CN108632185B - AIS system demodulation method and demodulation system of ship VDES system - Google Patents

AIS system demodulation method and demodulation system of ship VDES system Download PDF

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CN108632185B
CN108632185B CN201810462101.9A CN201810462101A CN108632185B CN 108632185 B CN108632185 B CN 108632185B CN 201810462101 A CN201810462101 A CN 201810462101A CN 108632185 B CN108632185 B CN 108632185B
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phase
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CN108632185A (en
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武光辉
吴鹏程
李英飞
刘向楠
李晓亮
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Beijing Research Institute of Telemetry
Aerospace Long March Launch Vehicle Technology Co Ltd
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Aerospace Long March Launch Vehicle Technology Co Ltd
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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
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/36—Modulator circuits; Transmitter circuits
    • H04L27/362—Modulation using more than one carrier, e.g. with quadrature carriers, separately amplitude modulated
    • H04L27/364—Arrangements for overcoming imperfections in the modulator, e.g. quadrature error or unbalanced I and Q levels
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/38—Demodulator circuits; Receiver circuits
    • 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 demodulation method and a demodulation system of an AIS system of a ship VDES system, the method adopts special signal detection, frequency deviation estimation and frequency discrimination demodulation to realize signal demodulation, overcomes the defect of poor demodulation performance of the traditional differential demodulation, is suitable for data demodulation under any conditions of single/multiple time slots, short frame/long frame signals and the like, greatly widens the application range of data demodulation, and carries out signal detection by counting the sign bit change condition of adjacent code elements according to the characteristics of a training sequence in a data frame structure, and has the advantages of simple design, easy realization, accurate signal detection result and higher precision; the frequency discrimination demodulation is adopted to realize the signal demodulation, so that the demodulation performance of the system is obviously improved; by utilizing the principle of spectral analysis, the frequency difference between the local carrier signal and the AIS receiving signal is extracted from the carrier phase discrimination error, the carrier frequency of the AIS receiving signal can be rapidly recovered, the capture period is short, and the accuracy is high.

Description

AIS system demodulation method and demodulation system of ship VDES system
Technical Field
The invention relates to the technical field of modulation and demodulation of ship data exchange systems, in particular to an AIS system demodulation method and a demodulation system of a ship VDES system.
Background
The international telecommunication union ITU-R m.1371-5 recommendation 2/2014 proposes technical characteristics of an Automatic Identification System (AIS) using time division multiple access in a VHF marine mobile frequency band, and makes corresponding regulations on communication technical characteristics of an AIS system, but does not make corresponding regulations on implementation modes, particularly implementation modes in any cases of single/multiple time slots, frequent/burst signals and the like, which are not explicitly described.
In the existing AIS system communication system, the signal demodulation method adopts 1bit/2bit differential demodulation, and although the method is simple to implement, the demodulation performance is low, and the practicability is not strong; the method for estimating the carrier frequency offset of the received signal is realized by adopting correlation operation, and the method needs to be realized according to the frame structure characteristics and cannot be widely applied.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides an AIS system demodulation method of a ship VDES system, which adopts special signal detection, frequency offset estimation and frequency discrimination demodulation to realize signal demodulation, overcomes the defect of poor demodulation performance of the traditional differential demodulation, is suitable for data demodulation under any conditions of single/multiple time slots, short frame/long frame signals and the like, and greatly widens the application range of data demodulation.
Another object of the present invention is to provide an AIS system demodulation system for a marine VDES system.
The above purpose of the invention is mainly realized by the following technical scheme:
an AIS system demodulation method of a ship VDES system comprises the following steps:
sampling AIS receiving signals, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signals and local carrier signals with frequency fc to obtain phase discrimination errors, and further obtaining frequency difference delta f between the local carrier signals and the AIS receiving signals;
carrying out in-phase and quadrature frequency mixing on the AIS receiving signal and the carrier signal after frequency offset compensation to obtain an in-phase component and a quadrature component, wherein the frequency of the carrier signal after frequency offset compensation is fc + delta f;
low-pass filtering the in-phase component and the orthogonal component to remove high-frequency components;
performing frequency discrimination processing on the in-phase component and the quadrature component after the low-pass filtering to obtain a frequency discrimination demodulation result;
carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result to realize signal detection;
and decoding and checking the data for realizing the signal detection to finish the AIS system demodulation.
In the AIS system demodulation method for the marine VDES system, the specific method for sampling the AIS received signal, performing in-phase and quadrature frequency mixing on the sampled AIS received signal and the local carrier signal with the frequency fc to obtain a phase discrimination error, and further obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal includes the following steps:
(1.1) sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal with frequency fc, and outputting an in-phase component I (t) and a quadrature component Q (t);
(1.2) low-pass filtering the in-phase component I (t) and the quadrature component Q (t) to remove high-frequency components;
(1.3) carrying out phase discrimination processing on the in-phase component I (t) and the quadrature component Q (t) after low-pass filtering to obtain a phase discrimination error
Figure BDA0001661134220000021
(1.4) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, carrying out phase discrimination on the obtained phase discrimination error
Figure BDA0001661134220000022
And performing FFT operation, analyzing the FFT operation result to obtain the frequency difference delta f between the local carrier signal and the AIS receiving signal, otherwise, not performing the FFT operation.
In the AIS system demodulation method of the marine VDES system, in the step (1.3), the phase discrimination error is detected
Figure BDA0001661134220000031
The calculation formula of (a) is as follows:
Figure BDA0001661134220000032
in the AIS system demodulation method for the marine VDES system, in the step (1.4), the obtained phase discrimination error is determined
Figure BDA0001661134220000033
The specific method for obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal by performing FFT operation and analyzing the FFT operation result is as follows:
(1.4.1) detecting the phase error
Figure BDA0001661134220000034
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer;
(1.4.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found;
(1.4.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2, f is the estimation accuracy, wherein:
Figure BDA0001661134220000035
fs is the sampling rate of the AIS received signal.
In the AIS system demodulation method of the marine VDES system, the specific method for performing frequency discrimination processing on the in-phase component and the quadrature component after the low-pass filtering to obtain a frequency discrimination demodulation result is as follows:
Figure BDA0001661134220000036
wherein:
Figure BDA0001661134220000037
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
In the AIS system demodulation method of the ship VDES system, the specific method for performing statistical analysis on the sign bit variation of adjacent code elements of the frequency discrimination demodulation result to realize signal detection is as follows:
(5.1) demodulating the data according to the frequency discrimination
Figure BDA0001661134220000041
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure BDA0001661134220000042
The length of the middle training sequence is L, and L is an even number; n is the number of sampling points in each data symbol in the AIS receiving signal;
(5.2) continuously counting for L/2 times, if continuously detecting that L/2-1 sign bits jump, judging to finish signal detection, entering the step (5.3), otherwise, resetting the counting result, and returning to the step (5.1);
and (5.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
In the AIS system demodulation method for the ship VDES system, the specific method for decoding and verifying the data for realizing the signal detection to complete the AIS system demodulation is as follows:
(6.1) carrying out bipolar reverse non-return-to-zero decoding on the data for realizing the signal detection;
(6.2) searching a start mark and an end mark in the bipolar reverse non-return-to-zero decoded data, and storing the complete data frame in which the start mark and the end mark are searched in a memory;
and (6.3) reading the complete data frame from the memory, performing CRC check, and judging whether the AIS received data is correct.
In the AIS system demodulation method of the ship VDES system, an AIS receiving signal is divided into two paths, wherein one path is delayed, the other path is subjected to sampling processing, the sampled AIS receiving signal and a local carrier signal with frequency fc are subjected to in-phase and quadrature frequency mixing to obtain a phase discrimination error, and further obtain a frequency difference delta f between the local carrier signal and the AIS receiving signal; and carrying out in-phase and quadrature frequency mixing on the delayed AIS receiving signal and the carrier signal subjected to frequency offset compensation to obtain an in-phase component and a quadrature component, wherein the frequency of the carrier signal subjected to frequency offset compensation is fc + delta f.
The utility model provides a boats and ships VDES system's AIS system demodulation system, includes frequency offset estimation module, mixing module, low pass filter module, frequency discrimination demodulation module, signal detection module and decoding check module, wherein:
a frequency offset estimation module: sampling AIS receiving signals, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signals and local carrier signals with frequency fc to obtain phase discrimination errors, further obtaining frequency difference delta f between the local carrier signals and the AIS receiving signals, and outputting the frequency difference delta f to a frequency mixing module;
a frequency mixing module: receiving a frequency difference delta f output by a frequency offset estimation module to obtain a carrier signal after frequency offset compensation, carrying out in-phase and quadrature frequency mixing on the AIS received signal and the carrier signal after frequency offset compensation to obtain an in-phase component and a quadrature component, and outputting the in-phase component and the quadrature component to a low-pass filtering module, wherein the frequency of the carrier signal after frequency offset compensation is fc + delta f;
a low-pass filtering module: receiving an in-phase component and an orthogonal component output by a frequency mixing module, performing low-pass filtering on the in-phase component and the orthogonal component to remove high-frequency components, and outputting the in-phase component and the orthogonal component subjected to low-pass filtering to a frequency discrimination demodulation module;
a frequency discrimination demodulation module: receiving the in-phase component and the orthogonal component which are output by the low-pass filtering module and subjected to low-pass filtering, performing frequency discrimination processing on the in-phase component and the orthogonal component which are output by the low-pass filtering module to obtain a frequency discrimination demodulation result, and outputting the frequency discrimination demodulation result to the signal detection module;
the signal detection module: receiving a frequency discrimination demodulation result output by a frequency discrimination demodulation module, carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result to realize signal detection, and outputting data after the signal detection to a decoding and checking module;
a decoding and checking module: and receiving the data after the signal detection output by the signal detection module, decoding and checking the data for realizing the signal detection, and completing the AIS system demodulation.
In the AIS system demodulation system of the marine VDES system, the frequency offset estimation module performs sampling processing on the AIS received signal, performs in-phase and quadrature frequency mixing on the sampled AIS received signal and the local carrier signal with frequency fc to obtain a phase discrimination error, and further obtains a frequency difference Δ f between the local carrier signal and the AIS received signal, where the specific method includes the following steps:
(1.1) sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal with frequency fc, and outputting an in-phase component I (t) and a quadrature component Q (t);
(1.2) low-pass filtering the in-phase component I (t) and the quadrature component Q (t) to remove high-frequency components;
(1.3) carrying out phase discrimination processing on the in-phase component I (t) and the quadrature component Q (t) after low-pass filtering to obtain a phase discrimination error
Figure BDA0001661134220000061
(1.4) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, carrying out phase discrimination on the obtained phase discrimination error
Figure BDA0001661134220000062
Performing FFT operation, and analyzing the FFT operation result to obtain a local carrier signal and the AIS receiving signalAnd f, otherwise, not performing FFT operation.
In the AIS system demodulation system of the marine VDES system, the phase discrimination error in the step (1.3) is detected
Figure BDA0001661134220000063
The calculation formula of (a) is as follows:
Figure BDA0001661134220000064
the obtained phase discrimination error is processed in the step (1.4)
Figure BDA0001661134220000065
The specific method for obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal by performing FFT operation and analyzing the FFT operation result is as follows:
(1.4.1) detecting the phase error
Figure BDA0001661134220000066
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer;
(1.4.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found;
(1.4.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2, f is the estimation accuracy, wherein:
Figure BDA0001661134220000067
fs is the sampling rate of the AIS received signal.
In the AIS system demodulation system of the marine VDES system, the frequency discrimination demodulation module receives the low-pass filtered in-phase component and quadrature component output by the low-pass filtering module, and performs frequency discrimination processing on the low-pass filtered in-phase component and quadrature component to obtain a frequency discrimination demodulation result, which includes the following specific steps:
Figure BDA0001661134220000071
wherein:
Figure BDA0001661134220000072
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
In the AIS system demodulation system of the marine VDES system, the signal detection module receives the frequency discrimination demodulation result output by the frequency discrimination demodulation module, and performs statistical analysis on the sign bit variation of adjacent code elements of the frequency discrimination demodulation result, so as to implement the specific method of signal detection as follows:
(5.1) demodulating the data according to the frequency discrimination
Figure BDA0001661134220000073
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure BDA0001661134220000074
The length of the middle training sequence is L, and L is an even number; n is the number of sampling points in each data symbol in the AIS receiving signal;
(5.2) continuously counting for L/2 times, if continuously detecting that L/2-1 sign bits jump, judging to finish signal detection, entering the step (5.3), otherwise, resetting the counting result, and returning to the step (5.1);
and (5.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
In the AIS system demodulation system of the marine VDES system, the decoding and checking module receives data after signal detection output by the signal detection module, and decodes and checks the data for realizing the signal detection, and the specific method for completing the AIS system demodulation is as follows:
(6.1) carrying out bipolar reverse non-return-to-zero decoding on the data for realizing the signal detection;
(6.2) searching a start mark and an end mark in the bipolar reverse non-return-to-zero decoded data, and storing the complete data frame in which the start mark and the end mark are searched in a memory;
and (6.3) reading the complete data frame from the memory, performing CRC check, and judging whether the AIS received data is correct.
Compared with the prior art, the invention has the following beneficial effects:
(1) the AIS system demodulation method of the invention adopts special signal detection, frequency offset estimation and frequency discrimination demodulation to realize signal demodulation, overcomes the defect of poor demodulation performance of the traditional differential demodulation, is suitable for data demodulation under any conditions of single/multiple time slots, short frame/long frame signals and the like, and greatly widens the application range of data demodulation.
(2) The invention carries out signal detection (frame header detection) by counting the sign bit change condition of adjacent code elements according to the characteristics of the training sequence in the data frame structure, and the method has the advantages of simple design, easy realization, accurate signal detection result and higher precision.
(3) The invention adopts frequency discrimination demodulation to realize signal demodulation, thereby obviously improving the demodulation performance of the system.
(4) The invention extracts the frequency difference between the local carrier signal and the AIS receiving signal from the carrier phase discrimination error by utilizing the principle of spectral analysis, can quickly recover the carrier frequency of the AIS receiving signal, and has short capture period and high accuracy.
(5) The FFT operation is realized by adopting the IP core in the FPGA chip, so that the consumption of hardware resources is less, the accuracy is high, the defect that the prior art is limited by a hardware platform can be effectively overcome, and the cost is reduced.
Drawings
Fig. 1 is a schematic block diagram of an AIS system demodulation method of a marine VDES system according to the present invention.
Fig. 2 is a block diagram of a specific implementation of the AIS system demodulation method of the marine VDES system of the present invention.
Fig. 3 is a schematic structural diagram of an AIS system demodulation system of the marine VDES system of the present invention.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples:
fig. 1 is a schematic block diagram of an AIS system demodulation method of a ship VDES system, and fig. 2 is a specific implementation block diagram of the AIS system demodulation method of the ship VDES system. The AIS system demodulation implementation method of the ship VDES system specifically comprises the following steps:
(1) and sampling the AIS receiving signal by an analog-to-digital converter (ADC), wherein N sampling points are arranged in each data symbol.
(2) Carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal, and outputting an in-phase component I (t) and a quadrature component Q (t); wherein the local carrier signal frequency is fc.
(3) The in-phase component I (t) and the quadrature component Q (t) are low-pass filtered to remove high-frequency components.
(4) The in-phase component I (t) and the quadrature component Q (t) after the low-pass filtering are processed by phase discrimination through a phase discriminator to obtain phase discrimination error
Figure BDA0001661134220000091
Wherein the phase discrimination error
Figure BDA0001661134220000092
The calculation formula of (a) is as follows:
Figure BDA0001661134220000093
(5) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, comparing the energy with the threshold valueResulting phase discrimination error
Figure BDA0001661134220000094
And performing FFT operation, analyzing the FFT operation result to obtain the frequency difference delta f between the local carrier signal and the AIS receiving signal, otherwise, not performing the FFT operation.
In the embodiment of the invention, the energy of the sampling data when the AIS receiving signal does not exist is used as the set threshold, namely the energy of noise is used as the set threshold.
The specific method for obtaining the frequency difference delta f between the local carrier signal and the AIS receiving signal in the embodiment of the invention is as follows:
(5.1) detecting the phase error
Figure BDA0001661134220000095
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer and is the nth power of 2; the FFT operation is realized by IPcore in an FPGA chip.
(5.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found; the detection range is 1-M/2 operation data.
(5.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2 and represents the first operation data; f is the estimation accuracy, wherein:
Figure BDA0001661134220000096
fs is the sampling rate of the AIS received signal.
(6) Carrying out in-phase and quadrature frequency mixing on the delayed AIS receiving signal and the carrier signal after frequency offset compensation, and outputting an in-phase component I '(t) and a quadrature component Q' (t); wherein, the frequency of the carrier signal after the frequency offset compensation is fc + Δ f.
(7) Low-pass (FIR) filtering the in-phase component I '(t) and the quadrature component Q' (t) to remove high-frequency components;
(8) the in-phase component I '(t) and the orthogonal component Q' (t) after FIR filtering are subjected to frequency discrimination processing by a frequency discriminator to obtain frequency discrimination demodulation results
Figure BDA0001661134220000101
Figure BDA0001661134220000102
Wherein:
Figure BDA0001661134220000103
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
(9) According to the characteristics of training sequence in data frame structure, the frequency discrimination demodulation result is obtained
Figure BDA0001661134220000107
The change situation of the sign bit of the adjacent code elements is statistically analyzed to realize signal detection (frame header detection), and the specific method comprises the following steps:
(9.1) demodulating the data according to the frequency discrimination
Figure BDA0001661134220000104
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure BDA0001661134220000105
The length of the middle training sequence is L, and L is an even number; and N is the number of sampling points in each data symbol in the AIS receiving signal. For example, in the embodiments of the present invention
Figure BDA0001661134220000106
The training sequence in the frame structure is 110011001100110011001100 for a total of 24 bits.
And (9.2) continuously counting for L/2 times, if the jump of L/2-1 sign bits is continuously detected, judging that the signal detection is finished, entering the step (9.3), otherwise, clearing the counting result, and returning to the step (9.1).
And (9.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
(10) Performing bipolar reverse non-return to zero (NRZI) decoding on the data subjected to the frame header detection in the step (9.3);
(11) searching a starting mark and an ending mark in NRZI decoded data, and sequentially storing complete data frames in which the starting mark and the ending mark are searched in a memory Ram;
(12) and reading out the complete data frame from the memory Ram, performing CRC (cyclic redundancy check) to judge whether demodulation is correct or not, if the CRC is correct, indicating that the demodulation is correct, and simultaneously outputting the demodulated binary data, otherwise, indicating that the demodulation is wrong.
Specifically, in the embodiment of the present invention, the AIS receiving signal is divided into two paths, one of the two paths is delayed, the other path is subjected to sampling processing, and the sampled AIS receiving signal and the local carrier signal with frequency fc are subjected to in-phase and quadrature frequency mixing to obtain a phase discrimination error, and further obtain a frequency difference Δ f between the local carrier signal and the AIS receiving signal; and carrying out in-phase and quadrature frequency mixing on the delayed AIS receiving signal and the carrier signal subjected to frequency offset compensation to obtain an in-phase component and a quadrature component, wherein the frequency of the carrier signal subjected to frequency offset compensation is fc + delta f. The delay may be performed using a FIFO as shown in fig. 2.
As shown in fig. 3, which is a schematic structural diagram of the AIS system demodulation system of the marine VDES system of the present invention, it can be seen that the AIS system demodulation system of the marine VDES system of the present invention includes a frequency offset estimation module, a frequency mixing module, a low pass filtering module, a frequency discrimination demodulation module, a signal detection module, and a decoding verification module.
And the frequency offset estimation module is used for sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and the local carrier signal with the frequency fc to obtain a phase discrimination error, further obtaining a frequency difference delta f between the local carrier signal and the AIS receiving signal, and outputting the frequency difference delta f to the frequency mixing module.
And the frequency mixing module is used for receiving the frequency difference delta f output by the frequency offset estimation module to obtain a carrier signal after frequency offset compensation, carrying out in-phase and quadrature frequency mixing on the AIS received signal and the carrier signal after frequency offset compensation to obtain an in-phase component and a quadrature component, and outputting the in-phase component and the quadrature component to the low-pass filtering module, wherein the frequency of the carrier signal after frequency offset compensation is fc + delta f.
And the low-pass filtering module is used for receiving the in-phase component and the orthogonal component output by the frequency mixing module, performing low-pass filtering on the in-phase component and the orthogonal component, removing high-frequency components, and outputting the in-phase component and the orthogonal component subjected to low-pass filtering to the frequency discrimination demodulation module.
And the frequency discrimination demodulation module is used for receiving the in-phase component and the orthogonal component which are output by the low-pass filtering module and subjected to low-pass filtering, carrying out frequency discrimination processing on the in-phase component and the orthogonal component which are output by the low-pass filtering module to obtain a frequency discrimination demodulation result, and outputting the frequency discrimination demodulation result to the signal detection module.
And the signal detection module is used for receiving the frequency discrimination demodulation result output by the frequency discrimination demodulation module, carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result to realize signal detection and outputting the data after the signal detection to the decoding and checking module.
And the decoding and checking module is used for receiving the data after the signal detection output by the signal detection module, decoding and checking the data for realizing the signal detection and finishing AIS system demodulation.
Specifically, in the embodiment of the present invention, the specific method for obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal includes the following steps:
(1.1) sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal with frequency fc, and outputting an in-phase component I (t) and a quadrature component Q (t);
(1.2) low-pass filtering the in-phase component I (t) and the quadrature component Q (t) to remove high-frequency components;
(1.3) carrying out phase discrimination processing on the in-phase component I (t) and the quadrature component Q (t) after low-pass filtering to obtain a phase discrimination error
Figure BDA0001661134220000121
Phase discrimination error
Figure BDA0001661134220000122
The calculation formula of (a) is as follows:
Figure BDA0001661134220000123
(1.4) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, carrying out phase discrimination on the obtained phase discrimination error
Figure BDA0001661134220000124
And performing FFT operation, analyzing the FFT operation result to obtain the frequency difference delta f between the local carrier signal and the AIS receiving signal, otherwise, not performing the FFT operation.
The specific method for obtaining the frequency difference delta f between the local carrier signal and the AIS receiving signal in the embodiment of the invention is as follows:
(1.4.1) detecting the phase error
Figure BDA0001661134220000125
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer and is the nth power of 2; the FFT operation is realized by IPcore in an FPGA chip.
(1.4.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found; the detection range is 1-M/2 operation data.
(1.4.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2, f is the estimation accuracy, wherein:
Figure BDA0001661134220000131
fs is the sampling rate of the AIS received signal.
Specifically, in the embodiment of the present invention, the frequency discrimination demodulation module receives the low-pass filtered in-phase component and quadrature component output by the low-pass filtering module, and performs frequency discrimination processing on the low-pass filtered in-phase component and quadrature component, so as to obtain a frequency discrimination demodulation result by the following specific method:
Figure BDA0001661134220000132
wherein:
Figure BDA0001661134220000133
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
Specifically, in the embodiment of the present invention, the signal detection module receives the frequency discrimination demodulation result output by the frequency discrimination demodulation module, and performs statistical analysis on the sign bit variation of adjacent code elements of the frequency discrimination demodulation result, so as to implement the specific method of signal detection as follows:
(2.1) demodulating the data according to the frequency discrimination
Figure BDA0001661134220000134
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure BDA0001661134220000135
The length of the middle training sequence is L, and L is an even number; n is the number of sampling points in each data symbol in the AIS receiving signal;
(2.2) continuously counting for L/2 times, if continuously detecting that L/2-1 sign bits jump, judging to finish signal detection, entering the step (2.3), otherwise, resetting the counting result, and returning to the step (2.1);
and (2.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
Specifically, in the embodiment of the present invention, the decoding and checking module receives the data after signal detection output by the signal detection module, and decodes and checks the data for realizing signal detection, and the specific method for completing AIS system demodulation is as follows:
(3.1) performing bipolar reverse non-return-to-zero decoding on the data for realizing the signal detection;
(3.2) searching a start mark and an end mark in the bipolar reverse non-return-to-zero decoded data, and storing the complete data frame in which the start mark and the end mark are searched in a memory;
and (3.3) reading the complete data frame from the memory, performing CRC check, and judging whether the AIS received data is correct.
Example (b):
in this embodiment, the data rate Rb of the AIS system of the marine VDES system is 9.6kbps, the frame length is 928bit, the data start flag is 0X7E, the data end flag is 0X7E, the local carrier frequency fc is 6.4MHz, the order of the FIR filter is 16, the FFT operation point number M is 4096, and the CRC bit number is 16 bits.
Sampling AIS received signal by ADC at sampling rate fc153.6kHz, 16 sampling points (i.e. N16) in each data symbol, the sampled AIS received signal and the local carrier signal with frequency of 6.4MHz are subjected to in-phase and quadrature mixing low-pass filtering to obtain an in-phase component i (t) and a quadrature component q (t), and a phase detection error
Figure BDA0001661134220000141
To pair
Figure BDA0001661134220000142
And performing 4096-point FFT operation, performing maximum peak value detection on 1-2048 operation results, detecting the maximum value at the 1428 th point, obtaining carrier frequency deviation delta f which is 1428 × 153.6kHz/4096 which is 53.55kHz, and further obtaining the carrier frequency of the received signal which is 6.45355 MHz.
Carrying out in-phase and quadrature mixing low-pass filtering on the AIS receiving signal after FIFO delay and a local carrier signal with the frequency of 6.45355MHz to obtain an in-phase component I '(t) and a quadrature component Q' (t), and carrying out frequency discrimination demodulation on the in-phase component I '(t) and the quadrature component Q' (t) to obtain a demodulated signal
Figure BDA0001661134220000143
According to
Figure BDA0001661134220000144
The characteristics of the training sequence (110011001100110011001100, 24 bits in total, i.e., L-24) in the frame structure are determined
Figure BDA0001661134220000145
The change of adjacent code element sign bit in signal, counting one sign bit jump every 32 sampling points, when 11 jumps appear continuously, indicating finding out frame head (training sequence), and completing frame head detection
Figure BDA0001661134220000151
The signal is integrated and output according to 16 sampling points in sequence.
Then NRZI decoding, searching the initial mark (0X 7E)/the end mark (0X7E) and CRC checking are carried out, and the original binary data source can be recovered.
The above description is only one embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Those skilled in the art will appreciate that the invention may be practiced without these specific details.

Claims (12)

1. An AIS system demodulation method of a ship VDES system is characterized in that: the method comprises the following steps:
sampling AIS receiving signals, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signals and local carrier signals with frequency fc to obtain phase discrimination errors, and further obtaining frequency difference delta f between the local carrier signals and the AIS receiving signals;
carrying out in-phase and quadrature frequency mixing on the AIS receiving signal and the carrier signal after frequency offset compensation to obtain an in-phase component and a quadrature component, wherein the frequency of the carrier signal after frequency offset compensation is fc + delta f;
low-pass filtering the in-phase component and the orthogonal component to remove high-frequency components;
performing frequency discrimination processing on the in-phase component and the quadrature component after the low-pass filtering to obtain a frequency discrimination demodulation result;
carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result to realize signal detection;
decoding and checking the data for realizing the signal detection to complete AIS system demodulation;
wherein, the specific method for realizing signal detection by carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result is as follows:
(5.1) demodulating the data according to the frequency discrimination
Figure FDA0002765333640000011
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure FDA0002765333640000012
The length of the middle training sequence is L, and L is an even number; n is the number of sampling points in each data symbol in the AIS receiving signal;
(5.2) continuously counting for L/2 times, if continuously detecting that L/2-1 sign bits jump, judging to finish signal detection, entering the step (5.3), otherwise, resetting the counting result, and returning to the step (5.1);
and (5.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
2. The AIS system demodulation method of the marine VDES system according to claim 1, wherein: the specific method for sampling the AIS received signal, performing in-phase and quadrature frequency mixing on the sampled AIS received signal and the local carrier signal with the frequency fc to obtain a phase discrimination error, and further obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal includes the following steps:
(1.1) sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal with frequency fc, and outputting an in-phase component I (t) and a quadrature component Q (t);
(1.2) low-pass filtering the in-phase component I (t) and the quadrature component Q (t) to remove high-frequency components;
(1.3) carrying out phase discrimination processing on the in-phase component I (t) and the quadrature component Q (t) after low-pass filtering to obtain a phase discrimination error
Figure FDA0002765333640000025
(1.4) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, carrying out phase discrimination on the obtained phase discrimination error
Figure FDA0002765333640000026
And performing FFT operation, analyzing the FFT operation result to obtain the frequency difference delta f between the local carrier signal and the AIS receiving signal, otherwise, not performing the FFT operation.
3. According to claimThe AIS system demodulation method of the marine VDES system as claimed in claim 2, characterized in that: phase discrimination error in the step (1.3)
Figure FDA0002765333640000021
The calculation formula of (a) is as follows:
Figure FDA0002765333640000022
4. the AIS system demodulation method of the marine VDES system according to claim 2, wherein: the obtained phase discrimination error is processed in the step (1.4)
Figure FDA0002765333640000023
The specific method for obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal by performing FFT operation and analyzing the FFT operation result is as follows:
(1.4.1) detecting the phase error
Figure FDA0002765333640000024
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer;
(1.4.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found;
(1.4.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2, f is the estimation accuracy, wherein:
Figure FDA0002765333640000031
fs is the sampling rate of the AIS received signal.
5. The AIS system demodulation method of the marine VDES system according to claim 1, wherein: the specific method for performing frequency discrimination processing on the in-phase component and the quadrature component after the low-pass filtering to obtain a frequency discrimination demodulation result is as follows:
Figure FDA0002765333640000032
wherein:
Figure FDA0002765333640000033
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
6. The AIS system demodulation method of the marine VDES system according to claim 1, wherein: the specific method for decoding and verifying the data for realizing the signal detection and finishing the AIS system demodulation comprises the following steps:
(6.1) carrying out bipolar reverse non-return-to-zero decoding on the data for realizing the signal detection;
(6.2) searching a start mark and an end mark in the bipolar reverse non-return-to-zero decoded data, and storing the complete data frame in which the start mark and the end mark are searched in a memory;
and (6.3) reading the complete data frame from the memory, performing CRC check, and judging whether the AIS received data is correct.
7. The AIS system demodulation method of the marine VDES system according to claim 1, wherein: dividing the AIS receiving signal into two paths, wherein one path is delayed, the other path is sampled, the sampled AIS receiving signal and a local carrier signal with frequency fc are subjected to in-phase and quadrature frequency mixing to obtain a phase discrimination error, and further obtain a frequency difference delta f between the local carrier signal and the AIS receiving signal; and carrying out in-phase and quadrature frequency mixing on the delayed AIS receiving signal and the carrier signal subjected to frequency offset compensation to obtain an in-phase component and a quadrature component, wherein the frequency of the carrier signal subjected to frequency offset compensation is fc + delta f.
8. An AIS system demodulation system of a ship VDES system is characterized in that: the device comprises a frequency offset estimation module, a frequency mixing module, a low-pass filtering module, a frequency discrimination demodulation module, a signal detection module and a decoding check module, wherein:
a frequency offset estimation module: sampling AIS receiving signals, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signals and local carrier signals with frequency fc to obtain phase discrimination errors, further obtaining frequency difference delta f between the local carrier signals and the AIS receiving signals, and outputting the frequency difference delta f to a frequency mixing module;
a frequency mixing module: receiving a frequency difference delta f output by a frequency offset estimation module to obtain a carrier signal after frequency offset compensation, carrying out in-phase and quadrature frequency mixing on the AIS received signal and the carrier signal after frequency offset compensation to obtain an in-phase component and a quadrature component, and outputting the in-phase component and the quadrature component to a low-pass filtering module, wherein the frequency of the carrier signal after frequency offset compensation is fc + delta f;
a low-pass filtering module: receiving an in-phase component and an orthogonal component output by a frequency mixing module, performing low-pass filtering on the in-phase component and the orthogonal component to remove high-frequency components, and outputting the in-phase component and the orthogonal component subjected to low-pass filtering to a frequency discrimination demodulation module;
a frequency discrimination demodulation module: receiving the in-phase component and the orthogonal component which are output by the low-pass filtering module and subjected to low-pass filtering, performing frequency discrimination processing on the in-phase component and the orthogonal component which are output by the low-pass filtering module to obtain a frequency discrimination demodulation result, and outputting the frequency discrimination demodulation result to the signal detection module;
the signal detection module: receiving a frequency discrimination demodulation result output by a frequency discrimination demodulation module, carrying out statistical analysis on the sign bit change of adjacent code elements of the frequency discrimination demodulation result to realize signal detection, and outputting data after the signal detection to a decoding and checking module;
a decoding and checking module: receiving data after signal detection output by the signal detection module, decoding and verifying the data for realizing the signal detection, and completing AIS system demodulation;
the signal detection module receives the frequency discrimination demodulation result output by the frequency discrimination demodulation module, and performs statistical analysis on the sign bit variation of adjacent code elements of the frequency discrimination demodulation result, wherein the specific method for realizing signal detection is as follows:
(5.1) demodulating the data according to the frequency discrimination
Figure FDA0002765333640000051
Counting symbol bit jumping every 2N sampling points, and frequency discrimination demodulation result
Figure FDA0002765333640000052
The length of the middle training sequence is L, and L is an even number; n is the number of sampling points in each data symbol in the AIS receiving signal;
(5.2) continuously counting for L/2 times, if continuously detecting that L/2-1 sign bits jump, judging to finish signal detection, entering the step (5.3), otherwise, resetting the counting result, and returning to the step (5.1);
and (5.3) taking a sampling point at the next moment of finishing the signal detection as an initial sampling point of the effective data, integrating the continuous N sampling points to output the effective data, and outputting a frame of effective data by analogy.
9. The AIS system demodulation system for marine VDES system of claim 8, wherein: the frequency offset estimation module samples the AIS received signal, performs in-phase and quadrature frequency mixing on the sampled AIS received signal and the local carrier signal with the frequency fc to obtain a phase discrimination error, and further obtains a frequency difference delta f between the local carrier signal and the AIS received signal, wherein the specific method comprises the following steps:
(1.1) sampling the AIS receiving signal, carrying out in-phase and quadrature frequency mixing on the sampled AIS receiving signal and a local carrier signal with frequency fc, and outputting an in-phase component I (t) and a quadrature component Q (t);
(1.2) low-pass filtering the in-phase component I (t) and the quadrature component Q (t) to remove high-frequency components;
(1.3) carrying out phase discrimination processing on the in-phase component I (t) and the quadrature component Q (t) after low-pass filtering to obtain a phase discrimination error
Figure FDA0002765333640000053
(1.4) judging the energy of the AIS receiving signal, and if the energy is larger than a set threshold value, carrying out phase discrimination on the obtained phase discrimination error
Figure FDA0002765333640000061
And performing FFT operation, analyzing the FFT operation result to obtain the frequency difference delta f between the local carrier signal and the AIS receiving signal, otherwise, not performing the FFT operation.
10. The AIS system demodulation system for marine VDES systems according to claim 9, characterized in that: phase discrimination error in the step (1.3)
Figure FDA0002765333640000062
The calculation formula of (a) is as follows:
Figure FDA0002765333640000063
，
the obtained phase discrimination error is processed in the step (1.4)
Figure FDA0002765333640000064
The specific method for obtaining the frequency difference Δ f between the local carrier signal and the AIS received signal by performing FFT operation and analyzing the FFT operation result is as follows:
(1.4.1) detecting the phase error
Figure FDA0002765333640000065
Performing M-point FFT operation and outputting M operation data, wherein M is a positive integer;
(1.4.2) carrying out maximum peak detection on the output M operation data until a maximum peak is found;
(1.4.3) finding the position n of the operation data corresponding to the maximum peak value within the range of 1-M/2 according to the maximum peak value, and calculating the carrier frequency difference delta f:
Δf＝n×f
wherein: n is a value in the range of 1-M/2, f is the estimation accuracy, wherein:
Figure FDA0002765333640000066
fs is the sampling rate of the AIS received signal.
11. The AIS system demodulation system for marine VDES system of claim 8, wherein: the frequency discrimination demodulation module receives the in-phase component and the quadrature component after the low-pass filtering output by the low-pass filtering module, and performs frequency discrimination processing on the in-phase component and the quadrature component after the low-pass filtering to obtain a frequency discrimination demodulation result, wherein the specific method comprises the following steps:
Figure FDA0002765333640000067
wherein:
Figure FDA0002765333640000071
for the frequency discrimination demodulation result, Δ I '(t) is the difference between two adjacent sampling points of I' (t), and Δ Q '(t) is the difference between two adjacent sampling points of Q' (t); i '(t) and Q' (t) are in-phase components and quadrature components obtained by performing in-phase and quadrature mixing and low-pass filtering on the AIS receiving signal and the carrier signal after frequency offset compensation respectively.
12. The AIS system demodulation system for marine VDES system of claim 8, wherein: the decoding and checking module receives the data after the signal detection output by the signal detection module, decodes and checks the data for realizing the signal detection, and the specific method for completing the AIS system demodulation is as follows:
(6.1) carrying out bipolar reverse non-return-to-zero decoding on the data for realizing the signal detection;
(6.2) searching a start mark and an end mark in the bipolar reverse non-return-to-zero decoded data, and storing the complete data frame in which the start mark and the end mark are searched in a memory;
and (6.3) reading the complete data frame from the memory, performing CRC check, and judging whether the AIS received data is correct.
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