Anti-ais time slot conflict method based on double-antenna phase shift synthesis

Abstract

The application relates to the technical field of ship automatic identification system, and discloses an anti-AIS time slot conflict method based on double-antenna phase shift synthesis, which comprises the following steps: S1, receiving message signals from different AIS cells through two antennas respectively; when N AIS message signals of the same frequency point of different AIS cells are received in the same time slot, the N AIS message signals are preprocessed, and then phase shift processing is carried out by adding a phase shift factor, wherein N is a positive integer greater than 1; S2, synthesizing the N AIS signals after the phase shift processing, and adjusting the signal strength of the AIS cells to which the N AIS message signals belong by adjusting the phase shift factor, so that the signal strength of the AIS cells to which the N AIS message signals belong is continuously changed, thereby improving the detection probability of the messages of different AIS cells under the time slot conflict condition. By adjusting the weighting factor, the signal enhancement area and the signal weakening area are continuously changed, so that the detection probability of the messages of different cells under the time slot conflict condition can be improved.

Description

Technical Field

[0001] This invention relates to the field of Automatic Identification System (AIS) technology, specifically to a method for resisting AIS time slot conflicts based on dual-antenna phase-shift synthesis. Background Technology

[0002] AIS (Airborne Information System) is a general-purpose shipborne system that complies with the SOTDMA (Site-to-Depth Detection and Analysis) protocol. It operates autonomously within a range of approximately 40 nautical miles, primarily aiming to facilitate effective information exchange between ships and between ships and shore stations, thereby improving navigation safety and efficiency. Globe-based AIS systems offer a way to realize remote AIS surveillance applications, significantly expanding the surveillance and control range and further improving the efficiency of safety and navigation monitoring. They can also complement radar information to enhance target identification efficiency, making globe-based AIS a currently popular application area.

[0003] However, spherical AIS systems present several technical challenges and operational difficulties. Since spherical AIS systems typically fly at altitudes between 3 and 10 kilometers, with this altitude and the typical beamwidth of a spherical VHF antenna, the spherical field of view (FOV) spans hundreds of nautical miles, covering dozens of AIS cells. While ships within each AIS cell adhere to the SOTDMA protocol, preventing them from transmitting messages in the same time slot, the multiple AIS cells within the entire spherical FOV do not follow the SOTDMA protocol. This results in situations where multiple different AIS cells transmit messages in the same time slot. When the spherical AIS receiver receives AIS messages from different AIS cells in the same time slot, a time slot conflict occurs, significantly reducing the probability of message detection.

[0004] Currently, most marine AIS receivers use a single-antenna solution with a dedicated AIS modem chip from CML. When time slot collisions occur, this solution can only demodulate the strongest message. Furthermore, the strongest message can only be successfully demodulated if the power difference between the strongest and second-strongest messages exceeds a demodulation threshold, while the second-strongest messages cannot be demodulated. Therefore, this solution has a very low message detection success rate during time slot collisions, resulting in a low detection probability for spherical AIS receivers based on this solution. Often, a large number of AIS-enabled ships are visible within the spherical AIS receiver's field of view, but the receiver only detects a small number of ships. Summary of the Invention

[0005] This invention provides a method for resisting AIS time slot collisions based on dual-antenna phase-shift synthesis. When a ball-borne AIS receiver faces severe time slot collisions, it enables the ball-borne receiver to demodulate multiple messages simultaneously in the conflicting time slots, significantly improving the detection probability of the ball-borne AIS receiver.

[0006] This invention is achieved through the following technical solution:

[0007] A method for mitigating AIS time slot conflicts based on dual-antenna phase-shift synthesis includes:

[0008] S1. Receive message signals from different AIS cells through two antennas. When N AIS message signals from different AIS cells at the same frequency are received in the same time slot, the N AIS message signals are preprocessed and then phase-shifted by adding a phase-shifting factor. N is a positive integer not less than 1.

[0009] S2. The N AIS signals after phase shifting are synthesized, and the signal strength of the AIS cell to which the N AIS message signals belong is adjusted by adjusting the phase shift factor, so that the signal strength of the AIS cell to which the N AIS message signals belong changes continuously, thereby improving the detection probability of different AIS cell messages in the case of time slot conflict.

[0010] As an optimization, the preprocessing of the N AIS message signals specifically includes sequentially performing DC removal, digital AGC, down-conversion / low-pass filtering, and decimation processing on the N AIS message signals.

[0011] As an optimization, the preprocessed N channels of AIS signals are as follows:

[0012] ;

[0013] ;

[0014] in, , , , These represent the N AIS message signals at the same frequency received by the two antennas at time t. Indicates the first The amplitude of each AIS message signal reaching the two antennas respectively; Indicates the first The receiving frequency of each AIS message signal; Indicates the first Phase information corresponding to each AIS message signal; and They represent the first The initial phase of each AIS message signal to the two antennas. The local carrier frequency for down-converting the AIS message signals received by the two antennas is provided by the LPF, which is a low-pass filter. For receiving frequency With local carrier frequency The difference between them .

[0015] As an optimization and The following relationship exists between them:

[0016] ;

[0017] in, Indicates the first The difference in path distance between the two AIS message signals reaching the two antennas. Indicates the first The wavelength of each AIS message signal ≈1.85 meters.

[0018] As an optimization, the phase-shifted AIS message signal is as follows:

[0019] ;

[0020] ;

[0021] in, and These are the phase shift factors of the AIS signals for the two antennas, respectively.

[0022] As an optimization, in S2, the synthesized signal obtained by combining the N-channel AIS signals after phase shifting is:

[0023] .

[0024] As an optimization, the specific steps of S2 are as follows:

[0025] S2.1. Combine the N-channel AIS signals after phase shifting to obtain a composite signal;

[0026] S2.2 Determine whether the synchronization header of the synthesized signal is detected. If yes, proceed to S2.3; otherwise, proceed to S2.9.

[0027] S2.3. The synthesized signal is subjected to timing estimation algorithm and frequency offset estimation algorithm to perform timing estimation (determine the optimal decision point of the synthesized signal) and frequency offset estimation (determine the residual frequency offset of the synthesized signal), and the synthesized signal is compensated for frequency offset according to the frequency offset estimation result to obtain the first intermediate signal;

[0028] S2.4. Differential modulation is performed on the first intermediate signal to obtain the second intermediate signal;

[0029] S2.5. Perform Viterbi decoding on the second intermediate signal to obtain the decoding result, which is the bit information corresponding to the message with the strongest power.

[0030] S2.6 Perform CRC verification on the decoding result;

[0031] S2.7 Determine whether the verification is complete. If yes, proceed to S2.8 and S2.9 respectively; otherwise, proceed to S2.6.

[0032] S2.8 Determine if the verification is correct. If yes, report the AIS message signal indicating that the verification is correct; otherwise, proceed to S2.9.

[0033] S2.9 Add a new phase-shifting factor to perform phase-shifting processing on the preprocessed N channels of the AIS message signals, and return to S2.1.

[0034] As an optimization, the specific steps of S2.5 are as follows:

[0035] S2.5.1 Extract the phase shift of the second intermediate signal :

[0036] ;

[0037] Where k is the time for extracting the phase shift. This indicates the phase of the bit pair corresponding to the AIS message signal at time ki at time k. The contribution value, The phase shift caused by white noise, This represents the symbol corresponding to the AIS message signal at time k, where L is the AND phase. Related code segments;

[0038] Define state ,state ,in Represents the possible symbols at time k, starting from the state. Transition to state The corresponding phase shift It can be represented as

[0039] ;

[0040] S2.5.2

[0041] The differential decoding algorithm based on Viterbi calculates the various states (total) at each symbol end time k. The path metrics of the transition branches of each state at time k are calculated, and the path metrics of the transition branches of each state at time k-1 are added together. For each state with two transition paths, the surviving path of that state is selected according to the principle of minimizing the path metric, thus obtaining the total path metric of each state at time k. Among these, the path metrics of the transition branches are... The calculation formula is:

[0042] ;

[0043] S2.5.3 At the end time M of the last symbol, obtain the total path metric of each state at time M, and select the path with the smallest metric as the total surviving path. Perform backtracking decoding on the surviving path to obtain the bit information of the AIS message signal with the strongest power.

[0044] As an optimization, the phase-shifting factor is added in parallel.

[0045] As an optimization, the phase shift factor is added serially.

[0046] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0047] This invention performs weighted preprocessing on two AIS signals at the same frequency received by dual antennas, and then combines the two preprocessed signals. The weighting factor enhances the signal in one area of ​​the field of view of the ball-borne receiver while weakening the signal in another area, thereby reducing the mutual interference between different cells and increasing the detection probability of packets in the enhanced area. By adjusting the weighting factor, the signal enhancement area and the signal weakening area are continuously changed, thereby improving the detection probability of packets from different cells in the event of time slot conflicts. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0049] Figure 1 This is a general flowchart of an anti-AIS time slot collision method based on dual-antenna phase-shift synthesis as described in this invention.

[0050] Figure 2 This is a flowchart illustrating a method for resisting AIS time slot conflicts based on dual-antenna phase-shift synthesis as described in this invention.

[0051] Figure 3 A schematic diagram illustrating the path difference when two antennas receive the same AIS signal;

[0052] Figure 4 This is a block diagram illustrating the principle of the differential Viterbi decoding algorithm.

[0053] Figure 5 The execution flow of the Viterbi decoding algorithm;

[0054] Figure 6 A flowchart for digital preprocessing of AIS message signals. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0056] Traditional AIS receivers based on a single antenna can only demodulate the strongest packet when faced with time slot collisions. The strongest packet can only be successfully demodulated when the power difference between the strongest and second strongest packets exceeds the demodulation threshold, while the second strongest packets cannot be demodulated. Therefore, this solution has a very low packet detection success rate when time slot collisions occur.

[0057] Drawing on the principles of traditional phased array antennas while considering the implementation cost of spherical AIS receivers, this invention proposes a dual-antenna phase-shift synthesis method to combat AIS time slot conflicts. This method performs weighted preprocessing (i.e., phase shifting) on ​​N AIS message signals received from the same frequency point by both antennas. Then, the preprocessed N AIS message signals are synthesized. The weighting factor (phase shift factor) enhances the signal in one area of ​​the spherical receiver's field of view while weakening the signal in another area, thereby reducing mutual interference between different cells and increasing the detection probability of messages in the enhanced area. This is achieved by adjusting the weighting factor (phase shift factor). and This allows the signal enhancement and signal attenuation regions to continuously shift, thereby increasing the detection probability of packets from different cells in the event of time slot collisions and adjusting the phase shift factor. and Essentially, it involves changing the signal strength at different locations within the field of view of the sphere-mounted AIS receiver.

[0058] An embodiment of the method for resisting AIS time slot collisions based on dual-antenna phase-shift synthesis, such as Figure 1 As shown in the flowchart, the entire implementation process will be described in detail below.

[0059] include:

[0060] S1. Receive message signals from different AIS cells through two antennas. When N AIS message signals from the same frequency point of different AIS cells are received in the same time slot, the N AIS message signals are preprocessed and then phase-shifted by adding a phase-shifting factor, where N is a positive integer not less than 1.

[0061] In this embodiment, the preprocessing of the N-channel AIS message signals specifically includes sequentially performing DC removal, digital AGC, down-conversion / low-pass filtering, and decimation processing on the N-channel AIS message signals. (See attached image for signal preprocessing details.) Figure 6 The data rate is reduced sufficiently to decrease the amount of digital signal data processing. An automatic gain control (AGC) module is added to the digital preprocessing section to ensure that the received signal remains stable at a suitable level regardless of the signal dynamics.

[0062] The AIS message signals received by antennas 1 and 2 are down-converted and low-pass filtered to obtain a synthesized baseband signal. It is assumed that within a certain AIS time slot, both antennas 1 and 2 receive signals at the same frequency. These AIS message signals can be represented as:

[0063] (1)

[0064] (2)

[0065] in, , These represent the N AIS message signals at the same frequency received by antenna 1 and antenna 2 at time t. Indicates the first The amplitude of each AIS message signal reaching the two antennas respectively; Indicates the first The receiving frequency (including frequency offset) of each AIS message signal; Indicates the first Phase information corresponding to each AIS message signal; and They represent the first The initial phase of each AIS message signal to the two antennas. and The following relationship exists between them:

[0066] (3)

[0067] in, Indicates the first The difference in path distance between the two antennas for each AIS message signal, such as Figure 3 As shown, Indicates the first The wavelength of each AIS message signal ≈1.85 meters.

[0068] Using local carrier frequency The N AIS signals received by antennas 1 and 2 are down-converted to obtain pre-processed N AIS signals:

[0069] (4)

[0070] (5)

[0071] in, and These are the phase shift factors of the AIS signal for the two antennas, respectively. The local carrier frequency for down-converting the AIS message signals received by the two antennas is provided by the LPF, which is a low-pass filter. For receiving frequency With local carrier frequency The difference between them .

[0072] Using phase shift factor and Phase-shifting is performed on the baseband signals of antenna 1 and antenna 2 respectively, resulting in the following phase-shifted AIS message signals:

[0073] (6)

[0074] (7)

[0075] This is the AIS message signal obtained after phase shifting of antenna 1. This is the AIS message signal obtained after phase shifting of antenna 2.

[0076] Depending on the system hardware resources, two adjustment methods can be adopted: parallel and serial. Assuming there are N phase shift factors, the parallel adjustment method generates N phase shift factors simultaneously and performs N-way phase shift synthesis-differential demodulation on the dual-antenna input signals. This method consumes more hardware resources. The serial adjustment method first uses one phase shift factor for phase shift synthesis-differential demodulation, and after demodulation, it switches to a second phase shift factor for phase shift synthesis-differential demodulation, and so on, until all N phase shift factors are synthesized and demodulated. This method consumes more storage resources. Therefore, this invention adopts a combination of parallel and serial methods, which is applicable to a wider range of hardware resources.

[0077] S2. The N AIS signals after phase shifting are synthesized, and the signal strength of the AIS cell to which the N AIS message signals belong is adjusted by adjusting the phase shift factor, so that the signal strength of the AIS cell to which the N AIS message signals belong changes continuously, thereby improving the detection probability of different AIS cell messages in the case of time slot conflict.

[0078] In this embodiment, the specific steps of S2 are as follows:

[0079] S2.1. Combine the N-channel AIS signals after phase shifting to obtain a composite signal;

[0080] The synthesized signal can be represented as:

[0081] ; (8).

[0082] As can be seen from formula (8), the amplitude of the synthesized signal is , and phase shift factor and It's related to angles.

[0083] Differential Viterbi demodulation is performed on the synthesized signal, such as... Figure 4 The detailed steps are as follows:

[0084] S2.2 Determine whether the synchronization header of the synthesized signal is detected. If yes, jump to S2.3; otherwise, jump to S2.9.

[0085] The AIS protocol stipulates that each AIS message signal has a 24-bit synchronization header sequence. The ball-borne AIS receiver uses the matched correlation method to detect the synchronization header. When the correlation peak exceeds the threshold, it is considered that an AIS message has been detected, and proceeds to S2.3.

[0086] S2.3. Use timing estimation algorithm and frequency offset estimation algorithm to perform timing estimation (determine the optimal decision point of the synthesized signal) and frequency offset estimation (determine the residual frequency offset of the synthesized signal) on the synthesized signal, and perform frequency offset compensation on the synthesized signal according to the frequency offset estimation result to obtain the first intermediate signal; timing estimation is performed in order to determine the optimal decision point of the synthesized signal, and the residual frequency offset of the synthesized signal is determined by using the sampled value of the optimal decision point.

[0087] After detecting the synchronization head, timing estimation and frequency offset estimation algorithms are used to estimate the timing and frequency offset of the phase-shifted synthesized signal. Based on the frequency offset estimation results, the synthesized signal is then processed. Perform frequency offset compensation to obtain the first intermediate signal. .

[0088] (9)

[0089] Switch to S2.4, where, for The estimated value, under ideal circumstances ; .

[0090] S2.4, For the first intermediate signal Differential modulation is performed to obtain the second intermediate signal;

[0091] The first intermediate signal after frequency offset compensation Perform differential demodulation to obtain the second intermediate signal. :

[0092] (10)

[0093] Where T represents the symbol period, and nT corresponds to the optimal sampling time;

[0094] S2.5. Perform Viterbi decoding on the second intermediate signal to obtain the decoding result, which is the bit information corresponding to the message with the strongest power.

[0095] S2.5.1 Extracting the phase shift of the second intermediate signal :

[0096] .

[0097] Define state ,state ,in Represents the possible symbols at time k, starting from the state. Transition to state The corresponding phase shift It can be represented as:

[0098] ;

[0099] From the second intermediate signal Phase parameters can be extracted from it. In other words, instantaneous phase information:

[0100] (11)

[0101] The Viterbi detector detects the second intermediate signal. Extracting instantaneous phase information Then, the phase path is estimated using the Viterbi algorithm to obtain the demodulated data. If we consider the inter-symbol interference of the current symbol to the L symbols before and after it, then Within the k-th symbol interval phase shift for:

[0102] (12)

[0103] Where k is the time for extracting the phase shift. This indicates the phase of the bit pair corresponding to the AIS message signal at time ki at time k. The contribution value, The phase shift caused by white noise, This represents the symbol corresponding to the AIS message signal at time k, where L is the AND phase. Related code segments;

[0104] The AIS system uses Gaussian filtered MSK modulation (i.e., GMSK modulation). Gaussian filtering introduces inter-symbol interference, therefore, the phase shift at the current moment... The phase shift at time k is determined not only by the bits at the current moment, but also by the bits at the preceding and following L moments. These 2L bits together determine the phase shift at time k. Since each bit has only two choices, 0 and 1, 2L bits have a total of In Viterbi decoding, each combination is called a state, and is represented by... .because There are only two possible values, 0 and 1. Therefore, from the state... Jump to status There are only two paths, also known as transfer branches, which can be denoted as follows: and .

[0105] S2.5.2, The Viterbi-based differential decoding algorithm calculates the various states (total) at each symbol end time k. The path metrics of the transition branches of each state at time k are calculated, and the path metrics of the transition branches of each state at time k-1 are added together. For each state with two transition paths, the surviving path of that state is selected according to the principle of minimizing the path metric, thus obtaining the total path metric of each state at time k. Among these, the path metrics of the transition branches are... The calculation formula is:

[0106] ;

[0107] S2.5.3 At the end time M of the last symbol, obtain the total path metric of each state at time M, and select the path with the smallest metric as the total surviving path. Perform backtracking decoding on the surviving path to obtain the bit information of the AIS message signal with the strongest power.

[0108] Specifically, the Viterbi-based differential decoding method calculates the path metric of each transition branch leading to each state at the end of each symbol. The total path metric of each state transition branch is obtained by summing the path metric with the surviving path of the previous state. The current surviving path of the state is selected based on the principle of minimizing the path metric.

[0109] The state of the kth symbol interval Represented as:

[0110] (13)

[0111] From state Transition to state Corresponding branch phase measurement Defined as:

[0112] (14)

[0113] in,

[0114] (15)

[0115] From state Transition to state The corresponding symbol sequence is equivalent to .

[0116] No. Total phase path metric of +1 symbol interval Survival path metric stored at the k-th symbol interval The relationship between them is:

[0117] (16)

[0118] The differential decoding method based on Viterbi is based on , , , The iterative relationship formed by the formulas among the four factors is used to demodulate the phase path using the Viterbi algorithm. Detailed steps of the Viterbi algorithm can be found in [link to Viterbi algorithm]. Figure 5 .

[0119] S2.6 Perform CRC check on the decoding result;

[0120] S2.7 Determine whether the verification is complete. If yes, proceed to S2.8 and S2.9 respectively; otherwise, proceed to S2.6.

[0121] S2.8 Determine if the verification is correct. If yes, report the AIS message signal indicating that the verification is correct; otherwise, proceed to S2.9.

[0122] S2.9 Add a new phase-shifting factor to perform phase-shifting processing on the preprocessed N-channel AIS message signals, and return to S2.1.

[0123] It should be noted that the entire process of dual-antenna phase-shifting synthesis in this invention involves: performing phase compensation on the signals received by the two antennas, then synthesizing the compensated signals, and finally demodulating the synthesized signals. This process can be extended to multiple antennas, and the principle remains the same.

[0124] Decoding using the differential Viterbi algorithm yields excellent demodulation error rate performance, which can lower the demodulation threshold requirement. Combined with phase-shift synthesis, it can improve the AIS detection probability.

[0125] At the same time, the phase shift factor is adjusted according to the amount of hardware system resources. and The number of cells can be increased to achieve fine-tuning of signal energy at different locations within the field of view of the sphere-borne AIS receiver, reducing mutual interference between different cells and improving detection probability.

[0126] After processing based on the above three points, the ball-borne AIS receiver based on the dual-antenna phase-shift synthesis algorithm can significantly improve the detection probability of AIS packets in time slot collision scenarios, with limited increase in hardware cost, low implementation complexity, and meeting the real-time requirements of engineering applications, thus having a wider range of engineering applications. The anti-AIS time slot collision algorithm based on dual-antenna phase-shift synthesis can be applied to similar or other products, and this technical approach cannot be avoided.

[0127] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for resisting AIS time slot collisions based on dual-antenna phase-shift synthesis, characterized in that, include: S1. Receive message signals from different AIS cells through two antennas. When N AIS message signals from different AIS cells at the same frequency are received in the same time slot, the N AIS message signals are preprocessed and then phase-shifted by adding a phase-shifting factor. N is a positive integer not less than 1. S2. The N AIS signals after phase shifting are synthesized, and the signal strength of the AIS cell to which the N AIS message signals belong is adjusted by adjusting the phase shifting factor, so that the signal strength of the AIS cell to which the N AIS message signals belong changes continuously, thereby improving the detection probability of different AIS cell messages in the case of time slot conflict. The specific steps of S2 are as follows: S2.

1. Combine the N-channel AIS signals after phase shifting to obtain a composite signal; S2.2 Determine whether the synchronization header of the synthesized signal is detected. If yes, proceed to S2.3; otherwise, proceed to S2.

9. S2.

3. The synthesized signal is subjected to timing estimation algorithm and frequency offset estimation algorithm to perform timing estimation and frequency offset estimation, and the synthesized signal is compensated for frequency offset according to the frequency offset estimation result to obtain the first intermediate signal; S2.

4. Differential modulation is performed on the first intermediate signal to obtain the second intermediate signal; S2.

5. Perform Viterbi decoding on the second intermediate signal to obtain the decoding result, which is the bit information corresponding to the most powerful AIS message signal. S2.6 Perform CRC verification on the decoding result; S2.7 Determine whether the verification is complete. If yes, proceed to S2.8 and S2.9 respectively; otherwise, proceed to S2.

6. S2.8 Determine if the verification is correct. If so, report the AIS message signal indicating that the verification is correct. Otherwise, proceed to S2.9; S2.9 Add a new phase-shifting factor to perform phase-shifting processing on the preprocessed N channels of the AIS message signals, and return to S2.

1.

2. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 1, characterized in that, The preprocessing of the N AIS message signals specifically includes sequentially performing DC removal, digital AGC, down-conversion / low-pass filtering, and decimation processing on the N AIS message signals.

3. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 1, characterized in that, The preprocessed N channels of AIS signals are as follows: ； ； in, , , , These represent the N AIS message signals at the same frequency received by the two antennas at time t. Indicates the first The amplitude of each AIS message signal reaching the two antennas respectively; Indicates the first The receiving frequency of each AIS message signal; Indicates the first Phase information corresponding to each AIS message signal; and They represent the first The initial phase of each AIS message signal to the two antennas. The local carrier frequency for down-converting the AIS message signals received by the two antennas is provided by the LPF, which is a low-pass filter. For receiving frequency With local carrier frequency The difference between them .

4. The method for resisting AIS time slot conflicts based on dual-antenna phase-shifting synthesis according to claim 3, characterized in that, and The following relationship exists between them: ； in, Indicates the first The difference in path distance between the two AIS message signals reaching the two antennas. Indicates the first The wavelength of each AIS message signal.

5. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 3, characterized in that, The phase-shifted AIS message signal is as follows: ； ； in, and These are the phase shift factors of the AIS signals for the two antennas, respectively.

6. The method for resisting AIS time slot collisions based on dual-antenna phase-shift synthesis according to claim 5, characterized in that, In S2, the synthesized signal obtained by combining the N-channel AIS signals after phase shifting is: 。 7. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 1, characterized in that, The specific steps of S2.5 are as follows: S2.5.1 Extract the phase shift of the second intermediate signal : ； Where k is the time for extracting the phase shift. This indicates the phase of the bit pair corresponding to the AIS message signal at time ki at time k. The contribution value, The phase shift caused by white noise, This represents the symbol corresponding to the AIS message signal at time k, where L is the AND phase. Related code segments; Define state ,state ,in Represents the possible symbols at time k, starting from the state. Transition to state The corresponding phase shift It can be represented as ； S2.5.

2. The Viterbi-based differential decoding algorithm calculates the path metric of the transition branches reaching each state at each symbol end time k, and adds the path metric of the transition branches of each state at time k to the path metric of the corresponding state at time k-1. For each state with two transition paths, the surviving path of that state is selected according to the principle of minimizing the path metric, thus obtaining the total path metric of each state at time k. The path metric of the transition branches is... The calculation formula is: ； S2.5.3 At the end time M of the last symbol, obtain the total path metric of each state at time M, and select the path with the smallest metric as the total surviving path. Then, perform backtracking decoding on the surviving path to obtain the bit information of the AIS message signal with the strongest power.

8. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 1, characterized in that, The phase-shifting factor was added in parallel.

9. The method for resisting AIS time slot collisions based on dual-antenna phase-shifting synthesis according to claim 1, characterized in that, The phase-shifting factor was added serially.

Images