A high-sensitivity fast acquisition method and system for a dynamic satellite-to-ground environment

Abstract

The application discloses a high-sensitivity fast acquisition method and system in a star-ground dynamic environment, and the method comprises the following steps: S1, a transmitting end transmits a communication signal conforming to a target communication frequency, and a receiving end receives the communication signal; S2, a plurality of local signals with different frequency points are generated in a frequency search interval through a local parallel digital control oscillator (NCO) group, and FFT-IFFT acquisition determination is performed on the communication signal; S3, if the acquisition determination fails, the frequency search interval is adjusted, and the step S2 is performed again; if the acquisition determination fails in two adjacent frequency search intervals, sideband interval estimation is performed on the two adjacent frequency search intervals, and FFT-IFFT acquisition determination is performed according to the sideband interval estimation result; the step is repeated until the acquisition determination succeeds. Under the conditions that the detection probability is more than 99%, the minimum carrier-to-noise ratio is 34.66 dB, and the minimum signal-to-noise ratio is 16 dB, the receiver sensitivity of-130 dBm can be realized, the total Doppler frequency offset acquisition range can reach 140 kHz, and the acquisition time can be as low as 0.8 seconds.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-speed moving low-orbit micro-satellite inter-satellite and ground communication, and relates to a high-sensitivity fast acquisition method and system for inter-satellite and ground dynamic environment. BACKGROUND

[0002] In recent years, space technology has made significant progress, promoting the development of satellite constellation and formation technology. Especially low-orbit small satellites, due to its small size, short development cycle, low cost, flexible launch and other advantages, through the formation of cooperative work can achieve similar functions with traditional "big satellite", so it has become the focus of domestic and foreign research. The orbital characteristics of low-orbit satellites give them high efficiency and short observation window. However, this high-speed movement will cause the state of satellite-ground communication link to change rapidly, introducing tens of kHz or even hundreds of kHz Doppler shift to the signal, increasing the difficulty of signal demodulation. Therefore, the acquisition capability of inter-satellite and ground communication directly affects the performance of low-orbit satellites. Further, in the widely used pseudo-code spread spectrum TT&C transponder, the acquisition of pseudo-code is the first step to establish the communication link, and is also a crucial step. The success or failure of the pseudo-code acquisition process is directly related to the stability and reliability of the communication link. Only after successfully acquiring the pseudo-code and establishing a reliable inter-satellite and ground communication link, the satellite can receive remote control commands from the ground and transmit data back to the ground, so as to successfully complete its scheduled tasks.

[0003] Due to the high speed of low-orbit satellites, the long distance of inter-satellite and ground communication, and the particularity of the work task, the inter-satellite and ground communication faces the challenges of large Doppler shift, weak received signal power and signal burst reception. In view of these communication characteristics, an effective acquisition method with anti-frequency offset capability, high sensitivity and fast signal acquisition capability is needed. The traditional acquisition method can be divided into serial acquisition and parallel acquisition, where serial acquisition is also called sliding correlation method. The principle of serial acquisition is simple, easy to implement, and resource consumption is small, but the calculation amount is large, the acquisition time is long, and it is only suitable for low dynamic and short pseudo-code length signal acquisition. Parallel acquisition includes FFT acquisition method and matched filter method. FFT acquisition method uses fast Fourier transform instead of correlation operation in pseudo-code acquisition, which has higher efficiency than serial acquisition. The matched filter method uses a matched filter instead of an integrator for correlation operation in the acquisition process, which has good acquisition efficiency, but the resource consumption is extremely large, and the pseudo-code rate and sampling frequency have strict requirements. In addition, the traditional FFT fast acquisition method has the defect of low sweep efficiency, which directly affects the acquisition time of the receiver and limits the effective use of computing resources. Therefore, there is a certain optimization space for the traditional FFT acquisition method, and the acquisition method needs to be improved and improved to meet the needs of inter-satellite and ground communication. SUMMARY

[0004] To solve the above problems, the application provides a high-sensitivity fast acquisition method and system for a star-ground dynamic environment, which takes the FFT fast acquisition method of sideband estimation as the core, realizes high-dynamic and high-sensitivity fast acquisition of a pseudo-code spread spectrum communication signal between stars and the ground, and is verified by actual hardware to be feasible and has advantages in sensitivity, Doppler frequency offset resistance and acquisition time compared with the traditional FFT acquisition method, and has good practicability.

[0005] The technical scheme adopted by the application is as follows:

[0006] A high-sensitivity fast acquisition method for a star-ground dynamic environment, comprising the following steps:

[0007] S1. The transmitting end obtains a communication signal conforming to a target communication frequency after pulse coding modulation, spread spectrum and frequency modulation of communication data and transmits the communication signal, and the receiving end receives the communication signal;

[0008] S2. A plurality of local signals with different frequencies are generated in a frequency search interval by a local parallel digital control oscillator (NCO) group, and FFT-IFFT acquisition determination is performed on the communication signal;

[0009] S3. If the acquisition determination fails, the frequency search interval is adjusted, and step S2 is performed again; if the acquisition determination fails in two adjacent frequency search intervals, sideband interval estimation is performed on the two adjacent frequency search intervals, and FFT-IFFT acquisition determination is performed according to the sideband interval estimation result; this step is repeated until the acquisition determination succeeds.

[0010] Further, the FFT-IFFT acquisition determination comprises the following specific steps:

[0011] The plurality of local signals with different frequencies are used to perform down-conversion processing on the communication signal received by the receiving end, respectively, to obtain a plurality of baseband spread spectrum signals;

[0012] The plurality of obtained baseband spread spectrum signals are subjected to fast Fourier transform (FFT) processing, respectively, to obtain the FFT result of each baseband spread spectrum signal;

[0013] Meanwhile, the receiving end generates a local pseudo-code, and the local pseudo-code is subjected to FFT processing to obtain the FFT result of the local pseudo-code;

[0014] The FFT result of the local pseudo-code is conjugated, multiplied by the FFT result of each baseband spread spectrum signal bit by bit, and the product result is subjected to inverse fast Fourier transform (IFFT) to obtain a baseband signal corresponding to each baseband spread spectrum signal;

[0015] The baseband signals are subjected to non-coherent accumulation, respectively, and the root mean square value of the non-coherent accumulation result is taken as the detection peak value of each baseband signal;

[0016] Taking the maximum value in the detected peak value of each baseband signal, if the maximum value is greater than a preset detection threshold CT, it is determined that the acquisition is successful, otherwise it is determined that the acquisition fails.

[0017] Further, the sideband interval is a frequency interval outside the acquisition range corresponding to a group of baseband signals corresponding to the last frequency point of the first frequency search interval and a group of baseband signals corresponding to the first frequency point of the second frequency search interval of the adjacent two frequency search intervals.

[0018] Further, the sideband interval estimation is specifically: presetting a normalization threshold T hz , and the detected value of the baseband signal corresponding to the last frequency point of the first frequency search interval and the detected value of the baseband signal corresponding to the first frequency point of the second frequency search interval in the adjacent two frequency search intervals are subtracted; if the difference is greater than the normalization threshold T hz , then the FFT-IFFT acquisition determination is performed in the estimation interval one; if the difference is less than the negative value-T hz of the normalization threshold, then the FFT-IFFT acquisition determination is performed in the estimation interval three; otherwise, the FFT-IFFT acquisition determination is performed in the estimation interval two.

[0019] Further, the determination method of the estimation interval is:

[0020] The non-coherent accumulation results of the baseband signals corresponding to the last frequency point of the first frequency search interval and the baseband signals corresponding to the first frequency point of the second frequency search interval in the adjacent two frequency search intervals are subtracted to obtain a difference curve, and a part of the difference curve in the sideband interval is taken to obtain a monotonically decreasing curve; the frequency interval corresponding to the part of the monotonically decreasing curve exceeding the normalization threshold T hz is the estimation interval one, the frequency interval corresponding to the part of the monotonically decreasing curve less than the negative value-T hz of the normalization threshold is the estimation interval three, and the frequency interval corresponding to the remaining part is the estimation interval two.

[0021] Further, the FFT processing process of the baseband spread spectrum signal and the FFT processing process of the local pseudo code are designed to be performed at the same time, and the FFT results of the two are output synchronously.

[0022] Further, after the FFT-IFFT acquisition determination is successful, the pseudo code phase and the carrier frequency coarse estimation result of the communication signal are obtained, and then the pseudo code phase fine tracking and the carrier frequency fine tracking are started, the symbol synchronization is performed on the tracked signal, and the symbol decoding operation is performed.

[0023] A high-sensitivity fast acquisition system for a dynamic environment between a satellite and the ground is used to implement the above method, comprising:

[0024] The signal transceiving module is used for transmitting the communication data after pulse coding modulation, spread spectrum and frequency modulation to obtain the communication signal conforming to the target communication frequency and transmitting the communication signal, and receiving the communication signal by the receiving end;

[0025] The FFT-IFFT capture judgment module is used for generating a plurality of local signals with different frequency points in the frequency search interval by a local parallel digital control oscillator (NCO) group, and performing FFT-IFFT capture judgment on the communication signal.

[0026] The sideband interval estimation module is used for performing sideband interval estimation on the adjacent two frequency search intervals, and performing FFT-IFFT capture judgment according to the sideband interval estimation result.

[0027] Further, the signal transceiving module, the FFT-IFFT capture judgment module and the sideband interval estimation module are all realized on the FPGA chip.

[0028] Compared with the prior art, the present application has the following beneficial effects:

[0029] 1) In the present application, a grouping parallel structure is adopted, each frequency search interval is further subdivided, and a multi-channel local carrier frequency synthesizer with a fixed frequency difference is used to perform down-conversion modulation, pseudo-code spread spectrum demodulation and signal capture on the signal. The parallel structure fully utilizes the FPGA hardware resources, and significantly improves the capture speed of the Doppler frequency offset.

[0030] 2) In the present application, the sideband interval estimation method is adopted, the spectrum analysis process is optimized, the sideband information of the adjacent frequency search intervals of the two parallel captures is fully utilized, the resource use of the FFT-IFFT group is reduced, the data capture and processing time is significantly shortened, and as the maximum Doppler frequency offset range increases, the optimization effect will be improved.

[0031] 3) In the present application, the FFT processing of the signal and the pseudo-code is realized through timing control, and the results are output synchronously. According to the order of the FFT output, the signal Fourier transform result value is multiplied by the conjugate value of the pseudo-code Fourier transform in real time. Compared with the traditional method of storing the pseudo-code FFT result, this pipeline operation not only reduces the processing delay, but also reduces the hardware resources, improves the flexibility and effectiveness of the method.

[0032] 4) The present application is aimed at the communication demand between the low-orbit micro-satellite and the ground, relies on a small satellite communication task, and is based on the actual design of the hardware description language based on the method. After the method is implemented, the function is measured. The hardware test result shows that the function of the measurement and control responder and the high-sensitivity signal capture method of the present application is feasible, has good practicability, and can excellently complete the establishment task of the satellite-ground communication link. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The figure is a hardware overall block diagram of a high-sensitivity satellite-ground spread spectrum TT&C transponder in the embodiment of the present application.

[0034] Figure 2 The figure is a hardware structure diagram of a satellite-ground TT&C transponder in the embodiment of the present application.

[0035] Figure 3 The figure is a functional block diagram of an FPGA chip in the embodiment of the present application.

[0036] Figure 4 The figure is a functional schematic diagram of a high-sensitivity fast acquisition method in the embodiment of the present application.

[0037] Figure 5 The figure is a flow schematic diagram of an acquisition method in the embodiment of the present application.

[0038] Figure 6 The figure is a structure explanatory diagram of a sideband interval in the embodiment of the present application.

[0039] Figure 7 The figure is a sideband interval difference curve in the embodiment of the present application.

[0040] Figure 8 The figure is an actual measurement diagram of an analog satellite-ground communication hardware in the embodiment of the present application. DETAILED DESCRIPTION

[0041] The technical solutions of the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Embodiment 1

[0043] A high-sensitivity fast acquisition method for a satellite-ground dynamic environment, comprising the following steps:

[0044] S1. The transmitting end obtains a communication signal conforming to a target communication frequency after pulse code modulation, spread spectrum and frequency modulation of communication data, and transmits the communication signal, and the receiving end receives the communication signal;

[0045] S2. A plurality of local signals with different frequencies are generated in a frequency search interval by a local parallel digital control oscillator (NCO) group, and FFT-IFFT acquisition determination is performed on the communication signal;

[0046] S3. If the acquisition determination fails, the frequency search interval is adjusted, and step S2 is performed again; if the acquisition determination fails in two adjacent frequency search intervals, sideband interval estimation is performed on the two adjacent frequency search intervals, and FFT-IFFT acquisition determination is performed according to the sideband interval estimation result; this step is repeated until the acquisition determination succeeds.

[0047] Further, the FFT-IFFT acquisition determination comprises the following specific steps:

[0048] The communication signals received by the receiving end are respectively down-converted by several local signals with different frequencies to obtain several baseband spread spectrum signals;

[0049] The obtained several baseband spread spectrum signals are respectively subjected to fast Fourier transform (FFT) processing to obtain FFT results of each baseband spread spectrum signal;

[0050] The receiving end generates a local pseudo code, and the local pseudo code is subjected to fast Fourier transform (FFT) processing to obtain FFT results of the local pseudo code;

[0051] The FFT results of the local pseudo code are conjugated, and are respectively multiplied by the FFT results of each baseband spread spectrum signal, and the product results are subjected to inverse fast Fourier transform (IFFT) to obtain a baseband signal corresponding to each baseband spread spectrum signal;

[0052] The baseband signals are respectively subjected to non-coherent accumulation, and the root mean square value of the non-coherent accumulation results is taken as a detection peak value of each baseband signal;

[0053] The maximum value in the detection peak values of each baseband signal is taken, and if the maximum value is greater than a preset detection threshold CT, it is determined that the acquisition is successful, otherwise it is determined that the acquisition fails.

[0054] Further, the sideband interval is a frequency interval outside the acquisition range corresponding to a group of baseband signals corresponding to the last frequency point of the first frequency search interval of the adjacent two frequency search intervals and a group of baseband signals corresponding to the first frequency point of the second frequency search interval.

[0055] Further, the sideband interval is estimated as: a preset normalization threshold T hz The detection value of the baseband signal corresponding to the last frequency point of the first frequency search interval in the adjacent two frequency search intervals is subtracted from the detection value of the baseband signal corresponding to the first frequency point of the second frequency search interval; if the difference is greater than the normalization threshold T hz , FFT-IFFT acquisition determination is performed in the estimation interval one; if the difference is less than the negative value -T hz of the normalization threshold, FFT-IFFT acquisition determination is performed in the estimation interval three; otherwise, FFT-IFFT acquisition determination is performed in the estimation interval two.

[0056] Further, the determination method of the estimation interval is:

[0057] The non-coherent accumulation results of the baseband signals corresponding to the last frequency point of the first frequency search interval and the first frequency point of the second frequency search interval in the two adjacent frequency search intervals are subtracted to obtain a difference curve, and a part of the difference curve in a sideband interval is taken to obtain a monotone decreasing curve; the frequency interval corresponding to the part of the monotone decreasing curve exceeding a normalized threshold T hz corresponds to the estimation interval one, the frequency interval corresponding to the part of the monotone decreasing curve less than a negative value -T hz corresponds to the estimation interval three, and the frequency interval corresponding to the remaining part corresponds to the estimation interval two.

[0058] Further, the FFT processing process of the baseband spread spectrum signal and the FFT processing process of the local pseudo code are made to proceed at the same time by designing a timing constraint, and the FFT results of the two are output synchronously.

[0059] Further, after the FFT-IFFT capture determination succeeds, the pseudo code phase and the carrier frequency coarse estimation result of the communication signal are obtained, and then the pseudo code phase fine tracking and the carrier frequency fine tracking are started, the symbol synchronization of the tracked signal is performed, and the symbol decoding operation is performed.

[0060] Embodiment 2

[0061] The high-sensitivity fast capture method for the star-ground dynamic environment of the application has been applied to a large-scale low-orbit micro-satellite constellation independently developed by a certain university.

[0062] As Figure 1 shown, a high-dynamic high-sensitivity star-ground spread spectrum TT&C transponder includes hardware circuit design and digital signal processing module design, which is used to ensure the normal operation of the star-ground communication link in a complex electromagnetic environment. The spread spectrum signal capture hardware includes: an FPGA chip, which is used to perform pulse coding modulation, spread spectrum, frequency modulation and other processing on the data to be transmitted, down-convert the received signal, implement spread spectrum capture on the baseband spread spectrum signal after down-conversion, and perform de-spreading, demodulation and other processing; a transmitting radio frequency link, which is used to frequency modulate the baseband spread spectrum signal generated by the FPGA to a transmitting frequency and transmit it through a transmitting antenna; a transmitting antenna, which is used to radiate the high-frequency spread spectrum signal after frequency modulation; a receiving antenna, which is used to receive the high-frequency spread spectrum signal; and a receiver radio frequency link, which is used to down-convert the received high-frequency spread spectrum signal to a baseband signal and amplify the signal using a low-noise amplifier. The transmitting end and the receiving end radio frequency links both adopt a zero intermediate frequency structure, and the signal frequency is converted to a target frequency band through one-time up-conversion or down-conversion.

[0063] The digital signal processing module mainly implements signal acquisition and processing at the transmitting and receiving ends. The transmitting end signal processing includes: an encoding module, which performs pulse code modulation on the communication data to obtain encoding gain; a narrowband transmission module, which performs BPSK modulation on the encoded data; a spread spectrum transmission module, which performs BPSK modulation on the encoded data and pseudo-code spread spectrum modulation on the modulated data; and an up-conversion module, which up-converts the modulated signal to the communication frequency band.

[0064] The receiver signal acquisition processing includes: a down-conversion module, which down-converts the received high-frequency spread spectrum signal to baseband; a conventional serial frequency sweep module, which performs correlation operations through parallel FFT transformation and performs non-correlation accumulation; a sideband interval sweep module, which estimates the sideband interval between two adjacent frequency search intervals and performs FFT-IFFT acquisition determination based on the sideband interval estimation results; an acquisition decision module, which combines the results of the above modules to obtain the Doppler frequency offset estimate and performs acquisition decision; a despreading module, which despreads the baseband spread spectrum signal; a demodulation module, which performs BPSK demodulation on the despread signal; a symbol synchronization module, which makes symbol decisions based on the demodulated signal; and a decoding module, which decodes the synchronized symbols.

[0065] like Figure 2 As shown, the design of the satellite-to-ground telemetry and control transponder in this example mainly includes the radio frequency (RF) front-end and the digital signal processing section. The RF front-end is further divided into a transmit link and a receive link, while the digital signal processing section uses an FPGA chip as its core. The transmit and receive links can be integrated on the same transponder PCB board. By default, the transponder is in receive mode. When other protocol layers send data that requires communication, the transponder can automatically switch to transmit mode. In addition, the transponder can also be turned on and off via ground remote control commands, allowing both transmission and reception to be achieved with a single transponder.

[0066] Specifically: When the transponder is in transmit mode, the data is processed by the FPGA. The FPGA performs modulation and spread spectrum operations on the communication data, and then converts it into a baseband analog signal via a D / A converter. The baseband analog signal is up-converted once and modulated to the target communication frequency band. The signal then passes through an intermediate frequency amplifier and a band selection filter to remove out-of-band noise, and is transmitted into the channel via a power amplifier. When the transponder is in receive mode, the received communication signal first passes through a low-noise amplifier and a band selection filter, and then undergoes down-conversion to mix the signal to the intermediate frequency range. After the intermediate frequency signal is amplified and filtered to remove out-of-band noise, it is converted into an intermediate frequency digital signal by an A / D converter and sent to the FPGA chip for further processing. The transponder uses... Figure 2 The structure simplifies the transponder structure by achieving frequency shifting through a single up-conversion or down-conversion.

[0067] As Figure 3 shown, in the monitoring and control transponder of the application, the FPGA chip is mainly responsible for signal processing of the transmitting and receiving functions. In the transmitting mode, the transponder receives the communication data signal from other protocol layers. First, the data is subjected to pulse coding modulation operation. Second, the coded communication data is multiplied by the local pseudo code bit by bit, thereby realizing spread spectrum modulation. Finally, the spread spectrum direct spread spectrum signal is sent into the D / A converter after BPSK modulation. In the receiving mode, the FPGA receives the digital communication signal from the A / D converter, and captures the signal through the high-sensitivity fast capture method for the dynamic environment between the satellite and the ground of the application. When the spread spectrum signal is successfully captured, the transponder is subjected to despreading and demodulation operation. The demodulated signal is subjected to symbol synchronization processing, and the synchronized symbols are subjected to decoding operation. Finally, the decoded communication data frame will be sent to other protocol layers for further processing.

[0068] As Figure 4 shown, the flow chart of the high-sensitivity fast capture method for the dynamic environment between the satellite and the ground in the application is shown, which is realized through the FPGA. The core technology is to use the parallel FFT pseudo code phase capture method combined with the interval estimation module, thereby significantly improving the capture speed, capture sensitivity and Doppler frequency offset capture range, and enhancing the overall reliability of the capture method. The steps are shown in Figure 5 , and are described in detail as follows:

[0069] S1. The transmitting end further performs BPSK modulation on the communication data after pulse coding modulation, and performs direct spread spectrum modulation with the local pseudo code. The modulated signal is up-converted to the target communication frequency and transmitted, and the receiving end receives the communication signal;

[0070] S2. A plurality of local signals with different frequency points are generated in the frequency search interval through a local parallel numerically controlled oscillator (NCO) group, and the communication signal is subjected to FFT-IFFT capture determination;

[0071] S3. If the capture determination fails, adjust the frequency search interval and repeat step S2; if the capture determination fails in the adjacent two frequency search intervals, perform sideband interval estimation on the adjacent two frequency search intervals, and perform FFT-IFFT capture determination according to the sideband interval estimation result; repeat this step until the capture determination is successful.

[0072] The application adopts a multi-path parallel mode, that is, a plurality of local NCOs are used to generate a plurality of carrier frequencies with fixed frequency difference, and the received signal is subjected to down-conversion modulation, and then the plurality of down-converted baseband signals are subjected to subsequent processing, so as to improve the signal carrier frequency search efficiency.

[0073] The FFT-IFFT capture judgment comprises the following specific steps:

[0074] The communication signal received by the receiving end is down-converted into a plurality of baseband spread spectrum signals by a plurality of local signals with different frequencies, and the signals are amplified by a low noise amplifier (LNA);

[0075] The plurality of baseband spread spectrum signals are subjected to Fast Fourier Transform (FFT) processing respectively to obtain the FFT result of each baseband spread spectrum signal; meanwhile, the receiving end generates a local pseudo code and subjects the local pseudo code to FFT processing to obtain the FFT result of the local pseudo code; the FFT processing of the plurality of baseband spread spectrum signals and the FFT processing of the local pseudo code are made to be performed simultaneously by designing timing constraints, and the FFT results of the two are output synchronously.

[0076] The FFT result of the local pseudo code is conjugated and multiplied by the FFT result of each baseband spread spectrum signal bit by bit, and the product result is subjected to Inverse Fast Fourier Transform (IFFT) to obtain the baseband signal corresponding to each baseband spread spectrum signal;

[0077] In order to improve the capture sensitivity of the receiver, the IFFT result of the baseband signal is subjected to non-coherent accumulation, and the root mean square value of the non-coherent accumulation result is taken as the detection value of each baseband signal;

[0078] The maximum value in the detection value of each baseband spread spectrum signal is taken, and if the maximum value is greater than a preset detection threshold CT, it is determined that the capture is successful, otherwise it is determined that the capture fails.

[0079] As shown in Figure 6 The figure shows two adjacent frequency search intervals, which correspond to parallel NCO groups of center frequency 1 and center frequency 2 respectively. Each peak corresponds to an NCO frequency search range. When the detected peak value exceeds the set threshold CT, the system will perform signal capture at the corresponding NCO frequency. If capture fails in both adjacent frequency search intervals, the frequency interval outside the capture range corresponding to a group of baseband signals corresponding to the last frequency point of the first frequency search interval and a group of baseband signals corresponding to the first frequency point of the second frequency search interval is taken as the sideband interval, and the detailed interval range is defined by Figure 7 .

[0080] As shown in Figure 7As shown, the difference between the incoherent accumulation result (curve a) of the baseband signal corresponding to the last frequency point of the first frequency search interval and the incoherent accumulation result (curve b) of the baseband signal corresponding to the first frequency point of the second frequency search interval is obtained to obtain the difference curve (curve c). The difference curve is then taken in the sideband interval. Figure 7 From part ④), we obtain a monotonically decreasing curve; the monotonically decreasing curve exceeds the normalization threshold T. hz The frequency range corresponding to part of it is the estimated interval one ( Figure 7 In the middle ①), the negative value -T is less than the normalization threshold. hz The frequency range corresponding to part of it is the estimated interval three ( Figure 7 (③), the frequency range corresponding to the remaining part is the estimated interval two ( Figure 7 (Middle ②).

[0081] The sideband interval estimation specifically involves: setting a preset normalization threshold T. hz Normalized threshold T hz According to the performance test settings, the difference between the detected peak value of the baseband signal corresponding to the last frequency point of the first frequency search interval and the detected value of the baseband signal corresponding to the first frequency point of the second frequency search interval is calculated; if the difference is greater than the normalization threshold T... hz Then, FFT-IFFT capture determination is performed within the estimation interval; if the difference is less than the negative value of the normalization threshold -T hz If the result is positive, then FFT-IFFT capture determination is performed within estimation interval three; otherwise, FFT-IFFT capture determination is performed within estimation interval two.

[0082] After successful FFT-IFFT acquisition, a coarse estimate of the pseudocode phase and carrier frequency of the communication signal is obtained. The Doppler frequency shift is determined based on the uncorrelated accumulated value, and the local pseudocode is shifted according to the maximum value of the IFFT detection peak, thus achieving despreading of the received signal. Then, fine tracking of the pseudocode phase and carrier frequency is initiated to further accurately estimate the carrier frequency and pseudocode phase difference. Symbol synchronization is performed on the tracked signal, and the symbols are decoded.

[0083] In the above method, the improved parallel FFT fast capture method adds sideband interval estimation. Compared with the traditional FFT fast capture method, the main improvement of this method is:

[0084] 1) A grouped parallel structure is adopted, which further subdivides the frequency search interval and uses a multi-channel local carrier frequency synthesizer with a fixed frequency difference to perform down-conversion modulation, pseudo-code spread spectrum demodulation, and signal acquisition. The parallel structure makes full use of FPGA hardware resources and significantly improves the acquisition speed of Doppler frequency offset.

[0085] 2) The sideband interval estimation method is used to optimize the spectrum analysis process, fully utilize the sideband information of the adjacent frequency sweep interval captured twice in parallel, reduce the resource usage of FFT-IFFT group, significantly shorten the data capture and processing time, and as the maximum Doppler frequency deviation range increases, the optimization effect will be improved.

[0086] 3) In software implementation, the FFT processing of signals and pseudo codes is realized through timing control, and the results are output synchronously. According to the order of FFT output, the signal Fourier transform result value is multiplied by the conjugate value of the pseudo code Fourier transform. Compared with the traditional method of storing pseudo code FFT results, this pipeline operation not only reduces the processing delay, but also reduces the hardware resources, improves the flexibility and effectiveness of the method.

[0087] As shown in Figure 8 The hardware measurement diagram of the satellite-ground TT&C transponder of the application shows the composition of the TT&C transponder, including the power supply system, the transmitting end transponder, the receiving end transponder and the adjustable attenuator. The improved FFT fast capture method is realized by the hardware description language VHDL and is burned into the used chip by using the ISE software of Xilinx company. The hardware verification result shows that the limit sensitivity of the receiver of the application can reach-130dBm, the total Doppler frequency deviation capture range can reach 140kHz, and the capture time is as low as 0.8 seconds, which fully meets the actual application requirements.

[0088] The above specific embodiments are used to explain and illustrate the application, rather than limit the application, and any modifications and changes made to the application within the spirit and protection scope of the claims of the application all fall within the protection scope of the application.

Claims

1. A high-sensitivity fast acquisition method for a satellite-to-ground dynamic environment, characterized in that, The method comprises the following steps: S1. The transmitting end obtains a communication signal conforming to a target communication frequency after pulse code modulation, spread spectrum and frequency modulation of communication data, and transmits the communication signal, and the receiving end receives the communication signal; S2. A plurality of local signals with different frequencies are generated in a frequency search interval by a local parallel digital control oscillator (NCO) group, and FFT-IFFT capture determination is performed on the communication signal; S3. If capture determination fails, the frequency search interval is adjusted, and step S2 is performed again; if capture determination fails in two adjacent frequency search intervals, sideband interval estimation is performed on the two adjacent frequency search intervals, and FFT-IFFT capture determination is performed according to the sideband interval estimation result; the step is repeated until capture determination succeeds; The sideband interval is a frequency interval outside a capture range corresponding to a group of baseband signals corresponding to a last frequency point of a first frequency search interval and a group of baseband signals corresponding to a first frequency point of a second frequency search interval of the two adjacent frequency search intervals; The sideband interval estimation is specifically: presetting a normalization threshold , and the detection value of the baseband signal corresponding to the last frequency point of the first frequency search interval in the two adjacent frequency search intervals is subtracted from the detection value of the baseband signal corresponding to the first frequency point of the second frequency search interval; if the difference is greater than a normalization threshold , then FFT-IFFT capture determination is performed in the estimation interval one; If the difference is less than a negative value of a normalization threshold then perform FFT-IFFT acquisition decision in estimation interval three; otherwise perform FFT-IFFT acquisition decision in estimation interval two.

2. The method for high-sensitivity fast acquisition in the dynamic environment of satellite-to-ground according to claim 1, wherein, The FFT-IFFT capture determination comprises the following steps: A plurality of baseband spread spectrum signals are obtained by performing frequency down-conversion processing on the communication signal received by the receiving end through the plurality of local signals with different frequencies; FFT (Fast Fourier Transform) processing is performed on the plurality of baseband spread spectrum signals to obtain FFT results of each baseband spread spectrum signal; A local pseudo code is generated by the receiving end, and FFT processing is performed on the local pseudo code to obtain FFT results of the local pseudo code; The FFT results of the local pseudo code are conjugated, and multiplied by the FFT results of each baseband spread spectrum signal bit by bit, and IFFT (Inverse Fast Fourier Transform) processing is performed on the product results to obtain baseband signals corresponding to each baseband spread spectrum signal; Non-coherent accumulation is performed on the baseband signals, and a root mean square value of the non-coherent accumulation result is taken as a detection peak value of each baseband signal; The maximum value in the detection peak value of each baseband signal is taken, and if the maximum value is greater than a preset detection threshold CT, it is determined that capture succeeds, otherwise it is determined that capture fails.

3. The method for high-sensitivity fast acquisition in the dynamic environment of satellite-to-ground according to claim 1, wherein, The determination method of the estimation interval is: The non-coherent accumulation results of the baseband signals corresponding to the last frequency point in the first frequency search interval and the first frequency point in the second frequency search interval in the two adjacent frequency search intervals are subtracted to obtain a difference curve, and a part of the difference curve in the sideband interval is taken to obtain a monotone decreasing curve; the frequency interval corresponding to the part of the monotone decreasing curve exceeding the normalized threshold is the estimation interval one, the frequency interval corresponding to the part of the monotone decreasing curve less than the negative value of the normalized threshold is the estimation interval three, and the frequency interval corresponding to the remaining part is the estimation interval two. The non-coherent accumulation results of the baseband signals corresponding to the last frequency point in the first frequency search interval and the first frequency point in the second frequency search interval in the two adjacent frequency search intervals are subtracted to obtain a difference curve, and a part of the difference curve in the sideband interval is taken to obtain a monotone decreasing curve; the frequency interval corresponding to the part of the monotone decreasing curve exceeding the normalized threshold is the estimation interval one, the frequency interval corresponding to the part of the monotone decreasing curve less than the negative value of the normalized threshold is the estimation interval three, and the frequency interval corresponding to the remaining part is the estimation interval two.​ 4. The method for high-sensitivity fast acquisition in the dynamic environment of satellite-to-ground according to claim 2, wherein, The FFT processing of the baseband spread spectrum signal and the FFT processing of the local pseudo code are performed simultaneously by designing timing constraints, and the FFT results of the two are output synchronously.

5. The method for high-sensitivity fast acquisition in the dynamic environment of satellite-to-ground according to claim 1, wherein, After the FFT-IFFT capture determination succeeds, the pseudo code phase and the carrier frequency coarse estimation result of the communication signal are obtained, and then the pseudo code phase fine tracking and the carrier frequency fine tracking are started, the symbol synchronization is performed on the tracked signal, and the symbol decoding operation is performed.

6. A high-sensitivity fast acquisition system for dynamic space-earth environment for implementing the method according to any one of claims 1-5, characterized in that, It comprises: A signal transceiver module is configured to transmit a communication signal conforming to a target communication frequency after pulse code modulation, spread spectrum and frequency modulation of communication data by the transmitting end, and to receive the communication signal by the receiving end; An FFT-IFFT capture determination module is configured to generate a plurality of local signals with different frequencies in a frequency search interval by a local parallel digital control oscillator (NCO) group, and to perform FFT-IFFT capture determination on the communication signal; A sideband interval estimation module is configured to perform sideband interval estimation on two adjacent frequency search intervals, and to perform FFT-IFFT capture determination according to the sideband interval estimation result.

7. The high-sensitivity fast acquisition system for dynamic environment of satellite-to-ground according to claim 6, characterized in that, The signal transceiving module, the FFT-IFFT capture judging module and the sideband interval estimating module are all realized on an FPGA chip.

Images