CN111147406B - ASM system demodulation method based on constant false alarm signal detection and frequency offset correction - Google Patents
ASM system demodulation method based on constant false alarm signal detection and frequency offset correction Download PDFInfo
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Abstract
The invention provides an ASM system demodulation method based on constant false alarm signal detection and frequency offset correction, which has constant low false alarm signal detection probability and can adapt to ASM system application scenes with wide input level dynamic range, large frequency offset range and sudden signal.
Description
Technical Field
The invention relates to an ASM system demodulation method based on constant false alarm signal detection and frequency offset correction.
Background
With the increase of AIS users and the expansion of the application thereof, high link load appears in some busy areas, and when the load of an AIS data link is too high, serious problems such as information blocking and the like can be caused, and the navigation safety is influenced. The introduction of future electronic navigation needs to support the exchange of larger data volumes, and currently, AIS cannot meet the requirements of future data exchange.
In this regard, the international telecommunications union proposed a VDES system in 2012 (WRC-12). On the basis of integrating the existing AIS function, the VDES comprehensively strengthens the data transmission capability of ship communication by adding a special application message (ASM) and a broadband very high frequency data exchange (VDE) function, and provides powerful guarantee for the ship navigation safety.
The international telecommunication union ITU-R m.2092 recommendation makes corresponding provisions for the communication technology of the ASM system in the VDES system, but does not make corresponding provisions for the specific implementation.
The ASM adopts a burst signal system, a slow-rising sequence of a few symbols is arranged before a training sequence and is used for power amplifier opening, and a traditional capture method aiming at continuous signals fails because a carrier synchronization ring or a bit synchronization ring cannot guarantee locking before a frame header arrives. Detection for burst-mode signals generally employs a method of matching with a known training sequence. And outputting a peak value when the local sequence is matched with the received training sequence, and considering that a signal is detected if the peak value level is greater than a threshold. If a fixed threshold is adopted for judgment, when the amplitude of an input signal is higher, a false alarm is easy to generate, and when the amplitude of the input signal is too small, a false alarm is easy to generate. In a traditional signal detection method for directly performing correlation matching on a received baseband signal, a correlation peak value is reduced along with the increase of frequency offset, so that the detection probability is reduced or the detection cannot be performed.
The ASM signal adopts pi/4 QPSK modulation, and the receiving end generally adopts differential demodulation. Frequency deviation is caused by the difference of reference frequencies at the two ends of the transceiver and the Doppler frequency shift caused by relative motion. If the compensation and correction are not performed, the differential constellation is rotated, which affects the demodulation performance.
The traditional method for performing frequency offset estimation by using FFT has the problem that the estimation accuracy is limited by the number of FFT points (the estimation accuracy Δ f is fs/N), and FFT calculation and peak search are required, which results in higher calculation complexity.
Chinese patent publication No. CN 206341236U discloses a demodulator for ASM system. The system mainly comprises a DDC digital down-conversion module, a first square root raised cosine filter, a lead code detection module, a timing phase estimation module, a timing resampling module, a second square root raised cosine filter, a carrier frequency offset module, a carrier phase synchronization module, a demodulation mapping module, a Turbo FEC decoding module and the like. The patent only simply describes the connection relation and functions of the modules, and does not explicitly describe the specific implementation mode.
Chinese patent publication No. CN 107483078A discloses a method for estimating receiving frequency offset of an ASM system of a ship VDES system. According to the method, after an intermediate frequency signal passes through a digital down-conversion device and a phase discriminator, FFT calculation is carried out on a phase discrimination error, maximum peak detection is carried out on a calculation result to obtain the position of the maximum peak, then, the estimated frequency offset of the intermediate frequency signal is calculated according to the position, and the estimated frequency offset is compensated to the nominal frequency of a local carrier signal. The frequency offset estimation precision of the method disclosed by the patent is limited by the number of Fourier transform points and the calculation is relatively complex.
Disclosure of Invention
The invention aims to provide an ASM system demodulation method based on constant false alarm signal detection and frequency offset correction.
In order to solve the above problems, the present invention provides an ASM system demodulation method based on constant false alarm signal detection and frequency offset correction, comprising:
carrying out frequency mixing on the complex signals of the received AD samples, and moving the frequency spectrum to zero intermediate frequency to obtain signals after frequency mixing;
performing matched filtering processing on the mixed signal through a matched filter to obtain matched filtered data, wherein the matched filter and the transmitting end forming filter have the same frequency spectrum characteristic and adopt a root raised cosine forming filter;
carrying out delay difference on the matched and filtered data to obtain a delay difference baseband signal;
detecting a training sequence from the delayed differential baseband signal by using a known training sequence and adopting a constant false alarm adaptive threshold method;
after the training sequence is detected, pulling up a signal capture instruction, adjusting the initial value of the phase of a synchronous loop at the moment of matching a peak point, simultaneously carrying out conjugate multiplication on the received matched training sequence and a local sequence to obtain a baseband signal without symbol modulation, sending the baseband signal without symbol modulation into a rotating vector estimation module for calculation to obtain a rotating vector estimation result, and sending the rotating vector estimation result to a constellation correction module;
returning to the initial position of the training sequence, carrying out bit synchronization processing on the baseband signal by adopting a gardner algorithm to obtain a sampling signal;
performing rotation correction on the sampling signal according to the rotation vector estimation result to obtain a rotation-corrected sampling signal;
judging the rotation corrected sampling signal, sending the judgment to an information sequence matcher, and determining the frame length, the code rate and the extraction starting point of soft information;
and extracting the soft information based on the frame length, the code rate and the extraction starting point of the soft information, and sending the soft information to a subsequent module for processing including decoding and unframing.
Further, in the above method, the detecting of the training sequence from the baseband signal by using a constant false alarm adaptive threshold method using a known training sequence includes:
the correlation detector passes the baseband signal after the delay difference through a matched filter with a coefficient as the conjugate of the training sequence, and squares the output of the matched filter, when the training sequence passes through the correlation detector, the correlation detector outputs the maximum value, if the output of the correlator is detected to be larger than the self-adaptive threshold value G, the frame head is considered to be detected.
Further, in the above method, conjugate multiplication is performed on the matched and filtered data and a signal before one symbol period, and a baseband signal after delay difference is recorded as r (k), then:
wherein, y k To match the filtered data, y k-1 Is a signal one symbol period ahead, T b In the form of a symbol period, the symbol period,
to carry the phase difference of the modulated data information, n' (k) is the noise term resulting from the difference operation.
Further, in the method, in the detection of the training sequence from the baseband signal by using the known training sequence and using the constant false alarm rate adaptive threshold method,
the adaptive threshold is calculated by the M-1 reference unit values before the unit to be measured and the M-1 reference unit values after the unit to be measured, the base power p is obtained by taking the mean value, the obtained base power p is multiplied by a threshold factor to obtain the adaptive threshold, and the selection of the threshold factor can be obtained by actual simulation.
Further, in the above method, sending the baseband signal without symbol modulation to a rotation vector estimation module for calculation to obtain a rotation vector estimation result, including:
after the training sequence is detected, a receiving sequence corresponding to a peak point is obtained, and correlation operation is carried out on the receiving sequence and the conjugate of a local matching sequence to obtain a baseband signal which eliminates symbol modulation and only retains frequency offset information, namely the conjugate of r (k) and training sequence modulation information
Multiplication, denoted as z (k):
wherein n' (k) is a noise term obtained by performing a correlation operation,
a phase error introduced for the frequency offset Δ f in one data period, that is, an angle of rotation of a received signal constellation caused by the frequency offset;
and performing weighted average L-1 times in the whole training sequence to estimate a rotation vector R:
wherein the smoothing function is
Compared with the prior art, the invention has the following beneficial effects:
(1) the invention adopts a constant false alarm rate adaptive threshold signal detection method aiming at the characteristics of specific data frame structure and burst type signals of an ASM system, overcomes the problem that a fixed threshold method cannot adapt to overlarge dynamic range of signals, and improves the stability of signal detection.
(2) Compared with the traditional method of directly carrying out correlation without difference, the method of carrying out difference first and then carrying out correlation detection is adopted, the correlation peak value is not reduced along with the increase of frequency deviation, and the method can be suitable for signal detection under the condition of large-range frequency deviation.
(3) The invention uses N BPSK modulated training symbol energies to detect signals, can detect the signals under the environment of low signal to noise ratio, and is practical and reliable.
(4) Aiming at the problem of carrier frequency offset, the invention adopts a constellation rotation correction method, eliminates the influence of frequency offset, and greatly improves the performance compared with the traditional differential demodulation method without frequency offset correction.
(5) The frequency offset correction method provided by the invention does not need operations such as phase discrimination, FFT calculation, peak value search and the like, only needs to calculate the constellation rotation vector and then carries out rotation correction on the demodulated constellation, has lower calculation complexity than the traditional FFT algorithm, and is easy for engineering realization.
Drawings
Fig. 1 is a demodulation block diagram of a receiving end of an ASM system;
FIG. 2 is a diagram of an adaptive threshold detection architecture;
FIG. 3 is a graph of the correlator output versus the adaptive threshold for a signal-to-noise ratio of 20 db;
FIG. 4 is a constellation diagram before and after frequency offset correction under 20db SNR;
fig. 5 is a graph of error rate without frequency offset correction and after frequency offset correction under different frequency offsets.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
As shown in fig. 1, the present invention provides an ASM demodulation method based on constant false alarm signal detection and frequency offset correction, which includes:
step S1, mixing the complex signals of the received AD sampling, moving the frequency spectrum to zero intermediate frequency, and obtaining the signals after mixing;
step S2, performing matched filtering processing on the mixed signal through a matched filter to obtain matched filtered data, wherein the matched filter and the transmitting end forming filter have the same frequency spectrum characteristics and adopt root raised cosine forming filters;
preferably, the match filtered data is represented as:
where A is the signal amplitude, Δ f is the frequency offset,
in order to modulate the phase of the light,
is the initial phase, n is the noise;
similarly, the signal expression before one symbol period is:
step S3, carrying out delay difference on the matched and filtered data to obtain a baseband signal;
preferably, the performing delay difference on the matched and filtered data to obtain a baseband signal after delay difference includes:
conjugate multiplication is carried out on the formula 1 and the formula 2, and the baseband signal after delay difference is recorded as r (k), then:
wherein, T b In the form of a symbol period, the symbol period,
to carry the phase difference of the modulated data information, n' (k) is the noise term resulting from the difference operation.
Step S4, using known training sequence to detect the training sequence from the base band signal by using constant false alarm adaptive threshold method;
here, the purpose of the midamble correlation detection is to find the starting point of the midamble, i.e. the frame header. The correlation detector passes the baseband signal after the delay difference through a matched filter with a coefficient as the conjugate of the training sequence, and squares the output of the matched filter, when the training sequence passes through the correlation detector, the correlation detector outputs the maximum value, if the output of the correlator is detected to be larger than the self-adaptive threshold value G, the frame head is considered to be detected, and the rotation vector estimation and the subsequent processing can be carried out.
The calculation structure of the adaptive threshold is shown in fig. 2. The adaptive threshold is calculated by the M-1 reference unit values before the unit to be measured and the M-1 reference unit values after the unit to be measured, the base power p is obtained by taking the mean value, the obtained base power p is multiplied by a threshold factor to obtain the adaptive threshold, and the selection of the threshold factor can be obtained by actual simulation. The self-adaptive threshold is adopted to ensure that the false alarm probability is not influenced by the signal input amplitude, namely constant false alarm, and the greater the threshold factor is, the lower the false alarm probability is.
Step S5, after detecting the training sequence, pulling up the signal capture instruction, adjusting the initial value of the synchronous loop phase at the time of matching the peak point, simultaneously carrying out conjugate multiplication on the received matched training sequence and the local sequence to obtain a baseband signal without symbol modulation, sending the baseband signal without symbol modulation into a rotation vector estimation module for calculation to obtain a rotation vector estimation result, and sending the rotation vector estimation result to a constellation correction module;
preferably, the sending the baseband signal without symbol modulation to the rotation vector estimation module for calculation to obtain a rotation vector estimation result includes:
after the training sequence is detected, a receiving sequence corresponding to a peak point is obtained, and correlation operation is carried out on the receiving sequence and the conjugate of a local matching sequence to obtain a baseband signal which eliminates symbol modulation and only retains frequency offset information, namely the conjugate of r (k) and training sequence modulation information
Multiplication, denoted as z (k):
wherein n' (k) is a noise term obtained by performing a correlation operation,
the phase error introduced for the frequency offset Δ f within one data period, i.e. the angle by which the received signal constellation is rotated due to the frequency offset.
In practice, due to the influence of noise, the error of the estimated value obtained by using the result of only one-point difference operation is large. Therefore, the rotation vector R is estimated by performing weighted averaging L-1 times in the whole training sequence:
wherein the smoothing function is
Step S6, returning to the initial position of the training sequence, carrying out bit synchronization processing on the baseband signal by adopting a gardner algorithm to obtain a sampling signal;
step S7, according to the estimation result of the rotation vector, the rotation correction is carried out to the sampling signal to obtain the sampling signal after the rotation correction;
here, the constellation rotation correction module multiplies the complex signal after bit synchronization by the conjugate of the rotation vector
Obtaining baseband data after frequency offset correction;
step S8, judging the sampling signal after rotation correction, sending the signal to an information sequence matcher, and determining the frame length, the code rate and the extraction starting point of soft information;
and step S9, based on the frame length, code rate and the extraction starting point of the soft information, extracting the soft information and sending the soft information to the subsequent modules including decoding and de-framing for processing.
The invention provides an ASM system demodulation method based on constant false alarm rate adaptive threshold signal detection and frequency offset constellation rotation correction to overcome the defects of the prior art. The method has constant low false alarm signal detection probability, can adapt to ASM system application scenes with wide input level dynamic range, large frequency deviation range and signal burst, and in addition, the method adopts a constellation rotation mode to correct the frequency deviation, has higher precision compared with the traditional FFT frequency deviation estimation method, and is easy to realize in engineering.
FIG. 3 is a diagram of matlab simulation of correlator output value and adaptive threshold for a signal-to-noise ratio of 20 dB. It can be seen that except for the sampling point time that is perfectly matched with (or close to) the training sequence, the output amplitudes of the correlators at other points are all lower than the adaptive threshold, so as long as the output of the correlator is detected to be higher than the adaptive threshold, the frame header is considered to be found. Because a symbol has a plurality of sampling points, a plurality of points exist nearby the symbol and exceed the threshold, and the time of the maximum sampling point output by the correlator is taken as a synchronous output point. The two peaks appear in the figure because the data of the input bit sync is returned to the front of the training sequence after the peak is detected for the first time while compensating for the phase of the bit sync. The time of the second pass of the training sequence is enough for the synchronization loop to lock, and the correlator has a second peak at the end, and no processing is performed at this time.
In order to verify the effect of frequency offset correction, the constellation diagrams before and after frequency offset correction are compared in a simulation mode under the condition of 20db signal-to-noise ratio, as shown in fig. 4. It can be seen from the figure that the constellation diagram before correction is rotated, which will result in the degradation of the receiving performance of the system. After constellation rotation correction, the influence of frequency offset on demodulation performance is almost completely eliminated.
To further illustrate the effect brought by the frequency offset correction, the error rate curves after the frequency offset correction and the frequency offset correction are compared without performing the frequency offset correction under different frequency offsets, as shown in fig. 5. As can be seen from the figure, under the conditions of 400Hz frequency offset and 10dB signal-to-noise ratio, if not, the process is carried outCorrection of frequency offset with a bit error rate of 10 -3 After frequency deviation correction, the error rate is reduced to 10 -6 . Also achieving a bit error rate of 10 -6 In order of magnitude, the signal-to-noise ratio required by the frequency offset correction is about 8db lower than that of the signal-to-noise ratio not required by the frequency offset correction, or the required power consumption is reduced by 6.3 times, so that the performance of the frequency offset correction algorithm is improved considerably.
Compared with the prior art, the invention has the following beneficial effects:
(1) the invention adopts a constant false alarm rate adaptive threshold signal detection method aiming at the characteristics of specific data frame structure and burst type signals of an ASM system, overcomes the problem that a fixed threshold method cannot adapt to overlarge dynamic range of signals, and improves the stability of signal detection.
(2) Compared with the traditional method of directly carrying out correlation without difference, the method of carrying out difference first and then carrying out correlation detection is adopted, the correlation peak value is not reduced along with the increase of frequency deviation, and the method can be suitable for signal detection under the condition of large-range frequency deviation.
(3) The invention uses N BPSK modulated training symbol energies to detect signals, can detect the signals under the environment of low signal to noise ratio, and is practical and reliable.
(4) Aiming at the problem of carrier frequency offset, the invention adopts a constellation rotation correction method, eliminates the influence of frequency offset, and greatly improves the performance compared with the traditional differential demodulation method without frequency offset correction.
(5) The frequency offset correction method provided by the invention does not need operations such as phase discrimination, FFT calculation, peak value search and the like, only needs to calculate the constellation rotation vector and then carries out rotation correction on the demodulated constellation, has lower calculation complexity than the traditional FFT algorithm, and is easy for engineering realization.
(6) Compared with the traditional FFT frequency offset estimation compensation method, the frequency offset correction method provided by the invention has higher precision.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
Those of skill would further appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both, and that the various illustrative components and steps have been described above generally in terms of their functionality in order to clearly illustrate this interchangeability of hardware and software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the implementation. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (5)
1. An ASM system demodulation method based on constant false alarm signal detection and frequency offset correction is characterized by comprising the following steps:
carrying out frequency mixing on the complex signals of the received AD samples, and moving the frequency spectrum to zero intermediate frequency to obtain signals after frequency mixing;
performing matched filtering processing on the mixed signal through a matched filter to obtain matched filtered data, wherein the matched filter and the transmitting end forming filter have the same frequency spectrum characteristic and adopt a root raised cosine forming filter;
carrying out delay difference on the matched and filtered data to obtain a delay difference baseband signal;
detecting a training sequence from the delayed differential baseband signal by using a known training sequence and adopting a constant false alarm adaptive threshold method;
after the training sequence is detected, pulling up a signal capture instruction, adjusting the initial value of the phase of a synchronous loop at the moment of matching a peak point, simultaneously carrying out conjugate multiplication on the received matched training sequence and a local sequence to obtain a baseband signal without symbol modulation, sending the baseband signal without symbol modulation into a rotating vector estimation module for calculation to obtain a rotating vector estimation result, and sending the rotating vector estimation result to a constellation correction module;
returning to the initial position of the training sequence, carrying out bit synchronization processing on the baseband signal by adopting a gardner algorithm to obtain a sampling signal;
performing rotation correction on the sampling signal according to the rotation vector estimation result to obtain a rotation-corrected sampling signal;
judging the rotation corrected sampling signal, sending the judgment to an information sequence matcher, and determining the frame length, the code rate and the extraction starting point of soft information;
and extracting the soft information based on the frame length, the code rate and the extraction starting point of the soft information, and sending the soft information to a module for decoding and decoding subsequent steps.
2. The ASM demodulation method based on constant false alarm signal detection and frequency offset correction as claimed in claim 1, wherein the matched and filtered data is conjugate-multiplied with the signal before one symbol period, and the baseband signal after delay difference is r (k), then:
wherein, y k In order to match the filtered data the data is,
is a signal one symbol period ahead, T b In the form of a symbol period, the symbol period,
n' (k) is a noise term obtained by difference operation for the phase difference carrying the modulated data information; a is the signal amplitude and Δ f is the frequency offset.
3. The method for demodulating ASM based on constant false alarm rate signal detection and frequency offset correction according to claim 1, wherein the detecting of the training sequence from the baseband signal by using the known training sequence and using the constant false alarm rate adaptive threshold method comprises:
the correlation detector passes the baseband signal after delay difference through a matched filter with a coefficient as training sequence conjugate, and squares the output of the matched filter, when the training sequence passes through the correlation detector, the correlation detector outputs the maximum value, if the output of the correlator is detected to be larger than the self-adaptive threshold value G, the frame header is detected.
4. The ASM demodulation method based on constant false alarm signal detection and frequency offset correction as claimed in claim 1, wherein in the detection of training sequence from the baseband signal by using known training sequence and using constant false alarm adaptive threshold method,
the adaptive threshold is calculated by M-1 reference unit values before a unit to be measured and M-1 reference unit values after the unit to be measured, the base power p is obtained by adopting an averaging mode, the obtained base power p is multiplied by a threshold factor to obtain the adaptive threshold, and the selection of the threshold factor is obtained by actual simulation.
5. The ASM demodulation method based on constant false alarm signal detection and frequency offset correction as claimed in claim 1, wherein the step of sending the baseband signal without symbol modulation to the rotation vector estimation module for calculation to obtain the rotation vector estimation result comprises:
after the training sequence is detected, a receiving sequence corresponding to a peak point is obtained, and correlation operation is carried out on the receiving sequence and the conjugate of a local matching sequence to obtain a baseband signal which eliminates symbol modulation and only retains frequency offset information, namely the conjugate of r (k) and training sequence modulation information
Multiplication, denoted as z (k):
wherein n' (k) is a noise term obtained by performing a correlation operation,
a phase error introduced for the frequency offset Δ f in one data period, that is, an angle of rotation of a received signal constellation caused by the frequency offset;
and performing weighted average L-1 times in the whole training sequence to estimate a rotation vector R:
wherein the smoothing function is
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