CN113872908B - Short-time burst carrier synchronization method and system for low-orbit satellite Internet of things - Google Patents
Short-time burst carrier synchronization method and system for low-orbit satellite Internet of things Download PDFInfo
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- CN113872908B CN113872908B CN202111144482.4A CN202111144482A CN113872908B CN 113872908 B CN113872908 B CN 113872908B CN 202111144482 A CN202111144482 A CN 202111144482A CN 113872908 B CN113872908 B CN 113872908B
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- H—ELECTRICITY
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- 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
- H04L27/2659—Coarse or integer frequency offset determination and synchronisation
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- H—ELECTRICITY
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- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/185—Space-based or airborne stations; Stations for satellite systems
- H04B7/1851—Systems using a satellite or space-based relay
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- H—ELECTRICITY
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Abstract
The invention provides a short-time burst carrier synchronization method and system for a low-orbit satellite Internet of things. The method comprises the following steps: carrying out matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local sequence number, and calculating to obtain a coarse frequency offset estimation value; carrying out frequency sweep on the signal after the coarse frequency offset estimation to realize the fine frequency offset estimation; performing CRC (cyclic redundancy check) in the frequency sweeping process, and when the CRC is correct, exiting the frequency sweeping, and realizing the synchronization of short-time burst carriers by using the precise frequency offset estimated value obtained by calculation at the moment; and if the frequency range is exceeded in the frequency sweeping process, the frequency sweeping is exited, the frequency sweeping is carried out again after the frequency sweeping range is adjusted, and the frequency sweeping is exited until the CRC is correct, so that the short-time burst carrier synchronization is completed. According to the short-time burst carrier synchronization method of the low-orbit satellite Internet of things, carrier synchronization of MF-TDMA can be achieved through a mode that coarse frequency offset estimation and frequency sweep are combined.
Description
Technical Field
The invention relates to the field of communication, in particular to a short-time burst carrier synchronization method and system for the Internet of things of low-orbit satellites.
Background
It is predicted that by 2025 the world of things will connect more than 270 billions and that the internet of things will be involved in various areas of human activity. But for the internet of things which relies on wireless access, a communication network with wide coverage and mass access is required besides the terminal of the internet of things. A ground network employing a large number of base stations is satisfactory for most terrestrial areas. However, in areas such as ocean and gobi, the base station is difficult to build and maintain, the construction funds are expensive, and a large number of base stations are difficult to build. In addition to conventional terrestrial base stations, satellite communications are established to be a supplement and extension to terrestrial communications.
The satellite communication system uses satellites as relay stations to forward radio waves, thereby realizing wireless communication between terminals. The low-orbit satellite is a satellite relatively close to the earth, and a low-orbit satellite constellation can be formed by an inter-satellite networking mode. The satellite node is an integral body, so that the full coverage is realized on the global scope, and users can access the network at any time. The low orbit satellite has the characteristics of large communication capacity, wide coverage, global seamless connection, strong anti-destruction property and the like. From the aspect of the internet of things, the essence of the internet of things of the low-orbit satellite is that the low-orbit satellite is used as a base station for access, so that the commercial value of global everything interconnection is realized.
The low orbit satellites move at high speed relative to the earth and form doppler frequency offsets during communication. On the one hand, the Doppler frequency offset of the communication of the low orbit satellite Internet of things reaches 1200Hz, and the frequency offset change rate of 250Hz/s exists. On the other hand, the communication data volume of the Internet of things of the low orbit satellite is small, the symbol rate is low, and the Doppler frequency offset has a great influence on signal demodulation for the short burst communication. How to perform carrier synchronization, eliminating the influence of Doppler frequency offset, and simultaneously ensuring the precision and complexity of carrier synchronization is a key technology with challenges and research value.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention aims to provide a short-time burst carrier synchronization method and system for the Internet of things of a low-orbit satellite.
In order to achieve the above purpose of the present invention, the present invention provides a short-time burst carrier synchronization method for a low-orbit satellite internet of things, comprising the following steps:
performing matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local preamble sequence, and calculating to obtain a coarse frequency offset estimation value;
carrying out frequency sweep on the signal after coarse frequency offset estimation under each set frequency offset step length and frequency offset change rate step length to realize fine frequency offset estimation;
performing CRC (cyclic redundancy check) in the frequency sweeping process, and when the CRC is correct, exiting the frequency sweeping, and realizing the synchronization of short-time burst carriers by using the precise frequency offset estimated value obtained by calculation at the moment; and if the frequency range is exceeded in the frequency sweeping process, the frequency sweeping is exited, the frequency sweeping is carried out again after the frequency sweeping range is adjusted, and the frequency sweeping is exited until the CRC is correct, so that the short-time burst carrier synchronization is completed.
According to the short-time burst carrier synchronization method of the low-orbit satellite Internet of things, carrier synchronization of MF-TDMA can be achieved through a mode that coarse frequency offset estimation and frequency sweep are combined.
The preferable scheme of the short-time burst carrier synchronization method of the low-orbit satellite Internet of things is as follows:
sequentially sliding sampling the short-time burst carrier signals after matched filtering by adopting a sliding window, and carrying out correlation operation on the sliding signal x and the local sequence y to obtain a correlation signal r; and carrying out FFT (fast Fourier transform) on the related signal r to a frequency domain, searching a frequency domain signal peak value of the related signal r and a sequence position of the peak value, and capturing a frame header synchronous with a local preamble sequence to obtain a coarse frequency offset estimation value.
The method for calculating the coarse frequency offset estimation value comprises the following steps: obtaining the frequency domain point number p corresponding to the maximum correlation peak, and if p is greater than FFT point number/2, coarse frequency offset estimation value= - (FFT point number-p) fs/FFT point number; if p < FFT point number/2, coarse frequency offset estimation value=p×fs/FFT point number.
And during frequency sweep, calculating frequency offset compensation based on the coarse frequency offset estimation value under each frequency offset step length and frequency offset change rate step length, wherein the frequency offset compensation=the coarse frequency offset estimation value+the frequency sweep value, and taking the frequency offset compensation corresponding to the correct CRC as a fine frequency offset estimation value.
And compensating the short-time burst carrier signal by utilizing the frequency offset compensation, carrying out matched filtering on the compensated signal, then carrying out timing sampling based on a frame head captured during coarse frequency offset estimation, carrying out correlation calculation on the sampled signal and a local preamble sequence to obtain an estimated phase, and carrying out phase offset compensation on the sampled signal.
The preferable scheme of the short-time burst carrier synchronization method of the low-orbit satellite Internet of things is as follows: zero padding of relevant signal r tail to 2 n After the point, FFT conversion is carried out, n is a positive integer, 2 n >The number of actual frequencies of the short-time burst carrier signal. This further improves the synchronization speed and accuracy.
The application also provides a short-time burst carrier synchronization system of the low-orbit satellite Internet of things, which comprises a processor and a memory in communication connection with the processor, wherein the memory is used for storing at least one executable instruction, and the executable instruction enables the processor to execute the operation corresponding to the short-time burst carrier synchronization method of the low-orbit satellite Internet of things.
The beneficial effects of the invention are as follows: the invention can realize carrier synchronization of MF-TDMA Internet of things short burst, and satisfies parallel use of multiple paths of users; and the method adopts a fast Fourier transform peak detection and simple fast frequency sweep mode, has low hardware implementation complexity, is beneficial to hardware implementation, can quickly and reliably estimate frequency offset in the uplink transmission time of the low-orbit satellite Internet of things, can realize synchronization efficiently and reliably, has small error, and can be controlled within 1Hz.
Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
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The foregoing and/or additional aspects and advantages of the invention will become apparent and may be better understood from the following description of embodiments taken in conjunction with the accompanying drawings in which:
fig. 1 is a flow chart of a short-time burst carrier synchronization method of the internet of things of a low orbit satellite.
Detailed Description
Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to like or similar elements or elements having like or similar functions throughout. The embodiments described below by referring to the drawings are illustrative only and are not to be construed as limiting the invention.
In the description of the present invention, unless otherwise specified and defined, it should be noted that the terms "mounted," "connected," and "coupled" are to be construed broadly, and may be, for example, mechanical or electrical, or may be in communication with each other between two elements, directly or indirectly through intermediaries, as would be understood by those skilled in the art, in view of the specific meaning of the terms described above.
The invention provides an embodiment of a short-time burst carrier synchronization method of a low-orbit satellite Internet of things. In this embodiment, a low-orbit satellite internet of things communication uplink transmission system is designed based on a channel environment with high dynamic and low signal-to-noise ratio, and an MF-TDMA mode is adopted, and a 10ksps symbol rate is taken as an example, and a terminal continuously broadcasts a short-time burst carrier signal with a 100 symbol length in 10ms time, wherein a preamble sequence containing 16 symbols carries out synchronization estimation. In order to achieve the complexity, the processing time and the synchronization precision, the embodiment provides a short-time burst carrier synchronization method of the low-orbit satellite Internet of things, which can meet the parallel processing of multiple users, and the carrier synchronization precision reaches 1Hz.
As shown in fig. 1, the method includes performing matched filtering on a short-time burst carrier signal, and then performing coarse frequency offset estimation on the matched filtered signal, specifically, aiming at the problem of inaccurate time synchronization, sampling the short-time burst carrier signal by adopting a sliding window, and performing correlation calculation on a signal x sampled in each sliding and a local preamble sequence y to obtain r, namely r=x, conj (y). The length of the sliding window may be based on the actual possible time offset confirmation, covering the number of symbols of the time offset, the longer the window the more accurate, but the more time consuming. In this embodiment, the length of the sliding window is preferably, but not limited to, 40.
Zero padding the tail of the related signal r to 2 n After the point, FFT conversion is carried out to a frequency domain, n is a positive integer and 2 n >The number of actual frequency points of the short-time burst carrier signal is preferably 2048 points in this embodiment.
And searching frequency domain signal peak values and sequence positions where the peak values are located from all signals obtained by the sliding window, namely capturing a frame header (the frame header is the length of corresponding sliding when obtaining the maximum correlation peak, and the maximum correlation peak is the frequency domain point number corresponding to the maximum peak-to-average ratio), so that the short-time burst carrier signals are aligned with the local sequence, and obtaining a coarse frequency offset estimation value.
The method for calculating the coarse frequency offset estimation value comprises the following steps: obtaining the frequency domain point number p corresponding to the maximum correlation peak, if p>FFT point number/2, coarse frequency offset estimation value= - (FFT point number-p) ×fs/FFT point number; if p<FFT point number/2, coarse frequency offset estimate = p fsNumber of FFT points. In the present embodiment, FFT point number=2 n ＝2048。
For the Doppler frequency offset of 1200Hz and the frequency offset change rate of 250Hz/s, the coarse frequency offset estimation realizes carrier synchronization within the error of 10 Hz. After coarse frequency offset estimation, frequency sweep is carried out to improve the frequency offset estimation precision, CRC (cyclic redundancy check) is carried out in the frequency sweep process, when the CRC is correct, the frequency sweep is exited, and the synchronization of short-time burst carriers is realized by the precise frequency offset estimation value obtained by calculation at the moment; and if the frequency range is exceeded in the frequency sweeping process, the frequency sweeping is exited, the frequency sweeping is carried out again after the frequency sweeping range is adjusted, and the frequency sweeping is exited until the CRC is correct, so that the short-time burst carrier synchronization is completed.
In the embodiment, the frequency offset range from-9 Hz to 9Hz is searched by adopting a frequency offset step length of 3Hz in the whole frequency sweep, the frequency offset change range from-225 Hz/s to 225Hz/s is searched by adopting a frequency offset change rate step length of 45Hz/s, and the following steps are carried out under each frequency offset step length and frequency offset change rate step length:
and calculating frequency offset compensation based on the coarse frequency offset estimation value, wherein the frequency offset compensation=the coarse frequency offset estimation value+the sweep frequency value. The frequency sweep value is the frequency offset value adopted currently during frequency sweep. And performing frequency offset compensation on the short-time burst carrier signal by using the frequency offset compensation, namely q=u×exp (-j×2pi×Δf×w/Fs), wherein w= (0:1:len), len refers to the length of the signal u, Δf is a coarse frequency offset estimated value, fs is a sampling rate, u is the short-time burst carrier signal, and j is an imaginary part.
And carrying out matched filtering on the signal subjected to the coarse frequency offset compensation, and then carrying out timing sampling on the signal subjected to the matched filtering based on a frame header captured during coarse frequency offset estimation.
And carrying out correlation calculation on the sampled signal and the local preamble sequence y to obtain an estimated phase, and carrying out phase offset compensation on the sampled signal.
And when the CRC is correct, the sweep frequency is exited, the frequency offset compensation corresponding to the correct CRC is used as a fine frequency offset estimation value, and the fine frequency offset estimation value obtained by calculation at the moment is used for realizing the synchronization of short-time burst carriers. In the embodiment, the frequency sweeping process of the fine frequency offset estimation can achieve carrier synchronization precision of 1Hz. And if the CRC (cyclic redundancy check) is not passed in the scanning process beyond the scanning range, the scanning is stopped, the scanning is performed again after the scanning range is adjusted, and the scanning is stopped until the CRC is correct, so that the short-time burst carrier synchronization is completed.
This embodiment is equally applicable to carrier synchronization for multi-frequency point services. In addition, the embodiment adopts fast Fourier transform peak detection and simple fast frequency sweep, which is beneficial to hardware realization and can realize synchronization with high efficiency and reliability.
It should be noted that a preferred scheme of this embodiment is as follows:
when capturing the short-time burst carrier signal, carrying out FFT conversion on the related signal r to a frequency domain, and calculating the peak-to-average ratio of each FFT converted related signal c; peak-to-average ratio ρ=2 n * max (C)/Σc, max (C) is the frequency domain peak value of the correlation signal C, Σc is the spectrum array after the correlation signal fft conversion.
Taking the maximum k peak-to-average ratios in all the related signals c, wherein k is a positive integer, taking the corresponding sliding number as an offset frame head, and k is preferably but not limited to 3.
And sequentially verifying the sliding signals corresponding to the k peak-to-average ratios by utilizing the offset frame heads and the frequency offsets corresponding to the k peak-to-average ratios, and demodulating the signals to obtain a final capturing result, namely, the offset frame heads corresponding to the peak-to-average ratios passing verification are the frame heads captured during accurate frequency offset estimation.
The application also provides a short-time burst carrier synchronization system of the low-orbit satellite Internet of things, which comprises a processor and a memory in communication connection with the processor, wherein the memory is used for storing at least one executable instruction, and the executable instruction enables the processor to execute the operation corresponding to the short-time burst carrier synchronization method of the low-orbit satellite Internet of things.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
While embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that: many changes, modifications, substitutions and variations may be made to the embodiments without departing from the spirit and principles of the invention, the scope of which is defined by the claims and their equivalents.
Claims (5)
1. The short-time burst carrier synchronization method for the low-orbit satellite Internet of things is characterized by comprising the following steps of:
performing matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local preamble sequence, and calculating to obtain a coarse frequency offset estimation value;
the method for calculating the coarse frequency offset estimation value comprises the following steps: obtaining the frequency domain point number p corresponding to the maximum correlation peak, and if p is greater than FFT point number/2, coarse frequency offset estimation value= - (FFT point number-p) fs/FFT point number; if p is less than FFT point number/2, coarse frequency offset estimation value=p is fs/FFT point number;
carrying out frequency sweep on the signal after coarse frequency offset estimation under each set frequency offset step length and frequency offset change rate step length to realize fine frequency offset estimation;
performing CRC (cyclic redundancy check) in the frequency sweeping process, and when the CRC is correct, exiting the frequency sweeping, and realizing the synchronization of short-time burst carriers by using the precise frequency offset estimated value obtained by calculation at the moment; if the frequency range is exceeded in the frequency sweeping process, the frequency sweeping is exited, the frequency sweeping is carried out again after the frequency sweeping range is adjusted, the frequency sweeping is exited until CRC (cyclic redundancy check) is correct, and the short-time burst carrier synchronization is completed;
during frequency sweep, frequency offset compensation is calculated based on the coarse frequency offset estimation value under each frequency offset step length and frequency offset change rate step length, frequency offset compensation=coarse frequency offset estimation value+frequency sweep value, and frequency offset compensation is carried out on the short-time burst carrier signal by utilizing the frequency offset compensation, namely q=u×exp (-j×2pi×Δf×w/Fs), wherein w= (0:1:len) is the length of the len signal u, Δf is the coarse frequency offset estimation value, fs is the sampling rate, u is the short-time burst carrier signal, j is the imaginary part, and the frequency offset compensation corresponding to the CRC is used as the precise frequency offset estimation value.
2. The short-time burst carrier synchronization method for the low-orbit satellite Internet of things according to claim 1, wherein,
sequentially sliding sampling the short-time burst carrier signals after matched filtering by adopting a sliding window, and carrying out correlation operation on the sliding signal x and the local sequence y to obtain a correlation signal r; and carrying out FFT (fast Fourier transform) on the related signal r to a frequency domain, searching a frequency domain signal peak value of the related signal r and a sequence position of the peak value, and capturing a frame header synchronous with a local preamble sequence to obtain a coarse frequency offset estimation value.
3. The short-time burst carrier synchronization method of the low-orbit satellite internet of things according to claim 1, wherein the frequency offset compensation is utilized to compensate short-time burst carrier signals, matched filtering is carried out on the compensated signals, then timing sampling is carried out based on frame heads captured during coarse frequency offset estimation, correlation calculation is carried out on the signals obtained by sampling and a local preamble sequence to obtain estimated phases, and phase offset compensation is carried out on the signals obtained by sampling.
4. The short-time burst carrier synchronization method of the low-orbit satellite internet of things according to claim 2, wherein the tail of the correlation signal r is zero-padded to 2 n After the point, FFT conversion is carried out, n is a positive integer, 2 n >The number of actual frequencies of the short-time burst carrier signal.
5. The short burst carrier synchronization system of the low-orbit satellite internet of things, which is characterized by comprising a processor and a memory in communication connection with the processor, wherein the memory is used for storing at least one executable instruction, and the executable instruction enables the processor to execute the operation corresponding to the short burst carrier synchronization method of the low-orbit satellite internet of things according to any one of claims 1 to 4.
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| CN112910819A (en) * | 2021-01-29 | 2021-06-04 | 东方红卫星移动通信有限公司 | Deep spread spectrum low-orbit satellite carrier synchronization method and system in high dynamic scene |
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| CN103023831A (en) * | 2012-12-19 | 2013-04-03 | 中国船舶重工集团公司第七二二研究所 | Carrier frequency offset estimation method applicable to burst waveforms |
| CN112910819A (en) * | 2021-01-29 | 2021-06-04 | 东方红卫星移动通信有限公司 | Deep spread spectrum low-orbit satellite carrier synchronization method and system in high dynamic scene |
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