CN111884981B - A Signal Processing Method Applicable to High Sensitivity Spaceborne ADS-B Receiver - Google Patents
A Signal Processing Method Applicable to High Sensitivity Spaceborne ADS-B Receiver Download PDFInfo
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Abstract
本发明涉及一种适用于高灵敏度星载ADS‑B接收机的信号处理方法，属于航空监视技术领域，包括S1：天线接收的ADS‑B信号经射频解调得到中频信号，数模转换后数字下变频解调获得数字基带信号；S2：低通滤波：初步滤除带外噪声；S3：粗同步检测，提取粗同步信号段；S4：粗频/相偏估计和补偿；S5：升余弦滤波器：提升信噪比；S6：精同步检测，提取精同步信号段；S7：非相干解调，若通过校验，则输出ADS‑B消息；若未通过，则实施步骤S8；S8：进行精频/相偏估计和补偿，并相干解调，若通过校验，则输出ADS‑B消息；若未通过，则丢弃本条数据。本方法检测率高，虚警率低，译码成功率高。
The invention relates to a signal processing method suitable for a high-sensitivity spaceborne ADS-B receiver, belonging to the technical field of aviation surveillance. The digital baseband signal is obtained by down-conversion demodulation; S2: Low-pass filtering: Preliminarily filtering out out-of-band noise; S3: Coarse synchronization detection, extracting the coarse synchronization signal segment; S4: Coarse frequency/phase offset estimation and compensation; S5: Raised cosine filtering S6: fine synchronization detection, extracting the fine synchronization signal segment; S7: non-coherent demodulation, if the verification is passed, the ADS-B message is output; if not, the implementation of step S8; S8: carry out Accurate frequency/phase offset estimation and compensation, and coherent demodulation. If the verification is passed, the ADS‑B message will be output; if not, the data will be discarded. The method has high detection rate, low false alarm rate and high decoding success rate.
Description
Technical Field
The invention belongs to the technical field of aviation monitoring, and relates to a signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver.
Background
Because the distance between the satellite and the aircraft is long, the ADS-B signal is weak, the signal-to-noise ratio is low, the minimum power required to be processed by calculation is-102 dBm signals, the decoding rate needs to be more than 90%, a high-sensitivity satellite-borne ADS-B receiver is required, the ground-based ADS-B receiver can only process signals with the minimum power of-90 dBm, and the signal processing method is not applicable. The signal processing of the existing satellite-borne ADS-B receiver aims at the characteristics of a satellite-based ADS-B receiver, improves the signal processing method of the terrestrial-based ADS-B receiver, but does not meet the sensitivity requirement, cannot complete the effective reception of the ADS-B signal transmitted by an aircraft in the coverage area of a receiving antenna, and influences the usability of the satellite-borne ADS-B receiver.
At present, the signal processing of the satellite-based ADS-B receiver adopts the improvement based on the signal processing flow of the terrestrial-based ADS-B receiver. A traditional ADS-B signal processing method of a land-based receiver is shown in figure 1, a Mode S1090 MHz ADS-B signal received by an antenna is demodulated by radio frequency to obtain an intermediate frequency signal, digital-to-analog conversion is performed, digital down-conversion demodulation is performed to obtain a digital baseband signal, low-pass filtering is performed to filter out-of-band noise and then frame header detection is performed for synchronization, frame header detection is a method based on pulse edge, pulse shape and pulse power, pulse distortion is large under a satellite-borne condition, the detection rate cannot meet the sensitivity requirement of the satellite-borne ADS-B receiver under the method, bit judgment and confidence extraction are performed after frame header detection, bit judgment is performed by a central comparison method, information is few, CRC is performed on information bits after bit judgment, error detection and correction are not performed, the number of error bits is large under the satellite-borne condition, effective error correction is required to improve the decoding rate, and if correct CRC, the message is successfully decoded, and if the message is checked to be wrong, the message is discarded. The traditional ADS-B signal processing flow of the land-based receiver is simple, the operand is small, the sensitivity is improved by using a coherent demodulation method without considering, and the decoding of weak signals under the satellite-borne condition cannot be completed.
Disclosure of Invention
In view of the above, the present invention is directed to a signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver.
In order to achieve the purpose, the invention provides the following technical scheme:
a signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver comprises the following steps:
s1: the ADS-B signal received by the antenna is demodulated by radio frequency to obtain an intermediate frequency signal, and is demodulated by digital down-conversion after digital-to-analog conversion to obtain a digital baseband signal;
s2: carrying out low-pass filtering on the digital baseband signal to preliminarily filter out-of-band noise;
s3: carrying out coarse synchronization detection and extracting a coarse synchronization signal section;
s4: carrying out coarse frequency offset and phase offset estimation and compensation;
s5: the signal-to-noise ratio is improved through a raised cosine filter with a signal bandwidth;
s6: carrying out fine synchronization detection and extracting a fine synchronization signal section;
s7: carrying out incoherent demodulation, and outputting ADS-B information if the incoherent demodulation passes CRC check; if not, go to step S8;
s8: performing fine frequency offset and phase offset estimation and compensation, performing coherent demodulation, and outputting an ADS-B message if the coherent demodulation passes CRC (Cyclic redundancy check); if not, the data is discarded.
Further, the bandwidth of the low-pass filter performing the low-pass filtering in step S2 is the sum of the signal bandwidth and the reserved frequency offset value, so as to avoid the loss of signal power due to too small bandwidth, and convert to a proper sampling rate.
Further, the step S3 specifically includes the following steps:
s31: the signal passes through a low-pass filter, the signal energy is gathered, the energy peak value is extracted to obtain a time range possibly containing the ADS-B signal, and meanwhile, the signal-to-noise ratio of the message in the time range is estimated;
s32: setting an interval according to the signal-to-noise ratio, extracting signal segments within the interval range from a memory, wherein each signal segment only contains one ADS-B message, coarse synchronization detection and signal segment extraction are convenient for improving synchronous detection probability and reducing synchronous false alarm rate, and signal segment extraction is convenient for parallel implementation of subsequent steps.
Further, the step S4 specifically includes:
and (3) adopting fast Fourier transform in each signal segment, estimating coarse frequency offset and phase offset, compensating in the signal segment, and limiting the frequency offset in a certain range. The frequency of the envelope of the real part (imaginary part) of the signal is reduced, so that the resolution and the estimation range of the integral frequency offset and phase offset estimation are conveniently improved during the precise frequency offset and phase offset estimation, conditions are created for fitting the envelope of the real part (imaginary part) during the precise frequency offset and phase offset estimation, and in addition, due to certain frequency offset estimation and compensation, most of frequency offset can be eliminated, so that the low-pass filter with the available bandwidth approximate to the signal bandwidth filters more out-of-band noise, and the signal-to-noise ratio is improved.
Further, the step S6 specifically includes the following steps:
because the frame header and the first 3bit waveform of the ADS-B signal are fixed, a matched filtering method is adopted, the impulse response of the filter is set as the frame header and the fixed first 3bit of the ADS-B signal, filtering is carried out in a signal section after coarse frequency offset and phase offset compensation and filtering, the peak value is extracted, the position of the frame header is determined, and the signal section of the signal length after the position of the frame header is extracted in a memory.
The ADS-B information for completing synchronization is obtained at the moment, the synchronization algorithm combining coarse synchronization and fine synchronization in the invention fully utilizes the energy difference between the signal section and the noise and the fixed waveform information of the frame header and the fixed data bit, and compared with the original synchronization method based on the pulse edge, the pulse shape and the pulse power, the false alarm rate is low and the detection rate is high. Tests prove that when the signal-to-noise ratio is 1.01dB, if the detection rate is 100%, the false alarm rate of the 'only fine synchronization' method is up to 76.9%, but the synchronization method in the invention has no false alarm. When the signal-to-noise ratio is 2.01dB, the detection rate of the synchronization method is improved by 99% compared with that of the original method.
Further, the non-coherent demodulation in step S7 includes the steps of:
bit decision and confidence extraction: obtaining data and a confidence matrix of each possible message, wherein the confidence matrix is extracted by adopting a maximum likelihood ratio method;
error detection and correction: and performing CRC (cyclic redundancy check) on the data after the bit judgment, performing violent decoding by combining the confidence coefficient matrix under the condition that the CRC fails, and performing CRC on a violent decoding result to verify the decoding result.
Further, the fine frequency offset and phase offset estimation and compensation in step S8 specifically includes:
and after the frequency deviation and the phase deviation value are estimated by adopting a least square fitting method after linear interpolation, extracting a signal section from a memory for compensation.
Because the ADS-B signal is a pulse position modulation signal, the pulse position distribution is uneven, the time matrix of least square estimation can be fixed through interpolation, so that inversion operation is avoided, the complexity of matrix operation is reduced, and least square fitting is performed on the phase after interpolation, so that frequency offset and phase offset are further eliminated.
The frequency offset and phase offset estimation and compensation method designed for the satellite-based ADS-B signal in the invention improves the signal-to-noise ratio of the signal by about 3dB during decoding, while the original signal processing method does not use frequency offset and phase offset estimation and compensation to realize coherent demodulation and improve the decoding rate, and the general frequency offset and phase offset estimation and compensation generally only use fixed bits, but the method in the invention utilizes the whole segment of information, and through experimental verification, when the SNR (2MHz) is 5.061dB, the decoding rates of noncoherent demodulation and coherent demodulation are respectively 58.8% and 90.7%, the decoding rate is greatly improved, and the performance of the coherent demodulation method is obviously improved.
Further, the coherent demodulation step in step S8 is the same as the non-coherent demodulation step in step S7.
The invention has the beneficial effects that:
1. the sensitivity of the signal processing method provided by the invention is more than or equal to-102 dBm (the decoding rate is more than or equal to 90 percent), and the effective receiving of the ADS-B signal transmitted by the aircraft in the coverage area of the receiving antenna can be completed.
2. The signal processing method provided by the invention uses a coarse signal synchronization method and a fine signal synchronization method, the detection rate is high, the false alarm rate is low, and the decoding success rate is higher than that of the traditional method.
3. The signal processing method provided by the invention uses the methods of coarse frequency offset estimation and compensation and fine frequency offset and phase offset estimation and compensation, utilizes the phase information to improve the sensitivity, has good adaptability to the frequency/phase offset caused in the processes of a transmitter, a channel and receiving, and has higher decoding success rate compared with the traditional method when the frequency/phase offset is serious.
4. The signal processing method provided by the invention is designed aiming at the hardware resources on the satellite, and is convenient for hardware transplantation.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.
Drawings
For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a flow chart of a conventional ADS-B signal processing method for a terrestrial-based receiver;
FIG. 2 is a schematic flow chart of a signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver according to the present invention;
fig. 3 is a graph of the correct unpacking rate and the signal power according to the embodiment of the present invention.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.
Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.
The invention provides a signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver, as shown in figure 2, a Mode S1090 MHz ADS-B signal received by an antenna is subjected to radio frequency demodulation to obtain an intermediate frequency signal, digital down-conversion demodulation is carried out after digital-to-analog conversion to obtain a digital baseband signal, and out-of-band noise is primarily filtered through low-pass filtering (1), wherein the bandwidth of a low-pass filter is the sum of the signal bandwidth and a reserved frequency offset value, so that the loss of signal power caused by too small bandwidth is avoided, and the signal power is converted into a proper sampling rate.
And then carrying out coarse synchronization detection, enabling the signal to pass through a low-pass filter, gathering signal energy, extracting an energy peak value to obtain a time range possibly containing ADS-B signals, estimating the signal-to-noise ratio of the message in the time range, setting an interval according to the signal-to-noise ratio, extracting a signal section in the interval range in a memory, wherein each signal section only contains one ADS-B message, and the coarse synchronization detection and the signal section extraction are convenient for improving the synchronous detection probability and reducing the synchronous false alarm rate, and the signal section extraction is convenient for realizing the parallel of subsequent steps.
And then, carrying out coarse frequency/phase offset estimation and compensation, adopting fast Fourier transform in each signal section to estimate coarse frequency/phase offset, compensating in the signal section, limiting the frequency offset in a certain range after compensation, reducing the frequency of a real part (imaginary part) envelope of a signal, facilitating the improvement of the resolution and the estimation range of the whole frequency/phase offset estimation in the fine frequency/phase offset estimation, creating conditions for carrying out real part (imaginary part) envelope fitting in the fine frequency/phase offset estimation, and eliminating most of frequency offset due to certain frequency offset estimation and compensation, so that a low-pass filter with available bandwidth approximate to signal bandwidth filters more out-of-band noise, improves the signal-to-noise ratio, and passes through a raised cosine filter with signal bandwidth after estimation to further improve the signal-to-noise ratio for low-pass filter (2) in the graph.
And then carrying out fine synchronization detection, wherein the frame header of the ADS-B signal is fixed with the first 3bit waveform, a matched filtering method is adopted, the impulse response of the filter is set as the frame header of the ADS-B signal and the fixed first 3bit, filtering is carried out in a signal section after coarse frequency/phase offset compensation and filtering, a relevant peak value is extracted, the position of the frame header is determined, and a signal section of the signal length after the position of the frame header is extracted from a memory.
The ADS-B information for completing synchronization is obtained at the moment, the synchronization algorithm combining coarse synchronization and fine synchronization in the invention fully utilizes the energy difference between the signal section and the noise and the fixed waveform information of the frame header and the fixed data bit, and compared with the original synchronization method based on the pulse edge, the pulse shape and the pulse power, the false alarm rate is low and the detection rate is high. Tests prove that when the signal-to-noise ratio is 1.01dB, if the detection rate is 100%, the false alarm rate of the 'only fine synchronization' method is up to 76.9%, but the synchronization method in the invention has no false alarm. When the signal-to-noise ratio is 2.01dB, the detection rate of the synchronization method is improved by 99% compared with that of the original method.
Then firstly, non-coherent demodulation is carried out, wherein the non-coherent demodulation is divided into two steps including bit decision and confidence extraction, error detection and error correction. And performing bit judgment and confidence extraction to obtain data and a confidence matrix of each possible message, wherein the extraction of the confidence matrix adopts a maximum likelihood ratio method, performing CRC (cyclic redundancy check) on the data subjected to bit judgment, performing violent decoding by combining the confidence matrix under the condition that the CRC fails to pass, and performing the second CRC on a violent decoding result to verify the decoding result. If the non-coherent demodulation passes the CRC, outputting ADS-B information, and if not, performing fine frequency/phase offset estimation and compensation.
The fine frequency/phase offset estimation adopts a least square fitting method after linear interpolation, and after a frequency/phase offset value is estimated, a signal segment is extracted from a memory for compensation. Because the ADS-B signal is a pulse position modulation signal, the pulse position distribution is uneven, the time matrix of least square estimation can be fixed through interpolation, so that inversion operation is avoided, the complexity of matrix operation is reduced, and the least square fitting is performed on the phase after interpolation, so that the frequency/phase offset is further eliminated.
The frequency/phase offset estimation and compensation method designed aiming at the satellite-based ADS-B signal in the invention improves the signal-to-noise ratio of about 3dB when decoding the signal, while the frequency/phase offset estimation and compensation are not used in the original signal processing method to realize coherent demodulation and improve the decoding rate, and the general frequency/phase offset estimation and compensation generally only use fixed bits, but the method in the invention utilizes the whole segment of information, and experiments prove that when the SNR (2MHz) is 5.061dB, the decoding rates of noncoherent demodulation and coherent demodulation are respectively 58.8 percent and 90.7 percent, the decoding rate is greatly improved, and the performance of the coherent demodulation method is obviously improved.
And performing coherent demodulation again after the fine frequency/phase offset estimation and compensation, wherein the specific steps are the same as those of incoherent demodulation, if the coherent demodulation passes the verification, outputting an ADS-B message, and if the coherent demodulation does not pass the verification, discarding the data.
2000 ADS-B intermediate frequency signals with signal power of-103 dBm to-95 dBm are generated in simulation, a curve of a correct unpacking rate and the signal power is obtained through the signal processing method of the satellite-borne ADS-B receiver, the curve is shown in figure 3, the 2MHz bandwidth signal-to-noise ratio of the-102 dBm signal is 5.3511dB, the signal-to-noise ratio requirement obtained by link budget is met, the correct unpacking rate reaches 96.3%, and the decoding rate is more than or equal to 90% when the signal power is-102 dBm.
Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.
Claims (3)
1. A signal processing method suitable for a high-sensitivity satellite-borne ADS-B receiver is characterized by comprising the following steps: the method comprises the following steps:
s1: the ADS-B signal received by the antenna is demodulated by radio frequency to obtain an intermediate frequency signal, and is demodulated by digital down-conversion after digital-to-analog conversion to obtain a digital baseband signal;
s2: carrying out low-pass filtering on the digital baseband signal to preliminarily filter out-of-band noise;
s3: carrying out coarse synchronization detection and extracting a coarse synchronization signal section; the method specifically comprises the following steps:
the signal passes through a low-pass filter, the signal energy is gathered, the energy peak value is extracted to obtain a time range possibly containing the ADS-B signal, and meanwhile, the signal-to-noise ratio of the message in the time range is estimated;
setting an interval according to the signal-to-noise ratio, and extracting signal segments within the interval range from a memory, wherein each signal segment only contains one ADS-B message;
s4: carrying out coarse frequency offset and phase offset estimation and compensation; the method specifically comprises the following steps:
adopting fast Fourier transform in each signal segment, estimating coarse frequency offset and phase offset, compensating in the signal segment, and limiting the frequency offset in a certain range;
s5: the signal-to-noise ratio is improved through a raised cosine filter with a signal bandwidth;
s6: carrying out fine synchronization detection and extracting a fine synchronization signal section; the method specifically comprises the following steps:
adopting a matched filtering method, setting the impulse response of a filter as a frame header and fixed first 3 bits of an ADS-B signal, filtering in a signal section after coarse frequency offset and phase offset compensation and filtering, extracting a peak value, determining the position of the frame header, and extracting the signal section of the signal length after the position of the frame header in a memory;
s7: carrying out incoherent demodulation, and outputting ADS-B information if the incoherent demodulation passes CRC check; if not, go to step S8; the non-coherent demodulation comprises the following steps:
bit decision and confidence extraction: obtaining data and a confidence matrix of each possible message, wherein the confidence matrix is extracted by adopting a maximum likelihood ratio method;
error detection and correction: performing CRC on the data after the alignment judgment, performing violent decoding by combining the confidence coefficient matrix under the condition that the CRC does not pass, and performing CRC on a violent decoding result to verify the decoding result;
s8: performing fine frequency offset and phase offset estimation and compensation, performing coherent demodulation, and outputting an ADS-B message if the coherent demodulation passes CRC (Cyclic redundancy check); if not, discarding the data; the fine frequency offset and phase offset estimation and compensation specifically include:
and after the frequency deviation and the phase deviation value are estimated by adopting a least square fitting method after linear interpolation, extracting a signal section from a memory for compensation.
2. The signal processing method applicable to the high-sensitivity satellite-borne ADS-B receiver according to claim 1, wherein: the bandwidth of the low-pass filter for low-pass filtering in step S2 is the sum of the signal bandwidth and the reserved frequency offset value.
3. The signal processing method applicable to the high-sensitivity satellite-borne ADS-B receiver according to claim 1, wherein: the coherent demodulation step in step S8 is the same as the noncoherent demodulation step in step S7.
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| EP2330784B1 (en) * | 2009-11-27 | 2015-09-09 | STMicroelectronics S.r.l. | Method of estimating transmission channel response and difference of synchronization offsets introduced in a received stream of packets of OFDM data and relative receiver |
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| CN109188387A (en) * | 2018-08-31 | 2019-01-11 | 西安电子科技大学 | Distributed coherent radar target component estimation method based on Interpolation compensation |
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