CN111211866B - Novel power consistency verification method based on ADS-B - Google Patents
Novel power consistency verification method based on ADS-B Download PDFInfo
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- H04L1/00—Arrangements for detecting or preventing errors in the information received
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- H—ELECTRICITY
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- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector
- H04L1/203—Details of error rate determination, e.g. BER, FER or WER
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Abstract
The invention discloses a novel power consistency verification method based on ADS-B, and belongs to the field of communication signal processing. Aiming at the problems of false detection and missed detection in the existing method when the A/C interference influence is faced. The invention evaluates the power consistency of the DF field and the non-DF field by respectively calculating the power estimation values of the DF field and the non-DF field and utilizing the difference value between the power estimation values. Through theoretical analysis and test, the method can reduce the information loss condition when the header is detected aiming at the conditions (1), (2) and (3) in A/C interference, and can not pass consistency authentication aiming at the conditions (4) and (5), the information is abandoned, and the error caused by missed detection of inconsistent information is reduced.
Description
Technical Field
The invention belongs to the field of communication signal processing, and relates to a method for verifying the power consistency of two different fields in a signal.
Background
The Broadcast Automatic Dependent-Broadcast (ADS-B) technology is a technology for completing ground/air-air traffic control monitoring and air-air monitoring between an aircraft and the ground and between the aircraft and the ground in a data chain form based on satellite positioning information. The ADS-B signal consists of a header and a message, as shown in FIG. 1, the header length is 8us, wherein 0-0.5 us, 1-1.5 us, 3.5-4 us and 4.5-5 us are high level, and the other parts are low level; the message part is high and low levels after PPM coding modulation. The first 5us of a 112us length message of ADS-B is the DF field of the message, i.e. 8-13 us of ADS-B signals, if the chip in the field is wrong, the subsequent CRC error correction and detection process cannot detect the error of the DF field, and finally error codes are caused. Therefore, it is very necessary to perform power consistency authentication on the DF field of the ADS-B signal.
In the DF authentication process of ADS-B baseband signals, if the difference between the DF field power and the non-DF field power is large, the ADS-B signals are too severely interfered and have no decoding significance, and the signals are discarded. In the actual processing process, the influence of the a/C interference on the DF field is the most prominent, and the traditional DF authentication scheme is also proposed for the a/C interference. There are five cases of the effect of a/C interference on the DF field:
(1) the interference occurs before the whole frame signal and forms a false preamble with just the first two preamble pulses, as shown in fig. 2 a;
(2) the interference coincides with the first two pulses or the last two pulses of the preamble, resulting in inconsistent preamble and message power, as shown in fig. 2 b;
(3) the interference occurs between 0us and 8us, wherein (2) the interference is in two special cases; other situations besides (2) this special case may also cause header and message power inconsistency, as shown in fig. 2 c;
(4) interference occurs between 8-13 us, causing the power of the DF field to be inconsistent with the power of other parts, as shown in FIG. 2 d;
(5) interference occurs after 13us, causing the header and message power to be inconsistent, as shown in fig. 2 e.
In the prior art, referring to the "research on critical technology based on 1090mhz es data chain ADS-B" (wangfei), after detecting the header, the following processes are generally performed:
for case (1), the frame message is virtually complete, but the frame data is discarded because the presence of the dummy header prevents the frame data from being authenticated by the DF field.
For the cases (2), (3) and (4), the header and DF field power are not consistent and cannot pass the consistency verification, thus discarding the frame signal.
For case (5), the header and DF field power are consistent, thus passing the consistency authentication.
The prior art has the following disadvantages:
1. the condition of false detection is easy to occur: case (1) can actually demodulate, but information is lost due to the limitations of the conventional method; in the cases (2) and (3), as long as the header can be detected, the power of the DF field and the following non-DF field are consistent, and most of the actual data can be demodulated, but since the conventional scheme only compares the power consistency of the header and the DF field, the conventional scheme obviously cannot pass the consistency authentication, so that the information is lost;
2. the condition of missed detection is easy to occur: in the case (5), the power of the DF field is not actually consistent with the power of the following non-DF field, and the message can only be discarded, but in the conventional scheme, only the power of the header and the DF field is involved, so that the consistency authentication is passed, which may cause missed detection and eventually generate an error code.
Disclosure of Invention
The invention relates to estimation of ADS-B signal power by an ADS-B receiver and a processing scheme for 5 cases of A/C interference. The method is a verification method and implementation for the consistency of the DF field and the non-DF field.
The invention discloses a method for verifying signal power consistency, which comprises the following steps:
1) set X ═ X for DF field sampling point of ADS-B signalkDenotes that, let xk＝a+nk；a>0，
Estimating the power of the DF field by maximum likelihood method
Expressed as:
wherein, a represents DF field power of ADS-B signal digitized at transmitting end, nkRepresenting signal noise and N representing the number of DF field samples.
2) Is provided withThe sample of the non-DF field of the ADS-B signal is Y ═ Yi}; let yi＝b+ni；b>0，
Estimating non-DF field power by maximum likelihood method, estimating value
Expressed as:
wherein, B represents the non-DF field power of the ADS-B signal digitized by the transmitting end, niRepresenting signal noise, and M represents the number of non-DF field samples.
3) Let c be a-b, so
4) By using
Evaluates the power consistency of the DF and non-DF fields. The specific method comprises the following steps:
i. obtaining the bearable threshold of the message header power difference under different signal-to-noise ratios through a plurality of experiments, wherein the threshold is generally 3dB, and the threshold is stored in a ROM;
by comparing threshold values in the ROM memory at the signal-to-noise ratio of the target signal
Size: if it is
If the value is larger than the corresponding threshold value, the DF field of the signal is seriously interfered and does not have power consistency, and the ADS-B signal of the frame is abandoned; otherwise, the consistency of the power of the DF field and the power of the non-DF field is shownSubsequent demodulation operations can be performed.
Set X ═ XkIs the set of five high-level sample points of the DF field. The DF field is 5us in length, including five high levels and five low levels. Assuming a sampling rate of 20MHz, N is 50.
non-DF field sample Y ═ YiThe acquisition method comprises the following steps: all high level sample points in the last 107us (i.e., not DF field) of the message are sampled. If the sampling rate of the message is 20MHz, M is 1070.
The invention has the advantages that: aiming at five conditions of the influence of A/C interference on the DF field, tests prove that compared with the prior art, the method can greatly reduce the false detection rate and the missed detection rate.
1. And (3) reduction of false detection: the DF field power of the cases (1), (2) and (3) is consistent with the non-DF field power, and the consistency authentication can be passed as long as the header can be detected, so that the information loss condition when the header is detected is reduced;
2. and (3) detection omission is reduced: the DF field and the non-DF field in the cases (4) and (5) have inconsistent power and can not pass the consistency authentication, and the information must be abandoned, thereby reducing the error caused by missing the inconsistent information.
Drawings
FIG. 1 is a schematic diagram of ADS-B signal composition;
FIGS. 2 a-2 e are schematic diagrams of five cases of A/C interference;
FIG. 3 is a flow chart of the process of the present embodiment;
FIG. 4 is a comparison graph of false detection rate of example statistics;
FIG. 5 is a comparison graph of the false negative rate of the example statistics.
Detailed Description
The invention will be further illustrated and described with reference to specific embodiments. The technical features of the embodiments of the present invention can be combined correspondingly without mutual conflict.
When the method is applied to the embodiment, the flow chart for realizing the scheme is shown in fig. 3, an ADC (analog-to-digital converter) collects ADS-B signals received by a receiver, a DF field and a non-DF field are cached respectively, the average power of the DF field and the average power of the non-DF field are calculated, the difference between the two average powers is judged, whether the difference is larger than or equal to 3dB or not is judged, if yes, the average power of the DF field and the average power of the non-DF field are inconsistent, ADS-B data of the frame are discarded, and if not, the power consistency is authenticated, so that demodulation can be performed.
The embodiment can be generally divided into four parts, namely DF field power estimation, non-DF field power estimation, an experimental contrast comparison traditional method and the virtual inspection rate of the method, and an experimental contrast comparison traditional method and the omission factor of the method;
(1) DF field power estimation
The waveform of the ADS-B signal is schematically shown in fig. 1, assuming that the sampling rate is 20MHz, and the set X of five high-level sampling points in the DF field is ═ XkAnd (1. ltoreq. k.ltoreq.50).
In the actual channel, the signal will be interfered by noise to different degrees, let the observed sample xk＝a+nkKnown as a>0, and
since the signal-to-noise ratio in the actual engineering varies with the channel environment and cannot be controlled, therefore,
is an unknown quantity and thus is different from the commonly known noise power estimation problem.
a estimated value
I.e. the power estimate for the DF field.
The sample X follows a Gaussian distribution with a probability density function of
Taking logarithm at both sides to obtain partial derivative to obtain likelihood function L (X; a)
Making L (X; a) equal to 0, can obtain
Since a >0 of the first group is present,
solving for
To obtain
Where N is 50.
Therefore, the DF field power estimate of ADS-B can be obtained from equation (4).
(2) non-DF field power estimation
The method for estimating the power of the non-DF field is the same as that for estimating the power of the DF field, and only the samples are different, i.e., the method is also estimated by the equation (4), and the non-DF field sample is set as Y ═ YiI is more than or equal to 1 and less than or equal to 1070), and the observed sample yi＝b+niKnown as b>0，
In the same way as above, the first and second,
is an unknown quantity.
Obtaining the estimated value of the message power in the same way
Where M is 1070.
Sample Y ═ YiObservation method of and basis of M1070: non-DF field sample Y ═ YiThe acquisition method is to sample all high levels of 107us (i.e. non-DF field) behind the messageAnd sampling point, if the sampling rate of the message is 20MHz, M is 1070.
To this end, the power estimation value of the ADS-B non-DF field can be obtained by equation (5).
The method of the invention is used for processing the following 5 cases of A/C interference in the background art (shown in figures 2 a-2 e):
i. since case (1) has a "false header" phenomenon, the receiver recognizes two headers. The first step regards false header and subsequent signal as a complete ADS-B signal of a frame, through the step 1) -4) of the invention, because of the existence of false header, the signal of 4.5-9.5 us of the correct ADS-B signal is mistaken for DF field, the section of signal is low level for a period of time, the final power difference far exceeds 3dB, therefore not authenticated; and the signal formed by the second step real header is not interfered by A/C, can pass the conformance authentication, and carry on the demodulation work.
in the case (2) and (3), the DF field and the non-DF field are not affected by the a/C interference, so that the consistency authentication can be passed and the demodulation operation can be performed after the steps 1) to 4) of the present invention. In summary, the cases (1), (2) and (3) do not have false detection by the scheme.
in the case (4) and (5), the DF field is affected by the a/C interference, and since the DF field is affected by the a/C interference, the power difference between the DF field and the non-DF field is far beyond 3dB, the ADS-B signal of the frame cannot be discarded through the consistency authentication after the steps 1) to 4) of the present invention. In summary, the cases (4) and (5) do not have missing detection by the scheme.
(3) Experimental contrast comparison of false detection rates of conventional and present methods
In the five cases of the above-mentioned a/C interference, the cases (1), (2) and (3) are susceptible to false detection, so three groups of experimental samples are set to perform the experiment:
i.A/C interference is not changed in power, 10000 pieces of information are transmitted under different signal-to-noise ratios, A/C interference is added before the head of the information to form a false head, and Gaussian white noise is added. And (3) performing DF authentication in the receiver by using the traditional method and the method respectively, and counting the number of the messages which do not pass DF authentication under each signal-to-noise ratio and the proportion of the messages occupying 10000, wherein the proportion is the virtual inspection rate of the condition (1).
The power of A/C interference is unchanged, 10000 pieces of information are transmitted under different signal-to-noise ratios, and A/C interference and white Gaussian noise are added to the high level of the header of the information. And (3) performing DF authentication in the receiver by using the traditional method and the method respectively, and counting the number of the messages which do not pass DF authentication under each signal-to-noise ratio and the proportion of the messages occupying 10000, wherein the proportion is the virtual inspection rate of the condition (2).
And (iii) the power of the A/C interference is unchanged, 10000 pieces of information are transmitted under different signal-to-noise ratios, the A/C interference is added in the middle position of the head of the information, and meanwhile, Gaussian white noise is added. And (3) performing DF authentication in the receiver by using the traditional method and the method respectively, and counting the number of the messages which do not pass DF authentication under each signal-to-noise ratio and the proportion of the messages occupying 10000, wherein the proportion is the virtual inspection rate of the condition (3).
And (5) calculating the average value of the virtual detection rates of i.ii.iii to obtain the total virtual detection rate, and drawing a curve chart of the total virtual detection rate along with the change of the signal-to-noise ratio under the traditional scheme and the scheme as shown in fig. 4.
As can be seen from fig. 4, in general trend, the missed detection rate of both schemes is decreasing as the SNR of the signal to noise ratio increases. However, compared with the conventional scheme, when (1), (2) and (3) of the above five cases occur to the a/C interference, the scheme can greatly reduce the false detection rate, so that a large amount of ADS-B information which can be demodulated originally cannot be lost.
(4) The test contrast compares the omission factor of the traditional scheme and the scheme
In the five cases of the a/C interference, the cases (4) and (5) are prone to missing detection, so two sets of experimental samples are set to perform the experiment:
i. the SNR is unchanged, 10000 pieces of information are transmitted under different A/C interference power, A/C interference is added between 8-13 us of the information, and Gaussian white noise is added at the same time. And (3) performing DF authentication in the receiver by using the traditional method and the method respectively, and counting the information quantity passing the DF authentication under each signal-to-noise ratio and the proportion of 10000 messages, wherein the proportion is the omission factor of the condition (4).
And ii, transmitting 10000 pieces of information under different A/C interference power without changing the SNR, and adding the A/C interference and the white Gaussian noise between 13-120 us of the information. And (3) performing DF authentication in the receiver by using the traditional method and the method respectively, and counting the information quantity passing the DF authentication under each signal-to-noise ratio and the proportion of 10000 messages, wherein the proportion is the undetected rate of the condition (5).
And (5) calculating the average value of the missed detection rate of i.ii to obtain the total missed detection rate, and drawing a curve chart of the total missed detection rate along with the change of the signal-to-noise ratio under the traditional scheme and the scheme as shown in fig. 5.
As can be seen from fig. 5, as the power of the a/C interference gradually increases, the missed detection rates under the two schemes both increase, but when (4) and (5) of the above five cases occur for the a/C interference, the missed detection rate can be greatly reduced in the scheme compared with the conventional scheme, so that a large amount of ADS-B information which cannot be demodulated originally is discarded, and the error rate of the receiver is reduced.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that various changes and modifications can be made by those skilled in the art without departing from the spirit of the invention, and these changes and modifications are all within the scope of the invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (5)
1. A novel power consistency verification method based on ADS-B is characterized by comprising the following steps:
1) set X ═ X for DF field sampling point of ADS-B signalkDenotes that, let xk＝a+nk；a＞0，
Estimating the power of the DF field by maximum likelihood method
Expressed as:
wherein, a represents DF field power of ADS-B signal digitized by transmitting end, nkRepresenting signal noise, and N representing the number of DF field sampling points;
2) let the sample of the non-DF field of ADS-B signal be Y ═ Yi}; let yi＝b+ni；b＞0，
non-DF field power estimation using maximum likelihood method
Expressed as:
wherein B represents the non-DF field power of the ADS-B signal digitized at the transmitting end, niRepresenting signal noise, and M representing the number of non-DF field sampling points;
3) let c be a-b,
therefore, it is
4) By using
Size of DF and non-DF field to evaluate the power consistency
i. Storing the bearable threshold of the message header power difference in a ROM memory;
by comparing threshold values in the ROM memory at the signal-to-noise ratio of the target signal
Size: if it is
If the value is larger than the threshold value, the DF field of the signal is seriously interfered and does not have power consistency, and the ADS-B signal is abandoned; otherwise, the power of the DF field and the non-DF field is consistent, and the subsequent demodulation operation can be performed.
2. The ADS-B based novel power consistency verification method of claim 1, wherein:
set X ═ XkThe DF field is a sampling point set of five high levels; the DF field is 5us in length.
3. The ADS-B based novel power consistency verification method of claim 1, wherein:
non-DF field sample Y ═ YiThe acquisition method comprises the following steps: all high level sampling points in the later 107us of the message are sampled.
4. The ADS-B based novel power conformance verification method of claim 1, wherein:
the sampling rate is 20MHz, N is 50, and M is 1070.
5. The ADS-B based novel power consistency verification method of claim 1, wherein:
the tolerable threshold for the packet header power difference is 3 dB.
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