A method and system for integrating low-orbit satellite coverage enhancement, including code domain and codeword overlay, and communication and navigation.

Abstract

This invention discloses a method and system for integrating low-Earth orbit satellite coverage enhancement through codeword overlay in the code domain. It utilizes orthogonal time-frequency spatial demodulation technology to modulate the communication and navigation fusion signal onto a corresponding carrier to form a time-domain signal. A pseudo-random sequence is added before the time-domain signal as training symbols before transmission. The channel impulse response in the time-delay-Doppler domain is reconstructed, and the distance between the satellite and the ground receiver is measured using the training symbols for positioning calculation before being deleted. The received signal is converted from the time domain to the time-frequency domain using orthogonal time-frequency spatial demodulation technology, and signal equalization is performed based on the channel impulse response. The probability of all users transmitting all codewords is calculated. Navigation message information is read to obtain satellite operating status, satellite coordinate information, and navigation message information from at least four satellites. Combined with the obtained distance information, the receiver position is calculated. This addresses the problems of relatively isolated development and low integration efficiency of satellite communication and satellite navigation systems.

Description

Technical Field

[0001] This invention belongs to the field of wireless satellite communication and satellite navigation technology, specifically relating to an integrated method and system for enhancing low-orbit satellite coverage and navigation by overlaying code domains and codewords. Background Technology

[0002] In recent years, the information and communication field has witnessed concentrated development, with 5G mobile communication technology and artificial intelligence and related emerging technologies emerging in rapid succession. This has brought increasing attention to intelligent scenarios such as autonomous driving, vehicle-to-everything (V2X) communication, and the Internet of Things (IoT). The diversification of scenarios and the increasing complexity of needs are placing higher demands on communication and navigation positioning services in terms of both depth and breadth, thus creating new development opportunities for these technologies. Especially for location services, high performance and ubiquity will become essential characteristics, specifically reflected in high positioning accuracy, high reliability, and wide service coverage.

[0003] In the design of integrated satellite communication and navigation systems, the first consideration should be saving on system deployment and modification costs. This involves fully utilizing existing hardware and signal standards, and designing a fused communication and navigation signal to achieve integrated system functionality, thereby saving on the number of satellites and satellite spectrum resources. Secondly, considering that low-Earth orbit (LEO) satellite systems will occupy an important position in future satellite systems, it is necessary to study the integrated communication and navigation signal transmission mode for LEO satellite scenarios.

[0004] The isolated development of satellite communication and navigation systems is no longer suitable for the current multi-dimensional intelligent scenario requirements. Furthermore, the independent development and operation of these two systems not only incurs significant investment but also leads to increasing strain on satellite orbital and radio frequency resources. Therefore, the deep integration of satellite navigation and positioning with satellite communication functions, achieved through a single satellite system utilizing the same radio resources, has become a crucial solution attracting widespread attention from academia and industry. The commonalities between satellite communication and navigation systems in modulation / demodulation methods and signal systems provide a foundation and conditions for the realization of integrated communication and navigation technology. Orthogonal Time-Frequency System (OTFS) technology exhibits excellent characteristics in overcoming the influence of dual-selective satellite-to-ground channels, and OTFS channel estimation can achieve high-precision estimation of positioning-related parameters such as channel transmission delay and Doppler frequency, giving it an inherent advantage in positioning. Therefore, introducing OTFS technology into the design of integrated communication and navigation signals for low-Earth orbit satellites is of great significance. In addition, the design of integrated communication and navigation signal transmission schemes also requires minimizing mutual interference between communication and navigation signals to ensure the overall performance of the integrated communication and navigation system. At the same time, it is also necessary to improve the flexibility of the system to meet the diverse communication and navigation needs of different user services in different scenarios. Therefore, it is necessary to design an integrated signal transmission method for variable rate services.

[0005] Current integrated communication and navigation solutions are divided into two modes: system collaboration and deep integration. The system collaboration mode relies on existing satellite navigation and satellite communication systems, making adaptive modifications to the existing systems and ultimately integrating them at the terminal. However, this approach has high deployment costs and presents significant challenges in terminal design. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method and system for integrating low-orbit satellite coverage enhancement by overlaying code domain and codewords, in order to address the shortcomings of the prior art and solve the technical problem of the relatively isolated development and low integration efficiency of satellite communication systems and satellite navigation systems.

[0007] The present invention adopts the following technical solution:

[0008] The method for enhancing low-Earth orbit satellite coverage and integrating communication and navigation by overlaying code domains and codewords includes the following steps:

[0009] S1. Treat communication signals and navigation signals as signals from different users, and fuse them using sparse code division multiple access coding technology to obtain a communication and navigation fused signal;

[0010] S2. Modulate the grid points on the time-delay Doppler domain corresponding to the communication and navigation fusion signal obtained in step S1 onto the corresponding carrier to form a time-domain signal using orthogonal time-frequency modulation technology. Add a pseudo-random sequence as a training symbol before sending the time-domain signal.

[0011] S3. Use the pseudo-random sequence generated in step S2 as training symbols to perform two-dimensional matched filtering in time and frequency, restore the channel impulse response in the time delay-Doppler domain, use the training symbols to measure the distance between the satellite and the ground receiver for use in step S6 for positioning calculation, and delete the training symbols after completion.

[0012] S4. Convert the signal received in step S2 from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and perform signal equalization based on the channel impulse response in step S3.

[0013] S5. Use the SCMA decoding method to solve the signal after equalization in step S4 to obtain the probability that all users send all codewords.

[0014] S6. Based on the log-likelihood probability value of each bit of each user, determine whether the user sends 0 or 1 at that location. Solve all user signals to separate navigation information. Convert the binary data code into hexadecimal and restore the original data. Read the navigation message information to obtain the satellite operation status, satellite coordinate information, and navigation message information of at least four satellites. Combine the distance information obtained in step S3 to calculate the receiver position.

[0015] Specifically, step S1 is as follows:

[0016] S101. Determine the SCMA codebook parameters. Consider an SCMA system with J users and K resource blocks. The system has user modulation orders. d f This represents the number of users that a resource block in the system can simultaneously support, where each user's codebook contains N non-zero elements.

[0017] S102. Generate a parent constellation based on the golden angle modulation, construct a set of several N-dimensional constellations, and initially generate the required number of constellation points.

[0018] S103, Based on M / 2 N-dimensional constellation points CM m (m = 1, 2, ..., M / 2) Determine the remaining general constellation points;

[0019] S104. Add a rotation factor φ to the obtained constellation points. f =e j(n-1) / N Optimize;

[0020] S105. Introduce correlations into the optimized constellation points and regenerate the symbols for the second half of the dimensions.

[0021] S106. The other half of the modulation symbol Y' is generated from step S105. Then Y and Y' are interleaved to obtain the extended master codebook.

[0022] S107. Based on the power sharing situation of users on a certain resource block, the master codebook is divided to generate the codebooks of each user;

[0023] S108. Use the factor matrix F to represent the correspondence between different users and resource blocks;

[0024] S109. Each user selects the codeword from the corresponding column of their own codebook based on the binary code, and the codewords of all users are merged and sent.

[0025] Specifically, in step S107, each user's codebook x j Specifically:

[0026]

[0027] Where j = 1, 2, ..., J, V j Let MC be the mapping matrix for the j-th user, d be the parent codebook, and d be the mapping matrix for the j-th user. f J represents the number of users reused on each resource block.

[0028] Specifically, step S2 is as follows:

[0029] S201. Divide the time-frequency domain and the time-delay-Doppler domain into grids;

[0030] S202. The conduction-pass fusion signal is first directly represented in the time-delay-Doppler domain, and then converted to the time-frequency domain through inverse symplectic Fourier transform.

[0031] S203. The signal is converted from the time-frequency domain to the time domain and transmitted through the Heisenberg transform.

[0032] Furthermore, pseudo-random sequence codes are continuously sent for ranging. A pseudo-random sequence is generated and processed with the transmitted communication and navigation fusion signal to determine the data code frame header at the maximum value of the correlation value and the start time of signal transmission, thereby obtaining the signal propagation distance.

[0033] Specifically, in step S3, the channel impulse response in the time-delay-Doppler domain is reconstructed as follows:

[0034] r[n] = e(ω0n)s[n-δ0] + N0[n]

[0035]

[0036]

[0037] Where r(n) is the received signal in the time domain, <·> represents the correlation operation, N0(t) is additive white Gaussian noise, PN[n] is the locally generated pseudo-random sequence, ω0 is the channel frequency offset, δ0 is the channel delay offset, and M(r,PN)[δ,ω] is the correlation value obtained by correlating the received signal with the delay-Doppler shifted signal of the local pseudo-random sequence. and Let represent the errors between the correlation value and the ideal value of 1 under different conditions, and let e(ωn) be an exponential function. It is an integer field.

[0038] Specifically, step S4 is as follows:

[0039] S401. The received signal is represented in a continuous time-domain form;

[0040] S402. The received signal in the time domain is converted to the time-frequency domain using the Wigner transform.

[0041] S403. Use SFFT transformation to convert the received signal from the time-frequency domain to the time-delay-Doppler domain.

[0042] Specifically, step S5 is as follows:

[0043] S501. Initialization is divided into initialization at the resource node and initialization at the user node.

[0044] S502. Resource nodes transmit messages to user nodes connected to them. When transmitting a message to a user, the external information transmitted to the resource by the other two user nodes in the previous iteration is combined.

[0045] S503. When a user node sends a message to a resource node connected to it in the factor graph, it combines the message sent by another resource node in the previous iteration with the external information at this user node to obtain the message passed along the factor graph.

[0046] S504. If the iteration termination condition is not met, continue to execute steps S502 and S503 in sequence; if the iteration termination condition is met, output the output value of the current user node, that is, the log-likelihood probability at that point.

[0047] Specifically, in step S6, the transmitted bit is determined based on the log-likelihood probability (LLR) obtained in step S5. The determination is based on the following criteria:

[0048]

[0049] in, The bits sent at that location.

[0050] Secondly, embodiments of the present invention provide an integrated system for low-orbit satellite coverage enhancement and communication and navigation enhancement through code domain and codeword overlay, comprising:

[0051] A low-orbit satellite coverage enhancement and communication / navigation integrated system with code domain and codeword overlay, characterized in that it includes:

[0052] The fusion module treats communication signals and navigation signals as signals from different users and uses sparse code division multiple access coding technology to fuse them to obtain a communication and navigation fused signal.

[0053] Add a module to take the communication and navigation fusion signal obtained by the fusion module and the corresponding grid points on the time delay Doppler domain. Use orthogonal time-frequency modulation technology to modulate it onto the corresponding carrier to form a time domain signal. Add a pseudo-random sequence as a training symbol before sending the time domain signal.

[0054] The filtering module uses the pseudo-random sequence generated by the adding module as training symbols to perform two-dimensional matched filtering in time and frequency, restores the channel impulse response in the time delay-Doppler domain, uses the training symbols to measure the distance between the satellite and the ground receiver for the reading module to perform positioning calculation, and deletes the training symbol after completion.

[0055] The conversion module converts the signal received by the addition module from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and performs signal equalization based on the channel impulse response in the filtering module.

[0056] The solution module uses the SCMA decoding method to solve the signal after equalization by the conversion module to obtain the probability that all users send all codewords;

[0057] The reading module determines whether a user is sending a 0 or a 1 at a given location based on the log-likelihood probability value of each bit of each user. It then decodes all user signals to separate navigation information, converts binary data codes into hexadecimal to restore the original data, reads navigation message information, obtains satellite operating status, satellite coordinate information, and navigation message information from at least four satellites, and calculates the receiver position by combining the distance information obtained from the filtering module.

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] The code-domain codeword overlay method for enhancing low-Earth orbit (LEO) satellite coverage and integrating communication and navigation effectively reduces the peak-to-average power ratio (PAPR) and is beneficial to existing satellite hardware, taking into account the characteristics of LEO satellite integrated communication and navigation systems. Furthermore, while ensuring bit error rate performance and positioning accuracy, it considers the different transmission rate requirements of satellite communication and navigation signals and designs a more flexible signal fusion method.

[0060] Furthermore, combining satellite communication signals and satellite navigation signals for transmission using the same waveform can solve problems such as limited spectrum resources and transmission bandwidth for satellite navigation and communication signals.

[0061] Furthermore, by employing an orthogonal time-frequency conditioning mechanism, the signal and channel are characterized in the time-delay-Doppler domain. Compared to the traditional time-frequency domain, this makes the impulse response of the satellite-to-ground time-varying Doppler offset dual-select channel distinguishable, thereby minimizing the impact of the dual-select channel on signal transmission. Inserting a frame header ranging code and intra-frame training sequences in the time domain, existing as pseudo-random sequences, allows for simultaneous ranging and channel estimation. The frame header ranging code is used for received signal synchronization, ranging, and initial channel estimation, while other intra-frame training sequences continuously perform channel estimation to monitor channel changes in real time.

[0062] Furthermore, pseudo-random sequences have good autocorrelation and a sharp autocorrelation function. In satellite navigation systems, different pseudo-random sequences are often used to distinguish different satellites. The receiver generates a specific pseudo-random sequence code and performs correlation operations with the received signal. The magnitude of the correlation value is used to determine whether the received signal is a useful signal. At the same time, due to its autocorrelation characteristics, the local input signal of the correlation operation can be appropriately adjusted to match the time-frequency characteristics of the received signal, thereby reflecting how the signal is affected by the channel.

[0063] Furthermore, in order to understand the original transmitted signal, it is necessary to convert the time-domain signal directly received by the receiver back to the original time-delay-Doppler domain. This is because the signal will be greatly diffused and aliased in other domains due to various conversions, and cannot be directly restored.

[0064] Furthermore, the integrated signal transmission scheme based on the sparse code division multiple access concept, which combines code domain superposition for communication and navigation, satisfies the flexible and adjustable transmission rate requirements of communication and navigation services and effectively reduces the peak-to-average power ratio of the signal, thereby increasing the design freedom. This is achieved through a variable rate codebook based on disk-type golden modulation and a message passing decoding algorithm based on confidence.

[0065] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here.

[0066] In summary, this invention treats communication signal channels and navigation signal communication as two parallel users, allocating different SCMA sparse codebooks to each, thus enabling simultaneous access for both communication and navigation signals. Furthermore, considering the different rate requirements of communication and navigation services, a variable-rate SCMA codebook design method based on golden angle GAM modulation is applied, allowing the two signals to be transmitted at different rates. At the receiving end, a demodulation algorithm for the variable-rate codebook is employed, completing the signal transmission process of the SCMA-based integrated communication and navigation system in low-Earth orbit satellite scenarios.

[0067] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0068] Figure 1 This is the overall flowchart of the present invention;

[0069] Figure 2 A diagram showing the representation of an SCMA-coded signal with integrated conduction and signal transmission in the time-delay-Doppler domain.

[0070] Figure 3 This is a flowchart of the decoding process of the present invention;

[0071] Figure 4 This is a graph showing how the bit error rate of a communication signal changes with the signal-to-noise ratio.

[0072] Figure 5 This is a graph showing the variation of navigation signal bit error rate with signal-to-noise ratio.

[0073] Figure 6 This is a graph showing how the positioning error changes with the signal-to-noise ratio. Detailed Implementation

[0074] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0075] In the description of this invention, it should be understood that the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0076] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0077] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes such combinations. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this invention generally indicates that the preceding and following objects have an "or" relationship.

[0078] It should be understood that although terms such as first, second, third, etc., may be used in the embodiments of the present invention to describe the preset range, these preset ranges should not be limited to these terms. These terms are only used to distinguish the preset ranges from one another. For example, without departing from the scope of the embodiments of the present invention, the first preset range may also be referred to as the second preset range, and similarly, the second preset range may also be referred to as the first preset range.

[0079] Depending on the context, the word "if" as used here can be interpreted as "when," "when," "in response to determination," or "in response to detection." Similarly, depending on the context, the phrase "if determination" or "if detection (of the stated condition or event)" can be interpreted as "when determination," "in response to determination," "when detection (of the stated condition or event)," or "in response to detection (of the stated condition or event)."

[0080] The accompanying drawings illustrate various structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0081] This invention provides a method for enhancing low-orbit satellite coverage and integrating navigation and communication by superimposing code domains and codewords. It adopts a deep fusion mode to achieve deep integration of navigation and communication at the signal system level. It has good compatibility, low cost, and high reliability. It can substantially improve the utilization rate of satellite orbit, hardware resources and wireless spectrum resources, and enhance system compatibility and reduce terminal design difficulty. It has good application prospects and has become a hot topic in the field of satellite navigation and communication technology research in recent years.

[0082] Please see Figure 1 The present invention discloses a method for integrating low-orbit satellite coverage enhancement communication and navigation by superimposing code domains and codewords, comprising the following steps:

[0083] S1. The transmitting end generates communication signals and navigation signals, treating them as signals from different users. It uses sparse code division multiple access (SCMA) coding technology to load them into the transmission system. The communication signals and navigation signals are treated as signals from different users, and SCMA coding is used to fuse them to obtain a communication and navigation fused signal, which is then modulated onto limited resources for transmission.

[0084] S101. Determine the SCMA codebook parameters. Consider an SCMA system with J users and K resource blocks. The system has multiple user modulation orders, denoted as follows: d f This represents the number of users simultaneously supported by a resource block in the system. Each user's codebook contains N non-zero elements.

[0085] S102. Generate a parent constellation based on Golden Angle Modulation (GAM). Due to symmetry, construct a set of several N-dimensional constellations. The initial number of constellation points to be generated is:

[0086]

[0087] Then, the nth constellation point is represented as:

[0088]

[0089] in, r n Let r be the radius of the nth constellation point, in a disk-shaped GAMn The expression is:

[0090]

[0091]

[0092] in, For average power constraints;

[0093] S103, Obtain the M / 2 N-dimensional constellation points CM m After (m=1,2,...,M / 2), the remaining general constellation points can be obtained by symmetry;

[0094]

[0095] S104. Add a rotation factor φ to the obtained constellation points. f =e j(n-1) / N To further optimize, we obtain:

[0096]

[0097] S105. Introduce correlation and regenerate the symbols for the second half of the dimensions:

[0098] Y n+N / 2,: =∠Y n,: *fliplr(|Y n+N / 2,: |), n=1,2,...,N

[0099] Here, fliplr(·) represents flipping the matrix;

[0100] S106. The other half of the modulation symbol Y' is directly generated from the above steps. Then, Y and Y' are interleaved to obtain the extended master codebook (EMC).

[0101] Y' = -Y;

[0102]

[0103]

[0104] S107. Based on the power sharing situation of users on a certain resource block, the master codebook EMC is divided to finally generate the codebooks of each user.

[0105] The process of segmenting the mother codebook is represented as follows:

[0106]

[0107] Where γ = K / N, and it is stipulated that...

[0108] The final generated user codebooks are represented as follows:

[0109]

[0110] Here is a codebook example:

[0111]

[0112]

[0113]

[0114]

[0115]

[0116] S108. The correspondence between different users and resource blocks is represented by a factor matrix. An example of a factor matrix is ​​as follows:

[0117]

[0118] The columns of the factor matrix represent users, with each column corresponding to a different user. The rows represent resource blocks, with each row corresponding to a different resource block. 1 indicates signal transmission, and 0 indicates no signal transmission. From the factor matrix, it can be seen that for an SCMA system with J=6 and K=4, each user's signal is transmitted through two resource blocks. Therefore, the column weights of the factor matrix are d. v =2, and at the same time, a resource block in the system simultaneously carries the information of 3 users, so the row weight d of the factor matrix is ​​2. f =3.

[0119] S109. Each user selects the codeword from the corresponding column of their own codebook based on the binary code, and the codewords of all users are merged and sent.

[0120] S2. The communication and navigation fusion signal is obtained through the above steps, corresponding to grid points in the time-delay Doppler domain, such as... Figure 2 As shown. The signal is modulated onto the corresponding carrier using Orthogonal Time-Frequency Modulation (OTFS) technology. The OTFS modulation process is as follows:

[0121] S201. First, the time-frequency domain and the time-delay-Doppler domain are divided into grids. The dimensions of the two domains have a one-to-one correspondence and can be converted to each other. The time-frequency domain grid is represented as follows:

[0122] Λ={(nT,mΔf),n,m∈Z}

[0123] Where T and Δf represent the time interval and frequency interval, respectively, the signal is represented on the time-frequency grid as:

[0124] S[n,m],n=0,...N-1,m=0,...M-1

[0125] The system duration is NT seconds, and the total bandwidth is MΔfHz. Correspondingly, the signal is represented on the time-delay-Doppler grid as follows:

[0126] s[k,l],k=0,...N-1,l=0,...M-1

[0127] The two can be converted to each other using the symptotic Fourier transform (SFFT) and the inverse symptotic Fourier transform (ISFFT), that is:

[0128]

[0129]

[0130] Among them, s p [k,l] and S p [n,m] represent the periodic extensions of the signal's time-frequency domain representation and time-delay-Doppler domain representation, respectively;

[0131] S202. The conduction-channel fusion signal is first directly represented in the time-delay-Doppler domain, i.e., s[k,l], k=0,...,N-1, l=0,...M-1, and then transformed to the time-frequency domain through inverse symplectic Fourier transform (ISFFT), as follows:

[0132]

[0133] Among them, W tx [n,m] represents the square-integrable window function at the transmitting end, used to truncate the periodic expression of the signal;

[0134] S203. The signal is transformed from the time-frequency domain to the time domain for transmission using the Heisenberg transform, as detailed below:

[0135]

[0136] Among them, g tx (t) indicates the transmission pulse.

[0137] Correspondingly, the receiving end has a receive pulse, and the inner product of the transmitted pulse and the received pulse satisfies a biorthogonal relationship between time delay and frequency, as follows:

[0138]

[0139] To achieve positioning functionality, a sufficiently long pseudo-random sequence code needs to be continuously transmitted before sending the data code so that the receiver can perform ranging. Simultaneously, to overcome the influence of the dual-selection channel, the delay-Doppler channel impulse response of the dual-selection channel needs to be estimated. This process can also be achieved by transmitting pseudo-random codes between data frames. The signal transmission format in each domain is as follows: Figure 4 As shown.

[0140] The receiver locally generates the same pseudo-random sequence as the transmitter and continuously performs correlation operations with the received signal until it finds the maximum correlation value. This value is used to locate the data frame header and determine the start time of signal transmission. The receiver then reads the local signal reception time; the difference between the two is the signal propagation time. Multiplying the propagation time by the speed of light gives the signal propagation distance.

[0141] d=c*(t arrival -t send )

[0142] Where d represents the distance the signal travels, c represents the speed of light, and t represents the speed of light. arrival and t send These represent the arrival time and the sending time, respectively.

[0143] S3. At the receiving end, channel estimation in the time-delay-Doppler domain is performed on the dual-selection channel. This involves using a pseudo-random sequence as training symbols for two-dimensional matched filtering in time and frequency to reconstruct the channel impulse response in the time-delay-Doppler domain. This is then used for subsequent equalization to counteract the influence of the dual-selection channel. The process is as follows:

[0144] r[n] = e(ω0n)s[n-δ0] + N0[n]

[0145]

[0146]

[0147] To clearly illustrate the process of this two-dimensional matched filtering, the received signal in the time domain is represented in discrete form as r(n). This equation simply explains that the transmitted signal is affected by Doppler frequency shift and time delay after passing through the channel. <·> represents the correlation operation, N0(t) represents additive white Gaussian noise, PN[n] represents the locally generated pseudo-random sequence, and M(r,PN)[δ,ω] represents the correlation value obtained by correlating the received signal with the time-delayed Doppler-shifted signal of the local pseudo-random sequence. and These represent the errors between the correlation value and the ideal value 1 under different conditions. The above process can be described as successively changing the locally generated pseudo-random sequence by a certain time delay δ or Doppler frequency ω, while ensuring that δ and ω are points on the aforementioned time delay-Doppler grid. If, in a certain attempt, the estimated time delay δ and Doppler frequency ω match the actual time delay δ0 or Doppler frequency ω0, the correlation value reaches its maximum. Then, this estimated value can be used as an approximation of the actual time delay and Doppler offset of the channel for subsequent decoding.

[0148] S4. The received signal is transformed from the time domain to the time-frequency domain. This step is called the Wigner transform, and the specific steps are as follows:

[0149] S401, The received signal is expressed in continuous time domain form as follows:

[0150] r(t)=∫∫f(τ,ν)g tx (t-τ)e j2πν(t-τ) dνdτ+N0(t)

[0151] Where τ represents time delay, ν represents frequency, N0(t) represents additive white Gaussian noise, and f(τ,ν) represents the transmitted signal affected by the channel, expressed in the form of:

[0152]

[0153] Where h(τ,ν) is the channel impulse response in the time-delay-Doppler domain representation, * σ For the defined twisted convolution operation, the received signal above is processed using the received pulse:

[0154]

[0155] S402. The received signal in the time domain is converted to the time-frequency domain using the Wigner transform, as follows:

[0156]

[0157] S403. Use SFFT transformation to convert the received signal from the time-frequency domain to the time-delay-Doppler domain.

[0158] S5. For the received signal, the SCMA decoding algorithm is used for solving. For the same resource block, let the codewords sent by the user be m1, m2, and m3, then m1 has M1 possible values, m2 has M2 possible values, and m3 has M3 possible values. The steps of this decoding algorithm are as follows: Figure 3 As shown;

[0159] S501, Initialization

[0160] Initialization is divided into initialization at the resource node and initialization at the user node. First, the initialization at the user node is given. In the initial iteration, the probability of a user sending its various codewords is equal. This prior probability needs to be assigned to the information the user wants to transmit to the resource for the first time.

[0161]

[0162] Furthermore, for a given resource node k, which simultaneously receives codewords from three different users, there are a total of M1·M2·M3 possible codeword combinations. Here, it is necessary to calculate the residuals between the received signal and all transmitted codeword combinations to observe the distance between them. This residual is expressed as...

[0163]

[0164] in, C represents the Gaussian noise power. j,k (m j Let represent the codeword selected by the j-th user from their codebook. For a given codebook among all possible combinations of codebooks, its conditional probability in a Gaussian noise background is expressed as:

[0165] P(y k |x1,x2,x3)~exp(f k (y k ,m1,m2,m3));

[0166] S502, resource nodes need to send messages to the user nodes they are connected to, in d f When the number of user nodes is 3, a resource node needs to pass messages to 3 user nodes in sequence. When passing a message to a user, it needs to combine the information passed to the resource by the other two user nodes in the previous iteration. This is called external information.

[0167] In other words, the message that a resource node transmits to a target user node combines its own internal information with the external information of other user nodes. The transmitted message represents the probability of inferring all possible values ​​of the user node from the known internal and external information.

[0168]

[0169]

[0170]

[0171] S503. User nodes need to send messages to resource nodes connected to them in the factor graph. Similarly, a user's information requires the transmission of two resource blocks, i.e., d.v =2 indicates that in this iteration, a user node needs to transmit messages to two resource nodes in sequence.

[0172] When sending a message to a resource node, it is also necessary to combine it with the message passed from another resource node in the previous iteration. Combining this external information with the internal information at this user node, we can obtain the message passed along the factor graph, represented as:

[0173]

[0174]

[0175] Among them, Ap v (•) is the initial prior probability of user v, which is determined by the modulation order of the user, and m is also determined by the modulation order of user v.

[0176] S504. Determine whether the iteration termination condition is met.

[0177] If the iteration termination condition is not met, then continue to execute steps S502 and S503 in sequence.

[0178] If the iteration termination condition is met, the output value of the current user node needs to be output, i.e., the log-likelihood probability (LLR) at that point. At this time, at user node v, the guesses for all possible transmitted codewords m are combinations of all guesses and prior probabilities from all its neighboring resource nodes. In the logarithmic field, for a modulation symbol x transmitted by user j... j The APP is

[0179]

[0180] S6. After the above steps, obtain the probability of all possible codewords sent by all users. Finally, calculate the LLR value bit by bit, separate the navigation information, convert the binary data code into hexadecimal and restore the original data, read the navigation message information, and obtain information such as satellite operation status and satellite coordinates; obtain the ranging and navigation message information of at least four satellites, and calculate the receiver position according to the standard procedure.

[0181] The LLR value is as follows:

[0182]

[0183] Where χ represents all possible codewords that the user can send.

[0184] Then, the transmitted bits are determined based on the calculated LLR value, and the determination is based on the following criteria:

[0185]

[0186] In another embodiment of the present invention, a code domain and code word overlay low-orbit satellite coverage enhancement communication and navigation integrated system is provided. This system can be used to implement the above-mentioned code domain and code word overlay low-orbit satellite coverage enhancement communication and navigation integrated method. Specifically, the code domain and code word overlay low-orbit satellite coverage enhancement communication and navigation integrated system includes a fusion module, an addition module, a filtering module, a conversion module, a solving module, and a reading module.

[0187] The fusion module treats communication signals and navigation signals as signals from different users and uses sparse code division multiple access coding technology to fuse them to obtain a communication and navigation fused signal.

[0188] Add a module to take the communication and navigation fusion signal obtained by the fusion module and the corresponding grid points on the time delay Doppler domain. Use orthogonal time-frequency modulation technology to modulate it onto the corresponding carrier to form a time domain signal. Add a pseudo-random sequence as a training symbol before sending the time domain signal.

[0189] The filtering module uses the pseudo-random sequence generated by the adding module as training symbols to perform two-dimensional matched filtering in time and frequency, restores the channel impulse response in the time delay-Doppler domain, uses the training symbols to measure the distance between the satellite and the ground receiver for the reading module to perform positioning calculation, and deletes the training symbol after completion.

[0190] The conversion module converts the signal received by the addition module from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and performs signal equalization based on the channel impulse response in the filtering module.

[0191] The solution module uses the SCMA decoding method to solve the signal after equalization by the conversion module to obtain the probability that all users send all codewords;

[0192] The reading module determines whether a user is sending a 0 or a 1 at a given location based on the log-likelihood probability value of each bit of each user. It then decodes all user signals to separate navigation information, converts binary data codes into hexadecimal to restore the original data, reads navigation message information, obtains satellite operating status, satellite coordinate information, and navigation message information from at least four satellites, and calculates the receiver position by combining the distance information obtained from the filtering module.

[0193] In another embodiment of the present invention, a terminal device is provided, comprising a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions to achieve a corresponding method flow or corresponding function. The processor described in this embodiment can be used in the operation of a code domain codeword overlay low-orbit satellite coverage enhancement communication and navigation integrated method, including:

[0194] Communication and navigation signals are treated as signals from different users and fused using sparse code division multiple access (SDMA) coding technology to obtain a fused communication and navigation signal. The fused communication and navigation signal is then modulated onto corresponding carriers using orthogonal time-frequency modulation techniques to form a time-domain signal, corresponding to grid points in the time-delay Doppler domain. A pseudo-random sequence is added before the time-domain signal as a training symbol, and then the signal is transmitted. The generated pseudo-random sequence is used as the training symbol for two-dimensional matched filtering in time and frequency to reconstruct the channel impulse response in the time-delay Doppler domain. The distance between the satellite and the ground receiver is measured using the training symbol for positioning calculation. After completion, the training symbol is deleted. The received signal is converted from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and signal equalization is performed based on the channel impulse response. The equalized signal is solved using the SCMA decoding method to obtain the probability of all users transmitting all codewords. Based on the log-likelihood probability value of each bit of each user, it is determined whether the user transmits 0 or 1 at that point. All user signals are solved to separate navigation information. The binary data code is converted to hexadecimal and the original data is restored. Navigation message information is read to obtain satellite operating status, satellite coordinate information, and navigation message information of at least four satellites. Combined with the obtained distance information, the receiver position is calculated.

[0195] In another embodiment of the present invention, a storage medium is also provided, specifically a computer-readable storage medium (memory). This computer-readable storage medium is a memory device in a terminal device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the terminal device and extended storage media supported by the terminal device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, this storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device.

[0196] One or more instructions stored in a computer-readable storage medium can be loaded and executed by a processor to implement the corresponding steps of the integrated communication and navigation method for low-orbit satellite coverage enhancement related to code domain codeword overlay in the above embodiments; one or more instructions in the computer-readable storage medium are loaded and executed by the processor in the following steps:

[0197] Communication and navigation signals are treated as signals from different users and fused using sparse code division multiple access (SDMA) coding technology to obtain a fused communication and navigation signal. The fused communication and navigation signal is then modulated onto corresponding carriers using orthogonal time-frequency modulation techniques to form a time-domain signal, corresponding to grid points in the time-delay Doppler domain. A pseudo-random sequence is added before the time-domain signal as a training symbol, and then the signal is transmitted. The generated pseudo-random sequence is used as the training symbol for two-dimensional matched filtering in time and frequency to reconstruct the channel impulse response in the time-delay Doppler domain. The distance between the satellite and the ground receiver is measured using the training symbol for positioning calculation. After completion, the training symbol is deleted. The received signal is converted from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and signal equalization is performed based on the channel impulse response. The equalized signal is solved using the SCMA decoding method to obtain the probability of all users transmitting all codewords. Based on the log-likelihood probability value of each bit of each user, it is determined whether the user transmits 0 or 1 at that point. All user signals are solved to separate navigation information. The binary data code is converted to hexadecimal and the original data is restored. Navigation message information is read to obtain satellite operating status, satellite coordinate information, and navigation message information of at least four satellites. Combined with the obtained distance information, the receiver position is calculated.

[0198] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0199] Four real satellites were selected and navigation message information was acquired. This real satellite navigation message information all originated from real observation stations. The proposed OTFS-based code domain codeword overlay low-Earth orbit satellite coverage enhancement integrated communication and navigation scheme was applied. At the signal layer, SCMA encoding was used to simultaneously transmit communication signals and navigation message signals. At the receiving end, SCMA decoding was used to calculate the receiver position. The receiver positioning error varies with the signal-to-noise ratio as follows: Figure 6 As shown.

[0200] It should be noted that in this experiment, excessive positioning deviation is considered a positioning failure, and a uniform value of 99m is assigned. The satellite channel is set to Ricean channel. To verify the feasibility of the scheme, only the integrated communication and navigation transmission and reception process under line-of-sight (LOS) conditions is considered. In this scenario, the delay estimation result of the main path can be directly used as the basis for ranging to help with positioning, and can also be used as the channel estimation result to help with decoding.

[0201] The signal-to-noise ratio is defined as SNR = 10log(P0 / P). N ), where P0 is the power of the useful signal, P N The noise power was set to a range of [6, 30] dB, with a delay-frequency domain dimension of 256×16, a center frequency of 2 GHz, and a total bandwidth of 3.84 MHz. Each satellite independently generated a channel based on its propagation time, and each satellite needed to transmit 960 bits to transmit the navigation message signal. In the experiment, the noise and channel fading coefficient of each OTFS frame were generated independently.

[0202] Comparison of options:

[0203] An integrated signal transmission scheme based on orthogonal time-frequency-space (OTFS) modulation using power domain superposition is employed. In this scheme, navigation and communication data are mapped and superimposed with different power allocation factors to form a fused signal. The receiver uses interference cancellation for decoding, i.e., first decodes the signal with the higher power allocation, and then decodes the signal with the lower power allocation. The remaining OTFS modulation and demodulation process and ranging and positioning process are the same as those in this invention.

[0204] from Figure 4 As can be seen, the integrated communication and conduction signal based on power domain multiplexing has a higher bit error rate (BER), while the integrated communication and conduction signal based on code domain codeword superposition has a lower BER. This is because direct coincidence superposition in the power domain leads to more severe mutual interference during signal separation, and the interference cancellation detection method cannot guarantee good reliability. In contrast, the code domain superposition-based scheme is optimized in its codebook design to reduce the BER, and the receiver employs a message passing (MPA) algorithm, both of which effectively suppress noise, resulting in superior BER performance.

[0205] The variation of the navigation signal bit error rate with signal-to-noise ratio for the three schemes is as follows: Figure 4 , Figure 5 , Figure 6 As shown in the figure, due to the BCH encoding and spreading processing of the navigation bit transmission, it has high coding gain and spreading gain, thus the bit error rate is much lower than that of the standard bit error rate across the entire observation signal-to-noise ratio range. Figure 4 The bit error rate of the communication signal in the system.

[0206] Furthermore, the navigation signal bit error rate of the integrated communication and navigation scheme based on code domain and codeword superposition is 0, therefore... Figure 5 The diagram only shows the bit error rate of the navigation signal under the power domain superposition scheme. This is because SCMA coding, used to fuse navigation and communication signals, introduces interference in the code domain, which can be partially canceled out by the MPA receiving algorithm. In contrast, in the power domain superposition scheme, the communication and navigation signals directly compete for power. This results in insufficient resources allocated to the navigation signal when the communication signal has a higher power proportion, and noise still has a certain impact during decoding, leading to a relatively higher bit error rate.

[0207] Similarly, the variation of positioning error with signal-to-noise ratio can be obtained as follows: Figure 6 As shown, the results are consistent with Figure 5 The result is corresponding.

[0208] In this invention, different modulation orders are set for navigation signals and communication signals to achieve different signals being transmitted at different rates, thus fully coordinating the needs of different services. The parameters set in the experiment were 2, 4, and 8. However, we also simulated the case of different signals being transmitted at the same rate, setting the modulation order to 4 for all signals and using an SCMA codebook based on the moth-to-a-flame algorithm. Figure 4 As can be seen, the variable-rate codebook-based scheme has a significantly higher bit error rate than the constant-rate codebook-based scheme, especially under relatively low signal-to-noise ratio (SNR) conditions. This is because, in this experiment, the constant-rate codebook requires 24 constellation points to accommodate all user information, while the variable-rate codebook requires 28 constellation points. Increasing the number of constellation points reduces the distance between them, thus decreasing the decision domain of each constellation point and consequently reducing its noise resistance.

[0209] In summary, the present invention provides a method and system for enhancing communication and navigation coverage via code domain and codeword overlay on low-orbit satellites. It employs a variable-rate codebook design method based on golden GAM modulation. The codebook effectively reduces PAPR and assigns different mapping rules to different signal channels by setting different modulation orders, thereby making the transmission rates of communication and navigation signals flexibly adjustable.

[0210] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the above-described division of functional units and modules is merely an example. In practical applications, the above functions can be assigned to different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit. Furthermore, the specific names of the functional units and modules are only for easy differentiation and are not intended to limit the scope of protection of this application. The specific working process of the units and modules in the above system can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0211] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0212] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed in this invention can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0213] In the embodiments provided by this invention, it should be understood that the disclosed devices / terminals and methods can be implemented in other ways. For example, the device / terminal embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0214] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0215] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0216] If the integrated module / unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content included in the computer-readable medium can be appropriately added or removed according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, computer-readable media do not include electrical carrier signals and telecommunication signals.

[0217] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0218] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0219] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0220] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for integrating low-orbit satellite coverage enhancement, communication, and navigation by overlaying code domains and codewords, characterized in that: Includes the following steps: S1. Treat communication signals and navigation signals as signals from different users, and fuse them using sparse code division multiple access coding technology to obtain a communication and navigation fused signal; S2. Modulate the grid points on the time-delay Doppler domain corresponding to the communication and navigation fusion signal obtained in step S1 onto the corresponding carrier to form a time-domain signal using orthogonal time-frequency modulation technology. Add a pseudo-random sequence as a training symbol before sending the time-domain signal. S3. Use the pseudo-random sequence generated in step S2 as training symbols to perform two-dimensional matched filtering in time and frequency, restore the channel impulse response in the time delay-Doppler domain, use the training symbols to measure the distance between the satellite and the ground receiver for use in step S6 for positioning calculation, and delete the training symbols after completion. S4. Convert the signal received in step S2 from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and perform signal equalization based on the channel impulse response in step S3. S5. Use the SCMA decoding method to solve the signal after equalization in step S4 to obtain the probability that all users send all codewords. S6. Based on the log-likelihood probability value of each bit of each user, determine whether the user sends 0 or 1 in that bit. Solve all user signals to separate navigation information. Convert the binary data code into hexadecimal and restore the original data. Read the navigation message information to obtain the satellite operation status, satellite coordinate information, and navigation message information of at least four satellites. Combine the distance information obtained in step S3 to calculate the receiver position.

2. The integrated method for enhancing low-orbit satellite coverage and communication and navigation by superimposing code domain and codewords according to claim 1, characterized in that, Step S1 is as follows: S101. Determine the SCMA codebook parameters. Consider an SCMA system that has... One user, One resource block, the system contains user modulation order. , This represents the number of users simultaneously supported by a resource block in the system, where the number of non-zero elements in each user's codebook is . ; S102. Generate a parent constellation based on the golden angle modulation, and construct several... The set of constellations, the initial number of constellation points to be generated; S103, according to indivual Dimensional points Determine the remaining general constellation points; S104. Add a rotation factor to the obtained constellation points. Optimize to obtain ; S105. Introduce correlations into the optimized constellation points and regenerate the symbols for the second half of the dimensions. S106. The other half of the modulation symbol is generated from step S105. Next and Interleaving is performed to obtain an extended master codebook; S107. Based on the power sharing situation of users on a certain resource block, the master codebook is divided to generate the codebooks of each user; S108, Using factor matrix This indicates the correspondence between different users and resource blocks; S109. Each user selects the codeword from the corresponding column of their own codebook based on the binary code, and the codewords of all users are merged and sent.

3. The integrated method for enhancing low-orbit satellite coverage and communication and navigation by superimposing code domains and codewords according to claim 2, characterized in that, In step S107, each user's codebook Specifically: in, , Let be the mapping matrix for the j-th user. For the parent codebook, The number of users reused on each resource block. Number of users.

4. The integrated method for enhancing low-orbit satellite coverage and communication and navigation by superimposing code domains and codewords according to claim 1, characterized in that, Step S2 is as follows: S201. Divide the time-frequency domain and the time-delay-Doppler domain into grids; S202. The conduction-pass fusion signal is first directly represented in the time-delay-Doppler domain, and then converted to the time-frequency domain through inverse symplectic Fourier transform. S203. The signal is converted from the time-frequency domain to the time domain and transmitted through the Heisenberg transform.

5. The integrated method for low-orbit satellite coverage enhancement and communication / navigation enhancement based on code domain and codeword overlay according to claim 4, characterized in that, The pseudo-random sequence code is continuously sent for ranging. A pseudo-random sequence is generated and processed with the sent communication and navigation fusion signal to determine the data code frame header at the maximum value of the correlation value and the start time of signal transmission, thus obtaining the signal propagation distance.

6. The integrated method for low-orbit satellite coverage enhancement and communication / navigation enhancement based on code domain and codeword overlay according to claim 1, characterized in that, In step S3, the channel impulse response in the time-delay-Doppler domain is reconstructed as follows: in, For the received signal in the time domain, For related calculations, It is additive white Gaussian noise. A locally generated pseudo-random sequence. This is the channel frequency offset value. This is the channel delay offset value. The correlation value is obtained by correlating the received signal with the time-delayed Doppler shift signal of the local pseudo-random sequence. and These represent the errors between the correlation value and the ideal value of 1 under different conditions. It is an exponential function. It is an integer field.

7. The integrated method for low-orbit satellite coverage enhancement and communication / navigation enhancement based on code domain and codeword overlay according to claim 1, characterized in that, Step S4 is as follows: S401. The received signal is represented in a continuous time-domain form; S402. The received signal in the time domain is converted to the time-frequency domain using the Wigner transform. S403. Use SFFT transformation to convert the received signal from the time-frequency domain to the time-delay-Doppler domain.

8. The method for integrating low-orbit satellite coverage enhancement, communication, and navigation by code domain and codeword overlay as described in claim 1, characterized in that, Step S5 is as follows: S501. Initialization is divided into initialization at the resource node and initialization at the user node. S502. Resource nodes transmit messages to user nodes connected to them. When transmitting a message to a user, the external information transmitted to the resource by the other two user nodes in the previous iteration is combined. S503. When a user node sends a message to a resource node connected to it in the factor graph, it combines the message sent by another resource node in the previous iteration with the external information at this user node to obtain the message sent along the factor graph. S504. If the iteration termination condition is not met, continue to execute steps S502 and S503 in sequence; if the iteration termination condition is met, output the output value of the current user node, that is, the log-likelihood probability at that point.

9. The integrated method for low-orbit satellite coverage enhancement and communication / navigation enhancement based on code domain and codeword overlay according to claim 1, characterized in that, In step S6, based on the log-likelihood probability obtained in step S5... The criteria for determining the transmitted bits are as follows: in, The bits sent at that location.

10. A low-orbit satellite coverage enhancement and communication / navigation integrated system with code domain and codeword overlay, characterized in that, include: The fusion module treats communication signals and navigation signals as signals from different users and uses sparse code division multiple access coding technology to fuse them to obtain a communication and navigation fused signal. Add a module to take the communication and navigation fusion signal obtained by the fusion module and the corresponding grid points on the time delay Doppler domain. Use orthogonal time-frequency modulation technology to modulate it onto the corresponding carrier to form a time domain signal. Add a pseudo-random sequence as a training symbol before sending the time domain signal. The filtering module uses the pseudo-random sequence generated by the adding module as training symbols to perform two-dimensional matched filtering in time and frequency, restores the channel impulse response in the time delay-Doppler domain, uses the training symbols to measure the distance between the satellite and the ground receiver for the reading module to perform positioning calculation, and deletes the training symbol after completion. The conversion module converts the signal received by the addition module from the time domain to the time-frequency domain using orthogonal time-frequency demodulation technology, and performs signal equalization based on the channel impulse response in the filtering module. The solution module uses the SCMA decoding method to solve the signal after equalization by the conversion module to obtain the probability that all users send all codewords; The reading module determines whether a user sends a 0 or a 1 for each bit based on the log-likelihood probability value of each bit. It then decodes all user signals to separate navigation information, converts the binary data code into hexadecimal to restore the original data, reads navigation message information, obtains satellite operating status, satellite coordinate information, and navigation message information from at least four satellites, and calculates the receiver position by combining the distance information obtained from the filtering module.

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