CN113872908A - Low-earth-orbit satellite Internet of things short-time burst carrier synchronization method and system - Google Patents
Low-earth-orbit satellite Internet of things short-time burst carrier synchronization method and system Download PDFInfo
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Abstract
The invention provides a short-time burst carrier synchronization method and system for a low-earth-orbit satellite Internet of things. The method comprises the following steps: performing matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local serial number, and calculating to obtain a coarse frequency offset estimation value; sweeping the frequency of 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 quitting frequency sweeping when the CRC is correct, so as to realize the synchronization of the short-time burst carrier by the fine frequency offset estimation value obtained by calculation; and if the frequency sweep range is exceeded in the frequency sweep process, exiting the frequency sweep, adjusting the frequency sweep range, then re-carrying out the frequency sweep until the frequency sweep is exited when the CRC check is correct, and completing the short-time burst carrier synchronization. The short-time burst carrier synchronization method for the low-earth-orbit satellite Internet of things can realize the carrier synchronization of MF-TDMA (multi-frequency time division multiple access) by combining coarse frequency offset estimation and frequency sweep fine frequency offset estimation.
Description
Technical Field
The invention relates to the field of communication, in particular to a method and a system for synchronizing short-time burst carrier waves of a low-earth-orbit satellite internet of things.
Background
It is predicted that by 2025 the internet of things will be connected worldwide by more than 270 billion and will be involved in various areas of human activity. However, for the internet of things depending on wireless access, besides the terminal of the internet of things, a communication network with wide coverage and massive access is required. Ground networks employing a large number of base stations are satisfactory for most terrestrial regions. However, in areas such as the ocean and the Gobi, the base stations are difficult to establish and maintain, the construction capital is expensive, and a large number of base stations are difficult to construct. In addition to traditional terrestrial base stations, it is a practical solution to establish satellite communications as a complement and extension to terrestrial communications.
The satellite communication system uses a satellite as a relay station to relay radio waves, thereby realizing wireless communication between terminals. The low-orbit satellite is a satellite relatively close to the earth, and can form a low-orbit satellite constellation in an inter-satellite networking mode. The satellite is integrated, full coverage is achieved 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 destruction resistance and the like. From the perspective of the internet of things, the nature of the internet of things of low earth orbit satellites is that the low earth orbit satellites are used as base stations to access, and the commercial value of the global everything interconnection is realized.
The low earth orbit satellite moves at a high speed relative to the earth, and Doppler frequency offset is formed in the communication process. On one hand, the Doppler frequency offset of the low-earth orbit satellite Internet of things communication reaches 1200Hz, and the frequency offset change rate of 250Hz/s exists at the same time. On the other hand, the communication data volume of the low-orbit satellite internet of things is small, the symbol rate is low, and for the short burst communication, the influence of Doppler frequency offset on signal demodulation is huge. How to carry out carrier synchronization, eliminate Doppler frequency offset influence and simultaneously ensure the precision and complexity of carrier synchronization is a key technology with both challenge and research value.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention aims to provide a method and a system for synchronizing short-time burst carrier waves of the low-earth-orbit satellite internet of things.
In order to achieve the above object, the present invention provides a synchronization method for short-time burst carrier of the internet of things of low earth orbit satellites, comprising the following steps:
performing matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local leader sequence, and calculating to obtain a coarse frequency offset estimation value;
sweeping the frequency of the signal after the coarse frequency offset estimation under each set frequency offset step length and frequency offset change rate step length to realize the fine frequency offset estimation;
performing CRC (cyclic redundancy check) in the frequency sweeping process, and quitting frequency sweeping when the CRC is correct, so as to realize the synchronization of the short-time burst carrier by the fine frequency offset estimation value obtained by calculation; and if the frequency sweep range is exceeded in the frequency sweep process, exiting the frequency sweep, adjusting the frequency sweep range, then re-carrying out the frequency sweep until the frequency sweep is exited when the CRC check is correct, and completing the short-time burst carrier synchronization.
The short-time burst carrier synchronization method for the low-earth-orbit satellite Internet of things can realize the carrier synchronization of MF-TDMA (multi-frequency time division multiple access) by combining coarse frequency offset estimation and frequency sweep fine frequency offset estimation.
The preferred scheme of the short-time burst carrier synchronization method for the low-earth-orbit satellite Internet of things comprises the following steps:
sequentially sliding and sampling the short-time burst carrier signals after matched filtering by adopting a sliding window, and performing correlation operation on the sliding signals x and the local sequence y to obtain correlation signals r; and performing FFT (fast Fourier transform) on the correlation signal r to a frequency domain, searching a frequency domain signal peak value of the correlation signal r and a sequence position where the peak value is located, and capturing a frame header synchronous with a local leader sequence to obtain a coarse frequency offset estimation value.
The calculation method of the coarse frequency offset estimation value comprises the following steps: obtaining a frequency domain point p corresponding to the maximum correlation peak, and if p is greater than the FFT point/2, obtaining a coarse frequency offset estimation value- (FFT point-p) fs/FFT point; if p is less than FFT point number/2, the coarse frequency deviation estimated value is p fs/FFT point number.
And during frequency sweeping, calculating frequency offset compensation based on the coarse frequency offset estimation value under each frequency offset step length and each frequency offset change rate step length, wherein the frequency offset compensation is the coarse frequency offset estimation value plus the frequency sweep value, and the frequency offset compensation corresponding to correct CRC check is used as a fine frequency offset estimation value.
And compensating the short-time burst carrier signal by using the frequency offset compensation, performing matched filtering on the compensated signal, then performing timing sampling based on a frame header captured during coarse frequency offset estimation, performing correlation calculation on the sampled signal and a local leader sequence to obtain an estimated phase, and performing phase offset compensation on the sampled signal.
The preferred scheme of the short-time burst carrier synchronization method for the low-earth-orbit satellite Internet of things comprises the following steps: zero-filling the tail of the correlation signal r to 2nAfter the point, FFT conversion is carried out, n is a positive integer, 2n>The actual frequency point number 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 earth 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 earth orbit satellite internet of things.
The invention has the beneficial effects that: the invention can realize the carrier synchronization of the MF-TDMA Internet of things short burst, and can meet the parallel use of multiple paths of users; and moreover, by adopting a fast Fourier transform peak detection and simple fast frequency sweep mode, the hardware implementation complexity is low, the hardware implementation is facilitated, the frequency offset can be estimated quickly and reliably in the uplink transmission time of the low-earth-orbit satellite Internet of things, the synchronization can be realized efficiently and reliably, the error is small, and the control can be controlled within 1 Hz.
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 above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 is a flow chart of a low earth orbit satellite internet of things short-time burst carrier synchronization method.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are illustrative only for the purpose of explaining the present invention, and are not to be construed as limiting the present invention.
In the description of the present invention, unless otherwise specified and limited, it is to be noted that the terms "mounted," "connected," and "connected" are to be interpreted broadly, and may be, for example, a mechanical connection or an electrical connection, a communication between two elements, a direct connection, or an indirect connection via an intermediate medium, and specific meanings of the terms may be understood by those skilled in the art according to specific situations.
The invention provides an embodiment of a short-time burst carrier synchronization method for a low-earth-orbit satellite Internet of things. In the 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, an MF-TDMA manner is adopted, a terminal continuously broadcasts a short-time burst carrier signal with a length of 100 symbols in 10ms by taking a symbol rate of 10ksps as an example, and a preamble sequence containing 16 symbols is used for synchronous estimation. In order to take complexity, processing time and synchronization precision into account, the embodiment provides a short-time burst carrier synchronization method for the low-orbit satellite internet of things, which can meet the requirement of multi-user parallel processing, and the carrier synchronization precision reaches 1 Hz.
As shown in fig. 1, the method first performs matched filtering on a short-time burst carrier signal, and then performs coarse frequency offset estimation on the signal after the matched filtering, specifically, for the problem of inaccurate time synchronization, a sliding window is used to sample the short-time burst carrier signal, and a signal x obtained by each sliding sampling and a local preamble sequence y are subjected to correlation calculation to obtain r, that is, r is x conj (y). The length of the sliding window can be confirmed according to the actual possible time offset, the symbol number of the time offset is covered, and the longer the window is, the more accurate the window is, but the more time is consumed. In this embodiment, the length of the sliding window is preferably, but not limited to, 40.
Zero-filling to 2 for tail of correlation signal rnAfter the point, FFT is carried out to the frequency domain, n is a positive integer, 2n>The number of actual frequency points of the short-time burst carrier signal is preferably 2048 points in the present embodiment.
Searching frequency domain signal peak values and sequence positions of the peak values in all signals obtained by the sliding window, namely capturing a frame header (the frame header is the length corresponding to sliding when the maximum correlation peak is obtained, and the maximum correlation peak is the frequency domain point number corresponding to the maximum peak-to-average ratio), aligning the short-time burst carrier signals with a local sequence, and obtaining a coarse frequency deviation estimation value.
The calculation method of 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 deviation estimated value ═ - (FFT point number-p) × fs/FFT point number; if p is<FFT point number/2, and coarse frequency deviation estimated value is p & fs/FFT point number. In this embodiment, the number of FFT points is 2n＝2048。
And aiming at the Doppler frequency offset of 1200Hz and the frequency offset change rate of 250Hz/s, the coarse frequency offset estimation realizes the carrier synchronization within 10Hz error. After coarse frequency offset estimation, frequency sweep is carried out to improve frequency offset estimation precision, CRC check is carried out in the frequency sweep process, when the CRC check is correct, the frequency sweep is quitted, and synchronization of short-time burst carrier waves is realized by the fine frequency offset estimation value obtained by calculation at the moment; and if the frequency sweep range is exceeded in the frequency sweep process, exiting the frequency sweep, adjusting the frequency sweep range, then re-carrying out the frequency sweep until the frequency sweep is exited when the CRC check is correct, and completing the short-time burst carrier synchronization.
In this embodiment, a frequency deviation step size of 3Hz is used to search a frequency deviation range from-9 Hz to 9Hz in the whole frequency sweep, and a frequency deviation change rate step size of 45Hz/s is used to search a frequency deviation change range from-225 Hz/s to 225Hz/s, and the following steps are performed under each frequency deviation step size and frequency deviation change rate step size:
and calculating frequency offset compensation based on the coarse frequency offset estimation value, wherein the frequency offset compensation is the coarse frequency offset estimation value and the sweep frequency value. The sweep frequency value is the frequency offset value adopted currently when sweeping frequency. And performing frequency offset compensation on the short-time burst carrier signal by using the frequency offset compensation, namely q is u × exp (-j × 2 pi × Δ f × w/Fs), wherein w is (0:1: len), len refers to the length of the signal u, Δ f is a coarse frequency offset estimation value, Fs is a sampling rate, u is the short-time burst carrier signal, and j is an imaginary part.
And performing matched filtering on the signal subjected to the coarse frequency offset compensation, and then performing 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 a local leader sequence y to obtain an estimated phase, and carrying out phase offset compensation on the sampled signal.
And when the CRC check is correct, exiting the frequency sweeping, taking the frequency offset compensation corresponding to the correct CRC check as a fine frequency offset estimation value, and realizing the synchronization of the short-time burst carrier wave by using the fine frequency offset estimation value obtained by calculation at the moment. In this embodiment, the frequency sweep process of the fine frequency offset estimation can achieve a carrier synchronization accuracy of 1 Hz. And if the frequency sweep range is exceeded and the CRC check is still failed in the frequency sweep process, exiting the frequency sweep, adjusting the frequency sweep range, then re-carrying out the frequency sweep until the frequency sweep is exited when the CRC check is correct, and completing the short-time burst carrier synchronization.
This embodiment can be used to implement carrier synchronization for multi-frequency point services as well. In addition, the embodiment adopts fast Fourier transform peak detection and simple fast frequency sweep, which is not only beneficial to hardware realization, but also can realize synchronization efficiently and reliably.
It is worth mentioning a preferred solution of this embodiment:
when capturing the short-time burst carrier signal, performing FFT (fast Fourier transform) 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 2nMax (C)/Σ C, max (C) is the frequency domain peak of the correlation signal C, and Σ C is the spectrum array after fft transformation of the correlation signal.
Taking the maximum k peak-to-average ratios in all the correlation signals c, where k is a positive integer, and taking the corresponding sliding number as the offset frame header, where k is preferably, but not limited to, 3.
And sequentially verifying the slid signals corresponding to the k peak-to-average ratios by using the offset frame headers 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 headers corresponding to the k peak-to-average ratios which pass the verification are the frame headers captured during the fine frequency offset estimation.
The application also provides a short-time burst carrier synchronization system of the low earth 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 earth orbit satellite internet of things.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean 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 invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. 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 invention have been shown and described, it will be understood by those of ordinary skill in the art that: various changes, modifications, substitutions and alterations can be made to the embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.
Claims (7)
1. A short-time burst carrier synchronization method for a low earth orbit satellite Internet of things is characterized by comprising the following steps:
performing matched filtering on the short-time burst carrier signal, capturing a frame header synchronous with a local leader sequence, and calculating to obtain a coarse frequency offset estimation value;
sweeping the frequency of the signal after the coarse frequency offset estimation under each set frequency offset step length and frequency offset change rate step length to realize the fine frequency offset estimation;
performing CRC (cyclic redundancy check) in the frequency sweeping process, and quitting frequency sweeping when the CRC is correct, so as to realize the synchronization of the short-time burst carrier by the fine frequency offset estimation value obtained by calculation; and if the frequency sweep range is exceeded in the frequency sweep process, exiting the frequency sweep, adjusting the frequency sweep range, then re-carrying out the frequency sweep until the frequency sweep is exited when the CRC check is correct, and completing the short-time burst carrier synchronization.
2. The low earth orbit satellite internet of things short time burst carrier synchronization method as claimed in claim 1,
sequentially sliding and sampling the short-time burst carrier signals after matched filtering by adopting a sliding window, and performing correlation operation on the sliding signals x and the local sequence y to obtain correlation signals r; and performing FFT (fast Fourier transform) on the correlation signal r to a frequency domain, searching a frequency domain signal peak value of the correlation signal r and a sequence position where the peak value is located, and capturing a frame header synchronous with a local leader sequence to obtain a coarse frequency offset estimation value.
3. The method for synchronizing the short-time burst carrier of the internet of things of low earth orbit satellites according to claim 1, wherein the calculation method of the coarse frequency offset estimation value is as follows: obtaining a frequency domain point p corresponding to the maximum correlation peak, and if p is greater than the FFT point/2, obtaining a coarse frequency offset estimation value- (FFT point-p) fs/FFT point; if p is less than FFT point number/2, the coarse frequency deviation estimated value is p fs/FFT point number.
4. The method for synchronizing the short-time burst carrier of the internet of things of low-earth orbit satellites according to claim 1, wherein during frequency sweeping, frequency offset compensation is calculated based on the coarse frequency offset estimation value under each frequency offset step size and frequency offset change rate step size, wherein the frequency offset compensation is the coarse frequency offset estimation value + the frequency sweep value, and the corresponding frequency offset compensation when the CRC check is correct is used as the fine frequency offset estimation value.
5. The method for synchronizing the short-time burst carrier of the internet of things of low earth orbit satellites as claimed in claim 4, wherein the frequency offset compensation is used for compensating the short-time burst carrier signal, matched filtering is performed on the compensated signal, timing sampling is performed based on a frame header captured during coarse frequency offset estimation, correlation calculation is performed on the sampled signal and a local preamble sequence to obtain an estimated phase, and phase offset compensation is performed on the sampled signal.
6. The low earth orbit satellite internet of things short time burst carrier synchronization method as claimed in claim 2, wherein the tail of the correlation signal r is zero-padded to 2nAfter the point, FFT conversion is carried out, n is a positive integer, 2n>The actual frequency point number of the short-time burst carrier signal.
7. A short-time burst carrier synchronization system of the internet of things for the low earth orbit satellite, comprising a processor and a memory communicatively connected to the processor, wherein the memory is used for storing at least one executable instruction, and the executable instruction causes the processor to execute the operation corresponding to the short-time burst carrier synchronization method of the internet of things for the low earth orbit satellite according to any one of claims 1 to 6.
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