CN112118201B - LFM-based combined Doppler estimation method - Google Patents
LFM-based combined Doppler estimation method Download PDFInfo
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- CN112118201B CN112118201B CN202010958773.6A CN202010958773A CN112118201B CN 112118201 B CN112118201 B CN 112118201B CN 202010958773 A CN202010958773 A CN 202010958773A CN 112118201 B CN112118201 B CN 112118201B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
- H04L27/266—Fine or fractional frequency offset determination and synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B13/00—Transmission systems characterised by the medium used for transmission, not provided for in groups H04B3/00 - H04B11/00
- H04B13/02—Transmission systems in which the medium consists of the earth or a large mass of water thereon, e.g. earth telegraphy
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
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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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- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Abstract
The invention discloses a combined Doppler estimation method based on LFM, which comprises the following steps: s1, carrying out carrier capture and coarse synchronization by adding a starting frame of an LFM + CW signal; and S2, measuring the peak value deviation of the LFM signal to perform fine synchronization. The step S1 includes the steps of: after the first LFM signal is identified by the LFM pulse compression peak value obtained by matched filtering, the single carrier realizes capture and coarse synchronization through a COSTAS loop to obtain an initial Doppler estimation result, and the sampling rate is adjusted for the first time. Compared with the prior art, the method has the advantages of accurate and reliable estimation result, less computation amount, simple realization, strong real-time performance, easy realization and the like.
Description
The application case is a divisional application of a parent case, namely a patent application with the application number of 201810025927.9, the application date of 2018, april 01, apt 11 and the name of 'a combined Doppler estimation method applicable to an underwater acoustic communication system'.
Technical Field
The invention relates to the technical field of underwater acoustic communication, in particular to a combined Doppler estimation method based on an LFM.
Background
With the frequent development activities of oceans, the underwater acoustic communication system has great application value in the military field and more prominent application value in the civil field. Such as ocean water quality monitoring, ocean disaster early warning, resource exploration and the like. An underwater acoustic communication system based on a multiple-input multiple-output Orthogonal Frequency Division Multiplexing (OFDM) technology has the advantages of high data transmission rate, high reliability and the like, and thus becomes an important development direction in the field of underwater communication. However, OFDM has a high requirement on frequency accuracy, and when there is frequency offset, orthogonality of subcarriers is destroyed, which may cause mutual interference between subcarriers, and if the frequency offset is large, the performance of the system is seriously affected. In an underwater acoustic channel, frequency selective fading caused by multipath transmission and frequency shift caused by a doppler effect are main factors influencing the speed of an underwater acoustic communication system and the reliability of data transmission. Because Linear Frequency Modulation (LFM) signals can generate deformation of waveforms after pulse compression when affected by frequency offset, the frequency offset existing in the period of time can be preliminarily analyzed by judging the difference between waveforms generated by two frames before and after the pulse compression. However, when the initial frequency offset is too large and the multipath effect is obvious, the estimation accuracy is obviously greatly reduced under the influence of the multipath effect, and the performance of underwater acoustic communication is seriously influenced.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the combined Doppler estimation method is accurate and reliable in estimation result and is suitable for the underwater acoustic communication system.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a combined doppler estimation method suitable for use in an underwater acoustic communication system, comprising the steps of:
s1, carrying out carrier capture and coarse synchronization by adding an initial frame of an LFM + Continuous Wave (CW) signal;
and S2, measuring the peak value deviation of the LFM signal to perform fine synchronization.
The invention has the beneficial effects that: the method can avoid the influence of multipath effect in the initial stage of carrier synchronization, realize a larger capture range and realize a Doppler tracking loop with narrow noise bandwidth in the tracking stage. The method has the advantages of accurate and reliable estimation result, less calculation amount, simple implementation, strong real-time property and easy implementation.
Drawings
FIG. 1 is a frame format according to an embodiment of the present invention;
FIG. 2 is a costas loop structure according to an embodiment of the invention;
FIG. 3 is a waveform diagram obtained by matched filtering according to an embodiment of the present invention;
FIG. 4 is a waveform diagram of different phase offset down matched filtering according to an embodiment of the present invention;
FIG. 5 is a graph of the relationship between the Doppler factor and the left-right difference according to the embodiment of the present invention;
FIG. 6 is a logic block diagram of a combined Doppler estimation method according to an embodiment of the invention.
Detailed Description
In order to explain technical contents, achieved objects, and effects of the present invention in detail, the following description is made with reference to the accompanying drawings in combination with the embodiments.
The most key concept of the invention is as follows: the initial frame of LFM + CW is added to carry out carrier capture and coarse synchronization, and fine synchronization is carried out by measuring the offset of LFM peak value, so that the influence of multipath effect is avoided at the initial stage of carrier synchronization, a larger capture range is realized, and a Doppler tracking loop with narrow noise bandwidth is realized at the tracking stage.
A combined doppler estimation method suitable for use in an underwater acoustic communication system, comprising the steps of:
s1, carrying out carrier capture and coarse synchronization by adding an initial frame of an LFM + Continuous Wave (CW) signal;
and S2, measuring the peak value deviation of the LFM signal to perform fine synchronization.
The invention has the beneficial effects that: the method can avoid the influence of multipath effect in the initial stage of carrier synchronization, realize a larger capture range and realize a Doppler tracking loop with narrow noise bandwidth in the tracking stage. The method has the advantages of accurate and reliable estimation result, less calculation amount, simple realization, strong real-time performance and easy realization.
Further, the step S1 includes the steps of: after the first LFM signal is identified from the LFM pulse compression peak obtained by matched filtering, the single carrier realizes capture and coarse synchronization by a COSTAS loop (COSTAS loop, also called an in-phase quadrature loop method or an edge loop method), an initial doppler estimation result is obtained, and the sampling rate is adjusted for the first time according to the doppler estimation result.
Furthermore, the LFM pulse compression peak is obtained by correcting the phase spectrum to a linear function of frequency through a matched filter and then performing inverse fourier transform on the linear function.
Further, the specific operation of step S2 is as follows: the fuzzy estimation of the peak value is realized by jointly judging the offset of the peak value position and the change of the difference value of the left sampling point and the right sampling point (the left difference value and the right difference value for short), and the sampling rate is adjusted according to the fuzzy estimation of the peak value, so that the correction of the Doppler frequency shift is completed.
Further, when the peak value shifts more than one sampling point, i.e. the peak value is advanced or delayed relative to the position of the previous frame, the sampling rate adjustment in step S2 includes step S21 of correcting the doppler shift according to the position of the peak value; s22, adjusting the sampling rate according to the left and right difference values of the sampling points; wherein the correcting the Doppler shift according to the peak position is operative to:
assuming that the time length of a frame is fixed to be T, the original sampling rate is Fs, the total number of sampled sampling points is N, and when the peak value is advanced or delayed by N sampling points, the Doppler factor is
At this time, the sampling rate is adjusted to
I.e. the correction of the doppler shift according to the peak position is completed.
Further, the step S22 further includes adjusting the sampling rate by fitting a curve of the doppler factor and the left-right difference.
Further, when the peak value is shifted by less than one sampling point, the sampling rate adjustment process in step S2 is as follows: and adjusting the sampling rate according to the left and right difference values, and adjusting the sampling rate by fitting a curve of the Doppler factor and the left and right difference values.
From the above description, the beneficial effects of the present invention are: the sampling rate is adjusted through multiple estimation of the peak value, the correction of the Doppler frequency shift is completed, and the estimation result is accurate and reliable.
The first embodiment of the invention is as follows: a combined doppler estimation method suitable for use in an underwater acoustic communication system, comprising the steps of: s1, carrying out carrier capture and coarse synchronization by adding an initial frame of an LFM + Continuous Wave (CW) signal; and S2, measuring the peak value deviation of the LFM signal to perform fine synchronization.
Specifically, as shown in fig. 1, the format of the design frame is as follows: one frame has a length of 120ms, wherein the LFM is positioned at the position of the frame header, the length is 30ms, the initial frame is a synchronous frame, and 90ms except the LFM in the one frame is a 6kHz single carrier; the subsequent frame is a data frame, and 90ms is an OFDM modulation signal. When a signal is transmitted, a start frame and a data frame are continuously transmitted without an end frame.
After the first LFM signal is identified by the LFM pulse compression peak value obtained by matched filtering, the single carrier realizes capture and coarse synchronization through a COSTAS loop to obtain an initial Doppler estimation result, and the sampling rate is adjusted for the first time. Wherein the COSTAS loop structure is shown in fig. 2. The pulse compression of the LFM is realized by a matched filter, the process is equivalent to correcting the phase spectrum to a first linear function of frequency, and then performing inverse fourier transform on the corrected phase spectrum, and the frequency characteristic of the chirp signal is set as follows:
the frequency characteristic of the matched filter should be:
the time domain waveform expression obtained by multiplying the two expressions and performing inverse Fourier transform is as follows:
where D is the time-bandwidth product, B is the LFM signal bandwidth, and f0 is the initial frequency. The whole waveform is a signal which is influenced by aspects of time width, bandwidth and initial frequency on the basis of a Sinc function envelope.
In the system of the invention, an LFM signal with 30ms, 3kHz bandwidth and 6kHz initial frequency is adopted as a criterion of frame synchronization. The pulse-compressed time-domain waveform obtained after passing through the matched filter at a sampling rate of 38.4kHz is shown in fig. 3. Fig. 3 is an ideal waveform obtained by the area matching filtering, but in an actual process, due to the influence of phase offset, frequency offset and multipath of a signal, the waveform may have a large change, for example, if the LFM signal enters the matched filter and has a phase offset of 0 to 0.3 pi, the waveform may generate fluctuations of different degrees as shown in fig. 4.
Due to the influence of the unfixed phase relation between the sampling clock and the signal, the phase deviation often fluctuates within a sampling point, but the generated influence can shift the peak value. The shift in the peak will cause a change in the amplitude of the two samples to the left and right of the peak. Through the correlation calculation of the left and right difference values and the actual doppler shift, it is found that, in a certain range, the left and right difference values and the doppler shift factor are in positive correlation, as shown in fig. 5. Therefore, by jointly judging the shift of the peak position and the change of the difference value of the left and right sampling points, the fuzzy estimation of the peak can be realized, and the sampling rate is adjusted according to the fuzzy estimation of the peak to complete the correction of the doppler shift, and the logical structure diagram of the method is shown in fig. 6.
When the peak value deviation exceeds one sampling point, namely the peak value is advanced or delayed relative to the upper frame position, the sampling rate is firstly adjusted according to the peak value position. Assuming that the time length of a frame is fixed to be T, the original sampling rate is Fs, the total number of sampled sampling points is N, and when the peak value is advanced or delayed by N sampling points, the Doppler factor is
At this time, the sampling rate is adjusted to
I.e. the correction of the doppler shift according to the peak position is completed. And adjusting the sampling rate according to the left and right difference values, and adjusting the sampling rate by fitting a curve of the Doppler factor and the left and right difference values.
When the peak value deviation is less than one sampling point, the sampling rate can be adjusted according to the left and right difference values. The sampling rate is adjusted by fitting a curve of the doppler factor and the left and right difference values.
In summary, the combined doppler estimation method applicable to the underwater acoustic communication system provided by the invention can realize frequency correction of doppler factors below 0.2%, factor change rate of 0.02%/s in a communication process and total change of no more than 1% in a multipath channel through theoretical calculation, simulation verification and test verification, has a good effect and has little influence on bit error rate.
The above description is only an embodiment of the present invention, and not intended to limit the scope of the present invention, and all equivalent changes made by using the contents of the present specification and the drawings, or applied directly or indirectly to the related technical fields, are included in the scope of the present invention.
Claims (5)
1. A combined Doppler estimation method based on LFM is characterized in that: the method comprises the following steps:
s1, carrying out carrier capture and coarse synchronization by adding an initial frame of an LFM + CW signal; wherein, the length of one frame is 120ms, and the format of the frame is: the LFM is positioned at the position of a frame header, the length is 30ms, the bandwidth is 3kHz, the initial frequency is 6kHz, the initial frame is a synchronous frame, and 90ms except the LFM in one frame is a 6kHz single carrier;
s2, fine synchronization is carried out by measuring the peak value deviation of the LFM signal;
wherein the step S1 includes the steps of: after a first LFM signal is identified by an LFM pulse compression peak value obtained by matched filtering, capturing and coarse synchronization of a single carrier are realized through a COSTAS loop to obtain an initial Doppler estimation result, and the sampling rate is adjusted for the first time according to the Doppler estimation result;
and after the LFM pulse compression peak value is corrected into a linear function of frequency through a matched filter, the linear function is subjected to inverse Fourier transform to obtain the LFM pulse compression peak value.
2. The LFM-based combined doppler estimation method according to claim 1, characterized in that: the specific operation of step S2 is as follows: the fuzzy estimation of the peak value is realized by jointly judging the offset of the peak value position and the change of the difference value of the left sampling point and the right sampling point, and the sampling rate is adjusted according to the fuzzy estimation of the peak value to finish the correction of the Doppler frequency shift.
3. The LFM-based combined doppler estimation method according to claim 2, wherein: when the peak value offset exceeds one sampling point, the sampling rate adjustment in step S2 includes the following steps: s21, correcting the Doppler frequency shift according to the peak position; s22, adjusting the sampling rate according to the left and right difference values of the sampling points;
wherein the step S21 is specifically operable to correct the doppler shift according to the peak position:
assuming that the time length of a frame is fixed to be T, the original sampling rate is Fs, the total number of sampled sampling points is N, and when the peak value is advanced or delayed by N sampling points, the sampling rate is adjusted to be T
4. The LFM-based combined doppler estimation method according to claim 3, characterized in that: the step S22 further includes adjusting the sampling rate by fitting a curve of the doppler factor and the left and right difference values.
5. The LFM-based combined doppler estimation method according to claim 1, wherein: when the peak value deviation is less than one sampling point, adjusting the sampling rate according to the left and right difference values, wherein the sampling rate adjusting process in the step S2 is as follows: and the adjustment of the sampling rate is realized by fitting a curve of the Doppler factor and the left and right difference values.
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| CN109768812B (en) * | 2019-01-30 | 2020-12-22 | 杭州电子科技大学 | Underwater acoustic communication Doppler estimation and synchronization method based on chaotic frequency modulation |
| CN111342949B (en) * | 2020-02-19 | 2021-06-11 | 西北工业大学 | Synchronous detection method for underwater acoustic mobile communication |
| CN111711492B (en) * | 2020-05-09 | 2021-07-20 | 中国科学院声学研究所 | Underwater acoustic communication symbol timing estimation and compensation method and device for deep submersible vehicle |
| CN112003810B (en) * | 2020-10-28 | 2021-01-29 | 鹏城实验室 | Underwater acoustic communication synchronization method based on OFDM, intelligent terminal and storage medium |
| CN113627398B (en) * | 2021-10-11 | 2022-01-04 | 西安瀚博电子科技有限公司 | Signal characteristic detection method based on adaptive reconstruction filtering |
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