Satellite Spread Spectrum Communication System and Symbol Synchronization Method Based on Improved Square Variance

Abstract

The present invention provides a satellite spread spectrum communication system and symbol synchronization method based on improved square variation. The method includes the following steps: S1: a receiving end performs matched filtering despreading on all sampling points of an input signal to convert the input signal into a complex signal; S2: preprocessing the complex signal; S3: comparing the preprocessed input signal with a preset signal amplitude threshold T, and determining the moment when the signal amplitude is greater than the threshold T as the current optimal sampling time point; S4: determining the phase based on the optimal sampling time point and generating a timing pulse sequence signal; S5: sampling the preprocessed signal using the timing pulse sequence signal, calculating the sum of the variation between the sampling value of the optimal sampling point and the two preceding and following sampling values as a correction variable; and S6: correcting the phase of the timing pulse sequence signal based on the correction variable. The present invention has a low computational complexity and can quickly and accurately perform symbol synchronization between a transmitting end and a receiving end.

Description

Technical Field

[0001] The present invention relates to the field of satellite communications, and more particularly to a satellite spread spectrum communication system and a symbol synchronization method based on improved square variation. Background Art

[0002] Satellite communications boast irreplaceable advantages over terrestrial networks, including long transmission distances, rapid deployment, and robust resilience to natural disasters. These advantages make them crucial in the development of national communications strategies. Despite these advantages, satellite communications are also highly vulnerable to interference. In the event of war, communications satellites are highly susceptible to electromagnetic interference, or even physical destruction, disrupting communications services. To combat interference, spread spectrum communications are the primary choice.

[0003] Spread spectrum communications directly utilize a high-rate spreading code sequence using various modulation schemes to spread the signal spectrum at the transmitter. At the receiver, the same spreading code sequence is used for despreading, restoring the original information to the signal. Spread spectrum signals exhibit pseudo-random noise characteristics, requiring the received signal sequence to be strictly synchronized with a local reference sequence for accurate reception and demodulation. Synchronization in spread spectrum communication systems involves carrier synchronization, bit synchronization, and code synchronization, with symbol / spreading code synchronization being the most important. Symbol / spreading code synchronization requires the generation of a timing pulse at the receiver that matches the repetition frequency and phase of the received symbol, known as a synchronization clock. Due to factors such as instability in the transmit and receive clocks, time differences in the start of the spreading sequence, and propagation delays, the receiver may be unable to determine the starting phase of the spreading sequence in the received signal, resulting in failure to synchronize the local pseudo-random code generated by the receiver with the received pseudo-random code. Summary of the Invention

[0004] The present invention aims to address the technical problem that the local pseudo-random code generated by the receiving end in the current spread spectrum communication system is difficult to strictly synchronize with the received pseudo-random code, and to provide a satellite spread spectrum communication system and a symbol synchronization method based on improved square variation, which can accurately achieve symbol synchronization.

[0005] To achieve the above objectives, the present disclosure proposes the following technical solutions:

[0006] In a first aspect, an embodiment of the present disclosure provides a satellite spread spectrum communication symbol synchronization method based on improved square variation. The method is implemented at a receiving end of a satellite spread spectrum communication system, comprising the following steps:

[0007] S1. The receiving end performs matched filtering despreading on all sampling points of the input signal to convert the input signal into a complex signal;

[0008] S2. Preprocessing the complex signal to improve the signal-to-noise ratio of the complex signal to a preset requirement;

[0009] S3, comparing the pre-processed input signal with a preset signal amplitude threshold T, and taking the moment when the signal amplitude is greater than the threshold T as the current optimal sampling time point;

[0010] S4. Determine the phase based on the optimal sampling time point and generate a timing pulse sequence signal; perform sub-sampling, demodulation, and decoding on the complex signal after matched filtering and despreading based on the timing pulse sequence signal;

[0011] S5, sampling the pre-processed signal using the timing pulse sequence signal, and calculating the sum of the variation between the sampling value of the optimal sampling point and the sampling values of the two preceding and following sampling points as a correction variable;

[0012] S6. Correct the phase of the timing pulse sequence signal based on the correction variable.

[0013] As an optional implementation of the improved square variation-based satellite spread spectrum communication symbol synchronization method, the preprocessing method includes squaring the modulus of the complex signal to convert it into a positive real value. If the signal-to-noise ratio after squaring is low and cannot meet the requirements of subsequent threshold determination, integration can be performed after squaring to improve the signal-to-noise ratio.

[0014] For example, assuming the input signal is r(n), after performing matched despreading on all sampling points of r(n), the resulting complex signal is s(n): Where PN is a spreading code of length K, PN(k) is the kth bit of the spreading code, and SPS is the number of sampling points per chip of the spreading code. Square the modulus of the complex signal s(n) to make all the signal s(n) real positive values; then integrate s(n) to get the result: Where M is the integration multiple.

[0015] As an optional implementation of the satellite spread spectrum communication symbol synchronization method based on improved square variation, the following method may be used to generate a timing pulse sequence signal:

[0016] According to the initial value of mk, the timing pulse sequence signal p(n) with a period of K×SPS and a phase of mk can be generated:

[0017]

[0018] Where δ is the Dirac function. The subsampling of the signal can be completed by the instruction of the p(n) pulse. The receiving end demodulates and decodes the subsampled signal in sequence, and the symbol synchronization of the receiving end is preliminarily completed.

[0019] As an optional implementation of the satellite spread spectrum communication symbol synchronization method based on improved square variation, the method for calculating the correction variable is:

[0020] Calculate ukt = sign[t(2)-t(1)] / 2+sign[t(3)-t(2)] / 2, where t(2) represents the sampling value of the optimal sampling point, t(1) represents the sampling value of the sampling point before the optimal sampling point, and t(3) represents the sampling value of the sampling point after the optimal sampling point; t(2)-t(1) represents the variation between the sampling value of the optimal sampling point and the adopted value of the sampling point before it, and t(3)-t(2) represents the variation between the sampling value of the optimal sampling point and the adopted value of the sampling point after it; sign[x] represents the sign function, when x>0, sign[x]=1; when x=0, sign[x]=0; when x<0, sign[x]=-1. In the actual communication process, when the optimal sampling point gradually advances, the maximum value gradually moves from the middle value to the sampling point at the previous moment, and the ukt value moves from 0 to -1; when the optimal sampling point gradually lags behind, the maximum value gradually moves from the middle value to the sampling point at the next moment, and the ukt value moves from 0 to 1. At this time, the phase of the timing pulse sequence signal needs to be corrected.

[0021] As an optional implementation of the satellite spread spectrum communication symbol synchronization method based on improved square variation, a method for correcting the phase of the timing pulse sequence signal includes the following steps:

[0022] After the system is powered on, the average value of ukt within the preset time period T is calculated, and the calculated average value is used as the initial value of the time deviation uk;

[0023] Each time the receiver performs subsampling and correction, the updated uk value is: uk←uk+G×(ukt-uk); where G is the coefficient of the loop filter and is a positive number less than 1;

[0024] Then, update the value of mk based on the updated time bias uk:

[0025] Determine whether the value of uk at this time satisfies uk>+U. If so, update the value of mk to: mk←mk+1, and update the value of uk to: uk←uk-1. If not, determine whether the value of uk at this time satisfies uk<-U. If so, update the value of mk to: mk←mk-1, and update the value of uk to: uk←uk+1.

[0026] In a second aspect, an embodiment of the present disclosure proposes a satellite spread spectrum communication system based on improved square variation, the system including a transmitter and a receiver, and the receiver uses the symbol synchronization method to perform symbol synchronization with the transmitter.

[0027] As an optional implementation of the satellite spread spectrum communication system based on improved square variation, the receiving end may include a matched filter despreading module, a signal processing module, a variation statistics module, a timing pulse module, a subsampling module, a demodulation module, and a decoding module, each of which may be implemented using an FPGA. The matched filter despreading module is configured to perform matched filter despreading on all sampling points of the received input signal to convert the input signal into a complex signal.

[0028] The signal processing module is configured to pre-process the complex signal;

[0029] The variation statistics module is configured to perform a threshold judgment on the pre-processed signal to obtain an optimal sampling point; and receive the sampling value of the optimal sampling point and the sampling values of the preceding and following sampling points sent by the timing pulse module, calculate the correction variable, and correct the optimal sampling point based on the correction variable;

[0030] The timing pulse module is configured to generate a timing pulse sequence signal based on the optimal sampling point calculated by the variation statistics module; and to sample the preprocessed input signal based on the timing pulse sequence signal, and send the sampling value of the optimal sampling point and the sampling values of the preceding and following sampling points to the variation statistics module;

[0031] The sub-sampling module is configured to perform sub-sampling on the complex signal after matched filtering and despreading based on the timing pulse sequence signal;

[0032] The demodulation module is configured to demodulate the sub-sampling results;

[0033] The decoding module is configured to decode the demodulated signal.

[0034] In summary, the present invention has the following beneficial effects: the symbol synchronization scheme proposed in the present invention has a small amount of calculation, can quickly and accurately perform symbol synchronization between the transmitter and the receiver, and greatly improves the efficiency of symbol synchronization in the satellite spread spectrum communication system. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a flow chart of a symbol synchronization method for a satellite spread spectrum communication system based on improved square variation according to an embodiment of the present disclosure;

[0036] Figure 2 This is a structural diagram of a receiving end of a satellite spread spectrum communication system based on improved square variation, which is schematically provided in an embodiment of the present disclosure;

[0037] Figure 3 Schematic diagram of the internal structure of functional modules of a receiving end of a satellite spread spectrum communication system based on improved square variation involved in an embodiment of the present disclosure;

[0038] Figure 4 (a) A waveform diagram of the transmission information of the transmitting end of the satellite spread spectrum communication system involved in the embodiments of the present disclosure after being modulated, spread-spectrum-spread, and shaped and filtered;

[0039] Figure 4 (b) is a waveform diagram of the transmission information of the transmitting end of the satellite spread spectrum communication system involved in the embodiment of the present disclosure, after shaping filtering and adding time offset but without adding noise;

[0040] Figure 5 (a) is an original waveform diagram (with noise, time offset, phase offset, and time offset) received by the receiving end of the satellite spread spectrum communication system involved in the embodiment of the present disclosure;

[0041] Figure 5 (b) is a waveform diagram of the original waveform received by the receiving end of the satellite spread spectrum communication system involved in the embodiment of the present disclosure after being despread through matched filtering;

[0042] Figure 6 (a) is a waveform diagram of the complex signal despread by matched filtering at the receiving end of the satellite spread spectrum communication system involved in the embodiment of the present disclosure after being squared;

[0043] Figure 6 (b) is a waveform diagram of a signal obtained by integrating the squared signal according to an embodiment of the present disclosure;

[0044] Figure 7 : This is a simulation result diagram for different time biases uk involved in the embodiment of the present disclosure, wherein:

[0045] Figure 7 (a) is the time domain superposition diagram of t(n) when uk = 0.25;

[0046] Figure 7 (b) is the time domain superposition diagram of t(n) when uk = 0.5;

[0047] Figure 7 (c) is the time domain superposition diagram of t(n) when uk = 0.75;

[0048] Figure 7 (d) is the time domain superposition diagram of t(n) when uk=1;

[0049] Figure (8) shows the change of the value of mk over time in the embodiment of the present disclosure, where:

[0050] Figure 8 (a) is a schematic diagram of the change of mk value over time when uk = 0.25;

[0051] Figure 8 (b) is a schematic diagram of the change of mk value over time when uk = 0.5;

[0052] Figure 8 (c) is a schematic diagram of the change of mk value over time when uk = 0.75;

[0053] Figure 8 (d) is a schematic diagram of the change of the value of mk over time when uk=1. DETAILED DESCRIPTION

[0054] The present application is described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but are not intended to limit the present application in any form. It should be noted that those skilled in the art may make several variations and improvements without departing from the scope of the present application. These all fall within the scope of protection of the present application.

[0055] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0056] It should be noted that, if there is no conflict, the various features in the embodiments of the present application can be combined with each other and are all within the scope of protection of the present application. In addition, although the functional modules are divided in the device schematic and the logical order is shown in the flow chart, in some cases, the steps shown or described can be performed in a different order than the module division in the device or the order in the flow chart. In addition, the words "first", "second", "third", etc. used herein do not limit the data and execution order, but only distinguish between the same items or similar items with basically the same functions and effects.

[0057] Unless otherwise defined, all technical and scientific terms used in this specification have the same meanings as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification and in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the relevant listed items.

[0058] In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0059] Please refer to Figure 1 , Figure 1 A symbol synchronization method for a satellite spread spectrum communication system based on improved square variation according to an embodiment of the present disclosure is provided, which may include the following steps:

[0060] S1. The receiving end performs matched filtering despreading on all sampling points of the input signal to convert the input signal into a complex signal.

[0061] First, the input signal at the receiving end is despread by matched filtering. At this time, the optimal sampling point cannot be determined, so the despreading is matched despreading of all sampling points. Let the input signal be r(n). After matching despreading all sampling points of r(n), the complex signal obtained is s(n):

[0062]

[0063] Wherein, PN is a spreading code of length K, PN(k) is the kth bit of the spreading code, and SPS is the number of sampling points per chip of the spreading code.

[0064] S2. Preprocess the complex signal s(n) obtained in step S1 to improve the signal-to-noise ratio of the signal s(n) for subsequent processing.

[0065] The complex signal s(n) may be preprocessed by the following steps:

[0066] Square the modulus of the complex signal s(n) to make the signal s(n) all real positive values; if the signal-to-noise ratio is low after squaring and cannot meet the requirements of subsequent threshold judgment, you can integrate it after squaring to improve the signal-to-noise ratio of the signal. The integral calculation formula is:

[0067]

[0068] Where M is the integration multiple.

[0069] S3. Compare the pre-processed input signal with the preset signal amplitude threshold T, and take the moment when the signal amplitude is greater than the threshold T as the current optimal sampling time point, which is also the end position of the spread spectrum code PN. The calculation formula for calculating the optimal sampling time point is:

[0070] mk=index[t(n)>T]

[0071] Where mk represents the optimal sampling time point, and index[x] represents the index function, which is used to return the value that meets the conditions in the array t(n).

[0072] S4. Determine the phase based on the optimal sampling time point and generate a timing pulse sequence signal; perform sub-sampling, demodulation and decoding on the complex signal after matched filtering and despreading based on the generated timing pulse sequence signal.

[0073] Among them, the following methods can be used to generate pulse train signals:

[0074] According to the initial value of mk, the timing pulse sequence signal p(n) with a period of K×SPS and a phase of mk can be generated:

[0075]

[0076] Where δ is the Dirac function. The subsampling of the signal can be completed by the instruction of the p(n) pulse. The receiving end demodulates and decodes the subsampled signal in sequence, and the symbol synchronization of the receiving end is preliminarily completed.

[0077] S5. Calculate the sum of the variations between the sampling value of the optimal sampling point and the sampling values of the two preceding and following sampling points as a correction variable.

[0078] Due to various factors in satellite spread-spectrum communication systems, such as hardware clock frequency deviation and instability, the timing pulse sequence signal gradually mismatches the received signal symbol rate over time. Consequently, subsequent calculations are required to continuously adjust the timing pulse sequence signal phase to accommodate this change. This adjustment strategy utilizes the squared variation of the signal, specifically fine-tuning the timing pulse sequence signal based on the difference between the first, middle, and last sampling values at a specific time.

[0079] Specifically, the calculation formula for the correction variable is:

[0080] ukt=sign[t(2)-t(1)] / 2+sign[t(3)-t(2)] / 2

[0081] Wherein, t(2) represents the sampling value of the optimal sampling point, t(1) represents the sampling value of the sampling point before the optimal sampling point, t(3) represents the sampling value of the sampling point after the optimal sampling point, t(2)-t(1) represents the variation between the sampling value of the optimal sampling point and the value adopted by the sampling point before it, and t(3)-t(2) represents the variation between the sampling value of the optimal sampling point and the value adopted by the sampling point after it. sign[x] represents the sign function, when x>0, sign[x]=1; when x=0, sign[x]=0; when x<0, sign[x]=-1. When calculating the correction variable, only the relative size of the two sampling points before and after is considered, and the sign of the difference is taken for calculation, without caring about the specific difference. Under normal circumstances, the middle value of these three values is the largest, that is, t(2)>t(1) and t(2)>t(3). At this time, sign[t(2)-t(1)]=1, sign[t(3)-t(2)]=-1, so ukt=0, and there is no need to correct the timing pulse sequence signal. In the actual communication process, when the optimal sampling point gradually advances, the maximum value gradually moves from the middle value to the sampling point at the previous moment, and the ukt value moves from 0 to -1; when the optimal sampling point gradually lags, the maximum value gradually moves from the middle value to the sampling point at the next moment, and the ukt value moves from 0 to 1. At this time, it is necessary to correct the phase of the timing pulse sequence signal.

[0082] S6. Correct the phase of the timing pulse sequence signal based on the calculated correction variable.

[0083] After the system is powered on, the average value of ukt within a preset time period T is calculated and used as the initial value of the time offset uk. Each subsequent time the receiving end performs subsampling and correction, the updated value of uk is: uk←uk+G×(ukt-uk). Here, G is the coefficient of the loop filter, a positive number less than 1. In this embodiment, the value of G is 0.01.

[0084] Then, based on the updated time offset uk, the value of mk is updated using the following formula:

[0085] Determine whether the value of uk at this time satisfies uk>+U. If so, update the value of mk to: mk←mk+1, and update the value of uk to: uk←uk-1. If not, determine whether the value of uk at this time satisfies uk<-U. If so, update the value of mk to: mk←mk-1, and update the value of uk to: uk←uk+1.

[0086] Generally, the adjusted threshold value U is set at around 0.75. Its value is greater than 0.5 because it leaves a certain margin to avoid being affected by noise, causing it to swing back and forth around the threshold value.

[0087] The above-mentioned symbol synchronization method does not require loop gain adjustment based on the signal level value, and can correct the timing pulse sequence signal in a timely and accurate manner. It has a lower operating signal-to-noise ratio threshold, a smaller estimation variance, is more accurate, and is simpler to implement, greatly improving the efficiency and accuracy of symbol synchronization in satellite spread spectrum communication systems.

[0088] To implement the above symbol synchronization method, the embodiment of the present disclosure also provides a satellite spread spectrum communication system based on improved square variation, including a transmitting end and a receiving end. Figure 2 , Figure 2 The structure of the receiving end is shown as an example, including a matched filter despreading module, a signal processing module, a variation statistics module, a timing pulse module, a subsampling module, a demodulation module and a decoding module. Each module can be implemented by FPGA. Please refer to Table 1. Table 1 shows the internal and external interface definitions of the matched filter despreading module, signal processing module, variation statistics module, timing pulse module and subsampling module implemented based on FPGA.

[0089] Table 1 Internal and external interfaces of each module

[0090]

[0091]

[0092] Please refer to Figure 3 , Figure 3 Given Figure 2 The internal structure of each module in the matched filter despreading module. The matched filter despreading module includes a matched despreading module and a matched despreading buffer module. The matched despreading module is used to complete the matched despreading of all sampling points of the input signal r(n) to obtain the complex signal s(n). The matched despreading buffer module is a linear shift register. Every time the receiving end receives a new data, all the data are shifted right by one bit and the last data is discarded. The depth of the matched despreading buffer module is one symbol, that is, a complete spread spectrum code. The storage length of the matched despreading buffer module is K×SPS, where K is the spread spectrum code length and SPS is the number of sampling points per code chip. The total size of the matched despreading buffer module is 2W×(K×SPS) bits, where W is the signal quantization bit width and the coefficient 2 is used to indicate that the stored signal is a complex signal.

[0093] The signal processing module consists of a multiplier, an integration buffer module, and an integration summation module. The multiplier performs the squaring operation on the complex signal s(n). Since the input is a complex signal, the result is the square of the modulus of the complex signal, a positive real signal. The quantization bit width of both its input and output is W. The integration buffer module also uses a linear shift register. Each time a new squared data point is received, all data points are right-shifted by one bit, and the last data point is discarded. The integration buffer module has a length of M symbols, where M is the integration multiplier. It needs to store M complete spreading codes, with a length of M × K × SPS. Therefore, the size of the integration buffer module is W × (M × K × SPS) bits, where W is the quantization bit width of the squared signal. In practical implementation, this can be simplified to W × ((M-1) × K × SPS + 1) bits.

[0094] The integral summation module primarily performs integral summation between data updates. For high-speed communication systems, a hierarchical summation approach can be used. The quantization bit width of both its input and output is W. If the signal-to-noise ratio of the matched despread signal is sufficiently high, the integral buffer and integral summation operations can be omitted.

[0095] The variation statistics module mainly completes operations such as threshold judgment, square variation calculation and loop filtering, and adjustment of the decision threshold, and outputs the mk moment pulse and the mk adjustment pulse.

[0096] When the variance statistics module starts, it first checks the value of mk. If mk is 0, it needs to be initialized: a moment in the preprocessed input signal greater than a threshold value T is selected as the initial value of mk. The phase of the timing pulse train signal is then determined based on the value of mk, and the timing pulse train signal is then determined. The sample value corresponding to the optimal sampling point mk in the preprocessed input signal, as well as the previous and next sampling values of the optimal sampling point, are then read from the timing pulse train signal. The variance of these three sampling values is calculated to determine the correction variable. Finally, the phase of the timing pulse train signal is corrected based on the calculated correction variable: the time offset uk is calculated based on the correction variable. When the uk value reaches the adjusted threshold value U, the mk adjustment decision output is performed, and a direct output of +1 or -1 is sufficient.

[0097] The timing pulse module primarily consists of a phase modulation module, a period counter, and a timing pulse sequence signal module. These three modules are responsible for phase adjustment, period adjustment, and synchronous output of the timing pulse sequence signal, respectively. Initially, when mk is 0, the timing pulse sequence signal module is inactive. After mk is initialized, the timing pulse sequence signal module is synchronously activated to generate a timing pulse sequence signal with mk as the phase and a period of one symbol / spread spectrum code (K×SPS). During operation, the timing pulse module constantly monitors the output of the variation statistics module. When the output is +1 or -1, the period counter adjusts the calculated value of the current period by +1 or -1 to adjust the pulse phase.

[0098] The sub-sampling module completes the sub-sampling of the matched filtered despread signal according to the timing pulse sequence signal output by the timing pulse module.

[0099] To verify the technical effects of the embodiments of the present disclosure, simulation verification results are provided and explained below.

[0100] The parameters of the transmitter, receiver, and wireless channel in this simulation are set as follows:

[0101] (1) Transmitter

[0102] Modulation: QPSK;

[0103] Channel coding: None;

[0104] Spread spectrum multiple: K = 1024;

[0105] Spread spectrum sequence: PN code;

[0106] Forming coefficient: α=0.25.

[0107] (2) Wireless Channel

[0108] Time deviation: 0.1 times the symbol rate;

[0109] Phase bias: random;

[0110] Time deviation: adjustable from 0 to 1;

[0111] Noise: Eb / N0=3dB, SNR=-24dB.

[0112] (3) Receiver

[0113] Number of sampling points per chip: SPS = 4;

[0114] Integration multiple after despreading: M = 16;

[0115] Signal processing threshold: T = 0.4;

[0116] mk adjustment threshold: U=0.75;

[0117] Loop filter coefficient: G = 0.001.

[0118] The waveform of the transmission information at the transmitting end after modulation, spread spectrum and shaping filtering is as follows Figure 4 (a) shows the waveform after shaping filtering with time offset but without noise. Figure 4 (b) shown.

[0119] The original waveform received by the receiving end (with noise, time deviation, phase deviation and time deviation) is as follows Figure 5 (a) shows the original waveform after matched filtering despreading. Figure 5 (b) shown.

[0120] The waveform after the receiving end squares the complex signal after matched filtering despreading is as follows Figure 6 As shown in (a), Figure 6 (a) It can be seen that due to the low signal-to-noise ratio, the complex signal sn) cannot be directly processed after being squared, so it is necessary to integrate the squared signal. The waveform of the integrated signal t(n) is as follows Figure 6 (b) shown.

[0121] Based on the symbol synchronization method described in the embodiment of the present disclosure, simulations were performed for different time offsets uk (uk=0.25, 0.5, 0.75, 1). Figure 7 (a) is the time domain superposition diagram of t(n) when uk=0.25, Figure 7 (b) is the time domain superposition diagram of t(n) when uk=0.5, Figure 7 (c) is the time domain superposition diagram of t(n) when uk=0.75, Figure 7 (d) is the time domain superposition diagram of t(n) when uk=1. Figure 7 (a) to Figure 7 As can be seen in (d), as the uk value increases, the optimal sampling point gradually moves backward.

[0122] Taking uk=0.5 as an example, the optimal sampling point is in the middle of point 2 and point 3. In the ideal noise-free case, the variation between point 2 and point 1 is all +1, while the variation between point 2 and point 3 is 0. In the actual noise case, the variation between point 2 and point 1 is basically all +1, while the variation between point 2 and point 3 is half +1 and half -1. After a long period of loop filtering, the result of the actual noisy case can approach the result of the ideal noise-free case.

[0123] In the symbol synchronization process described in the embodiment of the present disclosure, Figure 7 (a) to Figure 7 (d) The corresponding uk and mk values change with time as shown in the following table: Figure 8 (a) to Figure 8 (d) In the simulation, the initial value of mk is set to the previous sampling point (mk=1) of the optimal sampling point when uk=0.

[0124] Depend on Figure 8 (a) to Figure 8(d) As can be seen, because the initial value of mk (mk = 1) is biased toward the previous sampling point, in all cases, the system performs a backward adjustment of the sampling point position at the start of operation. In subsequent operations, when uk is less than 0.5, the optimal sampling point is closer to the current sampling point, so no further adjustment is required, and the system's actual uk converges to the simulated uk. When the simulated uk is greater than 0.5, the optimal sampling point is closer to the next sampling point, so mk undergoes another backward adjustment. When the system stabilizes, the calculated uk converges to the inverse of the simulated uk value with respect to 1 (1-uk).

[0125] From the above analysis, it can be seen that the satellite spread spectrum communication symbol synchronization scheme based on improved square variation described in the embodiment of the present disclosure can achieve symbol synchronization quickly and accurately.

[0126] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiment. All technical solutions based on the concept of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A satellite spread spectrum communication symbol synchronization method based on improved square variation, characterized in that: Including steps: S1. The receiving end performs matched filtering despreading on all sampling points of the input signal to convert the input signal into a complex signal; S2. Preprocessing the complex signal to improve the signal-to-noise ratio of the complex signal to a preset requirement; S3, comparing the pre-processed input signal with a preset signal amplitude threshold T, and taking the moment when the signal amplitude is greater than the threshold T as the current optimal sampling time point; S4. Determine the phase based on the optimal sampling time point and generate a timing pulse sequence signal; perform sub-sampling, demodulation, and decoding on the complex signal after matched filtering and despreading based on the timing pulse sequence signal; S5, sampling the pre-processed signal using the timing pulse sequence signal, and calculating the sum of the variation between the sampling value of the optimal sampling point and the sampling values of the two preceding and following sampling points as a correction variable; S6. Correcting the phase of the timing pulse sequence signal based on the correction variable; Preprocessing the complex signal comprises the following steps: Complex signal The modulus of the signal is squared to make the signal All become real positive values; if the signal-to-noise ratio is low after squaring and cannot meet the requirements of subsequent threshold judgment, integration can be performed after squaring to improve the signal-to-noise ratio of the signal. The integral calculation formula is: ； Where n represents the sampling time, K represents the spreading code length, SPS represents the number of sampling points per chip, and M is the integration multiple; Sampling the pre-processed signal using the timing pulse sequence signal, and calculating the sum of the variation between the sampling value of the optimal sampling point and the sampling values of the preceding and following sampling points as a correction variable comprises the following steps: The calculation formula of the correction variable is: ukt=sign[t(2)-t(1)] / 2+sign[t(3)-t(2)] / 2 Wherein, t(2) represents the sampling value of the optimal sampling point, t(1) represents the sampling value of the sampling point before the optimal sampling point, and t(3) represents the sampling value of the sampling point after the optimal sampling point; t(2)-t(1) represents the variation between the sampling value of the optimal sampling point and the value adopted by the sampling point before it, and t(3)-t(2) represents the variation between the sampling value of the optimal sampling point and the value adopted by the sampling point after it; sign[x] represents the sign function, when x>0, sign[x]=1; when x=0, sign[x]=0; when x<0, sign[x]=-1; The method for correcting the phase of the timing pulse sequence signal comprises the following steps: After the system is powered on, the average value of ukt within the preset time period T is calculated, and the calculated average value is used as the initial value of the time deviation uk; Each time the receiving end performs subsampling and correction, the updated uk value is: ; Wherein, G is the coefficient of the loop filter, which is a positive number less than 1; Then, update the value of mk based on the updated time bias uk: Determine whether the value of uk at this time satisfies uk>+U. If so, update the value of mk to: , update the value of uk to: If not, then determine whether the value of uk satisfies uk<-U. If so, update the value of mk to: , update the value of uk to: .

2. A satellite spread spectrum communication system based on improved square variation, characterized in that: The satellite spread spectrum communication system adopts the method described in claim 1 to perform symbol synchronization.

3. The satellite spread spectrum communication system based on improved square variation according to claim 2, comprising a transmitting end and a receiving end, characterized in that: The receiving end includes a matched filter despreading module, a signal processing module, a variation statistics module, a timing pulse module, a subsampling module, a demodulation module and a decoding module; The matched filtering despreading module is configured to perform matched filtering despreading on all sampling points of the received input signal to convert the input signal into a complex signal; The signal processing module is configured to pre-process the complex signal; The variation statistics module is configured to perform a threshold judgment on the pre-processed signal to obtain an optimal sampling point; and receive the sampling value of the optimal sampling point and the sampling values of the preceding and following sampling points sent by the timing pulse module, calculate the correction variable, and correct the optimal sampling point based on the correction variable; The timing pulse module is configured to generate a timing pulse sequence signal based on the optimal sampling point calculated by the variation statistics module; and sampling the pre-processed input signal based on the timing pulse sequence signal, and sending the sampling value of the best sampling point and the sampling values of the two preceding and following sampling points to the variation statistics module; The sub-sampling module is configured to perform sub-sampling on the complex signal after matched filtering and despreading based on the timing pulse sequence signal; The demodulation module is configured to demodulate the sub-sampling results; The decoding module is configured to decode the demodulated signal.

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