Satellite narrowband communication terminal

Abstract

The invention belongs to the technical field of communication, and particularly relates to a satellite narrowband communication terminal. Comprising a radio frequency receiving module, a frequency tracking module and a radio frequency transmitting module, the radio frequency receiving module is used for receiving a radio frequency signal transmitted from a satellite link; and the frequency tracking module is used for selectively switching frequency tracking modes by setting a reasonable threshold value according to the Doppler frequency shift change rate of the baseband signal under different motion states of the satellite relative to the ground communication terminal. A proper threshold value is set through a Doppler frequency shift change rate, so that a fast frequency tracking mode and a fine frequency tracking mode are dynamically switched, a ground communication terminal can adaptively process different motion scenes and frequency shift change conditions of a satellite, and the stability and reliability of a communication link are ensured; and the method has very high practicability in application scenes of relative high-speed movement of satellites and a narrowband communication environment.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and in particular relates to a satellite narrowband communication terminal. Background Art

[0002] Narrowband communication technology is commonly used in the field of low-orbit satellite communications. Since low-orbit satellites and ground terminals have relatively high-speed motion phases and relatively stable motion phases, existing satellite communication terminals usually adopt fixed compensation strategies when processing Doppler frequency shift. However, due to the low spectrum bandwidth, narrowband communication signals are often greatly affected by the Doppler effect, and the motion state of low-orbit satellites varies greatly, making it difficult to effectively cope with the dynamic changes of frequency shift under different motion states.

[0003] This fixed-mode frequency compensation method is often only effective for scenarios where the frequency offset is large during a single high-speed satellite orbital motion or small during steady motion. It is difficult to ensure the stability of the communication link throughout the entire process, which in turn leads to data transmission interruption or unstable communication signals. Summary of the Invention

[0004] In order to solve the technical problem that the existing fixed-mode frequency compensation method in the above-mentioned background technology can often only be effectively used for a single high-speed satellite orbit motion scenario and it is difficult to ensure the stability of the communication link in all stages; the present invention provides a satellite narrowband communication terminal; innovatively introduces a frequency tracking mode switching mechanism based on the Doppler frequency shift change rate; by setting a reasonable threshold, the terminal can dynamically switch between the fast frequency tracking mode and the fine frequency tracking mode to ensure adaptation to different satellite motion states and frequency shift changes.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A satellite narrowband communication terminal, comprising a radio frequency receiving module, a frequency tracking module and a radio frequency transmitting module

[0007] The RF receiving module is used to receive the RF signal transmitted from the satellite link, and perform down-conversion, filtering and demodulation processing to convert it into a baseband signal;

[0008] The frequency tracking module is used to selectively switch the frequency tracking mode, including fast frequency tracking mode or fine frequency tracking mode, by setting a reasonable threshold according to the Doppler frequency shift change rate of the baseband signal under different motion states of the satellite relative to the ground communication terminal;

[0009] The fast frequency tracking mode or the fine frequency tracking mode uses different frequency shift compensation algorithms to dynamically compensate the transmitting frequency and receiving frequency of the ground communication terminal to ensure the stability of the communication link;

[0010] The RF transmission module is used to dynamically correct the transmission frequency of the ground communication terminal based on the real-time calculation results of the frequency tracking mode;

[0011] The RF receiving module is also used to dynamically correct the receiving frequency of the ground communication terminal based on the real-time calculation results of the frequency tracking mode;

[0012] The implementation process of the frequency tracking module includes:

[0013] Extract features from the received baseband signal and calculate the Doppler frequency shift rate in the baseband signal in real time by analyzing its frequency variation characteristics;

[0014] Setting a Doppler frequency shift change rate threshold to switch between different frequency tracking modes;

[0015] When the Doppler frequency shift change rate is greater than the set threshold, it switches to the fast frequency tracking mode. The fast frequency tracking mode uses the Doppler frequency shift formula to calculate the compensation amount to adapt to the change of large frequency offset;

[0016] When the Doppler frequency shift change rate is less than the set threshold, it switches to the fine frequency tracking mode. The fine frequency tracking mode uses the phase difference method to calculate the compensation amount to achieve higher-precision Doppler frequency shift compensation.

[0017] Preferably, the Doppler shift rate of change is calculated as follows:

[0018] The Doppler frequency shift rate is calculated by calculating the Doppler frequency shift rate over time. The calculation formula is:

[0019]

[0020] Where, is the rate of change of Doppler frequency shift in a specific time interval; dt is the specific time interval; Δf c is the Doppler frequency shift of the downlink signal.

[0021] Preferably, the set threshold value of the Doppler frequency shift change rate is a value between 600-800 Hz / s.

[0022] Preferably, the process of calculating the compensation amount using the Doppler frequency shift formula in the fast frequency tracking mode is as follows:

[0023] The frequency offset caused by the Doppler effect is calculated based on the relative speed between the satellite and the ground terminal; the calculation formula is:

[0024]

[0025] Where Δf u is the Doppler frequency shift of the uplink signal, fu is the standard frequency of the uplink signal; Δf c is the Doppler frequency shift of the downlink signal, f c is the standard frequency of the downlink signal; v is the relative speed between the satellite and the ground communication terminal, and c is the speed of light;

[0026] Compensation is performed according to the Doppler frequency shift. The compensation calculation formula for the transmit frequency in the fast frequency tracking mode is:

[0027]

[0028] Where, f u ′ is the transmitting frequency after compensating the frequency shift;

[0029] Similarly, the compensation calculation formula for the receiving frequency in fast frequency tracking mode is:

[0030]

[0031] Where, f c ′ is the received frequency after compensating the frequency shift.

[0032] Preferably, the process of calculating the compensation amount using the phase difference method in the fine frequency tracking mode is as follows:

[0033] S1) Sample the received baseband signal, set an appropriate sampling rate, and convert the continuous sampled data analog signal into a discrete digital signal, which can be expressed as:

[0034] x[n]=x(t), t=nT s , n=0,1,2,…;

[0035] Where x[n] is the discrete digital signal after sampling; T s is the sampling period, n is the sampling point number;

[0036] S2) low-pass filtering is performed on the sampled discrete digital signal to extract the baseband frequency component; the filtering formula for the baseband frequency component is:

[0037]

[0038] Where y[n] represents the baseband frequency component; f c is the standard frequency of the downlink signal, Indicates a mixing operation;

[0039] S3) Calculate the signal phase information in the baseband frequency component, and then extract the signal phase sequence to analyze the change of signal frequency. The signal phase sequence can be expressed as:

[0040]

[0041] Where θ[n] is the phase sequence of the signal; is the imaginary part of the signal, is the real part of the signal;

[0042] S4) Frequency offset will cause the phase to change over time. The instantaneous frequency offset is calculated by the phase difference of adjacent sampling points. The calculation formula is:

[0043] Δθ[n]=θ[n]-θ[n-1];

[0044]

[0045] Where Δθ[n] is the phase difference, which represents the phase change between adjacent sampling points; f o [n] is the instantaneous frequency offset; T s is the sampling period, n is the sampling point number;

[0046] S5) performing corresponding compensation according to the instantaneous frequency offset, wherein the compensation calculation formula for the transmit frequency in the fine frequency tracking mode is:

[0047] f′ u =f u -f o [n];

[0048] Where, f u ′ is the transmitting frequency after compensating the frequency shift; f u It is the standard frequency of the uplink signal;

[0049] The compensation calculation formula for the receiving frequency in fine frequency tracking mode is:

[0050] f c ′=f c -f o [n];

[0051] Where, f c ′ is the receiving frequency after compensating the frequency shift; f c It is the standard frequency of the downlink signal;

[0052] Preferably, between step S4) and step S5), the method further includes: performing Kalman filtering or moving average filtering on the calculated instantaneous frequency offset, and then performing subsequent corresponding compensation calculation.

[0053] Preferably, the relative speed between the satellite and the ground communication terminal, the standard frequency of the uplink signal of the communication terminal, and the standard frequency of the downlink signal can all be obtained from known satellite orbit information.

[0054] Preferably, the radio frequency receiving module includes a down converter, a local oscillator, a filter and a demodulator;

[0055] The downconverter is used to convert the high-frequency satellite signal into a baseband signal;

[0056] The local oscillator is used to dynamically adjust the local oscillator frequency according to the received frequency after compensating the frequency shift, ensuring that the received RF signal is correctly mixed in conjunction with the downconverter;

[0057] The filter is used to filter out interference signals and retain the target frequency band signal;

[0058] The demodulator is used to modulate the baseband signal and convert it into a baseband digital signal or analog signal.

[0059] Preferably, the radio frequency transmission module includes a local oscillator and a power amplifier; the local oscillator is used to dynamically adjust the local oscillator frequency according to the transmission frequency after compensating the frequency shift;

[0060] The power amplifier is used to amplify the modulated and frequency-adjusted RF signal.

[0061] Preferably, the frequency tracking module includes a phase calculator, a pilot timer and a phase-locked loop controller;

[0062] The phase calculator is used to calculate the phase, i.e., the argument, based on the real and imaginary parts of the baseband signal;

[0063] The pilot timer is used to convert the pilot signal in the baseband signal into a time signal and provide a time count in the phase difference calculation process;

[0064] The phase-locked loop controller is used to calculate the Doppler frequency shift change rate in real time, and compare the Doppler frequency shift change rate with the set threshold to determine the difference, and then selectively switch between different frequency tracking modes; the phase-locked loop controller is also used to calculate the Doppler frequency shift compensation amount in real time and output the transmitting frequency or receiving frequency after the frequency shift is compensated.

[0065] Beneficial effects of the present invention:

[0066] 1. The communication terminal of the present invention consists of a radio frequency receiving module, a frequency tracking module, and a radio frequency transmitting module, each responsible for functions such as radio frequency signal reception and processing, frequency tracking and compensation, and transmit frequency adjustment. This modular design facilitates independent optimization of each function, improving overall system performance.

[0067] 2. The Doppler frequency shift change rate is used to set an appropriate threshold to achieve dynamic switching between fast frequency tracking mode and fine frequency tracking mode, enabling ground communication terminals to adaptively handle different satellite motion scenarios and frequency shift changes, ensuring the stability and reliability of the communication link.

[0068] 3. Fast frequency tracking mode uses the Doppler shift formula to calculate compensation to accommodate large frequency offsets. Fine frequency tracking mode uses a phase differential method to achieve higher-precision Doppler shift compensation. The communication terminal can adjust its transmit and receive frequencies based on the relative motion of the satellite and ground terminal through precise frequency tracking and real-time compensation. This makes it highly practical for applications involving relatively high-speed satellite motion and narrowband communication environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0070] Figure 1 The figure is a schematic diagram of the framework structure of a satellite narrowband communication terminal according to the present invention.

[0071] Figure 2 This is a flow chart of the frequency tracking module of the present invention.

[0072] Figure 3 This is a flow chart of the calculation method of the fine tracking mode of the present invention. DETAILED DESCRIPTION

[0073] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0074] See also Figure 1 As shown, a satellite narrowband communication terminal includes a radio frequency receiving module, a frequency tracking module and a radio frequency transmitting module

[0075] The RF receiving module is used to receive the RF signal transmitted from the satellite link, and perform down-conversion, filtering and demodulation processing to convert it into a baseband signal;

[0076] The frequency tracking module is used to selectively switch the frequency tracking mode, including fast frequency tracking mode or fine frequency tracking mode, by setting a reasonable threshold according to the Doppler frequency shift change rate of the baseband signal under different motion states of the satellite relative to the ground communication terminal;

[0077] The fast frequency tracking mode or the fine frequency tracking mode uses different frequency shift compensation algorithms to dynamically compensate the transmitting frequency and receiving frequency of the ground communication terminal to ensure the stability of the communication link;

[0078] The RF transmission module is used to dynamically correct the transmission frequency of the ground communication terminal based on the real-time calculation results of the frequency tracking mode;

[0079] The radio frequency receiving module is also used to dynamically correct the receiving frequency of the ground communication terminal according to the real-time calculation results of the frequency tracking mode.

[0080] Furthermore, the radio frequency receiving module includes a down converter, a local oscillator, a filter and a demodulator;

[0081] The downconverter is used to convert the high-frequency satellite signal into a baseband signal;

[0082] The local oscillator is used to dynamically adjust the local oscillator frequency according to the received frequency after compensating the frequency shift, ensuring that the received RF signal is correctly mixed in conjunction with the downconverter;

[0083] The filter is used to filter out interference signals and retain the target frequency band signal;

[0084] The demodulator is used to modulate the baseband signal and convert it into a baseband digital signal or analog signal.

[0085] Furthermore, the radio frequency transmission module includes a local oscillator and a power amplifier; the local oscillator is used to dynamically adjust the local oscillator frequency according to the transmission frequency after compensating the frequency shift;

[0086] The power amplifier amplifies the modulated and frequency-adjusted RF signal to achieve the required transmit power. After adjusting the transmit frequency, the power amplifier transmits the signal to the satellite at the appropriate power level.

[0087] Further, the frequency tracking module includes a phase calculator, a pilot timer and a phase-locked loop controller;

[0088] The phase calculator is used to calculate the phase, i.e., the argument, based on the real and imaginary parts of the baseband signal;

[0089] The pilot timer is used to convert the pilot signal in the baseband signal into a time signal and provide a time count in the phase difference calculation process;

[0090] The phase-locked loop controller is used to calculate the Doppler frequency shift change rate in real time, and compare the Doppler frequency shift change rate with the set threshold to determine the difference, and then selectively switch between different frequency tracking modes; the phase-locked loop controller is also used to calculate the Doppler frequency shift compensation amount in real time and output the transmitting frequency or receiving frequency after the frequency shift is compensated.

[0091] See also Figure 2 and Figure 3 As shown, the implementation process of the frequency tracking module includes:

[0092] Extract features from the received baseband signal and calculate the Doppler frequency shift rate in the baseband signal in real time by analyzing its frequency variation characteristics;

[0093] Setting a Doppler frequency shift change rate threshold to switch between different frequency tracking modes;

[0094] When the Doppler frequency shift change rate is greater than the set threshold, it switches to the fast frequency tracking mode. The fast frequency tracking mode uses the Doppler frequency shift formula to calculate the compensation amount to adapt to the change of large frequency offset;

[0095] When the Doppler frequency shift change rate is less than the set threshold, it switches to the fine frequency tracking mode. The fine frequency tracking mode uses the phase difference method to calculate the compensation amount to achieve higher-precision Doppler frequency shift compensation.

[0096] Specifically, low-Earth orbit (LEO) satellites typically operate at altitudes between 500 and 2000 km and at speeds of approximately 7-8 km / s. For the common narrowband satellite communication frequency band (L-band, 1-2 GHz), the Doppler shift can reach several kHz or even higher. Therefore, the rate of change of frequency offset varies across different frequency bands.

[0097] Furthermore, the Doppler shift rate of change is calculated as follows:

[0098] The Doppler frequency shift rate is calculated by calculating the Doppler frequency shift rate over time. The calculation formula is:

[0099]

[0100] Where, is the rate of change of Doppler frequency shift in a specific time interval; dt is the specific time interval; Δf c is the Doppler frequency shift of the downlink signal.

[0101] Specifically, assuming that the relative speed of the satellite increases from 7.5 km / s to 7.7 km / s in a short period of time, the change time is 1 second. At this time, the frequency offset change rate can be calculated as:

[0102]

[0103] Substitute f c =1GHz, Δt=1s; the calculated rate of change of the frequency offset can reach 667Hz; if the rate of change exceeds this value, the communication system can switch to fast frequency tracking mode.

[0104] Based on the above analysis, a reasonable threshold for the Doppler frequency shift rate can be set between 600 Hz / s and 800 Hz / s, depending on factors such as the design of the ground communication terminal, the operating frequency band, the satellite's orbital altitude and speed. This range covers the changes in the rapid movement and relatively stable phases of most LEO satellites.

[0105] Furthermore, the set threshold value of the Doppler frequency shift change rate is a value between 600-800 Hz / s.

[0106] This invention effectively addresses the frequency offset problem caused by the Doppler effect during satellite communications through the collaborative operation of a radio frequency receiving module, a frequency tracking module, and a radio frequency transmitting module. The frequency tracking module selects the appropriate tracking mode based on a set threshold. Two tracking modes can be switched based on the relative motion of the satellite and ground terminal, respectively, using different frequency shift compensation algorithms to compensate for the transmit and receive frequencies, ensuring the continuity and stability of the satellite communication link.

[0107] The specific working mechanism is as follows: Fast frequency tracking mode: Suitable for scenarios where the satellite moves rapidly or the frequency changes drastically. The frequency compensation amount is quickly calculated through a relatively simple Doppler frequency shift formula to adapt to large frequency offset changes.

[0108] Fine frequency tracking mode: Suitable for scenarios where the relative motion of satellites is slow or the frequency changes are stable. It calculates the compensation amount through the high-precision phase difference method to achieve higher-precision Doppler shift compensation.

[0109] Furthermore, the process of calculating the compensation amount using the Doppler frequency shift formula in the fast frequency tracking mode is as follows:

[0110] The frequency offset caused by the Doppler effect is calculated based on the relative speed between the satellite and the ground terminal; the calculation formula is:

[0111]

[0112] Where Δf u is the Doppler frequency shift of the uplink signal, f u is the standard frequency of the uplink signal; Δf c is the Doppler frequency shift of the downlink signal, f c is the standard frequency of the downlink signal; v is the relative speed between the satellite and the ground communication terminal, and c is the speed of light;

[0113] Compensation is performed according to the Doppler frequency shift. The compensation calculation formula for the transmit frequency in the fast frequency tracking mode is:

[0114]

[0115] Where, f u ′ is the transmitting frequency after compensating the frequency shift;

[0116] Similarly, the compensation calculation formula for the receiving frequency in fast frequency tracking mode is:

[0117]

[0118] Where, f c ′ is the received frequency after compensating the frequency shift.

[0119] Specifically, its factual logic can be intuitively understood as:

[0120] When a ground terminal approaches a satellite, the Doppler shift is positive, so the ground terminal's transmit frequency needs to be appropriately reduced so that the frequency returns to the standard uplink frequency when the satellite receives the signal. At the same time, the receive frequency will be higher than the standard downlink frequency; compensation can correct the receive frequency back to the standard frequency.

[0121] When the ground terminal is farther away from the satellite, the Doppler shift is negative, and the ground terminal's transmit frequency needs to be increased to match the satellite's received frequency with the standard uplink frequency. At the same time, the received frequency will be lower than the standard downlink signal frequency. Compensation can also be used to correct the received frequency back to the standard frequency.

[0122] Furthermore, the process of calculating the compensation amount using the phase difference method in the fine frequency tracking mode is as follows:

[0123] S1) Sample the received baseband signal, set an appropriate sampling rate, and convert the continuous sampled data analog signal into a discrete digital signal, which can be expressed as:

[0124] x[n]=x(t), t=nT s , n=0,1,2,…;

[0125] Where x[n] is the discrete digital signal after sampling; T s is the sampling period, n is the sampling point number;

[0126] S2) low-pass filtering is performed on the sampled discrete digital signal to extract the baseband frequency component; the filtering formula for the baseband frequency component is:

[0127]

[0128] Where y[n] represents the baseband frequency component; f c is the standard frequency of the downlink signal, Indicates a mixing operation;

[0129] S3) Calculate the signal phase information in the baseband frequency component, and then extract the signal phase sequence to analyze the change of signal frequency. The signal phase sequence can be expressed as:

[0130]

[0131] Where θ[n] is the phase sequence of the signal; is the imaginary part of the signal, is the real part of the signal;

[0132] S4) Frequency offset will cause the phase to change over time. The instantaneous frequency offset is calculated by the phase difference of adjacent sampling points. The calculation formula is:

[0133] Δθ[n]=θ[n]-θ[n-1];

[0134]

[0135] Where Δθ[n] is the phase difference, which represents the phase change between adjacent sampling points; f o [n] is the instantaneous frequency offset; T s is the sampling period; n is the sampling point number;

[0136] S5) performing corresponding compensation according to the instantaneous frequency offset, wherein the compensation calculation formula for the transmit frequency in the fine frequency tracking mode is:

[0137] f′ u =f u -f o [n];

[0138] Where, f u ′ is the transmitting frequency after compensating the frequency shift; f u It is the standard frequency of the uplink signal;

[0139] The compensation calculation formula for the receiving frequency in fine frequency tracking mode is:

[0140] f′ c =f c -f o [n];

[0141] Where, f c ′ is the receiving frequency after compensating the frequency shift; f c It is the standard frequency of the downlink signal;

[0142] Furthermore, after step S4), the calculated instantaneous frequency offset is subjected to Kalman filtering or moving average filtering, and then the corresponding compensation calculation is performed, thereby reducing the noise effect in the phase difference calculation and further improving the accuracy of frequency compensation.

[0143] Furthermore, the relative speed between the satellite and the ground communication terminal, the standard frequency of the uplink signal of the communication terminal, and the standard frequency of the downlink signal can all be obtained from the known satellite orbit information.

[0144] It should be noted that the orbital information of the satellite mentioned in the present invention, including orbital altitude, relative speed, etc., can be accurately calculated through a satellite orbital mechanics model (such as the Kepler orbit model).

[0145] The standard frequencies for uplink and downlink signals in satellite communication systems are pre-planned by standards organizations such as the International Telecommunication Union (ITU) and are typically fixed frequency bands. Therefore, the standard frequencies for uplink and downlink signals are known. Therefore, this invention does not perform specific calculations.

[0146] This invention utilizes a frequency tracking mode switching mechanism based on the Doppler shift rate. By setting appropriate thresholds, the terminal can dynamically switch between fast and fine frequency tracking modes, ensuring adaptation to varying satellite motion and frequency shift variations. Fast frequency tracking mode utilizes the Doppler shift formula to account for large frequency offsets, while fine frequency tracking mode achieves high-precision compensation through phase differential methods, ensuring accurate frequency correction in varying frequency shift scenarios.

[0147] This dynamic switching mechanism not only significantly improves the stability and reliability of the communication link, but also precisely adjusts the ground terminal's transmit and receive frequencies based on the real-time motion state. It is particularly suitable for scenarios involving high-speed satellite motion, demonstrating its broad practicality and advancement. Compared to existing technologies, this solution can more flexibly and efficiently address frequency shift issues, ensuring smooth narrowband communications.

[0148] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working processes of the terminals, modules and electronic devices described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0149] In the several embodiments provided in this application, it should be understood that the disclosed terminals, modules, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interface, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0150] In addition, the functional units in the various embodiments of the present application may be integrated into a single processing unit, or each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0151] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, read-only memory), random access memory (RAM, random access memory), disk or optical disk, and other media that can store program code.

[0152] Throughout the specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0153] The above content is merely an example and explanation of the structure of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in a similar manner. As long as they do not deviate from the structure of the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A satellite narrowband communication terminal, characterized in that: It includes a radio frequency receiving module, a frequency tracking module and a radio frequency transmitting module; The RF receiving module is used to receive the RF signal transmitted from the satellite link, and perform down-conversion, filtering and demodulation processing to convert it into a baseband signal; The frequency tracking module is used to selectively switch the frequency tracking mode, including the fast frequency tracking mode or the fine frequency tracking mode, by setting a reasonable threshold value according to the Doppler frequency shift change rate of the baseband signal under different motion states of the satellite relative to the ground communication terminal; The fast frequency tracking mode or the fine frequency tracking mode adopts different frequency shift compensation algorithms to dynamically compensate the transmitting frequency and receiving frequency of the ground communication terminal to ensure the stability of the communication link; The radio frequency transmission module is used to dynamically correct the transmission frequency of the ground communication terminal according to the real-time calculation results of the frequency tracking mode; The RF receiving module is also used to dynamically correct the receiving frequency of the ground communication terminal according to the real-time calculation results of the frequency tracking mode; The implementation process of the frequency tracking module includes: Extract features from the received baseband signal, and calculate the Doppler frequency shift rate in the baseband signal in real time by analyzing its frequency change characteristics; A threshold value of a Doppler frequency shift change rate is set to switch between different frequency tracking modes; When the Doppler frequency shift change rate is greater than the set threshold, it switches to the fast frequency tracking mode, which uses the Doppler frequency shift formula to calculate the compensation amount; When the Doppler frequency shift change rate is less than the set threshold, it switches to the fine frequency tracking mode, which uses the phase difference method to calculate the compensation amount.

2. A satellite narrowband communication terminal according to claim 1, characterized in that: The calculation method of the Doppler frequency shift change rate is: The Doppler frequency shift change rate is calculated by calculating the Doppler frequency shift change over time, and the calculation formula is: In the formula, is the rate of change of Doppler frequency shift in a specific time interval; dt is the specific time interval; Δf c is the Doppler frequency shift of the downlink signal.

3. A satellite narrowband communication terminal according to claim 1, characterized in that: The process of calculating the compensation amount using the Doppler frequency shift formula in the fast frequency tracking mode includes: According to the relative speed between the satellite and the ground terminal, the frequency offset caused by the Doppler effect is calculated; the calculation formula is: Where Δf u is the Doppler frequency shift of the uplink signal, f u is the standard frequency of the uplink signal; Δf c is the Doppler frequency shift of the downlink signal, f c is the standard frequency of the downlink signal; v is the relative speed between the satellite and the ground communication terminal, and c is the speed of light; The corresponding compensation is performed according to the Doppler frequency shift, where the compensation calculation formula for the transmission frequency in the fast frequency tracking mode is: In the formula, f u ′ is the transmitting frequency after compensating the frequency shift; The compensation calculation formula for the receiving frequency in fast frequency tracking mode is: In the formula, f c ′ is the received frequency after compensating the frequency shift.

4. A satellite narrowband communication terminal according to claim 1, characterized in that: The process of calculating the compensation amount using the phase difference method in the fine frequency tracking mode includes: S1) samples the received baseband signal, sets an appropriate sampling rate, and converts the continuous sampled data analog signal into a discrete digital signal, which can be expressed as: x[n]=x(t),t=nT s ,n=0,1,2,…; Where x[n] is the discrete digital signal after sampling; T s is the sampling period, n is the sampling point number; S2) low-pass filtering the sampled discrete digital signal to extract the baseband frequency component; the filtering formula of the baseband frequency component is: Where y[n] represents the baseband frequency component; f c is the standard frequency of the downlink signal, Indicates a mixing operation; S3) calculates the signal phase information in the baseband frequency component, and then extracts the signal phase sequence for analyzing the change of signal frequency. The signal phase sequence can be expressed as: In the formula, θ[n] is the phase sequence of the signal; is the imaginary part of the signal, is the real part of the signal; S4) Frequency offset will cause the phase to change over time. The instantaneous frequency offset is calculated by the phase difference of adjacent sampling points. The calculation formula is: Δθ[n]=θ[n]-θ[n-1]; Where Δθ[n] is the phase difference, which represents the phase change between adjacent sampling points; f o [n] is the instantaneous frequency offset; T s is the sampling period, n is the sampling point number; S5) performing corresponding compensation according to the instantaneous frequency offset, wherein the compensation calculation formula for the transmitting frequency in the fine frequency tracking mode is: f′ u =f u -f o [n]; In the formula, f u ′ is the transmitting frequency after compensating the frequency shift; f u It is the standard frequency of the uplink signal; The compensation calculation formula for the receiving frequency in fine frequency tracking mode is: f c ′=f c -f o [n]; In the formula, f c ′ is the receiving frequency after compensating the frequency shift; f c It is the standard frequency of the downlink signal.

5. A satellite narrowband communication terminal according to claim 4, characterized in that: Between step S4) and step S5), the method further includes: performing Kalman filtering or moving average filtering on the calculated instantaneous frequency offset, and then performing subsequent corresponding compensation calculation.

6. A satellite narrowband communication terminal according to claim 3 or 4, characterized in that: The relative speed between the satellite and the ground communication terminal, the standard frequency of the uplink signal of the communication terminal, and the standard frequency of the downlink signal are all obtained from the known satellite orbit information.

7. A satellite narrowband communication terminal according to any one of claims 1 to 4, characterized in that: The RF receiving module includes a downconverter, a local oscillator, a filter and a demodulator; Downconverters are used to convert high-frequency satellite signals into baseband signals; The local oscillator is used to dynamically adjust the local oscillator frequency according to the received frequency after compensating the frequency shift, so as to ensure the correct mixing of the received RF signal in cooperation with the down converter; The filter is used to filter out interference signals and retain the target frequency band signal; The demodulator is used to modulate the baseband signal and convert it into a baseband digital signal or an analog signal.

8. A satellite narrowband communication terminal according to any one of claims 1 to 4, characterized in that: The radio frequency transmission module includes a local oscillator and a power amplifier; the local oscillator is used to dynamically adjust the local oscillator frequency according to the transmission frequency after compensating the frequency shift; The power amplifier is used to amplify the modulated and frequency adjusted RF signal.

9. A satellite narrowband communication terminal according to any one of claims 1 to 4, characterized in that: The frequency tracking module includes a phase calculator, a pilot timer and a phase-locked loop controller; The phase calculator is used to calculate the phase, i.e. the argument, according to the real part and the imaginary part of the baseband signal; The pilot timer is used to convert the pilot signal in the baseband signal into a time signal and provide a time count in the phase difference calculation process; The phase-locked loop controller is used to calculate the Doppler frequency shift change rate in real time, and compare the Doppler frequency shift change rate with the set threshold value to determine the difference in frequency tracking mode. The phase-locked loop controller is also used to calculate the Doppler frequency shift compensation amount in real time and output the transmitting frequency or receiving frequency after the compensation frequency shift.

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