Design and detection method of chirp and zc sequence concatenated pilot in low earth orbit satellite internet of things

Abstract

The application relates to the technical field of communication, and particularly discloses a design and detection method of Chirp and ZC sequence concatenated pilots in a low-orbit satellite Internet of Things, which comprises the following steps: constructing a low-orbit satellite Internet of Things scene model; constructing ZC sequences and Chirp sequences; obtaining ZC conjugate sequences based on the ZC sequences and obtaining Chirp conjugate sequences based on the Chirp sequences; constructing Chirp and ZC sequence concatenated pilots based on the ZC sequences, the ZC conjugate sequences, the Chirp sequences and the Chirp conjugate sequences, that is, obtaining a preamble; and detecting the preamble. The method inserts the conjugate sequences of the selected Chirp sequences and ZC sequences in the preamble structure, so that the preamble detection does not involve the original sequences, but is based on the received preamble signal, the calculation complexity is reduced, and the parameter estimation is not affected by the preamble conflict.

Description

Technical Field

[0001] This invention relates to the field of communication technology, specifically to a design and detection method for cascaded pilot signals of Chirp and ZC sequences in a low-Earth orbit satellite Internet of Things (LEO). Background Technology

[0002] LEO satellite IoT integrates the characteristics of LEO. Key features of low Earth orbit (LEO) satellite communication include significant propagation delay due to long communication distances and substantial Doppler shift caused by high-speed satellite movement. The duration of CP and SEQ must exceed [a certain value]. Therefore, in LEO communication, exceptionally long CP and SEQ are required to meet this requirement. However, subcarrier spacing (SCS) is typically inversely proportional to symbol duration, meaning that increasing symbol duration reduces SCS. Furthermore, CFO disrupts the orthogonality between subcarriers, leading to inter-carrier interference (ICI). In high-speed mobile scenarios with large frequency offsets, such as low Earth orbit satellite communication, a larger SCS is needed because a larger SCS can withstand a larger CFO. Clearly, conventional preamble signals cannot simultaneously meet the requirements of long CP and large SCS.

[0003] Moreover, the most common method for preamble detection is to perform autocorrelation analysis between the received preamble and the original preamble sequence to determine the position of the peak. However, in GFRA, the selected preamble sequence is completely random, which means that the satellite cannot determine the specific content of the original preamble sequence. By traversing all known sequences and performing correlation operations with the received signal, and then finding the most significant correlation peak, this method increases the processing complexity of the receiver and prolongs the access time, especially when there are a large number of active user devices. It increases the computational complexity, and the parameter estimation is affected by preamble collisions. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a design and detection method for cascaded pilot signals of Chirp and ZC sequences in low-Earth orbit satellite Internet of Things (LEO). In the preamble detection process, to achieve frequency timing and frequency offset estimation, a conjugate sequence of a selected Chirp sequence and a ZC sequence is inserted into the preamble structure. This allows preamble detection to be performed without involving the original sequence, but rather based on the received preamble signal, reducing computational complexity and ensuring that parameter estimation is not affected by preamble collisions.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a design and detection method for Chirp and ZC sequence cascaded pilots in a low-Earth orbit satellite Internet of Things, comprising: S01: Construct a low-orbit satellite Internet of Things scenario model, the model including satellites covering multiple user terminals, among which the user terminals that transmit signals to the satellite are active users; S02: Construct the ZC sequence and the Chirp sequence; and obtain the ZC conjugate sequence based on the ZC sequence and the Chirp conjugate sequence based on the Chirp sequence; S03: Construct a Chirp and ZC sequence concatenated pilot based on the ZC sequence, ZC conjugate sequence, Chirp sequence, and Chirp conjugate sequence to obtain a preamble; the preamble includes a chirped signal segment, a short guard interval, a ZC segment, and a long guard interval arranged sequentially. S04: Detecting the preamble includes: an active user sending the preamble to the satellite; performing timing and frequency synchronization, user equipment identification, and channel estimation operations based on the preamble received by the satellite, thereby completing the detection of the preamble.

[0006] Preferably, in step S02, the ZC sequence is represented by the following formula: ; In the formula, It is a ZC sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

[0007] Preferably, in step S02, the Chirp sequence is represented by the following formula: ; In the formula, Chirp sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SF SF is the spreading factor.

[0008] Preferably, in step S02, the ZC conjugate sequence is represented by the following formula: ; In the formula, It is a ZC conjugate sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

[0009] Preferably, in step S02, the Chirp conjugate sequence is represented by the following formula: ; In the formula, It is a Chirp conjugate sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SF SF is the spreading factor.

[0010] Preferably, step S03 includes: A chirped signal segment is constructed based on the Chirp sequence and Chirp conjugate sequence; a short guard interval is constructed based on the ZC sequence; a ZC segment is constructed based on the ZC sequence and ZC conjugate sequence; and a long guard interval is constructed based on the Chirp sequence. The chirp and ZC sequence concatenated pilot is formed by the chirp signal segment, short guard interval, ZC segment and long guard interval set in sequence, and the chirp and ZC sequence concatenated pilot is the preamble.

[0011] Preferably, the number of Chirp conjugate sequences in the preamble is ; in, This represents the maximum round-trip latency difference between all devices. The duration of the short sequence symbol.

[0012] Preferably, the timing and frequency synchronization operation in step S04 includes: Coarse frequency offset estimation is performed based on the chirped signal segment in the preamble received by the satellite, and timing offset estimation and fine synchronization are performed based on the ZC segment received by the satellite, so as to achieve time and frequency synchronization of signal transmission between users and satellites.

[0013] Preferably, the coarse frequency offset estimation includes: multiplying the chirp sequence and the chirp conjugate sequence in the chirped signal segment received by the satellite to dechirp, and performing a fast Fourier transform on the dechirped signal; searching for a peak index based on the dechirped signal after the fast Fourier transform, and performing a coarse frequency offset estimation based on the peak index.

[0014] Preferably, the timing offset estimation and fine synchronization include: Based on the coarse frequency offset estimation results, frequency offset compensation is performed on the ZC sequence and ZC conjugate sequence in the ZC segment received by the satellite. The frequency offset compensated ZC sequence and ZC conjugate sequence are multiplied to obtain a product. An inverse Fourier transform is performed on the product to obtain a time domain correlation result. The position index of the peak is detected based on the time domain correlation result. Based on the position index of the peak, time series estimation and frequency offset estimation value correction are performed to obtain the ZC segment signal after time series and frequency offset correction.

[0015] The beneficial effects of the above technical solution are as follows: (1) This invention provides a design and detection method for Chirp and ZC sequence concatenated pilots in low-Earth orbit satellite Internet of Things (LEO). A LEO satellite Internet of Things scenario is constructed. Based on the characteristics of Internet of Things transmission, which are mainly short packets and data bursts, GFRA is used for simultaneous access multiple access. A preamble structure consisting of a linear frequency modulated (Chirp) sequence, a zero-mean (ZC) sequence, and their conjugate sequences is proposed and detected. This preamble structure combines the advantages of simple frequency offset estimation of linear frequency modulated sequences and accurate timing estimation of zero-mean sequences. During the detection process, chirped signals are used to solve the problems of signal acquisition and coarse frequency offset estimation, and high-precision synchronization and channel estimation are achieved through zero-sum cyclic sequences. It relies entirely on the received signal itself, reducing computational complexity. Since the original sequence is not involved, parameter estimation is not affected by preamble collisions. It provides the entire detection process after the satellite receives the preamble signal, including user equipment identification and LMMSE channel estimation based on timing and frequency offset estimation results.

[0016] (2) In the preamble detection process, in order to achieve frequency timing and frequency offset estimation, the present invention inserts a conjugate sequence of the selected Chirp sequence and ZC sequence into the preamble structure; thereby realizing that the preamble detection does not involve the original sequence, but is based entirely on the received preamble signal itself, which reduces the computational complexity and the parameter estimation is not affected by the preamble conflict. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating the design and detection method of Chirp and ZC sequence cascaded pilots in a low-Earth orbit satellite Internet of Things (LEO) according to an embodiment of the present invention; Figure 2 A schematic diagram of a low-Earth orbit satellite Internet of Things (IoT) scenario model provided for one embodiment of the present invention; Figure 3 A schematic diagram of the pilot structure of a preamble provided for one embodiment of the present invention; Figure 4 A schematic diagram illustrating the relationship between the number of user equipment accesses and communication performance, provided as an embodiment of the present invention; Figure 5 This is a schematic diagram illustrating the relationship between mean square error and signal-to-noise ratio, provided as an embodiment of the present invention. Detailed Implementation

[0019] The embodiments of this application will be described in further detail below. Obviously, the described embodiments are only a part of the embodiments of this application, and not an exhaustive list of all embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0020] The terms “first,” “second,” etc. (if applicable) in the specification and claims are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated (if applicable) or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion, such as a process, method, system, product, or apparatus that comprises a series of steps or units, not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0021] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0022] Example 1 like Figure 1 As shown, one embodiment of the present invention provides a design and detection method for cascaded pilot signals of Chirp and ZC sequences in a low-Earth orbit satellite Internet of Things (LEO), comprising the following steps: S01: Construct a low-orbit satellite Internet of Things scenario model, the model including satellites covering multiple user terminals, among which the user terminals that transmit signals to the satellite are active users; like Figure 2As shown, the constructed low-Earth orbit satellite IoT scenario model is an uplink GRFA LEO satellite IoT system, in which P single-antenna devices (user terminals) are covered by satellite S. This satellite is equipped with Q antennas capable of parallel reception. User devices are randomly distributed within this circular area and communicate directly with the satellite. In most cases, when there is no transmission request, the user devices are in a silent state and are considered inactive user devices; when ready to send a message, they become active (i.e., become active user devices).

[0023] S02: Construct the ZC sequence and the Chirp sequence; and obtain the ZC conjugate sequence based on the ZC sequence and the Chirp conjugate sequence based on the Chirp sequence; In GRFA, the active user equipment (UE) transmits a preamble followed by data signals. Due to the lack of prior information about the active user equipment (UE), synchronization and channel estimation are performed via the preamble, which gives the preamble some unique characteristics: the Zadoff-Chu (ZC) sequence. The ZC sequence is represented by the following formula: ; In the formula, It is a ZC sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

[0024] Typically, N is set to a prime number. Once the root index μ and sequence length N are determined, a ZC sequence can be generated. A sequence with a cyclic shift offset m is defined as follows: ; When cyclically shifted by m, the original sequence only has a significant correlation peak with the shifted sequence at position m, while the correlation peaks at other positions are 0.

[0025] When μ is set to 1, the normalized frequency of the ZC sequence is adjusted to cover the interval [-0.5, 0.5], thus obtaining the Chirp sequence (i.e., the original up-conversion sequence). The Chirp sequence is represented by the following formula: ; In the formula, Chirp sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SFSF is the spreading factor, representing the number of bits per symbol, typically set to {7, 8, 9, 10, 11, 12}. The frequency change of a linear frequency modulated signal is linear. Assume the initial frequency of the original linear frequency modulated signal is f0, and its frequency varies throughout the symbol duration T. s Inner linear change to f i Its bandwidth B is calculated as B =|f i -f0|,N=B×T s Given a bandwidth, increasing the spreading factor SF will extend the symbol duration T. s The formula for calculating the linear frequency modulation rate k is k = ±B / T. s When f i When the frequency is greater than f0, it is called uplink linear frequency modulation; otherwise, it is called downlink linear frequency modulation.

[0026] When cyclically shifted by m (m = 0, ..., N-1), the Chirp sequence can be represented as: ; To ensure phase continuity, it can be simplified to: ; Based on the ZC sequence, a ZC conjugate sequence is obtained, which is represented by the following formula: ; In the formula, It is a ZC conjugate sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

[0027] The Chirp conjugate sequence is obtained based on the Chirp sequence, and the Chirp conjugate sequence is represented by the following formula: ; In the formula, It is a Chirp conjugate sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SF SF is the spreading factor.

[0028] S03: Construct a Chirp and ZC sequence concatenated pilot based on the ZC sequence, ZC conjugate sequence, Chirp sequence, and Chirp conjugate sequence to obtain a preamble; the preamble includes a chirped signal segment, a short guard interval, a ZC segment, and a long guard interval arranged sequentially. In existing technologies, random access preambles consist of three parts: a cyclic prefix (CP), a sequence (SEQ), and a guard interval (GT). The length of each part must meet specific requirements: ; in, For cyclic prefix; For protection intervals; For sequence; This represents the maximum round-trip delay difference (RTD) between all devices. Representing maximum delay spread, N0 and N f These represent the noise spectral density and noise figure, respectively. It is the expected received signal power strength. It is the ratio of the sequence energy to thermal noise.

[0029] Key characteristics of low Earth orbit (LEO) satellite communications include significant propagation delay due to long communication distances and substantial Doppler shift caused by high-speed satellite movement. The durations of CP and SEQ must exceed [a certain value]. Therefore, in LEO communication, exceptionally long CP and SEQ are required to meet this requirement. However, subcarrier spacing (SCS) is typically inversely proportional to symbol duration, meaning that increasing symbol duration reduces SCS. Furthermore, large carrier frequency offset (CFO) disrupts the orthogonality between subcarriers, leading to inter-carrier interference (ICI). In high-speed mobile scenarios with large frequency offsets, such as low Earth orbit satellite communication, a larger SCS is needed because a larger SCS can withstand a larger CFO. Clearly, conventional preamble signals cannot simultaneously meet the requirements of long CP and large SCS.

[0030] Concatenating multiple short sequences into a long preamble sequence is an effective method to solve this problem. Chirp sequences perform exceptionally well in low signal-to-noise ratio and large carrier frequency offset (CFO) scenarios, making them ideal for LEO satellite IoT applications. Meanwhile, ZC sequences possess excellent zero autocorrelation sidelobes, enabling precise time synchronization, and their large quantity makes them suitable for large-scale random access scenarios. Therefore, combining the advantages of chirp and ZC sequences allows for concatenation to form a new preamble structure.

[0031] Suppose there exist M numbers of length N. chirpAn up-conversion sequence and L zero-mean cyclic shift sequences of length Nzc are available for selection. The up-conversion sequence can be generated using different spreading factors and frequency ranges, while the zero-mean cyclic shift sequences can be created using different root exponents. Both sequence pools are known to the base station and the satellite before communication. Each active user equipment (UE) randomly selects one up-conversion sequence (i.e., the chirp sequence) and one zero-mean cyclic shift sequence (i.e., the ZC sequence) to construct its own preamble.

[0032] The construction of a preamble based on the ZC sequence, ZC conjugate sequence, Chirp sequence, and Chirp conjugate sequence includes: A chirped signal segment is constructed based on the Chirp sequence and its conjugate sequence; a short guard interval (Short GT) is constructed based on the ZC sequence; a ZC segment is constructed based on the ZC sequence and its conjugate sequence; and a long guard interval (Long GT) is constructed based on the Chirp sequence. like Figure 3 As shown, the chirp and ZC sequence concatenated pilot is formed based on the chirp signal segment, short guard interval, ZC segment and long guard interval arranged in sequence, and the chirp and ZC sequence concatenated pilot is the preamble.

[0033] The chirped signal segment consists of a linear frequency modulation sequence (Chirp sequence) and its conjugate sequence (Chirp conjugate sequence) randomly selected by the active user equipment (UE), which is the so-called down-linear frequency modulation; Figure 3 Only one conjugate sequence is shown in the figure, but in reality, to utilize the CP function, the number of Chirp conjugate sequences should be [number missing]. , This represents the maximum round-trip latency difference between all devices. The duration of the short sequence symbol is defined. Compared to the traditional preamble which requires a CP segment, this invention achieves the function of a CP segment through a chirped signal segment. In addition to avoiding inter-carrier interference, the chirped signal can also be used to perform high-precision frequency offset estimation.

[0034] Short GT refers to the selected ZC sequence, whose main purpose is to eliminate time delay spread.

[0035] The ZC segment consists of a selected ZC sequence and its conjugate sequence, and is used for delay estimation and channel estimation. Depending on the requirements of delay and channel estimation, the number of conjugate sequences may need to be increased.

[0036] Long GT contains A series of Chirp sequences (i.e., down-conversion pulses) are used to absorb the propagation delay.

[0037] In one embodiment, preferably an Iridium system, the maximum differential delay The sequence length is 4.33 milliseconds. Assume the symbol frequency of the linear frequency modulated sequence is 10, the bandwidth B is 1000 kHz, and the symbol duration is 1.02 milliseconds. Therefore, This results in the number of lower linear frequency modulations in the linear frequency modulation band being 4.

[0038] This invention constructs a LEO satellite IoT scenario. Based on the characteristics of IoT transmission, which primarily involve short packets and data bursts, GFRA (Geometric Array of Multiple Access) is employed for simultaneous multiple access. A preamble structure consisting of a chirp sequence, a zero-mean (ZC) sequence, and their conjugate sequences is proposed, combining the advantages of simple frequency offset estimation using chirp sequences and precise timing estimation using ZC sequences. Chirped signals are used to solve the problems of signal acquisition and coarse frequency offset estimation, while high-precision synchronization and channel estimation are achieved through zero-sum cyclic sequences. It relies entirely on the received signal itself, reducing computational complexity. Since the original sequence is not involved, parameter estimation is not affected by preamble collisions.

[0039] S04: Detecting the preamble includes: performing timing and frequency synchronization, user equipment identification, and channel estimation operations based on the preamble (i.e., preamble signal) received by the satellite, thereby completing the detection of the preamble.

[0040] Assume that in time slot t, we have K( ( ) active users send preambles to the satellite. Assuming line-of-sight (LoS) transmission exists between the satellite and ground equipment, and neglecting multipath propagation, the preamble received by the satellite is: ; ; In the formula, K represents the preamble received by the satellite at time t, i.e., the pilot signal received by the satellite at time t; K represents the number of active users. To account for free-space path loss and atmospheric attenuation in the channel gain, the propagation delay... and frequency offset It is randomly generated; It is a complex additive white Gaussian noise (AWGN) with zero mean and variance σ²; k is the active user k, n is the nth bit of the received signal, N is the sequence length, and j is an imaginary number; To receive pilot signals; Indicates the remainder; This represents the frequency offset.

[0041] The satellite performs timing and frequency synchronization, user equipment identification, and channel estimation operations based on the preamble received by the satellite, including: A. Timing and frequency synchronization; Timing and frequency synchronization, specifically, involves estimating timing and frequency offsets. The most common method for estimating timing and frequency offsets is to perform autocorrelation analysis between the preamble received from the satellite and the original preamble sequence to determine the location of peaks. A similar method is used for dechirping linear frequency modulation (LFM) sequences. However, in GFRA, the selected preamble sequence is completely random, meaning the satellite cannot determine the specific content of the original preamble sequence. One approach to address this problem is to perform correlation operations between all known sequences and the received signal, and then find the most significant correlation peak. Obviously, this method increases the processing complexity of the receiver and extends access time, especially with a large number of active user equipment (UEs). Therefore, the method provided in this invention maximizes the use of the received signal.

[0042] To achieve frequency timing and frequency offset estimation, a conjugate sequence of a selected Chirp sequence and a ZC sequence is inserted into the preamble structure; the timing and frequency synchronization operations of this invention include: Coarse frequency offset estimation is performed based on the chirped signal segment in the preamble received by the satellite, and timing offset estimation and fine synchronization are performed based on the ZC segment received by the satellite, so as to achieve time and frequency synchronization of signal transmission between users and satellites.

[0043] (1) Coarse frequency offset estimation includes: coarsely estimating the frequency offset based on the chirped signal segment in the preamble structure; specifically, it includes: multiplying the Chirp sequence and the Chirp conjugate sequence in the chirped signal segment of the preamble received by the satellite to dechirp, and performing a fast Fourier transform on the dechirped signal; searching for a peak index based on the dechirped signal after the fast Fourier transform, and performing a coarse frequency offset estimation based on the peak index.

[0044] Assuming active user devices The selected linear frequency modulation sequence (i.e., the Chirp sequence) is: ; In the formula, It is a linear frequency modulation sequence; n is the nth bit of the received signal; j is an imaginary number; for Length.

[0045] The Chirp segment signal (i.e., the Chirp sequence and the Chirp conjugate sequence) received by the satellite is as follows: ; In the formula, The satellite received the Chirp sequence; The Chirp conjugate sequence received by the satellite; Add a delay to the sent Chirp sequence ; Add a delay to the transmitted Chirp conjugate sequence h represents the channel gain, which is assumed to remain constant during transmission. for Length; and These are timing and frequency offsets, respectively. It is complex additive white Gaussian noise (AWGN) with zero mean and variance σ².

[0046] The first step is to use Multiply However, due to It is unknown, so we adopt... Multiply To stop chirping; ; In the formula, Then, a Fast Fourier Transform (FFT) is performed on the dechirped signal. ; In the formula, These are the frequency domain representations of the k-th bit of the dechirped signal.

[0047] Search Peak Index : ; The frequency bias coarse estimate is: ; (2) Timing estimation and fine synchronization operations: Timing estimation and fine synchronization operations are mainly accomplished using ZC segment signals, including: performing frequency offset compensation on the ZC sequence and ZC conjugate sequence in the ZC segment signal of the preamble received by the satellite based on the coarse frequency offset estimation result; multiplying the frequency offset-compensated ZC sequence and ZC conjugate sequence to obtain a product; performing an inverse Fourier transform on the product to obtain a time-domain correlation result; detecting the position index of the peak based on the time-domain correlation result; and performing timing estimation and frequency offset estimation correction based on the position index of the peak to obtain the timing- and frequency offset-corrected ZC segment signal.

[0048] Assuming active user equipment The selected ZC sequence is Then the ZC segment signal (ZC sequence and ZC conjugate sequence) received by the satellite is: ; In the formula, The ZC sequence received by the satellite; The ZC conjugate sequence received by the satellite; Add a delay to the transmitted ZC sequence ; Add a delay to the conjugate of the transmitted ZC sequence. ; express Length.

[0049] First, frequency offset compensation is performed on the received ZC signal based on the coarse frequency offset estimation results, resulting in: ; In the formula, This is the result of coarse frequency offset estimation.

[0050] The most basic time synchronization method based on ZC sequences is achieved by performing time-domain autocorrelation with the original ZC sequence. To reduce computational complexity, the time-domain autocorrelation is converted into frequency-domain multiplication; then, an inverse Fourier transform (IFFT) is performed on the product to obtain the time-domain autocorrelation result.

[0051] ; In the formula, The result is the frequency domain multiplication of the received ZC signal and its conjugate after frequency offset compensation; The result of time-domain correlation of the received ZC signal and its conjugate after frequency offset compensation; The received ZC signal after frequency offset compensation; The received ZC conjugate signal is the signal after frequency offset compensation.

[0052] Detection peak position Peak location index and maximum value of related peaks : ; ; In the formula, To detect peak position Peak position index; To detect peak position The maximum value of the correlation peak of the peak.

[0053] The time series estimation result is as follows: ; Finally, the frequency offset estimate is corrected using the phase difference method: ; In the formula, The time-domain correlation results of the received ZC signal and its conjugate after frequency offset compensation The value of the bit; It is the conjugate of the maximum value of the relevant peak; This is the result of coarse frequency offset estimation; This is the correction value for the frequency offset estimation.

[0054] B. User identification; The ZC segment signal after timing and frequency offset correction is represented as follows: , After FFT transformation, Assume that the pool of all ZC sequences is The ZC sequence pool, after FFT transformation, becomes .Will and The sequences are multiplied sequentially, and then the result is transformed using IFFT. ; turn up Maximum value, and compare it with the threshold. Comparison. If the maximum value exceeds the threshold, then it is considered... Active user devices The selected sequence.

[0055] ; in, It is a coefficient between 0 and 1; To detect peak position The maximum value of the correlation peak of the peak.

[0056] C. Channel estimation; The result of least squares (LS) estimation is: ; In the formula, This is the channel least squares estimate; To send the preamble signal Transpose of; To receive preamble signals from satellites that have been frequency and time synchronized; To send the preamble signal Norm.

[0057] While the least squares method performs well under low signal-to-noise ratio (SNR) conditions, it has poor noise resistance. However, the LMMSE algorithm requires prior information, such as channel covariance and noise variance. In practical applications, this information can be estimated using sample covariance methods or parametric models. This invention employs a multi-sequence cascaded leader structure and uses the sample covariance method to estimate the channel covariance and noise variance. Assuming LS estimation is performed on the ZC sequence and its conjugate sequence, the estimation results can be obtained as follows: ; Where F represents the number of conjugate sequences. After removing the mean... Then, the estimated value of the channel covariance can be expressed as: ; In the formula, For the first LS estimation results for ZC conjugate sequences.

[0058] The noise variance calculated from the residuals estimated by LS is: ; In the formula, This is the ZC signal that is sent.

[0059] The LMMSE (Least Mean Square Error) estimation result is as follows: ; It can be used Represented as: ; In the formula, This is the linear minimum mean square error channel estimate; This is an estimate of the channel covariance; For noise variance; It is the identity matrix; This is the channel least squares estimate.

[0060] Figure 4 The graphs show the changes in active user equipment (UE) access probability, mean square error of channel estimation (MSE), and synchronization state relative to the number of active UEs. As the number of active UEs increases from 0 to 100, the access probability, the mean square error of channel estimation, and the synchronization state remain almost constant. This demonstrates that the method provided in this invention is highly robust to changes in the number of active UEs, indicating that the method is well-suited for large-scale access scenarios.

[0061] Figure 5 The mean square error of timing and frequency offset estimation, as well as the variation of Cramer-Rao (CRLB) under signal-to-noise ratio changes, are shown. It can be seen that the mean square error of the estimation by the method provided in this invention is very close to that of the CRLB, proving that the method provided in this invention has good estimation accuracy.

[0062] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A design and detection method for cascaded pilot signals of Chirp and ZC sequences in a low-Earth orbit satellite Internet of Things (LEO), characterized in that, include: S01: Construct a low-orbit satellite Internet of Things scenario model, the model including satellites covering multiple user terminals, among which the user terminals that transmit signals to the satellite are active users; S02: Construct the ZC sequence and the Chirp sequence; and obtain the ZC conjugate sequence based on the ZC sequence and the Chirp conjugate sequence based on the Chirp sequence; S03: Construct a concatenated pilot signal of Chirp and ZC sequences based on the ZC sequence, ZC conjugate sequence, Chirp sequence, and Chirp conjugate sequence to obtain a preamble; the preamble includes a chirped signal segment, a short guard interval, a ZC segment, and a long guard interval arranged sequentially; wherein, S03 includes: constructing a chirped signal segment based on the Chirp sequence and Chirp conjugate sequence; constructing a short guard interval based on the ZC sequence; constructing a ZC segment based on the ZC sequence and ZC conjugate sequence; and constructing a long guard interval based on the Chirp sequence; The chirp and ZC sequence concatenated pilot is constructed based on the sequentially arranged chirp signal segment, short guard interval, ZC segment, and long guard interval. This concatenated pilot is the preamble. The number of chirp conjugate sequences in the preamble is... ; in, This represents the maximum round-trip latency difference between all devices. The duration of short sequence symbols; S04: Detecting the preamble includes: an active user sending the preamble to the satellite; performing timing and frequency synchronization, user equipment identification, and channel estimation operations based on the preamble received by the satellite, thereby completing the preamble detection; the timing and frequency synchronization operations include: Coarse frequency offset estimation is performed based on the chirped signal segment in the preamble received by the satellite, and timing offset estimation and fine synchronization are performed based on the ZC segment received by the satellite, so as to achieve time and frequency synchronization of signal transmission between users and satellites.

2. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, In step S02, the ZC sequence is represented by the following formula: In the formula, It is a ZC sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

3. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, In step S02, the Chirp sequence is represented by the following formula: In the formula, Chirp sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SF SF is the spreading factor.

4. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, In step S02, the ZC conjugate sequence is represented by the following formula: In the formula, It is a ZC conjugate sequence; It is the root index; The sequence length; For the first received signal Bit; It is an imaginary number.

5. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, In step S02, the Chirp conjugate sequence is represented by the following formula: In the formula, It is a Chirp conjugate sequence; For the first received signal Bit; It is an imaginary number; Where N is the sequence length, N=2 SF SF is the spreading factor.

6. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, The coarse frequency offset estimation includes: multiplying the chirp sequence and the chirp conjugate sequence in the chirped signal segment received by the satellite to dechirp, and performing a fast Fourier transform on the dechirped signal; searching for a peak index based on the dechirped signal after the fast Fourier transform, and performing a coarse frequency offset estimation based on the peak index.

7. The design and detection method for Chirp and ZC sequence cascaded pilots in low-Earth orbit satellite IoT according to claim 1, characterized in that, The timing offset estimation and fine synchronization include: Based on the coarse frequency offset estimation results, frequency offset compensation is performed on the ZC sequence and ZC conjugate sequence in the ZC segment received by the satellite. The frequency offset compensated ZC sequence and ZC conjugate sequence are multiplied to obtain a product. An inverse Fourier transform is performed on the product to obtain a time domain correlation result. The position index of the peak is detected based on the time domain correlation result. Based on the position index of the peak, time series estimation and frequency offset estimation value correction are performed to obtain the ZC segment signal after time series and frequency offset correction.

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