CN114500188A - Frequency offset estimation method of automatic ship identification system - Google Patents
Frequency offset estimation method of automatic ship identification system Download PDFInfo
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- CN114500188A CN114500188A CN202111599318.2A CN202111599318A CN114500188A CN 114500188 A CN114500188 A CN 114500188A CN 202111599318 A CN202111599318 A CN 202111599318A CN 114500188 A CN114500188 A CN 114500188A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
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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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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
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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
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
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Abstract
The invention relates to a frequency offset estimation method of an Automatic Identification System (AIS) of a ship. The existing frequency offset estimation algorithm has the defects that the carrier frequency offset estimation precision is not high, or the existing frequency offset estimation algorithm depends on a modulation signal pattern to a greater extent, and the like. The invention is designed as follows: firstly, squaring a sampled AIS system received signal; performing fast Fourier transform on the square result and calculating the amplitude value of the square result to obtain a frequency spectrum sequence, respectively performing frequency spectrum shifting to the left and the right of the frequency spectrum sequence, and then performing superposition summation on the two sequences after the frequency spectrum shifting and the original frequency spectrum sequence; searching peak spectral lines in the maximum frequency offset range of the system in the summed sequence, and finally finishing frequency offset estimation. The invention can realize accurate estimation on the carrier frequency offset in the burst communication mode and is suitable for the automatic identification system of the ship.
Description
Technical Field
The invention belongs to the technical field of ship communication, and particularly relates to a frequency offset estimation method of an Automatic Identification System (AIS) of a ship.
Background
The AIS system employs a burst communication scheme, where the signal is discontinuous and aperiodic, and Gaussian Minimum Shift Keying (GMSK) is selected as the modulation scheme. The GMSK envelope has no sharp edge or inflection point, so that the GMSK envelope has high frequency band utilization rate. In communication, due to factors such as doppler effect and local oscillator offset, a carrier of a received signal has a certain frequency offset, and demodulation performance is affected by the frequency offset, thereby reducing reliability of the system.
Common GMSK frequency offset estimation algorithms include phase locked loop, frequency discrimination and Fast Fourier Transform (FFT). Phase-locked loops and frequency discrimination methods require longer sequences and are therefore not suitable for burst communications; the conventional FFT method is divided into two methods, one method is to utilize the 01 alternation characteristic of a 24bit leader sequence in AIS, obtain a signal frequency spectrum by using FFT and search a frequency spectrum peak value spectral line so as to determine the actual carrier frequency, but the frequency precision of the algorithm is limited by the number of the leader sequence points, the precision of frequency offset estimation is limited, and the algorithm is not beneficial to subsequent demodulation; another method is to use the data in the data frame in addition to the preamble sequence, obtain the frequency spectrum by using FFT after squaring the signal, and find out the 2 times upper limit cut-off frequency 2fhAnd 2 times lower cut-off frequency 2flSpectral lines, whereby the center carrier frequency is calculated, but whether there is a significant 2f in the spectrumhAnd 2flThe spectral peak will depend to a large extent on the modulation signal pattern.
Disclosure of Invention
The invention aims to solve the defects of the method and provides a high-precision frequency offset estimation method suitable for an AIS burst communication mode.
In order to achieve the above object, the present invention provides a frequency offset estimation method for an automatic ship identification system, which specifically includes the following steps:
step one, setting an expression of a receiving signal x (t) of the AIS system as shown in a formula (1).
x(t)＝cos[2π(fc+Δf)t+φ(t)+θ0] (1)
In the formula (1), fcIs a nominal carrier frequency, deltaf is a carrier frequency offset generated in the communication process, t is communication time, phi (t) is a GMSK modulation phase function, and theta0Is the carrier initial phase. X (T) is represented by TsSampling is performed for a sampling period, resulting in a time discrete form of x (t) as follows:
x(i)＝cos[2π(fc+Δf)iTs+φ(i)+θ0] (2)
in the formula (2), i is 0,1,2, …, M-1, M is N × P, which is the number of sampling points, N is the number of data bits required for FFT calculation, which is generally an even number, and P is Tb/TsIs an oversampling multiple, TbIs the symbol period.
The value of M is affected by the accuracy F of the frequency offset estimation, as shown in equation (3). The larger M, the better the frequency offset estimation accuracy F, but both the processing time and the computational complexity increase.
And step two, squaring x (i) to obtain y (i).
And step three, performing M-point FFT operation on the y (i) to obtain Y (k).
In the formula (5), the reaction mixture is,
step four, calculating the amplitude of Y (k) to obtain P (k), and then carrying out frequency spectrum left shift on P (k) according to the formula (6) to obtain PL(k)。
Step five, performing frequency spectrum right shift on P (k) according to the formula (7) to obtain PR(k)。
Step sixWill PL(k) P (k) and PR(k) Performing superposition to obtain Psum(k)。
Psum(k)＝PL(k)+P(k)+PR(k) (8)
Step seven, setting the maximum value of the AIS system frequency deviation as delta fMAXLooking for Psum(k) In the interval [2 (f)c-ΔfMAX)MTs,2(fc+ΔfMAX)MTs]Inner maximum value, set at sequence Psum(k) Where the corresponding index is index, the estimated frequency offset is expressed as
The invention has the beneficial effects that:
compared with the conventional frequency offset estimation method, the method makes full use of the received data information, and is not limited by the length of the leader sequence, so that the accuracy of frequency offset estimation is improved. Meanwhile, the information of characteristic spectral lines in the signals is fully utilized, and the dependence of the frequency offset estimation method on modulation signal patterns is effectively reduced.
Detailed Description
The present invention is further explained below.
In order to achieve the above object, the present invention provides a frequency offset estimation method for an automatic ship identification system, which specifically includes the following steps:
step one, setting an expression of a receiving signal x (t) of the AIS system as shown in a formula (1).
x(t)＝cos[2π(fc+Δf)t+φ(t)+θ0] (1)
In the formula (1), fcIs a nominal carrier frequency, delta f is carrier frequency deviation generated in the communication process, t is communication time, phi (t) is GMSK modulation phase function, and theta0Is the carrier initial phase. X (T) is represented by TsSampling is performed for a sampling period, resulting in a time discrete form of x (t) as follows:
x(i)＝cos[2π(fc+Δf)iTs+φ(i)+θ0] (2)
in the formula (2), i is 0,12, …, M-1, where M is N × P, which is the number of sampling points, N is the number of data bits required for FFT computation, which is generally an even number, and P is Tb/TsIs an oversampling multiple, TbIs the symbol period.
The value of M is affected by the accuracy F of the frequency offset estimation, as shown in equation (3). The larger M, the better the frequency offset estimation accuracy F, but both the processing time and the computational complexity increase.
And step two, squaring x (i) to obtain y (i).
And step three, performing M-point FFT operation on the y (i) to obtain Y (k).
In the formula (5), the reaction mixture is,
step four, calculating the amplitude of Y (k) to obtain P (k), and then carrying out frequency spectrum left shift on P (k) according to the formula (6) to obtain PL(k)。
Step five, performing frequency spectrum right shift on P (k) according to the formula (7) to obtain PR(k)。
Step six, adding PL(k) P (k) and PR(k) Performing superposition to obtain Psum(k)。
Psum(k)＝PL(k)+P(k)+PR(k) (8)
Step seven, setting the maximum value of the AIS system frequency deviation as delta fMAXLooking for Psum(k) In the interval [2 (f)c-ΔfMAX)MTs,2(fc+ΔfMAX)MTs]Inner maximum value, set at sequence Psum(k) Where the corresponding index is index, the estimated frequency offset is expressed as
In the embodiment of the frequency offset estimation method for the automatic identification system of the ship, the received data information is fully utilized, and is not limited to the length of the leader sequence, so that the accuracy of frequency offset estimation is improved. Meanwhile, the information of characteristic spectral lines in the signals is fully utilized, and the dependence of the frequency offset estimation method on modulation signal patterns is effectively reduced.
The above-described embodiments are only preferred embodiments of the present invention, and are not intended to limit the present invention in any way, and other variations and modifications may be made without departing from the spirit of the invention as set forth in the claims.
Claims (1)
1. A frequency offset estimation method of a ship automatic identification system is characterized by comprising the following specific steps:
firstly, setting an expression of an AIS system receiving signal x (t) as shown in a formula (1);
x(t)＝cos[2π(fc+Δf)t+φ(t)+θ0] (1)
in the formula (1), fcIs a nominal carrier frequency, deltaf is a carrier frequency offset generated in the communication process, t is communication time, phi (t) is a GMSK modulation phase function, and theta0Is the initial phase of the carrier; x (T) is represented by TsSampling is performed for a sampling period, resulting in a time discrete form of x (t) as follows:
x(i)＝cos[2π(fc+Δf)iTs+φ(i)+θ0] (2)
in the formula (2), i is 0,1,2, …, M-1, M is N × P, which is the number of sampling points, N is the number of data bits required for FFT calculation, which is generally an even number, and P is Tb/TsIs an oversampling multiple, TbIs the symbol period;
the value of M is influenced by the precision F of frequency offset estimation, as shown in formula (3); the larger M is, the better the frequency offset estimation precision F is, but the processing time and the calculation complexity are increased;
step two, squaring x (i) to obtain y (i);
step three, performing M-point FFT operation on the y (i) to obtain Y (k);
in the formula (5), the reaction mixture is,
k＝0，1，2，…，M-1；
step four, calculating the amplitude of Y (k) to obtain P (k), and then carrying out frequency spectrum left shift on P (k) according to the formula (6) to obtain PL(k)；
Step five, performing frequency spectrum right shift on P (k) according to the formula (7) to obtain PR(k)；
Step six, adding PL(k) P (k) and PR(k) Performing superposition to obtain Psum(k)；
Psum(k)＝PL(k)+P(k)+PR(k) (8)
Step seven, setting the maximum value of the AIS system frequency deviation as delta fMAXLooking for Psum(k) In the interval [2 (f)c-ΔfMAX)MTs，2(fc+ΔfMAX)MTs]Inner maximum value, set at sequence Psum(k) Where the corresponding index is index, the estimated frequency offset is expressed as
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Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016101658A1 (en) * | 2014-12-25 | 2016-06-30 | 中兴通讯股份有限公司 | Method and device for estimating frequency offset of microwave communication system channel |
| CN108632185A (en) * | 2018-05-15 | 2018-10-09 | 北京遥测技术研究所 | A kind of the AIS systems demodulation method and demodulating system of ship VDES systems |
| CN110691051A (en) * | 2019-09-29 | 2020-01-14 | 天津大学 | GMSK signal frequency offset estimation algorithm based on FFT |
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016101658A1 (en) * | 2014-12-25 | 2016-06-30 | 中兴通讯股份有限公司 | Method and device for estimating frequency offset of microwave communication system channel |
| CN108632185A (en) * | 2018-05-15 | 2018-10-09 | 北京遥测技术研究所 | A kind of the AIS systems demodulation method and demodulating system of ship VDES systems |
| CN110691051A (en) * | 2019-09-29 | 2020-01-14 | 天津大学 | GMSK signal frequency offset estimation algorithm based on FFT |
Non-Patent Citations (2)
| Title |
|---|
| 杨文东;蔡跃明;徐友云;: "一种基于连续导频的OFDM载波同步算法", 解放军理工大学学报(自然科学版), no. 03 * |
| 赵大伟;马社祥;王俊峰;孟鑫;: "基于频域相关的AIS信号频偏估计", 计算机仿真, no. 04 * |
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