CN112383494B - Burst communication receiving system based on DSSS-OQPSK - Google Patents
Burst communication receiving system based on DSSS-OQPSK Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7075—Synchronisation aspects with code phase acquisition
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7085—Synchronisation aspects using a code tracking loop, e.g. a delay-locked loop
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
- H04L27/22—Demodulator circuits; Receiver circuits
- H04L27/227—Demodulator circuits; Receiver circuits using coherent demodulation
- H04L27/2271—Demodulator circuits; Receiver circuits using coherent demodulation wherein the carrier recovery circuit uses only the demodulated signals
- H04L27/2273—Demodulator circuits; Receiver circuits using coherent demodulation wherein the carrier recovery circuit uses only the demodulated signals associated with quadrature demodulation, e.g. Costas loop
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
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- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
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Abstract
The invention discloses a burst communication receiving system based on DSSS-OQPSK, comprising: the device comprises a radio frequency front-end module, a signal acquisition module, a local carrier generation module, a carrier tracking module, a coherent demodulation module, a local spread spectrum code generation module, a correlation peak acquisition module, a code tracking module and a despreading module; the signal capturing module sequentially performs quadratic operation and high-pass filtering on the digital intermediate-frequency OQPSK signal, then completes coarse capturing on the burst signal through Fourier transformation, estimates frequency deviation at the receiving end and the transmitting end and performs coarse correction on a local carrier generated by the burst signal in real time; and the carrier tracking module adopts an improved costas loop facing the decision loop to complete carrier tracking according to the decision-oriented carrier phase estimation so as to complete frequency offset correction. The invention controls the frequency error in a very small range, thereby reducing the capture bandwidth and the locking time of the tracking phase-locked loop and improving the performance index of the system.
Description
Technical Field
The invention belongs to the technical field of communication systems, and particularly relates to a DSSS-OQPSK-based burst communication receiving system.
Background
In a DSSS (spread spectrum) system, in order to correctly recover user information, it is necessary to synchronize a spreading code for despreading generated at the receiving end with a spreading code at the transmitting end. Spreading code synchronization is generally performed in two steps: the first step is to search and capture the initial phase of the spread spectrum code, so that the code phase error with the transmitting end is less than 1bit, thus ensuring that the despread signal passes through a narrow-band intermediate frequency filter behind a correlator, namely the capture process; the second step is to further reduce the code phase error on the basis of the initial synchronization, so that the established synchronization is maintained, which is the code tracking process.
OQPSK (offset quadrature phase shift keying) is a constant envelope digital modulation improved over QPSK (quadrature phase shift keying), in which the symbols of the co-branch and the symbols of the quadrature branch are shifted in time by half a symbol period compared to QPSK signals. Therefore, in addition to the advantages of QPSK, OQPSK also eliminates the 180-degree phase jump phenomenon, and in a communication system with limited bandwidth, the envelope fluctuation is small, and no obvious power spectrum sidelobe hyperplasia effect is generated after the signal passes through the nonlinear power amplifier. Therefore, the OQPSK modulation has good spectrum efficiency and power efficiency, is widely applied in the field of satellite communication and the like, and has become one of the most common modulation modes in the nonlinear band-limited channel.
For a DSSS-OQPSK communication receiving system, the procedures of frequency offset correction, OQPSK demodulation, spread spectrum code synchronous acquisition, spread spectrum code synchronous tracking and despreading are generally carried out. Under the conventional condition, frequency offset correction is finished by depending on a phase-locked loop, OQPSK demodulation adopts coherent demodulation, the coherent demodulation is multiplied by a local carrier, and signals in two paths of I and Q are restored through integration and filtering processing; and restoring user information through the processes of synchronous capture of the spread spectrum codes, synchronous tracking of the spread spectrum codes and de-spreading. However, the phase-locked loop is used to correct the frequency offset, and there is a problem that, in the case of a relatively large frequency offset, the capture bandwidth of the phase-locked loop must be large, which increases the loop locking time and decreases the loop stability.
Disclosure of Invention
In order to solve the above problems, the present invention aims to provide a burst communication receiving system based on DSSS-OQPSK, which uses FFT frequency offset coarse estimation correction in combination with a loop-oriented costas loop to complete burst signal acquisition, frequency offset correction, and carrier tracking; the OQPSK signal is subjected to the quadratic operation, the high-pass filtering and the FFT to complete the capture of the burst signal, and the frequency error is controlled in a small range, so that the capture bandwidth and the locking time of the tracking phase-locked loop are reduced, and the system performance index is improved.
In order to achieve the above object, the present invention is achieved by the following means.
A DSSS-OQPSK based burst communication receiving system, comprising: the system comprises a radio frequency front-end module, a signal acquisition module, a local carrier generation module, a carrier tracking module, a coherent demodulation module, a local spread spectrum code generation module, a correlation peak acquisition module, a code tracking module and a despreading module; the radio frequency front end module converts burst OQPSK signals received by an antenna into digital intermediate frequency OQPSK signals and transmits the digital intermediate frequency OQPSK signals to the signal acquisition module and the coherent demodulation module respectively;
the local carrier generation module is used for generating two orthogonal local carriers and transmitting the local carriers to the coherent demodulation module;
the coherent demodulation module mixes the digital intermediate frequency OQPSK signals with two orthogonal local carriers respectively, obtains two demodulated signals of I and Q through an integral filter circuit, and transmits the two demodulated signals to the carrier tracking module and the related peak capturing module respectively;
the signal capturing module sequentially performs a fourth power operation and high-pass filtering on the digital intermediate frequency OQPSK signal to obtain a quadruple signal component; then Fourier transform is carried out on the quadruple signal component, coarse capture of the burst signaling is completed through spectrum peak detection, a captured signal is obtained, and the captured signal is transmitted to a carrier tracking module; estimating frequency offsets of the receiving end and the transmitting end, transmitting the frequency offsets to a local carrier generation module, and performing coarse correction on the generated local carrier in real time;
the carrier tracking module realizes carrier tracking through a carrier tracking loop, performs frequency tracking correction in real time, inputs frequency deviation into the local carrier generation module, and performs tracking correction on the local carrier generated by the local carrier generation module in real time;
the spread spectrum code generating module is used for generating a synchronous spread spectrum code in real time and transmitting the synchronous spread spectrum code to the correlation peak capturing module;
the related peak capturing module divides the first 128bit synchronous spread spectrum code into two parallel paths, each path of spread spectrum code is respectively in sliding correlation with the demodulated signals I and Q, and then the related peak is detected after passing through a matched filter, the detected position of the related peak is an initial synchronization position, the code capturing is completed, and the initial position is generated and transmitted to the code tracking module at regular time;
the code tracking module performs correlation operation on the timing of the demodulated signals of the I path and the Q path and other spreading codes according to the timing of the initial position to obtain a corresponding correlation peak position, and performs fine adjustment correction on the timing synchronous signal of the initial position according to the correlation peak position to obtain a fine-adjusted position timing signal;
and the despreading module performs despreading processing by adopting a spreading code corresponding to the position of the user information according to the finely adjusted bit timing signal to restore the user information.
Further, the digital intermediate frequency OQPSK signals are respectively mixed with two orthogonal local carriers, specifically: multiplying the digital intermediate frequency OQPSK signal by a cosine carrier, and obtaining an I-path signal through an integral filter circuit; and multiplying the digital intermediate frequency OQPSK signal by a sine carrier, and obtaining a Q-path signal through an integral filter circuit.
Furthermore, the carrier tracking loop comprises a coherent demodulation module, a phase discriminator, a loop filter and a local carrier generation module which are connected in sequence; the local carrier generation module is a numerical control oscillator.
Furthermore, the local carrier generation module depends on a numerically controlled oscillator to output a carrier consistent with the transmitting end, the numerically controlled oscillator generates the carrier according to the frequency control word, and the frequency control word generates the carrier according to f c *2 N /f clk Is generated wherein f c Is the carrier frequency, N is the carrier bit width, f clk Is the master clock frequency; and meanwhile, the local carrier is adjusted according to the frequency offset information of the signal acquisition module and the frequency offset information of the carrier tracking module.
Further, the process of detecting the correlation peak of the correlation peak capturing module is as follows: and performing correlation operation on a waveform generated by a local sequence generated by a local spread spectrum code generating module and a transmitting end waveform corresponding to the demodulated signals I and Q, generating a correlation peak when the two waveforms are completely matched, and positioning through the correlation peak to complete bit synchronization.
Further, the correlation operation is specifically: a correlator is adopted to circularly pass through at fixed time intervals, so that the demodulated signals of the two paths I and Q are subjected to correlation operation with a local sequence, whether the output result is greater than a threshold value or not is judged, if yes, the known signal sequence is determined to exist, and initial bit timing is obtained; otherwise, continuing to loop and repeating the process.
Furthermore, the threshold is determined dynamically, 200-300 relevant result data before the current time point are accumulated, and after the average value is obtained, the average value is multiplied by a threshold value to serve as the current threshold.
Further, the specific process of the despreading process is as follows: and carrying out multiplication and integration correlation operation by using the local spread spectrum code and the demodulated signals I and Q through the finely adjusted bit timing signal to obtain a sign bit of an integration result, and restoring the user information through serial-parallel conversion.
Compared with the prior art, the invention has the following beneficial effects:
(1) The invention improves the frequency deviation correction process, adopts FFT frequency deviation rough estimation correction, and combines with a loop-oriented costas loop to complete burst signal capture, frequency deviation correction and carrier tracking. By utilizing the characteristic that the carrier information can appear in the OQPSK signal quartic operation, the OQPSK signal is subjected to the quartic operation, the high-pass filtering is performed, then the capture of the burst signal is completed through FFT (fast Fourier transform), the frequency offset information is obtained, the carrier frequency offset correction is performed, the frequency error is controlled in a very small range, the capture bandwidth of a tracking phase-locked loop is small, the loop locking time is also reduced, and for the burst information, the smaller the loop locking time is, the smaller the system information loss is. Meanwhile, the carrier frequency error after locking is small, the sensitivity of coherent demodulation is improved, and the improvement of system performance indexes is facilitated.
(2) The invention enlarges the frequency deviation correction range, improves the carrier tracking locking time, reduces the carrier tracking phase error range and improves the performance of a receiving system aiming at burst signals.
(3) The invention adopts the modular design, has the characteristics of clear layers, simple structure, low complexity of FPGA hardware realization, less resource use, high utilization rate, stronger universality and the like, and has good maintainability and testability of software.
Drawings
The invention is described in further detail below with reference to the figures and the specific embodiments.
Fig. 1 is a schematic structural diagram of a DSSS-OQPSK-based burst communication receiving system according to the present invention;
FIG. 2 is a schematic diagram of the processing procedure of the coherent demodulation module of the present invention;
FIG. 3 is a schematic diagram of a frequency offset correction process of the present invention;
FIG. 4 is a schematic diagram of a local carrier generation module according to the present invention;
FIG. 5 is a schematic diagram of a code capture module according to the present invention;
fig. 6 is a schematic diagram of the despreading module processing procedure in accordance with the present invention.
Detailed Description
The embodiments and effects of the present invention will be described in further detail below with reference to the accompanying drawings.
In the embodiment, the information source of the originating signal is automatically generated by an originating user, the rate is 300bps, and the information source consists of a 50-bit synchronous code and 250-bit user information. After the user information is generated, the user information is sent out in a burst mode after meeting the sending condition. When the signal is sent, 300bit information is converted into odd-even two paths of signals, the odd-even two paths of signals are respectively multiplied by 128bit spreading codes, and the spreading codes are generated in real time according to m sequence generator polynomials. After the data after the spread spectrum is modulated, the data is mixed to radio frequency and then is sent out from an antenna port. The receiving is performed by a burst communication receiving system based on DSSS-OQPSK. The following definitions are made: frequency f of the master clock clk Is 31.104MHz; the intermediate frequency signal is 384KHz, the spreading code is m sequence, and the length is 128bit; the information data code rate is 300bps; carrier frequency f c Is 384KHz; intermediate frequency sampling frequency f s At 1536KHz.
Referring to fig. 1, the present invention provides a burst communication receiving system based on DSSS-OQPSK, including: the system comprises a radio frequency front-end module, a signal acquisition module, a local carrier generation module, a carrier tracking module, a coherent demodulation module, a local spread spectrum code generation module, a correlation peak acquisition module, a code tracking module and a despreading module; the radio frequency front end module converts burst OQPSK signals received by an antenna into digital intermediate frequency OQPSK signals and transmits the digital intermediate frequency OQPSK signals to the signal acquisition module and the coherent demodulation module respectively; specifically, a received radio frequency signal is amplified and mixed, then the received signal is moved to an intermediate frequency, and an analog-to-digital converter is used for performing analog-to-digital conversion to form a digital intermediate frequency signal;
the local carrier generation module is used for generating two orthogonal local carriers and transmitting the local carriers to the coherent demodulation module; the generation of specific local carrier depends on the carrier with NCO output consistent with the sending end, and the frequency control word is according to f c *2 N /f clk And generating and adjusting the local carrier according to the frequency offset information of the signal acquisition module and the frequency offset information of the carrier tracking module.
The coherent demodulation module mixes the digital intermediate frequency OQPSK signals with two orthogonal local carriers respectively, obtains demodulated signals of I and Q paths through an integral filter circuit, and transmits the demodulated signals to the carrier tracking module and the relevant peak capturing module respectively; specific I, Q two-path digital intermediate frequency signal and local carrier f generated by NCO c And (3) frequency mixing multiplication, wherein the I path is multiplied by cosine carrier, the Q path is multiplied by sine carrier, and the I and Q paths of information are restored through an integral filter circuit.
Referring to fig. 2, the oqpsk demodulation adopts coherent demodulation, and uses digital DDS to generate two local channels of orthogonal digital carriers, namely SIN and COS, which are multiplied by the received digital intermediate frequency signal to obtain two channels, I and Q, of signals, respectively. And filtering the signals of the I path and the Q path by a low-pass filter to filter high-frequency components. And performing integral operation on the filtered signals to obtain demodulated IQ two-path data signals, namely signals to be despread.
The signal capturing module sequentially performs quadratic operation and high-pass filtering on the digital intermediate-frequency OQPSK signal to obtain a quadruple signal component; then Fourier transform is carried out on the quadruple signal component, coarse capture of the burst signaling is completed through spectrum peak detection, a captured signal is obtained, and the captured signal is transmitted to a carrier tracking module; and estimating the frequency offset of the receiving and transmitting ends, transmitting the frequency offset to a local carrier generation module, and performing coarse correction on the generated local carrier in real time. The module filters low-frequency information by performing a quadratic operation on a digital intermediate-frequency OQPSK signal and a high-pass filter, and only four times of carrier components are reserved. Then, carrier frequency offset information can be obtained through FFT, coarse correction of carrier frequency offset is carried out, signal capture is completed, and then a carrier tracking module is started.
The carrier tracking module realizes carrier tracking through a carrier tracking loop, performs frequency tracking correction in real time, inputs frequency deviation into the local carrier generation module, and performs tracking correction on the local carrier generated by the local carrier generation module in real time; the specific carrier tracking module is combined with the local carrier generation module according to the phase estimation facing to the judgment carrier, so that a costas loop facing to the judgment loop is realized, the carrier tracking is completed, and an NCO (numerically controlled oscillator) is controlled to output the carrier consistent with a transmitting end in real time.
Referring to fig. 3, the frequency offset correction process of the present invention is as follows:
in practical communication applications, not only the influence of system frequency offset but also the influence of doppler shift exists. The range of doppler shift will generally exceed the range of ± 1KHz, and the acquisition bandwidth and the lock time are taken into account in carrier tracking, so the acquisition bandwidth cannot be too large. Therefore, the frequency offset correction is divided into two parts, one part is to roughly correct the signal by using FFT (fast Fourier transform), and pull the frequency offset into a reasonable carrier tracking range; and the other part adopts a carrier tracking loop to carry out carrier tracking correction. For a wideband OQPSK modulated wave, the carrier spectrum is not obtained by FFT.
Compared with QPSK, OQPSK has only a Q-path half-chip delay, so the derivation is taken as an example of a QPSK signal. The QPSK signal is represented as: acos (2 π f) c t)+bsin(2πf c t), the QPSK signal is subjected to a fourth power operation to obtain
As can be seen from the formula, 4f can be obtained only after high-pass filtering c Of the frequency spectrum of (c). Therefore, the low-frequency signal can be filtered by adopting a fourth power operation and a high-pass filter, and then the FFT operation is carried out to obtain a quadruple carrier frequency, so that the frequency deviation of the local carrier is obtained, and the coarse frequency deviation adjustment is carried out. And the acquisition of the burst signal can be completed at the same time through the estimation of the frequency spectrum.
For tracking of OQPSK signals, a multi-phase costas loop implementation is generally employed. In the implementation, a decision-oriented feedback loop is adopted for simplifying the operation. The working principle is as follows: firstly, coherent pre-demodulation is carried out on a received signal, and the demodulated signal is used for counteracting modulation information of the received signal, so that a phase error is obtained, carrier phase extraction is realized, and a loop phase difference is obtained.
The parameter design of the loop filter mainly considers the tracking acquisition bandwidth required by the system and the loop locking time. First, the loop noise bandwidth B is determined L And calculating the loop natural angular frequency omega n . In this example, the symbol rate is 19.2Kbps L Take 0.96kHz, according to formula (1)
Easily obtain, omega n =1810 (rad/s) =288Hz, wherein xi is a system constant, and 0.707 is taken.
The calculation formula of the loop gain K is as shown in formula (2), and the parameter C of the loop filter 1 、C 2 The calculation formula is as shown in formula (3) and formula (4):
in the formula, T dds For time delay, B loop Is the loop effective bandwidth;
adjusting design parameters, T, according to the system dds Is 4,B loop If the loop gain K is 29, the loop gain K is 0.7854, and the loop filter parameters are obtained by substituting equations (3) and (4). In the engineering implementation process, system parameters can be finely adjusted according to actual conditions. After the parameters of the loop filter are determined, whether the pole position is in the unit circle or not is calculated so as to ensure that the system works stably. TrackingAnd after the loop is established, the carrier tracking correction function is completed.
Referring to fig. 4, the digital DSS is generated with the IP core NCO of the FPGA. In coherent demodulation, to improve system performance, system frequency offset should be as small as possible, which requires high NCO output accuracy. NCO output frequency f out ＝M*f clk /2 N M is a frequency control word, if (bit width) N =32, then f out The resolution of (2) is 0.00035762786865234375Hz. After the control logic is started up, inputting an initial frequency control word M = f to the NCO c *2 N /f clk And controlling the NCO to output the local carrier wave. And after capturing the burst signal, controlling the frequency word of the NCO according to the coarse frequency offset value, and performing coarse correction on the output local carrier frequency offset. And then, correcting the output local carrier frequency offset in real time according to the carrier tracking output to generate an accurate local carrier signal.
The spread spectrum code generating module is used for generating a synchronous spread spectrum code in real time and transmitting the synchronous spread spectrum code to the correlation peak capturing module; and the spread spectrum code generating module generates the spread spectrum code in real time according to the system state.
The related peak capturing module divides the first two 128-bit synchronous spread spectrum codes into two parallel paths, each path of spread spectrum code is respectively in sliding correlation with the demodulated signals I and Q, and then the related peaks are detected after passing through a matched filter, the detected related peak positions are initial synchronization positions, code capturing is completed, and the initial positions are generated and transmitted to the code tracking module at regular time. For example, in the system, a 50-bit (25-bit for each of the I path and the Q path) synchronization code is available, and the IQ two-path signals are subjected to sliding correlation by using spreading codes of bit1, bit2, bit3 and bit4, respectively, to complete acquisition of a correlation peak.
The correlation peak acquisition module performs code acquisition, and referring to fig. 5, in the direct sequence spread spectrum system, the code acquisition is actually to implement bit synchronization, and the bit synchronization problem can be regarded as an attempt to synchronize samples between the local sequence and the originating sequence. In principle, matched filters or cross-correlations are the best way to establish synchronization. In the embodiment, a cross-correlation method is adopted, correlation operation is performed on a waveform generated by a local sequence and a received transmitting end waveform, when the waveforms are completely matched, a correlation peak is generated, and bit synchronization is completed through correlation peak positioning.
And the code tracking module performs correlation operation on the timing of the demodulated signals of the I path and the Q path and other spreading codes according to the timing of the initial position to obtain a corresponding correlation peak position, and performs fine adjustment correction on the timing synchronous signal of the initial position according to the correlation peak position to obtain a fine-adjusted position timing signal.
The code acquisition and code tracking module comprises the following correlation operation processes: the received sequence is correlated with the local sequence using a correlator that cycles through at regular intervals, and the output is compared to a threshold to determine if a known signal sequence is present. If the threshold is not exceeded, the loop continues, repeating the process until the threshold is exceeded, resulting in an initial bit timing.
The threshold value is determined in a dynamic manner, 200 pieces of relevant result data before the current time point are accumulated, and after the average value is obtained, the average value is multiplied by a threshold value to serve as the current threshold value. After the initial bit timing is obtained, other bits of the 50-bit synchronization bit are utilized to continue to carry out correlation peak detection, and according to the positions of the correlation peaks, bit timing signals are finely corrected to determine accurate bit timing signals.
And the despreading module performs despreading processing by adopting a spreading code corresponding to the position of the user information according to the finely adjusted bit timing signal to restore the user information.
Referring to fig. 6, spreading code despreading: through the finely adjusted bit timing signal, the local spread spectrum code and the receiving sequence are used for carrying out the correlation operation of multiplication and integration to obtain the sign bit of the integration result, and the user information is restored through serial-parallel conversion to complete the receiving function of the system.
In the embodiment of the invention, the acquisition of the OQPSK modulated signal of the burst adopts FFT estimation after the fourth power of the received signal, and the frequency value is detected to complete the acquisition of the burst signal.
In the embodiment of the invention, the frequency offset correction of the OQPSK modulated signal of burst adopts FFT estimation after the fourth power of the received signal to calculate the frequency offset and carry out rough correction; and then the carrier tracking loop carries out tracking correction.
In the embodiment of the invention, the code capture adopts a mode of sampling point sliding correlation and detecting the correlation peak, and the correlation peak detection threshold value is dynamically determined, so that the system difference can be dynamically adapted, the false alarm is effectively reduced, and the reliability and the universality of the sliding correlation detection are improved.
In the embodiment of the invention, for the modulated carrier, coherent pre-demodulation is firstly carried out on the received signal, the demodulated signal is used for counteracting the modulation information of the received signal, so that the phase error is obtained, the carrier phase extraction is realized, the loop phase difference is obtained, and the loop tracking is carried out. This approach reduces the implementation complexity of the carrier tracking loop.
The receiving system of the invention firstly carries out coherent demodulation, the demodulated signals of the I path and the Q path are then carried out correlation operation with the spread spectrum code to complete the code synchronization of burst information, and then the spread spectrum code tracking and de-spreading operation are started.
Performing a quadratic operation on the OQPSK signal, wherein the generated polynomial has information of fourfold carrier; filtering low-frequency information through a high-pass filter, and only reserving quadruple carrier wave components; then, carrier frequency offset information can be obtained through FFT, and coarse correction of carrier frequency offset is carried out. And according to the decision-oriented carrier phase estimation, adopting an improved costas loop oriented to a decision loop to finish carrier tracking and finish frequency offset correction. And carrying out coherent demodulation, carrying out correlation operation on the demodulated signals of the I path and the Q path and a spread spectrum code, completing code synchronization of burst information, starting spread spectrum code tracking and despreading operation, and restoring information sent by a user.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (8)
1. A DSSS-OQPSK based burst communication receiving system, comprising: the device comprises a radio frequency front-end module, a signal acquisition module, a local carrier generation module, a carrier tracking module, a coherent demodulation module, a local spread spectrum code generation module, a correlation peak acquisition module, a code tracking module and a despreading module; the radio frequency front-end module converts burst OQPSK signals received by an antenna into digital intermediate-frequency OQPSK signals and respectively transmits the digital intermediate-frequency OQPSK signals to the signal acquisition module and the coherent demodulation module;
the local carrier generation module is used for generating two orthogonal local carriers and transmitting the local carriers to the coherent demodulation module;
the coherent demodulation module mixes the digital intermediate frequency OQPSK signals with two orthogonal local carriers respectively, obtains demodulated signals of I and Q paths through an integral filter circuit, and transmits the demodulated signals to the carrier tracking module and the relevant peak capturing module respectively;
the signal capturing module sequentially performs quadratic operation and high-pass filtering on the digital intermediate-frequency OQPSK signal to obtain a quadruple signal component; fourier transform is carried out on the quadruple signal component, coarse capture of the burst signal is completed through spectrum peak detection, a captured signal is obtained and is transmitted to a carrier tracking module; estimating frequency offsets of the receiving end and the transmitting end, transmitting the frequency offsets to a local carrier generation module, and carrying out coarse correction on the generated local carrier in real time;
the carrier tracking module realizes carrier tracking through a carrier tracking loop, performs frequency tracking correction in real time, inputs frequency deviation into the local carrier generation module, and performs tracking correction on the local carrier generated by the local carrier generation module in real time;
the spread spectrum code generating module is used for generating a synchronous spread spectrum code in real time and transmitting the synchronous spread spectrum code to the correlation peak capturing module;
the related peak capturing module divides the first 128-bit synchronous spread spectrum code into two parallel paths, each path of spread spectrum code is respectively in sliding correlation with the demodulated signals of the I path and the Q path, and then the related peak is detected after passing through a matched filter, the detected position of the related peak is an initial synchronization position, the code capturing is completed, and the initial position is generated and transmitted to the code tracking module at regular time;
the code tracking module performs correlation operation on the timing of the demodulated signals of the two paths I and Q and other spread spectrum codes according to the timing of the initial bit to obtain a corresponding correlation peak position, and fine-tunes and corrects the timing synchronous signal of the initial bit according to the correlation peak position to obtain a fine-tuned bit timing signal;
and the despreading module performs despreading processing by adopting a spreading code corresponding to the position of the user information according to the finely adjusted bit timing signal to restore the user information.
2. The DSSS-OQPSK based burst communication receiving system according to claim 1, wherein the digital intermediate frequency OQPSK signals are mixed with two orthogonal local carriers, respectively, specifically: multiplying the digital intermediate frequency OQPSK signal by a cosine carrier, and obtaining an I-path signal through an integral filter circuit; and multiplying the digital intermediate frequency OQPSK signal by a sine carrier, and obtaining a Q-path signal through an integral filter circuit.
3. The DSSS-OQPSK based burst communication receiving system according to claim 1, wherein said carrier tracking loop includes a coherent demodulation module, a phase detector, a loop filter, and a local carrier generation module, connected in sequence; the local carrier generation module is a numerical control oscillator.
4. The DSSS-OQPSK based burst communication receiving system according to claim 1, wherein the local carrier generation module outputs a carrier in accordance with the origination based on a numerically controlled oscillator, the numerically controlled oscillator generates the carrier based on a frequency control word, the frequency control word is based on f c *2 N /f clk Is generated wherein f c Is the carrier frequency, N is the carrier bit width, f clk Is the master clock frequency; and meanwhile, the local carrier is adjusted according to the frequency offset information of the signal acquisition module and the frequency offset information of the carrier tracking module.
5. The DSSS-OQPSK based burst communication receiving system according to claim 1, wherein said correlation peak acquisition module detects the correlation peak by: and performing correlation operation on a waveform generated by a local sequence generated by a local spread spectrum code generating module and a transmitting end waveform corresponding to the demodulated signals I and Q, generating a correlation peak when the two waveforms are completely matched, and positioning through the correlation peak to complete bit synchronization.
6. The DSSS-OQPSK based burst communication receiving system according to claim 5, wherein said correlation operation is specifically: a correlator is adopted to circularly pass through at fixed time intervals, so that the demodulated signals of the I path and the Q path are subjected to correlation operation with a local sequence, whether the output result is greater than a threshold value or not is judged, if yes, the known signal sequence is determined to exist, and the initial bit timing is obtained; otherwise, continuing to loop and repeating the process.
7. The DSSS-OQPSK based burst communication receiving system according to claim 6, wherein the threshold is determined by dynamically accumulating 200-300 correlation results before the current time point, averaging, and multiplying by a threshold as the current threshold.
8. The DSSS-OQPSK based burst communication receiving system according to claim 1, wherein the despreading process is specifically performed by: and carrying out multiplication and integration correlation operation by using a local spread spectrum code and the demodulated signals I and Q through the finely adjusted bit timing signal to obtain a sign bit of an integration result, and restoring the user information through serial-parallel conversion.
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