CN110943956B - Signal demodulation method and system for satellite-borne automatic identification system AIS - Google Patents
Signal demodulation method and system for satellite-borne automatic identification system AIS Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/32—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
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/32—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
- H04L27/3818—Demodulator circuits; Receiver circuits using coherent demodulation, i.e. using one or more nominally phase synchronous carriers
Abstract
According to the technical scheme of the embodiment of the application, the AIS service information is subjected to GMSK modulation, the modulated AIS signal is subjected to band-pass filtering, digital down-conversion and low-pass filtering to obtain the AIS baseband signal, frame header detection, frequency offset estimation, frequency offset compensation, matched filtering and Viterbi decoding are carried out on the AIS baseband signal to obtain the demodulation signal of the AIS baseband signal, the good demodulation performance of a coherent demodulation algorithm is kept, the problems of carrier recovery and phase blind bit estimation in coherent demodulation are solved, and compared with the coherent demodulation algorithm, the implementation process is simpler. Finally, the obtained demodulated AIS baseband signal has low error rate and packet error rate, and the requirements for demodulating the AIS signal can be met.
Description
Technical Field
The embodiment of the application relates to the field of communication, in particular to a signal demodulation method and system for an Automatic Identification System (AIS) on board.
Background
An Automatic Identification System (AIS) is a ship navigation device which operates in a Very High Frequency (VHF) Frequency band at sea, adopts an Open System Interconnection (OSI) operating mode, can autonomously and continuously operate in all areas, and transmits and receives ship dynamic, static, navigation and safety information by using a Self-Organized Time Division Multiple Access (sodma) communication protocol. In the navigation process of the ship, the AIS can update data information transmission and reception in real time along with the change of information such as position, speed, course and the like, and display the received information on the AIS display screen after demodulation, so that the AIS can provide effective information and data for supervision departments and control personnel conveniently and quickly, and ship collision avoidance measures are taken. In the prior art, AIS service information is usually modulated by using a Minimum gaussian frequency Shift Keying (GMSK) Modulation method, where GMSK is a special Continuous Phase Modulation (CPM) method, and has the advantages of constant envelope, non-linear Phase, narrow bandwidth, large out-of-band attenuation, small interference to adjacent channels, and the like, but has the difficulty of demodulation technology. At present, the GMSK demodulation technology is mainly a coherent demodulation technology, and needs to recover a carrier with the same Frequency and phase as the carrier during modulation at a receiving end, so that the realization difficulty is high, the GMSK demodulation technology is very sensitive to receiving Frequency Offset (Frequency Offset), and if the carrier of a receiver and a transmitter has larger Frequency Offset, the coherent demodulation performance is sharply reduced.
Disclosure of Invention
In order to solve the above technical problem, embodiments of the present application provide a method and a system for demodulating AIS signals.
The AIS signal demodulation method provided by the embodiment of the application comprises the following steps:
performing Gaussian minimum frequency shift keying (GMSK) modulation on AIS service information to obtain an AIS signal of the GMSK signal system; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal;
detecting a frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal;
carrying out correlation operation on the AIS baseband signal of any frame and a local synchronous sequence to obtain a training sequence position of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame;
determining frequency offset estimation of the AIS baseband signal according to sampling input data at a training sequence position of the sampling input signal;
performing frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation; performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after matched filtering;
Generating a state table for performing Viterbi decoding of the match-filtered AIS baseband signal according to state information of the match-filtered AIS baseband signal, and determining a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal; and determining branch metrics of the matched and filtered AIS baseband signal based on the state table and the branch metric increment, and acquiring a demodulated signal for demodulating the AIS baseband signal according to the branch metrics.
In an optional embodiment of the present application, the correlation operation is performed on the any frame of AIS baseband signal and a local synchronization sequence, so as to obtain a training sequence position of a sampling input signal; the method comprises the following steps:
determining a training sequence of the AIS baseband signal of any frame, wherein the training sequence is delayed by one code element period;
conjugate multiplication is carried out on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period to obtain a conjugate multiplication value of the local training sequence;
carrying out conjugate multiplication on the sampled input signal and the sampled input signal delayed by one code element period to obtain a conjugate multiplication value of the sampled input signal;
According to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value of the accumulated values.
In an optional embodiment of the present application, the determining, according to the sampled input data at the training sequence position of the sampled input signal, a frequency offset estimate of the AIS baseband signal includes:
determining a frequency offset estimate of the AIS baseband signal according to the following formula:
fre＝α×arctan(Q÷I)÷(2π)
where fre represents the frequency offset estimate of the AIS baseband signal, α represents the frequency of the sampled input signal, arctan represents the arctan operation, Q represents the imaginary component of the sampled input data, and I represents the real component of the sampled input data.
In an optional embodiment of the present application, the performing, based on the frequency offset estimation, frequency offset compensation on the AIS baseband signal includes:
performing frequency offset compensation on the AIS baseband signal according to the following formula:
data_comp＝data×ej(2π×fre×t)
wherein, data _ comp represents the AIS baseband signal after the frequency offset compensation, data represents the AIS baseband signal, and t ═ N ÷ (p × α), where N is an integer between 1 and N, and N represents an effective data length of a frame of the AIS signal.
In an optional embodiment of the present application, the determining a branch metric of the match-filtered AIS baseband signal based on the status table and the branch metric increment, and obtaining a demodulated signal obtained by demodulating the AIS baseband signal according to the branch metric includes:
determining at least one branch metric of the match filtered AIS baseband signal based on the state table and the branch metric increment, the branch metric having a maximum value from among the at least one branch metric;
setting a path corresponding to the branch metric of the maximum value as a survival path; and determining a state corresponding to the starting point of the survivor path, and determining information of a moment corresponding to the state according to the state, thereby completing demodulation of the AIS baseband signal after the matched filtering.
The embodiment of the present application further provides a signal demodulation system of an automatic identification system AIS on board, the system includes:
the modulation unit is used for carrying out Gaussian minimum frequency shift keying GMSK modulation on the AIS service information to obtain an AIS signal of a GMSK signal system; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal;
The detection unit is used for detecting the frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal;
the acquisition unit is used for carrying out correlation operation on the AIS baseband signal of any frame and a local synchronization sequence to acquire the position of a training sequence of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame;
a determining unit, configured to determine a frequency offset estimation of the AIS baseband signal according to sampling input data at a training sequence position of the sampling input signal;
the compensation unit is used for carrying out frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation;
the filtering unit is used for performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after the matched filtering;
a generating unit, configured to generate a state table for viterbi decoding of the match-filtered AIS baseband signal according to state information of the match-filtered AIS baseband signal, and determine a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal;
A demodulation unit, configured to determine a branch metric of the match-filtered AIS baseband signal based on the state table and the branch metric increment, and obtain a demodulated signal obtained by demodulating the AIS baseband signal according to the branch metric.
In an optional embodiment of the present application, the obtaining unit is specifically configured to: determining a training sequence of any frame of AIS baseband signals, wherein the training sequence is delayed by one symbol period; carrying out conjugate multiplication on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period to obtain a conjugate multiplication value of the local training sequence; performing conjugate multiplication on the sampled input signal and the sampled input signal delayed by one code element period to obtain a conjugate multiplication value of the sampled input signal; according to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value in the accumulated values.
In an optional embodiment of the present application, the determining unit is specifically configured to: determining a frequency offset estimate of the AIS baseband signal according to the following formula:
fre＝α×arctan(Q÷I)÷(2π)
Where fre represents the frequency offset estimate of the AIS baseband signal, α represents the frequency of the sampled input signal, arctan represents the arctan operation, Q represents the imaginary component of the sampled input data, and I represents the real component of the sampled input data.
In an optional embodiment of the present application, the compensation unit is specifically configured to: performing frequency offset compensation on the AIS baseband signal according to the following formula:
data_comp＝data×ej(2π×fre×t)
wherein, data _ comp represents the AIS baseband signal after the frequency offset compensation, data represents the AIS baseband signal, and t ═ N ÷ (p × α), where N is an integer between 1 and N, and N represents an effective data length of a frame of the AIS signal.
In an optional embodiment of the present application, the demodulation unit is specifically configured to: determining at least one branch metric of the match filtered AIS baseband signal based on the state table and the branch metric delta, the branch metric having a maximum value from the at least one branch metric; setting a path corresponding to the branch metric of the maximum value as a survivor path; and determining a state corresponding to the starting point of the survivor path, and determining information of a moment corresponding to the state according to the state, thereby completing demodulation of the AIS baseband signal after the matched filtering.
According to the technical scheme of the embodiment of the application, the AIS signal of a GMSK signal system is obtained by carrying out Gaussian minimum shift keying GMSK modulation on the AIS service information; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal; detecting a frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal; carrying out correlation operation on the AIS baseband signal of any frame and a local synchronous sequence to obtain a training sequence position of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame; determining frequency offset estimation of the AIS baseband signal according to sampling input data at a training sequence position of the sampling input signal; performing frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation; performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after matched filtering; generating a state table used for Viterbi decoding of the match-filtered AIS baseband signal according to the state information of the match-filtered AIS baseband signal, and determining a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal; and determining branch metrics of the match-filtered AIS baseband signal based on the state table and the branch metric increment, and acquiring a demodulated signal for demodulating the AIS baseband signal according to the branch metrics. So, can be through carrying out band-pass filtering, digital frequency conversion and low pass filtering to the AIS signal of GMSK signal system, obtain the AIS baseband signal, and then carry out frequency offset compensation and matched filtering to the AIS baseband signal, realize the demodulation to the AIS baseband signal through the Viterbi decoding mode, good demodulation performance when having both kept adopting coherent demodulation algorithm to demodulate to the AIS baseband signal has avoided adopting coherent demodulation algorithm to carry out the carrier recovery and the blind position of phase place estimation problem in the demodulation process to the AIS baseband signal again, compare coherent demodulation algorithm, the realization process is comparatively simple.
Drawings
Fig. 1 is a schematic flowchart of a signal demodulation method for AIS according to an embodiment of the present disclosure;
fig. 2 is a flowchart of a signal demodulation process of AIS according to an embodiment of the present application;
fig. 3 is a bit error rate graph of signal demodulation of AIS according to an embodiment of the present disclosure;
fig. 4 is a packet error rate graph of signal demodulation of AIS according to an embodiment of the present disclosure;
fig. 5 is a schematic structural diagram of a component of an AIS signal demodulation system according to an embodiment of the present application.
Detailed Description
So that the manner in which the features and elements of the present embodiments can be understood in detail, a more particular description of the embodiments, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings.
Fig. 1 is a schematic flowchart of a signal demodulation method for AIS according to an embodiment of the present application, where as shown in fig. 1, the method includes the following steps:
s101: performing Gaussian minimum frequency shift keying (GMSK) modulation on AIS service information to obtain an AIS signal of the GMSK signal system; and performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal.
Specifically, the AIS system is referred to as an AIS signal, and after receiving the AIS signal, the AIS system performs bandpass filtering, digital frequency conversion, and low-pass filtering on the AIS signal of the GMSK signal system to output an AIS baseband signal.
The AIS signal of the GMSK signal system can comprise a plurality of different frequency points, the channel division of the AIS signal of the GMSK signal system of each frequency point in the plurality of frequency points is achieved through the FPGA, and the AIS signal of the GMSK signal system of the channel corresponding to each frequency point is subjected to band-pass filtering, digital down-conversion and low-pass filtering to obtain the AIS baseband signal.
For example, for AIS signals of a GMSK signal system of four different frequency points (main channel 1: 161.975 megahertz (MHz), main channel 2: 162.025MHz, spare channel 1: 156.775MHz, spare channel 2: 156.825MHz), an FPGA is used to implement parallel channel division, and then an AIS baseband signal is obtained through band-pass filtering, digital frequency conversion, and low-pass filtering, where the AIS baseband signal may be 4 × 9.6KHz sampling data, and a filtering value range of the band-pass filtering is: from 25 kilohertz (kHz) to 8.4 MHz.
S102: and detecting the frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal.
S103: carrying out correlation operation on the AIS baseband signal of any frame and a local synchronous sequence to obtain a training sequence position of a sampling input signal; wherein, the sampling input signal is the sampling input signal of any frame of AIS baseband signal.
In an optional embodiment of the present application, the correlation operation is performed on the AIS baseband signal of any frame and a local synchronization sequence, so as to obtain a training sequence position of a sampling input signal; the method comprises the following steps:
determining a training sequence of any frame of AIS baseband signals, wherein the training sequence is delayed by one symbol period;
carrying out conjugate multiplication on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period to obtain a conjugate multiplication value of the local training sequence;
performing conjugate multiplication on the sampled input signal and the sampled input signal delayed by one code element period to obtain a conjugate multiplication value of the sampled input signal;
according to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value in the accumulated values.
Specifically, in the AIS signal, since the synchronization sequence (24 bits (bit), 01010101 …) and the start flag (8 bits, 01111110) are determined digital sequences, the 32-bit digital sequence is taken as a local training sequence, any frame of the AIS baseband signal is correlated with the local synchronization sequence, and then the training sequence position of the sampling input signal is found, and the specific method for the sampling input signal to be 4 × 9.6kHz is as follows:
Firstly, a conjugate multiplication value of a local training sequence is obtained by multiplying a 32-bit local training sequence by a training sequence conjugate delayed by one symbol period, as shown in formula (1):
ΔtreSeq(n)＝treSeq(nT)×treSeq*((n+1)T) (1)
wherein, DeltatreSeq(n) represents the conjugate multiplication value of the local training sequence, n represents the position of the local training sequence, the value range is an integer between 1 and 31, T represents the code element period, treSeq represents the local training sequenceColumn, treSeq (nT) represents a local training sequence with start position n, treSeq*((n +1) T) represents the conjugate of the training sequence delayed by one symbol period.
Then, the conjugate multiplication value of the sampled input signal is obtained by conjugate multiplication of the sampled input signal and the sampled input signal delayed by one symbol period with different start bits, as shown in formula (2):
Δdata(n,k)＝data(k+nT)×data*(k+(n+1)T) (2)
wherein, Deltadata(n, k) represents a conjugate multiplication value of the sampled input signal, data represents a sampled input signal of 4 × 9.6kHz, and k represents a start bit of the sampled input signal.
Selecting different start bits k according to the sampling input signal, and calculating an accumulated value obtained by correspondingly multiplying a local training sequence conjugate multiplication value and a sampling input signal conjugate multiplication value, as shown in formula (3):
then, the maximum value in the accumulated values corresponding to different start bits is searched, the position corresponding to the maximum value is the training sequence position of 32 bits of the sampled input signal, and the position is set as m.
It should be noted that, in order to reduce the bit error rate and the packet error rate, a three-sampling-point serial search mode is adopted in the process of demodulating the AIS baseband signal, and on the basis of the original optimal single-sampling-point capturing demodulation scheme, the previous sampling point and the next sampling point at the position corresponding to the maximum value in the accumulated values are added to form three-time cycle calculation; and taking the error rate as a judgment reference, and jumping out of circulation to process the signal of the next time slot when the signal error rate is 0.
S104: and determining the frequency offset estimation of the AIS baseband signal according to the sampling input data at the training sequence position of the sampling input signal.
In an optional embodiment of the present application, the determining, according to the sampled input data at the training sequence position of the sampled input signal, a frequency offset estimate of the AIS baseband signal includes:
determining a frequency offset estimate of the AIS baseband signal according to the following formula:
fre＝α×arctan(Q÷I)÷(2π) (4)
where fre represents the frequency offset estimate of the AIS baseband signal, α represents the frequency of the sampled input signal, arctan represents the arctan operation, Q represents the imaginary component of the sampled input data, and I represents the real component of the sampled input data. Here, for 4X 9.6KHz of sampled data, α is 9600.
S105: and carrying out frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation.
In an optional embodiment of the present application, the performing, based on the frequency offset estimation, frequency offset compensation on the AIS baseband signal includes:
performing frequency offset compensation on the AIS baseband signal according to the following formula:
data_comp＝data×ej(2π×fre×t) (5)
wherein, data _ comp represents the AIS baseband signal after the frequency offset compensation, data represents the AIS baseband signal, and t ═ N ÷ (p × α), where N is an integer between 1 and N, and N represents an effective data length of a frame of the AIS signal.
S106: and performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after the matched filtering.
Here, the first term of the lorentry expansion is designed as a filter parameter, and the AIS baseband signal after frequency offset compensation is subjected to matched filtering. The corresponding relation of the matched filter is as follows:
h0(τ-nT)＝c(τ-nT+2T)·c(τ-nT+T)·c(τ-nT)·c(τ-nT-T) (6)
here, c represents a function c (t) whose expression is:
wherein h is a modulation coefficient, and h is 0.5 in the AIS system; q (t) is the unit smoothed response of the gaussian filter square pulse matching the filtered AIS baseband signal over time LT, L being the correlation length. Illustratively, L ═ 3 may be selected.
And the AIS baseband signal is subjected to matched filtering, so that whitening filtering can be removed, the influence of the correlation length on the detection performance is effectively avoided, and the complexity of the system is obviously reduced.
S107: generating a state table for Viterbi decoding of the match-filtered AIS baseband signal according to the state information of the match-filtered AIS baseband signal, and determining a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal.
Specifically, at time t ═ nT, the signal state S of the filtered AIS baseband signal is matchednCan be defined as:
wherein, thetanRepresenting the phase state at time t-nT,
mod represents the remainder operation.
Matching the signal state S of the filtered AIS baseband signal at time T ═ n + 1Tn+1Can be defined as:
wherein, thetan+1Representing the phase state at time T (n + 1). Here, due to toneCoefficient h is 0.5, so θnThe value of (A) can be 0, pi/2, pi and 3 pi/2.
Taking 4 frequency points as an example, the number of phase states of the AIS baseband signals after matched filtering is 4 multiplied by 2LSince the incoherent detection does not need to know theta in decodingnHere, θ can be set n0. The number of states N can be set when performing Viterbi (Viterbi) decodings＝2L. If L is 3, the state table for viterbi decoding, which generates the matching-filtered AIS baseband signal, is shown in table 1:
TABLE 1
Determining a branch metric increment of the matched and filtered AIS baseband signal according to a sequence in a table, as shown in a formula (10):
wherein alpha isnRepresenting the sequence of states in Table 1, bn Substitute for Chinese traditional medicineThe table matches the branch metric increments of the filtered AIS baseband signal,
xnrepresenting the match-filtered AIS baseband signal, NLIs an observation time window parameter. Here, N may be selectedL＝10。
According to the scheme of the embodiment of the application, when the state table is generated, only one of four phase states in the AIS signal of the GMSK modulated GMSK signal system is selected, the state number can be reduced, and the state number is reduced from 4 multiplied by 2 under the condition that the AIS signal of the GMSK signal system is 4 frequency pointsLIs reduced to 2LThe method can obviously reduce the operation complexity when the AIS baseband signal is demodulated by the Viterbi.
S108: and determining branch metrics of the match-filtered AIS baseband signal based on the state table and the branch metric increment, and acquiring a demodulated signal for demodulating the AIS baseband signal according to the branch metrics.
Here, T ═ nT is the current time, and T ═ n +1) T is the time next to the current time, and in the embodiment of the present application, the Viterbi decoding method is adopted to demodulate the AIS baseband signal after the matching filtering. The specific process of Viterbi decoding is as follows:
(1) all states of the AIS baseband signals after matching and filtering are put into a table state _ all for storage, wherein the size of the table state _ all is NsX L; putting the switching-in states of all the states of the AIS baseband signal subjected to matching filtering into a table state _ in for storage, wherein the size of the table state _ in is NsX 2; setting a backtracking length NTInitializing a survivor _ state and a branch metric table metric _ state of the AIS baseband signal after matching and filtering, wherein the size of the survivor _ state and the branch metric table metric _ state is Ns×NT。
(2) And calculating a branch metric increment of the matched and filtered AIS baseband signal. All possible states of the next moment of the matched filtered AIS baseband signal are Sn+1(i) Wherein i ranges from 0 to 2LBetween-1, calculating 2 branch metric increments that can enter the ith state at the next time according to table state _ in and equation (10)
(3) Calculating branch metrics of the match filtered AIS baseband signal. Based on the survival path that originally reaches the time t-nT 
The branch measurement increment of the other end node connected with the state node branch at the time t equal to nT
Adding to obtain the AIS baseband signal after matched filtering at the time T (n +1) TBranch metrics of numbers
(4) And finding a survival path. Selecting branch metrics for all matched filtered AIS baseband signals
And the path corresponding to the medium maximum value is taken as a survivor path.
(5) Survivor paths and branch metrics are saved. And saving the survivor path corresponding to each state and the branch metric information of the matched and filtered AIS baseband signal.
(6) If (N +1) T is less than or equal to NTRepeating the steps (2) to (5); otherwise, find out the state corresponding to the next time with the maximum branch metric
By backtracking decoding, find out
For the survival path of the end point, the state corresponding to the starting point of the path is found back along the path
From this state (N-N) can be solvedT+1) information of time T
(7) And entering the next code element period and updating the information. The data in the survivor path table survivor state and the branch metric table metric state are left shifted by one bit to store the survivor path at the next time instant and the match filtered AIS baseband signal branch metric using the last bit.
(8) And (5) repeating the steps (2) to (7) until all the information sequences of the AIS baseband signals after matching and filtering are adjusted.
Fig. 2 is a flowchart of a signal demodulation process of AIS according to an embodiment of the present application, where a process of processing an AIS signal in a GMSK signal system to obtain an AIS baseband signal includes: sampling AIS signals of GMSK signal systems of all channels at the frequency of 38.4MHz, processing the AIS signals by a band-pass filter at 8.4MHz, sampling at the frequency of 768KHz, outputting the signals, and obtaining the AIS baseband signals after the output signals are processed by digital down-conversion and low-pass filters and sampled at the frequency of 38.4 MHz. And then performing frame header detection, frequency offset estimation, frequency offset compensation, matched filtering and Viterbi decoding on the AIS signal in sequence according to the foregoing embodiment to obtain a demodulated signal obtained by demodulating the AIS baseband signal.
Fig. 3 and 4 are respectively a bit error rate and a packet error rate of the AIS baseband signal demodulated according to the AIS signal demodulation process of fig. 2. As can be seen from fig. 3 and 4, the demodulated AIS baseband signal obtained by the AIS signal demodulation method according to the embodiment of the present application has a low bit error rate and a low packet error rate, and can meet the requirement for demodulating the AIS signal.
According to the technical scheme, the business information is modulated, the AIS baseband signal is obtained through band-pass filtering, digital down-conversion and low-pass filtering, frame header detection, frequency offset estimation, frequency offset compensation and Viterbi decoding are carried out on the AIS baseband signal, the demodulation signal of the AIS baseband signal is obtained, good demodulation performance of a coherent demodulation algorithm is kept, the problems of carrier recovery and phase blind bit estimation in coherent demodulation are solved, and compared with the coherent demodulation algorithm, the implementation process is simple. Meanwhile, the demodulation capacity of the AIS baseband signal can be improved, the AIS baseband signal demodulation method has obvious low error code performance, the finally obtained demodulated AIS baseband signal has low error code rate and low packet error rate, the requirement for demodulating the AIS signal can be met, and the AIS baseband signal demodulation method has important practical value in engineering application.
An embodiment of the present application further provides a system for demodulating an AIS signal, as shown in fig. 5, the system includes:
a modulation unit 51, configured to perform gaussian minimum shift keying GMSK modulation on the AIS service information to obtain an AIS signal of a GMSK signal system; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal;
a detecting unit 52, configured to detect a frame header of the AIS baseband signal, and obtain any one frame of the AIS baseband signal;
an obtaining unit 53, configured to perform correlation operation on the AIS baseband signal of any frame and a local synchronization sequence, and obtain a training sequence position of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame;
a determining unit 54, configured to determine a frequency offset estimate of the AIS baseband signal according to the sampled input data at the training sequence position of the sampled input signal;
a compensating unit 55, configured to perform frequency offset compensation on the AIS baseband signal based on the frequency offset estimation, so as to obtain an AIS baseband signal after frequency offset compensation;
a filtering unit 56, configured to perform matched filtering on the AIS baseband signal after the frequency offset compensation, so as to obtain an AIS baseband signal after the matched filtering;
A generating unit 57, configured to generate a state table for viterbi decoding of the match-filtered AIS baseband signal according to the state information of the match-filtered AIS baseband signal, and determine a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal;
a demodulating unit 58, configured to determine a branch metric of the match-filtered AIS baseband signal based on the state table and the branch metric increment, and obtain a demodulated signal obtained by demodulating the AIS baseband signal according to the branch metric.
In an optional embodiment of the present application, the obtaining unit 53 is specifically configured to: determining a training sequence of any frame of AIS baseband signals, wherein the training sequence is delayed by one symbol period; carrying out conjugate multiplication on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period to obtain a conjugate multiplication value of the local training sequence; carrying out conjugate multiplication on the sampled input signal and the sampled input signal delayed by one code element period to obtain a conjugate multiplication value of the sampled input signal; according to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value in the accumulated values.
In an optional embodiment of the present application, the determining unit 54 is specifically configured to: determining a frequency offset estimate of the AIS baseband signal according to equation (4).
In an optional embodiment of the present application, the compensation unit 55 is specifically configured to perform frequency offset compensation on the AIS baseband signal according to formula (5).
In an optional embodiment of the present application, the demodulation unit 58 is specifically configured to: determining at least one branch metric of the match filtered AIS baseband signal based on the state table and the branch metric delta, the branch metric having a maximum value from the at least one branch metric; setting a path corresponding to the branch metric of the maximum value as a survivor path; and determining a state corresponding to the starting point of the survivor path, and determining information of a moment corresponding to the state according to the state, thereby completing demodulation of the AIS baseband signal after the matched filtering.
It will be understood by those skilled in the art that the functions of the units in the demodulation system for AIS signals shown in fig. 5 can be understood with reference to the foregoing description of the AIS signal demodulation method. The functions of the units in the AIS signal demodulation system shown in fig. 5 may be implemented by a program running on a processor, or may be implemented by specific logic circuits.
All functional units in the embodiments of the present application may be integrated into one second processing unit, or each unit may be separately regarded as one unit, or two or more units may be integrated into one unit; the integrated unit can be realized in a form of hardware, or in a form of hardware plus a software functional unit.
The above description is only for the specific embodiments of the present application, but the scope of the present application is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present application, and shall be covered by the scope of the present application.
Claims (4)
1. A signal demodulation method for an Automatic Identification System (AIS) on board a satellite is characterized by comprising the following steps:
carrying out Gaussian minimum frequency shift keying (GMSK) modulation on the AIS service information to obtain an AIS signal of a GMSK signal system; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal;
detecting a frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal;
Carrying out correlation operation on the AIS baseband signal of any frame and a local synchronous sequence to obtain a training sequence position of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame;
determining frequency offset estimation of the AIS baseband signal according to sampling input data at a training sequence position of the sampling input signal;
performing frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation;
performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after matched filtering;
generating a state table for performing Viterbi decoding of the match-filtered AIS baseband signal according to state information of the match-filtered AIS baseband signal, and determining a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal;
determining branch metrics of the match-filtered AIS baseband signal based on the status table and the branch metric increments, and obtaining a demodulated signal for demodulating the AIS baseband signal according to the branch metrics;
The performing correlation operation on the AIS baseband signal of any frame and a local synchronization sequence to obtain a training sequence position of a sampling input signal includes: determining a training sequence of the AIS baseband signal of any frame, wherein the training sequence is delayed by one code element period; conjugate multiplication is carried out on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period, and a conjugate multiplication value of the local training sequence is obtained; conjugate multiplication is carried out on the sampling input signal and the sampling input signal delayed by one code element period, and a conjugate multiplication value of the sampling input signal is obtained; according to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value in the accumulated values; and a process for the preparation of a coating,
the determining a frequency offset estimate of the AIS baseband signal from sampled input data at a training sequence position of the sampled input signal includes: determining a frequency offset estimate of the AIS baseband signal according to the following formula: fre ═ α × arctan (Q ÷ I) ÷ (2 pi), where fre represents the frequency offset estimate of the AIS baseband signal, α represents the frequency of the sampled input signal, arctan represents the arctangent operation, Q represents the imaginary part of the sampled input data, and I represents the real part of the sampled input data; and a process for the preparation of a coating,
The frequency offset compensation of the AIS baseband signal based on the frequency offset estimation comprises: performing frequency offset compensation on the AIS baseband signal according to the following formula: data _ comp ═ data × ej(2π×fre×t)In the formula, data _ comp represents the AIS baseband signal after the frequency offset compensation, data represents the AIS baseband signal, and t ═ N ÷ (p × α), where N is an integer between 1 and N, and N represents an effective data length of one frame of the AIS signal.
2. The method of claim 1, wherein said determining branch metrics for the match filtered AIS baseband signal based on the status table and the branch metric increments, and wherein deriving a demodulated signal from the AIS baseband signal based on the branch metrics comprises:
determining at least one branch metric of the match filtered AIS baseband signal based on the state table and the branch metric delta, determining a branch metric having a maximum value from the at least one branch metric;
setting a path corresponding to the branch metric of the maximum value as a survivor path; and determining a state corresponding to the starting point of the survivor path, and determining information of a moment corresponding to the state according to the state, thereby completing demodulation of the AIS baseband signal after the matched filtering.
3. A signal demodulation system for an automatic identification system AIS on-board a satellite, the system comprising:
the modulation unit is used for carrying out Gaussian minimum frequency shift keying GMSK modulation on the AIS service information to obtain an AIS signal of a GMSK signal system; performing band-pass filtering, digital frequency conversion and low-pass filtering on the AIS signal of the GMSK signal system to obtain an AIS baseband signal;
the detection unit is used for detecting the frame header of the AIS baseband signal to acquire any frame of AIS baseband signal in the AIS baseband signal;
the acquisition unit is used for carrying out correlation operation on the AIS baseband signal of any frame and a local synchronization sequence to acquire the position of a training sequence of a sampling input signal; wherein the sampling input signal is a sampling input signal of the AIS baseband signal of any frame;
a determining unit, configured to determine a frequency offset estimation of the AIS baseband signal according to sampling input data at a training sequence position of the sampling input signal;
the compensation unit is used for carrying out frequency offset compensation on the AIS baseband signal based on the frequency offset estimation to obtain the AIS baseband signal after frequency offset compensation;
the filtering unit is used for performing matched filtering on the AIS baseband signal after the frequency offset compensation to obtain the AIS baseband signal after the matched filtering;
A generating unit, configured to generate a state table for viterbi decoding of the match-filtered AIS baseband signal according to state information of the match-filtered AIS baseband signal, and determine a branch metric increment of the match-filtered AIS baseband signal based on the state table and the match-filtered AIS baseband signal;
a demodulation unit, configured to determine a branch metric of the match-filtered AIS baseband signal based on the state table and the branch metric increment, and obtain a demodulated signal obtained by demodulating the AIS baseband signal according to the branch metric;
wherein the obtaining unit is specifically configured to: determining a training sequence of any frame of AIS baseband signals, wherein the training sequence is delayed by one symbol period; conjugate multiplication is carried out on the local training sequence of any frame of AIS baseband signal and the training sequence delayed by one code element period, and a conjugate multiplication value of the local training sequence is obtained; carrying out conjugate multiplication on the sampled input signal and the sampled input signal delayed by one code element period to obtain a conjugate multiplication value of the sampled input signal; according to the conjugate multiplication value of the local training sequence and the conjugate multiplication value of the sampling input signal, correspondingly multiplying the multiplied accumulated value; determining a training sequence position of the sampled input signal; wherein the training sequence position of the sampled input signal is the maximum value in the accumulated values; and a process for the preparation of a coating,
The determining unit is specifically configured to: determining a frequency offset estimate of the AIS baseband signal according to the following formula: fre ═ α × arctan (Q ÷ I) ÷ (2 pi), where fre represents the frequency offset estimate of the AIS baseband signal, α represents the frequency of the sampled input signal, arctan represents the arctangent operation, Q represents the imaginary part of the sampled input data, and I represents the real part of the sampled input data; and a process for the preparation of a coating,
the compensation unit is specifically configured to: performing frequency offset compensation on the AIS baseband signal according to the following formula: data _ comp ═ data × ej(2π×fre×t)In the formula, data _ comp represents the AIS baseband signal after the frequency offset compensation, data represents the AIS baseband signal, and t ═ N ÷ (p × α), where N is an integer between 1 and N, and N represents an effective data length of one frame of the AIS signal.
4. The system of claim 3, wherein the demodulation unit is specifically configured to: determining at least one branch metric of the match filtered AIS baseband signal based on the state table and the branch metric delta, determining a branch metric having a maximum value from the at least one branch metric; setting a path corresponding to the branch metric of the maximum value as a survivor path; and determining a state corresponding to the starting point of the survivor path, and determining information of a moment corresponding to the state according to the state, thereby completing demodulation of the AIS baseband signal after the matched filtering.
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| CN106850478A (en) * | 2016-12-29 | 2017-06-13 | 北京遥测技术研究所 | A kind of data for ship automatic identification system are demodulated and synchronous method |
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| CN106850478A (en) * | 2016-12-29 | 2017-06-13 | 北京遥测技术研究所 | A kind of data for ship automatic identification system are demodulated and synchronous method |
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