Doppler change rate calculation method in low-orbit satellite communication system

Abstract

The invention relates to a Doppler change rate calculation method in a low-orbit satellite communication system, and belongs to the field of mobile communication. Aiming at a low-orbit satellite communication system of a non-ground communication network (NTN), the invention aims to solve the problem of Doppler change rate calculation under the condition that ephemeris information and frequency are not needed and timing fine synchronization is not needed. The method comprises the following steps of: extracting cyclic prefix data and symbol tail data of OFDM symbols by utilizing the structural characteristics of OFDM data frames, performing correlation calculation to obtain phase offset, calculating Doppler change rate by combining OFDM symbol interval time, and improving the calculation precision through multiple times of averaging. The method does not need to analyze the content of the data frame, is low in calculation complexity, is convenient for engineering implementation, is suitable for estimation and compensation of the Doppler frequency shift change rate in low-orbit satellite communication, and effectively improves the performance of a communication system.

Description

Technical Field

[0001] The invention belongs to the field of mobile communications and relates to a method for calculating a Doppler change rate in a low-orbit satellite communication system. Background Art

[0002] Non-terrestrial communication network (NTN) technology is an emerging communication technology that uses satellite constellations to expand the coverage of 5G networks, especially in remote areas where terrestrial networks are difficult to cover. The key features of NTN technology include the use of satellites in different orbits such as low earth orbit (LEO), medium earth orbit (MEO), geostationary orbit (GEO) and high altitude platforms (HAPs) to provide communication services. These satellites are capable of carrying transmission equipment relay nodes or base stations, thereby achieving broadband connectivity on a global scale.

[0003] The challenges faced by NTN technology include Doppler frequency shift, Doppler change rate and delay changes caused by the high-speed movement of satellites, as well as signal propagation delays caused by long-distance communications between satellites and ground user equipment (UE). In order to solve these problems, NTN technology requires technical innovations in time-frequency synchronization, mobility management, beam management and signal processing.

[0004] As 3GPP continues to advance the standardization of NTN technology, future Release 18 and beyond are expected to further enhance NTN technology to support a wider range of applications and services, including support for higher frequency bands (such as Ka band), improved mobility management and network optimization, and more comprehensive support for different types of satellites (including LEO and MEO). These advances will lay the foundation for a 5G network with seamless global coverage.

[0005] Among them, low-orbit satellite communication systems have shown great potential in providing global seamless coverage and enhancing signal quality, but they also face a series of technical challenges. First, due to the high-speed movement of low-orbit satellites relative to the ground, significant Doppler frequency shift and Doppler change rate will be generated, which requires the system to have precise time-frequency synchronization capabilities. Secondly, the random access process is also complicated by the satellite-to-ground propagation delay and time-frequency offset, and it is necessary to design efficient preamble codes and access strategies to reduce access delays and improve access success rates.

[0006] Without any frequency offset compensation, the mobile phone will face Doppler frequency shifts of tens of kilohertz or even megahertz, which poses a great challenge to the time-frequency synchronization between the mobile phone and the network. The traditional cross-correlation detection algorithm is prone to significantly degrade performance in a large frequency offset environment, resulting in synchronization failure. In order to solve this problem, an improved coarse synchronization algorithm is proposed. For example, the fast Fourier transform operation is used to replace the summation operation in the cross-correlation operation, and the corresponding integer frequency offset estimation is obtained from the exponential frequency offset in the algorithm, which simplifies the subsequent fractional frequency offset estimation to better adapt to the large frequency offset scenario.

[0007] The fast time-varying characteristics of Doppler shift in low-orbit satellite communication systems make the channel characteristics more complex, and it is crucial to quickly and accurately predict the time-varying characteristics of Doppler shift. In order to accurately calculate the Doppler shift of low-orbit satellites in real time, a geometric calculation method for the Doppler shift characteristics of low-orbit satellites with different eccentricities is proposed. This method does not require the longitude and latitude positions of ground terminals and the real-time coordinates of satellites. The Doppler shift characteristics in the current visible window can be obtained only from the satellite orbit parameters and the maximum visible elevation angle at the terminal. The algorithm calculation process is simple, low in complexity, and is conducive to on-chip programming. It can provide prior information for Doppler shift estimation and compensation of satellite communication receivers.

[0008] However, Doppler shift and Doppler rate of change in low-orbit satellite communication systems are two related but different concepts. Doppler shift refers to the phenomenon that the frequency of the wave received by the observer is different from the frequency emitted by the wave source due to the relative motion between the wave source (such as the satellite) and the observer (such as the ground receiving station). In low-orbit satellite systems, the high-speed motion of the satellite will produce a significant Doppler shift, which will change with the relative position between the satellite and the ground station.

[0009] The Doppler rate of change refers to the rate of change of the Doppler frequency shift over time, which reflects the dynamic characteristics of the Doppler frequency shift. In low-orbit satellite communication systems, due to the orbital motion of the satellite, the relative speed relative to the ground receiving station will change over time, so the Doppler frequency shift will also change rapidly, which leads to the existence of the Doppler rate of change. This rapidly changing Doppler frequency shift places higher requirements on signal synchronization, tracking and compensation.

[0010] TR38.811 gives the Doppler shift and Doppler change rate in various scenarios of NTN satellite communication, as shown in Table 1. It can be seen from the table that the maximum Doppler change rate reaches -8.16kHz / s. That is, every 1s, the Doppler shift will change by 8.16KHz.

[0011] Table 1

[0012]

[0013] Therefore, in low-orbit satellite communication systems, it is necessary to accurately estimate and compensate for Doppler shift and Doppler change rate to ensure the quality of communication signals. This includes developing efficient algorithms to predict the Doppler shift characteristics of satellites, and designing receivers and signal processing techniques that can adapt to the Doppler change rate.

[0014] According to the problem of Doppler change rate in NTN low-orbit satellite communication system, the present invention proposes a method for calculating in user terminal by utilizing the characteristics of NTN satellite communication signal. Summary of the invention

[0015] In view of this, an object of the present invention is to provide a method for calculating the Doppler change rate in a low-orbit satellite communication system.

[0016] In order to achieve the above object, the present invention provides the following technical solutions:

[0017] Aiming at the low-orbit satellite communication system in the non-terrestrial communication network (NTN), the present invention utilizes the characteristics of the wireless signal frame structure in the communication system and proposes a method for calculating the Doppler change rate at the receiving end. The non-terrestrial communication network (NTN) system belongs to an orthogonal frequency division multiple access (abbreviated as: OFDMA) communication system. The frame structure of wireless communication is composed of multiple OFDM symbols, and each OFDM symbol is composed of a cyclic prefix field (abbreviated as: CP) and OFDM symbol data. Assuming that the cyclic prefix length of the OFDM symbol is CPLen, the cyclic prefix data of each OFDM symbol and the CPLen length at the end of the OFDM symbol data field are used to perform correlation calculations to obtain the phase rotation angle of the transmitted data within the time length of an OFDM symbol. According to the interval time between two OFDM symbols, the Doppler change rate ε generated between the two OFDM symbols can be calculated. The Doppler change rate is calculated multiple times, and then the average value of the Doppler change rate is calculated. Figure 1 shown.

[0018] The invention consists of four modules, namely an OFDM symbol data module, a phase shift calculation module, a Doppler change rate calculation module and a Doppler change rate averaging module.

[0019] OFDM symbol data module, after the receiving end of the non-terrestrial communication network (NTN) system completes synchronization, it receives a complete wireless communication signal frame. The wireless communication signal frame consists of OFDM symbol data, and each OFDM symbol consists of an OFDM symbol cyclic prefix field and an OFDM symbol data field.

[0020] The phase offset calculation module takes out an OFDM symbol from the wireless communication signal data frame, takes out the cyclic prefix data from the cyclic prefix field of the OFDM symbol, takes out the OFDM symbol data of the cyclic prefix length from the end of the OFDM symbol data field, and performs correlation calculation on these two pieces of data to obtain the correlation peak value. The phase of the correlation peak value is the phase offset of the OFDM symbol.

[0021] The Doppler change rate calculation module calculates the Doppler change rate by using the phase deviation of two OFDM symbols and the interval between two OFDM symbols.

[0022] The Doppler change rate averaging module calculates the Doppler change rate once for every two OFDM symbols taken out from the wireless communication signal frame data, calculates the Doppler change rate multiple times using the same method, and then averages them.

[0023] The Doppler rate of change calculation process of non-terrestrial communication network (NTN) is as follows: Figure 2 shown.

[0024] Step 1: After the receiving end completes the base station signal search of the non-terrestrial system, the receiving end completes the downlink timing synchronization process with the base station, that is, the receiving end obtains the frame timing information and the wireless frame structure information. The receiving end receives the wireless frame data stream, which is recorded as a_rx_waveform. Figure 2 Middle 1 step.

[0025] Step 2: According to the radio frame structure parameters, the receiving end extracts multiple OFDM symbols from the radio frame data stream, each of which includes OFDM symbol cyclic prefix domain data and OFDM symbol data. The OFDM symbol is recorded as: a_rx_ofdm. Figure 2 Middle 2 steps.

[0026] Step 3: Take out the OFDM symbol cyclic prefix domain data from the OFDM symbol, record it as a_rx_CpData, take out the OFDM symbol data of the cyclic prefix length from the end of the OFDM symbol data domain, record it as a_rx_OfdmCpData, perform correlation calculation on a_rx_CpData and a_rx_OfdmCpData to obtain the phase angle of the correlation peak like Figure 2 Middle 3 steps.

[0027] The calculation formula is:

[0028] Among them: "angle()" represents the angle calculation formula; "conj()" represents the conjugate transpose calculation; "sum()" represents the sum calculation; "*" represents matrix multiplication.

[0029] Step 4: Assume that OFDM symbol 1 is taken from time t1 and OFDM symbol 2 is taken from time t2. The corresponding phase angle of the correlation peak is calculated as and like Figure 2 Medium 4.

[0030] The calculation formula of Doppler change rate is

[0031] Where: T is defined as the FFT size of the waveform divided by the sampling rate, which represents the time length of an OFDM symbol;

[0032] Step 5: Using the same principle to calculate the Doppler change rate N times, we can get a set of ε1,ε2,ε3,...,ε N , the average value of this group is the final Doppler change rate. Figure 2 Medium 4.

[0033] The calculation formula is as follows:

[0034]

[0035] The beneficial effects of the present invention are:

[0036] In low-orbit satellite communication systems, not only the impact of Doppler shift on communication needs to be considered, but also the impact of Doppler frequency shift change rate needs to be considered. This is different from the ground system, where only Doppler frequency shift compensation is required. The impact of Doppler frequency shift rate on communication is very small and can be ignored. The present invention provides a method for calculating the Doppler frequency shift change rate at the receiving end.

[0037] First: In low-orbit satellite communications, the receiving end usually calculates the Doppler frequency shift and the Doppler change rate with the help of the ephemeris information in the system message. The present invention provides a method for calculating the Doppler shift change rate without the need for the ephemeris information in the system message.

[0038] Second: In the process of calculating the Doppler change rate, there is no need for precise frequency synchronization or timing synchronization between the receiving end and the transmitting end. The Doppler change rate can be calculated by using the relationship between the received signal frame data.

[0039] Third: In the process of calculating the Doppler change rate, only the time domain signal is required to participate in the calculation, which greatly simplifies the calculation complexity and is conducive to practical engineering implementation.

[0040] Fourth: The present invention does not need to parse the specific content in the wireless communication data frame, and can be implemented with only the cyclic prefix of each symbol. Each OFDM symbol can be used as sample data for calculation, which is conducive to performing multiple calculations and then taking the average value, thereby improving the accuracy of calculating the Doppler change rate.

[0041] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0043] Figure 1 A method for calculating the Doppler change rate of an OFDM system;

[0044] Figure 2 The calculation process of Doppler change rate for OFDM system;

[0045] Figure 3 A schematic diagram of a cyclic prefix and symbol data of a single OFDM symbol;

[0046] Figure 4 Schematic diagram of calculating the Doppler rate of change for any two OFDM symbols;

[0047] Figure 5 Simulate links for low-orbit satellite communication systems;

[0048] Figure 6 is the Doppler change rate accuracy under different signal-to-noise ratio scenarios;

[0049] Figure 7 is the Doppler change rate accuracy under different synchronization deviations;

[0050] Figure 8 It is the Doppler change rate accuracy in different low-orbit satellite communication channel scenarios. DETAILED DESCRIPTION

[0051] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0052] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0053] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0054] In this embodiment, the mathematical derivation process of the present invention is firstly explained from the principle, and then the actual link simulation is explained.

[0055] In the low-orbit satellite communication of the NTN communication system, OFDM modulation is used for transmission. The structure of each OFDM symbol is as follows: Figure 3 As shown. A complete OFDM symbol consists of two parts, the OFDM symbol cyclic prefix field and the OFDM symbol data field. The OFDM symbol cyclic prefix field data is the same as the OFDM symbol data field tail. The OFDM symbol cyclic prefix field is recorded as: a_rx_CpData; the OFDM symbol data field tail data is recorded as a_rx_OfdmCpData.

[0056] Let's assume that the mathematical expression of the CP data sent by the transmitter is s(t), and the mathematical expression of the received signal is:

[0057]

[0058] Wherein, Δf represents the Doppler frequency shift; ε represents the Doppler change rate.

[0059] According to the cyclic prefix principle of the OFDM system, the mathematical expression of the data after time T is:

[0060]

[0061] Wherein, T represents the length of an OFDM symbol.

[0062] The correlation calculation is performed on the cyclic prefixes of the two parts, where t1 represents the start time of calculating the cyclic prefix

[0063]

[0064] Then Δφ1 can be expressed as:

[0065] Δφ1=2π(Δf+εt1)t1-j2π(Δf+ε(t1+T))(t1+T)=-4πεt1T-2πΔfT-2πεT2

[0066] It can be seen from the above formula that in the ground mobile communication system, assuming that the Doppler change rate ε is 0, it is a typical Doppler shift calculation formula.

[0067] Δφ1=-2πΔfT

[0068]

[0069] In low-orbit satellite communication systems, since the Doppler change rate ε does not tend to 0, it cannot be directly defaulted to 0 for processing. Then in the following formula, there are two unknown variables, namely the Doppler frequency shift Δf and the Doppler change rate ε.

[0070] Δφ1=-4πεt1T-2πΔfT-2πεT2

[0071] In order to calculate the Doppler frequency shift Δf and the Doppler change rate ε, another OFDM cyclic prefix data is required for calculation. Figure 4 shown.

[0072] The same principle is expressed as:

[0073] Δφ2=-4πεt2T-2πΔfT-2πεT2

[0074] Combine the two formulas into a system of equations

[0075]

[0076] Subtracting the two expressions yields the following expression.

[0077] Δφ1-Δφ2=-4πεt1T-4πεt2T=-4πε(t1-t2)T

[0078] Right now:

[0079]

[0080] Where: (t1-t2) is the time difference between two OFDM symbols.

[0081] In this embodiment, the above provides a derivation process of the calculation formula for calculating the Doppler change rate, and the following uses a Matlab link to simulate the performance analysis of the present invention in a specific embodiment.

[0082] Figure 5 A Matlab simulation link for calculating the Doppler change rate is given. In this simulation, the subcarrier spacing of the OFDM symbol is set to 15kHz, the FFT size of the OFDM symbol is 4096, denoted as fft_Size, and the simulation sends 2 frames (20ms) of data, that is, 20 time slots. There are 14 OFDM signals in each time slot, and the CP length of each time slot is [320 288 288 288 288 288 288 320 288 288 288 288 288 288], a total of 280 OFDM signals, and the sampling rate is 4096*15000, denoted as sampleRate.

[0083] In this embodiment, the Doppler change rate is calculated once in 20 ms, and the signal-to-noise ratio range is set to -20 dB to 30 dB, denoted as snr.

[0084] according to Figure 5 Link, the Matlab simulation process is introduced below.

[0085] Step 1: The bit data module randomly generates a bit data stream with a length of 24576 bits (4096x6=24576), which is recorded as a_tx_bitData. There are 4096 subcarriers in total, using 64QAM modulation, and each modulation symbol carries 6 bits of data.

[0086] Step 2: Perform 64QAM modulation on the bit data, that is, modulate 24576 bits of data into 4096 modulation symbols, recorded as a_txModSymbolSig.

[0087] Step 3: Map 4096 modulation symbols to 4096 subcarriers, perform inverse Fourier transform (IFFT), select 4096 IFFT points, and obtain the time domain data of an OFDM symbol, recorded as a_ofdm_waveform.

[0088] Step 4: Add the corresponding CP length data according to the position of the OFDM symbol in a time slot. The specific CP data length is [320 288 288 288 288 288 288 320 288 288 288 288 288 288], where 320 represents the CP length of the first OFDM symbol in the time slot.

[0089] Step 5: Carrying specific bit data does not affect performance evaluation, so the bit data carried by each OFDM symbol can be the same. Repeat 14 OFDM symbol data, and add CP to these 14 OFDM symbols in turn to form a complete time slot data. Repeat the time slot data 20 times to obtain 280 symbols (20ms), which is also 2 frames of time domain data. Recorded as a_tx_waveform.

[0090] Step 6: Pass the prepared a_tx_waveform time domain data through the NTN Channel model, which is completed using Matlab's NTN library function and is recorded as a_channel_waveform.

[0091] Step 7: Based on the NTN Channel output, add the Doppler frequency shift and Doppler rate of change to the transmit signal.

[0092]

[0093] Where a_channel_waveform represents the input and output signals; Δf represents the Doppler frequency shift; ε represents the Doppler change rate; T c Represents the inverse of the OFDM symbol sampling frequency.

[0094] Step 8: Add Gaussian white noise to the a_channel_waveform signal using Matlab's awgn function, denoted as a_rx_waveform. This is used to evaluate the accuracy of calculating the Doppler rate of change under different signal-to-noise ratios.

[0095] Step 9: Simulate the synchronization offset module. In the simulation of this embodiment, the default is the full synchronization scenario. In order to verify the performance of the synchronization offset on the Doppler change rate calculation, a fixed synchronization offset is added. That is, 0 is added before the a_rx_waveform sequence to indicate the number of synchronization offset points, recorded as a_rx_offSyncWaveform

[0096] Step 10: Complete OFDM signal reception, receive 20ms of time domain waveform data, that is, the time domain length of two frames of data, and obtain 280 symbols of time domain data. Record it as a_rx_ofdm array, which represents the time domain data of each OFDM symbol.

[0097] Note: Since the CP length of each OFDM symbol is different, not all OFDM symbols have the same time domain data length.

[0098] Step 11: Take out the CP data at the head of each OFDM symbol from the a_rx_ofdm array, record it as a_rx_CpData, and take out the data of length CP from the tail of the OFDM symbol, record it as a_rx_OfdmCpData. Calculate the angle offset values ​​of 14 OFDM symbols at the corresponding positions of every two time slots in turn, for a total of 19 groups of data. Then sum and average them to get a group of average angle offset values.

[0099]

[0100] Among them: "angle()" represents the angle calculation formula; "conj()" represents the conjugate transpose calculation; "sum()" represents the sum calculation; "*" represents matrix multiplication.

[0101] Step 12: Calculate a set of Doppler change rate values ​​using the set of average angle offset values. Assume that OFDM symbol 1 is taken from time t1 and OFDM symbol 2 is taken from time t2. The corresponding calculated phase angle of the correlation peak is and

[0102] The calculation formula of Doppler change rate is

[0103] Where: T represents the time length of OFDM symbol, refer to Figure 4 Annotation.

[0104] Step 13: Calculate the average of a set of Doppler change rate values ​​to obtain the final Doppler change rate.

[0105]

[0106] In order to illustrate the performance of the present invention in calculating the Doppler change rate in this embodiment, the following scenarios will be used for illustration.

[0107] Scenario 1: Performance of Doppler rate of change at different signal-to-noise ratios.

[0108] The simulation sets the Doppler change rate to 8KHz and evaluates the algorithm performance in the Gaussian white noise range of -20 to 30dB. The simulation results are as follows: Figure 6 As shown in Figure 2, the calculated Doppler rate fluctuations under different signal-to-noise ratio conditions.

[0109] When the signal-to-noise ratio is in the range of -20dB to -4dB, the calculated Doppler change rate is significantly different from its initial value of 8kHz and fluctuates significantly. This shows that under low signal-to-noise ratio conditions, the signal quality is poor due to noise, and the accuracy of signal feature extraction at the receiving end is reduced, resulting in large calculation errors and the estimated Doppler change rate deviating from the actual value. Noise interference will affect the phase recovery of the symbol, resulting in large fluctuations and instability in the estimated Doppler frequency shift. Therefore, in a low signal-to-noise ratio environment, the estimation accuracy of the Doppler frequency shift is insufficient and the system performance is limited.

[0110] Relatively speaking, when the signal-to-noise ratio is increased to the range of -4dB to 30dB, the calculated Doppler rate of change fluctuates slightly around 8kHz, indicating that the calculation accuracy is significantly improved at this time, and the estimated Doppler rate of change is closer to its true value. After the signal-to-noise ratio is improved, the interference of noise on the signal is significantly reduced, and the receiving end can more accurately restore the phase change of the signal, thereby improving the estimation accuracy of the Doppler frequency shift. Within this range, the system has a strong anti-interference ability and can effectively cope with the influence of channel fading and noise, ensuring that the estimation performance of the Doppler rate of change is relatively ideal.

[0111] Therefore, the signal-to-noise ratio is one of the important factors affecting the algorithm. In a low signal-to-noise ratio environment, noise will have a greater impact on phase recovery and frequency offset estimation, resulting in large fluctuations in the calculation results and high errors; while in a high signal-to-noise ratio environment, the calculation error is significantly reduced, and the algorithm can provide a more accurate and stable Doppler rate of change estimate.

[0112] In actual engineering, when the signal-to-noise ratio is -4dB, the signal-to-noise ratio requirements of the low-orbit satellite communication system have been met.

[0113] Scenario 2: Effect of synchronization bias on Doppler rate of change estimation performance

[0114] In actual engineering implementation, the Doppler change rate calculation is performed immediately after the coarse synchronization is completed, so it cannot be guaranteed that the receiving end is in a precise synchronization state at this moment. Therefore, in this embodiment, the performance impact of the synchronization deviation on the Doppler change rate calculation will be evaluated.

[0115] In the simulation, the offset is set to 0:30:600. Assume that the fixed signal-to-noise ratio is 20dB. The Doppler change rate under different synchronization deviation values ​​is calculated, and the preset true value 8kHz is subtracted and then plotted and output. The simulation results are as follows: Figure 7 shown.

[0116] When the receiving delay is within the CP length range, the cyclic prefix of the symbol can play a good protective role. Due to the periodic characteristics of OFDM symbols, the cyclic prefix is ​​the repeated part of the symbol, so that even if there is a small synchronization delay, the receiver can still calculate the phase offset of the symbol through the correlation of the CP. Such a synchronization mechanism ensures that when the delay does not exceed the CP length, the system's estimation accuracy of the phase offset will not be significantly affected, and the calculated Doppler frequency shift change rate will still have a high accuracy.

[0117] When the receiving delay is close to the maximum CP length but not exceeded, the symbol phase offset angle calculated based on the correlation peak has a large error. Therefore, in this case, this method can no longer accurately estimate the Doppler shift change rate.

[0118] When the receiving delay exceeds the CP length, the periodic characteristics of the symbols in the signal will be destroyed, resulting in the phase offset estimation of the OFDM symbol no longer being accurate, which in turn affects the calculation of the Doppler frequency shift rate of change and the demodulation performance of the symbol. In this case, the system can no longer rely on the cyclic prefix to correct the error caused by the offset, which will lead to significantly enhanced interference and a significant decrease in calculation accuracy. In an actual communication system, the CP length in the signal is greater than the maximum system delay, which can ensure the calculation accuracy of the present invention.

[0119] Scenario 3: Impact of different channel scenarios on Doppler rate of change estimation performance

[0120] In a low-orbit satellite communication system, only using a Gaussian white noise channel model does not have representative performance, so this embodiment uses Matlab to provide a satellite channel model function to evaluate the performance under different satellite communication models.

[0121] NTN channel parameter initialization: carrier frequency is 2GHz, orbit elevation angle is 50°, satellite orbit height is 600km, orbit speed is 7562.2m / s, mobile terminal speed is 0.83m / s, etc., simulating the downlink model in terminal and satellite communication. In this embodiment, the channel model consists of three parts: configuration parameters, setting channel model parameters, and using the channel model for simulation.

[0122] The simulation results are as follows Figure 8As shown. The four simulation curves represent the simulation results of estimating the mean square error of the Doppler shift change rate under different signal-to-noise ratios (SNRs) in four different channel scenarios (NTN-TDL-A, NTN-TDL-B, NTN-TDL-C, and NTN-TDL-D). The horizontal axis represents the signal-to-noise ratio, from -20dB to 30dB, and the vertical axis represents the mean square error (MSE) of the Doppler shift change rate. The higher the MSE, the less accurate the channel's estimate of the Doppler shift change rate. Blue represents the NTN-TDL-A channel, orange represents the NTN-TDL-B channel, yellow represents the NTN-TDL-C channel, and purple represents the NTN-TDL-D channel.

[0123] from Figure 8 It can be seen that in the low SNR range, each curve has large fluctuations and high errors. As the SNR increases to -5dB, the MSE begins to drop significantly. The mean square error of most channels in this range decreases, and the error fluctuation gradually stabilizes. In the high SNR range, the fluctuation tends to be a straight line under the estimation of the Doppler frequency shift change rate, and the calculation accuracy is high. NTN-TDL-A shows some fluctuations in the lower SNR range, but its overall trend is that the error decreases with the increase of SNR. NTN-TDL-C and NTN-TDL-D show relatively better stability after the SNR increases, especially NTN-TDL-D (purple curve), whose error decreases rapidly after -5dB.

[0124] Therefore, this algorithm shows stable Doppler rate of change estimation performance under different channel models.

[0125] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for calculating Doppler change rate in a low-orbit satellite communication system, the method comprising the following steps: Step 1: After the receiving end completes the base station signal search of the non-terrestrial system, it completes the downlink timing synchronization process with the base station, that is, the receiving end obtains the frame timing information and the wireless frame structure information; the receiving end receives the wireless frame data stream, which is recorded as a_rx_waveform; Step 2: According to the radio frame structure parameters, the receiving end extracts multiple orthogonal frequency division multiplexing (OFDM) symbols from the radio frame data stream. Each OFDM symbol includes OFDM symbol cyclic prefix field data and OFDM symbol data field, which is recorded as a_rx_ofdm. Step 3: Take out the OFDM symbol cyclic prefix domain data from the OFDM symbol, record it as a_rx_CpData, take out the OFDM symbol data of the cyclic prefix length from the end of the OFDM symbol data domain, record it as a_rx_OfdmCpData, perform correlation calculation on a_rx_CpData and a_rx_OfdmCpData to obtain the phase angle of the correlation peak The calculation formula is: Among them, angle() represents the angle calculation formula; conj() represents the conjugate transpose calculation; sum() represents the sum calculation; * represents matrix multiplication; Step 4: Assume that OFDM symbol 1 is taken from time t1 and OFDM symbol 2 is taken from time t2; the phase angles of the correlation peaks are calculated in sequence as follows: and The calculation formula of Doppler change rate is Where: T represents the time length of an OFDM symbol data domain; Step 5: Using the same principle to calculate the Doppler change rate N times, we get a set of ε1,ε2,ε3,...,ε N , the average value of this group is the final Doppler change rate; The calculation formula of Doppler rate of change is as follows:

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