Working method of receiver based on satellite-borne VDE-Link 20

Abstract

The working method of the satellite-borne VDE-Link 20-based receiver comprises the following steps: receiving a radio frequency signal, carrying out down-conversion and down-sampling to obtain baseband data, and carrying out frame header detection, frame synchronization and coarse frequency offset compensation on the baseband data; cPM de-spreading is carried out; conventional frequency offset and phase fine estimation and compensation are carried out, so that the absolute value of the frequency offset is smaller than a frequency offset threshold value; based on a candidate frequency offset range corresponding to the frequency offset threshold, performing frequency offset pre-compensation at an interval of 1Hz; and synchronously carrying out subsequent processing including CRC (Cyclic Redundancy Check) on the multiple groups of data, and finding a correct group of data and corresponding decoding according to a CRC result. According to the method, the frequency offset coverage range of the Link 20 receiver can be expanded, the estimation precision is improved, and the demodulation sensitivity of the receiver is improved.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication base stations or terminals, and specifically relates to a working method of a receiver based on a spaceborne VDE-Link20. Background Technology

[0002] VDES (VHF Data Exchange System) is a third-generation maritime communication technology that provides all-weather, 24 / 7 VHF data communication, data acquisition, and information management and services related to maritime IoT, with extremely broad application prospects. It aims to solve the problems of frequency congestion and insufficient data transmission capacity of traditional AIS (Automatic Identification System), meeting the modern shipping industry's demand for efficient and secure communication.

[0003] The VHF Data Exchange System (VDES) retains the basic functions of AIS while adding two major subsystems: ASM (Application Specific Messages) and VDE (VHF Data Exchange). ASM is used to support the transmission of application-specific data messages for maritime scenarios, while VDE is used for efficient VHF data communication. The signals of both subsystems rely on the Link20 physical layer frame format.

[0004] The VDES standard defines the Link20 physical layer frame format for random access channels. The physical layer employs QPSK-CPM spread spectrum modulation to ensure phase continuity of data during transitions from one symbol to the next. Specifically, VDE's CPM spread spectrum uses a CPM (Continuous Phase Modulation) waveform to spread the QPSK modulated signal 16 times.

[0005] The existing receiver processing flow consists of frame header detection, CPM despreading, and demodulation. The receiver solution addresses Link20 frame header detection and CPM despreading at a satellite altitude of 600km. However, the frame header detection algorithm is limited by the satellite's altitude, resulting in significant errors. The current solution only includes the basic Link20 demodulation process and lacks an effective overall receiving algorithm flow. Therefore, directly performing Link20 demodulation after CPM despreading yields poor performance, often suffering from noise interference and large residual frequency offset errors, leading to poor receiver sensitivity, especially at low noise-to-noise ratios. This severely impacts user access performance, with receiver sensitivity falling far short of protocol requirements.

[0006] Therefore, it is necessary to propose a receiver based on the spaceborne VDE-Link20, which includes a receiver overall algorithm flow of frame header detection, CPM despreading, and demodulation, to reduce noise interference and frequency offset residual interference, so as to improve the demodulation sensitivity of the receiver. Summary of the Invention

[0007] The purpose of this invention is to provide a method for operating a receiver based on a spaceborne VDE-Link20, so as to improve the frequency offset coverage of the Link20 receiver and improve the estimation accuracy, thereby improving the demodulation sensitivity of the receiver.

[0008] To achieve the above objectives, the present invention provides a method for operating a receiver based on a spaceborne VDE-Link20, comprising:

[0009] It receives the radio frequency signal from the onboard VDE-Link20 and performs downconversion and downsampling to obtain baseband data. It then performs frame header detection, frame synchronization, and coarse frequency offset compensation on the baseband data.

[0010] Perform CPM despreading to obtain the restored data;

[0011] Perform routine frequency offset and phase fine estimation and compensation to make the absolute value of the frequency offset less than the frequency offset threshold;

[0012] Based on the candidate frequency offset range corresponding to the frequency offset threshold, frequency offset pre-compensation is performed at intervals of 1Hz.

[0013] Multiple sets of data are simultaneously processed with CRC checks, and the correct set of data and corresponding decoding are found based on the CRC check results.

[0014] The frequency offset threshold is 2Hz, and the frequency offset pre-compensation with an interval of 1Hz includes 0, +1Hz, and -1Hz frequency offset pre-compensation.

[0015] The process of synchronizing multiple sets of data includes subsequent processing with CRC verification, such as phase compensation, data extraction, soft demodulation, descrambling, Turbo decoding, and CRC verification.

[0016] Perform routine frequency offset and phase fine-tuning and compensation to ensure that the absolute value of the frequency offset is less than the frequency offset threshold. Specifically, this includes:

[0017] Using the receive synchronization word in the restored data The local synchronization word is used for fine estimation and compensation, and then the pilot band in the restored data is used for fine estimation and compensation of the pilot.

[0018] Initial phase estimation and compensation are performed using the received synchronization word and the local synchronization word;

[0019] Pilot bands are used for pilot superposition estimation and compensation.

[0020] Both the synchronization word estimation and the pilot word estimation use the M&M algorithm.

[0021] Pilot superposition estimation and compensation using pilot bands specifically includes: using the pilot bands in the restored data as the received pilot signal. Combine it with the local pilot Conjugate summation yields the pilot superposition phase, which is then divided by time to obtain the increment of the pilot superposition phase over time. Subsequently, the increment of the pilot superposition phase over time is used to estimate the residual frequency offset. And provide compensation.

[0022] The CPM despreading includes synchronization word despreading and data segment despreading.

[0023] The synchronization word despreading includes: multiplying the spread spectrum sequence CPA / CPE of the local synchronization word with the conjugate of the received filtered data to obtain the received synchronization word of the restored data. And according to the received synchronization word Determine the time base for restoring the data;

[0024] Data segment despreading includes: locating the data segment portion of the restored data based on the time base of the restored data; splitting the data segment portion into symbol spreading sequences for each symbol; performing conjugate despreading on the first symbol spreading sequence using four sets of spreading coefficients, and comparing the amplitudes obtained from the despreading; the maximum conjugate amplitude is the required spreading coefficient; and performing conjugate despreading on all symbol spreading sequences using the required spreading coefficient.

[0025] After performing frame header detection, frame synchronization, and coarse frequency offset compensation on the baseband data, and before CPM despreading, the process includes matched filtering to obtain filtered data. The matched filter used in the matched filtering is one corresponding to the transmitter's shaping filter, with a roll-off factor of 0.25.

[0026] The operating method of the receiver based on the satellite-borne VDE-Link20 is applicable to satellite-to-ground communication systems based on the satellite-borne VDE-Link20, which include a transmitter located on the ground and a receiver located on the satellite.

[0027] This invention performs frequency offset pre-compensation at 1Hz intervals on the basis of conventional frequency offset estimation algorithms. As long as one demodulation is successful, the user demodulation is successful, so that the final residual frequency offset can be less than 1Hz, thereby suppressing noise and interference, improving estimation accuracy and thus improving receiver demodulation sensitivity.

[0028] Furthermore, this invention implements a stepped frequency offset estimation algorithm through coarse frequency offset compensation, fine estimation and compensation of synchronization word, fine estimation and compensation of pilot, pilot superposition estimation and compensation, and the aforementioned frequency offset pre-compensation with an interval of 1Hz. From the coarse estimation of frequency offset pre-compensation in frame header detection to the fine estimation of synchronization word and pilot, it ensures a larger estimation range and better estimation accuracy, resulting in higher demodulation sensitivity performance and stronger anti-interference capability.

[0029] In summary, compared with the prior art, the present invention can improve the frequency offset coverage of the Link20 receiver and improve the estimation accuracy, thereby improving the demodulation sensitivity of the receiver. Attached Figure Description

[0030] Figure 1 This is a flowchart of the operation method of the receiver based on the spaceborne VDE-Link20 of the present invention;

[0031] Figure 2 This is a flowchart of the existing operation method of the VDE-Link20-based transmitter;

[0032] Figure 3 This is a general diagram of the frame structure given by the VDES protocol;

[0033] Figure 4 This is a BLER curve graph under the Link20 link rule. Detailed Implementation

[0034] The preferred embodiments of the present invention are given below with reference to the accompanying drawings, and described in detail to enable a better understanding of the functions and features of the ultraviolet light monitoring device and method of the present invention.

[0035] The operating method of the receiver based on spaceborne VDE-Link20 of this invention is applicable to space-to-ground communication systems based on spaceborne VDE-Link20, which include a transmitter located on the ground and a receiver located on the satellite. Here, spaceborne VDE refers to the VHF data exchange subsystem carried on the satellite, and Link20 is the link rule of the VHF data exchange subsystem. In the space-to-ground communication system, the operating method of the transmitter is the same as in the prior art; only the operating method of the receiver differs from the prior art.

[0036] like Figure 1 As shown, the operating method of the receiver based on the spaceborne VDE-Link20 of the present invention includes:

[0037] Step S1: Receive the radio frequency signal from the onboard VDE-Link20 and perform downconversion and downsampling to obtain baseband data. Perform frame header detection, frame synchronization and coarse frequency offset compensation on the baseband data.

[0038] The receiver performs time-frequency two-dimensional correlation on the baseband data based on the pre-stored local frame header template to obtain the starting position of the user frame header (i.e. the moment when the correlation value is the largest) and the coarse frequency offset estimate (i.e. the frequency offset with the largest correlation value), and performs frame synchronization and coarse frequency offset compensation on the baseband data.

[0039] Step S2: Perform matched filtering to obtain filtered data;

[0040] When performing matched filtering, a matched filter corresponding to the transmitter's shaping filter is used, with a roll-off factor of 0.25. This reduces out-of-band noise.

[0041] Step S3: Perform CPM despreading to obtain the restored data;

[0042] The CPM despreading includes synchronization word despreading and data segment despreading.

[0043] CPM spreading refers to spreading each data symbol by 16 times and oversampling by 4 times, resulting in 64 "spreading chips" (i.e., spreading sequences) for each symbol. These 64 chips are divided into two parts: the first 32 are called the spreading sequence CPA (Continuous Phase Modulation, A), and the last 32 are called the spreading sequence CPE (Continuous Phase Modulation, E). The spreading sequences CPA and CPE together constitute the CPM (Continuous Phase Modulation) spreading sequence for one symbol. The spreading sequences CPA / CPE are the two parts of CPM.

[0044] During the spread spectrum process, each symbol selects one set from four fixed sets of spreading coefficients in the protocol and combines it with the spread spectrum sequence CPA / CPE to generate the final spread spectrum signal (the four sets of coefficients are preset by the protocol, similar to four fixed keys, and the receiver also stores these four sets of coefficients).

[0045] Synchronization word despreading includes: multiplying the spread spectrum sequence CPA / CPE of the local synchronization word with the conjugate of the received filtered data to obtain the received synchronization word of the restored data. And according to the received synchronization word Determine the time base for restoring the data.

[0046] Since the synchronization word is a known sequence, its spreading sequences CPA / CPE are also known. The received synchronization word can be directly obtained by multiplying the local spreading sequences CPA / CPE by the conjugate of the filtered data. This is used as the synchronization word portion in the restored data to determine the time base of the restored data. For example, suppose the original data is... Multiply by the spreading sequence CPA to obtain the transmitted baseband data. The receiver can then perform CPM despreading by multiplying by the conjugate of CPA to recover the restored data RxData, i.e. .

[0047] Figure 3 The general frame structure diagram given for the VDES protocol shows that the 48 symbols of the synchronization word and the 0 symbols in the link ID correspond to Link20 of this invention. The 27 symbols of the synchronization word or the 16 symbols of the link ID represent other links and are not within the scope of this discussion. Figure 3 As shown, the original data has a frame structure, which includes a synchronization word, a link ID, and data segments and pilot segments spaced apart from each other. Therefore, the reconstructed data will also include a synchronization word, a link ID, and data segments and pilot segments spaced apart from each other.

[0048] Data segment despreading includes: locating the data segment portion of the restored data based on the time base of the restored data; splitting the data segment portion into symbol spreading sequences for each symbol; performing conjugate despreading on the first symbol spreading sequence using four sets of spreading coefficients, and comparing the amplitudes obtained from the despreading; the maximum conjugate amplitude is the required spreading coefficient; and performing conjugate despreading on all symbol spreading sequences using the required spreading coefficient.

[0049] Data segment despreading also includes pilot segment despreading. Although the pilots are locally known, due to pilot dispersion and the characteristics of CPM spreading, they cannot be despread separately like synchronization words. The despreading method for pilots is exactly the same as that for data segments, or in other words, data segment despreading includes despreading of the pilot segments simultaneously.

[0050] Since the protocol has four fixed sets of spreading coefficients, the spreading sequence CPA / CPE of each symbol has four possible spreading coefficient results. Therefore, the amplitude obtained by despreading is obtained by multiplying the spreading sequence of each symbol with the conjugate of these four spreading coefficient results.

[0051] Step S4: Utilize the receive synchronization word in the restored data The local synchronization word is used for fine estimation and compensation, and then the pilot band in the restored data is used for fine estimation and compensation of the pilot.

[0052] Synchronization word fine estimation and pilot fine estimation are used to obtain corresponding fine frequency offset estimates, which are then used to compensate for frequency offset in the data. Synchronization word fine estimation is used to correct errors of tens of Hz, while pilot fine estimation is used to correct errors of tens of Hz in the restored data after synchronization word fine estimation and compensation.

[0053] In this embodiment, both the synchronization word fine estimation and the pilot fine estimation adopt the classic M&M algorithm (Mengali & Morelli algorithm).

[0054] In subsequent steps, each step is based on the compensation results of the previous step.

[0055] Step S5: Perform initial phase estimation and compensation using the received synchronization word and the local synchronization word;

[0056] Symbol for receiving synchronization word Symbols for syncing words with local local input Conjugate summation is used to obtain an estimate of the initial phase, which is then used to compensate for the phase.

[0057] The estimated value of the initial phase estimate for:

[0058]

[0059] It is the symbol for receiving the synchronization word. This is the symbol for a local synchronization word. It is the length of the synchronization character. This indicates the phase angle. This represents the estimated value of the initial phase estimate.

[0060] Step S6: Use pilot bands to perform pilot superposition estimation and compensation;

[0061] Step S6 specifically includes: restoring the pilot band in the data as the received pilot signal. Combine it with the local pilot Conjugate summation yields the pilot superposition phase, which is then divided by time to obtain the increment of the pilot superposition phase over time. Subsequently, the increment of the pilot superposition phase over time is used to estimate the residual frequency offset. And provide compensation.

[0062] Pilot superposition phase increment over time Represented as ,in Indicates residual frequency offset. Indicates the time elapsed.

[0063] Estimating residual frequency offset Satisfy the following formula:

[0064]

[0065] To receive the pilot signal, i.e., the pilot segment in the recovered data, This is a local pilot signal. The conjugate of the local pilot signal. The residual frequency offset is m, which is the timing sequence number of the pilot band. m = 1...12, indicating that there are a total of 12 pilots, starting from 1 and ending at 12. The position of the first pilot signal is fixed at 57. The distance between the two pilot signals is fixed at 17. Indicates a single sampling time. This indicates the phase angle. This represents the estimated residual frequency offset.

[0066] Steps S4 to S6 perform fine estimation and compensation of the conventional frequency offset and phase, ensuring that the absolute value of the frequency offset is less than the frequency offset threshold. In this embodiment, the frequency offset threshold is 2Hz.

[0067] Step S7: Based on the candidate frequency offset range corresponding to the frequency offset threshold, perform frequency offset pre-compensation with an interval of 1Hz.

[0068] In this embodiment, the frequency offset threshold is 2Hz, and the frequency offset pre-compensation with an interval of 1Hz includes 0, +1Hz, and -1Hz frequency offset pre-compensation, thus obtaining three sets of data after 0, +1Hz, and -1Hz frequency offset pre-compensation. In complex scenarios such as low signal-to-noise ratio, there is still a low probability that the residual frequency offset is higher than 1Hz. At this time, based on the data compensated in step S6, data after ±1Hz frequency offset pre-compensation is added respectively. All three signals enter the subsequent bit demodulation. As long as one of them is successfully demodulated, the user demodulation is successful, so that the final residual frequency offset can be less than 1Hz.

[0069] In other embodiments, since Link20 can theoretically only withstand a frequency offset error of about 1 Hz, after the above conventional frequency offset estimation, if implementation permits, the present invention is also applicable to a larger number of frequency offset pre-compensation intervals of 1 Hz, such as frequency offset compensation of [...2 1 0 -1 -2 ...] Hz, etc.

[0070] Step S8: Perform subsequent processing including CRC check on multiple sets of data synchronously, and find the correct set of data and the corresponding decoding based on the CRC check result.

[0071] The process of synchronizing multiple sets of data includes subsequent processing with CRC verification, such as phase compensation, data extraction, soft demodulation, descrambling, Turbo decoding, and CRC verification.

[0072] In this embodiment, during phase compensation, the three sets of data after pre-compensation of frequency offsets of 0, +1Hz, and -1Hz in step S7 are used to estimate the phase offset and compensate using the initial phase estimation and compensation method in step S5, resulting in three sets of compensated data.

[0073] During data extraction, the symbols for the data positions are extracted based on the frame structure of Link20.

[0074] During soft demodulation, the data symbols modulated by π / 4-QPSK are soft demodulated to obtain the corresponding soft bits.

[0075] During descrambling, the soft bits are descrambled in reverse order by sending the scrambling module to obtain the descrambled soft bits.

[0076] During Turbo decoding, the descrambled soft bits are sent to the Turbo decoding module to obtain the decoding information.

[0077] During CRC verification, the decoded information output by the Turbo decoding module undergoes CRC verification. Of the three sets of data, a successful CRC check indicates correct decoding, while a failure indicates incorrect decoding. This process is used to find the unique correct set of decoded data.

[0078] like Figure 2 As shown, the transmitter operates in the same way as existing technologies, including: adding CRC checksum; performing Turbo encoding; scrambling the data; performing π / 4-QPSK symbol mapping to achieve data modulation; inserting synchronization words (i.e., synchronization character symbols) and pilot bands (i.e., pilot symbols) to achieve data framing and obtain the required frame structure; performing CPM spread spectrum; performing shaping filtering; performing upsampling; and converting the baseband signal into an RF signal and transmitting it using the Link20 link protocol. The steps of the transmitter's operation correspond to the steps of the receiver's operation described above.

[0079] Experimental results:

[0080] The receiver simulation results under the AWGN channel are as follows. From Figure 4 The BLER (Block Error Rate) curve shown indicates that under the ideal Gaussian channel of this invention, BLER = 10. -2 The demodulation threshold Es (energy per symbol) / N0 (noise power spectral density) is -0.9dB, meeting the protocol requirements, and the loss across the entire link is also relatively small. This means that the present invention does indeed improve receiver sensitivity.

[0081] This invention performs frequency offset pre-compensation at 1Hz intervals on the basis of conventional frequency offset estimation algorithms. As long as one demodulation is successful, the user demodulation is successful, so that the final residual frequency offset can be less than 1Hz, thereby suppressing noise and interference, improving estimation accuracy and thus improving receiver demodulation sensitivity.

[0082] Furthermore, this invention implements a stepped frequency offset estimation algorithm through coarse frequency offset compensation, fine estimation and compensation of synchronization word, fine estimation and compensation of pilot, pilot superposition estimation and compensation, and the aforementioned frequency offset pre-compensation with an interval of 1Hz. From the coarse estimation of frequency offset pre-compensation in frame header detection to the fine estimation of synchronization word and pilot, it ensures a larger estimation range and better estimation accuracy, resulting in higher demodulation sensitivity performance and stronger anti-interference capability.

[0083] In summary, compared with the prior art, the present invention can improve the frequency offset coverage of the Link20 receiver and improve the estimation accuracy, thereby improving the demodulation sensitivity of the receiver.

[0084] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.

Claims

1. A method for operating a receiver based on a spaceborne VDE-Link20, characterized in that, include: It receives the radio frequency signal from the onboard VDE-Link20 and performs downconversion and downsampling to obtain baseband data. It then performs frame header detection, frame synchronization, and coarse frequency offset compensation on the baseband data. Perform CPM despreading to obtain the restored data; Perform routine frequency offset and phase fine estimation and compensation to make the absolute value of the frequency offset less than the frequency offset threshold; Based on the candidate frequency offset range corresponding to the frequency offset threshold, frequency offset pre-compensation is performed at intervals of 1Hz. Multiple sets of data are processed synchronously, including CRC check, and the correct set of data and corresponding decoding are found based on the CRC check result.

2. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 1, characterized in that, The frequency offset threshold is 2Hz, and the frequency offset pre-compensation with an interval of 1Hz includes 0, +1Hz, and -1Hz frequency offset pre-compensation.

3. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 1, characterized in that, The process of synchronizing multiple sets of data includes subsequent processing with CRC verification, such as phase compensation, data extraction, soft demodulation, descrambling, Turbo decoding, and CRC verification.

4. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 1, characterized in that, Perform routine frequency offset and phase fine-tuning and compensation to ensure that the absolute value of the frequency offset is less than the frequency offset threshold. Specifically, this includes: Using the receive synchronization word in the restored data The local synchronization word is used for fine estimation and compensation, and then the pilot band in the restored data is used for fine estimation and compensation of the pilot. Initial phase estimation and compensation are performed using the received synchronization word and the local synchronization word; Pilot bands are used for pilot superposition estimation and compensation.

5. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 4, characterized in that, Both the synchronization word estimation and the pilot word estimation use the M&M algorithm.

6. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 4, characterized in that, Pilot superposition estimation and compensation using pilot bands specifically includes: using the pilot bands in the restored data as the received pilot signal. Combine it with the local pilot Conjugate summation yields the pilot superposition phase, which is then divided by time to obtain the increment of the pilot superposition phase over time. Subsequently, the increment of the pilot superposition phase over time is used to estimate the residual frequency offset. And provide compensation.

7. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 1, characterized in that, The CPM despreading includes synchronization word despreading and data segment despreading.

8. The operating method of the receiver based on the spaceborne VDE-Link20 according to claim 7, characterized in that, The synchronization word despreading includes: multiplying the spread spectrum sequence CPA / CPE of the local synchronization word with the conjugate of the received filtered data to obtain the received synchronization word of the restored data. And according to the received synchronization word Determine the time base for restoring the data; Data segment despreading includes: locating the data segment portion of the restored data based on the time base of the restored data; splitting the data segment portion into symbol spreading sequences for each symbol; performing conjugate despreading on the first symbol spreading sequence using four sets of spreading coefficients, and comparing the amplitudes obtained from the despreading; the maximum conjugate amplitude is the required spreading coefficient; and performing conjugate despreading on all symbol spreading sequences using the required spreading coefficient.

9. The operating method of the receiver based on spaceborne VDE-Link20 according to claim 1, characterized in that, After performing frame header detection, frame synchronization, and coarse frequency offset compensation on the baseband data, and before performing CPM despreading, the process also includes: performing matched filtering to obtain filtered data; when performing matched filtering, the matched filter used is a matched filter corresponding to the transmitter's shaping filter, with a roll-off factor of 0.

25.

10. The operating method of the receiver based on spaceborne VDE-Link20 according to claim 1, characterized in that, The operating method of the receiver based on the spaceborne VDE-Link20 is applicable to the space-to-ground communication system based on the spaceborne VDE-Link20, which includes a transmitter located on the ground and a receiver located on the satellite.

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