A low-orbit satellite communication initial time-frequency offset estimation method

Abstract

This invention provides an initial time-frequency offset estimation method for low-Earth orbit (LEO) satellite communication, belonging to the field of satellite communication technology. In the initial time-frequency offset estimation of satellite communication, this invention first performs a coarse initial time-frequency offset estimation based on a first auxiliary synchronization signal, and then uses the estimation result to perform coarse time-frequency offset compensation on the received signal. Subsequently, a fine initial time-frequency offset estimation is performed based on a second synchronization signal. The compensated signal is then sent to the demodulation module for demodulation. This two-step time-frequency offset estimation method combines the advantages of symmetric Chirp reference signals and pseudocode sequences for time-frequency offset estimation, enabling fast and accurate time-frequency offset estimation in scenarios with large frequency offsets, thereby improving the performance of LEO satellite communication systems.

Description

Technical Field

[0001] This invention belongs to the field of satellite communication technology, and specifically relates to a method for estimating the initial time-frequency offset of low-orbit satellite communication. Background Technology

[0002] In the field of low-Earth orbit (LEO) satellite communications, time and frequency offset synchronization is a critical aspect of ensuring communication quality. LEO satellites, due to their high-speed mobility and rapid transit relative to ground terminals, face numerous challenges, among which Doppler shift and time offset are particularly prominent. Typical LEO satellite communications exhibit large dynamic Doppler shifts of up to several kilohertz, which vary over time. If this frequency offset is not compensated for in a timely manner, it will directly disrupt receiver carrier synchronization, causing demodulation errors and severely impacting the quality of satellite communication.

[0003] In traditional time-frequency offset acquisition methods, pilot sequences are embedded in the signal structure as reference signals. The receiver can achieve accurate time-frequency offset estimation by performing correlation operations between the received pilot sequence and a locally generated pilot sequence. Using pilot sequences for time-frequency offset estimation has the following advantages:

[0004] 1. It can provide high-precision synchronization information. Through detailed correlation calculations and analysis of the pilot sequence, the specific value of the time-frequency offset can be accurately determined, meeting the high-precision synchronization requirements of low-Earth orbit satellite communication;

[0005] 2. It has strong anti-interference capabilities. Due to the good autocorrelation and cross-correlation of the pilot sequence, it can still accurately detect synchronization information in complex channel environments and effectively resist the effects of multipath fading, noise interference and other factors.

[0006] Frequency offset estimation of pilot sequences depends on the periodicity or correlation of the sequence. When the pilot signal is affected by Doppler frequency shift, the following problems may occur:

[0007] 1. The pilot signal is "broadened" by the Doppler frequency shift, which causes the correlation peak to become blurred. When the Doppler frequency shift is too large, the probability of missed detection in signal acquisition increases, making it impossible to complete signal detection and time synchronization as required.

[0008] 2. If the Doppler frequency shift exceeds the pilot frequency range, the frequency offset between the local pilot and the received signal will be too large, and the correlation calculation will fail.

[0009] Chirp signals are linear frequency modulated signals whose instantaneous frequency changes linearly with time. This characteristic allows them to effectively cover a wide frequency range. By analyzing the received signal over a certain period, the approximate time-frequency offset range can be quickly estimated. Furthermore, chirp signal processing is relatively simple, requiring no complex algorithms or large computational resources to quickly achieve a preliminary estimate of the time-frequency offset. This feature enables terminal equipment to quickly acquire satellite signals and enter the tracking phase during the initial stages of low-Earth orbit satellite communication, significantly shortening communication establishment time.

[0010] However, using chirp signals for initial coarse synchronization of time and frequency offsets also has some drawbacks. First, its accuracy is limited. Since chirp signals mainly provide a rough estimate of time and frequency offsets over a wide range, they are difficult to provide high-precision synchronization information. In situations where low-Earth orbit satellites move at high speeds, this lower accuracy may not meet the requirements for subsequent high-quality communication. Second, its resistance to interference is poor. In actual communication environments, various interference factors exist, such as multipath fading and noise interference. The characteristics of chirp signals make them susceptible to these interferences, leading to inaccurate synchronization results. Summary of the Invention

[0011] The main purpose of this invention is to avoid the shortcomings of the two initial time-frequency offset estimation methods mentioned above, and to combine the advantages of the two methods to provide a new initial time-frequency offset estimation method for low-Earth orbit satellite communication, thereby achieving fast and accurate time-frequency offset synchronization in scenarios with large frequency offset and improving the performance of low-Earth orbit satellite communication systems.

[0012] This invention is achieved through the following technical solution:

[0013] A method for estimating the initial time-frequency offset in low-Earth orbit (LEO) satellite communication is applied to the receiver in LEO satellite communication scenarios. The receiver stores the same time-frequency offset estimation auxiliary signals as the transmitter, including a first auxiliary synchronization signal and a second auxiliary synchronization signal. Based on the time-frequency offset estimation auxiliary signals, the initial coarse and fine estimations of the received signal's time-frequency offset are performed sequentially, including the following process:

[0014] Step 1: The transmitting end sends a satellite communication signal containing a first auxiliary synchronization signal, a second auxiliary synchronization signal, and valid data to the low-Earth orbit communication satellite, and the low-Earth orbit communication satellite forwards the satellite communication signal to the receiving end.

[0015] Step 2: The receiving end receives the satellite communication signal relayed by the low-orbit communication satellite, performs radio frequency front-end processing and down-conversion operation on the received satellite communication signal to obtain the intermediate frequency signal;

[0016] Step 3: Sample the intermediate frequency signal and perform digital down-conversion to obtain the baseband signal;

[0017] Step 4: Use the first auxiliary synchronization signal to perform the first time-frequency offset estimation of the baseband signal to obtain a coarse estimate of the Doppler frequency offset. and rough estimate of time delay and utilize and Perform time-frequency coarse compensation on the baseband signal;

[0018] Step 5: Use the second auxiliary synchronization signal to perform a second time-frequency offset estimation on the baseband signal after coarse time-frequency offset compensation, and obtain accurate symbol timing. And accurate estimation of residual Doppler frequency deviation and based on and The baseband signal after coarse time-frequency offset compensation is then subjected to fine time-frequency offset compensation to obtain the compensated signal.

[0019] Furthermore, the first auxiliary synchronization signal is a symmetrical chirp reference signal, and its expression is:

[0020]

[0021] in, Represents the upper Chirp reference signal, This represents the lower Chirp reference signal. The starting frequency of the upper Chirp reference signal. The termination frequency of the upper Chirp reference signal. The duration of the upper and lower chirp reference signals. , where is the Chirp slope.

[0022] Furthermore, the second auxiliary synchronization signal is a ZC sequence or other pseudo-random sequence whose autocorrelation and cross-correlation properties meet the requirements, denoted as: ,in The sequence length is given.

[0023] Furthermore, the first time-frequency offset estimation process in step 4 is as follows:

[0024] Step 41: Calculate the extended ambiguity function of the baseband signal and the local matched filter, obtaining two peaks, the absolute values ​​of which are denoted as... and These correspond to the upper Chirp received signal and the lower Chirp received signal, respectively, and the corresponding times are denoted as . and ;

[0025] Step 42, will and Compared with a pre-given threshold, if and If the signal is found, then it is considered that a real signal has been found; otherwise, return to step 2 to continue the signal search.

[0026] Step 43: Using the obtained peak time, calculate a coarse estimate of the Doppler frequency offset. and rough estimate of time delay They are respectively:

[0027]

[0028] .

[0029] Furthermore, the local matched filter is an extended matched filter that takes into account maximum Doppler frequency offset compensation. Its expression is:

[0030] ;

[0031] in, , This is the absolute value of the maximum Doppler frequency offset that the signal may experience during transmission.

[0032] The extended fuzzy function is:

[0033]

[0034] in, For the frequency offset received by the receiver and latency Symmetrical Chirp received signal, superscript This represents the conjugate of the function; the corresponding peak time is:

[0035] .

[0036] Furthermore, the second time-frequency offset estimation process in step 5 is as follows:

[0037] Step 51, the second auxiliary synchronization signal in the baseband signal after time-frequency coarse compensation. With the second auxiliary synchronization signal stored locally Perform cross-correlation calculations:

[0038]

[0039] in, The sequence length of the second auxiliary synchronization signal is given. FFT represents the Fast Fourier Transform, and IFFT represents the Inverse Fast Fourier Transform. * This represents the conjugate of the Fast Fourier Transform;

[0040] Step 52, search results peak position ;

[0041] Step 53, based on After coarse time-frequency compensation, the baseband signal undergoes precise symbol timing adjustment, and the second auxiliary synchronization signal in the adjusted signal is... With the second auxiliary synchronization signal stored locally Conjugate multiplication yields the signal after modulation information has been eliminated. :

[0042]

[0043] in, This represents the residual Doppler frequency offset after coarse frequency offset synchronization. The symbol period of the pseudocode sequence, For carrier phase, It is complex Gaussian white noise;

[0044] Step 54, Calculate Autocorrelation function:

[0045] ;

[0046] Step 55, for conduct After smoothing, an accurate estimate of the residual Doppler frequency shift is obtained:

[0047]

[0048] in The autocorrelation length, It is the argument function of a complex number.

[0049] Furthermore, the optimal value of M is M=N / 2.

[0050] Compared with the prior art, the present invention has the following advantages:

[0051] 1. This invention uses a symmetric Chirp reference signal and a pseudo-random sequence as the first and second auxiliary synchronization signals to achieve coarse and fine estimation of the initial time-frequency offset in low-Earth orbit satellite communication scenarios, respectively. The method proposed in this invention combines the advantages of using symmetric Chirp reference signals and pseudo-code sequences for time-frequency offset estimation, which can achieve fast and accurate time-frequency offset estimation in scenarios with large frequency offset, thereby improving the performance of low-Earth orbit satellite communication systems.

[0052] 2. The method proposed in this invention, when using a symmetrical Chirp signal for coarse time-frequency offset estimation, employs an extended matched filter to calculate an extended ambiguity function with the received signal. This extended matched filter considers the influence of the maximum Doppler frequency offset, avoiding the peak value drop of the extended ambiguity function caused by the Doppler frequency offset, thus ensuring the sensitivity of symmetrical Chirp signal detection under the influence of the Doppler frequency offset. When using a pseudo-random sequence for fine time-frequency offset estimation, the autocorrelation function is smoothed to reduce noise interference with the signal and ensure the estimation accuracy of the residual Doppler frequency shift.

[0053] In summary, this invention achieves fast and accurate time-frequency offset estimation in low-Earth orbit satellite communication scenarios, thereby improving the performance of low-Earth orbit satellite communication systems. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the frame structure of the present invention.

[0055] Figure 2 This is a block diagram illustrating the principle of the initial time-frequency offset estimation method of this invention.

[0056] Figure 3 This is a schematic diagram of the symmetrical Chirp reference signal and the received signal of the present invention.

[0057] Figure 4 This is a schematic diagram illustrating the effect of Doppler frequency shift on the detection of the upper Chirp reference signal according to the present invention.

[0058] Figure 5 This is an illustration of the upper Chirp extended reference signal of the present invention.

[0059] Figure 6 This is an illustration of the lower Chirp extended reference signal of the present invention.

[0060] Figure 7 This is a block diagram illustrating the principle of time-frequency offset estimation for the second auxiliary synchronization signal in this invention. Detailed Implementation

[0061] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0062] Figure 1A schematic diagram of the signal frame structure used to implement the initial time-frequency offset estimation method for low-Earth orbit satellite communication proposed in this invention is given. The frame structure includes a first auxiliary synchronization signal 11, a second auxiliary synchronization signal 12, and valid data 13. Based on this frame structure, the implementation of this invention includes the following processes:

[0063] Step 1: The transmitting end first sends the satellite communication signal composed of the frame structure to the low-Earth orbit communication satellite, and the low-Earth orbit communication satellite forwards the satellite communication signal to the receiving end;

[0064] like Figure 2 As shown, the receiving end performs the following steps:

[0065] Step 2: The receiving end receives the satellite communication signal relayed by the low-orbit communication satellite, performs radio frequency front-end processing and down-conversion operation on the received radio frequency signal to obtain the intermediate frequency signal;

[0066] Step 3: Sample the intermediate frequency signal and perform digital down-conversion to obtain the baseband signal;

[0067] Step 4: Use the first auxiliary synchronization signal 11 to perform the first time-frequency offset estimation to obtain a coarse estimate of the Doppler frequency offset. and rough estimate of time delay and utilize and Perform time-frequency coarse compensation on the baseband signal;

[0068] Step 5: Using the second auxiliary synchronization signal 12, perform a second time-frequency offset estimation on the baseband received signal after coarse time-frequency offset compensation to obtain accurate symbol timing. And accurate estimation of residual Doppler frequency deviation and based on and The baseband received signal after coarse time-frequency offset compensation is further refined with fine time-frequency offset compensation, and the compensated signal is sent to the demodulation module for demodulation.

[0069] Furthermore, the first auxiliary synchronization signal 11 of the present invention is a symmetrical chirp reference signal, which is composed of an upper chirp reference signal and a lower chirp reference signal connected in series in the time domain. The instantaneous frequencies corresponding to the upper and lower chirp reference signals increase and decrease linearly with time, respectively, and the absolute values ​​of the chirp slopes of the two signals are the same.

[0070] The Chirp signal can be represented as:

[0071]

[0072] in, The Chirp slope is considered here. The situation described is the upper Chirp reference signal. The starting frequency of the Chirp signal. The termination frequency of the Chirp signal. The duration of the Chirp signal.

[0073] The instantaneous frequency of the Chirp reference signal is:

[0074]

[0075] When the Chirp reference signal propagates through the channel, a time delay is introduced. and Doppler shift The subsequent received signal is:

[0076] .

[0077] Based on this, Figure 3 A schematic diagram of the symmetrical Chirp reference signal and the received signal of this invention is given. When the start time of the symmetrical Chirp reference signal is... When, we can know that its expression is:

[0078]

[0079] in, Represents the upper Chirp reference signal 111, This represents the lower Chirp reference signal 112.

[0080] Furthermore, such as Figure 3 As shown, the symmetric Chirp reference signal introduces a time delay after transmission through the channel. and frequency offset The symmetrical Chirp received signal at this time can be expressed as:

[0081]

[0082] Include Chirp receives signal 113 and Chirp receives signal 114.

[0083] At the receiving end, the same matched filter as the transmitted signal is typically used to detect the received reference signal. Essentially, this involves cross-correlation between the received Chirp signal and the local Chirp reference signal; the result is called the ambiguity function of the Chirp reference signal. Therefore, the ambiguity function of the Chirp reference signal 111 is:

[0084]

[0085] in Representation function The conjugate of . Accordingly, the ambiguity function of the lower Chirp reference signal 112 is:

[0086]

[0087] The peaks of the two ambiguity functions mentioned above occur when the reference signal and the received signal coincide. For the upper chirp signal, this moment is determined by... If it is indicated, then there is

[0088]

[0089] The corresponding fuzzy function value at this time is:

[0090]

[0091] Correspondingly, the peak time of the ambiguity function of the lower Chirp signal Represented as:

[0092]

[0093] The corresponding fuzzy function value at this time is:

[0094] .

[0095] Figure 4 The diagram illustrating the effect of Doppler frequency offset on the detection of the upper Chirp reference signal, as shown in this invention, illustrates that due to the introduction of Doppler frequency offset during signal transmission, the time... When the reference signal and the received signal have only a partial overlap (as shown in Figure 115), the peak value is lower than in the case of no frequency offset. Furthermore, the larger the Doppler frequency offset, the larger the non-overlapping portion between the received and reference signals (as shown in Figure 116), and the greater the peak value reduction. This severely degrades the chirp signal detection performance and increases the probability of missed detection. When the Doppler frequency offset exceeds the tolerance limit of the chirp signal, the reference and received signals will have no overlap, causing peak detection to fail and making it impossible to estimate the Doppler frequency offset and time offset.

[0096] To address this, this invention proposes using an extended matched filter for symmetrical Chirp reception signal detection to eliminate peak performance loss caused by Doppler frequency offset. This considers the Doppler frequency offset experienced during signal transmission. Within the range, the symmetric Chirp extended reference signal used to calculate the extended ambiguity function in the extended matched filter is:

[0097]

[0098] in, , This represents the absolute value of the maximum Doppler frequency offset that the signal may experience during transmission; and

[0099] .

[0100] Figure 5 A diagram illustrating the upper chirp extended reference signal used in this embodiment is provided. It shows the relationship between the upper chirp reference signal, the upper chirp extended reference signal 117, and the upper chirp received signal. Figure 6 A diagram illustrating the lower Chirp extended reference signal used in this embodiment is provided. It shows the relationship between the lower Chirp reference signal, the lower Chirp extended reference signal 118, and the lower Chirp received signal. By frequency-domain extending the original reference signal, the resulting upper Chirp extended reference signal 117 and lower Chirp extended reference signal 118 cover the linear frequency range caused by Doppler frequency offset. The entire region eliminates the peak performance loss caused by Doppler frequency offset.

[0101] Based on the aforementioned symmetric Chirp extended reference signal, the extended ambiguity function of the received signal is calculated using an extended matched filter as follows:

[0102]

[0103] The above is approximately equal to the correlation value of 0 because the upper Chirp signal and the lower Chirp signal are uncorrelated.

[0104] Furthermore, when performing the first time-frequency offset estimation in step 4 after receiving the signal at the receiving end, the specific process is as follows:

[0105] Step 41: Calculate the extended ambiguity function of the received baseband signal and the local matched filter, obtaining two peak values, whose absolute values ​​are denoted as... and These correspond to the upper Chirp received signal and the lower Chirp received signal, respectively, and the corresponding times are denoted as . and :

[0106] ;

[0107] Step 42, will and Compared with a pre-given threshold, if and If the signal is found, then it is considered that a real signal has been found; otherwise, return to step 2 to continue the signal search.

[0108] Step 43: Using the obtained peak time, calculate a coarse estimate of the Doppler frequency offset. and rough estimate of time delay They are respectively:

[0109]

[0110] .

[0111] Furthermore, in this embodiment, the second auxiliary synchronization signal 12 is a ZC sequence or other pseudo-random sequence with excellent autocorrelation and cross-correlation characteristics, represented as follows: ,in The sequence length is given.

[0112] Figure 7 A block diagram illustrating the principle of frequency offset estimation for the second auxiliary synchronization signal is presented. The coarse estimate of the Doppler frequency offset obtained in step 4 is then used. and rough estimate of time delay The second auxiliary synchronization signal 12 is subjected to timing and frequency offset coarse compensation to obtain the compensated received pseudocode sequence as follows: Based on this, the second time-frequency offset estimation described in step 5 is performed as follows:

[0113] Step 51: Receive the second auxiliary synchronization signal from the baseband receiver after coarse time-frequency offset compensation. With the local second auxiliary synchronization signal 12 Perform cross-correlation calculations:

[0114]

[0115] in, The sequence length of the second auxiliary synchronization signal (12) is given by FFT, which represents the Fast Fourier Transform, and IFFT represents the inverse Fast Fourier Transform. * Represents the conjugate of the Fast Fourier Transform;

[0116] Step 52, search results peak position :

[0117] ;

[0118] Step 53, based on The received signal is precisely symbol-timing adjusted, and the adjusted second auxiliary synchronization signal is then received. With local pseudocode sequence Conjugate multiplication yields the signal after modulation information has been eliminated:

[0119]

[0120] in This represents the residual Doppler frequency offset after coarse frequency offset synchronization. The symbol period of the pseudocode sequence, For carrier phase, It is complex Gaussian white noise;

[0121] Step 54, Calculate Autocorrelation function:

[0122]

[0123] Receive signal Substituting into the above expression, we get:

[0124]

[0125] in, , These are the noise-related interaction terms in the autocorrelation process;

[0126] Step 55, for conduct Smoothing process yields residual Doppler frequency offset The precise estimate:

[0127]

[0128] in The autocorrelation length, It is the argument function of a complex number.

[0129] Furthermore, when the value of M is M = N / 2, The estimated variance is minimized, which is the optimal value of M, where N is the sequence length of the second auxiliary synchronization signal 12.

[0130] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for estimating the initial time-frequency offset in low-Earth orbit satellite communication, characterized in that, The receiver, used in low-Earth orbit satellite communication scenarios, stores the same time-frequency offset estimation auxiliary signals as the transmitter, including a first auxiliary synchronization signal and a second auxiliary synchronization signal. Based on the time-frequency offset estimation auxiliary signals, the receiver sequentially performs initial coarse and fine estimations of the received signal's time-frequency offset, including the following processes: Step 1: The transmitting end sends a satellite communication signal containing a first auxiliary synchronization signal, a second auxiliary synchronization signal, and valid data to the low-Earth orbit communication satellite, and the low-Earth orbit communication satellite forwards the satellite communication signal to the receiving end. Step 2: The receiving end receives the satellite communication signal relayed by the low-orbit communication satellite, performs radio frequency front-end processing and down-conversion operation on the received satellite communication signal to obtain the intermediate frequency signal; Step 3: Sample the intermediate frequency signal and perform digital down-conversion to obtain the baseband signal; Step 4: Use the first auxiliary synchronization signal to perform the first time-frequency offset estimation of the baseband signal to obtain a coarse estimate of the Doppler frequency offset. and rough estimate of time delay and utilize and Perform time-frequency coarse compensation on the baseband signal; Step 5: Use the second auxiliary synchronization signal to perform a second time-frequency offset estimation on the baseband signal after coarse time-frequency offset compensation, to obtain accurate symbol timing. And accurate estimation of residual Doppler frequency deviation and based on and The baseband signal after coarse time-frequency offset compensation is then subjected to fine time-frequency offset compensation to obtain the compensated signal. The first auxiliary synchronization signal is a symmetrical Chirp reference signal, and its expression is: in, Represents the upper Chirp reference signal, This represents the lower Chirp reference signal. The starting frequency of the upper Chirp reference signal. The termination frequency of the upper Chirp reference signal. The duration of the upper and lower chirp reference signals. , where is the Chirp slope; Specifically, the first time-frequency offset estimation process in step 4 is as follows: Step 41: Calculate the extended ambiguity function of the baseband signal and the local matched filter, obtaining two peaks, the absolute values ​​of which are denoted as... and These correspond to the upper Chirp received signal and the lower Chirp received signal, respectively, and the corresponding times are denoted as . and ; Step 42, and Compared with a pre-given threshold, if and If the signal is found, then it is considered that a real signal has been found; otherwise, return to step 2 to continue the signal search. Step 43: Using the obtained peak time, calculate a coarse estimate of the Doppler frequency offset. and rough estimate of time delay They are respectively: 。 2. The method for estimating the initial time-frequency offset of low-orbit satellite communication according to claim 1, characterized in that, The second auxiliary synchronization signal is a ZC sequence or other pseudo-random sequence whose autocorrelation and cross-correlation properties meet the requirements, denoted as: ,in The sequence length is given.

3. The method for estimating the initial time-frequency offset of low-orbit satellite communication according to claim 2, characterized in that, The local matched filter is an extended matched filter that takes into account maximum Doppler frequency offset compensation. Its expression is: in, , This is the absolute value of the maximum Doppler frequency offset that the signal may experience during transmission. The extended fuzzy function is: in, For the frequency offset received by the receiver and latency Symmetrical Chirp received signal, superscript This represents the conjugate of the function; the corresponding peak time is: 。 4. The method for estimating the initial time-frequency offset of low-orbit satellite communication according to claim 3, characterized in that, The second time-frequency offset estimation process in step 5 is as follows: Step 51, the second auxiliary synchronization signal in the baseband signal after time-frequency coarse compensation. With the second auxiliary synchronization signal stored locally Perform cross-correlation calculations: in, The sequence length of the second auxiliary synchronization signal is given. FFT represents the Fast Fourier Transform, and IFFT represents the Inverse Fast Fourier Transform. * This represents the conjugate of the Fast Fourier Transform; Step 52, search results peak position ; Step 53, based on After coarse time-frequency compensation, the baseband signal undergoes precise symbol timing adjustment, and the second auxiliary synchronization signal in the adjusted signal is... With the second auxiliary synchronization signal stored locally Conjugate multiplication yields the signal after modulation information has been eliminated. : in, This represents the residual Doppler frequency offset after coarse frequency offset synchronization. The symbol period of the pseudocode sequence, For carrier phase, It is complex Gaussian white noise; Step 54, Calculate Autocorrelation function: ； Step 55, for conduct After smoothing, an accurate estimate of the residual Doppler frequency shift is obtained: in, For autocorrelation length, , It is the argument function of a complex number.

5. The method for estimating the initial time-frequency offset of low-Earth orbit satellite communication according to claim 4, characterized in that, The optimal value of M is M=N / 2.

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