A distributed multi-satellite link cooperative transmission timing synchronization method

Abstract

The application discloses a kind of distributed multi-star link cooperative transmission timing synchronization method, it is applicable to low-orbit satellite communication and high dynamic node networking scene.The method includes: each satellite node is based on GNSS tamed clock generation synchronization sampling clock signal;According to ephemeris information, initial pre-compensation is carried out, and baseband signal containing independent non-overlapping DMRS is generated;Multi-star downlink signal is superimposed to form composite signal after space propagation in ground receiving end;Ground receiver carries out ADC sampling and OFDM demodulation to composite signal, extracts each link DMRS and carries out channel estimation, obtains each link delay deviation information;Delay deviation is fed back to corresponding satellite node through uplink;Each satellite node adjusts transmission time point offset, sampling clock phase and symbol timing alignment position according to this, forms closed loop delay synchronization control.The application overcomes the influence of inter-satellite delay difference on cooperative transmission, reduces on-board synchronization requirement, and is applicable to mobile phone direct connection satellite and the like scene.

Description

Technical Field

[0001] This invention relates to the field of low-Earth orbit satellite communication system applications, and proposes a distributed multi-satellite link collaborative transmission timing synchronization method, which is suitable for dense ultra-large-scale satellite constellation systems, and is especially suitable for vertical application scenarios such as direct mobile phone connection to satellites. Background Technology

[0002] To address the aforementioned issues, in recent years, the industry has proposed utilizing multiple satellites distributed in different spatial locations to collaboratively serve the same terminal, namely, using distributed MIMO technology to improve satellite-side spectral efficiency and increase satellite-to-ground link transmission rates. This has led to the development of distributed multi-satellite collaborative transmission systems. However, in these systems, the differentiated propagation paths between each satellite link and the user terminal introduce significant relative link delays. When employing multi-carrier modulation techniques such as orthogonal frequency division multiplexing (OFDM), this delay disrupts the timing synchronization of signals from multiple transmission links at the receiving end, resulting in inter-symbol / carrier interference. Simultaneously, the dynamically changing link delays further couple with the Doppler frequency shift caused by the high-speed motion of the satellites, exacerbating time-frequency selective fading and severely limiting system throughput and transmission reliability. Therefore, delay management and compensation between collaborative links are key challenges in achieving effective alignment of multi-satellite signals in the time-frequency domain and fully utilizing spatial diversity gain. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes a distributed multi-satellite link cooperative transmission timing synchronization method. It is applicable not only to vertical application scenarios such as direct satellite connection between mobile phones and satellites, but also to distributed node cooperative transmission systems with highly dynamic node networking, such as drones. It enables precise control of dynamic time delay differences between dynamic multi-node satellite-to-ground / air-to-ground links, ensuring that distributed multi-link signals arrive at the receiving end simultaneously and avoiding inter-symbol / inter-carrier interference caused by time delay differences.

[0004] The specific technical solution adopted in this invention is as follows: A distributed multi-satellite link cooperative transmission timing synchronization method includes the following steps; Step 1: Equip each satellite node with a GNSS module. Each satellite node generates a synchronized sampling clock signal based on the high-precision time reference provided by the GNSS module's disciplined clock. Step 2: Each satellite node performs initial pre-compensation on the transmitted signal using the GNSS module and ephemeris information and generates a baseband signal. The baseband signal includes an independent and non-overlapping DMRS assigned to each satellite. Step 3: Each satellite node transmits downlink baseband signals, which, after propagating through space, undergo natural electromagnetic superposition at the antenna of the ground receiving end to form a composite signal; the composite signal simultaneously contains information from all satellite links. Step 4: The ground receiver performs ADC sampling on the composite signal and sequentially performs symbol synchronization, cyclic prefix removal, serial-to-parallel conversion, and FFT transformation to convert the time-domain superimposed signal into received symbols on the frequency-domain resource grid. Step 5: On the acquired frequency domain resource grid, extract the DMRS corresponding to each link and perform channel estimation and equalization to obtain the time delay deviation information corresponding to each link. Step 6: The ground terminal feeds back the estimated delay deviation information of each link to the corresponding satellite node through the uplink; Step 7: Each satellite node dynamically adjusts its own launch parameters, including launch time offset, sampling clock phase and symbol timing alignment position, based on the feedback time delay deviation information, to achieve closed-loop time delay synchronization control.

[0005] Furthermore, in step 2, Based on the ephemeris information of each satellite node, the relativistic effect and the Doppler frequency shift caused by satellite motion are calculated, and the sampling clock signal generated in step 1 is phase-adjusted to compensate for the code rate deviation caused by the relativistic Doppler effect. Based on the geometric distance between each satellite node and the ground terminal, the transmission timestamp is aligned with an integer multiple of the sampling interval to ensure that the pre-compensated multi-link downlink signal can reach the ground terminal synchronously after propagation in space.

[0006] Furthermore, in step 2, The transmitters of each satellite node all use CP-OFDM waveforms; the CP-OFDM waveforms allocate idle resource elements, DMRS resource elements and data resource elements through a time-frequency two-dimensional grid; among them, the DMRS resource elements correspond to each satellite node, are independent and do not overlap; the total number of the DMRS resource elements meets the requirements of the terrestrial 5G NR protocol, and is in a sparse regular pattern in the time-frequency two-dimensional grid. Furthermore, in step 4, the ground receiver sequentially performs the following processing steps on the composite signal: first, it performs symbol synchronization and detects the start position of each OFDM symbol to align the FFT window; then, it removes the cyclic prefix and retains the valid OFDM symbols; next, it performs serial-to-parallel conversion to organize the serial time-domain samples into parallel blocks; finally, it performs FFT transformation to transform the time-domain OFDM symbols to the frequency domain, recovers the modulation symbols on each subcarrier, and outputs the received symbols on the frequency domain resource grid.

[0007] Furthermore, in step 5, Step 501: The ground receiver extracts the DMRS corresponding to each satellite on the frequency domain resource grid according to the independent DMRS allocated to each satellite node, and inputs the extracted DMRS into the least squares discrete Fourier transform channel estimator. After denoising and fitting operations, the channel frequency response of each satellite node and the time-frequency two-dimensional grid position of each satellite DMRS are obtained. Step 502: Calculate the vector sum of the frequency response of all satellite node links to obtain the equalizer tap coefficients, and extract the residual delay deviation of each link relative to the reference link by analyzing the relationship between the phase frequency response of each satellite node channel and the corresponding residual delay deviation.

[0008] Furthermore, in step 6, the specific method by which the ground terminal feeds back the obtained delay deviation information of each link to the corresponding satellite node through the uplink is as follows: the ground terminal encapsulates the satellite identifier of each link, the corresponding delay deviation value and the timestamp into an uplink feedback signaling and sends it to each satellite node; after receiving the feedback signaling through its own receiver, each satellite node extracts the delay deviation information corresponding to its own node.

[0009] Furthermore, in step 7, Step 701: Each satellite node dynamically adjusts its own transmission parameters based on the feedback delay deviation information, including the transmission time offset, sampling clock phase, and symbol timing alignment position. The transmission time offset is used to compensate for the residual propagation delay difference, the sampling clock phase adjustment is used to compensate for small time drifts, and the symbol timing alignment position is used to ensure consistency with the integer multiple alignment in Step 2. Step 702: By dynamically adjusting and coordinating with the initial pre-compensation in Step 2, a closed-loop feedback mechanism is formed to realize dynamic tracking and compensation of link delay; wherein, the adjusted transmission parameters will be used as the input of Step 2 in the next round of transmission, thereby realizing a closed-loop control loop.

[0010] Compared with the prior art, the beneficial effects of the present invention using the above technical solution are as follows: This invention discloses a distributed multi-satellite link cooperative transmission timing synchronization method, which addresses the impact of dynamic time delay differences between multiple links on the system's cooperative performance in multi-satellite cooperative transmission scenarios. Through a specially designed pilot pattern, the system can simultaneously estimate the channel impulse response of each downlink after interference superposition. This allows ground users to calculate the delay of each satellite link and then transmit the delay residuals back to multiple satellite nodes. The satellite terminal dynamically updates the delay compensation parameters in real time through a delay pre-compensation module, ensuring accurate synchronization when multiple satellite signals arrive at the ground receiver.

[0011] To address the dynamic time delay difference problem between multiple satellite links in a distributed multi-satellite cooperative transmission system, this patent proposes a timing synchronization method for distributed multi-satellite link cooperative transmission. This method utilizes a dedicated pilot design to achieve parallel high-precision estimation of the impulse responses of multiple downlink channels at the receiver, and then dynamically updates the on-board time delay pre-compensation parameters through a closed-loop feedback mechanism. Its core advantage lies in overcoming the dependence of traditional synchronization methods on precise inter-satellite clock synchronization. Under non-ideal inter-satellite synchronization conditions, it can achieve high-precision alignment of multi-satellite signal arrival times with the assistance of ground terminals, significantly reducing the system's requirements for on-board processing capabilities and providing a feasible synchronization solution for large-scale satellite cooperative transmission. Attached Figure Description

[0012] Figure 1 This is a structural diagram of the method according to an embodiment of the present invention. Detailed Implementation

[0013] To more clearly illustrate the present invention, the following description, in conjunction with embodiments and accompanying drawings, further clarifies the invention. Those skilled in the art should understand that the specific descriptions below are illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0014] Reference Figure 1 A distributed multi-satellite link cooperative transmission timing synchronization method includes the following steps: Step 1: Each satellite node generates a synchronous sampling clock signal based on the GNSS module's disciplined clock.

[0015] In this embodiment, the system includes at least two low-Earth orbit (LEO) satellite nodes and one ground terminal. Each satellite node is equipped with a GNSS module. Each satellite node receives GNSS signals through its GNSS module, disciplines its local crystal oscillator, and obtains a high-precision time reference. Based on this time reference, each satellite node generates a synchronized sampling clock signal to control the sampling timing of the transmitter's DAC and the receiver's ADC. The sampling clock frequencies of all satellite nodes are traceable to the same GNSS time reference, thereby achieving clock frequency synchronization among multiple nodes.

[0016] Step 2: Each satellite node performs initial pre-compensation and generates a baseband signal containing an independent DMRS.

[0017] Each satellite node performs the following initial pre-compensation operations based on its own position and ephemeris information provided by the GNSS module: Calculate the relativistic effects and Doppler frequency shift caused by the high-speed motion of the satellite, and perform phase adjustment on the sampling clock signal generated in step 1 to compensate for the code rate deviation caused by the relativistic Doppler effect.

[0018] Based on the geometric distance between each satellite node and the ground terminal, the signal propagation delay is calculated, and the transmission timestamp is aligned with an integer multiple of the sampling interval to ensure that the pre-compensated multi-link downlink signal can reach the ground terminal synchronously after propagation in space.

[0019] After pre-compensation, each satellite node generates a baseband signal. This baseband signal uses a CP-OFDM waveform, and each satellite is assigned an independent and non-overlapping demodulation reference signal (DMRS) on a time-frequency two-dimensional resource grid. The total number of DMRS resource elements conforms to the requirements of the terrestrial 5G NR protocol and is distributed in a sparse and regular pattern in the time-frequency two-dimensional grid. The DMRS of each satellite do not overlap with each other in the frequency or time domain, so that the receiver can distinguish between different links.

[0020] Step 3: Each satellite node transmits downlink baseband signals to form a composite signal.

[0021] Each satellite node simultaneously transmits downlink baseband signals according to the transmission parameters determined in step 2. After propagating through space, the signals undergo natural electromagnetic superposition at the antenna of the ground receiving end, forming a composite signal. This composite signal simultaneously contains the transmission information of all satellite links.

[0022] Step 4: The ground receiver performs ADC sampling and OFDM demodulation on the composite signal.

[0023] The ground receiver performs the following processing steps on the received composite signal in sequence: ADC sampling: converts radio frequency composite signals into digital baseband signals.

[0024] Symbol synchronization: Detect the starting position of each OFDM symbol and determine the alignment point of the FFT window.

[0025] Cyclic prefix removal: Removes the cyclic prefix before each OFDM symbol, preserving the valid symbol range.

[0026] Serial-to-parallel conversion: Converts serial time-domain samples into parallel block structures.

[0027] FFT transform: Transforms the time-domain OFDM symbols to the frequency domain, recovers the modulation symbols on each subcarrier, and outputs the received symbols on the frequency domain resource grid.

[0028] Step 5: Extract the DMRS of each link and perform channel estimation to obtain delay deviation information.

[0029] On the acquired frequency domain resource grid, the ground receiver performs the following operations: Step 501: Extract the DMRS corresponding to each satellite based on the independent DMRS assigned to each satellite node. Input the extracted DMRS into the least squares discrete Fourier transform (LS-DFT) channel estimator. After denoising and fitting operations, obtain the channel frequency response of each satellite node and the time-frequency two-dimensional grid position of each satellite's DMRS.

[0030] Step 502: Calculate the vector sum of the frequency responses of all satellite node links to obtain the equalizer tap coefficients. By analyzing the relationship between the phase frequency response of each satellite node channel and the corresponding residual delay deviation, extract the residual delay deviation of each link relative to the reference link.

[0031] Step 6: The ground terminal feeds back the delay deviation information to the corresponding satellite node via the uplink.

[0032] The ground terminal feeds back the estimated delay deviation information for each link obtained in step 5 to the corresponding satellite nodes. Specifically, the ground terminal encapsulates the satellite identifier, corresponding delay deviation value, and current timestamp for each link into an uplink feedback signaling message, which is then sent to each satellite node via the uplink. Each satellite node receives this feedback signaling message through its own receiver, parses it, and extracts the delay deviation information corresponding to its node.

[0033] Step 7: Each satellite node dynamically adjusts its launch parameters to achieve closed-loop time delay synchronization control.

[0034] Each satellite node dynamically adjusts its own transmission parameters based on the time delay deviation information fed back in step 6, specifically including: Step 701: Adjust the transmission time offset to compensate for the residual propagation delay difference; adjust the sampling clock phase to compensate for minor time drift; adjust the symbol timing alignment position to ensure consistency with the integer multiple alignment in Step 2.

[0035] Step 702: By combining the above dynamic adjustment with the initial pre-compensation in Step 2, a closed-loop feedback mechanism is formed to achieve dynamic tracking and compensation of link delay. The adjusted transmission parameters will serve as the input for Step 2 in the next round of transmission, thereby forming a closed-loop control loop and continuously maintaining the synchronous arrival of multi-link signals at the ground receiving end.

[0036] The above description is merely a preferred embodiment of the present invention and is 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 distributed multi-satellite link cooperative transmission timing synchronization method, characterized in that, Includes the following steps; Step 1: Equip each satellite node with a GNSS module. Each satellite node generates a synchronized sampling clock signal based on the high-precision time reference provided by the GNSS module's disciplined clock. Step 2: Each satellite node performs initial pre-compensation on the transmitted signal using the GNSS module and ephemeris information and generates a baseband signal. The baseband signal includes an independent and non-overlapping DMRS assigned to each satellite. Step 3: Each satellite node transmits downlink baseband signals, which, after propagating through space, undergo natural electromagnetic superposition at the antenna of the ground receiving end to form a composite signal; the composite signal simultaneously contains information from all satellite links. Step 4: The ground receiver performs ADC sampling on the composite signal and sequentially performs symbol synchronization, cyclic prefix removal, serial-to-parallel conversion, and FFT transformation to convert the time-domain superimposed signal into received symbols on the frequency-domain resource grid. Step 5: On the acquired frequency domain resource grid, extract the DMRS corresponding to each link and perform channel estimation and equalization to obtain the time delay deviation information corresponding to each link. Step 6: The ground terminal feeds back the estimated delay deviation information of each link to the corresponding satellite node through the uplink; Step 7: Each satellite node dynamically adjusts its own launch parameters, including launch time offset, sampling clock phase and symbol timing alignment position, based on the feedback time delay deviation information, to achieve closed-loop time delay synchronization control.

2. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 2, Based on the ephemeris information of each satellite node, the relativistic effect and the Doppler frequency shift caused by satellite motion are calculated, and the sampling clock signal generated in step 1 is phase-adjusted to compensate for the code rate deviation caused by the relativistic Doppler effect. Based on the geometric distance between each satellite node and the ground terminal, the transmission timestamp is aligned with an integer multiple of the sampling interval to ensure that the pre-compensated multi-link downlink signal can reach the ground terminal synchronously after propagation in space.

3. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 2, Each satellite node's transmitter uses CP-OFDM waveform; the CP-OFDM waveform allocates idle resource elements, DMRS resource elements, and data resource elements through a time-frequency two-dimensional grid; among them, the DMRS resource elements correspond to each satellite node, are independent and do not overlap; the total number of the DMRS resource elements meets the requirements of the terrestrial 5G NR protocol, and presents a sparse regular pattern in the time-frequency two-dimensional grid.

4. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 4, the ground receiver performs the following processing steps on the composite signal in sequence: First, it performs symbol synchronization and detects the start position of each OFDM symbol to align the FFT window; Then, the cyclic prefix is ​​removed, and valid OFDM symbols are retained. Next, serial-to-parallel conversion is performed to organize the serial time-domain samples into parallel blocks. Finally, FFT transformation is performed to transform the time-domain OFDM symbols to the frequency domain, recover the modulation symbols on each subcarrier, and output the received symbols on the frequency domain resource grid.

5. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 5 Step 501: The ground receiver extracts the DMRS corresponding to each satellite on the frequency domain resource grid according to the independent DMRS allocated to each satellite node, and inputs the extracted DMRS into the least squares discrete Fourier transform channel estimator. After denoising and fitting operations, the channel frequency response of each satellite node and the time-frequency two-dimensional grid position of each satellite DMRS are obtained. Step 502: Calculate the vector sum of the frequency response of all satellite node links to obtain the equalizer tap coefficients, and extract the residual delay deviation of each link relative to the reference link by analyzing the relationship between the phase frequency response of each satellite node channel and the corresponding residual delay deviation.

6. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 6, the ground terminal feeds back the obtained delay deviation information of each link to the corresponding satellite node through the uplink in the following way: the ground terminal encapsulates the satellite identifier, the corresponding delay deviation value and the timestamp of each link into an uplink feedback signaling and sends it to each satellite node; each satellite node receives the feedback signaling through its own receiver and extracts the delay deviation information corresponding to its own node.

7. The distributed multi-satellite link cooperative transmission timing synchronization method according to claim 1, characterized in that, In step 7 Step 701: Each satellite node dynamically adjusts its own transmission parameters based on the feedback delay deviation information, including the transmission time offset, sampling clock phase, and symbol timing alignment position. The transmission time offset is used to compensate for the residual propagation delay difference, the sampling clock phase adjustment is used to compensate for small time drifts, and the symbol timing alignment position is used to ensure consistency with the integer multiple alignment in Step 2. Step 702: By dynamically adjusting and coordinating with the initial pre-compensation in Step 2, a closed-loop feedback mechanism is formed to realize dynamic tracking and compensation of link delay; wherein, the adjusted transmission parameters will be used as the input of Step 2 in the next round of transmission, thereby realizing a closed-loop control loop.

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