CN114900405A - Acars signal demodulation method based on Soc - Google Patents
Acars signal demodulation method based on Soc Download PDFInfo
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- CN114900405A CN114900405A CN202210321567.3A CN202210321567A CN114900405A CN 114900405 A CN114900405 A CN 114900405A CN 202210321567 A CN202210321567 A CN 202210321567A CN 114900405 A CN114900405 A CN 114900405A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0008—Modulated-carrier systems arrangements for allowing a transmitter or receiver to use more than one type of modulation
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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
- H04B1/0003—Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain
- H04B1/0028—Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain wherein the AD/DA conversion occurs at baseband stage
- H04B1/0042—Digital filtering
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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
- H04B1/0003—Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain
- H04B1/0028—Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain wherein the AD/DA conversion occurs at baseband stage
- H04B1/0046—Decimation, i.e. data rate reduction techniques
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/02—Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation
- H04L27/06—Demodulator circuits; Receiver circuits
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
- H04L27/14—Demodulator circuits; Receiver circuits
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Abstract
The invention discloses an Acars signal demodulation method based on SoC, which relates to the technical field of communication software radio and solves the technical problems in the prior art, and the method starts from the down conversion to the demodulation end (adjustable frequency interception of ACARS signals, namely, in addition to a fixed frequency point of a control keeper, an ACARS signal interception frequency channel which can be set by a user in the frequency band range of 117.975 MHz-137 MHz is provided, so that the interception processing of the ACARS signals of the frequency point is realized), and an easily-realized structure is provided, so that the resource is saved, and the working efficiency of a receiver can be obviously improved. Under the condition of ensuring correct decoding rate, the algorithm of the invention is simpler to realize in hardware, thereby not only improving the working efficiency of the system, but also saving a large amount of resources and conforming to the SoC idea of software radio.
Description
Technical Field
The invention relates to the field of communication software radio, in particular to an Acars signal demodulation method based on Soc.
Background
The very high frequency communication radio station of the ACARS system adopts a double-sideband amplitude modulation technology, the ground-air communication mode is half duplex, and the ground-air communication mode is the same as the ground-air wireless telephone communication mode. A 'collision sense multiple access to Carrier Sense (CSMA)' communication protocol is used for data communication in a bidirectional wireless channel of a ground space, the transmission rate is 2400bps, and the channel interval is 25 KHz. The communication protocol enables all aircraft stations to monitor the radio channel activity through a common communication medium prior to communicating.
The ACARS signal adopts MSK/AM modulation form. The baseband signal adopts an MSK modulation mode (1.2K represents '1', 2.4K represents 0), and the code rate is 2.4K. The baseband signal has many benefits in the form of MSK modulation: firstly, 99.5% of MSK signal energy is concentrated in 1.5 times of code element rate, so that the out-of-band radiation of the signal is very small, the signal is very suitable for transmission in a narrow-band channel, and the interference to adjacent channels is also small; secondly, the MSK signal envelope is constant, so that a linear amplifier and a nonlinear amplifier can be used, and the error rate of demodulation is low.
Tasks such as identification and demodulation of the ACARS signal, clear code interpretation and the like need to be completed in a friend or foe identification system, and the accuracy of the signal identification and demodulation is mainly the MSK (minimum shift keying) demodulation error rate of a baseband signal.
Disclosure of Invention
The invention aims to: the invention provides an Acars signal demodulation method based on Soc, which is characterized in that under the condition of ensuring the decoding rate (the instantaneous dynamic range of a front stage is more than 90dB), the invention starts from the down conversion to the demodulation end (adjustable frequency interception of ACARS signals, namely, in addition to a fixed frequency point of control and guard, an ACARS signal interception frequency channel which can be set by a user is provided within the frequency band range of 117.975 MHz-137 MHz, so that the interception processing of the ACARS signals of the frequency point is realized), and an easily realized structure is provided, so that the structure saves resources and can also obviously improve the working efficiency of a receiver.
The invention specifically adopts the following technical scheme for realizing the purpose:
an Acars signal demodulation method based on Soc comprises the following steps:
step 1: firstly, acquiring an intermediate frequency signal output by a radio frequency front end by using an ADC (analog to digital converter), and converting an analog signal into a digital signal; secondly, setting a fixed frequency point and a non-fixed frequency point according to the frequency band range of the ACARS signal, wherein the non-fixed frequency point is the ACARS signal frequency point set by a user, the frequency of the frequency mixing is controlled by sending a frequency control word through an arm, the fixed frequency point does not need to be sent, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is converted into a baseband signal after frequency mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the signal after down-conversion are calculated;
and step 3: performing AM demodulation on I, Q paths of data subjected to down conversion, and solving a module I, Q by adopting a cordic algorithm;
and 4, step 4: compensating the data after AM demodulation, and moving the signal frequency point to zero intermediate frequency;
and 5: on the basis of the step 4, according to the requirements of bandwidth, rectangular coefficient, image rejection and sampling rate, an FIR filter is designed to filter and intercept signals;
and 6: on the basis of the step 5, the orthogonal component is subjected to phase shift pi/2, the homodromous component is kept unchanged, and then the two paths of signals are multiplied;
and 7: and 6, judging the multiplication result of the step 6 to obtain a demodulation result.
As a preferred technical solution, in the step 2, the digital intermediate frequency signal is converted into a baseband signal by using a digital down-conversion technique; the digital down-conversion needs to be subjected to frequency mixing, the NCO frequency is controlled by setting a frequency control word, and then the frequency of a signal after frequency mixing is controlled, and the system clock frequency of a known receiver is f clk Setting the value of the mixing frequency to f req The bit width of the phase accumulator is N according to the formula
The frequency point of the baseband signal after mixing is f-f req And the high frequency component is f + f req Where f is the signal frequency.
As a preferred technical solution, in the step 2, the mixed signal needs to be decimated and summedFiltering to obtain required baseband signal, extracting by using CIC extracting filter to convert signal sampling rate, and determining corresponding sampling rate f according to bandwidth out Thus, it can be determined that the sampling rate from the input terminal to the output terminal is reduced by a factor of D ═ f clk /f out 。
As a preferred technical solution, in the step 4, the AM demodulated signal and the digital local oscillator NCO are subjected to frequency mixing processing.
As a preferred technical solution, in the step 3, the output signals of the two paths of down-converted I, Q data are set as the same directional component X I (n), a forward component X Q (n) solving for X by using cordic algorithm I (n) and X Q Modulo x (n) of (n) is:
as a preferred technical solution, in the step 5, a MATLAB fdato tool is used to design a low-pass filter meeting the requirement, and the coefficient of the low-pass filter is derived, and then the filter coefficient is introduced into the program, and the signal passing through the low-pass filter also needs to be truncated.
As a preferable technical solution, in the step 6, the signal truncated in the step 5 is subjected to 1-bit differential demodulation, and transmitted data is determined according to a change of a phase within one information symbol; i.e. passing the orthogonal component through T b The delay is multiplied by the co-directional path component, where T b Is the information symbol width.
As a preferable technical solution, the result multiplied by step 6 is judged, the signal finally output in step 7 is set as y (t), the influence of noise is ignored, and if y (t) <0, the phase is reduced, and the judgment is 0; if y (t) >0, the phase is increased, and the judgment is 1;
therefore, the information code element can be recovered according to the positive and negative of the code element, thus completing the function of ACARS signal demodulation
The invention has the following beneficial effects:
1. under the condition of ensuring correct decoding rate, the algorithm of the invention is simpler to realize in hardware, thereby not only improving the working efficiency of the system, but also saving a large amount of resources and reducing the power consumption.
2. By using the idea of software radio, the signals of the fixed frequency point and the non-fixed frequency point can be decoded at the same time, and the ACARS signals can be received and processed by changing the parameters in the software for the non-fixed frequency point, so that the simultaneous reception of various different frequency bands is realized.
3. By utilizing the advantages of software radio technology, the ACARS system has low upgrading cost and is easy to transplant.
Drawings
FIG. 1 is a block diagram of a method implementation of the present invention;
FIG. 2 is a block diagram of an AM demodulation implementation of the present invention;
fig. 3 is a block diagram of an implementation of MSK demodulation in the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
An Acars signal demodulation method based on Soc comprises the following steps:
step 1: firstly, acquiring an intermediate frequency signal output by a radio frequency front end by using an ADC (analog to digital converter), and converting an analog signal into a digital signal; secondly, setting a fixed frequency point and a non-fixed frequency point according to the ACARS signal frequency range, wherein the non-fixed frequency point is an ACARS signal frequency point set by a user, the frequency of the mixing is controlled by issuing a frequency control word through an arm, the fixed frequency point does not need to be issued, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is converted into a baseband signal after frequency mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the signal after down-conversion are calculated;
and step 3: performing AM demodulation on I, Q paths of data subjected to down conversion, and solving a module I, Q by adopting a cordic algorithm;
and 4, step 4: compensating the data after AM demodulation, and moving the signal frequency point to zero intermediate frequency;
and 5: on the basis of the step 4, according to the requirements of bandwidth, rectangular coefficient, image rejection and sampling rate, an FIR filter is designed to filter and intercept signals;
step 6: on the basis of the step 5, the orthogonal component is subjected to phase shift pi/2, the homodromous component is kept unchanged, and then the two paths of signals are multiplied;
and 7: and 6, judging the multiplication result of the step 6 to obtain a demodulation result.
When in work: by using the idea of software radio, the signals of the fixed frequency point and the non-fixed frequency point can be decoded at the same time, and the ACARS signals can be received and processed by changing the parameters in the software for the non-fixed frequency point, so that the simultaneous reception of various different frequency bands is realized.
Example 2
Firstly, the ADC performs sampling, quantization and holding on an analog signal processed by the radio frequency terminal, and then encodes the analog signal into a digital signal, and performs digital down-conversion on the acquired digital signal, where the whole module includes three parts, namely, a Numerically Controlled Oscillator (NCO) module, a digital mixing module, and a sampling conversion module (including decimation, interpolation and digital filtering). The digital local oscillator NCO controls the NCO frequency by setting a frequency control word, and the system clock frequency of a known receiver is f clk Setting the mixing frequency to a value f req The bit width of the phase accumulator is N according to the formula
The frequency mixing module mixes the digital signal acquired by the AD with the generated digital local oscillator signal to generate a baseband signal and a high-frequency component, wherein the frequency point of the baseband signal is f-f req And the frequency point of the high-frequency component is f + f req Wherein f is the signal frequency; secondly, the signals after frequency mixing need to be extracted and filtered to obtain baseband signals needed by people, and the extraction is to convert the sampling rate of the signals by using a CIC (common information center) extraction filter; the coefficient of the CIC filter is 1, so that only addition and subtraction operation is performed, multiplication operation is not performed, the processing rate is high, and the CIC filter is very suitable for being used as a working mode of first-stage extraction and large extraction factor in an extraction system. Since the transition-band and stop-band attenuation characteristics of CIC are not very good, it is usually necessary to use multiple stages in cascade. Determining the corresponding sampling rate f according to the bandwidth out Thus, it can be determined that the sampling rate from the input terminal to the output terminal is reduced by a factor of D ═ f clk /f out . Thirdly, the FIR filter shapes and filters the channel, and the CIC filter cannot be used alone generally, so the FIR filter must be used for compensation after the CIC filter, and a pass band, a stop band, a sample and the like of the low-pass filter are designed by using a MATLAB self-band fdato design tool to derive the coefficient of the FIR filter, and then the filter coefficient is introduced into a program, and the signal passing through the low-pass filter needs to be cut off, so that the digital down-conversion function is completed. Fourthly, the AM demodulation module uses a digital quadrature demodulation method; the AM demodulation module performs square sum operation on the two I, Q orthogonal signals by using a multiplier and an adder, performs arithmetic square root on the result of the square sum by using a CORDIC algorithm, and filters the result through a low-pass filter to demodulate a modulation signal.
Example 3
The module mainly comprises three parts, namely a delay part, a difference multiplication part and a judgment part. The time delay part carries out pi/2 phase shift on the orthogonal component signal after frequency offset correction and delays one clock, the in-phase component signal also delays one clock, and the difference multiplication part multiplies the non-delayed orthogonal component signal and the non-delayed in-direction component signal by the complex multiplier, the quadrature component signal after pi/2 phase shift and the delayed in-direction component signal. The decision part mainly decides the signals after the difference multiplication, namely: when the result of the signal multiplied by the difference is greater than 0, 1 is judged, otherwise, 0 is judged. The above description is only for the purpose of illustrating the preferred embodiments of the present invention and should not be taken as limiting the scope of the present invention, and any modifications, equivalents and improvements made by those skilled in the art within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
1. An Acars signal demodulation method based on Soc is characterized by comprising the following steps:
step 1: firstly, acquiring an intermediate frequency signal output by a radio frequency front end by using an ADC (analog to digital converter), and converting an analog signal into a digital signal; secondly, setting a fixed frequency point and a non-fixed frequency point according to the frequency band range of the ACARS signal, wherein the non-fixed frequency point is the ACARS signal frequency point set by a user, the frequency of the frequency mixing is controlled by sending a frequency control word through an arm, the fixed frequency point does not need to be sent, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is converted into a baseband signal after frequency mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the signal after down-conversion are calculated;
and step 3: performing AM demodulation on I, Q paths of data subjected to down conversion, and solving a module I, Q by adopting a cordic algorithm;
and 4, step 4: compensating the data after AM demodulation, and moving the signal frequency point to zero intermediate frequency;
and 5: on the basis of the step 4, according to the requirements of bandwidth, rectangular coefficient, image rejection and sampling rate, an FIR filter is designed to filter and intercept signals;
step 6: on the basis of the step 5, the orthogonal component is subjected to phase shift pi/2, the homodromous component is kept unchanged, and then the two paths of signals are multiplied;
and 7: and 6, judging the multiplication result of the step 6 to obtain a demodulation result.
2. The Acars signal demodulation method based on Soc as claimed in claim 1, wherein in step 2, the digital intermediate frequency signal is converted into baseband signal by using digital down-conversion technique; the digital down-conversion needs to be subjected to frequency mixing, the NCO frequency is controlled by setting a frequency control word, and then the frequency of a signal after frequency mixing is controlled, and the system clock frequency of a known receiver is f clk Setting the value of the mixing frequency to f req The bit width of the phase accumulator is N according to the formula
The frequency point of the baseband signal after mixing is f-f req And the high frequency component is f + f req Where f is the signal frequency.
3. The Acars signal demodulation method based on Soc as claimed in claim 2, wherein in step 2, the mixed signal needs to be decimated and filtered to obtain the required baseband signal, the decimation is to convert the sampling rate of the signal by using a CIC decimation filter, and the corresponding sampling rate f is determined according to the bandwidth out Thus, it can be determined that the sampling rate from the input terminal to the output terminal is reduced by a factor of D ═ f clk /f out 。
4. The Acars signal demodulation method based on Soc as claimed in claim 2, wherein in step 4, the AM demodulated signal and the digital local oscillator NCO are mixed.
5. The method for demodulating Acars signal based on Soc according to claim 1, wherein in step 3, the output signals of I, Q data paths with frequency down-conversion are set as the same direction component X I (n), a forward component X Q (n) solving for X by using cordic algorithm I (n) and X Q Modulo x (n) of (n) is:
6. the method for demodulating Acars signals based on Soc as claimed in claim 1, wherein in step 5, MATLAB self-band fdato tool is used to design a low pass filter meeting the requirement, and derive its coefficients, then the filter coefficients are introduced into the program, and the signal passing through the low pass filter needs to be truncated.
7. The Acars signal demodulation method based on Soc as claimed in claim 1, wherein in step 6, the truncated signal in step 5 is subjected to 1-bit differential demodulation, and the transmitted data is determined according to the phase change in one information symbol; i.e. passing the orthogonal component through T b Multiplying the delayed version by the same way component, where T b Is the information symbol width.
8. The Acars signal demodulation method based on Soc according to claim 1, characterized in that the result of multiplication in step 6 is judged, the signal finally output in step 7 is set as y (t), the influence of noise is ignored, if y (t) <0, the phase is reduced, and the judgment is 0; if y (t) >0, the phase is increased, and the judgment is 1;
therefore, the information code element can be recovered according to the positive and negative of the code element, and the function of ACARS signal demodulation is completed.
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