the triangle fitting to cross-ambiguity functiondOptimization, obtains accurate Doppler's estimated value.
2. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that step 1
It is middle that direct expansion msk signal, the impulse response of filter h (t) are received using intermediate frequency matched filter h (t) are as follows:
Wherein, Tc=1/RcFor spreading code period, RcFor spread-spectrum code rate；fcFor carrier frequency.
3. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that step 1
Obtained in approximation DS-BPSK signal signal expression are as follows:
Wherein, A is signal amplitude,For initial phase, TsFor the sampling interval for receiving signal, d () is transmission symbol, c ()
For PN code, τ is the code phase error received between signal and local pseudo-code, fdFor Doppler shift.
4. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that step 2
In, receive signal result relevant to local PN code are as follows:
Wherein,For the copy of local PN code,For code phase difference estimated value,For
The Doppler-frequency estimation value of k-th of search unit, K are the unit sum of frequency search, fspFor frequency search stepping, N is phase
The total sample number of pass process.
5. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that step 3
In k-th frequency search unit the output of irrelevant integral result are as follows:
Wherein,TN=NTs, L is noncoherent integration length, Rc() is the normalizing of PN code
Change auto-correlation function.
6. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that step 3
In, it is assumed that the non-coherent integration results S in k-th of frequency search areakMore than the threshold value of CFAR detection, saving should be more than threshold value
Point and side frequency search in two points of same position these three points are defined as by P according to the sequence of amplitude sizemax,
PmidAnd Pmin, frequency and amplitude are respectively defined as fmax, fmidAnd fminAnd Amax, AmidAnd Amin, friendship is found using triangle fitting
Pitch the accurate vertex position of ambiguity function.
7. a kind of doppler frequency estimation method of direct expansion msk signal according to claim 1, which is characterized in that accurate
Doppler's estimated value are as follows:
Wherein,And α ∈ [- 0.5,0].
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