OFDM (Orthogonal Frequency Division Multiplexing)-based low-orbit satellite communication downlink communication link modeling method and system

Abstract

The invention relates to the technical field of low earth orbit satellite communication, and particularly discloses a low earth orbit satellite communication downlink communication link modeling method and system based on OFDM (Orthogonal Frequency Division Multiplexing), and the method comprises the steps: converting data needing to be transmitted into subcarriers through a transmitting end, and carrying out OFDM modulation on the subcarriers by using an IFFT (Inverse Fast Fourier Transform) method to generate OFDM signals; the method comprises the following steps: establishing a single-path channel model of Gaussian white noise, Doppler effect, satellite-ground free space attenuation and Ku frequency band rain attenuation according to a satellite communication link and an OFDM (Orthogonal Frequency Division Multiplexing) signal; doppler frequency shift compensation is carried out on the OFDM signals through a receiving end, OFDM demodulation is carried out on the OFDM signals through FFT operation, and channel estimation and channel correction are carried out; and visualizing and analyzing the OFDM signal sending process of the sending end and the OFDM signal receiving and demodulating result of the receiving end.

Description

Technical Field

[0001] The present invention relates to the technical field of low-orbit satellite communications, and in particular to a method and system for modeling a low-orbit satellite communication downlink link based on OFDM. Background Art

[0002] There are a lot of literatures on the performance of OFDM (Orthogonal Frequency Division Multiplexing) technology and OFDM-based high-order modulation technology (eg. OFDM-IM), and many new channel estimation algorithms have also emerged. The channel in OFDM is a multipath channel, and the Rice or Rayleigh model is often used in its implementation.

[0003] However, a lot of early research on both OFDM technology and higher-order modulation technology was conducted for ground base station applications. The scene environment affected by urban houses, trees, etc. is a common multipath environment, that is, there are multiple transmission paths, and the parameters of each path are different, such as attenuation and delay. However, the satellite communication channel is very different from the ground communication environment. In the environment where low-orbit constellations are gradually deployed on a large scale and ground terminals communicate at high elevation angles and with strong directionality, there are often strong Doppler effects, free space attenuation, rain attenuation and other channel environments. As OFDM technology is gradually confirmed to be used in low-orbit satellite communications, it is very meaningful to study its performance in satellite space-to-earth communication channels and the performance of various estimation and equalization methods in them. Summary of the invention

[0004] To achieve the purpose of the present invention, the present application provides a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM, comprising: Step S1: The transmitting end converts the data to be transmitted into subcarriers, and uses the IFFT fast Fourier transform method to perform OFDM modulation on the subcarriers to generate an OFDM signal; Step S2: establishing a single-path channel model including Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation and Ku-band rain attenuation according to the satellite communication link and the OFDM signal, so as to simulate the process in which the OFDM signal generates noise interference, phase shift and signal attenuation when passing through the channel environment; Step S3: performing Doppler frequency shift compensation on the OFDM signal through the receiving end, performing OFDM demodulation on the OFDM signal by using FFT operation, and performing channel estimation and channel correction; Step S4: Visualize and analyze the process of transmitting OFDM signals at the transmitting end and the result of receiving and demodulating OFDM signals at the receiving end.

[0005] In some specific embodiments, step S1 includes: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

[0006] In some specific embodiments, step S2 includes: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using a free space propagation model to calculate the attenuation degree of the OFDM signal; Step S22: Determine the Doppler frequency according to the predetermined Starlink satellite and ground receiving end parameters.

[0007] In some specific embodiments, in step S3, the performing of Doppler frequency shift compensation includes: the receiving end obtains the Doppler frequency shift value by calculating through symbol header information or querying according to public information to compensate for the OFDM signal.

[0008] In some specific embodiments, in step S3, the channel estimation adopts LS algorithm and MMSE algorithm, wherein the LS algorithm includes linear interpolation method and cubic spline interpolation method.

[0009] In some specific embodiments, in step S3, the OFDM signal is demodulated by 16QAM demapping, and the demodulated OFDM signal is converted from parallel to serial.

[0010] To achieve the same invention purpose, the present application also provides a low-orbit satellite communication downlink communication link modeling system based on OFDM, comprising: Signal sending module: used to convert the data to be transmitted into subcarriers through the sending end, perform OFDM modulation on the subcarriers using IFFT fast Fourier transform method, and generate OFDM signals; Signal transmission module: used to establish a single-path channel model of Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation and Ku-band rain attenuation according to the satellite communication link and the OFDM signal, so as to simulate the process of noise interference, phase shift and signal attenuation generated by the OFDM signal in the channel environment; Signal receiving module: used for performing Doppler frequency shift compensation on the OFDM signal through the receiving end, performing OFDM demodulation on the OFDM signal by using FFT operation, and performing channel estimation and channel correction; Result analysis module: used to visualize and analyze the process of sending OFDM signals at the transmitting end and the results of receiving and demodulating OFDM signals at the receiving end.

[0011] In some specific embodiments, the signal sending module is used to perform the following steps: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

[0012] In some specific embodiments, the signal transmission module is used to perform the following steps: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using a free space propagation model to calculate the attenuation degree of the OFDM signal; Step S22: Determine the Doppler frequency according to the predetermined Starlink satellite and ground receiving end parameters.

[0013] In some specific embodiments, in the signal receiving module, the Doppler frequency shift compensation includes: the receiving end directly compensates the OFDM signal by calculating the known Doppler frequency shift value through symbol header information or querying public information.

[0014] Beneficial effects of the above technical solution: (1) Compared with the existing technical solutions, this application does not use the existing common ground multipath Rayleigh or Ricean channel model in the OFDM signal transmission process, but instead establishes a single-path channel model including Gaussian white noise, Doppler effect, satellite-ground free space attenuation, and Ku-band rain attenuation, laying a foundation for exploring the application performance of OFDM system signals in low-orbit satellite communications.

[0015] (2) Compared with the existing OFDM system framework, the modeling framework of this application adds a mapping modulation part for the pilot sequence and a part for compensating for the Doppler frequency shift based on the original basis according to the actual satellite communication link conditions.

[0016] (3) This application compares the bit error rates of several different classical channel estimation algorithms in low-orbit satellite communication channels and multipath environments, and draws bit error rate curves, proving that different channel estimation algorithms have different effects in different environments (low-orbit satellite communication channel environment and multipath channel environment), indicating that the study of the application selection of different channel estimation algorithms in different environments is meaningful.

[0017] (4) In the result processing section, this application visualizes and analyzes the reception results of the communication link transmitter, cooperative receiver, and non-cooperative receiver, including IQ two-way waveform diagrams, spectrum diagrams, constellation diagrams, etc., to provide visual assistance for further understanding of the analysis process factors. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0019] Figure 1 A schematic flow chart of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Figure 2 A schematic diagram of bit error rate curves of different channel estimation algorithms under a low-orbit satellite communication channel of a method for modeling a low-orbit satellite communication downlink communication link based on OFDM provided by an embodiment of the present invention; Figure 3 A schematic diagram of bit error rate curves of different channel estimation algorithms in a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by one embodiment of the present invention in a low-orbit satellite communication channel and a multipath environment; Figure 4 A waveform diagram of a signal transmitted by a transmitter of a downlink communication link modeling method for low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Figure 5 A Q-path signal waveform diagram transmitted by a transmitter of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Figure 6 A spectrum diagram of a signal transmitted by a transmitter of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Figure 7 A transmission signal constellation diagram of a method for modeling a low-orbit satellite communication downlink link based on OFDM provided by an embodiment of the present invention; Figure 8 A waveform diagram of a non-cooperative environment receiving I-channel signal of a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by an embodiment of the present invention; Fig. 9 A non-cooperative environment receiving Q-path signal waveform diagram of a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by an embodiment of the present invention; Fig.10 A non-cooperative environment received signal spectrum diagram of a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by an embodiment of the present invention; Fig.11 A constellation diagram of a signal received in a non-cooperative environment of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by one embodiment of the present invention; Fig.12 A cooperative environment receiving I-channel signal waveform diagram of a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by an embodiment of the present invention; Fig.13 A cooperative environment receiving Q-path signal waveform diagram of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Fig.14 A cooperative environment received signal spectrum diagram of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by one embodiment of the present invention; Fig.15 A cooperative environment receiving signal constellation diagram of a method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM provided by an embodiment of the present invention; Fig.16 A cooperative environment receiving signal constellation diagram without channel equalization and Doppler frequency compensation of a low-orbit satellite communication downlink communication link modeling method based on OFDM provided by an embodiment of the present invention; Fig.17 A schematic structural diagram of an OFDM-based low-orbit satellite communication downlink link modeling system provided for one embodiment of the present invention. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0021] Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar symbols throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0022] Embodiment 1 The model simulation environment of this embodiment is Matlab 2022b, and the downlink parameters of the low-orbit satellite signal with an altitude of 550km and a channel center frequency of 11.325GHz are used as input to simulate the OFDM downlink signal, wherein the transmitter uses a comb pilot with 16 fixed position symbols, and the transmission channel uses a single-path channel including Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation, and Ku-band rain attenuation. Three common channel estimation methods are used at the receiving end to compare the effects, and the ground terminal receiving elevation angle is 80°. This simulation model has no channel coding, no up-conversion, and assumes perfect synchronization at the receiving end. In addition, it is assumed that the antenna efficiencies of the transmitting and receiving antennas during the simulation are both 1, and it is assumed that the system hardware has no loss.

[0023] A modeling method for the downlink communication link of low-orbit satellite communication based on OFDM, referring to Figure 1 As shown, including: Step S1: The transmitting end converts the data to be transmitted into subcarriers, and performs OFDM modulation on the subcarriers using the IFFT fast Fourier transform method to generate an OFDM signal.

[0024] In a specific embodiment of the present invention, step S1 includes: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

[0025] Specifically, the input parameter settings for the simulation of the low-orbit satellite communication downlink OFDM signal are shown in Table 1 below.

[0026] Table 1 Simulation parameter settings

[0027] The transmitter first generates a random binary bit stream to be transmitted and maps the data bits. The modulation method is selected according to the amount of data to be transmitted. This simulation selects the 16QAM digital signal modulation method. Then the serial-to-parallel conversion is performed to convert the serial data into 1004 parallel data subcarriers. The pilot sequence is set to a full 0 sequence and mapped using the QPSK diagonal modulation method. All transmission subcarriers are numbered 1 to 1024, and 16 fixed pilot sequences are inserted according to the index at the fixed subcarrier position of each symbol according to the identification information, and the data signal is inserted at the remaining 1004 carrier positions except for the 4 empty subcarrier troughs in the middle. Due to the large number of subcarriers, the IFFT fast Fourier transform method is used for OFDM modulation to generate an OFDM signal. Then the last 32 subcarriers are intercepted and added to the front end of the signal as a cyclic prefix to prevent ISI in signal transmission. Finally, the parallel data is converted into serial data and then the digital-to-analog conversion is performed to transmit the transmission signal from the transmitting antenna.

[0028] Step S2: A single-path channel model of Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation, and Ku-band rain attenuation is established according to the satellite communication link and the OFDM signal to simulate the process in which the OFDM signal generates noise interference, phase shift, and signal attenuation when passing through the channel environment.

[0029] In a specific embodiment of the present invention, step S2 includes: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using the free space propagation model to calculate the attenuation of the OFDM signal; Step S22: Determine the Doppler frequency based on the Starlink satellite and ground receiving terminal parameters with an altitude of 550 km, a channel center frequency of 11.325 GHz, and a ground terminal receiving elevation angle of 80°.

[0030] Furthermore, the Ku-band rain attenuation effect is added to the channel environment at a frequency of 11.325 GHz and a rainfall intensity of 10 mm / h, where the simulated rain clouds are cumulonimbus clouds or undulating clouds, and the rain attenuation path is 1500 m.

[0031] In addition, since both satellites and ground terminals are equipped with highly directional phased array antennas, and there are a large number of low-orbit satellite constellations, they can be flexibly switched according to signal quality. This simulation considers the situation of high-elevation main lobe communication, and there is no obstruction in the communication path, so it is chosen to create a single-path channel instead of the multipath channel scenario commonly seen in OFDM signals. The simulation signal-to-noise ratio range is set to 1~20dB, and it is run 1000 times under each signal-to-noise ratio environment.

[0032] Step S3: performing Doppler frequency shift compensation on the OFDM signal through the receiving end, and then performing OFDM demodulation on the OFDM signal by using FFT operation, and performing channel estimation and channel correction; In a specific embodiment of the present invention, in step S3, the Doppler frequency shift compensation includes: the receiving end directly compensates the OFDM signal by calculating the known Doppler frequency shift value through symbol header information or querying public information.

[0033] In a specific embodiment of the present invention, in step S3, the channel estimation adopts LS algorithm and MMSE algorithm, wherein the LS algorithm includes linear interpolation method and cubic spline interpolation method.

[0034] In a specific embodiment of the present invention, in step S3, the OFDM signal is demodulated by 16QAM demapping, and the demodulated OFDM signal is converted into parallel and serial signals.

[0035] Specifically, after receiving the downlink signal using the receiving antenna and performing analog-to-digital conversion, the cyclic prefix part is first removed, and the remaining data and pilot symbol parts are OFDM demodulated through FFT operation, and then Doppler shift compensation, channel estimation, and channel correction operations are performed in sequence. Among them, the Doppler shift pre-compensation method is used for Doppler shift compensation, that is, it is assumed that the receiving end performs direct compensation by calculating the known Doppler shift value through symbol header information or querying public information. Channel estimation uses LS algorithm and MMSE algorithm, among which LS algorithm includes linear interpolation method and cubic spline interpolation method. Next, after removing the pilot, the data part is 16QAM demapped, and finally the signal is converted from parallel to serial and output as a bit stream for subsequent result processing.

[0036] Step S4: Visualize and analyze the process of transmitting the OFDM signal at the transmitting end and the result of receiving and demodulating the OFDM signal at the receiving end.

[0037] Reference Figure 2 , Figure 3 As shown in the figure, in the result processing part, the effects of different channel estimation algorithms on the OFDM signal in the low-orbit satellite communication scenario are compared, and the transmission results in the low-orbit satellite communication environment and the multipath environment are compared, and the bit error rate diagrams are drawn respectively. In addition, the waveforms and spectrum diagrams of the time domain and frequency domain when the signal is sent and received are drawn, and the receiving constellation diagrams of the cooperative and non-cooperative parties at the receiving end are compared.

[0038] Specifically, Figure 4-Figure 7The waveform, spectrum and constellation diagram of the transmitter signal when the signal-to-noise ratio is 10dB. The waveform of the transmitter signal shows obvious regularity, and the distribution of its subcarriers can be observed from the spectrum diagram. There are valleys in the spectrum, which is consistent with the characteristics of the OFDM signal spectrum diagram.

[0039] Specifically, Figure 8-Figure 11 The signal waveform, spectrum and constellation diagram of the receiving end in a non-cooperative environment when the signal-to-noise ratio is 10dB. In a non-cooperative environment, that is, a blind reconnaissance environment, the received signal is generally a complex time domain waveform that has not been demodulated by OFDM, as shown in the figure, and it is difficult to extract useful features. Compared with the transmitted signal, the power of the received signal is significantly smaller, and the waveform shows significantly greater fluctuations due to noise interference and attenuation. The constellation diagram is messy and irregular, and the signal quality is reduced.

[0040] Specifically, Figure 12-Figure 15 The signal waveform, spectrum and constellation diagram of the receiving end in a cooperative environment when the signal-to-noise ratio is 10dB. The constellation diagram after channel equalization and Doppler frequency compensation is drawn using the processing result of the most effective MMSE channel equalization method. In a cooperative environment, the receiving end knows the spectrum, subcarrier distribution, modulation method and other information of the transmitted signal, and can perform cooperative signal processing. The processed signal waveform and spectrum have restored a certain regularity, the waveform is more regular, and the power is increased, but it is still subject to certain noise interference. The demodulated signal constellation diagram shows obvious 16QAM modulation characteristics. Referring to Fig.16 As shown in the figure, the constellation diagram without channel equalization and Doppler frequency compensation has a phase offset error compared with the former, and the whole is rotated by a certain angle, and the distance between points is closer, which increases the probability of misjudgment of the received signal and weakens the anti-interference ability.

[0041] Embodiment 2 An embodiment of the present invention provides a low-orbit satellite communication downlink communication link modeling system based on OFDM, referring to Fig.17 As shown, including: The signal transmission module 10 is used to convert the data to be transmitted into subcarriers through the transmitting end, perform OFDM modulation on the subcarriers using the IFFT fast Fourier transform method, and generate an OFDM signal; Signal transmission module 20: used to establish a single-path channel model of Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation, and Ku-band rain attenuation according to the satellite communication link and the OFDM signal, so as to simulate the process of noise interference, phase shift, and signal attenuation generated by the OFDM signal passing through the channel environment; The signal receiving module 30 is used to perform Doppler frequency shift compensation on the OFDM signal through the receiving end, and then perform OFDM demodulation on the OFDM signal by using FFT operation, and perform channel estimation and channel correction; The result analysis module 40 is used to visualize and analyze the process of sending OFDM signals by the transmitting end and the result of receiving and demodulating OFDM signals by the receiving end.

[0042] In some specific embodiments, the signal sending module 10 is used to perform the following steps: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

[0043] In some specific embodiments, the signal transmission module 20 is used to perform the following steps: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using a free space propagation model to calculate the attenuation degree of the OFDM signal; Step S22: Determine the Doppler frequency based on the Starlink satellite and ground receiving terminal parameters with an altitude of 550 km, a channel center frequency of 11.325 GHz, and a ground terminal receiving elevation angle of 80°.

[0044] In some specific embodiments, in the signal receiving module 30, the Doppler frequency shift compensation includes: the receiving end directly compensates the OFDM signal by calculating the known Doppler frequency shift value through symbol header information or querying public information.

[0045] During OFDM signal transmission, this application does not use the existing common ground multipath Rayleigh or Ricean channel models, but instead establishes a single-path channel model that includes Gaussian white noise, Doppler effect, satellite-ground free space attenuation, and Ku-band rain attenuation, laying the foundation for exploring the application performance of OFDM system signals in low-orbit satellite communications.

[0046] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

[0047] Each embodiment in this specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referenced to each other. The embodiments of the present invention are described with reference to the flowcharts and / or block diagrams of the methods, terminal devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing terminal device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing terminal device generate instructions for implementing the functions in the process. Figure 1 A process or multiple processes and / or boxes Figure 1 These computer program instructions can also be stored in a computer-readable memory that can guide a computer or other programmable data processing terminal device to work in a specific way, so that the instructions stored in the computer-readable memory produce a product including an instruction device, which implements the functions specified in the process. Figure 1 A process or multiple processes and / or boxes Figure 1 These computer program instructions can also be loaded onto a computer or other programmable data processing terminal device, so that a series of operation steps are executed on the computer or other programmable terminal device to produce a computer-implemented process, so that the instructions executed on the computer or other programmable terminal device provide for implementing the process in the process. Figure 1 A process or multiple processes and / or boxes Figure 1The steps of the functions specified in one or more boxes. Although the preferred embodiments of the embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the attached claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the embodiments of the present invention. Finally, it should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or terminal device including a series of elements includes not only those elements, but also other elements that are not explicitly listed, or also includes elements inherent to such process, method, article or terminal device. In the absence of more restrictions, the elements defined by the sentence "including one..." do not exclude the existence of other identical elements in the process, method, article or terminal device including the elements.

[0048] The method and device provided by the present invention are introduced in detail above. Specific examples are used in this article to illustrate the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

[0049] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", "one specific embodiment" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for modeling a downlink communication link of a low-orbit satellite communication based on OFDM, characterized in that: include: Step S1: The transmitting end converts the data to be transmitted into subcarriers, and uses the IFFT fast Fourier transform method to perform OFDM modulation on the subcarriers to generate an OFDM signal; Step S2: establishing a single-path channel model including Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation and Ku-band rain attenuation according to the satellite communication link and the OFDM signal, so as to simulate the process in which the OFDM signal generates noise interference, phase shift and signal attenuation when passing through the channel environment; Step S3: performing Doppler frequency shift compensation on the OFDM signal through the receiving end, performing OFDM demodulation on the OFDM signal by using FFT operation, and performing channel estimation and channel correction; Step S4: Visualize and analyze the process of transmitting OFDM signals at the transmitting end and the result of receiving and demodulating OFDM signals at the receiving end.

2. The OFDM-based low-orbit satellite communication downlink link modeling method according to claim 1, characterized in that: Step S1 includes: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

3. The OFDM-based low-orbit satellite communication downlink link modeling method according to claim 1, characterized in that: Step S2 includes: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using a free space propagation model to calculate the attenuation degree of the OFDM signal; Step S22: Determine the Doppler frequency according to the predetermined Starlink satellite and ground receiving end parameters.

4. The OFDM-based low-orbit satellite communication downlink link modeling method according to claim 1, characterized in that: In step S3, the Doppler frequency shift compensation includes: the receiving end obtains the Doppler frequency shift value by calculating through symbol header information or querying according to public information to compensate for the OFDM signal.

5. The OFDM-based low-orbit satellite communication downlink link modeling method according to claim 1, characterized in that: In step S3, the channel estimation adopts LS algorithm and MMSE algorithm, wherein the LS algorithm includes linear interpolation method and cubic spline interpolation method.

6. The OFDM-based low-orbit satellite communication downlink link modeling method according to claim 1, characterized in that: In step S3, the OFDM signal is demodulated by 16QAM demapping, and the demodulated OFDM signal is converted into parallel and serial signals.

7. A low-orbit satellite communication downlink link modeling system based on OFDM, characterized in that: include: Signal sending module: used to convert the data to be transmitted into subcarriers through the sending end, perform OFDM modulation on the subcarriers using IFFT fast Fourier transform method, and generate OFDM signals; Signal transmission module: used to establish a single-path channel model including Gaussian white noise, Doppler effect, satellite-to-ground free space attenuation and Ku-band rain attenuation according to the satellite communication link and the OFDM signal, so as to simulate the process of noise interference, phase shift and signal attenuation generated by the OFDM signal in the channel environment; Signal receiving module: used to perform Doppler frequency shift compensation on the OFDM signal through the receiving end, and then perform OFDM demodulation on the OFDM signal by using FFT operation, and perform channel estimation and channel correction; Result analysis module: used for visualizing and analyzing the process of sending OFDM signals at the transmitting end and the result of receiving and demodulating OFDM signals at the receiving end.

8. The OFDM-based low-orbit satellite communication downlink link modeling system according to claim 7, characterized in that: The signal sending module is used to perform the following steps: Step S11: The sending end generates a random binary bit stream; Step S12: performing serial-to-parallel conversion on the random binary bit stream, converting serial data into parallel subcarriers; Step S13: Mapping the subcarriers using a QPSK diagonal modulation method, and inserting a fixed pilot sequence; Step S14: performing OFDM modulation on the subcarrier to generate an OFDM signal; Step S15: converting the parallel data of the OFDM signal into serial data, and transmitting the OFDM signal after digital-to-analog conversion.

9. The OFDM-based low-orbit satellite communication downlink link modeling system according to claim 7, characterized in that: The signal transmission module is used to perform the following steps: Step S21: adding Gaussian white noise to the channel environment to simulate the noise inside the device, and using a free space propagation model to calculate the attenuation degree of the OFDM signal; Step S22: Determine the Doppler frequency according to the predetermined Starlink satellite and ground receiving end parameters.

10. The OFDM-based low-orbit satellite communication downlink link modeling system according to claim 7, characterized in that: The signal receiving module obtains a Doppler frequency shift value by calculating the symbol header information or querying the public information to compensate the OFDM signal.

Images