CN111510410B - Anti-interference DS-GMSK receiving method and device suitable for satellite communication - Google Patents
Anti-interference DS-GMSK receiving method and device suitable for satellite communication Download PDFInfo
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- H—ELECTRICITY
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- H04B1/707—Spread spectrum techniques using direct sequence modulation
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Abstract
The invention discloses an anti-interference DS-GMSK receiving method and device suitable for satellite communication, and belongs to the technical field of satellite communication. The DS-GMSK has the advantages of a direct sequence spread spectrum system, works in a low signal-to-noise ratio environment, and has strong anti-jamming capability; meanwhile, the characteristics of high GMSK modulation spectrum utilization rate and low out-of-band radiation power are combined, the anti-noise and anti-interference capability is enhanced, the spectrum utilization rate is improved, and the out-of-band radiation is reduced. By using FFT and IFFT operation, the operation efficiency of the capture and demodulation algorithm is obviously improved, and the anti-interference DS-GMSK receiving method suitable for satellite communication is more suitable for realizing an FPGA platform. The device comprises a down-conversion module, a frequency deviation compensation module, a code phase tracking and adjusting module, a capturing module and a de-spreading and demodulating module. The DSSS and GMSK are combined, so that the anti-noise and anti-interference capability is enhanced, the frequency spectrum utilization rate is improved, out-of-band radiation is reduced, and the DSSS and GMSK combined broadband satellite communication system has a wide application prospect in the field of satellite communication.
Description
Technical Field
The invention relates to an anti-interference DS-GMSK (direct sequence spread spectrum Gaussian minimum frequency shift keying) receiving method and device suitable for satellite communication, and belongs to the technical field of satellite communication.
Background
In this information and globalization era, satellite communication is an important and indispensable communication method. Compared with a common civil communication mode, satellite communication is often required to work in a communication environment with low signal-to-noise ratio and strong interference, and meanwhile, high requirements on confidentiality and interception resistance are also provided.
In satellite communication, modulation modes such as BPSK are often used, but BPSK causes spectrum broadening due to inter-symbol carrier phase mutation, a power spectrum generates strong side lobe components, and the spectrum utilization rate is not high; BPSK has the disadvantage of envelope collapse, and nonlinear distortion is large when high power amplifier is used. MSK (minimum shift keying) is a constant envelope modulation with continuous phases, and as a constant envelope modulation, it can improve the utilization rate of power amplifier to the maximum extent. And MSK is a modulation mode with continuous carrier phase, can effectively restrain sidelobe components and concentrates energy on a main lobe. GMSK (gaussian minimum shift keying) modulation is an improvement of MSK modulation by adding a gaussian low pass filter to pre-filter before MSK modulation to even further smooth the phase change of MSK. GMSK has faster out-of-band attenuation and more compact spectrum than MSK, so it has higher power efficiency and spectrum efficiency than MSK, and at the same time, it can make maximum use of the power amplifier performance.
The Direct Sequence Spread Spectrum (DSSS) has the characteristics of multipath fading resistance, strong anti-interference capability, low transmission power, low interception rate, good confidentiality and the like. The DSSS is combined with the GMSK, so that the spectrum utilization rate can be improved and out-of-band radiation can be reduced while the advantages of spread spectrum communication are obtained, and the DSSS has wide application prospects in the field of satellite communication.
Disclosure of Invention
The invention discloses an anti-interference DS-GMSK receiving method and device suitable for satellite communication, which aims to solve the technical problems that: the DSSS is combined with the GMSK, so that the anti-noise and anti-interference capability is enhanced, the frequency spectrum utilization rate is improved, out-of-band radiation is reduced, and the DSSS has a wide application prospect in the field of satellite communication.
The purpose of the invention is realized by the following technical scheme.
The invention discloses an anti-interference DS-GMSK receiving method suitable for satellite communication, which comprises the following steps:
the method comprises the following steps: and carrying out quadrature down-mixing and low-pass filtering on the received intermediate-frequency DS-GMSK signals to obtain IQ two-path DS-GMSK baseband signals. Inputting the DS-GMSK baseband signal subjected to frequency offset and code phase compensation into a capture module, and performing frequency offset compensation and code offset adjustment on the signal to be 0 before the capture module successfully captures the signal.
Step two: the DS-GMSK baseband signal enters a capture module, FFT operation is carried out on data with one symbol length of the signal, the data are stored in an RAM, a local template 1 is used for multiplying the FFT result in the RAM, then IFFT operation is carried out, the IFFT operation results of M1 symbols are subjected to modulus taking and M1 times of accumulation, the peak-to-average ratio of the accumulated results is calculated and compared with a capture threshold, if the peak-to-average ratio exceeds the threshold, the capture is successful, otherwise, the capture is continued. And outputting the peak position of the IFFT accumulation result after successful acquisition, namely obtaining the code phase information. And simultaneously outputting complex information at the position of the peak of the subsequent M2 IFFT result to obtain demodulated information of M2 symbols, performing M2-point FFT operation on the information of the M2 symbols, wherein the position of the peak of the FFT result represents information of frequency offset, and outputting the information of the frequency offset.
Step three: after the capturing module successfully captures the signals, the frequency offset information output by the capturing module is fed back to the front-end frequency offset compensation module, the code phase information output by the capturing module is fed back to the front-end code phase tracking and adjusting module, and the capturing and de-spreading demodulation switching module controls the de-spreading demodulation module to start working.
Step four: the DS-GMSK baseband signal compensated by the frequency offset compensation module and the code offset compensated by the code phase tracking adjustment module is input into the de-spreading demodulation module. And performing FFT operation on DS-GMSK baseband information with one symbol length, and storing the FFT operation result into the RAM. Dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as that of the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1. And subtracting the two paths of output despreading and demodulation results to obtain complete despreading and demodulation soft information. And finally, completing the acquisition, the de-spreading and the demodulation of the DS-GMSK signals, and judging the demodulation result to obtain the final data information.
Step five: the DS-GMSK has the advantages of a direct sequence spread spectrum system, works in a low signal-to-noise ratio environment, is high in anti-jamming capability, and simultaneously improves the spectrum utilization rate and reduces out-of-band radiation while enhancing the anti-noise and anti-jamming capabilities by combining the characteristics of GMSK that the spectrum utilization rate is high and the out-of-band radiation power is low. And implementing the despreading demodulation of the DS-GMSK through the steps one to four. By using FFT and IFFT operation in the second step and the third step, the operation efficiency of the capture and demodulation algorithm is obviously improved, and the anti-interference DS-GMSK receiving method suitable for satellite communication is more suitable for realizing an FPGA platform.
The invention also discloses an anti-interference DS-GMSK receiving device suitable for satellite communication, which is used for realizing the anti-interference DS-GMSK receiving method suitable for satellite communication.
And the down-conversion module is used for carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals and outputting IQ two paths of DS-GMSK baseband signals.
And the frequency offset compensation module generates a compensation carrier wave through the DDS according to the frequency offset value output by the capture module and performs frequency offset compensation on the DS-GMSK baseband signal after the down-mixing.
And the code phase tracking and adjusting module adjusts the data initial position input into the capturing module or the despreading and demodulating module according to the code phase information output by the capturing module so as to ensure that the data subjected to FFT operation at the rear end is aligned with the local template data.
And the capturing and de-spreading demodulation switching module is used for switching and inputting the DS-GMSK baseband signals subjected to frequency offset compensation and code phase adjustment to the de-spreading demodulation module after capturing is finished.
And the acquisition module is used for acquiring the DS-GMSK baseband signal and feeding back frequency offset and code phase information to the front end.
And the de-spreading demodulation module is used for de-spreading and demodulating the signals subjected to frequency offset and code phase compensation and outputting demodulation soft information.
Converting PN codes used for spreading into opposite polarity sequences; performing GMSK complex baseband modulation on the positive PN code, performing FFT operation, and conjugating the FFT result to obtain a template 0; and performing GMSK complex baseband modulation on the negative PN code, performing FFT operation, and conjugating the FFT result to obtain a template 1.
Carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals to obtain IQ two-path DS-GMSK baseband signals, and inputting the IQ two-path DS-GMSK baseband signals to a frequency offset compensation module; and the frequency offset compensation module converts the frequency offset information output by the capture module into frequency control words, adjusts the carrier frequency for correcting the frequency offset in real time by using a DDS method, generates two paths of orthogonal carriers, and multiplies the two paths of orthogonal carriers by IQ paths of a DS-GMSK complex baseband signal respectively to complete frequency offset compensation. And inputting the output signal of the frequency offset compensation module into a code phase tracking and adjusting module, and adjusting the position of an initial point for FFT (fast Fourier transform) operation according to the captured and output code offset information so as to align the code phase. Before the acquisition module does not successfully acquire the signal, the frequency offset compensation and the code offset adjustment of the signal are both 0; inputting the DS-GMSK baseband signals after the adjustment of the 0 frequency offset and the 0 code offset into a capture module; the acquisition module performs FFT operation on an input DS-GMSK baseband signal, multiplies the input DS-GMSK baseband signal by a local template 1, performs IFFT operation and modulus accumulation, judges whether the acquisition is successful or not by calculating a peak-to-average ratio, outputs a peak position after the acquisition is successful to obtain code offset information, takes out an IFFT peak position result of 2 symbols, and performs FFT at M2 points to obtain frequency offset information; after the capturing and judging are successful, feeding back the frequency offset information and the code offset information to the front-end frequency offset compensation module and the code phase tracking adjustment module to compensate the frequency offset and the code offset, and simultaneously controlling to input the DS-GMSK baseband signals after the frequency offset and the code offset are compensated to the de-spreading demodulation module; in the de-spreading demodulation module, FFT operation is carried out on DS-GMSK baseband information with one symbol length, and FFT operation results are stored in an RAM. Dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as that of the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1. And subtracting the two paths of output despreading and demodulation results to obtain complete despreading and demodulation soft information.
Advantageous effects
1. The invention discloses an anti-interference DS-GMSK receiving method and device suitable for satellite communication, wherein the DS-GMSK has the advantages of a direct sequence spread spectrum system, works in an environment with a low signal-to-noise ratio and has strong anti-interference capability; meanwhile, the characteristics of high GMSK modulation spectrum utilization rate and low out-of-band radiation power are combined, the anti-noise and anti-interference capability is enhanced, the spectrum utilization rate is improved, and the out-of-band radiation is reduced.
2. The anti-interference DS-GMSK receiving method and device suitable for satellite communication, disclosed by the invention, have the advantages that the calculation efficiency of a capturing and demodulating algorithm is obviously improved by using FFT and IFFT calculation, and the anti-interference DS-GMSK receiving method suitable for satellite communication is more suitable for realizing an FPGA platform.
Drawings
Fig. 1 is a flow chart of an anti-interference DS-GMSK receiving apparatus suitable for satellite communication according to the present invention.
FIG. 2 is a schematic diagram of local template generation.
Fig. 3 is a flow chart of the capture module.
Fig. 4 is a flowchart of a despreading demodulation module.
Fig. 5 is a graph of simulated bit error rate.
Detailed Description
The invention is further illustrated and described in detail below with reference to the figures and examples.
Example 1:
as shown in fig. 1, the anti-interference DS-GMSK receiving apparatus suitable for satellite communication disclosed in this embodiment includes a down-conversion module, a frequency offset compensation module, a code phase tracking adjustment module, an acquisition and despreading demodulation switching module, an acquisition module, and a despreading demodulation module.
And the down-conversion module is used for carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals and outputting IQ two paths of DS-GMSK baseband signals.
And the frequency offset compensation module generates a compensation carrier wave through the DDS according to the frequency offset value output by the capture module and performs frequency offset compensation on the DS-GMSK baseband signal after the down-mixing.
And the code phase tracking and adjusting module adjusts the data initial position input into the capturing module or the despreading and demodulating module according to the code phase information output by the capturing module so as to ensure that the data subjected to FFT operation at the rear end is aligned with the local template data.
And the capturing and de-spreading demodulation switching module is used for switching and inputting the DS-GMSK baseband signals subjected to frequency offset compensation and code phase adjustment to the de-spreading demodulation module after capturing is finished.
And the acquisition module is used for acquiring the DS-GMSK baseband signal and feeding back frequency offset and code phase information to the front end.
And the de-spreading demodulation module is used for de-spreading and demodulating the signals subjected to frequency offset and code phase compensation and outputting demodulation soft information.
Converting PN codes used for spreading into opposite polarity sequences; performing GMSK complex baseband modulation on the positive PN code, performing FFT operation, and conjugating the FFT result to obtain a template 0; and performing GMSK complex baseband modulation on the negative PN code, performing FFT operation, and conjugating the FFT result to obtain a template 1.
Carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals to obtain IQ two-path DS-GMSK baseband signals, and inputting the IQ two-path DS-GMSK baseband signals to a frequency offset compensation module; and the frequency offset compensation module converts the frequency offset information output by the capture module into frequency control words, adjusts the carrier frequency for correcting the frequency offset in real time by using a DDS method, generates two paths of orthogonal carriers, and multiplies the two paths of orthogonal carriers by IQ paths of a DS-GMSK complex baseband signal respectively to complete frequency offset compensation. And inputting the output signal of the frequency offset compensation module into a code phase tracking and adjusting module, and adjusting the position of an initial point for FFT (fast Fourier transform) operation according to the captured and output code offset information so as to align the code phase. Before the acquisition module does not successfully acquire the signal, the frequency offset compensation and the code offset adjustment of the signal are both 0; inputting the DS-GMSK baseband signals after the adjustment of the 0 frequency offset and the 0 code offset into a capture module; the acquisition module performs FFT operation on an input DS-GMSK baseband signal, multiplies the input DS-GMSK baseband signal by a local template 1, performs IFFT operation and modulus accumulation, judges whether the acquisition is successful or not by calculating a peak-to-average ratio, outputs a peak position after the acquisition is successful to obtain code offset information, takes out an IFFT peak position result of 2 symbols, and performs FFT at M2 points to obtain frequency offset information; after the capturing and judging are successful, feeding back the frequency offset information and the code offset information to the front-end frequency offset compensation module and the code phase tracking adjustment module to compensate the frequency offset and the code offset, and simultaneously controlling to input the DS-GMSK baseband signals after the frequency offset and the code offset are compensated to the de-spreading demodulation module; in the de-spreading demodulation module, FFT operation is carried out on DS-GMSK baseband information with one symbol length, and FFT operation results are stored in an RAM. Dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as that of the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1. And subtracting the two paths of output despreading and demodulation results to obtain complete despreading and demodulation soft information.
In this example, the time bandwidth constant BT of GMSK modulation is taken to be 0.3, the number of acquisition accumulation points M1 is taken to be 128, the number of frequency offset estimation points M2 is taken to be 64, the spreading ratio N is taken to be 64, and the symbol rate is taken to be 250 KHz.
The anti-interference DS-GMSK receiving method applicable to satellite communication disclosed in this embodiment includes the following data processing procedures at the receiving end:
the method comprises the following steps: and carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals to obtain IQ two-path DS-GMSK baseband signals. Inputting the DS-GMSK baseband signal subjected to frequency offset and code phase compensation into a capture module, and performing frequency offset compensation and code offset adjustment on the signal to be 0 before the capture module successfully captures the signal.
Step two: the DS-GMSK baseband signal enters a capture module, as shown in FIG. 3, FFT operation is carried out on data of one symbol length of the signal, the data are stored in an RAM, a local template 1 is used for multiplying the FFT result in the RAM, then IFFT operation is carried out, the IFFT operation results of 128 symbols are subjected to module taking and accumulation for 128 times, the peak-to-average ratio of the accumulated results is calculated and compared with a capture threshold, if the peak-to-average ratio exceeds the threshold, the capture is successful, otherwise, the capture is continued. And outputting the peak position of the IFFT accumulation result after successful acquisition, namely obtaining the code phase information. Meanwhile, complex information at the position of the peak of the IFFT result of 64 subsequent times is output to obtain information demodulated by 64 symbols, 64-point FFT operation is carried out on the information of the 64 symbols, the position of the peak of the FFT result represents information of frequency offset, and the information of the frequency offset is output.
Step three: after the capturing module successfully captures the signals, the frequency offset information output by the capturing module is fed back to the front-end frequency offset compensation module, the code phase information output by the capturing module is fed back to the front-end code phase tracking and adjusting module, and the capturing and de-spreading demodulation switching module controls the de-spreading demodulation module to start working.
Step four: the DS-GMSK baseband signal compensated for frequency offset by the frequency offset compensation module and compensated for code offset by the code phase tracking adjustment module is input to the de-spreading demodulation module, as shown in fig. 4. And performing FFT operation on DS-GMSK baseband information with one symbol length, and storing the FFT operation result into the RAM. Dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as that of the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1. And subtracting the two paths of output despreading and demodulation results to obtain complete despreading and demodulation soft information. And finally, completing the acquisition, the de-spreading and the demodulation of the DS-GMSK signals, and judging the demodulation result to obtain the final data information.
Step five: the DS-GMSK has the advantages of a direct sequence spread spectrum system, works in a low signal-to-noise ratio environment, is high in anti-jamming capability, and simultaneously improves the spectrum utilization rate and reduces out-of-band radiation while enhancing the anti-noise and anti-jamming capabilities by combining the characteristics of GMSK that the spectrum utilization rate is high and the out-of-band radiation power is low. And implementing the despreading demodulation of the DS-GMSK through the steps one to four. By using FFT and IFFT operation in the second step and the third step, the operation efficiency of the capture and demodulation algorithm is obviously improved, and the anti-interference DS-GMSK receiving method suitable for satellite communication is more suitable for realizing an FPGA platform. As shown in fig. 2, a PN code with a length of 64 is mapped to positive and negative 1, GMSK baseband modulation with a time bandwidth constant BT of 0.3 is performed on the mapped PN code, FFT operation is performed on the modulated waveform, and a conjugate is taken from the FFT result to obtain a template 0; and performing GMSK baseband modulation on the inverse number of the mapped PN code, performing FFT operation on the modulated waveform, and conjugating the FFT result to obtain the template 1. Template 0 and template 1 are stored locally for acquisition and despreading demodulation.
Fig. 5 is a direct-spread GMSK error rate curve obtained by simulation under the above receiver system, where no influence of frequency offset is added in the simulation. It can be seen that the curve is very close to the theoretical error rate curve for a typical GMSK non-coherent reception. When the frequency offset is within the range of positive Rs/2 and negative Rs/2, the error rate is almost unchanged, and the frequency offset compensation module can correctly estimate and compensate the frequency offset with the accuracy of 3.90625 KHz; when the frequency deviation is more than positive and negative Rs/2, namely positive and negative 125KHz, the system can not correctly estimate and compensate the frequency deviation, and the error rate is sharply reduced, so that the receiver is only suitable for the receiving condition of small frequency deviation.
The above description is only an example of the present invention, and is not intended to limit the embodiments of the present invention. Variations and modifications in other variations will occur to those skilled in the art upon reading the foregoing description. This list is not exhaustive of the many embodiments. All obvious changes and modifications of the present invention are within the scope of the present invention.
Claims (2)
1. An anti-interference DS-GMSK receiving method suitable for satellite communication is characterized in that: comprises the following steps of (a) carrying out,
the method comprises the following steps: orthogonal down-mixing and low-pass filtering a received intermediate frequency direct sequence spread spectrum Gaussian minimum frequency shift keying (DS-GMSK) signal to obtain IQ two-path DS-GMSK baseband signals; inputting the DS-GMSK baseband signal subjected to frequency offset and code phase compensation into a capture module, and performing frequency offset compensation and code offset adjustment on the signal to be 0 before the capture module successfully captures the signal;
step two: the DS-GMSK baseband signal enters a capture module, FFT operation is carried out on data with one symbol length of the signal, the data are stored in an RAM, a local template 1 is used for multiplying the FFT result in the RAM, then IFFT operation is carried out, the IFFT operation results of M1 symbols are subjected to modulus taking and M1 times of accumulation, the peak-to-average ratio of the accumulated results is calculated and compared with a capture threshold, if the peak-to-average ratio exceeds the threshold, the capture is successful, otherwise, the capture is continued; after the acquisition is successful, outputting the peak position of the IFFT accumulation result, namely obtaining code phase information; meanwhile, complex information at the position of the peak of the subsequent M2 IFFT result is output, namely information demodulated by M2 symbols is obtained, M2-point FFT operation is carried out on the information of the M2 symbols, the position of the peak of the FFT result represents information of frequency offset, and the information of the frequency offset is output;
step three: after the capturing module successfully captures the data, the frequency offset information output by the capturing module is fed back to the front-end frequency offset compensation module, the code phase information output by the capturing module is fed back to the front-end code phase tracking and adjusting module, and the capturing and de-spreading demodulation switching module controls the de-spreading demodulation module to start working;
step four: the DS-GMSK baseband signal compensated by the frequency offset compensation module and the code offset compensated by the code phase tracking adjustment module is input into a de-spreading demodulation module; performing FFT operation on DS-GMSK baseband information with one symbol length, and storing the FFT operation result into an RAM; dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1; subtracting the two paths of output despreading demodulation results to obtain complete despreading demodulation soft information; and finally, completing the acquisition, the de-spreading and the demodulation of the DS-GMSK signals, and judging the demodulation result to obtain the final data information.
2. An anti-interference DS-GMSK receiving apparatus suitable for satellite communication, for implementing an anti-interference DS-GMSK receiving method suitable for satellite communication according to claim 1, wherein: the device comprises a down-conversion module, a frequency offset compensation module, a code phase tracking and adjusting module, a capturing module and a de-spreading and demodulating module;
the down-conversion module is used for carrying out orthogonal down-mixing on the received intermediate frequency DS-GMSK signals and outputting IQ two paths of DS-GMSK baseband signals;
the frequency offset compensation module generates a compensation carrier through the DDS according to the frequency offset value output by the capture module, and performs frequency offset compensation on the DS-GMSK baseband signal after down-mixing;
the code phase tracking and adjusting module adjusts the initial position of data input into the capturing module or the despreading and demodulating module according to the code phase information output by the capturing module so as to ensure that data subjected to FFT operation at the rear end is aligned with local template data;
the acquisition and de-spreading demodulation switching module is used for switching and inputting the DS-GMSK baseband signals subjected to frequency offset compensation and code phase adjustment to the de-spreading demodulation module after the acquisition is finished;
the acquisition module is used for acquiring the DS-GMSK baseband signal and feeding back frequency offset and code phase information to the front end;
the de-spread demodulation module is used for de-spread demodulating the signals subjected to frequency offset and code phase compensation and outputting demodulation soft information;
converting PN codes used for spreading into opposite polarity sequences; performing GMSK complex baseband modulation on the positive PN code, performing FFT operation, and conjugating the FFT result to obtain a local template 0; performing GMSK complex baseband modulation on the negative PN code, performing FFT operation, and conjugating the FFT result to obtain a local template 1;
carrying out quadrature down-mixing on the received intermediate frequency DS-GMSK signals to obtain IQ two-path DS-GMSK baseband signals, and inputting the IQ two-path DS-GMSK baseband signals to a frequency offset compensation module; the frequency offset compensation module converts the frequency offset information output by the capture module into frequency control words, adjusts the carrier frequency for correcting the frequency offset in real time by using a DDS method, generates two paths of orthogonal carriers, and multiplies the two paths of orthogonal carriers by IQ paths of a DS-GMSK complex baseband signal respectively to complete frequency offset compensation; inputting a signal output by the frequency offset compensation module into a code phase tracking adjustment module, and adjusting the position of an initial point for FFT operation according to the captured and output code offset information so as to align the code phase; before the acquisition module does not successfully acquire the signal, the frequency offset compensation and the code offset adjustment of the signal are both 0; inputting the DS-GMSK baseband signals after the adjustment of the 0 frequency offset and the 0 code offset into a capture module; the acquisition module performs FFT operation on an input DS-GMSK baseband signal, multiplies the input DS-GMSK baseband signal by a local template 1, performs IFFT operation and modulus accumulation, judges whether the acquisition is successful or not by calculating a peak-to-average ratio, outputs a peak position after the acquisition is successful to obtain code offset information, extracts IFFT peak position results of M2 symbols, and performs M2-point FFT to obtain frequency offset information; after the capturing and judging are successful, feeding back the frequency offset information and the code offset information to the front-end frequency offset compensation module and the code phase tracking adjustment module to compensate the frequency offset and the code offset, and simultaneously controlling to input the DS-GMSK baseband signals after the frequency offset and the code offset are compensated to the de-spreading demodulation module; in the de-spreading demodulation module, FFT operation is carried out on DS-GMSK baseband information with one symbol length, and FFT operation results are stored in an RAM; dividing the FFT operation result into two paths for processing, wherein one path reads the FFT result from the RAM and multiplies the FFT result by a local template 0, the product result is subjected to IFFT operation, the peak value of the IFFT operation result is output in a modulus mode, and the path outputs a despreading and demodulation result of a symbol with symbol information of 0; the other path reads the FFT result from the RAM and multiplies the FFT result by the local template 1, the subsequent processing process is the same as the first path, and the path finally outputs the despreading and demodulation result of the symbol with the symbol information of 1; and subtracting the two paths of output despreading and demodulation results to obtain complete despreading and demodulation soft information.
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