Low-orbit internet satellite communication system and method and device for initial synchronization thereof

Abstract

The application discloses a low-orbit Internet satellite communication system and a method and device for initial synchronization thereof. The method comprises the following steps: a ground terminal device generates a local reference sequence by performing DPSK modulation on a fixed-length m sequence; a downlink signal transmitted by a low-orbit Internet satellite is listened to, and the listened downlink signal is received and preprocessed; the preprocessed downlink signal is intercepted in a sliding mode, the length of each time of interception is the same as the length of the local reference sequence, each time of intercepted signal sequence is sequentially correlated with the local reference sequence; a peak value of a correlation peak is detected, if the peak value of the correlation peak appears continuously and is greater than a detection threshold, it is determined that the downlink signal contains a plurality of synchronization sequences matched with the local reference sequence, and the starting position of a signal segment of the downlink signal is determined according to the peak value position of the last synchronization sequence. The application can realize initial synchronization between a satellite and a ground terminal device in a high dynamic scene with Doppler frequency shift.

Description

Technical Field

[0001] This application relates to the field of low-Earth orbit satellite synchronization technology, and more specifically to a low-Earth orbit internet satellite communication system and its initial synchronization method and apparatus. Background Technology

[0002] With the continuous development of low-Earth orbit (LEO) internet satellites and mobile communication technologies, users' demands for high-speed, low-latency, and flexible mobile communication system switching are constantly increasing. Compared to terrestrial mobile communication systems, the biggest advantage of LEO internet satellite communication systems, which consist of LEO internet satellites and ground terminal equipment, is their wider coverage; multiple satellites forming a constellation can achieve global coverage. However, it also has a significant disadvantage: due to the relatively low orbital altitude of LEO satellites, their orbital speed can reach several kilometers per second, resulting in a large Doppler frequency shift between the ground terminal equipment and the satellites, which in turn affects the synchronization and demodulation in subsequent signal processing.

[0003] Orthogonal Frequency-Division Multiplexing (OFDM) technology, a key technology in terrestrial fourth-generation (4G) and fifth-generation (5G) mobile communications, is a typical multi-carrier technology. It significantly improves spectrum utilization and thus data transmission rates by using multiple orthogonal subcarriers. When developing the 5G standard, the 3GPP organization proposed several contingency plans for space-ground converged communication. Among them, using OFDM as the multi-carrier basic waveform in low-Earth orbit (LEO) satellite mobile communication is becoming a current research hotspot. However, the application of OFDM systems in LEO satellite communication also faces some problems that need to be solved. The main issue is that the large-scale, large-dynamic Doppler frequency offset caused by the high-speed movement of satellites and ground terminals introduces significant subcarrier interference (ICI) into the OFDM system, making it impossible for existing LEO satellite mobile communication to achieve accurate frequency synchronization, thereby reducing the system's transmission quality. Summary of the Invention

[0004] To overcome the problem of low synchronization accuracy caused by large Doppler frequency shift in existing low-Earth orbit satellite mobile communications, this application provides a low-Earth orbit internet satellite communication system and its initial synchronization method and apparatus.

[0005] According to a first aspect of this application, an initial synchronization method for a low-Earth orbit internet satellite communication system is provided, the method being executed by a ground terminal device, comprising the following steps:

[0006] S1, Generate a fixed-length m-sequence, and perform symmetric DPSK modulation on the m-sequence to generate a local reference sequence;

[0007] S2, monitor the downlink signal transmitted by the low-orbit internet satellite. Once detected, receive the downlink signal completely and perform down-conversion and filtering preprocessing. The downlink signal segment is pre-constructed with multiple synchronization sequences, and the length of each synchronization sequence is the same as the length of the local reference sequence.

[0008] S3, perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and perform cross-correlation processing between each truncation signal sequence and the local reference sequence in sequence;

[0009] S4. Perform peak detection on the cross-correlation processing results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence. The starting position of the signal segment of the downlink signal is determined based on the peak position of the correlation peak of the last synchronization sequence.

[0010] According to a second aspect of this application, an initial synchronization device for a low-Earth orbit internet satellite communication system is provided. The device is deployed on a ground terminal device and includes the following modules:

[0011] A local reference sequence generation module is used to generate a fixed-length m-sequence and perform symmetric DPSK modulation on the m-sequence to generate a local reference sequence.

[0012] The monitoring module is used to monitor downlink signals transmitted by low-Earth orbit internet satellites. Once detected, the downlink signal is fully received and preprocessed by downconversion and filtering. The downlink signal is preceded by multiple synchronization sequences, and the length of each synchronization sequence is the same as the length of the local reference sequence.

[0013] The cross-correlation processing module is used to perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and to perform cross-correlation processing on the signal sequence obtained each time with the local reference sequence in sequence.

[0014] The peak detection module is used to perform peak detection on the cross-correlation processing results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence, and the starting position of the signal segment of the downlink signal is determined according to the peak position of the correlation peak of the last synchronization sequence.

[0015] According to a third aspect of this application, a low-Earth orbit (LEO) internet satellite communication system is provided, the system comprising a LEO internet satellite and multiple ground terminal devices, wherein:

[0016] Each of the ground terminal devices is used to generate a fixed-length m-sequence, and to perform symmetrical DPSK modulation on the m-sequence to generate a local reference sequence;

[0017] The low-orbit internet satellite is used to transmit downlink signals to multiple ground terminal devices. The downlink signal has multiple synchronization sequences pre-constructed before the signal segment. Each synchronization sequence has the same length and matches a local reference sequence generated by a certain ground terminal device.

[0018] Each of the aforementioned ground terminal devices is also used to monitor the downlink signals transmitted by the low-orbit internet satellite. Once detected, the downlink signals are fully received and preprocessed by downconversion and filtering. The preprocessed downlink signals are then truncated, with each truncation being the same length as the local reference sequence. The truncated signal sequences are then cross-correlated with the local reference sequence in sequence. Peak detection is performed on the cross-correlation results. If the peak values ​​of consecutive correlation peaks are greater than the detection threshold, it is determined that the downlink signals contain multiple synchronization sequences that match the local reference sequence. The starting position of the downlink signal segment is determined based on the peak position of the correlation peak of the last synchronization sequence.

[0019] The above-described technical solutions adopted in the embodiments of this application can achieve the following beneficial effects:

[0020] Ground terminal equipment generates a local reference sequence by performing symmetrical DPSK modulation on a fixed-length m-sequence. Compared with conventional binary modulation, this not only reduces spectral leakage but also improves the estimation accuracy of Doppler and timing uncertainties common in satellite communications. The ground terminal equipment monitors and receives downlink signals transmitted by low-Earth orbit internet satellites. Multiple synchronization sequences are pre-constructed before the signal segments of these downlink signals. Each synchronization sequence has the same length as the local reference sequence, giving the multiple synchronization sequences good autocorrelation characteristics. Compared to the traditional method of constructing only a single synchronization sequence, this method allows for coherent accumulation, achieving high Doppler estimation accuracy even at low signal-to-noise ratios (SNR), thus enabling... To achieve precise time and frequency synchronization, the ground terminal equipment performs sliding truncation on the received downlink signal, with each truncation length being the same as the length of the local reference sequence. The truncation sequence is then cross-correlated with the local reference sequence in turn. By detecting the peak values ​​of the correlation peaks, it is determined whether the downlink signal contains a synchronization sequence that matches the local reference sequence. If the peak values ​​of consecutive correlation peaks are greater than the detection threshold, the starting position of the downlink signal segment is determined based on the peak position of the correlation peak of the last synchronization sequence. This enables stable initial synchronization between the low-Earth orbit internet satellite and the ground terminal equipment in high-dynamic scenarios with large Doppler frequency shifts, laying the groundwork for subsequent signal demodulation and decoding. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application, and those skilled in the art can obtain other drawings based on these drawings. In the drawings:

[0022] Figure 1 A flowchart illustrating an initial synchronization method for a low-Earth orbit internet satellite communication system provided in an embodiment of this application is shown.

[0023] Figure 2 This illustration shows a schematic diagram of the cross-correlation processing results between the local reference sequence and the satellite downlink signal provided in an embodiment of this application;

[0024] Figure 3 This paper shows a schematic diagram of the structure of an initial synchronization device for a low-Earth orbit internet satellite communication system according to an embodiment of this application;

[0025] Figure 4 A schematic diagram of the architecture of a low-Earth orbit internet satellite communication system provided in an embodiment of this application is shown. Detailed Implementation

[0026] Embodiments of this application will now be described in more detail with reference to the accompanying drawings. These embodiments are provided to enable a more thorough understanding of this application and to fully convey the scope of this application to those skilled in the art. Although exemplary embodiments of this application are shown in the drawings, it should be understood that this application can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0027] Figure 1 This paper illustrates a flowchart of an initial synchronization method for a low-Earth orbit internet satellite communication system according to an embodiment of this application. The method is executed by a ground terminal device and includes steps S1 to S4:

[0028] S1, Generate a fixed-length m-sequence, and perform symmetric DPSK modulation on the m-sequence to generate a local reference sequence.

[0029] Step S1 specifically includes:

[0030] First, a fixed-length m-sequence is generated by a linear feedback shift register (LFSR).

[0031] m-sequences are a widely used type of pseudo-random sequence with broad applications in the communications field, such as spread spectrum communication, code division multiple access in satellite communication, and encryption, scrambling, synchronization, and bit error rate measurement in digital data. Among all pseudo-random sequences, m-sequences are the most important and fundamental type. They are easy to generate, exhibit strong regularity, and possess good autocorrelation and cross-correlation properties.

[0032] Each bit of the m-sequence represents a positive or negative π / 2 phase rotation. A fixed-length 128-bit m-sequence can be generated using a 7th-order Fibonacci LFSR (external LFSR, also known as many-to-one), whose primitive polynomial is 1+D. 3 +D 7 Its initial value is (a -1 ,...,a -7 ) = (0,0,1,1,0,1,0). Assume the outputs of the external LFSR are a0,a1,...,a 126 It is stored as a 127-bit number, with a0 as the most significant bit (MSB, i.e., the leftmost bit). 126 This is the least significant bit (LSB, i.e., the rightmost bit in a binary digit). Appending 0 to this number yields a 128-bit m-sequence q. pss The hexadecimal form of this number is shown below:

[0033] q pss =1A9C F685 5F4A 371F C3BC B240 898B AD83.

[0034] Next, the generated m-sequence q pss Symmetrical differential phase shift keying (DPSK) modulation is performed to obtain the time-domain waveform of the local reference sequence:

[0035]

[0036]

[0037] Among them, b l Let m be the m-sequence after symmetric DPSK modulation, p(k) be the local reference sequence, N be the length of the local reference sequence, and k represent the sampling points, 1≤k≤N.

[0038] Compared with conventional binary modulation, symmetrical DPSK modulation of a fixed-length m-sequence can not only reduce spectral leakage, but also improve the estimation accuracy of Doppler and the timing uncertainty commonly found in satellite communications.

[0039] S2, monitor the downlink signal transmitted by the low-orbit internet satellite. Once detected, receive the downlink signal completely and perform down-conversion and filtering preprocessing. The downlink signal has multiple synchronization sequences pre-constructed before the signal segment, and the length of each synchronization sequence is the same as the length of the local reference sequence.

[0040] Considering that the downlink signal of low-orbit satellites is in TDM (Time Division Modulation) mode and usually appears in burst form, once the ground terminal equipment detects the downlink signal, it needs to intercept the entire burst in order to receive the downlink signal completely, and perform preprocessing such as downconversion and filtering on the received signal.

[0041] The synchronization sequence in the downlink signal of a low-Earth orbit internet satellite is closely related to the local reference sequence. To ensure accurate initial synchronization with ground terminal equipment, the following synchronization sequences are pre-constructed before the signal segment of the downlink signal:

[0042]

[0043] Where, p s (x) represents the time-domain waveform of multiple synchronization sequences in the downlink signal, 1 p (x) is the indicator function, where x represents the sampling point, 1 ≤ x ≤ M, M is the total length of the synchronization sequence, Q is the number of synchronization sequences, M = Q * N, and N is the length of the local reference sequence. This indicates that the process repeats every N; when 1 ≤ x < N, 1 p The value of (x) is 1 when N≤x≤M. p The value of (x) is 0.

[0044] The downlink signal of low-Earth orbit (LEO) internet satellites has a cyclic prefix, also known as the primary synchronization subsequence. Assuming eight synchronization sequences are constructed in the downlink signal, Q=8, then the primary synchronization subsequence is repeated eight times, with a phase reversal performed the first time. The primary synchronization subsequence... It is obtained by symmetrical DPSK modulation of a fixed-length m-sequence.

[0045] By pre-constructing multiple synchronization sequences before the downlink signal segment, with each synchronization sequence having the same length as the local reference sequence, the multiple synchronization sequences have good autocorrelation characteristics. Compared to a traditional single synchronization sequence, they can achieve high Doppler estimation accuracy even at low signal-to-noise ratio (SNR) through coherent accumulation, thus enabling precise time-frequency synchronization.

[0046] S3, perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and sequentially perform cross-correlation processing between each truncation signal sequence and the local reference sequence.

[0047] The satellite downlink signal, after preprocessing such as filtering and downconversion, is subjected to sliding truncation. Each truncation is of the same length as the local reference sequence. Then, the truncation signal sequence is cross-correlated with the local reference sequence.

[0048] This step S3 specifically includes:

[0049] The preprocessed downlink signal s(t) is truncated by sliding, and each truncation is of the same length as the local reference sequence and is N, where 1≤t≤T, and T is the sequence length of the downlink signal;

[0050] Each extracted signal sequence is sequentially cross-correlated with the local reference sequence, as shown in the following formula:

[0051]

[0052] Where p(k) is the local reference sequence, m represents the range of the sliding cut, and s(m+k) is the signal sequence obtained by the sliding cut. * This indicates finding the conjugate, |x| 2 This indicates the goal of finding the energy of x.

[0053] S4. Perform peak detection on the cross-correlation processing results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence. The starting position of the signal segment of the downlink signal is determined based on the peak position of the correlation peak of the last synchronization sequence.

[0054] Considering that multiple synchronization sequences are constructed in the downlink signal of this application, multiple correlation peaks will appear. If the downlink signal is sent to the local receiving terminal, the peak value of the consecutive correlation peaks will be greater than the detection threshold through peak detection.

[0055] Figure 2 This illustration shows a schematic diagram of the cross-correlation processing results between the local reference sequence and the satellite downlink signal provided in an embodiment of this application, as shown below. Figure 2 As shown, eight distinct correlation peaks can be observed. This indicates that the downlink signal contains multiple synchronization sequences that match the local reference sequence. Furthermore, the starting position of the downlink signal segment can be determined based on the peak position of the correlation peak of the last synchronization sequence. Thus, stable initial synchronization between the low-Earth orbit internet satellite and ground terminal equipment is achieved in high-dynamic scenarios with large Doppler shifts.

[0056] In some preferred embodiments of this application, the method further includes the steps of:

[0057] S5, when it is determined that the downlink signal contains a synchronization sequence that matches the local reference sequence, multiple sets of synchronization sequences are extracted from the downlink signal according to the peak detection result of the correlation peak, the phase change rate of the multiple sets of synchronization sequences is solved to obtain the frequency offset estimation result, and the received downlink signal is subjected to phase correction processing according to the frequency offset estimation result.

[0058] This step S5 specifically includes:

[0059] The frequency offset estimation result is obtained by solving the phase change rate of multiple sets of synchronization sequences using the following formula:

[0060]

[0061]

[0062] Where r(i,n) represents multiple sets of synchronization sequences extracted from the downlink signal s(t), Q is the number of synchronization sequences, and T s Let be the sampling interval of the signal, N be the length of the synchronization sequence, z be the cross-correlation result between multiple synchronization sequences, and Δf be the frequency offset estimation result;

[0063] Based on the frequency offset estimation results, the received downlink signal is phase-corrected using the following formula:

[0064]

[0065] in, This represents the downlink signal after phase correction.

[0066] Thus, by solving the phase change rate of multiple synchronization sequences, the frequency offset estimation result is obtained. Based on the frequency offset estimation result, the phase offset of the received downlink signal is corrected, which can lay the groundwork for subsequent signal demodulation and decoding.

[0067] Similar to the aforementioned initial synchronization method, this application also provides an initial synchronization device. Figure 3 This illustration shows a schematic diagram of the structure of an initial synchronization device for a low-Earth orbit internet satellite communication system according to an embodiment of this application. The device of this embodiment is deployed on a ground terminal device and includes the following modules:

[0068] The local reference sequence generation module 31 is used to generate a fixed-length m-sequence and perform symmetric differential phase shift keying (DPSK) modulation on the m-sequence to generate a local reference sequence.

[0069] The monitoring module 32 is used to monitor the downlink signals transmitted by the low-orbit internet satellite. Once the downlink signals are detected, they are fully received and preprocessed by downconversion and filtering. The downlink signals are pre-constructed with multiple synchronization sequences before the signal segments, and the length of each synchronization sequence is the same as the length of the local reference sequence.

[0070] The cross-correlation processing module 33 is used to perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and to perform cross-correlation processing on the signal sequence obtained each time with the local reference sequence in sequence.

[0071] The peak detection module 34 is used to perform peak detection on the cross-correlation processing results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence, and the starting position of the signal segment of the downlink signal is determined according to the peak position of the correlation peak of the last synchronization sequence.

[0072] In some embodiments, see still Figure 3 As shown, the initial synchronization device in this application embodiment further includes:

[0073] The phase correction module 35 is used to extract multiple sets of synchronization sequences from the downlink signal based on the peak detection result of the correlation peak when it is determined that the downlink signal contains a synchronization sequence that matches the local reference sequence, solve the phase change rate of the multiple sets of synchronization sequences to obtain the frequency offset estimation result, and perform phase correction processing on the received downlink signal based on the frequency offset estimation result.

[0074] Figure 3 The implementation process of each module in the device can be found in the aforementioned method embodiments, and will not be repeated here.

[0075] Similar to the aforementioned initial synchronization method, this application also provides a low-orbit internet satellite communication system. Figure 4 A schematic diagram of the architecture of a low-Earth orbit (LEO) internet satellite communication system provided in an embodiment of this application is shown. The system of this embodiment includes a LEO internet satellite 40 and multiple ground terminal devices (41, 42, 43, ...), wherein:

[0076] Each of the ground terminal devices (41, 42, 43, ...) is used to generate a fixed-length m-sequence and perform symmetric differential phase shift keying (DPSK) modulation on the m-sequence to generate a local reference sequence;

[0077] The low-orbit internet satellite 40 is used to transmit downlink signals to multiple ground terminal devices. The downlink signal has multiple synchronization sequences pre-constructed before the signal segment. Each synchronization sequence has the same length and matches a local reference sequence generated by a ground terminal device.

[0078] Each of the ground terminal devices (41, 42, 43, ...) is also used to monitor the downlink signals transmitted by the low-orbit internet satellite 40. Once detected, the downlink signals are fully received and preprocessed by downconversion and filtering. The preprocessed downlink signals are then truncated, with each truncation length being the same as the length of the local reference sequence. The truncated signal sequences are then cross-correlated with the local reference sequence in sequence. Peak detection is performed on the cross-correlation results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signals contain multiple synchronization sequences that match the local reference sequence. The starting position of the signal segment of the downlink signal is determined based on the peak position of the correlation peak of the last synchronization sequence.

[0079] Furthermore, each of the ground terminal devices (41, 42, 43, ...) is also used to extract multiple sets of synchronization sequences from the downlink signal based on the peak detection result of the correlation peak when it is determined that the downlink signal contains a synchronization sequence that matches the local reference sequence, solve the phase change rate of the multiple sets of synchronization sequences to obtain the frequency offset estimation result, and perform phase correction processing on the received downlink signal based on the frequency offset estimation result.

[0080] Figure 4 The implementation process of the system shown can be found in the aforementioned method embodiments, and will not be repeated here.

[0081] In summary, ground terminal equipment generates a local reference sequence by performing symmetrical DPSK modulation on a fixed-length m-sequence. Compared with conventional binary modulation, this not only reduces spectral leakage but also improves the estimation accuracy of Doppler and the timing uncertainty commonly found in satellite communications. The ground terminal equipment monitors and receives downlink signals transmitted by low-Earth orbit internet satellites. Multiple synchronization sequences are pre-constructed before the signal segments of these downlink signals. Each synchronization sequence has the same length as the local reference sequence, giving the multiple synchronization sequences good autocorrelation characteristics. Compared to the traditional method of constructing only a single synchronization sequence, this method allows for coherent accumulation, achieving high Doppler estimation accuracy even at low signal-to-noise ratios (SNR). This enables precise time-frequency synchronization. The ground terminal equipment performs sliding truncation on the received downlink signal, with each truncation length being the same as the length of the local reference sequence. The truncation sequence is then cross-correlated with the local reference sequence. By detecting the peak values ​​of the correlation peaks, it is determined whether the downlink signal contains a synchronization sequence that matches the local reference sequence. If the peak values ​​of consecutive correlation peaks exceed the detection threshold, the starting position of the downlink signal segment is determined based on the peak position of the correlation peak of the last synchronization sequence. This achieves stable initial synchronization between the low-Earth orbit internet satellite and the ground terminal equipment in high-dynamic scenarios with large Doppler shifts, laying the groundwork for subsequent signal demodulation and decoding.

[0082] Finally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0083] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A method for initial synchronization of a low earth orbit internet satellite communication system, characterized by, The method is executed by ground terminal equipment and includes the following steps: S1, Generate a fixed-length m-sequence, and perform symmetric differential phase shift keying (DPSK) modulation on the m-sequence to generate a local reference sequence; S2, monitor the downlink signal transmitted by the low-orbit internet satellite. Once detected, receive the downlink signal completely and perform down-conversion and filtering preprocessing. The downlink signal is preceded by multiple synchronization sequences, and the length of each synchronization sequence is the same as the length of the local reference sequence, so that the multiple synchronization sequences have autocorrelation characteristics. S3, perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and perform cross-correlation processing between each truncation signal sequence and the local reference sequence in sequence; S4. Perform peak detection on the cross-correlation processing result. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence. The starting position of the signal segment of the downlink signal is determined according to the peak position of the correlation peak of the last synchronization sequence. Step S1 specifically includes: A fixed-length m-sequence is generated by a linear feedback shift register (LFSR); Symmetrical DPSK modulation is applied to the m-sequence to obtain the time-domain waveform of the local reference sequence: ； ； wherein, is a fixed length m-sequence, is a m-sequence after symmetric DPSK modulation, is a local reference sequence, is a length of the local reference sequence, denotes a sampling point, .

2. The method of claim 1, wherein, The following multiple synchronization sequences are pre-constructed before the signal segment of the downlink signal: ； wherein is a time-domain waveform of the plurality of synchronization sequences in the downlink signal, is an indicator function, denotes a sampling point, , is a total length of the synchronization sequence, is a number of the synchronization sequences, , is a length of the local reference sequence, denotes every repetition once; when , the value of is 1 when , the value of is 0.

3. The method according to claim 2, characterized in that, Step S3 specifically includes: For the preprocessed downlink signal Perform sliding truncation, with each truncation having the same length as the local reference sequence. ,in , The sequence length of the downlink signal; Each extracted signal sequence is sequentially cross-correlated with the local reference sequence, as shown in the following formula: ； in For local reference sequence, Indicates the range of the sliding cutoff. The signal sequence is obtained by sliding cut-off. This indicates finding the conjugate. Indicates request Energy.

4. The method according to any one of claims 1-3, characterized in that, The method further includes the following steps: S5, when it is determined that the downlink signal contains a synchronization sequence that matches the local reference sequence, multiple sets of synchronization sequences are extracted from the downlink signal according to the peak detection result of the correlation peak, the phase change rate of the multiple sets of synchronization sequences is solved to obtain the frequency offset estimation result, and the received downlink signal is subjected to phase correction processing according to the frequency offset estimation result.

5. The method according to claim 4, characterized in that, Step S5 specifically includes: The frequency offset estimation result is obtained by solving the phase change rate of the multiple sets of synchronization sequences using the following formula: ； ； in, Indicates from downlink signal Multiple sets of synchronization sequences were extracted from the data. The number of synchronization sequences. The sampling interval of the signal. The length of the synchronization sequence. The results of cross-correlation between multiple sets of synchronization sequences, This is the result of frequency offset estimation; Based on the frequency offset estimation results, the downlink signal of the low-Earth orbit internet satellite is phase-corrected using the following formula: ； in, This represents the downlink signal after phase correction.

6. An initial synchronization device for a low-Earth orbit internet satellite communication system, characterized in that, The device is deployed on a ground terminal device and includes the following modules: A local reference sequence generation module is used to generate a fixed-length m-sequence and perform symmetric differential phase shift keying (DPSK) modulation on the m-sequence to generate a local reference sequence. The monitoring module is used to monitor the downlink signals transmitted by low-Earth orbit internet satellites. Once detected, the downlink signals are received completely and preprocessed by downconversion and filtering. The downlink signals are pre-constructed with multiple synchronization sequences before the signal segments, and the length of each synchronization sequence is the same as the length of the local reference sequence, so that the multiple synchronization sequences have autocorrelation characteristics. The cross-correlation processing module is used to perform sliding truncation on the preprocessed downlink signal, with each truncation length being the same as the length of the local reference sequence, and to perform cross-correlation processing on the signal sequence obtained each time with the local reference sequence in sequence. The peak detection module is used to perform peak detection on the cross-correlation processing results. If the peak value of consecutive correlation peaks is greater than the detection threshold, it is determined that the downlink signal contains multiple synchronization sequences that match the local reference sequence, and the starting position of the signal segment of the downlink signal is determined according to the peak position of the correlation peak of the last synchronization sequence. The local reference sequence generation module is also used for: A fixed-length m-sequence is generated by a linear feedback shift register (LFSR); Symmetrical DPSK modulation is applied to the m-sequence to obtain the time-domain waveform of the local reference sequence: ； ； in, Given a fixed-length m-sequence, The m-sequence after symmetric DPSK modulation. For local reference sequence, The length of the local reference sequence. Indicates the sampling point. .

7. The apparatus according to claim 6, characterized in that, The device further includes: The phase correction module is used to extract multiple sets of synchronization sequences from the downlink signal based on the peak detection result of the correlation peak when it is determined that the downlink signal contains a synchronization sequence that matches the local reference sequence, solve the phase change rate of the multiple sets of synchronization sequences to obtain the frequency offset estimation result, and perform phase correction processing on the received downlink signal based on the frequency offset estimation result.

8. A low-Earth orbit internet satellite communication system, characterized in that, The system includes low-Earth orbit internet satellites and multiple ground terminal devices, wherein: Each of the ground terminal devices is used to generate a fixed-length m-sequence, and to perform symmetric differential phase shift keying (DPSK) modulation on the m-sequence to generate a local reference sequence; The low-orbit internet satellite is used to transmit downlink signals to multiple ground terminal devices. The downlink signal has multiple synchronization sequences pre-constructed before the signal segment. Each synchronization sequence has the same length and matches a local reference sequence generated by a ground terminal device, so that the multiple synchronization sequences have autocorrelation characteristics. Each of the aforementioned ground terminal devices is also used to monitor the downlink signals transmitted by the low-orbit internet satellite. Once detected, the downlink signals are received completely and preprocessed by downconversion and filtering. The preprocessed downlink signals are then truncated, with each truncation length being the same as the length of the local reference sequence. The truncated signal sequences are then cross-correlated with the local reference sequence in sequence. Peak detection is performed on the cross-correlation results. If the peak values ​​of consecutive correlation peaks are greater than the detection threshold, it is determined that the downlink signals contain multiple synchronization sequences that match the local reference sequence. The starting position of the signal segment of the downlink signal is determined based on the peak position of the correlation peak of the last synchronization sequence. The low-Earth orbit internet satellite is also used to generate a fixed-length m-sequence from a linear feedback shift register (LFSR); the m-sequence is then subjected to symmetrical DPSK modulation to obtain the time-domain waveform of a local reference sequence. ； ； in, Given a fixed-length m-sequence, The m-sequence after symmetric DPSK modulation. For local reference sequence, The length of the local reference sequence. Indicates the sampling point. .

9. The system according to claim 8, characterized in that, Each of the aforementioned ground terminal devices is further configured to, when determining that the downlink signal contains a synchronization sequence matching the local reference sequence, extract multiple sets of synchronization sequences from the downlink signal based on the peak detection result of the relevant peak, solve the phase change rate of the multiple sets of synchronization sequences to obtain a frequency offset estimation result, and perform phase correction processing on the received downlink signal based on the frequency offset estimation result.

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