A low-orbit navigation signal high-precision time delay measurement method based on an OFDM system

Abstract

The application discloses a kind of low orbit navigation signal high-precision time delay measurement method based on OFDM system, belong to low orbit navigation technical field.The application is received to the low orbit navigation signal of OFDM system, selects signal frequency conversion and the OFDM symbol data after FFT processing, carries out bit-by-bit decision and peels off in the resource grid of navigation signal corresponding OFDM symbol data, obtains time delay estimation complex signal data sample, and carries out amplitude angle calculation and over-limit adjustment, finally the amplitude angle calculation result of time delay estimation complex signal data sample after adjustment is carried out time delay estimation calculation, finally obtains the high-precision time delay measurement result of low orbit navigation signal, can be used for application end to carry out positioning solution processing.The application solves the technical problem that when OFDM system is used as low orbit navigation signal, time delay pseudorange cannot be estimated using spread spectrum code, realizes accurate demodulation of electric text, and also realizes high-precision estimation of time delay, further improves the precision of pseudorange measurement.

Description

Technical Field

[0001] This invention belongs to the field of low-Earth orbit navigation technology, and specifically refers to a high-precision time delay measurement method for low-Earth orbit navigation signals based on OFDM. Background Technology

[0002] While traditional satellite navigation methods are widely used, their low ground voltage levels pose significant vulnerabilities, leading to service interruptions in obscured spaces and complex electromagnetic environments. Therefore, using integrated communication and navigation signals for positioning and navigation has become a research hotspot. Currently, the most advanced modulation method in the communication field is OFDM (Orthogonal Frequency Division Multiplexing) modulation, represented by 4G and 5G mobile communications. Consequently, using OFDM signals to carry navigation services has become the mainstream approach.

[0003] However, considering that traditional navigation signals use a tracking loop with pseudo-random spreading codes to accurately estimate arrival delay, while OFDM signals use signals after inverse Fourier transform for spatial transmission, it is difficult to use pseudo-random spreading codes for tracking processing before and after the Fourier transform. In addition, OFDM navigation signals are generally broadcast intermittently, and using traditional detection synchronization estimation mechanisms can only make the estimation accuracy reach the size of the sampling interval, which seriously restricts the improvement of measurement accuracy. Summary of the Invention

[0004] The purpose of this invention is to provide a high-precision time delay measurement method for low-Earth orbit (LEO) navigation signals based on the OFDM system. This method solves the technical problem that when OFDM is used as a LEO navigation signal, the spread spectrum code cannot be used to estimate the pseudorange delay. This invention achieves accurate message demodulation while also enabling high-precision time delay estimation, further improving the accuracy of pseudorange measurement.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A high-precision time delay measurement method for low-orbit navigation signals based on OFDM includes the following steps:

[0007] (1) Receive the low-orbit navigation signal of OFDM system, and after signal frequency conversion and FFT processing, select the OFDM symbol data corresponding to the resource cell where the navigation signal is located;

[0008] (2) Perform message information bit-by-bit judgment on the OFDM symbol data corresponding to the resource cell where the navigation signal is located;

[0009] (3) Remove the message information from the OFDM symbol data in the resource cell where the navigation signal is located, obtain the time delay estimation complex signal data sample, and perform the amplitude calculation;

[0010] (4) Adjust the amplitude calculation results of the time delay estimation complex signal data samples to ensure that the amplitude calculation results are within the range of [-π,π].

[0011] (5) Perform time delay estimation calculation on the amplitude calculation results of the adjusted time delay estimation complex signal data samples, and finally obtain the high-precision time delay measurement results of the low-orbit navigation signal.

[0012] Furthermore, the specific method of step (1) is as follows:

[0013] (101) The low-orbit navigation signal output after analog-to-digital conversion is an OFDM intermediate frequency signal S(n). After stripping the intermediate frequency signal carrier, the low-orbit navigation OFDM baseband signal S0(n) is obtained. The stripping method is as follows:

[0014]

[0015] Among them, F s f is the sampling frequency of the OFDM intermediate frequency signal. c f is the center frequency of the OFDM intermediate frequency signal. cT is the center frequency of the resource grid where the navigation information is located in the OFDM low-orbit navigation signal, and j is the imaginary unit;

[0016] (102) Perform a Fourier transform on the frequency-converted low-orbit navigation OFDM baseband signal to obtain the OFDM symbol data M1(n) corresponding to the resource cell where the navigation signal is located:

[0017] M1(n) = DFT(S0(n))

[0018] Wherein, DFT stands for Discrete Fourier Transform.

[0019] Furthermore, the specific method of step (2) is as follows:

[0020] (201) Set the start bit of the low-orbit navigation signal modulation information to 1, i.e., I(1) = 1, where I(n) is the sequence of modulation information, n = 1, 2, ... N. F N F For information length;

[0021] (202) Perform the decision calculation for each information bit in sequence according to the following formula:

[0022] I(k)=0.5+[(I(k-1)-0.5)]×sign[real(M1(k-1))×real(M1(k))+imag(M1(k-1))×imag(M1(k))],k=2,3,…N F

[0023] Where real is the real part operation, imag is the imaginary part operation, and sign is the sign operation.

[0024] Furthermore, the specific method of step (3) is as follows:

[0025] (301) Remove the message information from the OFDM symbol data corresponding to the resource cell where the navigation signal is located. M2(n) is the OFDM symbol data corresponding to the resource cell where the navigation signal is located after removing the message information, i.e., the delay estimation complex signal data sample. The processing method is as follows:

[0026] M2(n) = M1(n) × I(n)

[0027] (302) Calculate the phase sequence A1(n) of the time-delay estimated complex signal data samples. The processing method is as follows:

[0028]

[0029] Where real is the operation of extracting the real part, and imag is the operation of extracting the imaginary part.

[0030] Furthermore, the specific method of step (4) is as follows:

[0031]

[0032] The above formula is used to complete the over-limit adjustment of the argument calculation result, where A2(n) is the argument calculation result of the time delay estimation complex signal data sample after adjustment.

[0033] Furthermore, the specific method of step (5) is as follows:

[0034] (501) Generate a reference matrix B for time delay estimation. The reference matrix is ​​as follows:

[0035]

[0036] (502) Generate the reference vector C(n) according to the following formula;

[0037] C(n) = [B × B T ] -1 ×B

[0038] Where T represents the transpose of the matrix, and -1 represents the inversion of the matrix;

[0039] (503) Generate the time delay estimation vector D(n) according to the following formula;

[0040] D(n) = C(n) × A2(n) T

[0041] (504) Calculate the high-precision time delay measurement result Delay of the low-orbit navigation signal according to the following formula:

[0042]

[0043] Where D(1) represents the first term of vector D(n).

[0044] The beneficial effects of this invention compared to the prior art are as follows:

[0045] 1. Traditional methods use the cyclic prefix of OFDM to synchronize the low-orbit OFDM navigation signal and then perform time delay estimation. However, the synchronization time delay estimation based on the cyclic prefix can only achieve the resolution accuracy of the signal sampling interval. This method uses the signal after Fourier transform for time delay estimation, which can greatly improve the measurement accuracy of time delay and ensure the accuracy of positioning when using low-orbit OFDM navigation signals.

[0046] 2. This invention performs accurate measurement of the time delay of low-orbit OFDM navigation signals after Fourier transform at the receiving end, ensuring accurate positioning based on these signals. This invention solves the technical problem of being unable to estimate the pseudorange using spreading codes when OFDM is used as a low-orbit navigation signal. Therefore, while achieving accurate message demodulation, it also achieves high-precision estimation of the time delay, further improving the accuracy of pseudorange measurement. Attached Figure Description

[0047] Figure 1 This is a flowchart of a high-precision time delay measurement method for low-orbit navigation signals based on the OFDM system. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0049] A high-precision time delay measurement method for low-Earth orbit (LEO) navigation signals based on the OFDM system is proposed. This method first receives the OFDM LEO navigation signal, selects the OFDM symbol data after frequency conversion and FFT processing, performs bit-by-bit message information judgment and stripping on the corresponding OFDM symbol data in the resource cell containing the navigation signal, obtains a time delay estimation complex signal data sample, performs amplitude calculation and over-limit adjustment, and finally calculates the time delay based on the amplitude calculation result of the adjusted time delay estimation complex signal data sample. The final result is a high-precision time delay measurement of the LEO navigation signal, which can be used for positioning and processing at the application end.

[0050] like Figure 1 As shown, the method specifically includes the following steps:

[0051] (1) Receive the low-orbit navigation signal of OFDM system, and after signal frequency conversion and FFT processing, select the OFDM symbol data corresponding to the resource cell where the navigation signal is located;

[0052] After passing through the antenna and RF front-end, the OFDM low-orbit navigation RF signal reaches the receiving processing board. The low-orbit navigation signal output after quantization by the AD converter on the board is the OFDM intermediate frequency signal S(n). The carrier of the intermediate frequency signal is stripped to obtain the low-orbit navigation OFDM baseband signal S0(n). The stripping method is as follows:

[0053]

[0054] Among them, F s f is the sampling frequency of the OFDM intermediate frequency signal. c f is the center frequency of the OFDM intermediate frequency signal. cT This refers to the center frequency of the resource grid where the navigation information is located in the OFDM low-orbit navigation signal.

[0055] The frequency-converted low-orbit navigation OFDM baseband signal is subjected to Fourier transform to obtain the OFDM symbol data M1(n) corresponding to the resource cell where the navigation signal is located. At this time, the data contains the modulated message and the phase offset caused by the time delay:

[0056] M1(n) = DFT(S0(n))

[0057] (2) Perform message information bit-by-bit judgment on the OFDM symbol data corresponding to the resource cell where the navigation signal is located;

[0058] The start bit of the low-orbit navigation signal modulation is set to 1, i.e., I(1) = 1, where I(n) is the sequence of modulation information, n = 1, 2, ... N. F N F For information length;

[0059] The decision calculation for each information bit is completed sequentially according to the following formula. The calculation process requires first using I(1) to calculate I(2), then using I(2) to calculate I(3), and so on:

[0060] I(k)=0.5+[(I(k-1)-0.5)]×sign[real(M1(k-1))×real(M1(k))+imag(M1(k-1))×imag(M1(k))],k=2,3,…N F

[0061] Where real is the real part operation, imag is the imaginary part operation, and sign is the sign operation.

[0062] (3) Remove the message information from the OFDM symbol data in the resource cell where the navigation signal is located, obtain the time delay estimation complex signal data sample, and perform the amplitude calculation;

[0063] First, the message information is stripped from the OFDM symbol data corresponding to the resource cell where the navigation signal is located. M2(n) represents the OFDM symbol data corresponding to the resource cell where the navigation signal is located after the message information has been stripped, i.e., the delay estimation complex signal data sample. The processing method is as follows:

[0064] M2(n) = M1(n) × I(n)

[0065] Then, the amplitude sequence A1(n) of the time-delay estimated complex signal data samples is calculated using the following method:

[0066]

[0067] (4) Adjust the amplitude calculation results of the time delay estimation complex signal data samples to ensure that the data range of the amplitude calculation results meets the value range of [-π,π].

[0068]

[0069] The above formula is used to complete the over-limit adjustment of the argument calculation result, where A2(n) is the argument calculation result of the time delay estimation complex signal data sample after adjustment.

[0070] (5) Perform time delay estimation calculation on the amplitude calculation results of the adjusted time delay estimation complex signal data samples, and finally obtain the high-precision time delay measurement results of the low-orbit navigation signal.

[0071] Generate a reference matrix B for time delay estimation. The reference matrix is ​​as follows:

[0072]

[0073] After generating the reference matrix, the reference vector C(n) is generated using the reference matrix according to the following formula;

[0074] C(n) = [B × B T ] -1 ×B

[0075] The time delay estimation vector D(n) is generated using the reference vector and the argument calculation results according to the following formula;

[0076] D(n) = C(n) × A2(n) T

[0077] The high-precision time delay measurement results of the low-orbit navigation signal can be calculated using the following formula to obtain a high-precision estimate of the time delay. Combined with the arrival time estimated during synchronization, a highly accurate pseudorange observation can be obtained:

[0078]

[0079] In summary, this invention solves the technical problem that when OFDM is used as a low-orbit navigation signal, it is impossible to estimate the time delay pseudorange using the spreading code. While achieving accurate demodulation of the message, it also achieves high-precision estimation of the time delay, further improving the accuracy of pseudorange measurement.

Claims

1. A high-precision time delay measurement method for low-orbit navigation signals based on OFDM, characterized in that, Includes the following steps: (1) Receive the low-orbit navigation signal of OFDM system, and after signal frequency conversion and FFT processing, select the OFDM symbol data corresponding to the resource cell where the navigation signal is located; (2) Perform bit-by-bit message information judgment on the OFDM symbol data corresponding to the resource cell where the navigation signal is located; the specific method is as follows: (201) Set the start bit of the low-orbit navigation signal modulation to 1, that is ,in The sequence of modulated information. , For information length; (202) Perform the decision calculation for each information bit in sequence according to the following formula: = ,k=2,3,… Where real is the real part operator, imag is the imaginary part operator, and sign is the sign operator. This refers to the OFDM symbol data corresponding to the resource cell where the navigation signal is located; (3) Remove the message information from the OFDM symbol data corresponding to the resource cell where the navigation signal is located, obtain the time delay estimation complex signal data sample, and perform the amplitude calculation; (4) Adjust the calculation results of the amplitude of the time delay estimation complex signal data sample to ensure that the calculation results of the amplitude are within the range of [-π,π]. (5) Perform time delay estimation calculation on the phase angle calculation results of the adjusted time delay estimation complex signal data samples to finally obtain the high-precision time delay measurement results of the low-orbit navigation signal; the specific method is as follows: (501) Generate a reference matrix for time delay estimation. The reference matrix is: (502) Generate the reference vector according to the following formula. ; Where T represents the transpose of a matrix, and -1 represents the inversion of a matrix; (503) Generate the time delay estimation vector according to the following formula. ; in, The result of calculating the amplitude of the adjusted time-delay estimation complex signal data sample; (504) Calculate the high-precision time delay measurement result Delay of the low-orbit navigation signal according to the following formula: in, Let D(1) be the sampling frequency of the OFDM intermediate frequency signal, and let D(1) represent the vector. The first item.

2. The method for high-precision time delay measurement of low-orbit navigation signals based on OFDM system according to claim 1, characterized in that, The specific method for step (1) is as follows: (101) The low-orbit navigation signal output after analog-to-digital conversion is an OFDM intermediate frequency signal S(n). After stripping the intermediate frequency signal carrier, the low-orbit navigation OFDM baseband signal S0(n) is obtained. The stripping method is as follows: in, The sampling frequency of the OFDM intermediate frequency signal. The center frequency of the OFDM intermediate frequency signal. is the center frequency of the resource grid where the navigation information is located in the OFDM low-orbit navigation signal, and j is the imaginary unit; (102) Perform Fourier transform on the frequency-converted low-orbit navigation OFDM baseband signal to obtain the OFDM symbol data corresponding to the resource cell where the navigation signal is located. : Wherein, DFT stands for Discrete Fourier Transform.

3. The method for high-precision time delay measurement of low-orbit navigation signals based on OFDM system according to claim 2, characterized in that, The specific method for step (3) is as follows: (301) Remove the message information from the OFDM symbol data corresponding to the resource cell where the navigation signal is located. To remove the OFDM symbol data corresponding to the navigation signal in the resource cell containing the message information, i.e., to estimate the complex signal data samples by delay, the processing method is as follows: (302) Calculate the time delay to estimate the phase sequence of the complex signal data samples. The solution is as follows: Where real is the operation of extracting the real part, and imag is the operation of extracting the imaginary part.

4. The method for high-precision time delay measurement of low-orbit navigation signals based on OFDM system according to claim 3, characterized in that, The specific method of step (4) is as follows: The above formula is used to adjust the over-limit result of the argument calculation, where The result is the calculated amplitude of the adjusted time delay estimation complex signal data sample.

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