CN107493117B - The two-dimentional joint acquisition method of direct expansion msk signal under a kind of high dynamic - Google Patents
The two-dimentional joint acquisition method of direct expansion msk signal under a kind of high dynamic Download PDFInfo
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Abstract
The present invention provides a kind of two-dimentional joint acquisition method of direct expansion msk signal under high dynamic, comprising the following steps: handles the received switched filter of intermediate frequency direct expansion msk signal, down coversion, low-pass filtering, extraction, obtains approximate DS-BPSK signal signal；Pairing approximation DS-BPSK signal signal and local code carry out joint acquisition, obtain the estimated value of code phase error and the estimated value of Doppler shift；It is subsequent to carry out accurate code tracking and despread-and-demodulation after carrying out Doppler effect correction based on above-mentioned estimated value pairing approximation DS-BPSK signal signal, obtain information.The present invention has that algorithm complexity is small, is completed at the same time pseudo-code phase and Doppler shift capture, the advantages such as evaluated error is small under high dynamic, is highly suitable for the application environment of high dynamic low signal-to-noise ratio.
Description
Technical Field
The invention relates to a digital communication technology, in particular to a two-dimensional combined capturing method of a direct sequence spread spectrum (MSK) signal under high dynamic condition.
Background
At present, BPSK and QPSK modulation modes are mostly adopted in a spread spectrum system, but the two modulation modes cannot be applied to application environments with serious nonlinear distortion, Doppler frequency shift and multipath fading. The direct spread MSK communication system has the advantages of low interception performance, multi-user random site selection capability, strong anti-interference performance and the like of a spread spectrum system, has the advantages of constant MSK signal envelope, high frequency spectrum utilization rate, energy concentration, fast side lobe attenuation, low out-of-band radiation power, insensitivity to nonlinear distortion and the like, and is widely applied to the fields of tactical data links, civil aviation ground-air data links, missile guidance instruction transmission, satellite communication and the like. Therefore, the direct spread MSK signal still has good application prospect in the field that the direct spread BPSK/QPSK signal can not be applied.
The classical MSK signal capturing method has two types, one is code capturing based on sliding correlation, and the method is simple to realize and long in capturing time; the other is matched filter based code acquisition. The acquisition correlation peaks of the two methods are extremely easily influenced by Doppler frequency offset, and obviously are not suitable for acquisition under high dynamic conditions.
In a high dynamic environment, the most classical estimation method of doppler frequency offset is nonlinear transformation combined with FFT, and the specific method is to perform square or multiple nonlinear transformation on a received signal and then obtain an estimated value of frequency offset according to an index of a frequency spectrum peak after signal FFT processing, however, this method cannot work under a low signal-to-noise ratio. G.j.r.povey et al first propose a capture model based on the combination of a digital partial matched filter and FFT (PMF-FFT), which alleviates the effect of doppler frequency offset on the pseudo code capture performance to a certain extent and realizes two-dimensional capture of pseudo code phase and carrier frequency offset, but which is mainly adapted to MPSK signals and has a small search range of doppler frequency offset, which is still not applicable in a high dynamic environment. In recent years, although some researchers have proposed a code acquisition method under high dynamic conditions such as differential acquisition and two-step acquisition, it is difficult to balance low snr with estimation accuracy. Therefore, how to accurately and quickly complete the joint estimation of the pseudo code phase and the Doppler frequency offset of the spread spectrum signal under the high dynamic environment becomes a key technology of the direct spread MSK full digital receiver.
Disclosure of Invention
The invention aims to provide a two-dimensional combined capturing method of a direct spread MSK signal under high dynamic, which realizes the capturing of the direct spread MSK signal under large Doppler frequency offset, so that the direct spread MSK signal firstly realizes Doppler frequency offset compensation under the high dynamic environment, and realizes the code capturing of the signal under the condition that residual frequency offset does not influence a related peak. And under the condition of low signal-to-noise ratio, higher detection probability can be obtained, and higher precision is obtained at the same time, so that the received signals are ensured to obtain better coarse synchronization before entering a signal tracking (fine synchronization) processing module.
The invention comprises the following steps:
the intermediate frequency direct spread MSK signal is processed by a receiving conversion filter, down conversion, low pass filtering and extraction to obtain an approximate direct spread BPSK signal,
the approximate direct spread BPSK signal and the local code are jointly captured to obtain the estimated value of the code phase error and the estimated value of the Doppler frequency offset,
performing Doppler compensation and de-spreading on the approximate direct spread BPSK signal based on the estimation value, acquiring information, modulating the information by MSK and transmitting the information; wherein
The joint capture comprises:
and respectively performing delay processing on the approximate direct spread BPSK signal and the local code and then performing convolution, when the local code is aligned with the approximate direct spread BPSK signal, acquiring an estimated value of a code phase error at the position of the maximum correlation peak obtained by the convolution, and acquiring an estimated value of the Doppler frequency offset from the phase of the signal at the correlation peak.
As an improvement of the invention, the delay sampling point is changed, a plurality of delayed approximate direct spread BPSK signals and local codes are obtained, the convolution is carried out on the two signals under the same delay sampling point, and the average value of all the obtained Doppler frequency offset estimation values is taken as the Doppler compensation and de-spread estimation values.
As an improvement of the invention, a Doppler frequency offset estimation value obtained by averaging the phases of a plurality of correlation peaks obtained after convolution is used as an estimation value of Doppler compensation and de-spread.
As an improvement of the invention, all correlation operations in the algorithm can replace time domain convolution operations by FFT.
Compared with the traditional direct sequence spread spectrum MSK signal capturing method, the invention has the following advantages: (1) all the related operations in the algorithm can replace time domain convolution operation by FFT, so that the complexity of the algorithm is greatly reduced; (2) the pseudo code phase and the carrier frequency offset are searched simultaneously, the capturing time is M spread spectrum code periods, and a parameter M can be flexibly selected according to the capturing time and the capturing probability performance; (3) the received signal and the local code are subjected to delay multiplication processing, correlation is performed to obtain a correlation peak, a code phase difference and Doppler frequency offset are estimated, the Doppler frequency search range can be controlled by adjusting delay amount and delay number, balance between the Doppler frequency search range and estimation precision is achieved, and the method is suitable for a high dynamic environment; (4) the direct spread MSK signal is processed by the receiving conversion filter and converted into the direct spread BPSK signal, so that the problem of complex MSK signal form is solved, and the rapid acquisition method suitable for the direct spread BPSK signal can be applied to the direct spread MSK signal.
The invention is further described below with reference to the accompanying drawings.
Drawings
FIG. 1 is a block diagram of the system components of an embodiment of the present invention.
Fig. 2 is a signal processing flow diagram of a joint estimation module according to an embodiment of the invention.
Fig. 3 is a diagram illustrating the variation of the detection probability and the false alarm probability with the input signal-to-noise ratio according to an embodiment of the present invention.
Detailed Description
The condition for accurate despreading and demodulation of the direct-spread MSK signal is the synchronization of the signal, including the synchronization of a pseudo code and a carrier. The signal synchronization is divided into coarse synchronization and fine synchronization, and the invention focuses on the coarse synchronization of signals, namely pseudo code and Doppler frequency offset capture.
The transmission signal adopts a form of pilot frequency code + transmission data, and M all-1 data symbols are inserted in front of the transmission data for the spread spectrum code and carrier synchronization at the receiver end. After the pilot frequency code compensates Doppler frequency offset, code phase and the like, the data signal part can be subjected to accurate de-spread demodulation.
The direct spread MSK signal can be generated in serial mode, i.e. the spread signal and the carrier cos (2 pi f)1t) carrying out BPSK modulation to obtain a direct spread BPSK signal, and then generating a direct spread MSK signal through a conversion filter g (t). The impulse response of the conversion filter is
Wherein,fcto be loadedThe wave frequency, T, is the spreading code chip width.
Aiming at the serial generation mode of the direct sequence spread MSK signal, a filter h (t) matched with a conversion filter g (t) is adopted at a receiving end for receiving, and the impulse response of the corresponding receiving conversion filter is
The output signal of the conversion filter is then passed through f1And performing down-conversion, low-pass filtering and K-time extraction to obtain an approximate direct spread BPSK baseband signal. The decimation factor is consistent with the oversampling factor, and the output baseband signal can be expressed as
Wherein τ is the code phase error;is the initial phase, set to 0 for ease of analysis; rcIs the spreading code rate; f. ofdIs the Doppler frequency offset; omeganIs Gaussian complex white noise; since the pilot code part data is 1, equation (3) can be rewritten as:
jointly capturing an approximate direct spread BPSK signal and a local code, comprising the steps of:
step S101, delay the received signal y (n), i.e. the received signal is delayed by D samples and then multiplied by the conjugate of the undelayed signal, the process is as follows:
where ω' (n) is the noise after the delay processing.
Step S102, correcting the local code c (n), i.e. performing the same delay processing:
cD(n)＝c*(n)c(n+D) (6)
step S103, the signal y after the delay processing is carried outD(n) and local correction code cD(n) convolving to obtain:
where N is the spreading code period, the convolution may be performed by a correlator.
Step S104, as can be seen from equation (7), when the local code is aligned with the received signal, the correlation peak obtained by convolution is the largest:
to this end, an estimate of the code phase errorCan be obtained from the position of the correlation peak.
Compared with the traditional code acquisition algorithm, the method obviously eliminates the influence of Doppler frequency on the peak-to-peak value of the correlation of the acquisition output.
Step S105, obtaining an estimated value of Doppler frequency offset from the phase of the signal at the correlation peak as
Wherein arg [ z ]]Is an angle of complex number z;indicating the correlation peak output result.
The estimated value expressed by equation (9) is based on only one delay, and the concept of multiple delays is introduced to improve accuracy. Thus, the joint capture process changes to:
in step S201, the initialization delay amount D is Dmax-d,DmaxIs the maximum delay amount, D is the number of delays, where the maximum delay amount DmaxAnd the delay d is controllable to obtain a greater probability of detection.
Step S202: at the same time, the received signal y (n) and the local code c (n) are delayed in the same manner as the above steps S102 and S103 to obtain yD(n) and cD(n)。
Step S203, adding yD(n) and cD(n) inputting the correlation device to perform convolution, searching the position of the correlation peak in the correlation output to obtain M correlation peaks, wherein the position of the correlation peak is the estimated value of the code phase error
In step S204, the estimated value of Doppler frequency offset can be obtained from the phase of the signal at the correlation peak (9).
Step S205, determine whether D > DmaxIf the determination condition is negative, changing the value of D, where D is D +1, and returning to step S202; if the determination condition is yes, the process proceeds to step S206.
Step S206, averaging all the obtained Doppler frequency offset estimation values to obtain the final Doppler frequency offset estimation value
Wherein d is a delay number, and when d is 0, the delay is a single delay; dmaxIs the maximum delay.
The above processes are all doppler frequency offset estimation based on the correlation peak of a data symbol, where the accuracy can be improved by using the average value of the correlation peak phases of the pilot codes of M symbols instead of the correlation peak phase of a symbol, so the acquisition process is modified as follows:
in step S301, the initialization delay amount D is Dmax-d,DmaxIs the maximum delay amount, D is the number of delays, where the maximum delay amount DmaxAnd the delay d is controllable to obtain a greater probability of detection.
Step S302: at the same time, the received signal y (n) and the local code c (n) are delayed in the same manner as the above steps S102 and S103 to obtain yD(n) and cD(n)。
Step S303, mixing yD(n) and cD(n) inputting the correlation device to perform convolution, searching the position of the correlation peak in the correlation output to obtain M correlation peaks, wherein the position of the correlation peak is the estimated value of the code phase error
In step S304, the estimated value of the doppler frequency offset can be obtained from the phase of the signal at the correlation peak (equation 9).
Step S305, averaging the phases of the M correlation peaks:
wherein,is the resulting average of the delay DA phase value;is the output value of the correlation peak.
Step S306, judging whether D is larger than DmaxIf the determination condition is negative, changing the value of D, where D is D +1, and returning to step S302; if the determination condition is yes, the process proceeds to step S307.
Step S307, averaging all the obtained Doppler frequency offset estimation values to obtain the final Doppler frequency offset estimation value
Wherein d is a delay number, and when d is 0, the delay is a single delay; dmaxIs the maximum delay.
According to the equation (12), the doppler frequency offset search range of the present invention is:
then, the maximum doppler estimation frequency offset is:
the innovation point of the invention is that the related peak value is output by utilizing the matching of the delay multiplication signal, the estimated values of the pseudo code phase and the Doppler frequency offset can be obtained simultaneously, the two-dimensional combined capture of the pseudo code and the Doppler frequency offset is realized, and the delay number D and the delay maximum value DmaxAre controllable to balance the detection probability with the Doppler estimation range.
Examples
The invention relates to a two-dimensional combined capturing method for a direct sequence spread spectrum (MSK) signal under high dynamic conditions. As shown in fig. 1, the received intermediate frequency direct spread MSK signal is first subjected to a receive conversion filter, down conversion, low pass filtering, and decimation processing to obtain an approximate direct spread BPSK signal, and then the approximate direct spread BPSK signal is sent to a joint estimation module to obtain an estimated value of a code phase error and a doppler frequency offset, thereby completing the work of a code capture link. After the received signal passes through a Doppler compensation/de-spread module, the MSK modulation signal can be obtained.
System sampling frequency fs245.52MHz, intermediate frequency fc76.725MHz, 12 oversampling multiple K, and R spreading code ratec20.46Mchip/s, 20kbps data rate, Gold sequence for spreading code, 1023 code length N, and 20 pilot code symbols M.
And receiving the intermediate frequency direct spread MSK signal, and obtaining an approximate direct spread BPSK signal through a receiving conversion filter with impulse response of h (t). The impulse response of the receiving transform filter is
Wherein,is the spreading code period. The frequency response of the conversion filter is
The design of the filter adopts a convex optimization technology, firstly, the design problem of the filter needs to be converted into a convex optimization problem, and the Chebyshev approximation of the conversion filter can be established as a convex optimization model:
wherein sup is the infimum border; ω 2 pi f is the angular frequency; d (ω) is a given frequency response function; h (ω) is the frequency response of the designed filter,h (N) is the filter coefficient, N0Is the filter order.
The actually applied filter coefficients h (n) were determined by the cvx kit of Matlab software.
The signal after the receiving and converting filter passes through f1Down conversion of whereinLow-pass filtering is carried out after down-conversion, and then K times of extraction is carried out, wherein the extraction multiple is consistent with the oversampling multiple. Since the pilot code part data is 1, the baseband signal is obtained
Wherein τ is the code phase error;is the initial phase; rcIs the spreading code rate; f. ofdIs the Doppler frequency offset; omeganIs Gaussian complex white noise; for convenience of analysis, the initial phase is set
As shown in fig. 2, the steps of combining the capture modules are as follows:
step 1: initialization delay amount D ═ Dmax-d。
DmaxIs the maximum delay amount, D is the number of delays, where the maximum delay amount DmaxAnd the delay d is controllable to obtain a greater probability of detection.
Step 2: meanwhile, the received signal y (n) and the local code c (n) are delayed.
The delay processing process comprises the following steps: delaying the signal by D sampling points, multiplying the delayed signal by the undelayed signal after conjugation to obtain a delayed signal yD(n) and local correction code cD(n)。
And step 3: will yD(n) and cD(n) inputting the correlator, searching the position of the correlation peak at the correlation output, and obtaining M correlation peaks. The position of the correlation peak is the estimated value of the code phase error
And 4, step 4: the phases of the resulting M correlation peaks are averaged:
wherein,the resulting average phase value for delay D;is the output value of the correlation peak.
And 5: judging whether D is larger than Dmax。
If the judgment condition is negative, changing the value of D, and returning to the step 2 if D is D + 1; if the determination condition is yes, the process proceeds to step 6.
Step 6: averaging the (d +1) phase calculation results, and calculating a Doppler frequency offset estimation value:
according to the equation (20), the doppler frequency offset search range of the present invention is:
then, the maximum doppler estimation frequency offset is:
FIG. 3 shows the input SNR of [ -30dB, -5dB [ -30dB ]]，fd280kHz, maximum delay Dmax36, delay d 5, false alarm probability PfWhen the signal-to-noise ratio is 0.001, the obtained detection probability and false alarm probability are changed along with the signal-to-noise ratio. The calculation formula (21) can be obtained: the Doppler frequency offset estimation range is [ -284.17kHz, 284.17kHz]。
It can be seen from the figure that when the signal-to-noise ratio reaches-20 dB, the detection probability reaches 0.9155, and the false alarm probability is hardly affected by the signal-to-noise ratio due to the constant false alarm threshold setting criterion. Therefore, the pseudo code-Doppler joint capturing method designed by the invention can accurately capture the pseudo code phase and the carrier frequency offset in a high dynamic environment.
Therefore, compared with the existing capturing method, the two-dimensional joint estimation method for the direct sequence spread spectrum MSK signal under the high dynamic condition has the advantages of small algorithm complexity, capability of simultaneously completing capturing of the pseudo code phase and the Doppler frequency offset, small estimation error under the high dynamic condition and the like, is very suitable for the application environment with high dynamic condition and low signal-to-noise ratio, and has high practical value.
Claims (5)
1. A two-dimensional combined capturing method for directly spread MSK signals under high dynamic conditions is characterized by comprising the following steps:
the intermediate frequency direct spread MSK signal is processed by a receiving conversion filter, down conversion, low pass filtering and extraction to obtain an approximate direct spread BPSK signal,
the approximate direct spread BPSK signal and the local code are jointly captured to obtain the estimated value of the code phase error and the estimated value of the Doppler frequency offset,
after Doppler compensation is carried out on the approximate direct spread BPSK signal based on the two estimation values, accurate code tracking and despreading demodulation are carried out to obtain information; wherein
The joint capture comprises:
and respectively performing delay processing on the approximate direct spread BPSK signal and the local code and then performing convolution, when the local code is aligned with the approximate direct spread BPSK signal, acquiring an estimated value of a code phase error at the position of the maximum correlation peak obtained by the convolution, and acquiring an estimated value of Doppler frequency offset from the phase of the signal at the correlation peak.
2. The method of claim 1, wherein the delayed samples D are changed to obtain a plurality of delayed substantially direct spread BPSK delayed signals and local correction codes, and convolving the two signals under the same delayed sample, and averaging all the obtained Doppler frequency offset estimates to obtain Doppler compensated and despread Doppler frequency offset estimates, wherein the averaged Doppler frequency offset estimate is the Doppler frequency offset estimate
Wherein the phase of the correlation peakD is a delayed sample, DmaxIs the maximum delay amount, d is the delay number, RD(x) is the signal after the approximate direct spread BPSK signal and the local code are respectively delayed and convolved,for the code phase error estimate, RcIs the spreading code rate.
3. The method of claim 2, wherein the doppler frequency offset estimate is obtained by averaging the phases of a plurality of correlation peaks obtained after the convolution, wherein the averaging of the phases of the plurality of correlation peaks is
The Doppler frequency offset estimation value obtained by averaging the phases of a plurality of correlation peaks is
Where M is the number of correlation peaks.
4. The method according to claim 1, 2 or 3, characterized in that the intermediate frequency direct spread MSK signal is generated in a serial manner, specifically: spread signal and carrier cos (2 pi f)1t) carrying out BPSK modulation to obtain a direct spread BPSK signal, and generating a direct spread MSK signal by a conversion filter, wherein the impulse response of the conversion filter is
Wherein,fct is the carrier frequency and the spreading code chip width.
5. The method of claim 4, wherein the intermediate frequency direct spread MSK signal is received by a filter h (t) matched to the conversion filter g (t), and the impulse response of the corresponding receiving conversion filter is
Wherein,fcis carrier frequency, T is spreading codeChip width.
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| CN109088838B (en) * | 2018-09-08 | 2020-11-06 | 南京理工大学 | Pseudo code-Doppler fast capturing method of direct sequence spread spectrum DPSK signal under high dynamic condition |
| CN109150233B (en) * | 2018-09-13 | 2021-02-12 | 南京理工大学 | Modulation and demodulation method for direct sequence spread spectrum DPSK signal |
| CN109412644B (en) * | 2018-09-13 | 2021-02-12 | 南京理工大学 | Doppler frequency estimation method for direct sequence spread spectrum MSK signal |
| CN109617570B (en) * | 2018-12-25 | 2020-09-18 | 西安空间无线电技术研究所 | Full-digital synchronization method for broadband frequency hopping direct sequence spread spectrum signal without data assistance |
| CN110601718B (en) * | 2019-09-05 | 2022-11-04 | 珠海市杰理科技股份有限公司 | Spread spectrum communication capturing method, device and system |
| CN111082835A (en) * | 2019-12-03 | 2020-04-28 | 南京理工大学 | Pseudo code and Doppler combined capturing method of direct sequence spread spectrum signal under high dynamic condition |
| CN114531329B (en) * | 2022-01-28 | 2023-08-22 | 西安电子科技大学 | Multipath MSK signal carrier frequency estimation method, system and application |
| CN116938657B (en) * | 2023-09-15 | 2023-12-29 | 武汉船舶通信研究所(中国船舶集团有限公司第七二二研究所) | DSSS-OQPSK signal demodulation method and device |
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| CN104639100B (en) * | 2015-02-12 | 2017-09-22 | 南京理工大学 | Produce the method and switched filter design method of DS msk signal |
| CN104833988B (en) * | 2015-04-30 | 2017-12-22 | 西安空间无线电技术研究所 | The capture systems and method of the lower high data rate direct sequence signal of large doppler dynamic |
| CN105141340B (en) * | 2015-07-24 | 2018-01-05 | 南京理工大学 | The digital method of reseptance of DS msk signal |
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