Self-adaptive capturing method for multi-rate direct sequence spread spectrum signals of low earth orbit satellite

Abstract

The invention discloses a self-adaptive capturing method for a multi-rate direct sequence spread spectrum signal of a low earth orbit satellite, and belongs to the field of low earth orbit satellite communication. According to the method, the multi-rate direct sequence spread spectrum signal self-adaptive capturing method is constructed, multiple sets of direct sequence spread spectrum signal receivers at all rates do not need to be achieved, signal detection and synchronization can be conducted on the multi-rate direct sequence spread spectrum signals only through one set of receiver algorithm, and hardware resource consumption of the receivers is reduced; meanwhile, the capturing method is compatible with processing of multi-rate direct sequence spread spectrum signals, so that risks caused by program reconstruction and interruption of communication services can be avoided; in addition, under the condition that the transmitting power is not changed, the ground terminal adjusts the transmitting rate, on one hand, the transmitting rate can be increased, and the signal exposure time can be shortened; on the other hand, the transmission rate can be reduced to ensure the communication reliability, and the communication is more flexible; according to the invention, on the basis of a DSSS receiver algorithm, signal detection and synchronization of spread spectrum signals with different rates are completed through a self-adaptive adjustment capturing method.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of low earth orbit satellite communication, and relates to an adaptive acquisition method for direct sequence spread spectrum (DSSS) signals with multiple spreading ratios and bandwidths. BACKGROUND

[0002] Compared with high orbit or geostationary orbit satellites (GEO), a low earth orbit satellite (LEO) can achieve global time-sharing coverage, and its transmission delay is relatively low due to its close distance to the ground. This advantage has led to an unprecedented research and development boom in low earth orbit satellite communication systems in recent years, which provides reliable connection support for personal mobile communication. In a low earth orbit satellite communication system, in order to improve transmission effectiveness, there is a demand to increase the communication rate to reduce the transmission time under the premise of fixed transmission power, although this may sacrifice certain communication reliability. Due to the high-speed movement of the low earth orbit satellite, the position relationship between the satellite and the ground terminal changes rapidly with time, resulting in the continuous change of the channel state. Under the condition of fixed terminal transmission power, the signal-to-noise ratio received on the satellite may vary significantly due to the change of the satellite-ground distance and the channel state. In order to ensure the reliability of communication, the terminal usually transmits the uplink signal at a high power to ensure that reliable communication can be carried out in the case of the worst channel condition and the farthest distance. When the signal-to-noise ratio received on the satellite is high, the satellite can respond through the downlink to inform the terminal of the current uplink access state, so that the terminal can adjust the rate of transmitting information according to the response information. In this way, without increasing the original transmission power, the existence time of the uplink signal in the electromagnetic space is reduced, which not only improves the concealment of communication, but also helps the terminal to save energy.

[0003] However, the acquisition, despreading, frame synchronization and other processes of the satellite receiver are extremely sensitive to the communication rate, and are usually designed in advance for a specific communication rate. Although theoretically, different information rates can be adapted by continuous program reconstruction, not only is the reconstruction itself risky, but it may also force the ongoing communication service on the satellite to be interrupted. Based on this, in order to realize the fast synchronization and reliable reception of low earth orbit satellite signals with multiple rates, it is necessary to propose an adaptive acquisition method to complete the detection and synchronization of different direct sequence spread spectrum signals (signals with different rates, different spreading ratios and bandwidths). SUMMARY

[0004] In order to avoid the reconstruction risk caused by the reconstruction of the program to adapt to different service rates and the problem that the ongoing communication service on the satellite is forced to be interrupted, the application aims to provide a low-orbit satellite multi-rate direct sequence spread spectrum signal adaptive acquisition method, by constructing a multi-rate direct sequence spread spectrum signal adaptive acquisition method, without implementing a plurality of sets of direct sequence spread spectrum signal receivers under each rate, only one set of receiver algorithms can perform signal detection and synchronization on multiple types of direct sequence spread spectrum signals, reducing the hardware resource consumption of the receiver; at the same time, since the acquisition method is compatible with the processing of multi-rate direct sequence spread spectrum signals, the risk caused by program reconstruction and the interruption of communication service can be avoided; in addition, under the condition that the transmission power is unchanged, the ground terminal adjusts the transmission rate, on the one hand, the transmission rate can be improved to reduce the signal exposure time, more specifically, the application is based on the DSSS receiver algorithm, and the signal detection and synchronization of the spread spectrum signal of different rates are completed by adaptively adjusting the acquisition method; on the other hand, the transmission rate can be reduced to ensure the communication reliability, so that the communication is more flexible.

[0005] The application aims to achieve the following technical solutions:

[0006] The application discloses a low-orbit satellite multi-rate direct sequence spread spectrum signal adaptive acquisition method, characterized by:

[0007] Step one, obtaining a baseband digital signal r[n] through sampling and quantization;

[0008] Step two, performing initial frequency offset compensation on r[n] to obtain a baseband digital signal r'[n] after half-symbol compensation;

[0009] Step three, generating a local spread spectrum code sequence b[k] through a longest linear feedback shift register;

[0010] Step four, performing Fourier transform on the inverse sequence result b'[k] of b[k], and performing cyclic shift on the Fourier transform result B'[k] to obtain a plurality of groups of frequency domain template sequences for compensating step frequency offset

[0011] Step five, performing Fourier transform on r[n] and r'[n] to obtain R[n] and R'[n], and performing complex multiplication operation on R[n] and R'[n] and respectively, and performing inverse Fourier transform on the complex multiplication operation result to obtain an acquisition plane result P o [u τ ,z f ];

[0012] Step six, performing differential coherent accumulation on the acquisition plane result P o [u τ ,z f ] of each symbol to obtain a multi-symbol accumulation result P acc[u τ ,z f ] by detecting the peak position of P acc [u τ ,z f ] to achieve the detection and synchronization of the multi-rate direct sequence spread spectrum signal.

[0013] Further, the step one implementation method is as follows:

[0014] The received radio frequency signal r(t) at the receiving end is represented as:

[0015]

[0016] Wherein, t is time; A r is the gain coefficient of the reception; s(t) is the baseband signal; f c is the carrier frequency; θ r is the initial phase of the received signal; w(t) is the additive white Gaussian noise AWGN introduced in the receiving process.

[0017] r(t) is down-converted with the local quadrature local oscillator to move the spectrum of the radio frequency signal to the baseband, and the baseband signal is sampled and quantized through analog-to-digital conversion (ADC) to obtain the baseband digital signal r[n], which is represented as:

[0018]

[0019] Wherein, n is discrete time; T s is the sampling interval; τ is the time delay between the transmitter and the receiver; f d is the Doppler frequency offset; is the clock difference between the transmitter and the receiver; w[n] is the discrete noise signal. τ and are caused by the inability to accurately synchronize the transmitter and the receiver. f d is the Doppler effect caused by the relative motion between the transmitter and the receiver, which is represented as:

[0020]

[0021] Wherein, v represents the relative motion speed of the transmitter and the receiver, c represents the speed of light 3×10 8 m / s, f c represents the radio frequency carrier frequency.

[0022] In formula (2), at the receiving end, f d and can be superimposed, and the time delay τ brings the change of the phase of the sampling signal, so formula (2) is simplified as:

[0023]

[0024] Wherein, The combined frequency offset effect, The combined time delay effect. Formula (4) is further simplified as: r And Equivalent to a constant, formula (4) is further simplified as:

[0025]

[0026] Where A is a constant, expressed as A r The effect of superimposing with τ on the baseband signal.

[0027] Further, the implementation method of step two is as follows:

[0028] A fixed frequency offset carrier signal γ[n] is generated using a direct digital frequency synthesizer DDS, expressed as:

[0029]

[0030] Where f0 is the initial carrier frequency of compensation, which is related to the speed of the signal, and the value is R s Indicates the symbol rate of the signal before spread spectrum. γ[n] is multiplied by r[n] to obtain the initial frequency offset compensated signal r′[n], and the expression of r′[n] is:

[0031]

[0032] Where, It still conforms to the Gaussian distribution, consistent with w[n].

[0033] Further, the implementation method of step three is as follows:

[0034] The local spread spectrum code sequence at the receiving end is generated by using the longest linear feedback shift register, which is the same as the transmitting end. The local spread spectrum code sequence b[k] is expressed as:

[0035]

[0036] Where x i is the current data stored in the i-th register; c i is the feedback coefficient of the i-th register, which means that the i-th register participates in feedback when it is 1; l reflects the specific speed of the direct spread spectrum signal. 2 l Indicates the length of the local spread spectrum code sequence b[k], that is, 0≤k≤2 l -1, the spread spectrum code period of different speed spread spectrum signals is inconsistent.

[0037] Further, the implementation method of step four is as follows:

[0038] b[k] has good autocorrelation, and the autocorrelation of b[k] is represented by the following formula:

[0039]

[0040] wherein, R[ρ] represents the result of cyclic shift of b[k] by ρ bits. Formula (9) represents that R[ρ] is 1 only when ρ=0, and R[ρ] is close to 0 in other cases. By using the good autocorrelation characteristics of the local spreading code, if the baseband signal in r[n] or r'[n] is correlated with the local spreading code sequence by sliding, a larger value appears only when the code phases are completely aligned, and the values of other cases are close to 0. When the noise component in r[n] or r'[n] is correlated with the local spreading code sequence by sliding, since the noise component conforms to Gaussian random distribution, the noise component and the local spreading code sequence cannot be matched, and the sliding correlation result still conforms to Gaussian random distribution.

[0041] The sliding correlation operation can be converted into convolution operation, that is, convolution operation of a sequence and the inverse sequence of another sequence. Further, the convolution operation in the time domain can be converted into complex multiplication operation in the frequency domain and then inverse Fourier transform.

[0042] r[n] and r'[n] are correlated with b[k] by sliding after gradually compensating for the frequency offset, and the step frequency offset compensation of r[n] and r'[n] is converted into negative step frequency offset compensation of b[k].

[0043] To convert the operation in the time domain to the frequency domain, b[k] is first processed in inverse order to obtain b'[k], and then b'[k] is compensated for step frequency offset, and the following relationship exists by using the cyclic shift property of Fourier transform:

[0044]

[0045] wherein, B'[k] is the result of cyclic shift of the Fourier transform of b'[k] by ρ. It can be seen from formula (10) that when B'[k] is cyclically right-shifted, a negative frequency carrier is compensated, and when B'[k] is cyclically left-shifted, a positive frequency carrier is compensated.

[0046] Further, the method of step five is as follows:

[0047] According to formula (11), r[n] and r'[n] are Fourier transformed to obtain R[n] and R'[n]:

[0048]

[0049] Formula (11) and formula (10) both involve 2 l point discrete Fourier transform. For any N=2 lThe sequence of points x[n] whose discrete Fourier transform is

[0050]

[0051] where the rotation factor v = 1, 2,..., N - 1.

[0052] Based on the "decimation in time (DIT) base-2-FFT algorithm" calculation formula (12), x[n] is divided into x1[n] and x2[n] according to the parity of n, which is expressed as follows:

[0053]

[0054] where,

[0055] Substitute formula (13) into formula (12) to derive:

[0056]

[0057] where, Therefore, formula (14) is transformed into the following formula:

[0058]

[0059] where X1[v] and X2[v] represent v taking 0, 1, 2,..., N - 1, respectively. The Fourier transform results of the time series x1[n] and x2[n].

[0060] Based on formula (15), the other half X[v] is converted:

[0061]

[0062] Observing formula (15) and formula (16), it is found that the N-point DFT calculation can be converted into two N / 2-point DFT calculations. Based on this decomposition idea, all N-point DFTs can be iterated into several 2-point DFT calculations. A single 2-point DFT calculation is recorded as a base-2 butterfly unit. The expression of the base-2 butterfly unit is:

[0063]

[0064] where d in1 and d in2 are the two points input to the base-2 butterfly unit, and the initial values are the values of the adjacent two points of x[n]; W in is the corresponding rotation factor; d out1 and d out2 are the base-2 butterfly unit output results, which are used as the input for the next calculation.

[0065] The N-point inverse Fourier transform can also be completed by equation (17), which requires first conjugating the sequence to be inverse Fourier transformed, then performing N-point Fourier transform, multiplying the Fourier transform result by the coefficient after conjugating it, and performing inverse Fourier transform , which is the result of N-point inverse Fourier transform.

[0066] Based on the iterative operation of equation (17), the conversion between inverse Fourier transform and Fourier transform, the Fourier transform results R[n], R'[n] and B'[k] of r[n], r'[n] and b'[k] can be obtained. The following takes R[n], R'[n] respectively and performs complex multiplication operation, and then inverse Fourier transform to obtain multiple sets of correlation results. The multiple sets of correlation results of inverse Fourier transform are recorded as capture correlation planes, and the capture plane P o is expressed by the following relation:

[0067] P o [u τ ,z f ] = S o [u τ ,z f ] + W o [u τ ,z f ] (18)

[0068] wherein u τ represents time delay dimension, z f represents frequency offset dimension, and S o represents the result of sliding correlation between signal component in the oth symbol period and local spreading code sequence; W o represents the result of sliding correlation between noise component in the oth symbol period and local spreading code sequence. Equation (18) represents the size of capture plane and u τ ,z f , wherein the size of capture plane is related to u τ , which is affected by the direct spread signal rate.

[0069] Further, the step six is implemented as follows:

[0070] The symbol capture planes obtained in step five are differentially coherently accumulated.

[0071] The conjugate operation is performed on P o-1 of the previous symbol, and the conjugate result of P o-1 is multiplied by P o of the next symbol. The multiple sets of differential calculation results are added and the modulus value is taken to obtain the multi-symbol accumulation result P acc , which is expressed by the following equation:

[0072]

[0073] wherein, represents the square value of the signal component in the 1st symbol capture plane in the multi-symbol accumulation; represents the conjugate value of the noise component in the dth symbol capture plane in the multi-symbol accumulation; W d+1 [u τ ,z f ] represents the noise component in the d+1th symbol capture plane in the multi-symbol accumulation; S d [u τ ,z f ] represents the signal component in the dth symbol capture plane; represents the conjugate value of the signal component in the d+1th symbol capture plane. Analysis by formula (19) shows that, in the multi-symbol accumulation process, the component containing noise has less influence on P acc with the increase of the accumulation number, because the noise correlation of the two symbols before and after is poor.

[0074] At a certain [u τ ,z f ] position, the multi-symbol accumulation result P acc has a correlation peak, and the capture plane results P o [u τ ,z f ] of the symbols are differentially coherently accumulated to obtain the multi-symbol accumulation result P acc [u τ ,z f ], and the multi-rate direct sequence spread spectrum signal is detected and synchronized by detecting the peak position of P acc [u τ ,z f ], the peak position of P acc corresponds to the actual position of the signal, and the [u τ ,z f ] at the position is the synchronization result.

[0075] Further, the low-orbit satellite multi-rate direct sequence spread spectrum signal is synchronized by the adaptive capture method, that is, the detection and synchronization of the compatible multi-rate direct sequence spread spectrum signal are realized, and the consumption of hardware resources caused by using multiple sets of direct sequence spread spectrum receivers at different rates is avoided.

[0076] Further, the signal detection and synchronization of the compatible multi-type direct sequence spread spectrum signal are realized by the adaptive capture method, and the risk caused by program reconstruction and the interruption of communication services can be avoided.

[0077] Further, the adaptive acquisition method of the low-orbit satellite multi-rate direct sequence spread spectrum signal realizes adaptive acquisition of the signal, and under the condition that the transmission power is constant, the ground terminal can adjust the transmission rate, on the one hand, can improve the transmission rate to reduce the signal exposure time, and on the other hand, can reduce the transmission rate to ensure the communication reliability, so that the communication is more flexible.

[0078] Beneficial effects:

[0079] 1. The adaptive acquisition method of the low-orbit satellite multi-rate direct sequence spread spectrum signal discloses a multi-point compatible FFT calculation method, which is compatible with the detection and synchronization of the multi-rate direct sequence spread spectrum signal. The hardware resource consumption caused by using multiple sets of direct sequence spread spectrum receivers under each rate is avoided.

[0080] 2. The adaptive acquisition method of the low-orbit satellite multi-rate direct sequence spread spectrum signal discloses a self-designed acquisition method, which can complete the acquisition of the multi-rate direct sequence spread spectrum signal by using only one receiver. Since the adaptive acquisition method is compatible with the signal detection and synchronization of multiple types of direct sequence spread spectrum signals, the risk and communication service interruption caused by program reconstruction can be avoided.

[0081] 3. The adaptive acquisition method of the low-orbit satellite multi-rate direct sequence spread spectrum signal can complete the detection and synchronization of the multi-rate direct sequence spread spectrum signal by adaptively calculating the multi-symbol accumulation result of each rate direct sequence spread spectrum signal. Therefore, under the condition that the transmission power is constant, the ground terminal can adjust the transmission rate, on the one hand, can improve the transmission rate to reduce the signal exposure time, and on the other hand, can reduce the transmission rate to ensure the communication reliability, so that the communication is more flexible. BRIEF DESCRIPTION OF DRAWINGS

[0082] In order to further illustrate the specific schemes in the embodiments of the present application or the prior art, the required drawings will be briefly introduced. Obviously, the drawings described below are only examples of the embodiments of the present application, and are intended to provide a clearer understanding for those skilled in the art. It should be noted that based on these drawings, those skilled in the art can deduce other related drawings without creative work.

[0083] Figure 1 It is a two-dimensional search plane schematic diagram for the acquisition process;

[0084] Figure 2 It is a flow chart of the adaptive acquisition method of the low-orbit satellite multi-rate direct sequence spread spectrum signal of the present application;

[0085] Figure 3 It is a hardware structure diagram of m sequence;

[0086] Figure 4 It is a hardware structure diagram of multi-point compatible FFT;

[0087] Figure 5 Fig. 3 is a schematic diagram of a multi-rate DSSS acquisition result, wherein Figure 5 (a) is the acquisition result of a symbol rate 8Ksps DSSS signal, and (b) is the acquisition result of a symbol rate 32Ksps DSSS signal. DETAILED DESCRIPTION

[0088] In order to make the present application clearer, the present application will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0089] Embodiment 1

[0090] In order to verify the feasibility of the method, a BPSK-DSSS modulation system is taken as an example, wherein the system parameters are as follows: signal symbol rate R s : 8Ksps and 32Ksps; signal spreading ratio L: 4096 and 1024; signal signal-to-noise ratio 5dB; radio frequency f c : 1.5GHz.

[0091] In the acquisition, the range of the frequency offset and the code phase delay is determined by searching the signal at each frequency offset and code phase, and the access time of the signal is determined. In this process, the acquisition can be regarded as searching in a very large two-dimensional plane, as shown in Figure 1 , a two-dimensional search is performed on the frequency offset and the time delay of the access signal within a certain range, and after the frequency offset and the time delay of the access signal are found, the detection and synchronization result is output. As shown in Figure 2 , the adaptive acquisition method for a low-orbit satellite multi-rate DSSS signal disclosed in the present example has the following specific implementation steps:

[0092] Step 1: Sampling and quantization of the radio frequency signal are completed based on the AD9371 platform to obtain a baseband digital signal r[n]. The sampling frequency of the analog-to-digital converter ADC is

[0093] The baseband digital signal r[n] needs to be buffered before subsequent step processing. In the present example, the sampled baseband digital signal is subjected to "ping-pong buffering" at M=2 times the oversampling rate: in the first storage period, the data for acquisition is stored in the first storage block; in the subsequent second storage period, the input data is stored in the second storage block, and in this process, the data is extracted from the first storage block at the same time for subsequent calculation and processing; in the third storage period, the new data is overwritten to the first storage block, and at the same time, the data in the second storage block is extracted for processing. Such ping-pong operation realizes efficient switching between data storage and data reading in storage processing, ensures that the data can be seamlessly transmitted to subsequent processing, and thus optimizes the performance of the entire system.

[0094] In this example, there are two storage blocks, and the depth of each storage module is 16384. For the 2 times oversampled digital baseband signal r[n], for the spread spectrum ratio L is 1024 and 4096, one storage period can store 8 and 2 symbol length codes respectively, and one storage period can also extract and seamlessly transfer the 16384 codes stored in a single storage block to the subsequent processing.

[0095] Step two, for the r[n] cached and output in step one, R s / 2 half symbol carrier compensation is performed. At the same time, the uncompensated signal and the compensated signal are output at the same time. The method for R s / 2 carrier compensation is described as follows:

[0096] In the half symbol compensation step of the embodiment of the application, for the received direct sequence spread spectrum signal, there can be multiple symbol rates R s and multiple spread spectrum ratios L, and the expression of the stored carrier sequence is

[0097]

[0098] It can be seen that the carrier sequence to be stored in the half symbol compensation memory is related to the spread spectrum ratio L and the oversampling rate M, and is irrelevant to the symbol rate R s .

[0099] Therefore, for the received direct sequence spread spectrum signal, there can be multiple symbol rates, and here only the carrier sequence corresponding to the maximum L·M needs to be stored, and other carrier sequences can be obtained by transforming the read address of the memory. Multiply r[n] by the carrier sequence represented by formula (20) to obtain the half symbol compensated baseband digital signal r'[n].

[0100] Finally, r[n] and r'[n] are output to the subsequent steps for processing at the same time.

[0101] Step three, direct spread spectrum technology realizes the expansion of the original signal spectrum by directly multiplying the signal with a spread spectrum code with a wide spectrum, so that the signal is difficult to be distinguished in the noise. In order to successfully despread and recover the signal at the receiving end, the same spread spectrum code as the sending end must be used, and this mechanism gives the spread spectrum signal high security.

[0102] In this example, the spread spectrum code is required to have good randomness to reduce the risk of signal interception and analysis, and finally the longest linear feedback shift register sequence, also known as m-sequence, is used, and its implementation architecture is shown in Figure 3 .

[0103] The sequence generates the local spreading code sequence b[k] by performing linear multiplication and addition operation on the value in the register and feeding back the result to the first bit of the register, and performing linear shift operation on the value in the register, wherein k takes values related to the signal rate and the spreading ratio, and is [0, L-1].

[0104] Step four, after inverse sequence processing of the stored local spreading code sequence b[k], the sequence is subjected to cyclic shift processing to simulate the carrier to be compensated in each frequency offset channel on the acquisition plane, thereby obtaining the frequency domain template sequence of each group of compensated step frequency offset.

[0105] Step five, r[n] and r'[n] are first decimated to become single-over-sampling four-way signals. The four-way signals are subjected to FFT calculation, and four-way FFT calculation results are outputted.

[0106] In the example, the FFT calculation step uses an independently designed FFT hardware, and the implementation architecture thereof is as shown in Figure 4 It can be compatible with 2 l , l = 6, 7, 8, 9, 10, 11, 12 or 13 point calculation: when performing 4096 point FFT calculation, it needs 11 levels of butterfly operation; when performing 1024 point FFT calculation, it needs 9 levels of butterfly operation. Each level of butterfly operation needs to be performed 4096 times of base 2 butterfly calculation, and this calculation process is realized by time-sharing multiplexing 8 base 2 butterfly units.

[0107] Take the FFT results R[n] and R'[n] of the signal respectively, multiply them with , and call the FFT hardware to perform IFFT calculation on the multiplied results, store the IFFT calculation results, and obtain the acquisition plane results.

[0108] Step six, 64 symbols are accumulated by using the differential coherent accumulation method, and finally the multi-symbol accumulation result P acc is outputted. Based on the set decision threshold, the multi-symbol accumulation result is judged, and the final detection result and the estimated frequency shift and time delay result are outputted. The adaptive acquisition results of the two types of spread spectrum signals are as shown in Figure 5 .

[0109] The above detailed description further describes the purpose, technical scheme and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application, and is not used to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. An adaptive acquisition method of a low-orbit satellite multi-rate direct sequence spread spectrum signal, characterized in that: Step one, obtaining a baseband digital signal r[n] through sampling and quantization; Step two, performing initial frequency offset compensation on r[n] to obtain a baseband digital signal r'[n] after half-symbol compensation; Step three, generating a local spreading code sequence b[k] through a longest linear feedback shift register; Step four, Fourier transform the inverse sequence result b'[k] of b[k], and circularly shift the Fourier transform result B'[k] to obtain a plurality of groups of frequency domain template sequences compensating for step frequency offset Step five, Fourier transform r[n] and r'[n] to get R[n] and R'[n], multiply R[n] and R'[n] with respectively, and inverse Fourier transform the result after multiplication to get the capture plane result P o [u τ ,z f ] Step six, the multi-symbol accumulation result P o [u τ ,z f ] is obtained by differentially coherent accumulation of the symbol-by-symbol capture plane results P acc [u τ ,z f ], and the detection and synchronization of the multi-rate direct sequence spread spectrum signal is achieved by detecting the peak position of P acc [u τ ,z f ].

2. The adaptive acquisition method for low-Earth orbit satellite multi-rate direct sequence spread spectrum signals as described in claim 1, characterized in that: The implementation method of step one is as follows: The radio frequency signal r(t) received by the receiving end is expressed as: where t is time; A r is the received gain coefficient; s(t) is the baseband signal; f c is the carrier frequency; θ r is the initial phase of the received signal; w(t) is the additive white Gaussian noise (AWGN) introduced during reception; r(t) is down-converted with a local oscillator in quadrature to move the frequency spectrum of the radio frequency signal to the baseband, and the baseband signal is sampled and quantized through analog-to-digital conversion (ADC) to obtain a baseband digital signal r[n], which is expressed as: where n is the discrete time; T s is the sampling interval; τ is the time delay between transmission and reception; f d is the Doppler frequency offset; is the difference between the local oscillators of the transmitter and receiver; w[n] is the discrete noise signal; τ and are caused by the inaccurate synchronization between the transmitter and receiver; f d is the Doppler effect caused by the relative motion between the transmitter and receiver, which is expressed as: wherein v represents the relative motion speed of the transceiver, c represents the speed of light 3 x 10 8 m / s, f c represents the radio frequency carrier frequency; In Equation (2), at the receiving end, f d With This can be represented superimposed, the delay τ brings the change of the phase of the sampling signal, so Equation (2) is simplified as: wherein is the combined frequency offset influence, is the combined delay influence; A r and equivalent to a constant, equation (4) further simplifies to: Where A is a constant, expressed as A r The effect of τ on the baseband signal after superposition.

3. The adaptive acquisition method for low-Earth orbit satellite multi-rate direct sequence spread spectrum signals as described in claim 2, characterized in that: The implementation method of step two is as follows: A carrier signal γ[n] with a fixed frequency offset is generated using a direct digital frequency synthesizer (DDS), and the expression is: where f0 is the initial carrier frequency of the compensation, which is related to the signal rate, and has a value of R s denotes the symbol rate of the signal before spreading; and γ[n] is multiplied by r[n] to obtain the initial frequency offset compensated signal r'[n], which has the expression: r'[n] = γ[n]r[n] wherein, It is still Gaussian, consistent with w[n].

4. The adaptive acquisition method for low-Earth orbit satellite multi-rate direct sequence spread spectrum signals as described in claim 3, characterized in that: The implementation method of step three is as follows: A longest linear feedback shift register is used to generate a local spreading code sequence for the receiving end, which is the same as that of the sending end; the local spreading code sequence b[k] is expressed as: Where, x i The current data is stored in the i-th level register; c i It is the feedback coefficient of the i-th level register, where 1 indicates that the i-th level register participates in the feedback; l reflects the specific rate level of the direct-sequence spread spectrum signal, 2 l This represents the length of the local spreading code sequence b[k], i.e., 0≤k≤2. l -1, the spreading code period is inconsistent for spreading signals with different rates.

5. A method for adaptive acquisition of a low orbit satellite multi-rate direct sequence spread spectrum signal as recited in claim 4, further characterized by: The implementation method of step four is as follows: b[k] has good autocorrelation, and the autocorrelation of b[k] is expressed by the following formula: wherein, R[ρ] represents the result of b[k] cyclic shift ρ bits; R[ρ] represented by formula (9) is 1 only when ρ = 0, and is close to 0 in other cases; by using the good autocorrelation characteristics of the local spreading code, if the baseband signal in r[n] or r'[n] is slidingly correlated with the local spreading code sequence, a larger value will only appear when the two code phases are completely aligned, and the values in other cases are close to 0; when the noise component in r[n] or r'[n] is slidingly correlated with the local spreading code sequence, since the noise component conforms to Gaussian random distribution, the noise component and the local spreading code sequence cannot be matched, and the sliding correlation result still conforms to Gaussian random distribution; After step-by-step compensation of the frequency offset, r[n] and r'[n] are subjected to a sliding correlation operation with b[k], and the step-by-step compensation of the frequency offset of r[n] and r'[n] can be converted into the compensation of a negative step-by-step frequency offset of b[k]; b[k] is processed in reverse order to obtain b'[k], and then b'[k] is compensated for a step-by-step frequency offset, and the Fourier transform circular shift property exists the following relationship: wherein B'[k] is the result of the Fourier transform of b'[k] followed by a circular shift of p; as can be seen from equation (10) a circular right shift of B'[k] corresponds to a compensation of the negative frequency carrier and a circular left shift corresponds to a compensation of the positive frequency carrier.

6. A method for adaptive acquisition of a low orbit satellite multi-rate direct sequence spread spectrum signal as recited in claim 5, further characterized by: The implementation method of step five is as follows: According to formula (11), r[n] and r'[n] are subjected to Fourier transform to obtain R[n] and R'[n]: Both equation (11) and equation (10) involve the discrete Fourier transform of 2 l points; for any N = 2 l point sequence x[n] whose discrete Fourier transform is wherein the rotation factor v = 1, 2,..., N - 1; Based on the "decimation in time (DIT) base-2-FFT algorithm", formula (12) is calculated, x[n] is divided into x1[n] and x2[n] according to the parity of n, and the following formula is used to express: wherein Substitute formula (13) into formula (12) to derive: wherein So equation (14) is transformed into the following equation: wherein X1[v] and X2[v] represent the v-th element of X1 and X2, respectively the Fourier transform results of the time series x1[n] and x2[n] Based on formula (15), the other half X[v] is converted: From formula (15) and formula (16), it is found that N-point DFT calculation can be converted into two N / 2-point DFT calculations, and based on this decomposition idea, all N-point DFTs can be finally iterated into several 2-point DFT calculations, and a single 2-point DFT calculation is recorded as a base-2 butterfly unit; the expression of the base-2 butterfly unit is: where d in1 and d in2 are the values of the two adjacent points of the initial value x[n] input to the base-2 butterfly unit; W in is the corresponding rotation factor; d out1 and d out2 are the output results of the base-2 butterfly unit, serving as the input for the next calculation; In addition, the N-point inverse Fourier transform can also be completed by formula (17), which needs to first conjugate the sequence to be inverse Fourier transformed, then perform N-point Fourier transform, multiply the Fourier transform result by the coefficient after conjugating the Fourier transform result That is, the result of the N-point inverse Fourier transform; Based on the iterative operation of formula (17), the conversion of inverse Fourier transform and Fourier transform, the Fourier transform results R[n], R'[n] and B'[k] of r[n], r'[n] and b'[k] can be obtained; the following takes R[n], R'[n] respectively and After the complex multiplication operation, the inverse Fourier transform is performed to obtain a plurality of groups of correlation results, the plurality of groups of correlation results of the inverse Fourier transform are recorded as a capture correlation plane, and the capture plane P o The following relationship is used to express: P o [μ τ ,z f ]=S o [u τ ,z f ]+W o [u τ ,z f ] (18) where u τ represents the time delay dimension, z f represents the frequency offset dimension, and represents the horizontal and vertical coordinate axes of the acquisition plane; S o represents the result of the sliding correlation of the signal component in the oth symbol period with the local spreading code sequence; W o represents the result of the sliding correlation of the noise component in the oth symbol period with the local spreading code sequence; and equation (18) represents the size of the acquisition plane and u τ , z f , where the DSSS signal rate affects u τ and thus the size of the acquisition plane.

7. A method for adaptive acquisition of a low-orbit satellite multi-rate direct sequence spread spectrum signal as claimed in claim 6, characterized by: The implementation method of step six is as follows: The symbol acquisition planes obtained in step five are subjected to differential coherent accumulation; P o-1 of the previous symbol is multiplied by the P o-1 of the next symbol; and o the conjugate result of the P The multi-signature accumulation result P is obtained by adding the multiple sets of difference calculation results and taking the modulus value. acc is expressed by the following equation: wherein represents the square value of the signal component in the 1st symbol capture plane during multi-symbol accumulation; represents the conjugate value of the noise component in the dth symbol capture plane during multi-symbol accumulation; W d+1 [u τ ,z f ] represents the noise component in the d+1th symbol capture plane during multi-symbol accumulation; S d [u τ ,z f ] represents the signal component in the dth symbol capture plane; represents the conjugate value of the signal component in the d+1th symbol capture plane; analysis by equation (19) shows that, during multi-symbol accumulation, the component containing noise has less influence on P acc as the number of accumulations increases, because the noise correlation between the two symbols is poor. In a certain [u τ ,z f Location, multi-symbol accumulation result P acc A correlation peak appears, which will capture the plane result P of each symbol. o [u τ ,z f The multi-symbol accumulation result P is obtained by performing differential coherent accumulation. acc [u τ ,z f ], by detecting P acc [u τ ,z f The peak position of P reaches the detection and synchronization of multi-rate direct sequence spread spectrum signals. acc The peak position corresponds to the actual position of the signal, and the [μ] at that position τ ,z f This is the synchronization result.

8. A method for adaptive acquisition of a low orbit satellite multi-rate direct sequence spread spectrum signal as recited in claims 1, 2, 3, 4, 5, 6, or 7, characterized by: In the adaptive acquisition method, a multi-point compatible FFT calculation method is designed to realize the synchronization of a low-orbit satellite multi-rate direct sequence spread spectrum signal, that is, to realize the detection and synchronization of a multi-rate direct sequence spread spectrum signal, thereby avoiding the consumption of hardware resources caused by using multiple sets of direct sequence spread spectrum receivers at different rates.

9. A method for adaptive acquisition of a low orbit satellite multi-rate direct sequence spread spectrum signal as recited in claims 1, 2, 3, 4, 5, 6, or 7, characterized by: Through the adaptive acquisition method, the signal detection and synchronization of multiple types of direct sequence spread spectrum signals are realized, and only one receiver is needed to complete the acquisition of multi-rate direct sequence spread spectrum signals, which can avoid the risk of program reconstruction and the interruption of communication services.

10. A method for adaptive acquisition of a low orbit satellite multi-rate direct sequence spread spectrum signal as recited in claims 1, 2, 3, 4, 5, 6, or 7, characterized by: The multi-rate direct spread signal is detected and synchronized by self-adapting calculation of multi-symbol accumulation results of each rate direct spread signal. Under the condition of invariable transmitting power, the ground terminal can adjust the transmitting rate, which can improve the transmitting rate and reduce the signal exposure time, and can reduce the transmitting rate to ensure the communication reliability, so that the communication is more flexible.

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