A ground network assisted frequency synchronization method and device for a satellite-ground multi-mode terminal

Abstract

The application relates to the field of communication technology and discloses a ground network assisted satellite-ground multi-mode terminal frequency synchronization method and device, which comprises the following steps: a target terminal accesses a ground network, and the crystal oscillator frequency of the target terminal is calibrated based on the downlink synchronization signal and the downlink reference signal of the ground network; in response to a satellite connection request, it is checked whether the crystal oscillator calibration state of the target terminal is valid, and a corresponding Doppler frequency offset pre-compensation method is selected according to the checking result to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses a satellite network uplink. The application calibrates the terminal crystal oscillator frequency through the downlink signal of the ground network, lays a high-precision frequency reference for satellite communication, reduces the crystal oscillator cost and power consumption, checks the crystal oscillator calibration state and adapts the frequency offset pre-compensation method when responding to the satellite connection request, accurately calculates the uplink frequency offset compensation value, solves the problem of the superposition of the Doppler frequency shift and the crystal oscillator frequency offset in satellite-ground communication, and improves the success rate and stability of the terminal access to the satellite network.

Description

Technical Field

[0001] This invention relates to the field of communication technology, and specifically to a method and apparatus for frequency synchronization of satellite-to-ground multimode terminals assisted by a terrestrial network. Background Technology

[0002] In low-Earth orbit (LEO) satellite communication systems, due to their wide coverage and low transmission latency, they serve as an important supplement to terrestrial mobile communication networks. Frequency synchronization is a core component for achieving effective communication between terminals and LEO satellites. However, the continuous high-speed motion of satellites relative to ground terminals generates a significant Doppler shift effect. This shift is much greater than that of geostationary orbit satellite systems, and its value is not fixed but exhibits a wide range of continuous time-varying characteristics, changing dynamically with the relative position of the satellite's nadir trajectory and the terminal. This large-scale, time-varying Doppler shift poses a significant challenge to the carrier frequency synchronization and signal reception / demodulation design of the communication system.

[0003] To address the frequency offset issue during the initial access phase, common technical solutions include: (i) improving the accuracy of the terminal crystal oscillator to control the frequency offset within a smaller range; (ii) utilizing the Global Navigation Satellite System to obtain the terminal's precise position, velocity, and time, and calculating the theoretical Doppler frequency shift value in real time to achieve frequency offset compensation; and (iii) optimizing the design of the uplink access channel, adopting modulation and demodulation methods more robust to frequency offset, and employing more complex receiving algorithms to improve tolerance to frequency offset. While existing technologies have alleviated the frequency synchronization problem in low-Earth orbit satellite communication to some extent, they are insufficient to meet the diverse, low-cost, and highly reliable application requirements of low-Earth orbit satellite communication systems. Summary of the Invention

[0004] This invention provides a ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method and apparatus to solve the problem of difficulty in meeting the diverse, low-cost, and high-reliability requirements of low-Earth orbit satellite communication systems.

[0005] In a first aspect, the present invention provides a terrestrial network-assisted frequency synchronization method for satellite-to-ground multimode terminals, the method comprising: The target terminal accesses the terrestrial network, and the crystal oscillator frequency of the target terminal is calibrated based on the downlink synchronization signal and downlink reference signal of the terrestrial network; In response to a satellite connection request, the system checks whether the crystal oscillator calibration status of the target terminal is valid, and selects the corresponding Doppler frequency offset pre-compensation method based on the check results. The selected Doppler frequency offset pre-compensation method is then used to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

[0006] The ground network-assisted frequency synchronization method for satellite-to-ground multimode terminals provided by this invention calibrates the terminal crystal oscillator frequency through downlink synchronization and reference signals of the ground network, compressing the crystal oscillator frequency offset to an extremely low level. This establishes a high-precision frequency reference for satellite communication. When responding to satellite connection requests, the method verifies the crystal oscillator calibration status and adapts to the Doppler frequency offset pre-compensation method to accurately calculate the uplink access frequency offset compensation value. This effectively solves the problem of the superposition of Doppler frequency shift and crystal oscillator frequency offset in satellite-to-ground communication. It avoids the high cost and high power consumption of high-precision crystal oscillators and improves the success rate and stability of terminal access to the satellite network, adapting to the efficient and reliable communication needs of satellite-to-ground multimode terminals in various scenarios.

[0007] In one optional implementation, calibrating the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network includes: The downlink synchronization signal of the terrestrial network is acquired, and the current downlink carrier frequency of the target terminal is aligned with the downlink carrier frequency of the downlink synchronization signal by adjusting the control voltage of the crystal oscillator, so as to obtain the coarse downlink carrier frequency of the target terminal. Based on the coarse adjustment of the downlink carrier frequency, the downlink reference signal of the terrestrial network is obtained, and the control voltage of the crystal oscillator is dynamically adjusted based on the downlink reference signal to calibrate the crystal oscillator frequency.

[0008] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided by this invention uses a two-stage calibration logic of coarse adjustment and fine adjustment. First, it uses the ground downlink synchronization signal to align the terminal carrier frequency and complete the coarse calibration of the crystal oscillator. Then, it dynamically adjusts the crystal oscillator control voltage based on the downlink reference signal to achieve precise calibration of the crystal oscillator frequency, quickly reduce the precise frequency deviation, dynamically ensure calibration accuracy, and rapidly compress the crystal oscillator frequency deviation to an extremely low level. It does not require reliance on high-precision crystal oscillators, reduces terminal cost and power consumption, and effectively balances calibration efficiency and practicality.

[0009] In one optional implementation, checking whether the crystal oscillator calibration status of the target terminal is valid includes: Obtain the duration of the disconnection between the target terminal and the ground network, and the crystal oscillator frequency drift; If the duration exceeds a preset time threshold, or the crystal oscillator frequency drift exceeds a preset range, the crystal oscillator calibration status of the target terminal will be invalid; otherwise, the calibration status will be valid.

[0010] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided by this invention uses both the duration of disconnection and crystal frequency drift to determine the validity of the crystal oscillator calibration status. This avoids relying on expired calibration, which leads to satellite access frequency offset compensation errors, reduces unnecessary repeated calibration overhead, and improves the accuracy of crystal oscillator calibration status determination through crystal frequency drift.

[0011] In one optional implementation, a corresponding Doppler frequency offset pre-compensation method is selected based on the inspection results, and the selected Doppler frequency offset pre-compensation method is used to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink, including: If the crystal oscillator calibration status is valid, the downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal crystal oscillator, and the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network is calculated based on the downlink reference signal transmitted by the satellite network. If the crystal oscillator calibration status is invalid and the terrestrial network is available, the crystal oscillator frequency of the target terminal is repeatedly calibrated based on the downlink synchronization signal and downlink reference signal of the terrestrial network until the crystal oscillator calibration status is valid. The downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the crystal oscillator of the target terminal, and the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network is calculated based on the downlink reference signal transmitted by the satellite network. If the crystal oscillator calibration status is invalid and the ground network is unavailable, then determine whether the preset operating conditions of the ephemeris prediction Doppler method are met. If the preset operating conditions of the ephemeris prediction Doppler method are met, then use the ephemeris prediction Doppler method to predict the uplink Doppler frequency offset value as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

[0012] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided by this invention directly calculates the compensation value using the satellite downlink reference signal when the crystal oscillator calibration is effective, ensuring accuracy and efficiency. When the crystal oscillator calibration is ineffective but the ground network is available, the crystal oscillator is recalibrated before calculation to ensure the reliability of the compensation benchmark. When the crystal oscillator calibration is ineffective and the ground network is unavailable, a fallback method using ephemeris estimation is used to cover network-free scenarios. This method adapts to different network environments, ensures the accuracy of frequency offset compensation, improves the success rate and stability of terminal access to the satellite network, and avoids the limitations of over-reliance on a single method, thus balancing practicality and reliability in multiple scenarios.

[0013] In one optional implementation, the downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal's crystal oscillator, and the Doppler frequency offset pre-compensation value for the target terminal's uplink access to the satellite network is calculated based on the downlink reference signal transmitted by the satellite network, including: The downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal crystal oscillator, and the downlink carrier frequency offset value is estimated based on the downlink reference signal transmitted by the satellite network. The downlink carrier frequency offset value is used as the downlink Doppler frequency offset estimate. Combined with the preset relationship between the uplink Doppler spectrum and the downlink Doppler frequency offset, the Doppler frequency offset pre-compensation value for the target terminal when accessing the satellite network uplink is determined based on the downlink Doppler frequency offset estimate.

[0014] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided by this invention receives the satellite downlink reference signal based on the downlink carrier frequency calibrated by the terminal. It first estimates the downlink carrier frequency offset, and then uses the preset relationship between uplink and downlink Doppler frequency offsets to derive the uplink pre-compensation value from the downlink frequency offset. This method relies on the stable frequency reference after ground calibration to ensure the accuracy of downlink frequency offset estimation, and efficiently obtains the uplink compensation value through the correlation rules between uplink and downlink frequency offsets, avoiding the complex process of independently calculating the uplink frequency offset.

[0015] In one optional implementation, the uplink Doppler frequency offset value is estimated using the ephemeris prediction Doppler method as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink, including: Obtain the ephemeris of the target satellite and determine its position and velocity based on the ephemeris; The system acquires the target terminal's position and velocity, and determines the relative position between the satellite and the terminal based on the satellite position and the terminal position, and determines the relative velocity between the satellite and the terminal based on the satellite velocity and the terminal velocity. The relative radial velocity is calculated based on the relative position and relative velocity. Based on the relative radial velocity, the Doppler frequency offset value is estimated using the Doppler frequency shift formula as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network for uplink.

[0016] The ground network-assisted frequency synchronization method for satellite-to-ground multi-mode terminals provided by this invention determines the satellite's position and velocity using ephemeris, combines this with the terminal's own position and velocity, calculates the relative radial velocity, and derives the uplink Doppler frequency offset compensation value. Even when the ground network is unavailable, frequency offset pre-compensation can still be achieved based on ephemeris, filling the gap in frequency offset calculation in scenarios without a ground network. At the same time, the accurate derivation of relative motion parameters ensures the accuracy of Doppler frequency offset prediction, providing a reliable frequency compensation basis for the terminal's uplink access to the satellite network. This not only improves the success rate of satellite access in remote, network-free areas but also avoids strong dependence on the ground network, enhancing the scenario adaptability and communication stability of the satellite-to-ground multi-mode terminal.

[0017] Secondly, the present invention provides a terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization device, the device comprising: The frequency calibration module is used for the target terminal to access the terrestrial network and to calibrate the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network. The uplink access Doppler frequency offset pre-compensation module is used to respond to satellite connection requests, check whether the crystal oscillator calibration status of the target terminal is valid, select the corresponding Doppler frequency offset pre-compensation method according to the check results, and calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network using the selected Doppler frequency offset pre-compensation method.

[0018] Thirdly, the present invention provides an electronic device, comprising: a memory and a processor, the memory and the processor being communicatively connected to each other, the memory storing computer instructions, and the processor executing the computer instructions to perform the method described in the first aspect or any corresponding embodiment thereof.

[0019] Fourthly, the present invention provides a computer-readable storage medium storing computer instructions for causing a computer to perform the method described in the first aspect or any corresponding embodiment thereof.

[0020] Fifthly, the present invention provides a computer program product, including computer instructions for causing a computer to perform the method described in the first aspect or any corresponding embodiment thereof. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of an application scenario according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the first step of the ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method according to an embodiment of the present invention. Figure 3 This is a schematic diagram of the process of calibrating the crystal oscillator by connecting the terminal to the ground network in the frequency synchronization method of satellite-to-ground multimode terminal assisted by the ground network according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the second process of the ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the process of a terminal accessing a satellite network in a ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method according to an embodiment of the present invention. Figure 6 This is a structural block diagram of a ground network-assisted satellite-to-ground multimode terminal frequency synchronization device according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the hardware structure of an electronic device according to an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] It is understood that before using the technical solutions disclosed in the various embodiments of the present invention, users should be informed of the types, scope of use, and usage scenarios of the personal information involved in the present invention and their authorization should be obtained in accordance with relevant laws and regulations through appropriate means.

[0025] As an optional application scenario of this invention, such as Figure 1 As shown, the satellite-to-ground multi-mode terminal frequency synchronization system may include at least one terminal device, at least one ground network device, and at least one satellite. Figure 1 The example shows that the system includes a terrestrial network device 101, a terminal device 102, and a satellite 103, and the terminal device 102 can be connected to the terrestrial network device 101 and the satellite 103 respectively.

[0026] Specifically, terminal devices can be smartphones, tablets, laptops, PDAs, desktop computers, game consoles, smart TVs, smart wearable devices, in-vehicle terminals, VR (Virtual Reality) devices, AR (Augmented Reality) devices, etc.; terrestrial network equipment can be base stations, etc.; satellites can be aviation satellites. Terminal devices may choose to connect to terrestrial network equipment or satellites depending on the actual situation.

[0027] During the initial access phase, the frequency offset detected by the terminal from the downlink signal It consists of two parts: crystal frequency offset. and Doppler shift , At this point, a feedback loop has not yet been formed with the satellite gateway, making it impossible to obtain the Doppler estimation of the uplink signal from the satellite gateway, thus hindering the calculation of... and .

[0028] At a certain moment, Doppler shift ,in The speed at which the satellite moves relative to the ground terminal. This is the angle between the relative movement direction and the carrier propagation direction. Assume the downlink carrier frequency is... The uplink carrier frequency is Downlink Doppler frequency shift is Ignore the satellite's relative speed to the ground terminal caused by the time variation between downlink reception and uplink transmission times. and To compensate for the uplink Doppler frequency shift during uplink transmission, the uplink Doppler frequency shift should be pre-compensated in the opposite direction. .

[0029] If the terminal detects the frequency offset If all of these are treated as Doppler frequency shifts and compensated for during uplink transmission, then the uplink transmission frequency offset is: Uplink receiving frequency offset is .

[0030] Therefore, the uplink receive frequency offset is approximately twice the uplink carrier frequency offset caused by the crystal oscillator frequency offset. For low-cost terminals, if a digitally compensated crystal oscillator (DCXO) is used, the crystal oscillator frequency offset may be a relatively large value (e.g., 10ppm, which is 20kHz at 2G carrier frequency). The uplink carrier frequency offset caused by the crystal oscillator frequency offset being twice the value may cause the uplink signal to deviate from the receiving frequency band of the satellite gateway, resulting in access failure.

[0031] To address the frequency offset issue during the initial access phase, several methods are commonly used, but each method has its own drawbacks, as detailed below: Option 1: Improve the accuracy of the terminal crystal oscillator by using a higher-precision and more stable temperature-compensated crystal oscillator (TCXO) or even a temperature-controlled crystal oscillator (OCXO) to reduce the crystal oscillator frequency deviation. It was controlled within a very small range, but this increased the cost and power consumption of the terminal.

[0032] Option 2 involves terminals that typically integrate a Global Navigation Satellite System (GNSS) module (such as GPS or BeiDou), enabling them to obtain their precise position, velocity, and time. Satellite position and velocity information (ephemeris) can be obtained through pre-installation on the terminal, downloading via a terrestrial network, or satellite downlink broadcasting. Using their known position and satellite positions, the terminal can calculate the theoretical Doppler shift value for the current and future timeframes in real time. However, this option suffers from ephemeris errors and update delays. Furthermore, positioning errors in the terminal's GNSS under adverse conditions introduce Doppler prediction errors. Low-cost terminals without GNSS cannot obtain absolute time and terminal position in advance, making it impossible to use ephemeris information for Doppler prediction.

[0033] Option 3 employs a more robust design for the uplink access channel, using a more complex receiving algorithm to improve its tolerance to frequency offset. However, this option does not reduce frequency offset, but only improves its tolerance. The trade-off may be reduced spectral efficiency, reduced transmission rate, and increased network-side processing latency and power consumption.

[0034] Based on the problems existing in the above-mentioned related technologies, the present invention provides a ground network-assisted satellite-ground multi-mode terminal frequency synchronization method. By connecting to the ground network to calibrate the residual frequency offset of the crystal oscillator, and calculating the total frequency offset of the access satellite network based on the effective residual frequency offset of the crystal oscillator, the method can meet the diverse, low-cost, and high-reliability requirements of low-orbit satellite communication systems.

[0035] According to an embodiment of the present invention, a method for frequency synchronization of satellite-ground multi-mode terminals assisted by a terrestrial network is provided. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Furthermore, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0036] This embodiment provides a terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization method, which can be used in the aforementioned mobile terminals, such as mobile phones and tablet computers. Figure 2 This is a flowchart of a ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method according to an embodiment of the present invention, such as... Figure 2 As shown, the process includes the following steps: Step S201: The target terminal accesses the terrestrial network and calibrates the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network.

[0037] Specifically, terrestrial networks (such as 5G / 6G base stations) are the core reliance for daily terminal communication, possessing advantages such as low latency, high bandwidth, and high-density access. They mainly cover densely populated areas such as cities and towns, providing high-speed services to terminals and supporting bandwidth-sensitive needs such as high-definition video. Terrestrial networks can provide a high-precision frequency reference for the terminal's crystal oscillator through the downlink synchronization signal and downlink reference signal of the base station, while simultaneously updating data such as satellite ephemeris and access parameters.

[0038] Therefore, before accessing the satellite network, the target terminal first accesses the terrestrial network. The target terminal completes coarse downlink frequency synchronization using the downlink synchronization signal from the terrestrial network. Then, it continuously uses the specific downlink reference signal from the base station or demodulates the downlink reference signal for channel estimation and fine frequency synchronization. At this point, the frequency offset is calibrated to a level close to the base station's reference source. Based on the requirements of 4G and 5G networks for residual frequency offset, the crystal oscillator residual frequency offset... It should be within ±0.1 ppm.

[0039] If the target terminal maintains a connection with the terrestrial network, the automatic frequency control module adjusts the correction voltage configured for the crystal oscillator to ensure that the terminal carrier frequency generated by the crystal oscillator in the target terminal is consistent with the downlink signal frequency, and the residual frequency offset of the crystal oscillator is minimized. It remains within ±0.1ppm; if the target terminal loses connection with the ground network due to movement or upper-level control, the automatic frequency control module maintains the correction voltage configured for the crystal oscillator before the disconnection. ,in This is for fine-tuning of the correction voltage during continuous frequency tracking. The terminal carrier frequency remains at the value before disconnection.

[0040] Meanwhile, the target terminal can download the latest satellite ephemeris in advance through the terrestrial network to assist in Doppler calculations and obtain parameters such as access frequency points, thereby improving access speed and reducing terminal power consumption.

[0041] Step S202: In response to the satellite connection request, check whether the crystal oscillator calibration status of the target terminal is valid, and select the corresponding Doppler frequency offset pre-compensation method according to the check result. Calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink using the selected Doppler frequency offset pre-compensation method.

[0042] Specifically, satellite networks serve as an extension and backup for terrestrial networks, offering advantages such as global coverage and strong disaster resilience, covering areas such as oceans, deserts, remote mountainous regions, and aerial (aircraft, drones) areas where terrestrial networks are often unavailable. The target terminal initiates a satellite connection request based on the application layer's requirements.

[0043] First, check if the calibration status of the terminal carrier frequency is valid. If valid, select the corresponding Doppler frequency offset pre-compensation method to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink. If invalid, the crystal oscillator frequency needs to be recalibrated before selecting the corresponding Doppler frequency offset pre-compensation method to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

[0044] like Figure 3 The diagram shows the overall process of frequency synchronization for a satellite-to-ground multi-mode terminal. The terminal connects to the ground network to lock onto the frequency. Then, it determines whether to maintain a connection with the ground network. If the connection is maintained, the terminal receives the ground network signal and performs automatic frequency control (AFC) tracking. If the connection is lost, the terminal's AFC tracking module retains the configuration it had before the connection with the ground network was lost.

[0045] The ground network-assisted frequency synchronization method for satellite-to-ground multimode terminals provided in this embodiment calibrates the terminal crystal oscillator frequency through downlink synchronization and reference signals from the ground network, compressing the crystal oscillator frequency offset to an extremely low level. This establishes a high-precision frequency reference for satellite communication. When responding to satellite connection requests, the method verifies the crystal oscillator calibration status and adapts to the Doppler frequency offset pre-compensation method to accurately calculate the uplink access frequency offset compensation value. This effectively solves the problem of the superposition of Doppler frequency shift and crystal oscillator frequency offset in satellite-to-ground communication. It avoids the high cost and high power consumption of high-precision crystal oscillators and improves the success rate and stability of terminal access to the satellite network, adapting to the efficient and reliable communication needs of satellite-to-ground multimode terminals in various scenarios.

[0046] This embodiment provides a terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization method, which can be used in the aforementioned mobile terminals, such as mobile phones and tablet computers. Figure 4 This is a flowchart of a ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method according to an embodiment of the present invention, such as... Figure 4 As shown, the process includes the following steps: Step S301: The target terminal accesses the terrestrial network and calibrates the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network.

[0047] Specifically, step S301 includes: Step S3011: Obtain the downlink synchronization signal of the terrestrial network, and align the current downlink carrier frequency of the target terminal with the downlink carrier frequency of the downlink synchronization signal by adjusting the control voltage of the crystal oscillator, thereby obtaining the coarse downlink carrier frequency of the target terminal.

[0048] Specifically, the target terminal achieves frequency locking through the terrestrial network, including downlink frequency coarse synchronization and channel-level frequency fine synchronization, gradually converging the frequency offset using the high-precision time and frequency reference of the base station. The crystal oscillator frequency is adjusted by changing its control voltage. The specific relationship between the control voltage and the crystal oscillator frequency depends on factors such as the crystal oscillator model, and can be obtained from the crystal oscillator's product manual; it will not be elaborated here.

[0049] Base stations achieve high-precision time and frequency synchronization through the Global Positioning System (GPS) or ground-based atomic clocks. The synchronization signal carries strict frame timing and frequency references. The target terminal first acquires the downlink synchronization signal from the terrestrial network, captures the frame boundaries and carrier frequency of the synchronization signal through relevant detection algorithms, completes coarse synchronization, aligns the terminal's crystal oscillator frequency with the downlink carrier frequency, initially eliminates large frequency offsets, and obtains the coarsely adjusted downlink carrier frequency of the target terminal.

[0050] Step S3012: Based on the coarse adjustment of the downlink carrier frequency, obtain the downlink reference signal of the terrestrial network, and dynamically adjust the control voltage of the crystal oscillator based on the downlink reference signal to calibrate the crystal oscillator frequency.

[0051] Specifically, the base station inserts a cell-specific terrestrial network downlink reference signal into the data channel. This terrestrial network downlink reference signal is used for frequency offset estimation and synchronization tracking. After the target terminal completes coarse synchronization, it continuously receives the terrestrial network downlink reference signal and extracts residual frequency offset information based on the terrestrial network reference signal using a frequency offset estimation algorithm.

[0052] The automatic frequency control module in the target terminal dynamically adjusts the control voltage of the crystal oscillator based on the residual frequency deviation information to achieve precise synchronization and converge the residual frequency deviation of the crystal oscillator to within ±0.1ppm, thereby obtaining the terminal fine-tuned carrier frequency as the terminal carrier frequency.

[0053] By providing a more accurate frequency reference, the composition of system frequency error is altered, thereby mitigating the impact of ephemeris prediction errors on overall system performance. Ephemeris errors and update delays introduce Doppler prediction errors. The positioning accuracy of the terminal's own GNSS module degrades in harsh environments (urban canyons, indoors, jungles), potentially dropping from meters to tens of meters or even worse, which also leads to Doppler calculation errors. In contrast, the frequency reference of the ground base station is real-time, high-precision, and unaffected by terminal position errors. Therefore, in the ground network-assisted frequency synchronization scheme, even with a small amount of Doppler prediction error, the crystal oscillator reference is accurate, resulting in a much smaller overall residual system frequency error.

[0054] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided in this embodiment uses a two-stage calibration logic of coarse adjustment and fine adjustment. First, it uses the ground downlink synchronization signal to align the terminal carrier frequency and complete the coarse calibration of the crystal oscillator. Then, it dynamically adjusts the crystal oscillator control voltage based on the downlink reference signal to achieve precise calibration of the crystal oscillator frequency, quickly reduce the crystal oscillator frequency deviation, dynamically ensure calibration accuracy, and rapidly compress the crystal oscillator frequency deviation to an extremely low level. It does not require reliance on high-precision crystal oscillators, reduces terminal cost and power consumption, and effectively balances calibration efficiency and practicality.

[0055] Step S302: In response to the satellite connection request, check whether the crystal oscillator calibration status of the target terminal is valid, and select the corresponding Doppler frequency offset pre-compensation method according to the check result. Calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink using the selected Doppler frequency offset pre-compensation method.

[0056] Specifically, step S302 includes: Step S3021: Obtain the duration of the disconnection between the target terminal and the ground network, and the crystal oscillator frequency drift.

[0057] Specifically, after receiving a satellite connection request, the target terminal must first check its own frequency calibration status. Prolonged disconnection from the terrestrial network or excessive temperature drift of the crystal oscillator can cause the crystal oscillator to be in an uncalibrated state, meaning the crystal oscillator calibration has failed. If the target terminal disconnects from the terrestrial network, the duration of the disconnection is recorded, i.e., the time since the last frequency calibration.

[0058] The crystal oscillator temperature sensor is integrated inside the crystal oscillator (or closely attached to the crystal oscillator housing). Temperature changes directly cause crystal oscillator frequency drift; the further the temperature deviates from the crystal oscillator's nominal operating temperature, the more severe the frequency drift. Therefore, by continuously monitoring the changes in the crystal oscillator temperature sensor readings, the corresponding crystal oscillator frequency drift can be determined.

[0059] In step S3022, if the duration is greater than a preset time threshold, or the crystal oscillator frequency drift is greater than a preset range, the crystal oscillator calibration status of the target terminal is invalid; otherwise, the calibration status is valid.

[0060] Specifically, if the duration exceeds a preset time threshold, or if the crystal oscillator frequency drifts beyond a preset range, the calibrated terminal carrier frequency will be inaccurate, thus causing the terminal's crystal oscillator calibration status to fail. If neither of these two situations occurs, the calibration status is considered valid.

[0061] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided in this embodiment judges the validity of the crystal oscillator calibration status by dually judging the duration of disconnection and crystal oscillator frequency drift. This avoids relying on expired calibration, which leads to satellite access frequency offset compensation errors, reduces unnecessary repeated calibration overhead, and improves the accuracy of crystal oscillator calibration status judgment by judging crystal oscillator frequency drift.

[0062] Step S3023: If the crystal oscillator calibration status is valid, the downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal crystal oscillator, and the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network is calculated based on the downlink reference signal transmitted by the satellite network.

[0063] Specifically, if the crystal oscillator calibration status is valid, it indicates that the downlink carrier frequency generated by the target terminal's crystal oscillator is accurate. Therefore, the downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal's crystal oscillator, and the Doppler frequency offset pre-compensation value for the target terminal's uplink access to the satellite network is calculated based on the downlink reference signal transmitted by the satellite network. The total frequency offset in the satellite reference signal received by the target terminal includes two parts: the Doppler frequency offset generated during the transmission of the satellite downlink reference signal from the satellite to the terminal, and the residual frequency offset of the crystal oscillator itself when the terminal receives the satellite reference signal. The basic formula for frequency offset calculation is as follows: ,in, This represents the total frequency offset in the satellite reference signal received by the target terminal. Indicates Doppler frequency shift. This indicates the residual frequency offset of the crystal oscillator. The residual frequency offset of the crystal oscillator is generally very small (within ±0.1ppm). The total frequency offset estimated by the target terminal based on the satellite reference signal is used as the downlink Doppler frequency offset. Based on the relationship between the downlink Doppler frequency offset and the uplink Doppler frequency offset, the Doppler frequency offset pre-compensation value when the terminal accesses the satellite network uplink is calculated.

[0064] Step S3024: If the crystal oscillator calibration status is invalid and the ground network is available, repeat the calibration of the target terminal's crystal oscillator frequency based on the downlink synchronization signal and downlink reference signal of the ground network until the crystal oscillator calibration status is valid. Receive the downlink reference signal transmitted by the satellite network based on the downlink carrier frequency generated by the target terminal's crystal oscillator, and calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink based on the downlink reference signal transmitted by the satellite network.

[0065] Specifically, if the calibration status of the terminal carrier frequency is invalid and the terrestrial network is available (the target terminal can connect to the terrestrial network), the target terminal is reconnected to the terrestrial network, and the process of calibrating the terminal carrier frequency of the target terminal based on the downlink reference signal and downlink synchronization signal of the terrestrial network is repeated until the terminal carrier frequency is accurate and the crystal oscillator calibration status is valid. The process of calculating the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink is the same as in step S3023, and will not be repeated here.

[0066] Addressing extreme dynamic scenarios and signal obstruction: High-speed terminal movement makes the overall frequency offset model extremely complex, significantly increasing the difficulty of prediction; in complex environments, satellite signals may be briefly obstructed, causing the terminal to lose lock. If the crystal oscillator drifts during signal reacquisition, pure ephemeris prediction algorithms cannot detect this new crystal oscillator error. As long as the terminal can briefly connect to the ground network, it only takes milliseconds to immediately resynchronize the crystal oscillator frequency to a high-precision reference, resetting all accumulated frequency errors.

[0067] Step S3025: If the crystal oscillator calibration status is invalid and the ground network is unavailable, determine whether the preset operating conditions of the ephemeris prediction Doppler method are met. If the preset operating conditions of the ephemeris prediction Doppler method are met, use the ephemeris prediction Doppler method to estimate the uplink Doppler frequency offset value as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

[0068] Specifically, if the terminal carrier frequency calibration status is invalid and the ground network is unavailable (the target terminal can connect to the ground network), the terminal carrier frequency cannot be calibrated through the ground network. In this case, it is necessary to determine whether the currently stored satellite ephemeris meets the preset operating conditions of the ephemeris prediction Doppler method. For example, if the target terminal has no GNSS or the GNSS is out of lock, or the currently stored satellite ephemeris data is expired, or if the target terminal integrates or connects some environmental sensors (detecting a sudden change in air pressure altitude) and determines that the terminal may have entered the underground, it means that the satellite ephemeris cannot meet the preset operating conditions of the ephemeris prediction Doppler method. In addition to the above three determination methods, machine learning algorithms can also be used to learn the user's historical behavior to optimize decision-making.

[0069] If the preset operating conditions of the ephemeris prediction Doppler method are met, the Doppler frequency shift value is estimated based on the currently stored satellite ephemeris and used as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network for uplink.

[0070] To differentiate the methods for calculating the Doppler frequency shift pre-compensation value when the target terminal accesses the satellite network uplink, a pre-estimated Doppler frequency shift compensation flag is set. When the downlink carrier frequency can be calibrated via a terrestrial network, the pre-estimated Doppler frequency shift compensation flag is set to 0, locking the downlink carrier frequency after crystal oscillator calibration. Based on the downlink carrier frequency, the satellite downlink reference signal transmitted by the satellite network is received, and the Doppler frequency shift pre-compensation value for the target terminal's uplink access to the satellite network is calculated based on the satellite downlink reference signal. When the downlink carrier frequency cannot be obtained by calibrating the crystal oscillator via a terrestrial network, and the preset operating conditions of the ephemeris-based Doppler estimation method are met, the pre-estimated Doppler frequency shift compensation flag is set to 1, and the Doppler frequency shift value estimated using the ephemeris-based Doppler estimation method is used as the Doppler frequency shift pre-compensation value for the target terminal's uplink access to the satellite network.

[0071] The ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method provided in this embodiment calculates the compensation value directly using the satellite downlink reference signal when the crystal oscillator calibration is effective, ensuring accuracy and efficiency. When the crystal oscillator calibration is ineffective but the ground network is available, the crystal oscillator is recalibrated before calculation to ensure the reliability of the compensation benchmark. When the crystal oscillator calibration is ineffective and the ground network is unavailable, an ephemeris prediction method is used as a fallback to cover network-free scenarios. This method adapts to different network environments, ensures the accuracy of frequency offset compensation, improves the success rate and stability of terminal access to the satellite network, and avoids the limitations of over-reliance on a single method, taking into account practicality and reliability in multiple scenarios.

[0072] In some optional implementations, step S3024 above, which involves receiving the downlink reference signal transmitted by the satellite network based on the downlink carrier frequency generated by the target terminal's crystal oscillator, and calculating the Doppler frequency offset pre-compensation value for the target terminal's uplink access to the satellite network based on the downlink reference signal transmitted by the satellite network, includes: Step a1: Receive the downlink reference signal transmitted by the satellite network based on the downlink carrier frequency generated by the target terminal crystal oscillator, and estimate the downlink carrier frequency offset value based on the downlink reference signal transmitted by the satellite network.

[0073] Specifically, since the crystal oscillator frequency of the target terminal is calibrated using the terrestrial network, the downlink carrier frequency offset is estimated by receiving the satellite downlink reference signal transmitted by the satellite network based on the downlink carrier frequency generated by the target terminal. The carrier frequency offset component caused by the frequency offset of the crystal oscillator is relatively small.

[0074] Step a2: The estimated downlink carrier frequency offset value is used as the downlink Doppler frequency offset estimate value. Combined with the preset relationship between the uplink Doppler spectrum and the downlink Doppler frequency offset, the Doppler frequency offset pre-compensation value for the target terminal when accessing the satellite network uplink is determined based on the downlink Doppler frequency offset estimate value.

[0075] Specifically, the initial total frequency offset of the satellite reference signal received by the target terminal consists of two parts:

[0076] in, This represents the initial total frequency offset. This indicates the reference Doppler frequency offset (which is the true value). This indicates the downlink carrier frequency offset caused by the residual frequency offset of the crystal oscillator.

[0077] The initial total frequency offset value estimated using the frequency offset estimation algorithm, based on the satellite network downlink carrier frequency received from the target terminal and the satellite network downlink reference signal transmitted by the satellite network, includes estimation error, i.e.:

[0078] in, This represents the initial total frequency offset estimate. The estimated frequency offset error for the terminal is typically within ±0.1 ppm. Residual frequency offset of the crystal oscillator. The error between the terminal frequency offset estimation and the error is generally very small, within ±0.1ppm. Therefore, within the preset error range, the initial total frequency offset estimate can be equivalent to the downlink Doppler frequency offset.

[0079] At a certain moment, Doppler shift ,in The speed at which the satellite moves relative to the ground terminal. This is the angle between the relative movement direction and the carrier propagation direction. Assume the downlink carrier frequency is... The uplink carrier frequency is Downlink Doppler frequency shift is Ignore the satellite's relative speed to the ground terminal caused by the time variation between downlink reception and uplink transmission times. and To compensate for the uplink Doppler frequency shift during uplink transmission, the uplink Doppler frequency shift should be pre-compensated in the opposite direction. .

[0080] The following is the verification process for this embodiment: When the estimated Doppler frequency shift compensation flag is 0, the true total frequency offset estimated using the satellite downlink signal is used as the estimated value of the Doppler frequency shift. After conversion, pre-compensation is applied to the uplink transmission frequency. At this point, the downlink frequency locked by the terminal is:

[0081] The estimated value of the downlink Doppler frequency shift is converted and used as the uplink Doppler frequency shift pre-compensation value:

[0082] Uplink transmission frequency for:

[0083] Therefore, the uplink transmission frequency and uplink carrier frequency The deviation is: =

[0084] The signal frequency and uplink carrier frequency received by the satellite gateway The deviation is:

[0085] From the above formula, we can see that It is approximately within the range of ±0.3 ppm.

[0086] The ground network-assisted satellite-to-ground multimode terminal frequency synchronization method provided in this embodiment receives the satellite downlink reference signal based on the downlink carrier frequency calibrated by the terminal. It first estimates the downlink carrier frequency offset, and then uses the preset relationship between uplink and downlink Doppler frequency offsets to derive the uplink pre-compensation value from the downlink frequency offset. This method relies on the stable frequency reference after ground calibration to ensure the accuracy of downlink frequency offset estimation, and efficiently obtains the uplink compensation value through the correlation rules between uplink and downlink frequency offsets, avoiding the complex process of independently calculating the uplink frequency offset.

[0087] In some optional implementations, step S3025 above, which uses the ephemeris prediction Doppler method to predict the uplink Doppler frequency offset value as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network, includes: Step b1: Obtain the ephemeris of the target satellite and determine the satellite position and velocity based on the ephemeris.

[0088] Specifically, the ephemeris contains the satellite's orbital parameters. The terminal uses the Kepler orbital model or numerical integration method, combined with the ephemeris timestamp, to calculate the satellite's three-dimensional position (x, y) at a given moment. s ,y s ,z s ) and three-dimensional velocity (v) sx ,v sy ,v sz ).

[0089] Step b2: Obtain the terminal position and terminal velocity of the target terminal, and determine the relative position between the satellite and the terminal based on the satellite position and the terminal position, and determine the relative velocity between the satellite and the terminal based on the satellite velocity and the terminal velocity.

[0090] Specifically, a rough location (x) can be obtained through cell positioning of a terrestrial network. t ,y t ,z t The accuracy is in the hundreds of meters range. Although the accuracy is slightly lower, it can meet the basic requirements for Doppler prediction.

[0091] Calculate the relative position vector between the satellite and the terminal using their position coordinates: Then calculate the magnitude of the vector. (Straight-line distance between satellite and terminal), and unit vector (The direction of the relative position).

[0092] The relative velocity of the satellite to the terminal: When the target terminal is stationary, its velocity is 0, i.e., v. tx =v ty =v tz =0.

[0093] Step b3: Calculate the relative radial velocity based on the relative position and relative velocity, and based on the relative radial velocity, use the Doppler frequency shift formula to estimate the Doppler frequency offset value as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network for uplink.

[0094] Specifically, the Doppler frequency shift is essentially a frequency change caused by the relative radial velocity (the velocity component along the signal propagation direction) between the satellite and the terminal. The relative radial velocity v is calculated... r (Projection of relative velocity onto the unit position vector): Finally, the Doppler frequency shift value is calculated using the Doppler frequency shift formula: If v r A positive value indicates that the satellite is close to the terminal, and the Doppler frequency shift value is positive; if v rA negative value indicates that the satellite is far from the terminal, and the Doppler frequency shift value is negative.

[0095] The following is the verification process for this embodiment: When the estimated Doppler frequency shift compensation flag is 1, the terminal calculates the theoretical Doppler frequency offset value based on the ephemeris and its own position. When the terminal sends an uplink signal, it directly compensates for this predicted Doppler frequency offset. .

[0096] Therefore, the actual uplink transmission frequency is:

[0097] The signal frequency and uplink carrier frequency received by the satellite gateway The deviation is:

[0098] in, The Doppler prediction error is typically within ±0.5 ppm. Satellite gateway receivers can maintain stable reception performance within a small frequency offset range. Therefore, It can be controlled within ±0.6ppm.

[0099] The ground network-assisted satellite-to-ground multi-mode terminal frequency synchronization method provided in this embodiment determines the satellite position and velocity using ephemeris, combines it with the terminal's own position and velocity, calculates the relative radial velocity, and derives the uplink Doppler frequency offset compensation value. Even when the ground network is unavailable, it can still achieve frequency offset pre-compensation based on ephemeris, filling the gap in frequency offset calculation in scenarios without a ground network. At the same time, through the accurate derivation of relative motion parameters, the accuracy of Doppler frequency offset prediction is ensured, providing a reliable frequency compensation basis for the terminal's uplink access to the satellite network. This not only improves the success rate of satellite access in remote, network-free areas but also avoids strong dependence on the ground network, enhancing the scenario adaptability and communication stability of the satellite-to-ground multi-mode terminal.

[0100] In one specific embodiment, such as Figure 5 The diagram shows the process of a terminal accessing a satellite network. The terminal automatically triggers the satellite network access preparation process and begins to detect the ground network frequency calibration status and ground network availability.

[0101] Check if the ground network frequency calibration is effective: The terminal retrieves the locally stored "Ground Network Frequency Calibration Record" and checks the duration of disconnection from the ground network and the crystal oscillator frequency drift. If the calibration status is effective (the disconnection time is less than the preset threshold and the crystal oscillator drift is within the satellite detection frequency range), it directly enters the "Terminal performs satellite network downlink synchronization" step; if the calibration status is invalid, it enters the "Ground network availability" judgment step.

[0102] Determining the availability of the terrestrial network: The terminal scans for surrounding 5G signals. If the terrestrial network is available (detection of urban 5G base station signals, RSRP > -100dBm), then the "terminal access to the terrestrial network for frequency locking" is executed: using the synchronization and reference signals of the 5G base station, the residual frequency offset of the crystal oscillator is calibrated to within ±0.1ppm, and the latest satellite ephemeris data is downloaded. If the terrestrial network is unavailable (entering uninhabited mountainous areas with no 5G signal), then the "effectiveness of Doppler prediction based on ephemeris" judgment step is entered.

[0103] Determining the validity of Doppler shift based on ephemeris: The terminal checks the validity and integrity of the locally stored satellite ephemeris. If the ephemeris is valid (not expired and the data is complete), then the "Terminal uses ephemeris to estimate Doppler shift" function is executed. (1) Obtain the ephemeris of the target satellite and determine its position and velocity; (2) Calculate the relative position and relative velocity between the satellite and the terminal by combining the GNSS positioning position and the speed of movement of the terminal; (3) Based on the relative radial velocity, the theoretical Doppler frequency offset value is calculated using the Doppler frequency shift formula.

[0104] (4) If the ephemeris is invalid (expired or corrupted), return to the “Terminal initiates satellite network access preparation” step and try the process again.

[0105] The terminal performs downlink synchronization with the satellite network and sends uplink access signals: If the "ground network frequency calibration is valid" or "ground network calibration is successful", the terminal uses the calibrated crystal oscillator frequency as a reference to receive the downlink reference signal transmitted by the satellite network, completes the downlink synchronization with the satellite network, and then sends uplink access signals.

[0106] If the Doppler effect is predicted by the ephemeris, the terminal adjusts the transmission frequency and sends the uplink access signal by combining the residual frequency offset of the crystal oscillator and the predicted Doppler frequency offset.

[0107] If the number of failed access attempts exceeds N, the satellite gateway checks the access signal. If access is successful, satellite network communication is established, and the process ends. If access fails, the number of failures is recorded. If the number of failures is not greater than N (e.g., N=3), the process returns to the "Terminal performs satellite network downlink synchronization" step and attempts to access again. If the number of failures exceeds N, the process returns to the "Terminal initiates satellite network access preparation" step and restarts the entire process.

[0108] This embodiment also provides a terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization device, which is used to implement the above embodiments and preferred embodiments; details already described will not be repeated. As used below, the term "module" can refer to a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware implementation, or a combination of software and hardware, is also possible and contemplated.

[0109] This embodiment provides a terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization device, such as... Figure 6 As shown, it includes: The frequency calibration module 601 is used for the target terminal to access the terrestrial network and to calibrate the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network.

[0110] The uplink access Doppler frequency offset pre-compensation module 602 is used to respond to satellite connection requests, check whether the crystal oscillator calibration status of the target terminal is valid, select the corresponding Doppler frequency offset pre-compensation method according to the check results, and calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network using the selected Doppler frequency offset pre-compensation method.

[0111] In some alternative implementations, the frequency calibration module 601 includes: The frequency coarse adjustment unit is used to acquire the downlink synchronization signal of the terrestrial network and align the current downlink carrier frequency of the target terminal with the downlink carrier frequency of the downlink synchronization signal by adjusting the control voltage of the crystal oscillator, thereby obtaining the coarse downlink carrier frequency of the target terminal.

[0112] The frequency fine-tuning unit is used to obtain the downlink reference signal of the terrestrial network based on the coarse adjustment of the downlink carrier frequency, and to dynamically adjust the control voltage of the crystal oscillator and calibrate the crystal oscillator frequency based on the downlink reference signal.

[0113] In some optional implementations, the uplink access Doppler frequency offset pre-compensation module 602 includes: The status data acquisition unit is used to obtain the duration of the disconnection between the target terminal and the ground network, and the crystal oscillator frequency drift.

[0114] The calibration status judgment unit is used to determine if the crystal oscillator calibration status of the target terminal is invalid if the duration is greater than a preset time threshold or the crystal oscillator frequency drift is greater than a preset range; otherwise, the calibration status is valid.

[0115] The first frequency offset pre-compensation value calculation unit is used to receive the downlink reference signal transmitted by the satellite network based on the downlink carrier frequency generated by the target terminal crystal oscillator if the crystal oscillator calibration state is valid, and to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network for uplink based on the downlink reference signal transmitted by the satellite network.

[0116] The second frequency offset pre-compensation value calculation unit is used to repeatedly calibrate the crystal frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network if the crystal oscillator calibration state is invalid and the terrestrial network is available, until the crystal oscillator calibration state is valid. Based on the downlink carrier frequency generated by the crystal oscillator of the target terminal, the unit receives the downlink reference signal transmitted by the satellite network and calculates the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink based on the downlink reference signal transmitted by the satellite network.

[0117] The third frequency offset pre-compensation value calculation unit is used to determine whether the preset operating conditions of the ephemeris prediction Doppler method are met if the crystal oscillator calibration state is invalid and the ground network is unavailable. If the preset operating conditions of the ephemeris prediction Doppler method are met, the uplink Doppler frequency offset value is estimated by the ephemeris prediction Doppler method as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

[0118] The terrestrial network-assisted satellite-to-ground multi-mode terminal frequency synchronization device provided in this embodiment of the invention can execute the terrestrial network-assisted satellite-to-ground multi-mode terminal frequency synchronization method provided in any embodiment of the invention, and has the corresponding functional modules and beneficial effects for executing the method. Further functional descriptions of the above modules and units are the same as in the corresponding embodiments described above, and will not be repeated here.

[0119] Figure 7 This is a schematic diagram of the structure of an electronic device provided in an embodiment of the present invention.

[0120] The following is a detailed reference. Figure 7 This diagram illustrates a suitable structural schematic for implementing an electronic device according to embodiments of the present invention. The electronic device may include a processor (e.g., a central processing unit, graphics processor, etc.) 701, which can perform various appropriate actions and processes based on a program stored in read-only memory (ROM) 702 or a program loaded from memory 708 into random access memory (RAM) 703. The RAM 703 also stores various programs and data required for the operation of the electronic device. The processor 701, ROM 702, and RAM 703 are interconnected via a bus 704. An input / output (I / O) interface 705 is also connected to the bus 704.

[0121] Typically, the following devices can be connected to I / O interface 705: input devices 706 including, for example, touchscreens, touchpads, keyboards, mice, cameras, microphones, accelerometers, gyroscopes, etc.; output devices 707 including, for example, liquid crystal displays (LCDs), speakers, vibrators, etc.; memory devices 708 including, for example, magnetic tapes, hard disks, etc.; and communication devices 709. Communication device 709 allows electronic devices to exchange data via wireless or wired communication with other devices. Although Figure 7 Electronic devices with various devices are shown, but it should be understood that it is not required to implement or have all of the devices shown, and more or fewer devices may be implemented or have instead.

[0122] In particular, according to embodiments of the present invention, the processes described above with reference to the flowcharts can be implemented as computer software programs. For example, embodiments of the present invention include a computer program product comprising a computer program carried on a non-transitory computer-readable medium, the computer program containing program code for performing the methods shown in the flowcharts. In such embodiments, the computer program can be downloaded and installed from a network via a communication device 709, or installed from a memory 708, or installed from a ROM 702. When the computer program is executed by the processor 701, it performs the functions defined in the terrestrial network-assisted satellite-to-ground multimode terminal frequency synchronization method of the embodiments of the present invention.

[0123] Figure 7 The electronic device shown is merely an example and should not be construed as limiting the functionality and scope of use of the embodiments of the present invention.

[0124] This invention also provides a computer-readable storage medium. The methods described above according to embodiments of the invention can be implemented in hardware or firmware, or implemented as computer code that can be recorded on a storage medium, or implemented as computer code downloaded via a network and originally stored on a remote storage medium or a non-transitory machine-readable storage medium and then stored on a local storage medium. Thus, the methods described herein can be processed by software stored on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. The storage medium can be a magnetic disk, optical disk, read-only memory, random access memory, flash memory, hard disk, or solid-state drive, etc.; further, the storage medium can also include combinations of the above types of memory. It is understood that computers, processors, microprocessor controllers, or programmable hardware include storage components capable of storing or receiving software or computer code, which, when accessed and executed by the computer, processor, or hardware, implements the methods shown in the above embodiments.

[0125] A portion of this invention can be applied as a computer program product, such as computer program instructions, which, when executed by a computer, can invoke or provide the methods and / or technical solutions according to the invention through the operation of the computer. Those skilled in the art will understand that the forms in which computer program instructions exist in a computer-readable medium include, but are not limited to, source files, executable files, installation package files, etc. Correspondingly, the ways in which computer program instructions are executed by a computer include, but are not limited to: the computer directly executing the instructions, or the computer compiling the instructions and then executing the corresponding compiled program, or the computer reading and executing the instructions, or the computer reading and installing the instructions and then executing the corresponding installed program. Here, the computer-readable medium can be any available computer-readable storage medium or communication medium accessible to a computer.

[0126] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A ground network-assisted frequency synchronization method for satellite-to-ground multi-mode terminals, characterized in that, The method includes: The target terminal accesses the terrestrial network, and the crystal oscillator frequency of the target terminal is calibrated based on the downlink synchronization signal and downlink reference signal of the terrestrial network; In response to a satellite connection request, the system checks whether the crystal oscillator calibration status of the target terminal is valid, and selects a corresponding Doppler frequency offset pre-compensation method based on the check result. The selected Doppler frequency offset pre-compensation method is then used to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink. Checking the validity of the crystal oscillator calibration status of the target terminal includes: obtaining the duration of the disconnection between the target terminal and the ground network, and the crystal oscillator frequency drift. If the duration is greater than a preset time threshold, or the crystal oscillator frequency drift is greater than a preset range, then the crystal oscillator calibration status of the target terminal is invalid; otherwise, the calibration status is valid.

2. The method according to claim 1, characterized in that, The crystal oscillator frequency of the target terminal is calibrated based on the downlink synchronization signal and downlink reference signal of the terrestrial network, including: The downlink synchronization signal of the terrestrial network is acquired, and the current downlink carrier frequency of the target terminal is aligned with the downlink carrier frequency of the downlink synchronization signal by adjusting the control voltage of the crystal oscillator, so as to obtain the coarse downlink carrier frequency of the target terminal. Based on the coarse adjustment of the downlink carrier frequency, the downlink reference signal of the terrestrial network is obtained, and the control voltage of the crystal oscillator is dynamically adjusted based on the downlink reference signal to calibrate the crystal oscillator frequency.

3. The method according to claim 1, characterized in that, Based on the inspection results, select the corresponding Doppler frequency offset pre-compensation method, and use the selected Doppler frequency offset pre-compensation method to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink, including: If the crystal oscillator calibration status is valid, the downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal crystal oscillator, and the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network is calculated based on the downlink reference signal transmitted by the satellite network. If the crystal oscillator calibration status is invalid and the terrestrial network is available, the crystal oscillator frequency of the target terminal is repeatedly calibrated based on the downlink synchronization signal and downlink reference signal of the terrestrial network until the crystal oscillator calibration status is valid. The downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the crystal oscillator of the target terminal, and the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network is calculated based on the downlink reference signal transmitted by the satellite network. If the crystal oscillator calibration status is invalid and the ground network is unavailable, it is determined whether the preset operating conditions of the ephemeris prediction Doppler method are met. If the preset operating conditions of the ephemeris prediction Doppler method are met, the uplink Doppler frequency offset value is estimated by the ephemeris prediction Doppler method as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink.

4. The method according to claim 3, characterized in that, Based on the downlink carrier frequency generated by the target terminal's crystal oscillator, the target terminal receives the downlink reference signal transmitted by the satellite network, and calculates the Doppler frequency offset pre-compensation value for uplink access to the satellite network based on the downlink reference signal transmitted by the satellite network, including: The downlink reference signal transmitted by the satellite network is received based on the downlink carrier frequency generated by the target terminal crystal oscillator, and the downlink carrier frequency offset value is estimated based on the downlink reference signal transmitted by the satellite network. The downlink carrier frequency offset value is used as the downlink Doppler frequency offset estimate. Combined with the preset relationship between the uplink Doppler spectrum and the downlink Doppler frequency offset, the Doppler frequency offset pre-compensation value for the target terminal when accessing the satellite network uplink is determined based on the downlink Doppler frequency offset estimate.

5. The method according to claim 3, characterized in that, The method of ephemeris prediction to estimate the uplink Doppler frequency offset value is used as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network, including: Obtain the ephemeris of the target satellite, and determine the satellite position and velocity based on the ephemeris; The system acquires the target terminal's position and velocity, and determines the relative position between the satellite and the terminal based on the satellite position and the terminal position, and determines the relative velocity between the satellite and the terminal based on the satellite velocity and the terminal velocity. The relative radial velocity is calculated based on the relative position and relative velocity. Based on the relative radial velocity, the Doppler frequency offset value is estimated using the Doppler frequency shift formula as the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network for uplink.

6. A ground network-assisted satellite-to-ground multimode terminal frequency synchronization device, characterized in that, The device includes: A frequency calibration module is used for the target terminal to access the terrestrial network and to calibrate the crystal oscillator frequency of the target terminal based on the downlink synchronization signal and downlink reference signal of the terrestrial network. The uplink access Doppler frequency offset pre-compensation module is used to respond to a satellite connection request, check whether the crystal oscillator calibration status of the target terminal is valid, and select the corresponding Doppler frequency offset pre-compensation method according to the check result. It uses the selected Doppler frequency offset pre-compensation method to calculate the Doppler frequency offset pre-compensation value when the target terminal accesses the satellite network uplink. Checking whether the crystal oscillator calibration status of the target terminal is valid includes: obtaining the duration of the disconnection between the target terminal and the ground network and the crystal oscillator frequency drift; if the duration is greater than a preset time threshold, or the crystal oscillator frequency drift is greater than a preset range, then the crystal oscillator calibration status of the target terminal is invalid; otherwise, the calibration status is valid.

7. An electronic device, characterized in that, include: A memory and a processor, the memory and the processor being communicatively connected to each other, the memory storing computer instructions, the processor executing the computer instructions to perform the method of any one of claims 1 to 5.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer instructions for causing a computer to perform the method of any one of claims 1 to 5.

9. A computer program product, characterized in that, Includes computer instructions for causing a computer to perform the method of any one of claims 1 to 5.

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