CN114900405B - Soc-based Acars signal demodulation method - Google Patents
Soc-based Acars signal demodulation method Download PDFInfo
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- CN114900405B CN114900405B CN202210321567.3A CN202210321567A CN114900405B CN 114900405 B CN114900405 B CN 114900405B CN 202210321567 A CN202210321567 A CN 202210321567A CN 114900405 B CN114900405 B CN 114900405B
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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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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 prior technical problems, and the invention provides an ACARS signal detection frequency channel which can be automatically set by a user in the frequency range of 117.975 MHz-137 MHz except for a controlled fixed frequency point from the beginning of down-conversion to the end of demodulation (ACARS signal adjustable frequency detection), thereby realizing detection processing of ACARS signals of the frequency point, providing an easy-to-realize structure, saving resources by comparing the structure and obviously improving the working efficiency of a receiver. Under the condition of ensuring the correct decoding rate, the algorithm of the invention is simpler to realize in hardware, not only improves the working efficiency of the system, but also saves a large amount of resources, and accords with the SoC idea of software radio.
Description
Technical Field
The invention relates to the field of communication software radio, in particular to a Soc-based Acars signal demodulation method.
Background
The very high frequency communication radio station of the ACARS system adopts a double-sideband amplitude modulation technology, and the ground-air communication mode is half duplex and the same as the ground-air wireless telephone communication mode. The communication protocol of Carrier Sense Multiple Access (CSMA) for collision detection is adopted when data communication is carried out in a bidirectional wireless channel in a ground space, the transmission rate is 2400bps, and the channel interval is 25KHz. This communication protocol allows all aircraft stations to monitor the activity of the wireless channel via a common communication medium prior to communication.
The ACARS signal takes the form of MSK/AM modulation. The baseband signal adopts an MSK modulation mode (1.2K represents "1",2.4K represents 0), and the symbol rate is 2.4K. The benefits of using MSK modulation for the baseband signal are many: firstly, 99.5% of MSK signal energy is concentrated in 1.5 times code element rate, so 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 linear and nonlinear amplifiers can be used, and the demodulation error rate is low.
In a friend or foe identification system, tasks such as identification demodulation and clear code interpretation of ACARS signals are required to be completed, and the accuracy of signal identification demodulation is critical to the MSK (minimum shift keying) demodulation error rate of baseband signals.
Disclosure of Invention
The invention aims at: the invention provides a Soc-based Acars signal demodulation method, which provides a user with an ACARS signal detection frequency channel capable of being set by the user in a 117.975 MHz-137 MHz frequency band range except for a controlled fixed frequency point from the beginning of down-conversion to the end of demodulation (ACARS signal adjustable frequency detection) under the condition of ensuring a decoding rate (the instantaneous dynamic range of a front stage is larger than 90 dB), and provides an easy-to-realize structure, which saves resources and can obviously improve the working efficiency of a receiver.
The invention adopts the following technical scheme for realizing the purposes:
a Soc-based Acars signal demodulation method comprises the following steps:
step 1: firstly, an ADC is utilized to collect an intermediate frequency signal output by a radio frequency front end, and an analog signal is changed into a digital signal; secondly, setting fixed frequency points and non-fixed frequency points according to the ACARS signal frequency range, wherein the non-fixed frequency points are ACARS signal frequency points set by a user by the user, and the fixed frequency points are not required to be issued by an arm issuing frequency control word, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is changed into a baseband signal after mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the down-converted signal are calculated;
step 3: performing AM demodulation on the down-converted I, Q two paths of data, and obtaining a I, Q modulus by adopting a cordic algorithm;
step 4: compensating the AM demodulated data, and moving the signal frequency point to zero intermediate frequency;
step 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 truncate signals;
step 6: on the basis of the step 5, the phase shift pi/2 is carried out on the quadrature component, the same-direction component is kept unchanged, and then the two paths of signals are multiplied;
step 7: and (3) judging the multiplied result in the step (6) to obtain a demodulation result.
As a preferable technical solution, in the step 2, a digital intermediate frequency signal is converted into a baseband signal by using a digital down-conversion technology; the digital down conversion firstly needs to be mixed, NCO frequency is controlled by setting a frequency control word, and then the mixed signal frequency is controlled, and the system clock frequency of the receiver is known as 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.
In the step 2, the mixed signal is subjected to decimation and filtering to obtain a required baseband signal, the decimation is to transform 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 This makes it possible to determine the reduction in the sampling rate from the input to the output by a factor of d=f clk /f out 。
In the step 4, the AM demodulated signal and the digital local oscillator NCO are mixed.
As a preferable technical solution, in the step 3, it is assumed that output signals of the down-converted I, Q two paths of data are the same-direction component X I (n), forward component X Q (n) solving for X by using cordic algorithm I (n) and X Q The modulus X (n) of (n) is:
as a preferred technical solution, in the step 5, a MATLAB self-contained FDATOOL design tool is used to design a low-pass filter that meets the requirements, and the coefficients thereof are derived, and then the filter coefficients are introduced into a 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 truncated signal in the step 5 is subjected to 1bit differential demodulation, and in one information symbol, the transmitted data is determined according to the phase change; i.e. passing the quadrature component through T b Time delay and co-directional path component multiplication, where T b Is the information symbol width.
As a preferable technical scheme, the result of the multiplication in the step 6 is judged, the signal output finally in the step 7 is set as y (t), the influence of noise is ignored, if y (t) is less than 0, the phase is reduced, and the judgment is 0; if y (t) >0, indicating a phase increase, the decision 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 beneficial effects of the invention are as follows:
1. under the condition of ensuring the correct decoding rate, the algorithm of the invention is simpler to realize in hardware, not only improves the working efficiency of the system, but also saves a large amount of resources and reduces the power consumption.
2. By utilizing the software radio idea, signals of fixed frequency points and non-fixed frequency points can be decoded at the same time, ACARS signals can be received and processed by changing parameters in software for the non-fixed frequency points, and multiple different frequency bands can be received at the same time.
3. The ACARS system has the advantages of low upgrading cost and easy transplanting by utilizing the advantage of the software radio technology.
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 in the present invention;
fig. 3 is a block diagram of an MSK demodulation implementation in the present invention.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. The components of the 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 invention, as 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 made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Example 1
A Soc-based Acars signal demodulation method comprises the following steps:
step 1: firstly, an ADC is utilized to collect an intermediate frequency signal output by a radio frequency front end, and an analog signal is changed into a digital signal; secondly, setting fixed frequency points and non-fixed frequency points according to the ACARS signal frequency range, wherein the non-fixed frequency points are ACARS signal frequency points set by a user by the user, and the fixed frequency points are not required to be issued by an arm issuing frequency control word, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is changed into a baseband signal after mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the down-converted signal are calculated;
step 3: performing AM demodulation on the down-converted I, Q two paths of data, and obtaining a I, Q modulus by adopting a cordic algorithm;
step 4: compensating the AM demodulated data, and moving the signal frequency point to zero intermediate frequency;
step 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 truncate signals;
step 6: on the basis of the step 5, the phase shift pi/2 is carried out on the quadrature component, the same-direction component is kept unchanged, and then the two paths of signals are multiplied;
step 7: and (3) judging the multiplied result in the step (6) to obtain a demodulation result.
When in operation, the device comprises: by utilizing the software radio idea, signals of fixed frequency points and non-fixed frequency points can be decoded at the same time, ACARS signals can be received and processed by changing parameters in software for the non-fixed frequency points, and multiple different frequency bands can be received at the same time.
Example 2
Firstly, the ADC samples, quantizes and holds the analog signal processed by the radio frequency end, then encodes the analog signal into a digital signal, and digital down-conversion is performed on the acquired digital signal, and the whole module comprises three parts, namely a Numerical Control Oscillation (NCO) module, a digital mixing module and a sampling conversion module (comprising extraction, interpolation and digital filtering). The digital local oscillator NCO controls NCO frequency by setting frequency control word, and the system clock frequency of the 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 mixing module mixes the digital signals acquired by the AD and the generated digital local oscillation signals to generate a baseband signal and a high-frequency component, and 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 mixed signals need to be subjected to extraction and filtering to obtain baseband signals needed by users, and the extraction is to convert the sampling rate of the signals by using a CIC extraction filter; the CIC filter has a coefficient of 1, so that the CIC filter has the advantages of addition and subtraction, no multiplication and high processing speed, and is suitable for being used as a first-stage extraction and large extraction factor working mode in an extraction system. Since the transition and stop band attenuation characteristics of CIC are not very good, multistage cascading is typically required. Determining a corresponding sampling rate f based on bandwidth out This allows determining the multiple of the sample rate from the input to the outputFor d=f clk /f out . Thirdly, the FIR filter performs shaping filtering on the channel, the CIC filter cannot be used independently, therefore, the CIC filter is compensated by the FIR filter, the passband, stop band, sampling and the like of the low-pass filter are designed by utilizing MATLAB self-contained FDATOOL design tools, the coefficients of the FIR filter are derived, then the coefficients of the filter are imported into a program, and the signal passing through the low-pass filter also needs bit cutting, so that the digital down-conversion function is completed. Fourth, the AM demodulation module uses a digital quadrature demodulation method; the AM demodulation module performs square sum operation on two I, Q orthogonal signals by using a multiplier and an adder, performs arithmetic square root on the square sum result by using a CORDIC algorithm, and then filters the result by using a low-pass filter to demodulate the modulated signal.
Example 3
The module mainly comprises three parts, namely a delay part, a differential multiplication part and a judgment part. The delay part is used for performing pi/2 phase shift on the quadrature component signal after frequency offset correction and delaying one clock, the in-phase component signal is delayed by one clock, and the differential multiplication part is used for multiplying the undelayed quadrature component signal and the undelayed homodromous component signal with the quadrature component signal and the delayed homodromous component signal after pi/2 phase shift by using a complex multiplier. The decision part is mainly used for deciding the signals after differential multiplication, namely: when the result of the differential multiplied signal is greater than 0, a decision 1 is made, otherwise, a decision is 0. The above description is only of the preferred embodiments of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and modifications within the spirit and principles of the invention will become apparent to those skilled in the art.
Claims (6)
1. The method for demodulating the Acars signal based on Soc is characterized by comprising the following steps:
step 1: firstly, an ADC is utilized to collect an intermediate frequency signal output by a radio frequency front end, and an analog signal is changed into a digital signal; secondly, setting fixed frequency points and non-fixed frequency points according to the ACARS signal frequency range, wherein the non-fixed frequency points are ACARS signal frequency points set by a user by the user, and the fixed frequency points are not required to be issued by an arm issuing frequency control word, and the frequency control word is generated internally;
step 2: the digital intermediate frequency signal is changed into a baseband signal after mixing, then sampling rate conversion and digital filtering are carried out, and then the frequency point and the sampling rate of the down-converted signal are calculated;
step 3: performing AM demodulation on the down-converted I, Q two paths of data, and obtaining a I, Q modulus by adopting a cordic algorithm;
step 4: compensating the AM demodulated data, and moving the signal frequency point to zero intermediate frequency;
step 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 truncate signals;
step 6: on the basis of the step 5, the phase shift pi/2 is carried out on the quadrature component, the same-direction component is kept unchanged, and then the two paths of signals are multiplied;
step 7: judging the multiplied result in the step 6 to obtain a demodulation result;
in the step 2, the digital intermediate frequency signal is converted into a baseband signal by utilizing a digital down-conversion technology; the digital down conversion firstly needs to be mixed, NCO frequency is controlled by setting a frequency control word, and then the mixed signal frequency is controlled, and the system clock frequency of the receiver is known as 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 Wherein f is the signal frequency;
in the step 7, the result of the multiplication in the step 6 is judged, the last output signal is set as y (t), the influence of noise is ignored, if y (t) is less than 0, the phase is reduced, and the judgment is 0; if y (t) >0, indicating a phase increase, the decision is 1; and recovering the information code element according to the positive and negative of the code element, thus completing the function of ACARS signal demodulation.
2. The method of claim 1, wherein in the step 2, the mixed signal is subjected to decimation and filtering to obtain a baseband signal, the decimation is performed by converting 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 This makes it possible to determine the reduction in the sampling rate from the input to the output by a factor of d=f clk /f out 。
3. The method for demodulating the Soc-based Acars signal according to claim 1, wherein in the step 4, the AM-demodulated signal and the digital local oscillator NCO are mixed.
4. The method of claim 1, wherein in the step 3, the output signals of the two paths of data of I, Q subjected to down-conversion are set as the same-direction component X I (n), forward component X Q (n) solving for X by using cordic algorithm I (n) and X Q The modulus X (n) of (n) is:
5. the method according to claim 1, wherein in step 5, MATLAB is used to design a low-pass filter with FDATOOL, and the coefficients are derived, and then the filter coefficients are introduced into the program, so that the signal passing through the low-pass filter still needs to be truncated.
6. The Soc-based Acars signal demodulation method according to claim 1,the method is characterized in that in the step 6, 1bit differential demodulation is carried out on the truncated signal in the step 5, and the transmitted data is judged according to the phase change in one information code element; i.e. passing the quadrature component through T b Time delay and co-directional path component multiplication, where T b Is the information symbol width.
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