Space-air communication system and frequency offset correction method of space-air communication system

Abstract

The application relates to the field of satellite communication and discloses a space-air communication system and a frequency offset correction method of the space-air communication system. The method comprises the following steps: a satellite transmitting end calculates a beam direction vector based on received beam center position information, calculates a Doppler frequency offset pre-compensation value based on ephemeris information, and performs Doppler frequency offset pre-compensation on a transmitting signal; a ground receiving end establishes initial time synchronization and cell synchronization with the satellite transmitting end, demodulates a system information block, and obtains satellite ephemeris information; the ground receiving end performs frequency offset equation fitting based on self position information and satellite ephemeris information, and obtains a frequency offset fitting equation; the ground receiving end calculates a frequency offset estimation value, corrects the frequency offset estimation value based on the frequency offset fitting equation, and obtains a frequency offset correction value. The application provides an efficient frequency offset tracking method which can integrate ephemeris information, synchronization information, spatial position and dynamic state equation, so as to solve the problem of inaccurate frequency offset estimation and compensation in low-orbit satellite communication.

Description

Technical Field

[0001] This application relates to the field of satellite communication technology, and more specifically, to an aerospace communication system and a frequency offset correction method for the aerospace communication system. Background Technology

[0002] With the continuous development of global mobile communication technology, low-Earth orbit (LEO) satellite communication, as an important component of non-terrestrial networks (NTNs), is widely used for wireless information transmission over wide coverage areas. Compared to traditional terrestrial cellular networks, LEO satellite NTN communication can achieve seamless connectivity in remote areas, oceans, and aerospace scenarios that terrestrial base stations cannot cover, effectively improving network service capabilities and coverage.

[0003] The core architecture of a low-Earth orbit (LEO) satellite communication system consists of three parts: a LEO satellite constellation, user terminals (such as portable terminals, vehicle-mounted terminals, and shipborne terminals), and ground gateway stations. LEO satellites, acting as airborne signal transmitters and relay nodes, operate at high speeds along near-Earth orbits, reaching speeds of 7-8 km / s, far exceeding those of ground vehicles and medium- and high-Earth orbit satellites. User terminals are responsible for receiving satellite signals and transmitting data to the satellites, while ground gateway stations handle data exchange, signal scheduling, and network management between the satellites and the ground core network. These three components work together to complete wireless information transmission.

[0004] Due to the low orbital altitude and high speed of low-Earth orbit (LEO) satellites, the relative motion between the satellite, user terminals, and ground gateways is constantly changing at high speed. This dynamic characteristic directly causes a significant Doppler frequency offset effect, which is a core technical bottleneck faced by LEO satellite NTN communication systems. Doppler frequency offset refers to the phenomenon that the frequency of the wave signal received by the observer deviates from the frequency of the wave source transmission when there is relative motion between the satellite and the user terminal. The magnitude of the deviation is positively correlated with the relative motion speed, the signal propagation direction, and the signal frequency.

[0005] For low-Earth orbit satellite communications, the impact of Doppler frequency offset persists throughout the entire signal transmission process. If the frequency offset is not effectively compensated, it will lead to increased demodulation errors at the receiver, decreased clock synchronization accuracy, resource allocation conflicts, and in severe cases, even signal loss, directly restricting the system's communication rate, transmission reliability, and service quality. Summary of the Invention

[0006] This application provides an aerospace communication system and a frequency offset correction method for the aerospace communication system, so as to at least solve the technical problem of inaccurate frequency offset estimation in low-Earth orbit satellite communication in related technologies.

[0007] According to one aspect of the embodiments of this application, a frequency offset correction method for an aerospace communication system is provided, comprising: The satellite transmitter calculates the beam direction vector based on the received beam center position information and calculates the Doppler frequency offset pre-compensation value based on the ephemeris information in order to perform Doppler frequency offset pre-compensation on the transmitted signal; The ground receiver establishes initial time synchronization and cell synchronization with the satellite transmitter, and demodulates system information blocks to obtain satellite ephemeris information; The ground receiving end performs frequency offset equation fitting based on its own position information and the satellite ephemeris information to obtain the frequency offset fitting equation; The ground receiver calculates the frequency offset estimate, and corrects the frequency offset estimate based on the frequency offset fitting equation to obtain the frequency offset correction value.

[0008] In this embodiment of the application, before the satellite transmitter calculates the beam direction vector based on the received beam center position information, it further includes: The gateway station broadcasts the beam center location information to the satellite transmitter via the feeder link.

[0009] In this embodiment of the application, the satellite transmitter calculates the beam direction vector based on the received beam center position information and calculates the Doppler frequency offset pre-compensation value based on ephemeris information, including: Based on the beam center position information, the satellite transmitter uses a preset beamforming algorithm to calculate the beam direction vector in order to adjust the antenna beam direction to align with the target cell. Based on the ephemeris information, the real-time position vector and real-time velocity vector of the satellite in the Earth-fixed coordinate system are calculated, and the position vector of the beam center and the direction vector of the satellite pointing to the beam center are determined in combination with the beam center position information. Based on the real-time motion velocity vector, the real-time position vector, the position vector of the beam center, the direction vector of the satellite pointing to the beam center, the speed of light, and the center frequency of the satellite radio frequency carrier signal, the Doppler frequency offset pre-compensation value is calculated to perform frequency offset pre-compensation on the transmitted signal.

[0010] In this embodiment of the application, the ground receiving end establishes initial time synchronization and cell synchronization with the satellite transmitting end, and demodulates system information blocks to obtain satellite ephemeris information, including: The ground receiving end receives the downlink pilot sequence sent by the satellite transmitting end, and establishes initial time synchronization and cell synchronization with the satellite transmitting end based on the downlink pilot sequence; The ground receiving end demodulates the system information block and extracts the satellite ephemeris information from the system information block; The ground receiving end calculates the cell identifier based on the downlink pilot sequence and performs an integer multiple frequency offset search based on the downlink pilot sequence.

[0011] In this embodiment of the application, the ground receiving end performs frequency offset equation fitting based on its own position information and the satellite ephemeris information to obtain a frequency offset fitting equation, including: The ground receiving end uses an orbit recursion algorithm to calculate the satellite's real-time position vector and real-time velocity vector in the Earth-fixed coordinate system based on the satellite ephemeris information. The ground receiver obtains positioning parameters based on its integrated GNSS equipment, and calculates the real-time position vector of the ground receiver in the Earth-fixed coordinate system based on the positioning parameters. The ground receiver determines the relative motion relationship between the satellite and the ground receiver based on the satellite's real-time position vector, real-time velocity vector, and the ground receiver's real-time position vector, and calculates the Doppler theoretical frequency offset based on the relative motion relationship. Based on the Doppler theory frequency offset, a frequency offset fitting equation is constructed to characterize the frequency offset variation over time.

[0012] In this embodiment of the application, the ground receiver calculates a frequency offset estimate, and corrects the frequency offset estimate based on the frequency offset fitting equation to obtain a frequency offset correction value, including: The ground receiver calculates the frequency offset estimate based on the received signal; The parameters of the frequency offset fitting equation and the time synchronization measurement values ​​are used as state variables to construct the system state transition model and observation model; Based on the system state transition model and the observation model, the frequency offset estimate is recursively corrected using a nonlinear filter to output the Doppler frequency offset correction value.

[0013] In this embodiment of the application, after the ground receiving end calculates the frequency offset estimate and corrects the frequency offset estimate based on the frequency offset fitting equation to obtain the frequency offset correction value, the method further includes: The ground receiver acquires the Doppler frequency offset correction value; The ground receiver uses a digital down-conversion module to compensate for the frequency offset of the baseband signal using the Doppler frequency offset correction value; or the ground receiver uses the Doppler frequency offset correction value to adjust the local oscillator to eliminate residual Doppler frequency offset in the received signal.

[0014] According to another aspect of the embodiments of this application, an aerospace communication system is provided, including a satellite transmitter and a ground receiver; The satellite transmitter is used to calculate the beam direction vector based on the received beam center position information and to calculate the Doppler frequency offset pre-compensation value based on the ephemeris information, so as to perform Doppler frequency offset pre-compensation on the transmitted signal. The ground receiving end is used to establish initial time synchronization and cell synchronization with the satellite transmitting end, and to demodulate system information blocks to obtain satellite ephemeris information; The ground receiving end is used to perform frequency offset equation fitting based on its own position information and the satellite ephemeris information to obtain the frequency offset fitting equation. The ground receiver is used to calculate the frequency offset estimate, and based on the frequency offset fitting equation, correct the frequency offset estimate to obtain the frequency offset correction value.

[0015] In this embodiment of the application, a gateway station is also included; The gateway station is used to broadcast the beam center location information to the satellite transmitter via the feeder link.

[0016] In this embodiment of the application, the ground receiving end is used to calculate a frequency offset estimate based on the received signal; The parameters of the frequency offset fitting equation and the time synchronization measurement values ​​are used as state variables to construct the system state transition model and observation model; Based on the system state transition model and the observation model, the frequency offset estimate is recursively corrected using a nonlinear filter to output the Doppler frequency offset correction value.

[0017] The technical solutions provided in this application embodiment may include the following beneficial effects: This application performs pre-compensation for frequency offset based on ephemeris information at the satellite end. The receiver establishes initial time synchronization and cell synchronization with the satellite transmitter. Then, it uses a frequency offset fitting equation to track relative motion changes in real time and update the frequency offset correction value. Compared with the existing single-point estimation mode, it can accurately capture the continuous frequency offset change trend caused by the high-speed motion of the satellite, avoid compensation lag, and improve the signal synchronization stability in high-dynamic scenarios. This application constructs a frequency offset fitting equation by integrating satellite ephemeris information, user location, synchronization information, etc., and then performs frequency offset correction. This effectively suppresses the influence of noise interference and ephemeris error. Compared with instantaneous estimation methods, it can correct errors through the correlation between historical and real-time data, significantly improving the accuracy of frequency offset estimation and meeting the stringent requirements of high-speed communication for frequency offset control. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a flowchart of a frequency offset correction method for an aerospace communication system according to an embodiment of this application; Figure 2 This is a schematic diagram of an aerospace communication system according to an embodiment of this application; Figure 3 This is a module composition diagram of the transmitter and receiver according to an embodiment of this application; Figure 4 This is a flowchart of the operation of an aerospace communication system according to an embodiment of this application; Figure 5 This is a flowchart of a frequency offset correction method for an aerospace communication system according to an embodiment of this application; Figure 6 This is a flowchart illustrating a frequency offset correction method for an aerospace communication system according to an embodiment of this application. Figure 7 This is a graph showing the impact of the change in the order of the fitting equation according to the embodiments of this application on the performance of the ephemeris-assisted frequency offset tracking algorithm; Figure 8 This is a graph used for performance evaluation of the frequency offset tracking algorithm according to an embodiment of this application; Figure 9 This is a block diagram of a frequency offset correction system for an aerospace communication system according to an embodiment of this application. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0020] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0021] like Figure 2As shown in the embodiment of this application, the space-air communication system includes a low-Earth orbit (LEO) satellite transmitter, a user receiver, and a satellite gateway station. The LEO satellite transmitter is equipped with a transmitting antenna for transmitting signals; the user receiver is equipped with a receiving antenna for receiving signals; the satellite gateway station is communicatively connected to the LEO satellite transmitter and the user receiver to realize information relay and exchange, and the three communicate with each other via a space-air link and a ground link.

[0022] like Figure 3 As shown, both the transmitting and receiving ends of the aerospace communication system include a baseband unit for processing baseband signals, an radio frequency unit for processing radio frequency signals, and an antenna unit for providing gain. The antenna unit at the transmitting end is the transmitting antenna, used to radiate transmitted signals; the antenna unit at the receiving end is the receiving antenna, used to receive signals from the aerospace link.

[0023] like Figure 4 As shown, the operation of the space-air communication system includes four stages: gateway station broadcasting information, satellite transmitter signal processing, ground receiver access, and ground receiver frequency offset estimation and tracking. Specifically, the gateway station plans in advance according to the protocol and broadcasts beam center position information to the transmitter, providing a position reference for beam adjustment. The satellite transmitter aligns its beam direction with the target cell and performs frequency offset pre-compensation based on ephemeris information, completing the preprocessing of the transmitted signal. The ground receiver uses pilot sequences to establish initial time synchronization and cell synchronization with the satellite transmitter, and simultaneously demodulates SIB19 (System Information Block 19) to obtain satellite ephemeris information, completing access preparation. The ground receiver uses its integrated GNSS equipment to obtain its own position information. Based on its own position information and satellite ephemeris information, it performs frequency offset equation fitting, and combines the time synchronization information and the frequency offset equation fitting results to correct the frequency offset estimation results, ultimately achieving frequency offset tracking.

[0024] The frequency offset correction method of the aerospace communication system according to embodiments of this application will be described in detail below. Figure 1 As shown, the method mainly includes the following steps: The S101 satellite transmitter calculates the beam direction vector based on the received beam center position information and calculates the Doppler frequency offset pre-compensation value based on ephemeris information to perform Doppler frequency offset pre-compensation on the transmitted signal.

[0025] In this embodiment of the application, before the satellite transmitter calculates the beam direction vector based on the received beam center position information, the method further includes: the gateway station broadcasting the beam center position information to the satellite transmitter based on the feeder link.

[0026] First, the gateway station broadcasts the beam center position information to the satellite transmitter via the feeder link. The satellite transmitter receives and parses the information to obtain the beam center position data required for subsequent beam direction calculation.

[0027] In this embodiment of the application, the satellite transmitter calculates the beam direction vector based on the received beam center position information. This includes the satellite transmitter calculating the beam direction vector using a preset beamforming algorithm based on the beam center position information, so as to adjust the antenna beam direction to align with the target cell.

[0028] Furthermore, based on ephemeris information, the satellite's real-time position vector and real-time velocity vector in the Earth-fixed coordinate system are calculated, and combined with the beam center position information, the beam center position vector and the direction vector of the satellite pointing towards the beam center are determined. Based on the real-time velocity vector, real-time position vector, beam center position vector, satellite direction vector pointing towards the beam center, speed of light, and center frequency of the satellite radio frequency carrier signal, Doppler frequency offset pre-compensation values ​​are calculated to perform frequency offset pre-compensation on the transmitted signal.

[0029] Specifically, the Doppler frequency offset pre-compensation value can be calculated using the following formula:

[0030] in, Represents the satellite's velocity vector. Represents the satellite's position vector. The position vector representing the beam center. This represents the direction vector of the satellite pointing towards the center of the beam. Represents the speed of light. This represents the center frequency of the satellite's radio frequency carrier signal. The satellite's position vector and the beam center's position vector are in Earth-fixed coordinate system.

[0031] This application embodiment calculates the beam direction vector based on the received beam center position information at the satellite transmitter, and calculates the Doppler frequency offset pre-compensation value in combination with ephemeris information, thereby achieving accurate pre-compensation for the transmitted signal.

[0032] The S102 ground receiver establishes initial time synchronization and cell synchronization with the satellite transmitter, and demodulates system information blocks to obtain satellite ephemeris information.

[0033] In this embodiment of the application, the ground receiver establishes initial time synchronization and cell synchronization with the satellite transmitter, and demodulates the system information block to obtain satellite ephemeris information, including: the ground receiver receives the downlink pilot sequence sent by the satellite transmitter, and establishes initial time synchronization and cell synchronization with the satellite transmitter based on the downlink pilot sequence.

[0034] Specifically, the ground receiver first receives the downlink pilot sequence sent by the satellite transmitter. By detecting and analyzing the characteristics of the sequence, it completes the initial time synchronization with the satellite transmitter and identifies the basic parameters of the cell, thereby achieving cell synchronization and establishing basic synchronization conditions for subsequent communication processes.

[0035] The downlink pilot sequence is a predefined sequence of specific signals actively transmitted by the transmitter in a communication system and known to the receiver. It helps the receiver quickly detect signals in complex electromagnetic environments, determine the start position of data frames, and achieve time synchronization.

[0036] In one implementation, the ground receiver calculates the cell identifier based on the downlink pilot sequence and performs an integer multiple frequency offset search based on the downlink pilot sequence.

[0037] Because the high-speed movement of satellites or terminals will cause Doppler frequency shift, the receiver calculates and compensates for the Doppler frequency shift by comparing the frequency difference between the received pilot sequence and the local known sequence, so as to maintain carrier synchronization.

[0038] In satellite or cellular networks, different beams or cells may use different pilot sequences. By identifying the pilot sequence, the terminal can determine which specific satellite beam or cell it is currently connected to, thus achieving cell synchronization.

[0039] Specifically, the receiving device calculates the cell ID based on the synchronization sequence. The integer multiple frequency offset search can be calculated using the following formula:

[0040] in, This indicates the main synchronization sequence for the search. Indicates the FFT order. Indicates synchronization time. This indicates the obtained cell ID. This represents the search result with an integer multiple of the frequency offset. Indicates the received signal sequence. express conjugate, j It represents the imaginary unit.

[0041] In this embodiment, the ground receiver demodulates the system information block and extracts satellite ephemeris information from the system information block.

[0042] Specifically, the ground receiver demodulates the received system information block, parses and extracts the satellite ephemeris information, which contains the satellite's orbital parameters, enabling the ground receiver to determine the satellite's real-time position and trajectory, thereby providing data support for Doppler frequency offset compensation.

[0043] The S103 ground receiver performs frequency offset equation fitting based on its own position information and satellite ephemeris information to obtain the frequency offset fitting equation.

[0044] In this embodiment of the application, the ground receiver performs frequency offset equation fitting based on its own position information and satellite ephemeris information to obtain the frequency offset fitting equation.

[0045] First, the ground receiver uses an orbit recursion algorithm to calculate the satellite's real-time position vector and real-time velocity vector in the Earth-fixed coordinate system based on satellite ephemeris information.

[0046] Based on the extracted satellite ephemeris information, the ground receiver uses the preset orbit recursion algorithm SGP4 and the current system time to calculate the satellite's real-time position vector and real-time velocity vector in the Earth-fixed coordinate system, providing a dynamic reference for subsequent communication parameter calculations.

[0047] Furthermore, the ground receiver obtains positioning parameters based on its integrated GNSS equipment, and calculates the real-time position vector of the ground receiver in the Earth-fixed coordinate system based on the positioning parameters.

[0048] Furthermore, the ground receiver determines the relative motion relationship between the satellite and the ground receiver based on the satellite's real-time position vector, real-time velocity vector, and the ground receiver's real-time position vector, and calculates the Doppler theoretical frequency offset based on this relative motion relationship. The formula for calculating the Doppler theoretical frequency offset is:

[0049] in, , representing the direction vector between the satellite and the receiving equipment. Represents the satellite's velocity vector. Represents the speed of light. This indicates the center frequency of the satellite radio frequency carrier signal.

[0050] Based on the frequency offset of Doppler theory, a frequency offset fitting equation is constructed to characterize the variation of frequency offset over time.

[0051] The normalized residual Doppler bias can be expressed as:

[0052] in, Indicates the subcarrier spacing. and This represents the theoretical Doppler frequency offset and the satellite Doppler pre-compensation value calculated by the SGP4 algorithm.

[0053] The frequency offset fitting equation can be expressed as:

[0054] in, It is the order of the fitted polynomial. and Here, m is the polynomial parameter, and m is the subscript of the polynomial order. Indicates the first discrete time .

[0055] The S104 ground receiver calculates the frequency offset estimate, and then corrects the frequency offset estimate based on the frequency offset fitting equation to obtain the corrected frequency offset value.

[0056] In this embodiment, the ground receiver calculates a frequency offset estimate, and then corrects the frequency offset estimate based on the frequency offset fitting equation to obtain a corrected frequency offset value. First, the ground receiver calculates the frequency offset estimate based on the received signal.

[0057] Specifically, based on the received signal The formula for calculating frequency offset can be expressed as: ,in and The calculation formula can be expressed as:

[0058]

[0059] in, This indicates the length including the cyclic prefix, where n represents the index of the signal sampling point. This indicates the main synchronization sequence for the search. Indicates the FFT order. Indicates synchronization time. This represents the search result with an integer multiple of the frequency offset. Indicates the received signal sequence. express conjugate, j It represents the imaginary unit.

[0060] Furthermore, the parameters of the frequency offset fitting equation and the time-synchronized measurements are used as state variables to construct the system state transition model and observation model. By treating the model parameters and time-synchronized measurements as state variables, it can be represented that... The system state transition model and observation model are as follows:

[0061]

[0062] The state transition equation is:

[0063] Based on the system state transition model and observation model, the frequency offset estimate is recursively corrected using a nonlinear filter, outputting the corrected Doppler frequency offset value. The correction formula can be expressed as:

[0064] in, To utilize Time frequency deviation measurement value The corrected state variables are calculated using a filtering algorithm.

[0065] In this embodiment, a nonlinear filtering algorithm, such as unscented Kalman filtering, is used to dynamically correct the Doppler frequency offset. First, a state transition model incorporating the Doppler frequency shift and its rate of change is established to predict the state at the next moment. Then, an observation model is constructed, using the actual demodulated frequency offset measurement as the observation input. The Kalman gain is recursively calculated using a nonlinear filter, and the predicted and observed values ​​are fused to suppress measurement noise interference, thereby outputting a smooth and accurate Doppler frequency offset correction value.

[0066] In this embodiment of the application, the ground receiver calculates the frequency offset estimate, corrects the frequency offset estimate based on the frequency offset fitting equation, and after obtaining the corrected frequency offset value, the method further includes: The ground receiver acquires the Doppler frequency offset correction value; the ground receiver uses the Doppler frequency offset correction value to compensate for the frequency offset of the baseband signal based on the digital down-conversion module; or the ground receiver uses the Doppler frequency offset correction value to adjust the local oscillator to eliminate the residual Doppler frequency offset in the received signal.

[0067] In this embodiment, after the ground receiver obtains the Doppler frequency offset correction value, it can achieve frequency offset compensation in two optional ways: one is to use the correction value to perform digital domain frequency offset correction on the baseband signal based on the digital down-conversion module; the other is to use the correction value to adjust the output frequency of the local oscillator to eliminate the residual Doppler frequency offset in the received signal in hardware, thereby ensuring the accuracy of signal demodulation.

[0068] To facilitate understanding of the methods in the embodiments of this application, the following description is provided in conjunction with the appendix. Figure 5 Further description, such as Figure 5 As shown, firstly, the transmitter obtains information through the gateway station; then, through the communication link with the gateway station, the transmitter obtains the beam center position information, providing basic data for subsequent beam adjustment and frequency offset pre-compensation.

[0069] Furthermore, the beam direction and Doppler frequency offset pre-compensation are calculated; the transmitter calculates the beam direction vector through a beamforming algorithm based on the acquired beam center position information, and at the same time calculates the Doppler frequency offset pre-compensation value based on ephemeris information, thus completing the beam direction adjustment and frequency offset pre-compensation of the transmitted signal.

[0070] Furthermore, the frequency offset fitting equation is calculated based on ephemeris and GNSS equipment parameters; the receiver calculates the satellite's real-time position and motion vector based on the SGP4 orbit recursive algorithm and the acquired satellite ephemeris information; at the same time, the receiver's real-time position is calculated based on its integrated GNSS equipment parameters; finally, based on the relative motion relationship between the satellite position, satellite motion vector and receiver position, the Doppler theoretical frequency offset is calculated and the frequency offset fitting equation is constructed, which can be expressed in polynomial form.

[0071] Finally, a nonlinear filter is constructed using the frequency offset fitting equation. The frequency offset value of the space-air communication system is then corrected using the frequency offset fitting equation. Specifically, this includes: constructing a system state transition model and an observation model based on the frequency offset fitting equation; treating the model parameters of the frequency offset fitting equation and the time synchronization measurement values ​​as state variables to construct a state transition equation; and using the constructed system state transition model, observation model, and nonlinear filter, the frequency offset estimate is recursively corrected to output the optimal Doppler frequency offset correction value, thereby achieving dynamic tracking of time-varying frequency offset.

[0072] To facilitate understanding of the frequency offset correction method in this application, the following is in conjunction with the appendix. Figure 6 Further description, such as Figure 6 As shown, firstly, the satellite calculates the beam direction and performs Doppler frequency offset pre-compensation. The receiver performs cell search and integer multiple frequency offset search, completing time synchronization and cell synchronization. The receiver calculates the frequency offset fitting equation, estimates the frequency offset, and constructs a system state transition and observation model based on the frequency offset fitting equation. Frequency offset tracking is then performed based on this model.

[0073] Figure 7 and Figure 8 The performance evaluation results of the proposed method are presented. In the ephemeris-assisted frequency offset tracking scenario, the performance evaluation for different modeling and filtering strategies can be carried out. Under the condition of given measurement interval and fitting model, the method of this application can be applied to verify and correct the algorithm parameter configuration and expected performance, thereby more accurately evaluating the engineering usability of the tracking system.

[0074] Taking a correction interval of 20 ms as an example, Figure 7The impact of the fitting equation order on algorithm performance is evaluated. Simulation results show that both the traditional modeling method using Kalman filtering and the novel modeling method using Kalman filtering are largely insensitive to changes in the fitting order. Under high noise conditions, the traditional modeling method degenerates into a simple linear model, making it difficult to effectively utilize prior ephemeris information; while the Kalman filtering algorithm, unable to characterize the nonlinear relationships in observations, is also insensitive to the fitting order. In contrast, the novel modeling method proposed in this application, using unscented Kalman filtering, can more fully capture the higher-order dynamic characteristics of frequency shifts as the fitting order increases, with a significant reduction in mean square error; when the fitting order is greater than four, the performance tends to converge and remain stable. Therefore, in engineering implementation, fourth order and above can be considered as the preferred range for fitting orders to balance performance and complexity under complex dynamic and noisy conditions.

[0075] Furthermore, Figure 8 The mean square error tracking performance of three ephemeris-assisted algorithms under different correction intervals was compared. The results show that as the correction interval increases, the performance of the traditional modeling method using unscented Kalman filtering deteriorates significantly; while the modeling method in this application only shows a slight performance decrease when solved by unscented Kalman filtering and Kalman filtering algorithms respectively, indicating that the method is not sensitive to changes in the measurement interval and has higher robustness and adaptability.

[0076] It should be noted that if the impact of the fitting order and measurement interval on algorithm performance is not fully considered during the system design phase, it may lead to biased evaluation of key indicators (such as steady-state mean square error, transient tracking error, and jitter), thereby causing risks of improper parameter configuration and insufficient performance margin. Figure 7 and Figure 8 The results shown indicate that the algorithm parameters can be selected and modified in a targeted manner using the method of this application, so as to obtain more reliable tracking performance prediction and engineering configuration basis.

[0077] Existing technologies rely solely on instantaneous pilot and ephemeris information for single-point estimation, neglecting the continuity of satellite motion and the value of multi-source data. This application constructs a frequency offset tracking state equation, integrates satellite ephemeris, user GNSS position, and timing synchronization information to establish a relative motion model, and then iteratively optimizes it using nonlinear filtering algorithms such as Kalman filtering. This effectively suppresses the influence of noise interference and ephemeris errors. Compared to instantaneous estimation methods, it can correct errors through the correlation between historical and real-time data, significantly improving the accuracy of frequency offset estimation and meeting the stringent requirements of high-speed communication for frequency offset control.

[0078] Existing technologies are prone to delays in frequency offset compensation due to ephemeris update delays. This application adopts a closed-loop design of "pre-compensation + real-time estimation + iterative correction". The satellite performs pre-compensation for frequency offset based on ephemeris information in advance. The receiver achieves timing synchronization through pilot sliding correlation, and then uses the state equation to track relative motion changes in real time. Combined with filtering algorithms, the frequency offset correction value is dynamically updated. Compared with the existing single-point estimation mode, it can accurately capture the continuous change trend of frequency offset caused by the high-speed motion of the satellite, avoid compensation lag, and improve the signal synchronization stability in high dynamic scenarios.

[0079] According to another aspect of the embodiments of this application, an aerospace communication system for implementing the above-described frequency offset correction method is also provided, the system comprising a satellite transmitter and a ground receiver; The satellite transmitter is used to calculate the beam direction vector based on the received beam center position information and to calculate the Doppler frequency offset pre-compensation value based on the ephemeris information in order to perform Doppler frequency offset pre-compensation on the transmitted signal. The ground receiver is used to establish initial time synchronization and cell synchronization with the satellite transmitter, and to demodulate system information blocks to obtain satellite ephemeris information; The ground receiver is used to fit the frequency offset equation based on its own position information and satellite ephemeris information to obtain the frequency offset fitting equation. The ground receiver is used to calculate the frequency offset estimate. Based on the frequency offset fitting equation, the frequency offset estimate is corrected to obtain the frequency offset correction value.

[0080] In this embodiment of the application, a gateway station is also included; The gateway station is used to broadcast beam center location information to the satellite transmitter via the feeder link.

[0081] In this embodiment of the application, the ground receiver is used to calculate a frequency offset estimate based on the received signal; The parameters of the frequency offset fitting equation and the time synchronization measurement values ​​are used as state variables to construct the system state transition model and observation model; Based on the system state transition model and observation model, the frequency offset estimate is recursively corrected by a nonlinear filter, and the Doppler frequency offset correction value is output.

[0082] To facilitate understanding of the aerospace communication system of this application, the following is in conjunction with the appendix. Figure 9 Further description, such as Figure 9 As shown, the frequency offset correction system includes a parameter acquisition module 11, an initial synchronization module 12, a Doppler estimation module 13, a position and beam calculation module 14, a fitting equation construction module 15, a frequency offset tracking module 16, and a compensation and feedback module 17. Wherein: Parameter acquisition module 11 is used to acquire equipment parameters, receiver location information, and transmitter ephemeris or orbit information of the space-air communication system. The initial synchronization module 12 is used to complete time synchronization, cell / signal source identification, and carrier coarse frequency offset estimation using the pilot signal from the receiver. Doppler estimation module 13 is used to obtain instantaneous Doppler frequency offset measurement based on pilot sequence and baseband processing; The position and beam calculation module 14 is used to calculate the position vector and velocity vector of the transmitter in the ECEF or inertial coordinate system based on ephemeris information, calculate the transmit-receive direction vector in combination with the position of the receiver, and calculate the theoretical Doppler. The fitting equation construction module 15 is used to establish a frequency offset fitting equation based on the receiver position information, transmitter position information and timing synchronization information. The frequency offset tracking module 16 is used to combine the fitted equation and the system state equation to construct a state-space model for recursive estimation, and to use a nonlinear filtering algorithm to estimate and correct the residual Doppler and the fitted parameters. The compensation and feedback module 17 is used to apply the Doppler frequency offset correction value obtained by filtering to the local oscillation adjustment or baseband frequency offset compensation at the receiver.

[0083] In this embodiment, the parameter acquisition module 11 is specifically used for: acquiring and maintaining the equipment parameter database of the aerospace communication system, including radio frequency parameters (center frequency, bandwidth, transmit power, pre-compensation value, etc.) of the transmitter, receiver, and ground gateway station, antenna parameters (aperture, radiation pattern, maximum main lobe gain, sidelobe characteristics, polarization, etc.), local oscillator and sampling parameters; acquiring real-time position information and velocity and clock deviation information of the receiver (latitude / longitude, velocity vector, clock deviation and accuracy indicators PDOP / HDOP, etc. from GNSS); acquiring ephemeris or orbit information of the transmitter (e.g., SGP4 orbital elements, broadcast ephemeris or precise ephemeris, etc.) and cell location information and version number broadcast by the gateway station.

[0084] In this embodiment, the initial synchronization module 12 is specifically used for: extracting the synchronization signal block and pilot sequence from the received signal, calculating the symbol timing deviation through sliding correlation operation, and completing time synchronization calibration; performing cell ID search and matching based on the primary synchronization signal and the secondary synchronization signal to achieve signal source identification and cell synchronization; and using a pilot-based cyclic correlation coarse frequency offset estimation algorithm to estimate the carrier coarse frequency offset and generate a reverse compensation signal to perform coarse frequency offset compensation on the received signal, thereby reducing the dynamic range of subsequent accurate frequency offset estimation and ensuring the stability of baseband signal processing.

[0085] In this embodiment of the application, the Doppler estimation module 13 is specifically used to: extract the pilot sequence from the baseband signal after initial synchronization and coarse frequency offset compensation, construct a reference pilot signal based on a preset pilot pattern, and perform sliding correlation operation on the received pilot signal and the reference pilot signal to obtain the instantaneous Doppler frequency offset measurement value.

[0086] In this embodiment, the position and beam calculation module 14 is specifically used for: loading SGP4 orbital elements or broadcast ephemeris data provided by the parameter acquisition module 11; calling the SGP4 orbital recursion function to calculate the real-time position vector and velocity vector of the satellite transmitter in the ECEF coordinate system; converting the GNSS position information of the receiver into a position vector in the ECEF coordinate system; and calculating the spatial direction unit vector between the transmitter and the receiver.

[0087] In this embodiment of the application, the fitting equation construction module 15 is specifically used to: collect the timing synchronization timestamp and receiver location information output by the initial synchronization module 12, calculate the theoretical Doppler frequency offset value according to the Doppler frequency offset formula, and construct the frequency offset fitting equation.

[0088] In this embodiment, the frequency offset tracking module 16 is specifically used to: construct a state vector by using the coefficients of the fitting equation and the residual Doppler frequency offset as state variables; construct a state transition equation based on the time-varying characteristics of the polynomial coefficients; recursively iterate and output the optimal state estimate to obtain the corrected residual Doppler frequency offset and fitting parameters.

[0089] In the embodiments of this application, the compensation and feedback module 17 is specifically used to: receive the Doppler frequency offset correction value output by the frequency offset tracking module 16, and perform frequency offset compensation on the baseband signal through the digital down-conversion module.

[0090] It should be noted that the above-described aerospace communication system, when implementing the frequency offset correction method, is only illustrated by the division of the aforementioned functional modules. In practical applications, the functions described above can be assigned to different functional modules as needed, that is, the internal structure of the device can be divided into different functional modules to complete all or part of the functions described above. Furthermore, the aerospace communication system and the frequency offset correction method embodiments of the aerospace communication system belong to the same concept, and their implementation process is detailed in the method embodiments, which will not be repeated here.

[0091] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A frequency offset correction method for an aerospace communication system, characterized in that, include: The satellite transmitter calculates the beam direction vector based on the received beam center position information and calculates the Doppler frequency offset pre-compensation value based on the ephemeris information in order to perform Doppler frequency offset pre-compensation on the transmitted signal; The ground receiver establishes initial time synchronization and cell synchronization with the satellite transmitter, and demodulates system information blocks to obtain satellite ephemeris information; The ground receiving end performs frequency offset equation fitting based on its own position information and the satellite ephemeris information to obtain the frequency offset fitting equation; The ground receiver calculates the frequency offset estimate, and corrects the frequency offset estimate based on the frequency offset fitting equation to obtain the frequency offset correction value.

2. The method according to claim 1, characterized in that, Before calculating the beam direction vector based on the received beam center position information, the satellite transmitter also includes: The gateway station broadcasts the beam center location information to the satellite transmitter via the feeder link.

3. The method according to claim 1, characterized in that, Based on the received beam center position information, the satellite transmitter calculates the beam direction vector and, based on ephemeris information, calculates the Doppler frequency offset pre-compensation value, including: Based on the beam center position information, the satellite transmitter uses a preset beamforming algorithm to calculate the beam direction vector in order to adjust the antenna beam direction to align with the target cell. Based on the ephemeris information, the real-time position vector and real-time velocity vector of the satellite in the Earth-fixed coordinate system are calculated, and the position vector of the beam center and the direction vector of the satellite pointing to the beam center are determined in combination with the beam center position information. Based on the real-time motion velocity vector, the real-time position vector, the position vector of the beam center, the direction vector of the satellite pointing to the beam center, the speed of light, and the center frequency of the satellite radio frequency carrier signal, the Doppler frequency offset pre-compensation value is calculated to perform frequency offset pre-compensation on the transmitted signal.

4. The method according to claim 1, characterized in that, The ground receiver establishes initial time synchronization and cell synchronization with the satellite transmitter, and demodulates system information blocks to obtain satellite ephemeris information, including: The ground receiver receives the downlink pilot sequence sent by the satellite transmitter, and establishes initial time synchronization and cell synchronization with the satellite transmitter based on the downlink pilot sequence; The ground receiving end demodulates the system information block and extracts the satellite ephemeris information from the system information block; The ground receiving end calculates the cell identifier based on the downlink pilot sequence and performs an integer multiple frequency offset search based on the downlink pilot sequence.

5. The method according to claim 1, characterized in that, The ground receiver performs frequency offset equation fitting based on its own location information and the satellite ephemeris information to obtain the frequency offset fitting equation, including: The ground receiving end uses an orbit recursion algorithm to calculate the satellite's real-time position vector and real-time velocity vector in the Earth-fixed coordinate system based on the satellite ephemeris information. The ground receiver obtains positioning parameters based on its integrated GNSS equipment, and calculates the real-time position vector of the ground receiver in the Earth-fixed coordinate system based on the positioning parameters. The ground receiver determines the relative motion relationship between the satellite and the ground receiver based on the satellite's real-time position vector, real-time velocity vector, and the ground receiver's real-time position vector, and calculates the Doppler theoretical frequency offset based on the relative motion relationship. Based on the Doppler theory frequency offset, a frequency offset fitting equation is constructed to characterize the frequency offset variation over time.

6. The method according to claim 1, characterized in that, The ground receiver calculates a frequency offset estimate, and based on the frequency offset fitting equation, corrects the frequency offset estimate to obtain a frequency offset correction value, including: The ground receiver calculates the frequency offset estimate based on the received signal; The parameters of the frequency offset fitting equation and the time synchronization measurement values ​​are used as state variables to construct the system state transition model and observation model; Based on the system state transition model and the observation model, the frequency offset estimate is recursively corrected using a nonlinear filter to output the Doppler frequency offset correction value.

7. The method according to claim 1, characterized in that, The ground receiver calculates the frequency offset estimate, corrects the frequency offset estimate based on the frequency offset fitting equation, and then, after obtaining the corrected frequency offset value, further includes: The ground receiver acquires the frequency offset correction value; The ground receiver uses a digital down-conversion module to compensate for the frequency offset of the baseband signal using the frequency offset correction value; or the ground receiver uses the frequency offset correction value to adjust the local oscillator to eliminate residual Doppler frequency offset in the received signal.

8. A space-air communication system, characterized in that, Including satellite transmitter and ground receiver; The satellite transmitter is used to calculate the beam direction vector based on the received beam center position information and to calculate the Doppler frequency offset pre-compensation value based on the ephemeris information, so as to perform Doppler frequency offset pre-compensation on the transmitted signal. The ground receiving end is used to establish initial time synchronization and cell synchronization with the satellite transmitting end, and to demodulate system information blocks to obtain satellite ephemeris information; The ground receiving end is used to perform frequency offset equation fitting based on its own position information and the satellite ephemeris information to obtain the frequency offset fitting equation. The ground receiver is used to calculate the frequency offset estimate, and based on the frequency offset fitting equation, correct the frequency offset estimate to obtain the frequency offset correction value.

9. The system according to claim 8, characterized in that, This also includes customs stations; The gateway station is used to broadcast the beam center location information to the satellite transmitter via the feeder link.

10. The system according to claim 8, characterized in that, The ground receiver is used to calculate the frequency offset estimate based on the received signal; The parameters of the frequency offset fitting equation and the time synchronization measurement values ​​are used as state variables to construct the system state transition model and observation model; Based on the system state transition model and the observation model, the frequency offset estimate is recursively corrected using a nonlinear filter to output the Doppler frequency offset correction value.

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