CN114785425B - Enhanced Rowland system data demodulation method based on log likelihood ratio - Google Patents
Enhanced Rowland system data demodulation method based on log likelihood ratio Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention provides a method for demodulating enhanced Roland system data based on log likelihood ratio, which is used for receiving and sampling enhanced Roland signals containing noise, wherein the sampling range at least comprises a complete pulse group signal; generating three reference pulse modulation form sequences by using the first two pulses, wherein the three reference pulse modulation form sequences correspond to three modulation forms respectively; circularly extracting a single modulation pulse sequence in a received signal, wherein i represents the index of a pulse; calculating three reference pulse sequences and correlation calculation values of the pulse sequences respectively, and marking the three reference pulse sequences as Y (i, 1), Y (i, 2) and Y (i, 3); combining Y (i, 1) and Y (i, 2), combining Y (i, 1) and Y (i, 3), and calculating log-likelihood ratios LLR (i, 1) and LLR (i, 2) of the two combinations, respectively; and carrying out modulation judgment through the LLR value. The invention can realize the correct demodulation of the information under the condition of lower signal-to-noise ratio, thereby realizing the correct decoding of the information and greatly reducing the error rate of the information demodulation and decoding.
Description
Technical Field
The invention belongs to the technical field of land-based positioning time service, and relates to a data demodulation method which can be applied to an enhanced Roland receiving system and can reduce the demodulation error rate of Roland signals.
Background
Modulation of the enhanced rowland system (eLoran) data link usually adopts a tri-state pulse displacement word balance modulation (Pulse Position Modulation, PPM) mode, namely Pulse Position Modulation (PPM) mode, namely information is modulated onto the 3 rd to 8 th pulse of a group repetition period by a transmission time control method, and the modulation quantity of time control is +/-1 mu s. This modulation scheme produces three signal patterns: the modulation results of the three modes are shown in fig. 1, (1) representing the information "0" modulation if the transmission phase is not shifted, (2) representing the information "-" modulation if the transmission phase is advanced by 1 μs, and (3) representing the information "+" modulation if the transmission phase is retarded by 1 μs.
The information demodulation process of the enhanced Roland system is to correctly judge that the phase modulation information applied to each pulse of the pulse group signal of the enhanced Roland system is 0 "-" or "+" modulation. Because the signal has errors in the propagation and receiving processes, a certain error rate exists in the information in the demodulation process, and the data decoding and the recovery of the text information behind the Roland data link are directly affected. Therefore, the demodulation method becomes a critical ring in the Roland data chain processing. In the traditional Roland information processing process, a cross-correlation detection mode is generally adopted for demodulation, when the signal-to-noise ratio is high, the error rate for representing the demodulation accuracy is low, but along with the severe change of a noise environment, the signal-to-noise ratio is reduced, the error rate of information demodulation is greatly increased, and the decoding performance of a receiver is seriously influenced. Therefore, the conventional information demodulation method is difficult to meet the requirement of the modern receiver on information demodulation, and other methods are needed to improve the effectiveness of information demodulation in PPM modulation.
Log likelihood ratio is a common soft demodulation processing method in communication systems, and can be applied to many demodulation fields in combination with hard decision conditions. In the demodulation process based on the log likelihood ratio method, the likelihood ratio calculation process is related to a modulation mode, the common modulation modes are BPSK, QPSK, 16-QAM and the like, and the number of states corresponding to the modulated data is even, which is different from the PPM tri-state modulation mode of the enhanced Roland system. Therefore, the conventional log-likelihood ratio calculation process and decision method cannot be directly applied to PPM modulation.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides an enhanced Rowland system data demodulation method based on a log likelihood ratio, which utilizes the correlation between a single pulse receiving signal containing noise and a reference signal to calculate the log likelihood ratio of the single pulse signal, and then judges phase modulation information. The invention corrects the log-likelihood ratio method and the judgment condition, so that the method can be applied to PPM modulation, improves the reliability of demodulation, and can effectively improve the demodulation and decoding precision of the enhanced Rowland system information.
The technical scheme adopted by the invention for solving the technical problems comprises the following steps:
1) Receiving and sampling the enhanced Roland signal containing noise, wherein the sampling range at least comprises a complete pulse group signal;
2) Generating three reference pulse modulation form sequences by using the first two pulses, wherein the three reference pulse modulation form sequences correspond to three modulation forms of 0, 0 and plus respectively;
3) Circularly extracting a single modulation pulse sequence in a received signal, wherein i represents the index of a pulse; calculating correlation calculation values of three reference pulse sequences and the pulse sequences respectively, and recording the result of the correlation of the '0' modulation reference pulse and the single modulation pulse sequence as Y (i, 1), the result of the correlation of the '-' modulation reference pulse and the single modulation pulse sequence as Y (i, 2), and the result of the correlation of the 'plus' modulation reference pulse and the single modulation pulse sequence as Y (i, 3);
4) Combining Y (i, 1) and Y (i, 2), combining Y (i, 1) and Y (i, 3), and calculating log-likelihood ratios LLR (i, 1) and LLR (i, 2) of the two combinations, respectively;
5) The modulation judgment is made by the value of LLR, and when LLR (i, 1) >0 and LLR (i, 2) >0, the pulse is judged to be "0" modulation, when LLR (i, 1) <0 and LLR (i, 2) >0, the pulse is judged to be "-" modulation, and when LLR (i, 1) >0 and LLR (i, 2) <0, the pulse is judged to be "+" modulation.
Step 2) takes the average value of the two pulse sequences as the reference pulse in the form of '0' modulation, and the '-' and the 'plus' are generated by data translation according to the time control modulation quantity and the sampling rate.
Step 4) calculates likelihood probability of Y (i, 1) and Y (i, 2) combination, probability of 0 modulation of ith single pulseProbability of occurrence of "-" modulation>Log likelihood ratioSimilarly, the likelihood probability of Y (i, 1) and Y (i, 3) combination is calculated, the probability of "0" modulation occurring +.>Probability of occurrence of "+" modulation->Log likelihood ratio
The beneficial effects of the invention are as follows: by combining the characteristics of the PPM tri-state signal modulation mode of the enhanced Rowland system, the common log-likelihood ratio method is modified, and the judgment condition is updated to be combined judgment, so that a new signal demodulation method based on the log-likelihood ratio and applicable to the enhanced Rowland system is provided. The invention can realize the correct demodulation of the information under the condition of lower signal-to-noise ratio, thereby realizing the correct decoding of the information and greatly reducing the error rate of the demodulation and decoding of the information. Compared with the traditional demodulation mode of cross-correlation detection, the performance of the invention is improved by about 7-8dB, and the specific performance result is shown in figure 4.
Drawings
FIG. 1 is a Pulse Position Modulation (PPM) schematic of an enhanced Roland system signal;
FIG. 2 is a schematic diagram of a noisy enhanced Roland pulse set signal (signal-to-noise ratio of-5 dB);
FIG. 3 is a flow chart of a method process;
fig. 4 is a comparative schematic diagram of demodulation performance.
Detailed Description
The invention will be further illustrated with reference to the following figures and examples, which include but are not limited to the following examples.
The technical scheme of the invention comprises the following steps:
1) The noise-containing enhanced rowland signal is received and sampled over a range of at least one complete pulse set signal, since signal demodulation is only for the 3-8 th pulse of the pulse set, the following description and processing only involves pulses containing information modulation, and the phase encoding of the pulses is not considered.
2) Three reference pulse modulation form sequences are generated by using the first two pulses, and the three reference pulse modulation form sequences respectively correspond to three modulation forms of 0, 0 and plus. Since the first two pulses of the enhanced rowland signal are not modulated, the average value of the two pulse sequences can be taken as a reference pulse in the form of "0" modulation, and the forms of "-" and "+" can be generated by shifting the data according to the time-controlled modulation amount and the sampling rate.
3) A single modulated pulse sequence in the received signal is cyclically extracted, with i representing the index of the pulse. And then calculating the correlation calculated values of the three reference pulse sequences and the pulse sequences respectively, and recording the result of the correlation of the '0' modulation reference pulse and the single modulation pulse sequence as Y (i, 1), the result of the correlation of the '-' modulation reference pulse and the single modulation pulse sequence as Y (i, 2), and the result of the correlation of the 'plus' modulation reference pulse and the single modulation pulse sequence as Y (i, 3).
4) A log-likelihood ratio is calculated. Y (i, 1) and Y (i, 2) are combined, Y (i, 1) and Y (i, 3) are combined, and log likelihood ratios of the two combinations are calculated respectively. First, likelihood probabilities of a combination of Y (i, 1) and Y (i, 2) are calculated. The probability of the occurrence of a "0" modulation for the ith single pulse is expressed as:
the probability of occurrence of a "-" modulation is expressed as:
the log-likelihood ratio can be expressed as:
similarly, the likelihood probability of a combination of Y (i, 1) and Y (i, 3) is calculated, and the probability of a "0" modulation occurring is expressed as:
the probability of occurrence of a "+" modulation is expressed as:
the log-likelihood ratio can be expressed as:
5) Modulation judgment is made by the value of LLR, and when LLR (i, 1) >0 and LLR (i, 2) >0, the pulse is judged to be "0" modulation, when LLR (i, 1) <0 and LLR (i, 2) >0, the pulse is judged to be "-" modulation, and when LLR (i, 1) >0 and LLR (i, 2) <0, the pulse is judged to be "+" modulation.
In this embodiment, the pulse group signal is modulated using a modulation pattern of "0- + -0+". To avoid loss of generality, the first two unmodulated pulses are added before the modulation sequence. Random noise is added into the signal, and the signal to noise ratio is-5 dB in the example, so that the signal is used for replacing the received signal of the enhanced Rowland system. The signal demodulation new method based on log likelihood ratio calculation comprises the following specific steps:
1) The enhanced Roland signal containing noise is received and sampled at a sampling rate of 10MHz (the modulation amount controlled by PPM modulation time is + -1 mu s, and the sampling rate should be greater than 1 MHz). The received signal is sampled as shown in fig. 2.
2) Three reference pulse modulation sequences are generated by using the first two pulses, and the three reference pulse modulation sequences respectively correspond to three modulation forms of '0', '-' and 'plus'. The "0" modulation form reference sequence is the average of the first two pulse sequences, denoted as X (i, 1), the sampling rate of the reference pulse is likewise 10MHz, and the sampling length is 1000 mus. When the modulation interval is 1 μs, the reference sequence of the modulation form of the corresponding data number of 10, "-" can eliminate the first 10 values of X (i, 1) and be zero-padded at the back. The sequence of the "+" modulation form can be padded with zeros at the last 10 values of the culled X (i, 1).
3) A single modulated pulse sequence in the received signal is cyclically extracted, with i representing the index of the pulse. The total time span of the received signal is 8000 mus, comprising 8 pulses. The first two pulses are used for generating a reference sequence, the last 6 pulses are respectively related to three reference pulse sequences, and the result of the correlation between the '0' modulation reference pulse and the single modulation pulse sequence is recorded as Y (i, 1), the result of the correlation between the '-' modulation reference pulse and the single modulation pulse sequence is recorded as Y (i, 2), and the result of the correlation between the 'plus' modulation reference pulse and the single modulation pulse sequence is recorded as Y (i, 3). Where i=1, 2, …,8.
4) A log-likelihood ratio is calculated. The log-likelihood ratio is calculated with a combination of Y (i, 1) and Y (i, 2) and a combination of Y (i, 1) and Y (i, 3). The values of the log likelihood ratios of the respective pulses obtained according to the calculation formula are shown in the following table:
| pulse index | LLR(i，1) | LLR(i，2) |
| 1 | - | - |
| 2 | - | - |
| 3 | 0.4075 | 0.3323 |
| 4 | -0.3573 | 0.7193 |
| 5 | 1.0653 | -0.4797 |
| 6 | -0.1225 | 1.0392 |
| 7 | 0.1639 | 0.0159 |
| 8 | 1.2629 | -0.0360 |
5) And carrying out modulation judgment through the LLR value, wherein the demodulated pattern result is 0-minus 0+', and the demodulated pattern result is consistent with the pattern before modulation.
As can be seen from the above embodiments, in the signal demodulation method based on the log likelihood ratio, the method can still correctly demodulate the modulation pattern under the condition of low signal-to-noise ratio, thereby reducing the error rate in the demodulation process of the enhanced rowland system signal. Simulation results show that the method can be improved by 7-8dB compared with the traditional cross-correlation demodulation method.
Claims (1)
1. The enhanced Rowland system data demodulation method based on the log likelihood ratio is characterized by comprising the following steps of:
1) Receiving and sampling the enhanced Roland signal containing noise, wherein the sampling range at least comprises a complete pulse group signal;
2) Generating three reference pulse modulation form sequences by using the first two pulses, wherein the three reference pulse modulation form sequences correspond to three modulation forms of 0, 0 and plus respectively;
step 2) taking the average value of the first two pulses as a reference pulse in a 0 modulation form, wherein the '-' and '+' are generated by data translation according to the time control modulation quantity and the sampling rate;
3) Circularly extracting a single modulation pulse sequence in a received signal, wherein i represents the index of a pulse; calculating correlation calculation values of three reference pulse modulation form sequences and the pulse sequence respectively, and recording that the result of the correlation of the reference pulse modulation form sequence of ' 0 ' and the single modulation pulse sequence is Y (i, 1), the result of the correlation of the reference pulse modulation form sequence of ' to the single modulation pulse sequence is Y (i, 2), and the result of the correlation of the reference pulse modulation form sequence of ' plus ' and the single modulation pulse sequence is Y (i, 3);
4) Combining Y (i, 1) and Y (i, 2), combining Y (i, 1) and Y (i, 3), and calculating log-likelihood ratios LLR (i, 1) and LLR (i, 2) of the two combinations, respectively;
step 4) calculates likelihood probability of Y (i, 1) and Y (i, 2) combination, probability of 0 modulation of ith single pulseProbability of occurrence of "-" modulation>Log likelihood ratioSimilarly, the likelihood probability of Y (i, 1) and Y (i, 3) combination is calculated, the probability of "0" modulation occurring +.>The "+" modulation occursProbability of->Log likelihood ratio
5) The modulation judgment is made by the value of LLR, and when LLR (i, 1) >0 and LLR (i, 2) >0, the pulse is judged to be "0" modulation, when LLR (i, 1) <0 and LLR (i, 2) >0, the pulse is judged to be "-" modulation, and when LLR (i, 1) >0 and LLR (i, 2) <0, the pulse is judged to be "+" modulation.
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| JP4771996B2 (en) * | 2006-06-20 | 2011-09-14 | パナソニック株式会社 | Code error detection apparatus, radio system, and error detection method |
| US11255663B2 (en) * | 2016-03-04 | 2022-02-22 | May Patents Ltd. | Method and apparatus for cooperative usage of multiple distance meters |
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|---|---|---|---|---|
| US4821038A (en) * | 1987-11-03 | 1989-04-11 | Megapulse Incorporated | Method of and apparatus for Loran-C message communication with reduced skywave navigation location errors and the like |
| CN102571120A (en) * | 2011-11-23 | 2012-07-11 | 周隽 | Timing demodulation method of Loran-C signals under condition of low signal to noise ratio |
| CN103379049A (en) * | 2012-04-14 | 2013-10-30 | 兰州理工大学 | Wireless optical communication channel estimation and soft-demodulation method by using 4PPM digital modulation mode |
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