Synchronization sequence generation method for low-orbit satellite broadband communication based on linear frequency modulation

Abstract

The present invention discloses a method for generating synchronization sequences for low-orbit satellite broadband communications based on linear frequency modulation. The method comprises the following steps: generating up-and-down chirp frequency signals using a fractional slope coefficient of linear frequency modulation spread spectrum modulation; generating up-and-down chirp phase signals using a phase-frequency relationship; generating up-and-down chirp time domain signals using a phase signal index; generating a signaling signal using the slope coefficient and modulation symbol value of the linear frequency modulation spread spectrum; and generating a synchronization sequence frame structure using the generated up-and-down chirp and signaling signal. When applied to a low-orbit satellite broadband communication system, the present invention can achieve time-frequency synchronization in low signal-to-noise ratio and large-scale Doppler frequency shift environments. It can also flexibly adjust the sequence frequency domain position to avoid partial interference in the synchronization channel, thereby improving the anti-interference capability of the synchronization system.

Description

Technical Field

[0001] The present invention belongs to the field of communications technology, and further relates to a method for generating a synchronization sequence for low-orbit satellite broadband communications based on linear frequency modulation in satellite communications technology. The present invention can be used to generate the required frame synchronization sequence in a low-orbit satellite broadband system, achieving precise time and frequency synchronization in the system. Background Art

[0002] Compared to terrestrial communications, low-orbit satellite communications primarily face challenges such as long propagation delays and path losses due to long transmission distances, as well as Doppler shifts caused by the high-speed satellite motion. Low-orbit satellites orbit the Earth at high speeds, causing signals to experience Doppler shifts of tens to hundreds of kilohertz during transmission, compared to terrestrial mobile communication systems, which typically only require frequency offsets of a few hundred to a few thousand kilohertz. Low-orbit satellite communication scenarios place higher demands on receiver frequency offset estimation capabilities. Furthermore, the significant signal fading caused by propagation distances also requires highly reliable synchronization at lower signal-to-noise ratios. In OFDM systems, both time and frequency offsets are critical issues. Accurate timing synchronization and frequency offset estimation are crucial for correct demodulation in broadband mobile communication systems based on OFDM waveforms.

[0003] Chengdu Xinglian Xintong Technology Co., Ltd. and Wuxi Xinglian Xintong Technology Co., Ltd. jointly filed a patent application titled "A Joint Estimation Method for Counteracting Large Frequency Deviation and Large Delay in Low-Earth Orbit Broadband Satellite Systems" (patent application number 202110358118.1, application publication number CN 113179233 A). This patent application discloses a method for synchronizing low-orbit satellite broadband communications to combat large frequency deviation and large delay in low-orbit broadband satellite systems. The method uses a single pilot signal to estimate both the frequency deviation and delay, enabling timely correction of user signals without having to wait for multiple pilot signals. Furthermore, the method simultaneously estimates both the delay and frequency deviation in a single step, achieving a single estimate of both quantities, which is simpler than a two-step estimation method. Furthermore, the method divides the frequency deviation into two steps: integer frequency deviation and fractional frequency deviation. This eliminates the frequency deviation estimation range, which is not limited by the pilot signal allocation interval in the time domain. Finally, the method performs both timing and fractional frequency deviation estimation simultaneously, avoiding the problem of traditional ZC sequence timing being affected by integer frequency deviation. However, the synchronization sequence used in this method to achieve time-frequency synchronization still has shortcomings. In the case of lower signal-to-noise ratio, the synchronization performance is limited. The range of frequency offset estimation of the single pilot sequence used is very limited, and the maximum estimation range is only 1 / 6 of the OFDM subcarrier spacing. It is difficult to meet the synchronization requirements in the large-scale Doppler frequency shift scenario of low-orbit satellite communications. In addition, this method cannot meet the time-frequency synchronization problem when the synchronization channel is interfered.

[0004] Chongqing Liangjiang Satellite Mobile Communications Co., Ltd., in its patent application, "A Method and System for Estimating Time and Frequency Offsets for Low-Earth Orbit Broadband Satellites" (patent application number 202111653150.9, publication number CN 114285713 A), discloses a method for estimating time and frequency offsets for low-orbit broadband satellites. The method includes the following steps: S1: acquiring a time-domain signal; S2: transforming the time-domain signal into a frequency-domain signal after coarse frequency offset estimation; S3: performing resource demapping on the frequency-domain signal to extract a pilot signal; S4: acquiring a local pilot signal and multiplying the pilot signal by the conjugate of the local pilot signal to obtain a channel estimate at the pilot signal's location; S5: performing time offset estimation on the channel estimate to obtain a precise time offset estimate; and S6: performing precise frequency offset estimation on the channel estimate to obtain a precise frequency offset estimate. This invention improves the accuracy of frequency offset estimation, thereby enhancing the overall channel's frequency offset resistance and acquisition probability; it also reduces the computational complexity of the frequency offset estimation method. However, the synchronization sequence used in this method to achieve time-frequency synchronization still has the following shortcomings: the range of frequency deviation estimation is too small to meet the large range of frequency deviations caused by the high-speed movement of low-orbit satellites. It also does not consider the interference of the synchronization channel during low-orbit satellite communication with the time-frequency synchronization system of the low-orbit satellite broadband system. Summary of the Invention

[0005] The purpose of the present invention is to address the problems existing in the above-mentioned prior art and provide a method for generating synchronization sequences for low-orbit satellite broadband communications based on linear frequency modulation, so as to solve the problems in low-orbit satellite communication scenarios where broadband communication systems have difficulty in time-frequency synchronization in low signal-to-noise ratio and large Doppler shift channel environments, and where the time-frequency synchronization performance seriously deteriorates when the synchronization channel is interfered with.

[0006] In order to achieve the above-mentioned purpose, the idea of ​​the present invention is to introduce linear frequency modulation spread spectrum technology for the first time in the method for constructing synchronization sequences of low-orbit satellite broadband systems as a basic unit for generating synchronization sequences. Since linear frequency modulation spread spectrum technology has the characteristics of strong anti-interference, anti-multipath and anti-frequency deviation capabilities, the generated synchronization sequence also has the above characteristics and can be used for broadband system time-frequency synchronization in low-orbit satellite communication scenarios with low signal-to-noise ratio and large Doppler frequency shift. At the same time, a fractional slope coefficient is introduced into the linear frequency modulation spread spectrum technology. By adjusting the slope coefficient and the modulation symbol value, the generated synchronization sequence can be flexibly adjusted in the frequency domain position and bandwidth, which is suitable for solving the time-frequency synchronization problem in scenarios where the synchronization channel is interfered. First, the specific parameters of the generated sequence are determined according to the actual communication channel conditions. Then, the frequency variation signal f of the up and down chirp sequence in the synchronization sequence is determined according to the parameters. up (t; a) and f down (t; a+KM), using the relationship between phase and frequency to process f up (t; a) and f down(t; a + KM) obtains the time domain signals of upchirp and downchirp, the basic units that constitute the synchronization sequence. Then, linear frequency modulation spread spectrum technology is used to generate a signaling sequence to carry the frequency domain position information of the synchronization sequence. Finally, the generated upchirp, downchirp and signaling sequence are combined into a synchronization sequence frame according to a fixed format to complete the synchronization sequence generation.

[0007] To achieve the above object, the specific steps implemented by the present invention are as follows:

[0008] Step 1, generating up and down chirp frequency signals using a fractional multiple slope coefficient of linear frequency modulation spread spectrum modulation;

[0009] Step 2: Generate up-chirp and down-chirp phase signals using the relationship between phase and frequency;

[0010] Step 3, generating up-chirp and down-chirp time domain signals through the phase signal index;

[0011] Step 4: Generate a signaling signal using the slope coefficient of the linear frequency modulation spread spectrum and the modulation symbol value;

[0012] Step 5: Generate a synchronization sequence frame structure using the generated up-chirp, down-chirp and signaling signal.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] First, the present invention introduces linear frequency modulation spread spectrum technology to construct the basic unit of synchronization sequences during synchronization sequence generation for low-orbit satellite broadband systems. This generates synchronization sequences with strong anti-interference, multipath, and frequency offset capabilities. This overcomes the shortcomings of existing techniques in generating sequences in low-orbit satellite communication systems, such as poor timing synchronization performance and a small frequency offset estimation range. This enables the present invention to achieve precise time-frequency synchronization even in scenarios with low signal-to-noise ratios and large Doppler frequency offsets on low-orbit satellites.

[0015] Second, the present invention introduces a fractional slope coefficient design into linear frequency modulation spread spectrum technology. Using fractional slope coefficients combined with dynamic modulation symbols to generate synchronization sequences enables flexible frequency domain position adjustment, avoiding interfering signals. This overcomes the existing shortcomings of synchronization performance degradation when the synchronization channel is interfered with. This makes the present invention's low-orbit satellite broadband system time-frequency synchronization highly resistant to interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a flow chart of the present invention;

[0017] Figure 2 It is a schematic diagram of the structure of the frame synchronization sequence generated by the present invention;

[0018] Figure 3It is a curve showing the change of synchronization sequence frequency over time and the relationship between spectrum positions in simulation experiment 1 of the present invention;

[0019] Figure 4 This is a curve showing the change of synchronization sequence frequency over time and a spectrum position relationship diagram in simulation experiment 2 of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be described in further detail below with reference to the accompanying drawings and specific examples. The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0021] Reference Figure 1 , further describing the specific steps of implementing the embodiment of the present invention.

[0022] Step 1: Generate up and down chirp frequency signals according to the following formula:

[0023]

[0024] Among them, f up (t; a) represents the up-chirp signal of the modulation symbol a generated by the synchronization sequence in the low-orbit satellite broadband system, where the frequency changes with time t, 0≤t≤T s ;T s represents the length of a symbol period of the low-orbit satellite broadband system, B represents the signal bandwidth of the low-orbit satellite broadband system, and M represents the total number of transmitted code chips in a symbol period of the low-orbit satellite broadband system, M = 2 SF SF represents the spreading factor of the linear frequency modulation spread spectrum modulation, and its value is determined by the number of OFDM subcarriers in the low-orbit satellite broadband system. K represents the slope coefficient of the linear frequency modulation spread spectrum modulation. <K<1,f down (t; a+KM) represents the down-chirp signal in which the frequency of the modulation symbol a+KM required for synchronization sequence generation in the low-orbit satellite broadband system increases with time t.

[0025] Step 2: Obtain the phase signals of the up-chirp and down-chirp signals according to the following formula:

[0026]

[0027] in, represents the phase-frequency relationship of the up-chirp signal used to form the synchronization sequence in the low-orbit satellite broadband system, It represents the phase-frequency relationship of the down-chirp signal used to form the synchronization sequence in the low-orbit satellite broadband system.

[0028] Step 3: Generate up-chirp and down-chirp time domain signals according to the following formula:

[0029]

[0030] Among them, x up (t; a) represents the up-chirp signal that constitutes the synchronization sequence in the low-orbit satellite broadband system, e (·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit, π represents pi, x down (t; a+KM) represents the down-chirp signal that constitutes the synchronization sequence in the low-orbit satellite broadband system.

[0031] Step 4: Generate a signaling sequence according to the following formula:

[0032]

[0033] Where x(t; a+p) represents the signaling sequence constituting the synchronization sequence of the low-orbit satellite broadband system, which is used to carry the frequency domain position information of the synchronization sequence. p represents the KM frequency point numbers within the frequency variation interval [aB / M, aB / M+KB], p∈{0,1,…,KM-1}, τ p Indicates the time turning point of frequency, τ p =(KM-p) / KB.

[0034] Step 5: Generate a synchronization sequence frame structure.

[0035] The synchronization sequence generated by the method of the present invention is composed of four consecutive sequences A, two consecutive sequences B and sequence C. Figure 2 As shown. Among them, sequence A is the up-chirp signal x up (t; a), which is used for data frame capture and fractional frequency offset estimation. Sequence B is the down-chirp signal x down (t; a + KM), which is used for integer frequency offset estimation and precise timing synchronization estimation. Sequence C is the signaling sequence x(t; a + p). The purpose of this sequence is to carry the information of the frequency domain starting position a of the synchronization sequence so that the receiver can obtain the frequency domain position information of the synchronization sequence and correct the integer frequency offset.

[0036] The effect of the present invention can be further demonstrated through the following simulation.

[0037] 1. Simulation experiment conditions.

[0038] The software platforms for the simulation experiment of the present invention are: Windows 10 operating system and Matlab R2021b.

[0039] 2. Simulation content and result analysis.

[0040] Simulation experiments 1 and 2 of the present invention are both further simulations of the time-frequency domain characteristics of the low-orbit satellite broadband synchronization sequence generated by the method of the present invention.

[0041] The number of OFDM subcarriers used in the LEO satellite broadband system in Simulation Experiment 1 of the present invention is N = 1024, the carrier spacing Δf = 15 kHz, and the signal bandwidth B = 15.36 MHz. The linear frequency modulation spread spectrum parameters used to generate the synchronization sequence are: spreading factor SF = 10, slope coefficient K = 1 / 8, and the modulation symbol value a is adjusted based on the actual channel interference situation, with a range of a = 8b, b = 0, 1, 2, ..., 127. In Simulation Experiment 1, a = 128 is selected.

[0042] The number of OFDM subcarriers used in the LEO satellite broadband system in Simulation Experiment 2 of the present invention is N = 1024, the carrier spacing Δf = 15 kHz, and the LEO satellite broadband system signal bandwidth B = 15.36 MHz. The linear frequency modulation spread spectrum parameters used to generate the synchronization sequence are: spreading factor SF = 10, slope coefficient K = 1 / 4, and the modulation symbol value a is adjusted based on the actual channel interference situation. The value range is a = 4b, b = 0, 1, 2, ..., 255. In Simulation Experiment 2, a = 128 is selected.

[0043] Simulation Experiment 1 and Simulation Experiment 2 of the present invention both adopt the method of the present invention and the above-mentioned different parameter configurations to obtain the frequency values ​​and signal frequency domain values ​​at 1024 time sampling points under the two configurations respectively, and then plot the obtained frequency domain time correspondence and frequency domain signal into Figure 3 and Figure 4 Middle curve.

[0044] The following combination Figure 3 and Figure 4 The simulation diagram of the present invention is further described.

[0045] Figure 3 The horizontal axis of (a) represents the sampling time points in the synchronization sequence of the low-orbit satellite broadband system, and the vertical axis represents the frequency value of the modulation symbol of the synchronization sequence. Figure 3 The curve in (a) shows the frequency variation curve of the synchronization sequence of the low-orbit satellite broadband system in simulation experiment 1 over time. The curve is drawn under the conditions of simulation experiment 1 using the chirped frequency signal generated in step 1 of the present invention.

[0046] Figure 3 The abscissa in (b) represents the bandwidth B of the LEO satellite broadband system synchronization sequence, in kHz, and the ordinate represents the signal power spectral density, in dB. The curve in the figure represents the signal spectrum of the LEO satellite broadband system synchronization sequence in Simulation Experiment 1. This curve was drawn by performing a DFT transformation on the up-chirped time-domain signal generated in Step 3 of the present invention under the conditions of Simulation Experiment 1.

[0047] Combine Figure 3 (a) and Figure 3(b) It can be seen that under the conditions of simulation experiment 1, the signal bandwidth of the low-orbit satellite broadband synchronization sequence generated by the present invention is B / 8, and the starting frequency of the synchronization spectrum is consistent with the time variation range of the frequency, and the variation range is [aB / M, aB / M+B / 8].

[0048] Figure 4 The horizontal axis of (a) represents the sampling time points in the synchronization sequence of the low-orbit satellite broadband system, and the vertical axis represents the frequency values ​​of the synchronization sequence modulation symbols. The curve in the figure shows the frequency variation of the synchronization sequence of the low-orbit satellite broadband system over time in Simulation Experiment 2. This curve was drawn using the chirped frequency signal generated in step 1 of the present invention under the conditions of Simulation Experiment 1.

[0049] Figure 4 The abscissa in (b) represents the bandwidth B of the LEO satellite broadband system synchronization sequence, in kHz, and the ordinate represents the signal power spectral density, in dB. The curve in the figure represents the signal spectrum of the LEO satellite broadband system synchronization sequence in Simulation Experiment 2. This curve was drawn by performing a DFT transformation on the up-chirped time-domain signal generated in Step 3 of the present invention under the conditions of Simulation Experiment 2.

[0050] Combine Figure 4 (a) and Figure 4 (b) It can be seen that under the conditions of simulation experiment 2, the signal bandwidth of the low-orbit satellite broadband synchronization sequence generated by the present invention is B / 4, and the starting frequency of the synchronization spectrum is consistent with the time variation range of the frequency, and the variation range is [aB / M, aB / M+B / 4].

[0051] To sum up, in the synchronization sequence generated by the present invention, the slope coefficient K determines the bandwidth of the synchronization sequence signal, and the modulation symbol a determines the frequency domain position of the synchronization signal. By configuring different slope coefficients and modulation symbols a, the synchronization sequence of the low-orbit satellite broadband system with different bandwidths and frequency domain positions can be flexibly determined. The use of the method of the present invention to generate a sequence can ensure the time-frequency performance of the low-orbit satellite broadband communication system when the synchronization channel is interfered with, and meet the synchronization requirements of the low-orbit satellite broadband communication system.

Claims

1. A method for generating synchronization sequences for low-orbit satellite broadband communications based on linear frequency modulation, characterized in that: The method comprises the following steps: generating up-and-down chirp frequency signals by using a fractional multiple slope coefficient of linear frequency modulation spread spectrum modulation; and generating a synchronization sequence frame structure by using the generated up-and-down chirp and signaling signal. Step 1: Generate up and down chirp frequency signals using the fractional slope coefficient of linear frequency modulation spread spectrum modulation as follows: Among them, f up (t; a) represents the up-chirp frequency signal of the modulation symbol a generated by the synchronization sequence in the low-orbit satellite broadband system, which changes with time t, 0≤t≤T s ;T s represents the length of a symbol period of the low-orbit satellite broadband system, B represents the signal bandwidth of the low-orbit satellite broadband system, and M represents the total number of transmitted code chips in a symbol period of the low-orbit satellite broadband system, M = 2 SF SF represents the spreading factor of the linear frequency modulation spread spectrum modulation, and its value is determined by the number of OFDM subcarriers in the low-orbit satellite broadband system. K represents the slope coefficient of the linear frequency modulation spread spectrum modulation. <K<1,f down (t; a+KM) represents the down-chirped frequency signal of the modulation symbol a+KM required for synchronization sequence generation in the low-orbit satellite broadband system, where the frequency increases with time t; Step 2: Generate up-chirp and down-chirp phase signals using the relationship between phase and frequency as follows: in, represents the up-chirped phase signal used to form the synchronization sequence in the low-orbit satellite broadband system, Represents the down-chirped phase signal used to form the synchronization sequence in the low-orbit satellite broadband system; Step 3: Generate up-chirp and down-chirp time domain signals using the phase signal index as follows: Among them, x up (t; a) represents the up-chirped time domain signal that constitutes the synchronization sequence in the low-orbit satellite broadband system, e (·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit, π represents pi, x down (t; a + KM) represents the down-chirped time domain signal constituting the synchronization sequence in the low-orbit satellite broadband system; Step 4: Generate a signaling signal using the slope coefficient of the linear frequency modulation spread spectrum and the modulation symbol value as follows: Where x(t; a+p) represents the signaling sequence constituting the synchronization sequence of the low-orbit satellite broadband system, which is used to carry the frequency domain position information of the synchronization sequence. p represents the KM frequency point numbers within the frequency variation interval [aB / M, aB / M+KB], p∈{0,1,…,KM-1}, τ p Indicates the time turning point of frequency, τ p =(KM-p) / KB; Step 5: Use the generated up-chirp, down-chirp and signaling signal to generate a synchronization sequence frame structure of [AAAA BB C], where A is the up-chirp time domain signal x up (t; a), B is the down-chirped time domain signal x down (t; a+KM), C is the signaling sequence x(t; a+p).

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