CN111200466A - Confidence threshold optimization method for digital signal demodulation - Google Patents

Confidence threshold optimization method for digital signal demodulation Download PDF

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CN111200466A
CN111200466A CN201911259127.4A CN201911259127A CN111200466A CN 111200466 A CN111200466 A CN 111200466A CN 201911259127 A CN201911259127 A CN 201911259127A CN 111200466 A CN111200466 A CN 111200466A
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罗亨
罗喜伶
张昌明
李云波
金晨
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Hangzhou Innovation Research Institute of Beihang University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B14/00—Transmission systems not characterised by the medium used for transmission
    • H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
    • H04B14/04—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using pulse code modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B14/00—Transmission systems not characterised by the medium used for transmission
    • H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
    • H04B14/04—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using pulse code modulation
    • H04B14/046—Systems or methods for reducing noise or bandwidth
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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Abstract

The invention discloses a confidence threshold optimization method for digital signal demodulation, and relates to the field of communication signal processing. The method aims to solve the problem that the threshold accuracy can only be evaluated by the final error rate and the threshold performance cannot be quantized in the prior art. The method can evaluate a better or even optimal threshold value by calculating and comparing the curve of the Q value of each threshold value gamma along with the change of the signal-to-noise ratio, improve the demodulation performance of a receiver and finally achieve the aim of reducing the error rate.

Description

Confidence threshold optimization method for digital signal demodulation
Technical Field
The invention belongs to the field of communication signal processing, and relates to a confidence threshold reliable optimization method for digital signal demodulation.
Background
PPM, Pulse Position Modulation, for symbols "0" and "1", the waveform after PPM is shown in fig. 1.
When performing PPM demodulation at the receiving end, the waveform is sampled, assuming that the number of sampling points of one symbol is 20, the left and right halves are 10, respectively, the values of all the sampling points are represented by the set D ═ x1, x2, x3, …, x20, and the schematic diagram of the sampling points is shown in fig. 2.
A common demodulation method is to sum the left and right halves of the waveform separately, i.e.
Figure BDA0002311128280000011
Figure BDA0002311128280000012
Comparing the magnitude of the A and B values, it is possible to determine whether the waveform represents the symbol "1" or the symbol "0".
In an ideal channel, the accuracy of this method should be 100%, i.e., the confidence is highest. In a real channel, however, the signal is disturbed by noise, and the degree of the disturbance depends on the magnitude of the signal-to-noise ratio λ. The larger λ, the higher the demodulation accuracy and thus the higher the confidence, the smaller λ, the lower the accuracy and thus the lower the confidence.
There are many methods for determining the confidence level, and these methods also have strong and weak performance, that is, strong and weak accuracy of the confidence level determination.
The existing technical scheme directly evaluates the accuracy of the confidence coefficient threshold value according to the final error rate, the method can only qualitatively judge whether the confidence coefficient threshold value is reasonable, the threshold value performance cannot be quantized, and the prior art has no systematic method for quantitatively analyzing and evaluating the accuracy of the confidence coefficient threshold value gamma.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a confidence threshold optimization method for digital signal demodulation.
The technical scheme of the invention is as follows:
the reliable estimator of the confidence threshold value of the digital signal demodulation comprises the following steps:
SO1: sampling the waveform to obtain a sample set when PPM demodulation is carried out; acquiring the high and low levels +/-a of a sample when the sample is not interfered;
s02: acquiring a signal-to-noise ratio lambda in an actual channel;
s03: the Q function was constructed as follows:
Figure BDA0002311128280000021
wherein gamma is a confidence threshold for digital signal demodulation;
and in the value range of the gamma to be optimized, taking the maximum value of the Q function as an optimization target, and obtaining the gamma value corresponding to the maximum value of the Q function as the optimal confidence threshold value for demodulating the digital signal.
In a preferred embodiment of the present invention, in the step S01, the sampling interval is fixed when the waveform is sampled. In step S01, when the waveform is sampled, the number of the left and right sampling points of the waveform is the same. In step S01, the average method or the maximum likelihood method is used to obtain the high and low levels ± a.
The invention further discloses a method for demodulating the digital signal, which comprises the following steps:
SO1, sampling the waveform to obtain a sample set when PPM demodulation is carried out; acquiring the high and low levels +/-a of a sample when the sample is not interfered;
s02: acquiring a signal-to-noise ratio lambda in an actual channel;
s03: the Q function was constructed as follows:
Figure BDA0002311128280000022
wherein gamma is a confidence threshold for digital signal demodulation;
within the value range of gamma to be optimized, taking the maximum value of the Q function as an optimization target, obtaining the gamma value corresponding to the maximum value of the Q function, and taking the gamma value as the confidence coefficient threshold gamma of the optimal digital signal demodulation0；
S04: the sampling points on the left and right halves of the waveform are summed separately,
Figure BDA0002311128280000023
Figure BDA0002311128280000024
wherein m is the number of sampling points positioned on the left half of the waveform, and n is the total number of the sampling points; n is 2 m;
comparing the values of A and B, it can judge that said waveform represents symbol "1" or symbol "0";
defining C ═ A-B |, comparing C with confidence threshold gamma of optimal digital signal demodulation0If C is not less than gamma0Then the confidence is 1, if C<γ0If yes, the confidence coefficient is 0;
the obtained confidence coefficient is used for CRC, firstly, the number of chips with the confidence coefficient of 0 is counted, if the number is more than 5, the message is seriously interfered, and the message is abandoned; and if the number of the chips is less than or equal to 5, performing error correction and detection analysis, and correcting the wrong chips to obtain correct chips. At this point, the demodulation operation is completed.
The present invention also provides a digital signal demodulation apparatus, comprising:
an ADC signal acquisition module; it is used for collecting ADC signals;
a header data caching module; the ADC signal acquisition module is connected with the ADC signal acquisition module and is used for caching header data of the acquired ADC signals;
a signal-to-noise ratio lambda estimation module of a channel; the device is connected with a header data caching module and is used for acquiring the signal-to-noise ratio lambda in an actual channel;
a high level a value estimation module in the absence of noise; the device is connected with a header data caching module and used for acquiring the high and low levels +/-a of the sample when the sample is not interfered;
a Q value searching module; according to the input confidence threshold, the built-in Q function calculation is carried out to obtain the confidence threshold gamma of the digital signal demodulation with the Q function taking the maximum value0；
A confidence threshold acquisition module; it produces confidence threshold value for Q function calculation according to input confidence threshold value or confidence threshold value range;
a message data processing module; it buffers and demodulates the message data of ADC signal, and demodulates confidence threshold gamma according to the obtained digital signal0Obtaining a confidence coefficient;
and the CRC check module is used for comparing, correcting and detecting errors aiming at the chips with low confidence coefficient.
The method can evaluate a better or even optimal threshold value by calculating and comparing the curve of the Q value of each threshold value gamma along with the change of the signal-to-noise ratio, improve the demodulation performance of the receiver and finally achieve the aim of reducing the bit error rate.
Drawings
FIG. 1 is a waveform diagram after PM encoding;
FIG. 2 is a schematic diagram of the sampling point;
FIG. 3 is a sample diagram of a symbol "1" after sampling in the embodiment;
FIG. 4 is a normal distribution diagram represented by formula (6);
FIG. 5 is a normal distribution diagram represented by formula (10);
FIG. 6 is a flow chart of a confidence threshold optimization method for digital signal demodulation according to the present invention;
fig. 7 is a graph of the variation of different thresholds gamma with the signal to noise ratio lambda.
Detailed Description
The invention is further described with reference to the drawings and the specific embodiments in the following description.
As shown in fig. 6, which is a flowchart of a confidence threshold optimization method for digital signal demodulation according to the present invention, in an embodiment, a signal-to-noise ratio λ is changed by changing a signal power at a transmitting end, a receiving end receives 10000 frames of ADS-B data samples altogether, where in a certain frame of ADS-B data, a sample of a certain PPM-modulated chip sampling point is as shown in fig. 3, and the sample is represented by a set of X ═ { X1, X2, X3, X4, X5, X6, X7, X8, X9, X10, X11, X12, X13, X14, X15, X16, X17, X18, X19, X20}, and specific values of the set of samples are:
sample sequence x1 x2 x3 x4 x5 x6 x7 x8 x9 x10
Value of 455 543 620 618 587 602 585 496 466 410
Sample sequence x11 x12 x13 x14 x15 x16 x17 x18 x19 x20
Value of 353 297 208 315 298 319 189 248 320 411
The specific size of the high and low levels a of the undisturbed samples is related to the header of each frame data sample, and is constant for the current sample, and does not represent that value. For example, the header corresponding to the current sample estimates that the value of a is 538, and this value only represents the reference value of the current sample, and the specific embodiment has 10000 frame data samples in total, and cannot list all values, so a constant a is used to represent this value.
Each sampling point is interfered by gaussian white noise, if the high and low levels are ± a (a is constant and can be estimated by averaging or maximum likelihood method, the present invention is not expanded), then each point in the set can be represented as a sampling point interfered by gaussian white noise, that is:
Figure BDA0002311128280000041
wherein
Figure BDA0002311128280000042
The first half and the second half are summed separately,
Figure BDA0002311128280000043
Figure BDA0002311128280000044
then, the difference is made between A and B,
C＝A-B＝20a+N1-N2＝20a-N (4)
it is known that
Figure BDA0002311128280000051
Therefore, it is
Figure BDA0002311128280000052
Thus N to N (0, sigma)2)，C～N(20a,σ2)
Wherein, σ1,σ2,σk(k is 1 to 20) are all constants, and
Figure BDA0002311128280000053
there is a constant γ, the confidence is 1 if C ≧ γ, and the confidence is 0 if C < γ.
The value of constant gamma determines the confidence level, the confidence level determines the error correction and detection capability during the subsequent CRC check, if the confidence level is unreliable, the originally correct demodulation result of a certain chip is judged as low confidence level, the Hamming distance of a check bit is wasted, therefore, the value of gamma also directly influences the PPM demodulation performance, the reliability degree of gamma is expressed by the probability P (C is more than or equal to gamma), and the meaning of P (C is more than or equal to gamma) is as follows: the probability that a demodulation result that is originally correct for a certain chip is determined to be of high confidence is higher, indicating that the higher the probability is, the higher the reliability of γ is.
The following treatment was performed for P (C.gtoreq.gamma):
P(C≥γ)＝P(20a-N≥γ)＝P(-N≥-20a+γ) (5)
order to
Figure BDA0002311128280000054
Then there is
Figure BDA0002311128280000055
Since N to N (0, sigma)2) Therefore, it is
Figure BDA0002311128280000056
Equation (6) represents the shaded area shown in FIG. 4
The signal-to-noise ratio is known as λ, and λ satisfies
Figure BDA0002311128280000057
Thus, the noise power PNSatisfy the requirement of
Figure BDA0002311128280000058
From the formulae (7) and (8)
Figure BDA0002311128280000059
Since λ and a are both constants, so
Figure BDA0002311128280000061
At this time, let
Figure BDA0002311128280000062
Then there is
Figure BDA0002311128280000063
Therefore, the expression (10) represents the hatched area shown in FIG. 5,
in this case, the parameters can be calculated
Figure BDA0002311128280000064
To evaluate the reliability of the confidence threshold gamma.
Figure BDA0002311128280000065
As can be derived from the above, the value of Q is P (C ≧ gamma), and the meaning of P (C ≧ gamma) is: the probability that a demodulation result which is originally correct for a certain chip is judged to be high confidence is higher, and the error rate is lower. Therefore, the size of Q is a quantitative index for evaluating the demodulation performance of the receiver, and the larger the Q value is, the stronger the demodulation performance of the receiver is.
By selecting different gamma values gamma 1, gamma 2 and gamma 3, the Q value (reliability) is calculated, and the variation along with the signal-to-noise ratio lambda is shown in figure 7.
The values of γ 1, γ 2, and γ 3 are obtained in a confidence threshold value obtaining module in the receiver, the confidence threshold value obtaining module of this embodiment has three sub-modules, and the inputs of the three sub-modules are all the high-level average value of the current ADS-B data header minus the overall header average value, which is represented by Δ x, in this embodiment, the three word modules are respectively set to be 9 times, 10 times, and 11 times of Δ x, and γ 1, γ 2, and γ 3 are respectively 9 Δ x, 10 Δ x, and 11 Δ x. Similarly, the value of Δ X is only related to the ADS-B data header of the frame where the current sample is located, and Δ X of the data header where the sample X is located is 269 after calculating the high level mean and the overall mean of the header, so γ 1, γ 2, and γ 3 are 2421, 2690, and 2959, respectively.
It can be seen that in the set of data, the reliability of the threshold γ 1 is higher than γ 2 and γ 3, so γ 1 is selected as the confidence threshold of the one frame ADS-B data where the sample X is located in the current embodiment. Since the sample X is interfered to a greater extent, only γ 1 is less than C, and both γ 2 and γ 3 are greater than C, it is more reliable to select γ 1 as the confidence threshold of the sample X from the perspective of a single sample.
In summary, the method of the present invention may calculate and compare the curves of the Q value of each threshold γ along with the change of the signal-to-noise ratio, where a higher Q value represents a lower false determination rate of the chip, and the error rate is lower. Therefore, a better or even optimal confidence threshold can be selected according to the Q value curve, the demodulation performance of the receiver is improved, and the purpose of reducing the error rate is finally achieved.
The above-mentioned embodiments only express one embodiment 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, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (7)

1. A method for confidence threshold optimization for digital signal demodulation, comprising the steps of:
SO1: sampling the waveform to obtain a sample set when PPM demodulation is carried out; acquiring high and low levels of a sample when the sample is not interfered;
s02: acquiring a signal-to-noise ratio lambda in an actual channel;
s03: the Q function was constructed as follows:
Figure FDA0002311128270000011
wherein gamma is a confidence threshold for digital signal demodulation;
and in the value range of the gamma to be optimized, taking the maximum value of the Q function as an optimization target, and obtaining the gamma value corresponding to the maximum value of the Q function as the optimal confidence threshold value for demodulating the digital signal.
2. The method of claim 1, wherein in step S01, the sampling interval is fixed when the waveform is sampled.
3. The method of claim 1, wherein in step S01, when the waveform is sampled, the number of samples on the left and right sides of the waveform is the same.
4. The method of claim 1, wherein in step S01, the level ± a is obtained by averaging or maximum likelihood.
5. A method of demodulating a digital signal, comprising the steps of:
SO1, sampling the waveform to obtain a sample set when PPM demodulation is carried out; acquiring the high and low levels +/-a of a sample when the sample is not interfered;
s02: acquiring a signal-to-noise ratio lambda in an actual channel;
s03: the Q function was constructed as follows:
Figure FDA0002311128270000012
wherein gamma is a confidence threshold for digital signal demodulation;
within the value range of gamma to be optimized, taking the maximum value of the Q function as an optimization target, obtaining the gamma value corresponding to the maximum value of the Q function, and taking the gamma value as the confidence coefficient threshold gamma of the optimal digital signal demodulation0；
S04: the sampling points on the left and right halves of the waveform are summed separately,
Figure FDA0002311128270000013
Figure FDA0002311128270000021
wherein m is the number of sampling points positioned on the left half of the waveform, and n is the total number of the sampling points; n is 2 m;
comparing the values of A and B, if A is greater than or equal to B, the waveform represents symbol "1", otherwise, the waveform represents symbol "0";
defining C ═ A-B |, comparing C with confidence threshold gamma of optimal digital signal demodulation0If C is not less than gamma0Then the confidence is 1, if C<γ0If yes, the confidence coefficient is 0;
s05: and performing CRC check according to the obtained confidence.
6. The method according to claim 5, wherein the step S05 is specifically as follows: counting the number of chips with the confidence coefficient of 0, and if the number of chips is more than 5, discarding the message; and if the error is less than or equal to 5, carrying out error correction and detection analysis, correcting the wrong chip to obtain a correct chip, and finishing the demodulation work.
7. An apparatus for demodulating a digital signal, comprising:
an ADC signal acquisition module; it is used for collecting ADC signals;
a header data caching module; the ADC signal acquisition module is connected with the ADC signal acquisition module and is used for caching header data of the acquired ADC signals;
a signal-to-noise ratio lambda estimation module of a channel; the device is connected with a header data caching module and is used for acquiring the signal-to-noise ratio lambda in an actual channel;
a high level a value estimation module in the absence of noise; the device is connected with a header data caching module and used for acquiring the high and low levels +/-a of the sample when the sample is not interfered;
a Q value searching module; according to the input confidence threshold, the built-in Q function calculation is carried out to obtain the confidence threshold gamma of the digital signal demodulation with the Q function taking the maximum value0；
A confidence threshold acquisition module; it produces confidence threshold value for Q function calculation according to input confidence threshold value or confidence threshold value range;
a message data processing module; it buffers and demodulates the message data of ADC signal, and demodulates confidence threshold gamma according to the obtained digital signal0Obtaining a confidence coefficient;
and the CRC check module is used for carrying out error correction and error detection on the chips with low confidence coefficient.
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