CN115134205B - AIS frame synchronization method and equipment based on sliding window sectional cross correlation - Google Patents
AIS frame synchronization method and equipment based on sliding window sectional cross correlation Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention discloses an AIS frame synchronization method and device based on sliding window piecewise cross correlation. Receiving an AIS signal; calculating a correlation peak value of the AIS signal for the received leading sequence of the AIS signal by adopting a piecewise cross-correlation algorithm; and if the correlation peak value of the AIS signal is not greater than the stored correlation peak threshold value, determining that the received AIS signal is invalid and ignoring the received AIS signal. The invention realizes the frame synchronization function of the AIS signal by using a sliding window sectional cross correlation method, the frame synchronization error rate is lower than 0.1 percent under the condition of 8dB signal-to-noise ratio, and the carrier frequency offset within +/-500 Hz can be resisted.
Description
Technical Field
The invention relates to the technical field of AIS frame synchronization, in particular to an AIS frame synchronization method and device based on sliding window piecewise cross correlation.
Background
The AIS (Automatic Identification System) is a System that is based on global satellite positioning System information and can provide relevant information for shore stations and ship stations to track and monitor marine vessels. The system works in VHF (very high frequency) maritime frequency bands (161.975 MHz and 162.025 MHz), and can send static information, dynamic information, voyage information and the like of a ship, wherein the static information comprises an MMSI (Mobile multimedia subscriber identity) number, a call sign, a ship name, ship length and width information, ship types and the like of the ship; the dynamic information comprises the current navigation position, navigation speed, course and the like of the ship; the voyage information comprises the draft, the cargo type, the destination port and the like of the voyage.
The AIS system uses GMSK (Gaussian Filtered Minimum Shift Keying) modulation mode, the format of the training sequence used for synchronization is 010101 \8230type, the autocorrelation characteristic of the sequence is poor, and the correlation peak side lobe is large, and the anti-frequency offset capability is weak.
Disclosure of Invention
The invention aims to provide an AIS frame synchronization method and equipment based on sliding window piecewise cross correlation, aiming at the defects in the prior art.
To achieve the above object, in a first aspect, the present invention provides an AIS frame synchronization method based on sliding window segment cross-correlation, including:
receiving an AIS signal;
calculating a correlation peak value of the AIS signal for the received leading sequence of the AIS signal by adopting a piecewise cross-correlation algorithm;
comparing the correlation peak value of the AIS signal with a stored correlation peak threshold value, judging whether the correlation peak value of the AIS signal is larger than the stored correlation peak threshold value, if the correlation peak value of the AIS signal is larger than the stored correlation peak threshold value, judging that a valid signal is captured, then updating the correlation peak value of the AIS signal to the stored correlation peak threshold value, outputting a timing starting point to a subsequent demodulator, if the correlation peak value of the AIS signal is not larger than the stored correlation peak threshold value, judging that the received AIS signal is invalid, and ignoring the received AIS signal.
Further, the correlation peak is calculated in the following specific manner:
wherein,
is the correlation peak value at the time n, and K is the total segment of the segment correlation of the AIS signalA number, M being the length of each segment and satisfying K M =24, x (.) being a received time domain signal,
m is the sequence number in the segment, k is the segment number.
Furthermore, the value of K is 3, and the value of M is 8.
In a second aspect, the present invention provides an AIS frame synchronization device based on sliding window segment cross-correlation, comprising:
the receiving module is used for receiving the AIS signal;
the calculation module is used for calculating the correlation peak value of the AIS signal for the leader sequence of the received AIS signal by adopting a piecewise cross-correlation algorithm;
a correlator to store a correlation peak threshold;
and the comparison and judgment module is used for comparing the correlation peak value of the AIS signal calculated by the calculation module with the correlation peak threshold value stored by the correlator, judging whether the correlation peak value of the AIS signal calculated by the calculation module is greater than the correlation peak threshold value stored by the correlator, if the correlation peak value of the AIS signal calculated by the calculation module is greater than the correlation peak threshold value stored by the correlator, judging that an effective signal is captured, then sending the correlation peak value of the AIS signal calculated by the calculation module as the correlation peak threshold value to the correlator for updating and storing, and outputting a timing starting point to a subsequent demodulator, if the correlation peak value of the AIS signal calculated by the calculation module is not greater than the correlation peak threshold value stored by the correlator, judging that the received AIS signal is invalid, and neglecting the received AIS signal.
Further, the correlation peak is calculated in the following specific manner:
wherein,
for the correlation peak at time n, K is the total number of segments of the segmented correlation, M is the length of each segment, and K M =24, x (.) is satisfied for the received time domain signal,
is a training sequence time domain signal, m is a sequence number in a segment, and k is a segment number.
Further, the value of K is 3, and the value of M is 8.
Has the advantages that: the invention realizes the frame synchronization function of the AIS signal by using a sliding window sectional cross correlation method, the frame synchronization error rate is lower than 0.1 percent under the condition of 8dB signal-to-noise ratio, and the carrier frequency offset within +/-500 Hz can be resisted.
Drawings
FIG. 1 is a flow chart of a method for discriminating the timing start point of an AIS signal;
FIG. 2 is a simulation of the system frame sync error rate under different segments;
fig. 3 is a functional block diagram of an AIS frame synchronization apparatus based on sliding window segment cross-correlation.
Detailed Description
The present invention will be further illustrated with reference to the accompanying drawings and specific examples, which are carried out on the premise of the technical solution of the present invention, and it should be understood that these examples are only for illustrating the present invention and are not intended to limit the scope of the present invention.
The embodiment of the invention provides an AIS frame synchronization method based on sliding window subsection cross-correlation, which comprises the following steps:
an AIS signal is received. The training sequence of the AIS signal is a series of sequences formed by alternating 0 and 1 with the total length of 24 bits, the code pattern is 01010101 \8230, and before GMSK modulation, NRZI coding is needed, so that the time domain signal is asymmetric.
And calculating a correlation peak value of the AIS signal for the preamble sequence of the received AIS signal by adopting a piecewise cross-correlation algorithm. Specifically, the correlation peak is calculated as follows:
wherein,
k is the total number of segments of the segment correlation of the AIS signal for a correlation peak at time n, M is the length of each segment, and K M =24, x (.) is the received time domain signal,
m is the sequence number in the segment, k is the segment number. Referring to fig. 2, as the training sequence of AIS is relatively short and only has 24 bits, the system is verified by simulation, K is selected to be 3, and m is selected to be 8, so that the frame synchronization error rate is lower than 0.1% under the baseband signal-to-noise ratio of 8dB, and the carrier frequency offset within +/-500 Hz can be resisted.
In order to continuously realize the function of continuously capturing the AIS signals and simultaneously reduce storage and calculation resources generated in the capturing process, the invention continuously judges and calculates the mutual peaks in real time based on the sliding window autocorrelation peak, thereby avoiding the process of calculating and storing the background noise in the traditional scheme and greatly reducing the hardware requirement on a calculation unit. Referring to fig. 1 specifically, the present invention compares the correlation peak value of the AIS signal with the stored correlation peak threshold value, determines whether the correlation peak value of the AIS signal is greater than the stored correlation peak threshold value, determines that a valid signal is captured if the correlation peak value of the AIS signal is greater than the stored correlation peak threshold value, then updates the correlation peak value of the AIS signal to the stored correlation peak threshold value, and outputs a timing start point to the subsequent demodulator, determines that the currently received AIS signal is invalid if the correlation peak value of the AIS signal is not greater than the stored correlation peak threshold value, and ignores the currently received AIS signal.
Based on the above embodiments, those skilled in the art can easily understand that the present invention provides an AIS frame synchronization device based on sliding window segment cross-correlation, which includes a receiving module 1, a calculating module 2, a correlator 3 and a comparison and determination module 4.
The receiving module 1 is configured to receive the AIS signal. The AIS signal training sequence is a string of 0 and 1 alternating sequence with the total length of 24 bits, the code type is 01010101 \8230, before GMSK modulation, NRZI coding is needed, and the time domain signal is asymmetric.
The calculating module 2 is used for calculating a correlation peak value of the AIS signal for the preamble sequence of the received AIS signal by adopting a piecewise cross-correlation algorithm. Specifically, the correlation peak is calculated as follows:
wherein,
k is the total number of segments of the segment correlation of the AIS signal for a correlation peak at time n, M is the length of each segment, and K M =24, x (.) is the received time domain signal,
m is the sequence number in the segment, k is the segment number. Referring to fig. 2, as the training sequence of the AIS is relatively short and only 24 bits are available, the system is verified by simulation, K is selected to be 3, and m is selected to be 8, so that the frame synchronization error rate is lower than 0.1% under the baseband signal-to-noise ratio of 8dB, and the system can resist the carrier frequency offset within +/-500 Hz.
The correlator 3 is used to store a correlation peak threshold. After power-on, the correlator is initialized, setting the stored correlation peak threshold to 0.
In order to continuously realize the function of continuously capturing the AIS signal and simultaneously reduce storage and calculation resources generated in the capturing process, the method and the device continuously judge and calculate the mutual peaks in real time based on the sliding window autocorrelation peak, so that the process of calculating and storing the background noise in the traditional scheme is omitted, and the hardware requirement on a calculation unit is greatly reduced. Specifically referring to fig. 1, the invention compares the correlation peak value of the AIS signal calculated by the calculation module 2 with the correlation peak threshold value stored by the correlator 3 through the comparison and determination module 4, determines whether the correlation peak value of the AIS signal calculated by the calculation module 2 is greater than the correlation peak threshold value stored by the correlator 3, determines that a valid signal is captured if the correlation peak value of the AIS signal calculated by the calculation module 2 is greater than the correlation peak threshold value stored by the correlator 3, then sends the correlation peak value of the AIS signal calculated by the calculation module 2 as the correlation peak threshold value to the correlator 3 for updating and storing, and outputs a timing start point to the subsequent demodulator, determines that the received AIS signal is invalid if the correlation peak value of the AIS signal calculated by the calculation module 2 is not greater than the correlation peak threshold value stored by the correlator 3, and ignores the received AIS signal.
The foregoing is merely a preferred embodiment of the present invention, and it should be noted that other parts not specifically described are within the skill or common general knowledge of one of ordinary skill in the art. Numerous modifications and adaptations may be made without departing from the principles of the present invention and such modifications and adaptations are intended to be within the scope of the present invention.
Claims (4)
1. An AIS frame synchronization method based on sliding window segment cross-correlation is characterized by comprising the following steps:
receiving an AIS signal;
calculating a correlation peak value of the AIS signal for the received leading sequence of the AIS signal by adopting a piecewise cross-correlation algorithm;
comparing the correlation peak value of the AIS signal with a stored correlation peak threshold value, judging whether the correlation peak value of the AIS signal is larger than the stored correlation peak threshold value, if the correlation peak value of the AIS signal is larger than the stored correlation peak threshold value, judging that an effective signal is captured, then updating the correlation peak value of the AIS signal to the stored correlation peak threshold value, outputting a timing starting point to a subsequent demodulator, if the correlation peak value of the AIS signal is not larger than the stored correlation peak threshold value, judging that the AIS signal received this time is invalid, and ignoring the AIS signal received this time;
the calculation method of the correlation peak value is specifically as follows:
wherein,
k is the total number of segments of the segment correlation of the AIS signal for a correlation peak at time n, M is the length of each segment, and K M =24, x (.) is the received time domain signal,
m is the sequence number in the segment, k is the segment number.
2. The AIS frame synchronization method based on sliding window segment cross-correlation according to claim 1, wherein the value of K is 3, and the value of M is 8.
3. An AIS frame synchronization device based on sliding window segment cross-correlation, comprising:
the receiving module is used for receiving the AIS signal;
the calculation module is used for calculating the correlation peak value of the AIS signal for the leader sequence of the received AIS signal by adopting a piecewise cross-correlation algorithm;
a correlator for storing a correlation peak threshold;
a comparison and determination module, configured to compare the correlation peak value of the AIS signal calculated by the calculation module with the correlation peak threshold stored by the correlator, determine whether the correlation peak value of the AIS signal calculated by the calculation module is greater than the correlation peak threshold stored by the correlator, determine that an effective signal is captured if the correlation peak value of the AIS signal calculated by the calculation module is greater than the correlation peak threshold stored by the correlator, send the correlation peak value of the AIS signal calculated by the calculation module as the correlation peak threshold to the correlator for update and storage, and output a timing start point to a subsequent demodulator, determine that the received AIS signal is invalid if the correlation peak value of the AIS signal calculated by the calculation module is not greater than the correlation peak threshold stored by the correlator, and ignore the received AIS signal;
the calculation method of the correlation peak value is specifically as follows:
wherein,
for the correlation peak at time n, K is the total number of segments of the segmented correlation, M is the length of each segment, and K M =24, x (.) is satisfied for the received time domain signal,
is a training sequence time domain signal, m is a sequence number in a segment, and k is a segment number.
4. The AIS frame synchronization device based on sliding window segment cross correlation according to claim 3, wherein K is 3, and M is 8.
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| CN101577580A (en) * | 2008-05-09 | 2009-11-11 | 北京大学 | Frame synchronization method |
| JP2011166769A (en) * | 2010-02-11 | 2011-08-25 | Fujitsu Ltd | Primary synchronization sequence detecting method and apparatus |
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| CN101577580A (en) * | 2008-05-09 | 2009-11-11 | 北京大学 | Frame synchronization method |
| JP2011166769A (en) * | 2010-02-11 | 2011-08-25 | Fujitsu Ltd | Primary synchronization sequence detecting method and apparatus |
| CN110311747A (en) * | 2019-07-16 | 2019-10-08 | 京信通信系统（中国）有限公司 | Resist big frequency deviation frame synchornization method, apparatus and system |
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