KR101728218B1 - Apparatus and method for removing error of the turbo decoder - Google Patents

Abstract

An apparatus and method for eliminating the error floor phenomenon of a turbo decoder and correcting errors are presented. The turbo decoder includes a detector for performing a cyclic redundancy check (CRC) on an input bit and detecting an error, and a detector for detecting an error when the detector detects the error, And outputs the converted data.

Description

[0001] APPARATUS AND METHOD FOR REMOVING ERROR OF THE TURBO DECODER [0002]

And more particularly to an apparatus and method for eliminating an error floor phenomenon of a turbo decoder and correcting errors.

A satellite communication system based on wireless communication has an unstable channel environment due to causes such as fading, non-linearity, and the like. Therefore, there is a need for research on error correction and error control techniques to provide improved quality of service. In error correction techniques, turbo codes are widely used because of their excellent error correction capability and easy code rate extension.

A turbo code is a code that improves the error correction capability by reducing the probability of occurrence of such a code string when a code string having a minimum distance is small compared to other codes. Therefore, the code sequences generated by the turbo code generally have a larger hamming distance than the code sequences generated by the other codes. However, certain code sequences generated by turbo codes have a hamming distance smaller than that generated by other codes. These specific code sequences cause an error floor phenomenon in which the bit error rate (BER) is not reduced even though the signal to noise ratio (SNR) of the input bit increases. do.

In DVB-RCS2 (Digital Video Broadcasting-Return Channel via Satellite Second Generation), which is a reverse communication standard of the satellite wireless communication system, the state of the RSC (recursive systematic code) of the turbo code is extended to 16 states to improve the error correction capability, The phenomenon was reduced. However, the turbo code of RCS2 is also a minimum distance increase compared with RCS1 (Return Channel via Satellite First Generation) and still has a small minimum distance compared with the algebra-based code, resulting in an error floor phenomenon.

According to one side, a turbo decoder is provided. The turbo decoder includes a detector for performing a cyclic redundancy check (CRC) on an input bit and detecting an error, and a detector for detecting an error when the detector detects the error, And outputs the converted data. The transform unit may store the bit stream having the hamming distance between the bit streams generated by the turbo code less than a preset value as the candidate bit stream. The converting unit may preferentially change the bit string having the smallest hamming distance from the input bit among the stored candidate bit strings.

According to an exemplary embodiment, the turbo decoder may further include a determination unit for comparing a magnitude of the threshold with a signal-to-noise ratio corresponding to the input bit. The determination unit may turn on the conversion unit when the signal-to-noise ratio corresponding to the input bit is greater than the threshold value. The determination unit may set the signal-to-noise ratio at which the error floor phenomenon is detected in the output bits of the turbo decoder to the threshold value.

According to another embodiment of the present invention, the detector performs the cyclic redundancy check on any one of the bit strings and detects an error, and when the detecting unit detects the error, the converter converts the input bit into the pre- It can be changed to another bit string and output.

According to another aspect of the present invention, there is provided a method for performing a cyclic redundancy check on an input bit and detecting the presence or absence of an error, and, when the error exists in the input bit, And converting the bit string into a bit string and outputting the bit string. The converting and outputting to any one of the bit strings preferably converts the bit string having a small hamming distance with the input bit among the pre-stored bit string candidates to one of the bit strings.

According to an embodiment of the present invention, the error correcting method using the turbo code is configured to perform the cyclic redundancy check on any one of the converted bit strings, detect the presence or absence of an error, and, when the error exists, And re-converting the pre-stored bit string candidates to another one of the bit string candidates. The re-conversion step may be repeatedly performed and the cyclic redundancy check is performed on the other bit string that has been retransformed, so that the error can be discontinued when no error is detected.

According to another embodiment of the present invention, the error correction method using the turbo code further includes the step of storing the input bit and the bit stream having the hamming distance less than a preset value as a pre-stored bit stream candidate corresponding to the input bit.

Meanwhile, the error correction method using the turbo code may further include a step of comparing a signal-to-noise ratio corresponding to the input bit and a predetermined threshold value. According to an embodiment, the step of converting and outputting to any one of the bit strings may be performed when the signal-to-noise ratio corresponding to the input bit is greater than the predetermined threshold value in the step of comparing the preset threshold values . The threshold may be a signal-to-noise ratio that causes an error floor effect on the output bits.

According to another aspect, a bit string substitution for a turbo decoder is provided. A bit string substitution unit for the turbo decoder includes a calculation unit for calculating a hamming distance between bit strings generated based on the turbo code, a storage unit for storing a set of the bit strings having the hamming distance below the threshold as bit string candidates, And a replacing unit replacing the bit string with another bit string of the bit string candidate corresponding to the bit string.

According to an embodiment, when the turbo decoder detects an error in the bit stream, the replacing unit may replace the bit stream candidate with the other bit stream corresponding to the bit stream. The replacing unit may replace the different bit sequence of the bit string candidate corresponding to the bit string when the signal-to-noise ratio of the bit string is equal to or greater than a signal-to-noise ratio at which an error floor phenomenon is found. The storage unit may include a ROM table and may store the bit string candidates in the ROM table.

According to an embodiment, the calculation unit may calculate the signal-to-noise ratio as an error floor phenomenon when the bit error rate corresponding to the signal-to-noise ratio of the bit string changes below a predetermined rate of change.

1 is a block diagram illustrating a turbo decoder according to an embodiment of the present invention.
FIG. 2 shows a bit error rate graph of a turbo decoder according to an embodiment.
FIG. 3 illustrates a structure of a turbo decoder according to an embodiment of the present invention.
4 is a block diagram illustrating an error correction method using a turbo code according to an embodiment.
5 is a block diagram illustrating a bit string substitution unit according to one embodiment.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. However, the scope of the rights is not limited or limited by these embodiments. Like reference symbols in the drawings denote like elements.

The terms used in the following description are chosen to be generic and universal in the art to which they are related, but other terms may exist depending on the development and / or change in technology, customs, preferences of the technician, and the like. Accordingly, the terminology used in the following description should not be construed as limiting the technical thought, but should be understood in the exemplary language used to describe the embodiments.

Also, in certain cases, there may be a term chosen arbitrarily by the applicant, in which case the meaning of the detailed description in the corresponding description section. Therefore, the term used in the following description should be understood based on the meaning of the term, not the name of a simple term, and the contents throughout the specification.

1 is a block diagram illustrating a turbo decoder according to an embodiment of the present invention.

Referring to FIG. 1, a turbo decoder 100 includes a detector 110, a determiner 120, and a transformer 130. The turbo decoder 100 according to an embodiment improves the error correction capability for the input bits and can eliminate the error flooring phenomenon of the turbo code. The detection unit 110 detects an error existing in the input bit of the turbo decoder 100. In one embodiment, the detector 110 may perform cyclic redundancy checking (CRC) on the input bits of the turbo decoder 100. For example, the detection unit 110 may divide the input bit by a predetermined polynomial and check whether the remaining value is 0, to check whether there is an error in the transmitted and / or received input bits.

The determination unit 120 compares the signal-to-noise ratio corresponding to the input bit of the turbo decoder 100 with the threshold value. The determination unit 120 causes the conversion unit to operate when the signal-to-noise ratio corresponding to the input bit of the turbo decoder 100 is greater than the threshold value. Illustratively, but not limited to, the determination unit 120 may set the signal-to-noise ratio at which the error floor phenomenon is detected in the output bits of the turbo decoder to a threshold value. Some of the code lines generated by the turbo code have an error floor effect. The error floor phenomenon is a phenomenon in which the bit error rate does not decrease even though the signal to noise ratio of the input bit increases. In other words, even if the minimum distance of some code sequences generated by the turbo code is too small, the error correction and improvement effect is not large and the bit error rate is not improved even when the signal-to-noise ratio is increased.

According to another embodiment, the determination unit 120 measures the rate of change of the bit error rate corresponding to the bit error rate and the signal-to-noise ratio. The determination unit 120 may set the threshold value of the rate of change of the bit error rate. The determination unit 120 may set the signal-to-noise ratio corresponding to the threshold value to the threshold value when the rate of change of the measured bit error rate is smaller than the threshold value. The determination unit 120 may turn on the conversion unit 130 when the signal-to-noise ratio corresponding to the input bit is greater than the threshold value.

The converting unit 130 may convert the input bit of the turbo decoder 100 into one of the previously stored candidate bit streams and output the modified bit stream. When the detecting unit 110 detects an error with respect to the input bit of the turbo decoder, the converting unit 130 may convert the input bit into a bit string of one of the previously stored candidate bit strings and output the bit string.

According to another embodiment, the transforming unit 130 may store the bit stream having the hamming distance between the bit streams generated by the turbo code less than a predetermined value as the candidate bit stream. The Hamming distance is associated with channel coding and is associated with the detection and correction of errors. The Hamming distance means the number of different character pairs present in two bit strings or code strings. Illustratively, the hamming distance of bits 010 and 000 is 1, and the hamming distance of 010 and 111 is 2. Therefore, as the Hamming distance is as large as possible between different code sequences, the error correction capability becomes higher.

In one embodiment, the converting unit 130 may preferentially change any one of the stored bitstreams having the smallest hamming distance from the input bitstream. As described above, since the bit streams having shorter Hamming distances may cause errors with a higher probability, the transformer 130 of the present invention may be configured such that, for example, It is possible to output it first.

FIG. 2 shows a bit error rate graph of a turbo decoder according to an embodiment.

In FIG. 2, the x-axis of the graph represents the signal-to-noise ratio (E _S / N ₀ ) of the input bit of the turbo decoder and the y-axis represents the bit error rate. In one embodiment, the modulation scheme is Quadrature Phase Shift Keying (QPSK), and the bit error rate is shown when the coding rate is 1/3 and the number of repetitions is eight. As can be seen from the graph of FIG. 2, the bit error rate of the y-axis tends to decrease while the signal-to-noise ratio of the x-axis increases. The determination unit 120 may calculate the rate of change of the bit error rate corresponding to the bit error rate and the signal-to-noise ratio.

Referring to FIG. 2, in one embodiment, the rate of change of the bit error rate can be calculated when the signal-to-noise ratio varies by 0.2. The rate of change of the bit error rate 210 when the signal-to-noise ratio is between 0.0 and 0.2, the rate of change of the bit error rate 220 when between 0.2 and 0.4 and the rate of change of the bit error rate 230 when between 0.4 and 0.6, do.

According to another embodiment, the determination unit 120 may set the signal-to-noise ratio at which the error floor phenomenon occurs to a threshold value. In one embodiment, the determination unit 120 may detect an error floor phenomenon if the bit error rate does not decrease by 10 times or more in response to the signal-to-noise ratio increase of 0.2.

Referring to FIG. 2, the rate of change 210 of the bit error rate between 0.0 and 0.2 and the rate of change 220 of the bit error rate between 0.2 and 0.4 indicate a rate of reduction of 1 or more, i.e., 10 times or more, in the y axis of the logarithmic scale. However, the rate of change of the bit error rate 230 between 0.4 and 0.6 indicates a reduction rate of 1 or less, i.e., 10 times or less, in the y-axis of the logarithmic scale. Accordingly, the determination unit 120 may detect an error floor phenomenon corresponding to the rate of change of the bit error rate 230 and set the signal-to-noise ratio 0.4 at this time as a threshold value.

FIG. 3 illustrates a structure of a turbo decoder according to an embodiment of the present invention.

The turbo decoder 300 includes a first MAP (Maximum A Posteriori) decoder 310, a second MAP decoder 320, an interleaver 330, a deinterleaver 340, a detector 350, a determiner 360, (370). The first MAP decoder 310 and the second MAP decoder 320 may encode input bits according to a decision rule that maximizes the posterior probability. Also, the first MAP decoder 310 and the second MAP decoder 320 may determine the received input bit to minimize the error probability. In one embodiment, the first MAP decoder 310 and the second MAP decoder 320 can determine an input bit using the Log-Map decoding algorithm of Equation (1). Equation 1 is as follows.

L (d _k ) represents the output probability of each of the first MAP decoder 310 and the second MAP decoder 320. L (d _k ) can be expanded as shown in Equation (2) below.

,

및

을 이용하여 전개할 수 있다.

는 순방향 상태 매트릭(Forward State Metric),

는 역방향 상태 매트릭(Backward State Metric),

는 가지 매트릭(Branch Metric) 함수를 나타낸다. 각각의 함수는 아래의 수학식 3, 4, 5와 같이 전개될 수 있다.
L (d _k )

,

And

Can be developed.

A forward state metric,

A backward state metric,

Represents a branch metric function. Each function can be expanded as shown in Equations (3), (4) and (5) below.

The first MAP decoder 310 and the second MAP decoder 320 may calculate the output likelihood ratio (LLR) using Equations (3), (4) and (5).

The interleaver 330 rearranges the order of the input bits or the input data string of the turbo decoder 300 in a predetermined unit. As an example, the interleaver 330 may distribute the data stream of the turbo decoder 300 to correspond to each of the columns and rows of the block. The deinterleaver 340 arranges input bits or input data rearranged by the interleaver 330 in their original positions. The interleaver 330 and the deinterleaver 340 arrange such that errors that may occur in transmission and reception of input data are scattered rather than concentrated.

In one embodiment, the operations of the detector 350, the determiner 360, and the converter 370 are the same as those described above with reference to FIG.

4 is a block diagram illustrating an error correction method using a turbo code according to an embodiment.

The method 400 for error correction using turbo codes comprises comparing 410 the magnitude of the signal-to-noise ratio and the threshold of the input bit of the turbo decoder, performing CRC (Cyclic Redundancy Checking) error detection 420, transforming the bit stream 430, and performing CRC error detection and error correction 440.

Step 410 compares the signal-to-noise ratio of the input bit with a predetermined threshold magnitude. As an example, the threshold may be set to the signal-to-noise ratio of the input bit where an error floor effect occurs for the output bits. If the signal-to-noise ratio of the input bit is greater than the predetermined threshold size, the error correction method 400 using the turbo code includes a step 430 of transforming the bit stream.

Step 420 is a step of performing CRC-based error detection. Step 420 divides the transmitted input bit by a CRC-based polynomial and determines whether the remainder is 0 or not to see if there is an error in the input bit.

Step 430 is a step of outputting an input bit for which an error is detected to a bit string of any one of the previously stored bit string candidates corresponding to the input bit. In one embodiment, step 430 may include storing the input bit and the bit stream having the Hamming distance less than or equal to a preset value as a bit stream candidate corresponding to the input bit.

In another embodiment, step 430 may convert the bit string having a small Hamming distance with the input bit among the pre-stored bit string candidates to one of the bit strings. As described above, as the hamming distance between the two bit streams is shortened, the error occurrence probability is further increased. Therefore, error correction can be attempted for a bit stream having a higher error occurrence probability by preferentially replacing a bit stream having a small Hamming distance. In addition, step 430 may be repeatedly performed with step 420.

Illustratively, step 420 may be performed again for any one transformed bit stream. CRC error detection is performed on the newly transformed bitstream through the rule 420, and if there is an error, step 430 is performed again. The step 430 re-converts any one of the bit strings into another one of the previously stored bit string candidates corresponding to the input bit. According to one embodiment, steps 420 and 430 are repeatedly performed and may be performed until no further error is detected for the other bit stream that has been re-transformed.

Step 440 performs CRC-based error detection and error correction. In one embodiment, step 440 may perform error correction by requesting retransmission of an error frame for an input bit based on an Automatic Repeat request (ARQ). In another embodiment, step 440 may correct the error through Forward Error Correction (FEC). Error correction codes can be used to detect errors and attempt to restore them to their original values.

5 is a block diagram illustrating a bit string substitution unit according to one embodiment.

The bit string substitution unit 500 includes a calculation unit 510, a storage unit 520, and a replacement unit 530. The bit string substitution unit 500 is an apparatus that performs error correction by repeatedly replacing bit strings determined to have a high error occurrence probability because the Hamming distance is smaller than the threshold value. The calculation unit 510 calculates the hamming distance between the bit strings generated based on the turbo code. In addition, the calculation unit 510 calculates the signal-to-noise ratio when the bit error rate corresponding to the signal-to-noise ratio of the bit string changes below a predetermined rate of change. The calculation unit 510 determines the signal-to-noise ratio as a signal-to-noise ratio at which the error floor phenomenon starts.

The storage unit 520 may store a set of bit strings having the hamming distance below the threshold as bit string candidates. In addition, the storage unit 520 may store the value of the signal-to-noise ratio at which the error flooring computed by the calculation unit 510 starts. As an example, the storage unit 520 may include a ROM table and may store values of bit string candidates or signal-to-noise ratios in the ROM table.

The replacing unit 530 may replace the bit string with another bit string candidate of the bit string candidate corresponding to the bit string. In one embodiment, the replacing unit 530 may replace the other bit string of the bit string candidate corresponding to the bit string when the turbo decoder detects an error with respect to the bit string. In addition, the replacing unit 530 may replace the different bit string of the bit string candidate corresponding to the bit string when the signal-to-noise ratio of the bit string is equal to or greater than the signal-to-noise ratio at which the error floor phenomenon is found.

In another embodiment, the replacing unit 530 replaces the bit string with the different bit string of the bit string candidate corresponding to the bit string in order of decreasing Hamming distance. In one embodiment, the replacing unit 530 may replace the bit string in the bit string candidate until no error is detected in the bit string.

Accordingly, although the embodiments have been described with reference to specific embodiments and drawings, various modifications and variations are possible in light of the above teachings. For example, it is to be understood that the techniques described may be performed in a different order than the described methods, and / or that components of the described systems, structures, devices, circuits, Lt; / RTI > or equivalents, even if it is replaced or replaced.

Therefore, other implementations, other embodiments, and equivalents to the claims are also within the scope of the following claims.

Claims (20)

A detector for performing a cyclic redundancy check (CRC) on an input bit string of a turbo decoder and detecting an error; And
And a conversion unit for converting the input bit string into a bit string of a previously stored candidate bit string and outputting the bit string when the detection unit detects the error,
Lt; / RTI >
Wherein the conversion unit preferentially changes any one bit string having the smallest hamming distance from the input bit string among the previously stored candidate bit strings and performs the cyclic redundancy check on the changed bit string, Turbo decoder to stop changing. The method according to claim 1,
A determination unit for comparing a magnitude of a signal to noise ratio (SNR) of the input bit stream with a threshold value;
Lt; / RTI >
Wherein the determination unit turns on the conversion unit when the signal-to-noise ratio of the input bit stream is larger than the threshold value
Turbo decoder. 3. The method of claim 2,
Wherein the determination unit sets the signal-to-noise ratio at which an error floor phenomenon is detected in the output bits of the turbo decoder to the threshold value
Turbo decoder. The method according to claim 1,
Wherein the converting unit stores the bit string having the Hamming distance between the bit strings generated by the turbo code less than a preset value as the candidate bit string
Turbo decoder. The method according to claim 1,
Wherein the detecting unit detects the error by performing the cyclic redundancy check on any one of the converted bit strings,
Wherein the converting unit changes the input bit string to another bit string of the previously stored candidate and outputs it when the detecting unit detects the error
Turbo decoder. delete Performing a cyclic redundancy check on an input bit string and detecting the presence or absence of an error; And
Converting the input bit stream into any one of the bit stream candidates stored in advance corresponding to the input bit stream and outputting the input bit stream if the error exists in the input bit stream
Lt; / RTI >
The step of converting and outputting any one of the bit strings may include:
A bit string having a small hamming distance with the input bit string among the pre-stored bit string candidates is preferentially converted into any one of the bit strings, and the cyclic redundancy check is performed on the converted bit string to detect no error Error Correction Method Using Turbo Codes Interrupted in Case. 8. The method of claim 7,
And performing a cyclic redundancy check on any one of the converted bit strings and detecting the presence or absence of an error, and when there is an error, extracting a bit string of another one of the previously stored bit string candidates corresponding to the input bit string Conversion step
The error correction method using the turbo code. 9. The method of claim 8,
And the re-conversion step is repeatedly performed, and is interrupted when the error is not detected by performing the cyclic redundancy check on the other bit string re-transformed. 8. The method of claim 7,
Storing the input bit stream and the bit stream whose hamming distance is less than or equal to a preset value as a pre-stored bit stream candidate corresponding to the input bit stream
The error correction method using the turbo code. delete 8. The method of claim 7,
Comparing the signal-to-noise ratio of the input bit stream with a predetermined threshold value
Further comprising:
The converting and outputting to any one of the bit strings is performed when the signal-to-noise ratio of the input bit string is larger than the predetermined threshold value in the step of comparing the predetermined threshold values
An Error Correction Method Using Turbo Codes. 13. The method of claim 12,
The threshold value is a signal-to-noise ratio that causes an error floor phenomenon to occur with respect to an output bit of the turbo decoder
An Error Correction Method Using Turbo Codes. A calculation unit for calculating a hamming distance between bit strings generated based on the turbo code;
A storage unit for storing a set of the bit strings having the hamming distances equal to or less than a threshold value as bit string candidates, respectively; And
And replacing the bit string with another bit string of the bit string candidate corresponding to the bit string,
Lt; / RTI >
Wherein the replacing unit replaces the bit string with the different bit string of the bit string candidate corresponding to the bit string in the ascending order of the Hamming distance, Bit string substitution for turbo decoder. 15. The method of claim 14,
When the turbo decoder detects an error with respect to the bit stream, the replacing unit replaces the bit stream with the different bit stream of the bit stream candidate corresponding to the bit stream
Bit - column substitution for turbo decoder. 16. The method of claim 15,
Wherein the replacing unit replaces the bit string with the different bit string of the bit string candidate corresponding to the bit string when the signal-to-noise ratio of the bit string is equal to or larger than a signal-to-
Bit - column substitution for turbo decoder. 15. The method of claim 14,
Wherein the calculation unit calculates the signal-to-noise ratio as an error floor phenomenon when the bit error rate corresponding to the signal-to-noise ratio of the bit string changes below a predetermined rate of change
Bit - column substitution for turbo decoder. delete delete 15. The method of claim 14,
Wherein the storage unit includes a ROM table and stores the bit string candidate in the ROM table
Bit - column substitution for turbo decoder.

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