A construction method and upgrade method of OFDM-BOC navigation signal

Abstract

This invention belongs to the technical field of navigation signal design, specifically to a method for constructing an OFDM-BOC navigation signal. The method comprises the following steps: generating a preliminary OFDM-BOC navigation signal; and performing performance analysis on the generated preliminary OFDM-BOC navigation signal using the power spectral density of the designed navigation signal. A method for upgrading the OFDM-BOC navigation signal is also disclosed. This method fully utilizes spectrum resources, achieving excellent navigation performance while maintaining strong compatibility with existing navigation signals. It can be used in broadband navigation signal design and integrated communication and navigation signal design.

Description

Technical Field

[0001] The present invention belongs to the technical field of navigation signal design, and in particular relates to a construction method and an upgrading method of an OFDM-BOC navigation signal. Background Art

[0002] With the rapid development of autonomous driving, 5G, and Internet technologies, especially the rise of concepts such as the Internet of Things and smart cities, the demand for location-based services (LBS) continues to increase. At present, the Global Navigation Satellite System (GNSS) can provide high-precision positioning services outdoors, and has achieved rapid development with its high-precision PNT (Positioning, Navigation, and Timing) capabilities and ubiquitous, high-speed information transmission capabilities. However, due to factors such as weak landing power of navigation signals, susceptibility to interference from the physical space environment, low navigation message transmission rate, and a small number of navigation services, GNSS cannot meet the personalized needs of users in complex environments, especially in terms of positioning accuracy, serviceability, and anti-interference performance. This has greatly limited the scope and depth of satellite navigation applications. The vigorous development of low-orbit satellites and broadband communications has provided an opportunity for the development of integrated communication and navigation technology.

[0003] However, (1) the current navigation modulation method is generally narrowband navigation signal modulation, which cannot fully utilize the large-bandwidth spectrum resources of low-orbit satellites and broadband communication equipment; (2) the current navigation modulation does not fully consider compatibility design and cannot be compatible with the old system during the upgrade process. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for constructing and upgrading an OFDM-BOC navigation signal, which solves the problem that the current navigation modulation method cannot fully utilize the large-bandwidth spectrum resources of low-orbit satellites and broadband communication equipment; and the current navigation modulation upgrade process cannot be compatible with the old system.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for constructing an OFDM-BOC navigation signal includes the following steps:

[0007] S1. Generate preliminary OFDM-BOC navigation signal:

[0008] S101. For any OFDM-BOC navigation signal, first determine the basic parameters of the navigation signal;

[0009] S102. Select a pseudo-random sequence to achieve orthogonality between different subcarriers and implement orthogonal frequency division multiplexing.

[0010] S103 constructs the navigation signal of the i-th subcarrier;

[0011] S104 sequentially performs S103 operation on the subcarrier signals from 1 to K;

[0012] S105. The navigation signals of the K subcarriers are accumulated to obtain a multi-carrier OFDM-BOC navigation signal, thereby generating a preliminary OFDM-BOC navigation signal;

[0013] S2. Using the designed power spectrum density G(f) of the OFDM-BOC navigation signal, a performance analysis is performed on the generated preliminary OFDM-BOC navigation signal. If the preliminary OFDM-BOC navigation signal meets the requirements, a qualified OFDM-BOC navigation signal is generated.

[0014] If the preliminary OFDM-BOC navigation signal does not meet the requirements, S101 to S105 are repeated until the requirements are met.

[0015] Furthermore, in S101, the basic parameters of the navigation signal include: the number of subcarriers K, the reference frequency f0, the rate of the pseudo-random code f C And the square wave subcarrier frequency f corresponding to the i-th subcarrier i .

[0016] Furthermore, in S102, the pseudo-random sequence is generated in the following two ways:

[0017] (1) Construct different pseudo-random sequences for different subcarriers;

[0018] (2) Use the same pseudo-random sequence, but perform appropriate cyclic shifts on the pseudo-random sequences used by different sub-carriers to ensure orthogonality of different sub-carriers.

[0019] Furthermore, S103 is specifically as follows:

[0020] Construct the navigation signal of the i-th subcarrier. Each subcarrier contains I branch and Q branch. The navigation message is D Ii (k) and D Qi (k), the pseudo-random sequences generated in S102 are PN Ii (k) and PN Qi (k) Construct the modulation signal a of the navigation message and pseudo-random sequence i (k) and b i (k):

[0021]

[0022] Generate a square wave subcarrier signal χ by using sign function and trigonometric function Ii (t) and χ Qi (t):

[0023]

[0024]

[0025] Among them, f S,i represents the frequency corresponding to the i-th subcarrier; T C Indicates the duration of a pseudo code chip;

[0026] Then we get the navigation modulation signal s of branch I i,cos (t) and the navigation modulation signal s of the Q branch i,sin (t):

[0027]

[0028] The navigation signal of the I branch is subtracted from the navigation signal of the Q branch to obtain the navigation signal s of the i-th branch. i (t):

[0029] s i (t) = s i,cos (t)-js i,sin (t) (6).

[0030] Furthermore, in S105, the navigation signals of the K subcarriers are accumulated to obtain a multi-carrier OFDM-BOC navigation signal, which is expressed as:

[0031]

[0032] Furthermore, in S2, the design of the power spectrum density G(f) of the OFDM-BOC navigation signal specifically includes the following steps:

[0033] S201. Calculate the duration of the i-th subcarrier chip

[0034]

[0035] According to α i and β calculate n i :

[0036]

[0037] α i and β are based on the pseudo random code rate f C , the square wave subcarrier frequency f corresponding to the i-th subcarrier i , and the reference frequency f0: β=f C / f0;α i =f i / f0;

[0038] S202.f represents the frequency. The power spectrum density function S of the ith subcarrier I branch is calculated according to formula (8) and formula (9): cos,ii (f):

[0039]

[0040] S203. Calculate the power spectral density function S of the i-th subcarrier Q branch sin,ii (f):

[0041]

[0042] S204. The I branch and the Q branch are orthogonal, and the power spectral density of the I branch and the Q branch is 0. Different subcarriers are orthogonal to each other, and the power spectral density of two different subcarriers is also 0.

[0043] S205 repeats S201 to S203 for each subcarrier from 1 to K, and calculates the power spectral density of each subcarrier;

[0044] S206. According to formula (10) and formula (11), the power spectrum density of each subcarrier is superimposed to obtain the power spectrum density G(f) of the navigation signal OFDM-BOC:

[0045]

[0046] The invention also discloses an OFDM-BOC navigation signal generated by the construction method.

[0047] The present invention also discloses a method for upgrading the OFDM-BOC navigation signal. When the external environment changes, the OFDM-BOC navigation signal needs to be upgraded. A progressive navigation upgrade method is adopted, which specifically includes the following steps:

[0048] S301. Determine the navigation signal S to be upgraded ori (t), plot the power spectrum density G of the navigation signal to be upgraded ori (f);

[0049] S302. According to S ori (t) and G ori (f) Determine the parameters of the superimposed OFDM-BOC navigation signal:

[0050] S303. Generate the superimposed OFDM-BOC navigation signal S according to S1 based on the parameters of the superimposed OFDM-BOC navigation signal generated in S302. add (t);

[0051] S304. The superimposed OFDM-BOC navigation signal S add (t) and the navigation signal S to be upgraded ori (t) Superposition to obtain the upgraded navigation signal S update (t);

[0052] S305. Construction S update (t) power spectral density PSD, find the parameter PAR2 of OFDM-BOC with similar PSD;

[0053] S306. Generate OFDM-BOC navigation signal S using S1 according to parameter PAR2 update，sim (t), thereby simplifying the generation structure of the navigation signal.

[0054] Furthermore, S302 specifically includes the following steps:

[0055] 3021. Determine the reference frequency f0 based on the pseudo code rate of the navigation signal to be upgraded;

[0056] 3022. According to G ori (f) Navigation signal bandwidth, which determines the bandwidth of the OFDM-BOC navigation signal subcarriers;

[0057] 3023. The subcarrier spacing of the OFDM-BOC navigation signal is equal to the subcarrier bandwidth;

[0058] 3024. According to the allowed radio frequency bandwidth, the number of subcarriers is determined, thereby determining the square wave subcarrier frequency α of the subcarrier. i f0.

[0059] Compared with the prior art, the present invention has the following beneficial technical effects:

[0060] The present invention provides a method for constructing an Orthogonal Frequency Division Multiplexing Binary Square Wave Carrier (OFDM-BOC) navigation signal. Existing navigation signals are primarily narrowband modulated, and their power spectrum density exhibits multiple peaks and valleys. As communication bandwidths become increasingly wider, current modulation methods are unable to fully utilize broadband resources. The OFDM-BOC navigation modulation proposed in the present invention, however, achieves a flat, gate-shaped power spectrum density across the entire bandwidth, resulting in higher spectrum utilization and the full utilization of broadband spectrum resources.

[0061] During the design phase, navigation signal performance analysis is a crucial basis and reference for navigation signal design. The tracking accuracy of the navigation signal can be inferred from its power spectral density, making it the core of performance analysis. This paper presents an analytical expression for OFDM-BOC's power spectral density, providing strong support for navigation signal design.

[0062] This invention discloses a method for upgrading OFDM-BOC navigation signals. Existing navigation signals have weak scalability, making it impossible to upgrade the navigation system by adding new navigation signals while retaining the existing ones, making them inadequate for meeting the requirements of next-generation navigation systems. The OFDM-BOC proposed in this invention exhibits excellent scalability, allowing the existing navigation signal to be retained as part of the upgrade signal. The newly added signal has no negative impact on the receiver of the original navigation signal, resulting in excellent iterative upgrade performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 Generate a flow chart for OFDM-BOC navigation signals;

[0064] Figure 2 is the time domain waveform of I branch and Q branch;

[0065] Figure 3 ACF comparison diagram of several different navigation modulations;

[0066] Figure 4 The power spectral density (PSD) comparison diagram of several different navigation modulations is shown below:

[0067] Figure 5 Comparison of tracking errors for several navigation modulation schemes; (a) RF bandwidth 36 MHz, code tracking loop lower bound; (b) RF bandwidth 55 MHz, code tracking loop lower bound; (c) RF bandwidth 36 MHz, NELP tracking error; (d) RF bandwidth 55 MHz, NELP tracking error.

[0068] Figure 6 This is the autocorrelation function curve before and after the navigation modulation upgrade;

[0069] Figure 7 The power spectral density curves before and after the navigation modulation upgrade. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical solutions and advantages of the present invention more clear, the following is a further detailed description with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. That is, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments.

[0071] The components described and illustrated in the drawings and embodiments of the present invention may be arranged and designed in a variety of different configurations. Therefore, the detailed description of the embodiments of the present invention provided in the following drawings is not intended to limit the scope of the claimed invention, but merely represents a selected embodiment of the present invention. All other embodiments derived by those skilled in the art based on the drawings and embodiments of the present invention without inventive effort shall fall within the scope of protection of the present invention.

[0072] This invention discloses a method for modulation, performance analysis, and progressive navigation signal system upgrade for an Orthogonal Frequency Division Multiplexing (OFDM) binary offset subcarrier (OFDM-BOC) navigation signal. The navigation signal generation method is the foundation of the entire invention, and the performance analysis provides a reference for navigation signal design. The progressive navigation upgrade method provides a solution to the incompatibility of existing satellite navigation upgrades. Specifically, the method includes the following steps:

[0073] S1.OFDM-BOC navigation signal generation method;

[0074] S101. For any OFDM-BOC navigation signal, it is necessary to determine the basic parameters of the navigation signal: the number of subcarriers K, the reference frequency f0, the pseudo-random code rate f C , the frequency f of the square wave subcarrier corresponding to the i-th subcarrier i ;

[0075] S102. Orthogonality between different subcarriers is achieved by selecting an appropriate pseudo-random sequence, thereby achieving orthogonal frequency division multiplexing. The present invention provides two pseudo-random sequence generation methods:

[0076] (1) Construct different pseudo-random sequences for different subcarriers;

[0077] (2) Use the same pseudo-random sequence, but perform appropriate cyclic shifts on the pseudo-random sequences used by different sub-carriers to ensure orthogonality of different sub-carriers.

[0078] S103. Figure 1 As shown, the navigation signal of the i-th subcarrier is constructed. Each subcarrier contains two branches, I and Q (I and Q branches can work independently). The navigation message is D Ii (k) and D Qi (k), pseudo-random sequence PN Ii (k) and PN Qi (k) is generated in S102. Through the navigation message and the pseudo-random sequence, the modulation signal a of the navigation message and the pseudo-random sequence can be constructed. i (k) and b i (k):

[0079]

[0080] Next, the square wave subcarrier signal χ is generated by the sign function and trigonometric function. Ii (t) and χ Qi (t)

[0081]

[0082] Then, the navigation modulation signals of the in-phase branch and the quadrature branch of the i-th subcarrier can be obtained. The in-phase branch and the quadrature branch can also be called the I branch and the Q branch. The navigation modulation signals s i,cos (t) and s i,sin (t) is:

[0083]

[0084] Navigation signal s through I branch i,cos (t) and the navigation signal s of the Q branch i,sin (t), get the navigation signal s of the i-th branch i Plural form of (t):

[0085] s i (t) = s i,cos (t)-js i,sin (t) (6)

[0086] S104 sequentially performs S103 operation on the subcarrier signals from 1 to K;

[0087] S105. Accumulate the navigation signals of K subcarriers to obtain a multi-carrier OFDM-BOC navigation signal:

[0088]

[0089] S2.OFDM-BOC Navigation Signal Performance Analysis

[0090] S201. Calculate the duration of the i-th subcarrier chip

[0091]

[0092] Calculate n i :

[0093]

[0094] Where, α i and β are based on the pseudo random code rate f C , the square wave subcarrier frequency f corresponding to the i-th subcarrier i , and the reference frequency f0: β=f C / f0;αi =f i / f0;

[0095] S202.f represents the frequency. The power spectrum density function S of the ith subcarrier I branch is calculated according to formula (8) and formula (9): cos,ii (f):

[0096]

[0097] Where π represents the ratio of the circumference of a circle to its circumference.

[0098] S203. Calculate the power spectral density function S of the i-th subcarrier Q branch sin,ii (f):

[0099]

[0100] S204. The I branch and the Q branch are orthogonal, so the power spectral density of the I branch and the Q branch is 0. Different subcarriers are orthogonal to each other, so the power spectral density of two different subcarriers is also 0.

[0101] S205 repeats S201 to S203 for each subcarrier from 1 to K, and calculates the power spectral density of each subcarrier;

[0102] S206. According to formula (10) and formula (11), the power spectrum density of each subcarrier is superimposed to obtain the power spectrum density G(f) of the navigation signal OFDM-BOC:

[0103]

[0104] S3.OFDM-BOC progressive navigation signal upgrade method:

[0105] S301. Determine the navigation signal S to be upgraded ori (t), plot the power spectrum density G of the navigation signal to be upgraded ori (f);

[0106] S302. According to S ori (t) and G ori (f) Determine the parameters of the superimposed OFDM-BOC navigation signal:

[0107] 3021. Determine the reference frequency f0 based on the pseudo code rate of the navigation signal to be upgraded;

[0108] 3022. According to G ori(f) Navigation signal bandwidth: Determine the bandwidth of the OFDM-BOC navigation signal subcarrier. Generally, the subcarrier bandwidth is the same as the navigation signal bandwidth. Alternatively, a suitable ratio k can be selected such that k times the subcarrier bandwidth equals the bandwidth of the navigation signal to be upgraded.

[0109] 3023. The subcarrier spacing of the OFDM-BOC navigation signal is equal to the subcarrier bandwidth;

[0110] 3024. According to the allowed radio frequency bandwidth, the number of subcarriers is determined, thereby determining the square wave subcarrier frequency α of the subcarrier. i f0.

[0111] S303. Generate the superimposed OFDM-BOC navigation signal S according to S1 based on the parameters of the superimposed OFDM-BOC navigation signal generated in S302. add (t);

[0112] S304. The superimposed OFDM-BOC navigation signal S add (t) and the navigation signal S to be upgraded ori (t) Superposition to obtain the upgraded navigation signal S update (t);

[0113] S305. Construction S update (t) Power spectral density (PSD), find the parameter PAR2 of OFDM-BOC with similar PSD;

[0114] S306 generates OFDM-BOC navigation signal S using S1 according to parameter PAR2 update，sim (t), thereby simplifying the generation structure of the navigation signal.

[0115] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0116] Implementation 1: Generate an OFDM-BOC navigation modulation signal

[0117] 1. Determine the parameters of the modulation signal. The reference frequency f0 = 1.023 MHz, the code rate of the pseudo-random sequence is also 1.023 MHz (β = 1), and there are 16 subcarriers [α1, α2, ..., α 16 ]=[1,2,3,…,16].

[0118] 2. The pseudo-random sequence uses CA code, and CA codes numbered 1 to 16 are used on different subcarriers;

[0119] 3. According to S103 to S104, the time domain waveform of each subcarrier is constructed in sequence;

[0120] 4. Superimpose each subcarrier, such as Figure 2 As shown, the OFDM-BOC navigation modulation signal with 16 subcarriers is obtained:

[0121] Implementation 2: Analyzing OFDM-BOC Navigation Signal Performance

[0122] BOC(10,5) and MBOC(6,1) are two widely used satellite navigation signals. This example uses OFDM-BOC (16:1:1,1) with 16 subcarriers and OFDM-BOC (27:1:1,1) with 27 subcarriers for comparison and analyzes the performance of navigation signals using several different modulation schemes.

[0123] The autocorrelation function (ACF) is obtained by inverse Fourier transforming the power spectral density (PSD). The autocorrelation function (ACF) and the power spectral density (PSD) can be used to qualitatively analyze the performance of the navigation signal.

[0124] Follow the steps in S2, such as Figure 3 and Figure 4 As shown, the ACF and PSD of OFDM-BOC (16:1:1,1) and OFDM-BOC (27:1:1,1) can be generated in sequence.

[0125] The main lobe width of ACF refers to the width of the zero-crossing point of the main peak of ACF. The narrower the main lobe, the better the tracking performance of the navigation signal. When the main lobe peak is normalized, the maximum amplitude of the side lobe can reflect the difficulty of the navigation receiver tracking. Figure 3 The ACF of several different navigation signals shows that the OFDM-BOC signal has a narrow main lobe width and good tracking accuracy; the side lobe amplitude is low, making it easy for the receiver to track the signal and less prone to peak false lock.

[0126] The flatter the PSD main lobe, the less likely the navigation signal is to be detected, and the stronger its anti-interception performance is. The smaller the maximum value of the main lobe, the stronger its anti-interference capability is. OFDM-BOC signals have a flat main lobe and a small maximum value, resulting in better anti-interference and anti-interception performance.

[0127] Furthermore, the power spectrum density Gs(f) can be used to qualitatively analyze the tracking accuracy of the navigation signal. The tracking lower bound of the receiver pseudo-code tracking loop is σ LB and the tracking error σ of the non-coherent lead-lag loop (NELP) NELP The calculation formulas are as follows:

[0128]

[0129]

[0130] Where B L is the bandwidth of the code tracking loop, β rms is the RMS bandwidth, T is the correlation integration time, β r is the bandwidth of the RF front end, C / N0 is the carrier-to-noise ratio, and Δ is the time interval between the leading branch and the lagging branch of the code tracking loop.

[0131] Figure 5 It shows that σ LB and σ NELP ,It can be found that OFDM-BOC has lower tracking error and higher tracking accuracy.

[0132] Tracking errors of several navigation modulations: (a) RF bandwidth 36 MHz, code tracking loop tracking lower limit; (b) RF bandwidth 55 MHz, code tracking loop tracking lower limit; (c) RF bandwidth 36 MHz, NELP tracking error; (d) RF bandwidth 55 MHz, NELP tracking error.

[0133] Implementation 3: Gradual upgrade method based on MBOC (6,1) navigation signal

[0134] MBOC(6,1) is a navigation modulation signal widely used in GPS, Galileo, and BeiDou satellite navigation systems. It consists of BOC(1,1) and BOC(6,1), with a power ratio of 10:1. Analysis of the power spectral density and modulation parameters of MBOC(6,1) reveals gaps in the center and outer edges of the BOC(1,1) and BOC(6,1) power spectra. Therefore, a reference frequency f0 of 1.023 MHz and a spreading code modulation rate of 1.023 MHz are selected. This allows for four subcarriers between BOC(1,1) and BOC(6,1), with more subcarriers on either side.

[0135] To the signal S to be upgraded MBOC(6,1) (t) Add a 4-carrier OFDM-BOC (5:1:2,1) and a 10-carrier OFDM-BOC (16:1:7,1), (Note: OFDM-BOC (α K :1:α1,β)-indicates that there are k subcarriers, and the set of parameters α is {α K :1:α1}), and get the upgraded navigation signal S I (t) and S II (t), its expression is:

[0136] s I (t) = s MBOC(6,1) (t)+s OFDM-BOC(5:1:2,1) (t) (15)

[0137] sII (t) = s I (t)+s OFDM-BOC(16:1:7,1) (t) (16)

[0138] Draw the ACF and PSD of the upgraded navigation signal. Figure 6 As shown in Figure 2, through ACF, it can be found that the main lobe width of the upgraded navigation signal has become narrower, indicating that the upgraded signal tracking accuracy has improved; the amplitude of the side lobe has decreased, indicating that the false lock probability of the receiver code tracking loop has decreased. Figure 7 As shown in the figure, by observing the PSD, it is found that the upgraded signal has a lower maximum main lobe power and a flatter main lobe, indicating that the upgraded signal has stronger anti-interference and anti-interception capabilities.

[0139] like Figure 6 and Figure 7 As shown in the figure, observing the ACF and PSD of OFDM-BOC (16:1:1,1) shows that its ACF is basically consistent with that of Upgrade II, indicating that their tracking performance is basically the same. Observing the PSD, except for some differences near f = ±6 MHz, the PSDs of OFDM-BOC (16:1:1,1) and Upgrade II are basically the same. To simplify the complexity of device implementation, in some low-cost situations, OFDM-BOC (16:1:1,1) can be used instead of Upgrade II to reduce equipment costs.

[0140] As shown in the table below, observing the spectral separation coefficient of the superimposed OFDM-BOC signal and MBOC(6,1), we can see that the spectral separation coefficient is very low, indicating that the upgraded signal will not adversely affect the original MBOC(6,1) receiver. This upgrade is a gradual navigation signal upgrade method.

[0141] Spectral separation coefficient of superimposed signal and MBOC(6,1)

[0142] KSSC(dB / Hz) MBOC(6,1) BPSK-R(1) -74.3 MBOC(6,1) -66.9 OFDM-BOC(5:1:2,1) -75.1 OFDM-BOC(16:1:7,1) -85.5

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for constructing an OFDM-BOC navigation signal, characterized in that: The following steps are involved: S1. Generate preliminary OFDM-BOC navigation signal: S101. For any OFDM-BOC navigation signal, first determine the basic parameters of the navigation signal; S102. Select a pseudo-random sequence to achieve orthogonality between different subcarriers and implement orthogonal frequency division multiplexing. S103 constructs the navigation signal of the i-th subcarrier; S104 sequentially performs S103 operation on the subcarrier signals from 1 to K; S105. The navigation signals of the K subcarriers are accumulated to obtain a multi-carrier OFDM-BOC navigation signal, thereby generating a preliminary OFDM-BOC navigation signal; S2. Using the designed power spectrum density G(f) of the OFDM-BOC navigation signal, a performance analysis is performed on the generated preliminary OFDM-BOC navigation signal. If the preliminary OFDM-BOC navigation signal meets the requirements, a qualified OFDM-BOC navigation signal is generated. If the preliminary OFDM-BOC navigation signal does not meet the requirements, repeat S101-S105 until the requirements are met; S103 is specifically: Construct the navigation signal of the i-th subcarrier. Each subcarrier contains I branch and Q branch. The navigation message is D Ii (k) and D Qi (k), the pseudo-random sequences generated in S102 are PN Ii (k) and PN Qi (k) Construct the modulation signal a of the navigation message and pseudo-random sequence i (k) and b i (k): a i (k)=D Ii (k)×PN Ii (k) b i (k)=D Qi (k)×PN Qi (k) (1) Generate a square wave subcarrier signal χ by using sign function and trigonometric function Ii (t) and χ Qi (t): Among them, f S,i represents the frequency corresponding to the i-th subcarrier; T C Indicates the duration of a pseudo code chip; Then we get the navigation modulation signal s of branch I i,cos (t) and the navigation modulation signal s of the Q branch i,sin (t): The navigation signal of the I branch is subtracted from the navigation signal of the Q branch to obtain the navigation signal s of the i-th branch. i (t): s i (t)=s i,cos (t)-js i,sin (t) (6)。 2. The method for constructing an OFDM-BOC navigation signal according to claim 1, wherein: In S101, the basic parameters of the navigation signal include: the number of subcarriers K, the reference frequency f0, the rate of the pseudo-random code f C And the square wave subcarrier frequency f corresponding to the i-th subcarrier i .

3. The method for constructing an OFDM-BOC navigation signal according to claim 1, wherein: In S102, the pseudo-random sequence is generated in the following two ways: (1) Construct different pseudo-random sequences for different subcarriers; (2) Use the same pseudo-random sequence, but perform appropriate cyclic shifts on the pseudo-random sequences used by different sub-carriers to ensure orthogonality of different sub-carriers.

4. The method for constructing an OFDM-BOC navigation signal according to claim 1, wherein: In S105, the navigation signals of the K subcarriers are accumulated to obtain a multi-carrier OFDM-BOC navigation signal, which is expressed as:

5. The method for constructing an OFDM-BOC navigation signal according to claim 1, wherein: In S2, the design of the power spectrum density G(f) of the navigation signal OFDM-BOC specifically includes the following steps: S201. Calculate the duration of the i-th subcarrier chip According to α i and β calculate n i : α i and β are based on the pseudo random code rate f C , the square wave subcarrier frequency f corresponding to the i-th subcarrier i , and the reference frequency f0: β=f C / f0;α i =f i / f0; S202.f represents the frequency. The power spectrum density function S of the ith subcarrier I branch is calculated according to formula (8) and formula (9): cos,ii (f): S203. Calculate the power spectral density function S of the i-th subcarrier Q branch sin,ii (f): S204. The I branch and the Q branch are orthogonal, and the power spectral density of the I branch and the Q branch is 0. Different subcarriers are orthogonal to each other, and the power spectral density of two different subcarriers is also 0. S205 repeats S201 to S203 for each subcarrier from 1 to K, and calculates the power spectral density of each subcarrier; S206. According to formula (10) and formula (11), the power spectrum density of each subcarrier is superimposed to obtain the power spectrum density G(f) of the navigation signal OFDM-BOC:

6. A method for upgrading an OFDM-BOC navigation signal generated by the construction method according to any one of claims 1 to 5, characterized in that: When the external environment changes, the OFDM-BOC navigation signal needs to be upgraded. A progressive navigation upgrade method is used, which specifically includes the following steps: S301. Determine the navigation signal S to be upgraded ori (t), plot the power spectrum density G of the navigation signal to be upgraded ori (f); S302. According to S ori (t) and G ori (f) Determine the parameters of the superimposed OFDM-BOC navigation signal: S303. Generate the superimposed OFDM-BOC navigation signal S according to S1 based on the parameters of the superimposed OFDM-BOC navigation signal generated in S302. add (t); S304. The superimposed OFDM-BOC navigation signal S add (t) and the navigation signal S to be upgraded ori (t) Superposition to obtain the upgraded navigation signal S update (t); S305. Construction S update (t) power spectral density PSD, find the parameter PAR2 of OFDM-BOC with similar PSD; S306. Generate OFDM-BOC navigation signal S using S1 according to parameter PAR2 update，sim (t), thereby simplifying the generation structure of the navigation signal.

7. The OFDM-BOC navigation signal upgrade method according to claim 6, characterized in that: S302 specifically includes the following steps: 3021. Determine the reference frequency f0 based on the pseudo code rate of the navigation signal to be upgraded; 3022. According to G ori (f) Navigation signal bandwidth, which determines the bandwidth of the OFDM-BOC navigation signal subcarriers; 3023. The subcarrier spacing of the OFDM-BOC navigation signal is equal to the subcarrier bandwidth; 3024. According to the allowed radio frequency bandwidth, the number of subcarriers is determined, thereby determining the square wave subcarrier frequency α of the subcarrier. i f0.

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