Low earth orbit satellite carrier loop based spread spectrum orthogonal frequency division multiplexing frequency offset estimation method

Abstract

The application provides a low-orbit satellite spread spectrum orthogonal frequency division multiplexing frequency offset estimation method based on a carrier loop, and belongs to the technical fields of satellite communication and signal processing. The method comprises the following steps: a receiver divides an integral frame sequence into multiple accumulation blocks with the same structure in time sequence; phase rotation factors of residual carriers corresponding to each symbol period in a current accumulation block are calculated respectively; carrier stripping values and carrier correlation values of each subcarrier in each symbol period in the current accumulation block are calculated in turn respectively; accumulation correlation values corresponding to each subcarrier in the current accumulation block are calculated respectively; the carrier loop state of the current accumulation block is judged according to the index of the current accumulation block, and the carrier frequency control word of the next accumulation block is calculated; the processing of all accumulation blocks is iterated, and the average value of all carrier frequency control words obtained after the carrier loop converges is taken as the frequency offset estimation value of the integral frame sequence. The application can provide high-precision and continuous stable frequency observation values.

Description

Technical Field

[0001] This application relates to the field of satellite communication and signal processing technology, and in particular to a frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop. Background Technology

[0002] Low Earth Orbit (LEO) satellite positioning systems, with their advantages of low orbital altitude, high signal landing power, and flexible constellation deployment, have become an important direction for the integrated development of navigation and communication in recent years. In these systems, Doppler frequency shift is not only a direct reflection of the relative motion between the satellite and the user terminal, but also one of the key observations for achieving high-precision velocity measurement and positioning calculation. The accuracy of frequency estimation largely determines the upper limit performance of the positioning calculation results. For LEO satellite positioning systems employing spread spectrum orthogonal frequency division multiplexing, the role of frequency estimation is no longer limited to eliminating interference between subcarriers to ensure the reliability of communication demodulation, but also lies in providing high-confidence Doppler observation information for positioning algorithms, thereby supporting positioning accuracy requirements at the meter or even decimeter level.

[0003] In a typical low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing (OFDM) communication system, the transmitting equipment spreads and modulates the digital signal using a spreading code, mapping it onto specific subcarriers to form the transmitted waveform. After achieving time-frequency synchronization with the transmitting equipment, the receiving equipment uses a locally generated spreading code sequence and the received signal sequence for despreading and demodulation. To improve the accuracy of frequency estimation, existing technologies generally employ open-loop estimation methods based on sequence correlation search. Specifically, the receiving equipment uses the demodulated complete frame signal sequence, sliding within a preset frequency step range to change the locally replicated frequency. By calculating the correlation peaks under different frequency assumptions, the frequency point corresponding to the correlation peak is searched as the frequency estimation result. This type of method attempts to theoretically reduce the frequency estimation error through the processing gain brought by the accumulation of long sequences, thereby providing observational input for the positioning algorithm.

[0004] However, the above-mentioned method based on sliding correlation search of complete frame signal sequences has obvious technical defects in practical engineering applications, specifically:

[0005] First, the accuracy of frequency estimation is limited by the length of the complete frame signal sequence. According to the basic principle of Fourier transform, frequency resolution is inversely proportional to the length of the observation time window. To further improve the accuracy of frequency estimation, the length of the complete frame signal sequence involved in the correlation calculation must be extended. This will directly lead to an exponential increase in the search computation and introduce a large processing delay, making it difficult to meet the stringent requirements for real-time performance and resource efficiency in low-Earth orbit satellite scenarios.

[0006] Secondly, this type of method is extremely sensitive to low signal-to-noise ratio environments. Under conditions of limited budget and tight margins for low-Earth orbit satellite links, noise power may mask the true correlation peaks or generate multiple spurious peaks within the search range, causing discontinuous jumps in the frequency estimation results. Such high-variance and unstable estimation outputs cannot provide reliable Doppler observations for high-precision positioning systems, severely limiting the system's positioning robustness and continuous availability in complex channel environments.

[0007] In summary, existing frequency offset estimation methods based on sliding correlation search of complete frame signal sequences have inherent defects in terms of estimation accuracy, sequence length constraints, and adaptability to low signal-to-noise ratios. They are difficult to meet the application requirements of low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing communication systems for high-precision, continuous, and stable frequency observations. Therefore, it is necessary to propose a scheme to improve one or more of the problems existing in the above-mentioned related technical solutions.

[0008] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0009] This application provides a frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop, the method comprising the following steps:

[0010] The receiver divides the received full frame sequence into multiple accumulation blocks with the same structure in chronological order. Each accumulation block contains multiple identical and consecutive subcarriers, and each subcarrier contains multiple identical and consecutive symbol periods.

[0011] The receiver calculates the phase rotation factor of the residual carrier for each symbol period in the current accumulation block based on the carrier frequency control word corresponding to the current accumulation block. Each phase rotation factor contains... Road component values ​​and Path component values;

[0012] Use each The path component values ​​correspond to each subcarrier after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping to obtain the value of each subcarrier in each symbol period of the current accumulation block. The carrier stripping value is used simultaneously for each The path component values ​​correspond to each subcarrier after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping to obtain the value of each subcarrier in each symbol period of the current accumulation block. Road carrier stripping value;

[0013] Use each The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the value of each subcarrier in each symbol period of the current accumulation block. The correlation value of the road carrier is used simultaneously for each The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the value of each subcarrier in each symbol period of the current accumulation block. Road carrier correlation value;

[0014] Each subcarrier in the current accumulation block is included separately. The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. Accumulate the correlation value and simultaneously add all the relevant values ​​of each subcarrier in the current accumulation block. The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. Road accumulation correlation value;

[0015] Based on the index of the current accumulation block in the entire frame sequence, determine the carrier loop state of the current accumulation block, and utilize all the data in the current accumulation block. Road accumulation related values ​​and all The relevant values ​​are accumulated to obtain the carrier frequency control word corresponding to the next accumulation block;

[0016] The process of calculating the phase rotation factor and obtaining the carrier frequency control word is repeated iteratively until all accumulated blocks in the entire frame sequence are processed. The average value of all carrier frequency control words obtained after the carrier loop converges is used as the frequency offset estimate of the entire frame sequence.

[0017] Furthermore, The expression for the road carrier stripping value is:

[0018] (1)

[0019] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Road component values, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Path component values;

[0020] The expression for the road carrier stripping value is:

[0021] (2)

[0022] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

[0023] Furthermore, The expression for the path carrier correlation value is:

[0024] (3)

[0025] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block The code value of the spreading code corresponding to each symbol period, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value;

[0026] The expression for the path carrier correlation value is:

[0027] (4)

[0028] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

[0029] Furthermore, The expression for the cumulative correlation value is:

[0030] (5)

[0031] in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, This indicates the number of symbol periods in each subcarrier within each accumulation block. Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value;

[0032] The expression for the cumulative correlation value is:

[0033] (6)

[0034] in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Correlation value of the road carrier.

[0035] Furthermore, based on the index of the current accumulation block in the entire frame sequence, the carrier loop state of the current accumulation block is determined, and all data in the current accumulation block are utilized. Road accumulation related values ​​and all The steps for accumulating relevant values ​​to obtain the carrier frequency control word corresponding to the next accumulation block include:

[0036] Set the total number of all accumulator blocks required for frequency-locked loop convergence. Set the total number of accumulator blocks required for phase-locked loop convergence to . All of the settings in the entire frame sequence have been obtained. Road accumulation related values ​​and all The index of the current accumulator block for the path accumulation correlation value is , , Indicates the index of the last accumulated block in the entire frame sequence;

[0037] when At this time, joint frequency and phase detection cannot be performed, and all data in the current accumulation block is saved. Road accumulation related values ​​and all The relevant values ​​are accumulated, and the next accumulation block is processed.

[0038] when When, combine all the data in the current accumulation block and the previous accumulation block. Road accumulation related values ​​and all The frequency difference estimate corresponding to the current accumulation block is obtained by performing joint frequency discrimination on the accumulated correlation values. The frequency difference estimate is then filtered by the loop filter of the frequency locking loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulation block is updated to obtain the carrier frequency control word corresponding to the next accumulation block.

[0039] when When the frequency-locked loop converges, it directly merges the current accumulator block with all the data in the previous accumulator block. Road accumulation related values ​​and all The phase difference estimate corresponding to the current accumulation block is obtained by performing joint phase detection on the accumulated correlation values. The phase difference estimate is then filtered by the loop filter of the phase-locked loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulation block is then updated to obtain the carrier frequency control word corresponding to the next accumulation block.

[0040] when At that time, both the frequency-locked loop and the phase-locked loop converged, and the execution was completed. The same processing procedure is followed, and the carrier frequency control word corresponding to the next accumulation block is recorded.

[0041] Furthermore, when When, combine all the data in the current accumulation block and the previous accumulation block. Road accumulation related values ​​and all The steps of performing joint frequency discrimination by accumulating correlation values ​​to obtain the frequency difference estimate corresponding to the current accumulator block, filtering the frequency difference estimate using the loop filter of the frequency-locked loop, outputting the corresponding carrier frequency control word, updating the carrier frequency control word corresponding to the current accumulator block, and obtaining the carrier frequency control word corresponding to the next accumulator block include:

[0042] Calculate the dot product and cross product of each subcarrier in the current accumulation block;

[0043] The expression for the dot product is:

[0044] (7)

[0045] in, Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value;

[0046] The expression for the cross product is:

[0047] (8)

[0048] in, Indicates the first The subcarrier at the _ ... Cross product values ​​in each accumulator block;

[0049] Calculate the frequency difference estimate corresponding to the current accumulation block using all dot product values ​​and all cross product values;

[0050] The expression for the frequency offset estimate is:

[0051] (9)

[0052] in, Indicates the first Frequency offset estimates corresponding to each accumulator block This indicates the number of all subcarriers. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block;

[0053] The frequency difference estimate corresponding to the current accumulator block is filtered by the loop filter of the frequency-locked loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulator block is updated to obtain the carrier frequency control word corresponding to the next accumulator block.

[0054] The expression for updating the carrier frequency control word using the loop filter of the frequency-locked loop is as follows:

[0055] (10)

[0056] in, Indicates the first The carrier frequency control word corresponding to each accumulator block Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain factor of the frequency-locked loop. , This represents the noise bandwidth of the frequency-locked loop. These represent the parameter values ​​of the loop filter in the frequency-locked loop. Indicates the first Frequency offset estimates corresponding to each accumulator block.

[0057] Furthermore, the expression for the phase difference estimate is:

[0058] (11)

[0059] in, Indicates the first The phase difference estimate corresponding to each accumulation block Represents the arctangent function in the four quadrants. Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... The cross product value in each accumulator block Indicates the number of all subcarriers;

[0060] The expression for updating the carrier frequency control word using the loop filter of a phase-locked loop is as follows:

[0061] (12)

[0062] in, Indicates the first The carrier frequency control word corresponding to each accumulator block This represents the gain factor of the phase-locked loop. , This represents the noise bandwidth of the phase-locked loop. These represent the parameter values ​​of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain coefficient of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, Indicates the first The phase difference estimate corresponding to each accumulation block.

[0063] Furthermore, the step of using the average value of all carrier frequency control words obtained after carrier loop convergence as the frequency offset estimate of the entire frame sequence includes:

[0064] Carrier loop convergence indicates that both the frequency-locked loop and the phase-locked loop have converged. The average value of all carrier frequency control words obtained at that time is used as the frequency offset estimate of the entire frame sequence.

[0065] This application provides a frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop, which has at least the following beneficial effects:

[0066] (1) This application introduces a spread spectrum orthogonal frequency division multiplexing communication system and utilizes its closed-loop feedback mechanism to achieve deep utilization of the spread spectrum code gain of the entire frame sequence, thereby enhancing the processing gain of the communication system in a low signal-to-noise ratio environment.

[0067] (2) This application determines the carrier loop state of the current accumulation block based on the index of the current accumulation block in the whole frame sequence, and utilizes all the current accumulation blocks. Road accumulation related values ​​and all The correlation values ​​are accumulated to obtain the carrier frequency control word corresponding to the next accumulation block. This process utilizes the low-pass smoothing characteristics of the loop filters of the frequency-locked loop and the phase-locked loop, which can effectively suppress additive white Gaussian noise. This fundamentally solves the problem that traditional frequency domain correlation search is susceptible to noise interference and correlation peak shift in low signal-to-noise ratio environments, thus causing frequency estimation jumps.

[0068] (3) This application can effectively improve the accuracy of frequency observation while ensuring the accuracy of Doppler frequency extraction, and provide reliable physical quantity support for subsequent high-precision positioning. Attached Figure Description

[0069] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0070] Figure 1 This illustration shows a step diagram of a low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing frequency offset estimation method based on carrier loop in an exemplary embodiment of this application;

[0071] Figure 2 A schematic diagram showing the frequency difference estimate of the frequency-locked loop output in a simulation experiment of an exemplary embodiment of this application;

[0072] Figure 3 A schematic diagram showing the phase difference estimate of the phase-locked loop output in a simulation experiment of an exemplary embodiment of this application;

[0073] Figure 4 This diagram illustrates a comparison of the errors of frequency offset estimates obtained using different frequency offset estimation methods in an exemplary embodiment of this application. Detailed Implementation

[0074] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided to make this application more comprehensive and complete, and to fully convey the concept of the exemplary embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0075] Furthermore, the accompanying drawings are merely illustrative of this application and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities. These functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0076] To address the problems of low estimation accuracy, significant limitations due to sequence length, and inability to provide continuous and smooth observations when using complete signal sequences for frequency domain correlation search in low signal-to-noise ratio (SNR) environments in existing low-Earth orbit (LEO) satellite spread spectrum orthogonal frequency division multiplexing (SS-OFDM) systems, this application proposes a carrier loop-based LEO satellite spread spectrum orthogonal frequency division multiplexing frequency offset estimation method, such as... Figure 1 As shown, the method may include the following steps:

[0077] In step S101 of this embodiment, the receiver divides the received whole frame sequence into multiple accumulation blocks with the same structure in chronological order. Each accumulation block contains multiple identical and continuous subcarriers, and each subcarrier contains multiple identical and continuous symbol periods.

[0078] In step S102 of this embodiment, the receiver calculates the phase rotation factor of the residual carrier corresponding to each symbol period in the current accumulation block according to the carrier frequency control word corresponding to the current accumulation block. Each phase rotation factor includes... Road component values ​​and Path component value.

[0079] Furthermore, the initial value of the carrier frequency control word is set to The carrier frequency control word corresponding to the current accumulator block is updated based on the loop filters of the frequency-locked loop and the phase-locked loop, and is calculated using formulas 10 and 12.

[0080] Furthermore, the phase rotation factor represents the effect of the carrier frequency control word on the phase rotation of the symbol, and its expression is as follows, using conventional calculation methods in this field: ,in, Indicates the first The first in the accumulator block Phase rotation factor corresponding to each symbol period Indicates the first The carrier frequency control word corresponding to each accumulator block This indicates the duration of each symbol period. Indicates the first The first in the accumulator block Phase rotation factor corresponding to each symbol period Indicates the first The first in the accumulator block Phase rotation factor corresponding to each symbol period.

[0081] Step S103 of this embodiment: Utilize each The path component values ​​correspond to each subcarrier after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping to obtain the value of each subcarrier in each symbol period of the current accumulation block. The carrier stripping value is used simultaneously for each The path component values ​​correspond to each subcarrier after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping to obtain the value of each subcarrier in each symbol period of the current accumulation block. Carrier stripping value. This step removes the influence of residual carriers on communication symbols, restoring communication symbols to the greatest extent possible.

[0082] Furthermore, The expression for the road carrier stripping value is:

[0083] (1)

[0084] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Road component values, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Path component value.

[0085] Furthermore, The expression for the road carrier stripping value is:

[0086] (2)

[0087] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

[0088] Step S104 of this embodiment: Utilize each The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the value of each subcarrier in each symbol period of the current accumulation block. The correlation value of the road carrier is used simultaneously for each The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the value of each subcarrier in each symbol period of the current accumulation block. The carrier correlation value. This step can remove the phase inversion effect of the communication symbol caused by the spreading code and restore the true communication symbol.

[0089] Furthermore, The expression for the path carrier correlation value is:

[0090] (3)

[0091] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block The code value of the spreading code corresponding to each symbol period, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

[0092] Furthermore, The expression for the path carrier correlation value is:

[0093] (4)

[0094] in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

[0095] Step S105 of this embodiment: Each subcarrier in the current accumulation block is... The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. Accumulate the correlation value and simultaneously add all the relevant values ​​of each subcarrier in the current accumulation block. The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. Accumulate the correlation values. This step can improve the signal-to-noise ratio and reduce the impact of noise.

[0096] Furthermore, The expression for the cumulative correlation value is:

[0097] (5)

[0098] in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, This indicates the number of symbol periods in each subcarrier within each accumulation block. Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Correlation value of the road carrier.

[0099] Furthermore, The expression for the cumulative correlation value is:

[0100] (6)

[0101] in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Correlation value of the road carrier.

[0102] In this embodiment, step S106: Based on the index of the current accumulating block in the entire frame sequence, determine the carrier loop state of the current accumulating block, and utilize all the data in the current accumulating block. Road accumulation related values ​​and all The relevant values ​​are accumulated to obtain the carrier frequency control word corresponding to the next accumulation block. Step S106 in this embodiment may include the following sub-steps:

[0103] Sub-step S1061: Set the total number of all accumulator blocks required for frequency-locked loop convergence to... This refers to the total number of accumulator blocks required for the frequency difference estimate corresponding to the current accumulator block in the frequency-locked loop to remain consistently and stably close to 0. The total number of accumulator blocks required for the phase-locked loop to converge is set to... This refers to the total number of accumulator blocks required for the phase difference estimate corresponding to the current accumulator block in the phase-locked loop to remain consistently and stably close to 0. This is set to the number of blocks for which all accumulator blocks in the entire frame sequence have been obtained. Road accumulation related values ​​and all The index of the current accumulator block for the path accumulation correlation value is , , This represents the index of the last accumulated block in the entire frame sequence.

[0104] Sub-step S1062: When At this time, joint frequency and phase detection cannot be performed, and all data in the current accumulation block is saved. Road accumulation related values ​​and all The relevant values ​​are accumulated, and the next accumulation block is processed.

[0105] Sub-step S1063: When When, combine all the data in the current accumulation block and the previous accumulation block. Road accumulation related values ​​and all The accumulated correlation values ​​are jointly discriminated to obtain the frequency difference estimate corresponding to the current accumulation block. The frequency difference estimate is then filtered using the loop filter of the frequency-locked loop, and the corresponding carrier frequency control word is output. This update is applied to the carrier frequency control word corresponding to the current accumulation block to obtain the carrier frequency control word for the next accumulation block. This joint frequency discrimination method can remove the influence of phase rotation corresponding to the binary phase shift keying symbols. The specific processing steps of sub-step S1063 include:

[0106] The first step is to calculate the dot product and cross product of each subcarrier in the current accumulation block.

[0107] Furthermore, the expression for the dot product value is:

[0108] (7)

[0109] in, Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block The cumulative correlation value of the road.

[0110] Furthermore, the expression for the cross product value is:

[0111] (8)

[0112] in, Indicates the first The subcarrier at the _ ... The cross product value in each accumulator block.

[0113] The second step is to use all dot product values ​​and all cross product values ​​to calculate the frequency difference estimate corresponding to the current accumulation block.

[0114] Furthermore, the expression for the frequency difference estimate is:

[0115] (9)

[0116] in, Indicates the first Frequency offset estimates corresponding to each accumulator block This indicates the number of all subcarriers. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each cumulative block.

[0117] The third step involves using the loop filter of the frequency-locked loop to filter the frequency difference estimate corresponding to the current accumulator block, outputting the corresponding carrier frequency control word, and updating the carrier frequency control word corresponding to the current accumulator block to obtain the carrier frequency control word corresponding to the next accumulator block. This step can filter out high-frequency noise and obtain a stable carrier frequency control word.

[0118] Furthermore, the expression for updating the carrier frequency control word using the loop filter of the frequency-locked loop is as follows:

[0119] (10)

[0120] in, Indicates the first The carrier frequency control word corresponding to each accumulator block Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain factor of the frequency-locked loop. , This represents the noise bandwidth of the frequency-locked loop. These represent the parameter values ​​of the loop filter in the frequency-locked loop. Indicates the first Frequency offset estimates corresponding to each accumulator block.

[0121] Furthermore, the summation in Formula 10 is derived from... At the beginning, and when At that time, the initial value of the carrier frequency control word is 0, and the phase rotation factor is also 0, which serves as the basis for the cumulative summation.

[0122] Furthermore, in this embodiment, the parameter value of the loop filter of the frequency-locked loop is set to 0.1~1, preferably. This enables a rapid response without excessive overshoot. The parameter values ​​of the loop filter in the frequency-locked loop are set to conventional settings in the art.

[0123] Sub-step S1064: When When the frequency-locked loop converges, it directly merges the current accumulator block with all the data in the previous accumulator block. Road accumulation related values ​​and all The phase difference estimate corresponding to the current accumulation block is obtained by performing joint phase detection on the accumulated correlation values. The phase difference estimate is then filtered by the loop filter of the phase-locked loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulation block is then updated to obtain the carrier frequency control word for the next accumulation block.

[0124] Furthermore, the expression for the phase difference estimate is:

[0125] (11)

[0126] in, Indicates the first The phase difference estimate corresponding to each accumulation block Represents the arctangent function in the four quadrants. Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... The cross product value in each accumulator block This indicates the number of all subcarriers. Using a joint phase detection method can eliminate the influence of phase rotation corresponding to binary phase shift keying symbols, thus improving the signal-to-noise ratio.

[0127] Furthermore, the expression for updating the carrier frequency control word using the loop filter of the phase-locked loop is as follows:

[0128] (12)

[0129] in, Indicates the first The carrier frequency control word corresponding to each accumulator block This represents the gain factor of the phase-locked loop. , This represents the noise bandwidth of the phase-locked loop. These represent the parameter values ​​of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain coefficient of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, Indicates the first The phase difference estimate corresponds to each accumulator block. Updating the carrier frequency control word using a phase-locked loop (PLL) loop filter can remove high-frequency noise.

[0130] Furthermore, in this embodiment, the parameter value of the loop filter of the phase-locked loop is set to 0.1~1, preferably. The gain coefficient of the loop filter of the phase-locked loop is set to These are all standard settings in this field.

[0131] Sub-step S1065: When At that time, both the frequency-locked loop and the phase-locked loop converged, and the execution was completed. The same processing procedure is followed, and the carrier frequency control word corresponding to the next accumulation block is recorded.

[0132] Here, when switching between frequency-locked loop and phase-locked loop, a stable strategy of first performing joint frequency discrimination and then joint phase discrimination is adopted, which can reliably find the corresponding carrier frequency control word.

[0133] In this embodiment, step S107 is to iteratively perform the above process from calculating the phase rotation factor to obtaining the carrier frequency control word until the processing of all accumulated blocks in the entire frame sequence is completed, and the average value of all carrier frequency control words obtained after the carrier loop converges is used as the frequency offset estimate of the entire frame sequence.

[0134] To verify the superiority of the proposed carrier loop-based low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing frequency offset estimation method, the following simulation experiments were conducted.

[0135] The parameters for this simulation experiment are set as follows:

[0136] The length of the spreading code sequence is set to 448, the number of all subcarriers is 230, and the total number of accumulator blocks required for frequency-locked loop convergence is [not specified]. The number of all accumulator blocks required for phase-locked loop convergence The signal-to-noise ratio is 3dB, the sampling rate is 35.84MHz, and each accumulation block includes 8 symbol periods. This simulation experiment is applicable to orthogonal frequency division multiplexing (OFDM) signals with binary phase shift keying spread spectrum modulation on multiple subcarriers. The length of the spreading code sequence should be greater than or equal to the sum of the convergence step size of the frequency-locked loop (LLL) and the phase-locked loop (PLL). Furthermore, the number of effective subcarriers should be greater than half the number of subcarriers in the OFDM signal. The proposed method achieves the best processing effect when the cyclic prefix is ​​less than 1 / 4 of the OFDM symbol length.

[0137] Figure 2The curve showing the estimated frequency difference of the output when the carrier loop is in a frequency-locked loop state in this simulation experiment is illustrated. Figure 2 As can be seen, when the cumulative block index is 13 to 16, the obtained frequency difference estimate has remained consistently close to 0. Therefore, Set it to 15. Figure 2 The horizontal axis represents the cumulative block index, and the vertical axis represents the frequency difference estimate.

[0138] Figure 3 The figure shows the phase difference estimate curve of the output when the carrier loop is in a phase-locked loop state in this simulation experiment. Figure 3 As can be seen, after the cumulative block index is 30, the obtained phase difference estimate has remained consistently close to 0. Therefore, Set it to 15, and start recording the obtained carrier frequency control word from the accumulator block index 32. Figure 3 The horizontal axis represents the cumulative block index, and the vertical axis represents the estimated phase difference.

[0139] exist Figure 4 In a and b, the horizontal axis represents the number of Monte Carlo simulations, and the vertical axis represents the error of the frequency offset estimate. Figure 4 Figure 'a' shows a graph illustrating the error of the frequency offset estimate obtained using the method proposed in this application. Figure 4 Figure b shows a graph illustrating the error of the frequency offset estimate obtained using the conventional time-domain correlation frequency offset estimation method. It can be seen that the error of the frequency offset estimate obtained using the method proposed in this application is significantly smaller than the error of the frequency offset estimate obtained using the conventional time-domain correlation frequency offset estimation method. This indicates that this application can extract the Doppler frequency with higher accuracy than the conventional frequency-domain correlation frequency offset estimation method, thus providing robust observational data for high-precision satellite positioning.

[0140] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0141] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0142] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the scope of the technology disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application.

[0143] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not disclosed herein.

Claims

1. A method for estimating frequency offset in low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop, characterized in that, The method includes the following steps: The receiver divides the received full frame sequence into multiple accumulation blocks with the same structure in chronological order. Each accumulation block contains multiple identical and consecutive subcarriers, and each subcarrier contains multiple identical and consecutive symbol periods. The receiver calculates the phase rotation factor of the residual carrier corresponding to each symbol period in the current accumulation block based on the carrier frequency control word corresponding to the current accumulation block. Each phase rotation factor includes... Road component values ​​and Path component values; Each of the above The path component values ​​correspond to each of the subcarriers after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping processing to obtain the subcarrier values ​​for each symbol period in the current accumulation block. The road carrier stripping value is used simultaneously for each of the aforementioned values. The path component values ​​correspond to each of the subcarriers after orthogonal frequency division multiplexing demodulation. The signal values ​​are subjected to carrier stripping processing to obtain the subcarrier values ​​for each symbol period in the current accumulation block. Road carrier stripping value; Each of the above The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the subcarrier's value in each symbol period of the current accumulation block. The path carrier correlation value is used simultaneously for each of the aforementioned values. The subcarrier stripping value is correlated with the code value of the corresponding spreading code in the locally generated spreading code sequence to obtain the subcarrier's value in each symbol period of the current accumulation block. Road carrier correlation value; Each of the subcarriers in the current accumulation block is respectively... The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. The path accumulates the correlation value, and simultaneously adds all the values ​​of each subcarrier in the current accumulation block. The correlation values ​​of the road carriers are coherently accumulated to obtain the corresponding values. Road accumulation correlation value; Based on the index of the current accumulation block in the entire frame sequence, the carrier loop state of the current accumulation block is determined, and all the current accumulation blocks are used as references. Road accumulation related values ​​and all mentioned The relevant values ​​are accumulated to obtain the carrier frequency control word corresponding to the next accumulation block; The process of calculating the phase rotation factor and obtaining the carrier frequency control word is iterated until all the accumulated blocks in the whole frame sequence are processed, and the average value of all the carrier frequency control words obtained after the carrier loop converges is used as the frequency offset estimate of the whole frame sequence.

2. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier ring as described in claim 1, characterized in that, The The expression for the road carrier stripping value is: (1) in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Road component values, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road signal value, Indicates the first The first in the accumulator block The corresponding symbol period Path component values; The The expression for the road carrier stripping value is: (2) in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

3. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier ring as described in claim 1, characterized in that, The The expression for the path carrier correlation value is: (3) in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block The code value of the spreading code corresponding to each symbol period, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value; The The expression for the path carrier correlation value is: (4) in, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier stripping value.

4. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier ring as described in claim 1, characterized in that, The The expression for the cumulative correlation value is: (5) in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, This indicates the number of symbol periods in each subcarrier within each accumulation block. Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Road carrier correlation value; The The expression for the cumulative correlation value is: (6) in, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... The first in the accumulator block Within a symbol period Correlation value of the road carrier.

5. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier ring as described in claim 1, characterized in that, The carrier loop state of the current accumulation block is determined based on the index of the current accumulation block in the entire frame sequence, and all the current accumulation blocks are used as references. Road accumulation related values ​​and all mentioned The step of accumulating the correlation values ​​to obtain the carrier frequency control word corresponding to the next accumulator block includes: Set the number of all the aforementioned accumulator blocks required for frequency-locked loop convergence. The number of all the accumulator blocks required for phase-locked loop convergence is set to... The entire frame sequence has been set to have obtained all the aforementioned... Road accumulation related values ​​and all mentioned The index of the current accumulation block for the path accumulation correlation value is , , Indicates the index of the last accumulated block in the entire frame sequence; when When joint frequency and phase detection are not possible, all values ​​in the current accumulation block are saved. Road accumulation related values ​​and all mentioned The relevant values ​​are accumulated, and the next accumulation block is processed. when When, combine all the current accumulation block and the previous accumulation block. Road accumulation related values ​​and all mentioned The frequency difference estimate corresponding to the current accumulation block is obtained by accumulating the correlation values ​​of the accumulator and performing joint frequency discrimination. The frequency difference estimate is then filtered using the loop filter of the frequency-locked loop to output the corresponding carrier frequency control word. The carrier frequency control word corresponding to the current accumulation block is then updated to obtain the carrier frequency control word corresponding to the next accumulation block. when When the frequency-locked loop converges, it directly merges the current accumulator block with all the values ​​in the previous accumulator block. Road accumulation related values ​​and all mentioned The phase difference estimate corresponding to the current accumulation block is obtained by accumulating the correlation values ​​of the accumulation block and performing joint phase detection. The phase difference estimate is then filtered using the loop filter of the phase-locked loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulation block is then updated to obtain the carrier frequency control word corresponding to the next accumulation block. when At that time, both the frequency-locked loop and the phase-locked loop converge, and the operation is performed in accordance with the current... The same processing procedure is followed, and the carrier frequency control word corresponding to the next accumulation block is recorded.

6. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop according to claim 5, characterized in that, The when When, combine all the current accumulation block and the previous accumulation block. Road accumulation related values ​​and all mentioned The steps of performing joint frequency discrimination by accumulating correlation values ​​to obtain the frequency difference estimate corresponding to the current accumulating block, filtering the frequency difference estimate using the loop filter of the frequency-locked loop, outputting the corresponding carrier frequency control word, updating the carrier frequency control word corresponding to the current accumulating block, and obtaining the carrier frequency control word corresponding to the next accumulating block include: Calculate the dot product and cross product of each subcarrier in the current accumulation block; The expression for the dot product value is: (7) in, Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value, Indicates the first The subcarrier at the _ ... In each accumulated block Road accumulation correlation value; The expression for the cross product value is: (8) in, Indicates the first The subcarrier at the _ ... Cross product values ​​in each accumulator block; Using all the dot product values ​​and all the cross product values, calculate the frequency difference estimate corresponding to the current accumulation block; The expression for the frequency offset estimate is: (9) in, Indicates the first Frequency offset estimates corresponding to each accumulator block This indicates the number of all subcarriers. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block; The frequency difference estimate corresponding to the current accumulator block is filtered using the loop filter of the frequency-locked loop, and the corresponding carrier frequency control word is output. The carrier frequency control word corresponding to the current accumulator block is then updated to obtain the carrier frequency control word corresponding to the next accumulator block. The expression for updating the carrier frequency control word using the loop filter of the frequency-locked loop is as follows: (10) in, Indicates the first The carrier frequency control word corresponding to each accumulator block Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain factor of the frequency-locked loop. , This represents the noise bandwidth of the frequency-locked loop. These represent the parameter values ​​of the loop filter in the frequency-locked loop. Indicates the first Frequency offset estimates corresponding to each accumulator block.

7. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop as described in claim 5, characterized in that, The expression for the phase difference estimate is: (11) in, Indicates the first The phase difference estimate corresponding to each accumulation block Represents the arctangent function in the four quadrants. Indicates the first The subcarrier at the _ ... The dot product value in each cumulative block, Indicates the first The subcarrier at the _ ... The cross product value in each accumulator block Indicates the number of all subcarriers; The expression for updating the carrier frequency control word using the loop filter of the phase-locked loop is as follows: (12) in, Indicates the first The carrier frequency control word corresponding to each accumulator block This represents the gain factor of the phase-locked loop. , This represents the noise bandwidth of the phase-locked loop. These represent the parameter values ​​of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, This represents the gain coefficient of the loop filter in a phase-locked loop. Indicates the first The length of all symbol periods in a accumulator block, , Indicates the first The end time of each accumulation block, Indicates the first The end time of each accumulation block, Indicates the first The phase difference estimate corresponding to each accumulation block.

8. The frequency offset estimation method for low-Earth orbit satellite spread spectrum orthogonal frequency division multiplexing based on carrier loop according to claim 5, characterized in that, The step of using the average value of all carrier frequency control words obtained after carrier loop convergence as the frequency offset estimate of the entire frame sequence includes: The convergence of the carrier loop indicates that both the frequency-locked loop and the phase-locked loop have converged. The average value of all the carrier frequency control words obtained at that time is used as the frequency offset estimate of the entire frame sequence.

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