JP2001237718A - Viterbi decoder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the path memory of a Viterbi decoder and to reduce deterioration due to truncation errors. SOLUTION: A likelihood determining part 1-6 selects a most likely path metric 1-11 and selects a corresponding information sequence 1-12. The selected information sequence 1-12 is divided and stored into information memories 1 to (n) at each time. An output determining part 1-8 compares the information sequences which are likelihood-determined at different times as information of the same time. The information sequence is selected by majority decision to correct the error included in the received sequence. Thus, path memories 1-5 are eliminated, and deterioration due to the truncation errors can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Viterbi decoder used for decoding a convolutional code, and more particularly to a Viterbi decoder having a reduced path memory.

[0002]

2. Description of the Related Art A conventional Viterbi decoder is disclosed in
Like the Viterbi decoder disclosed in Japanese Patent Application Laid-Open No. 6-77845, the Viterbi decoder includes a branch metric calculation unit, an ACS unit, a path metric memory, and a path memory, and decodes an information sequence by performing maximum likelihood determination. As shown in FIG.
Is compared with a code sequence corresponding to a branch representing a state transition inside the convolutional coder by a branch metric calculation unit 8-2, and a branch metric 8-8 as a difference is calculated. In ACS8-3,
The path metric memory 8-4 is updated by sequentially adding to the path metric 8-9 of the immediately preceding state. The information sequence 8-10 corresponding to the branch that transitions from the immediately preceding state to the current state is stored in the path memory 8-5. The path with the smallest path metric 8-9 is regarded as the most likely path, and the maximum likelihood determination unit 8-6 performs the maximum likelihood determination. The information sequence 8-10 stored in the path memory 8-5 corresponding to the path is used as the decoded output 8-7. In such a Viterbi decoder, as shown in FIG. 9, when the constraint length of a convolutional codeword is k, a path memory 8-5 having a constraint length k that is approximately six times the constraint length k is used for all 2 ^k states. I need.

[0003]

However, in the conventional Viterbi decoder as described above, it is necessary to secure path memories corresponding to the length of the received sequence for the number of internal states of the convolutional encoder. In general, the reception sequence is longer than the path memory size, and a large amount of path memory is required to secure sufficient path memory for all states, and it is difficult to secure such a memory size. is there. In such a case, a path memory of about 5 to 6 times the constraint length of the convolutional code is secured for all states, and when this memory becomes full, the oldest information is output as a decoding result. A decoding process using the back method is used. But,
When such a decoding process is performed, the decoding process is interrupted halfway, and maximum likelihood decoding is performed. Therefore, there is a problem that deterioration due to an error in the interruption cannot be avoided.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Viterbi decoder which solves the above-mentioned conventional problems, reduces the number of path memories, and reduces deterioration caused by a truncation error.

[0005]

In order to solve the above-mentioned problems, the present invention provides a Viterbi decoder comprising: a maximum likelihood determining section for comparing a received sequence with a codeword sequence to determine the maximum likelihood; A plurality of information memories that store the information sequence of the maximum likelihood path determined by the maximum likelihood path at different times, and an output determination unit that determines the information sequence based on the contents of the plurality of information memories and obtains a decoding result. Configuration.

[0006] With this configuration, the information sequence corresponding to the maximum likelihood path can be stored in the information memory for each time without providing a huge path memory in the Viterbi decoder, and the same time can be stored in the information memory. An error can be corrected and decoded by comparing and determining the plurality of information sequences thus obtained.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to FIGS.

According to an embodiment of the present invention, an information sequence corresponding to the path metric selected by the maximum likelihood determining unit is divided and stored for each time in an information memory, and the information sequence subjected to the maximum likelihood determination at different times is This is a Viterbi decoder that compares information as the same time and selects an information sequence by majority decision.

FIG. 1 is a functional block diagram of a Viterbi decoder according to an embodiment of the present invention. In FIG. 1, a branch metric calculation unit 1-2 is means for calculating a branch metric from a received sequence. The ACS 1-3 is a means for performing an ACS process using the path metric stored in the path metric memory and the branch metric calculated by the branch metric calculation unit. The path metric memory 1-4 is a means for storing a path metric. The path memory 1-5 is a means for storing an information sequence corresponding to a path metric. The maximum likelihood determining unit 1-6 is a means for comparing the path metrics in each state and selecting the most likely path metric. The information memories 1 (1-7-1) to n (1-7-n) are means for dividing an information sequence corresponding to a path metric for each time and storing it. The output determination unit 1-8 is means for comparing and determining the information series as information at the same time.

FIG. 2 is a configuration diagram of the path memory of the Viterbi decoder according to the embodiment of the present invention. FIG. 3 is a configuration diagram of the information memory of the Viterbi decoder according to the embodiment of the present invention. 4, 5, 6, and 7 are state transition diagrams of the Viterbi decoder according to the embodiment of the present invention.

The operation of the Viterbi decoder according to the embodiment of the present invention configured as described above will be described. In the Viterbi decoder 1-0, the branch metric calculation unit 1-2 calculates a branch metric 1-10 from the received sequence 1-1. In ACS 1-3, the path metric 1-11 stored in the path metric memory 1-4
And the branch metric 1-1 calculated by the branch metric calculation unit 1-2
With 0, the ACS process is performed, and the path metric 1-11 and the path metric memory 1-4 are updated. At the same time, an information sequence 1-12 corresponding to the path metric 1-11 is stored in the path memory 1-5.

FIG. 2 shows the configuration of the path memory 1-5. The size of the path memory 1-5 depends on the number of states of the convolutional encoder and the depth of the traceback time. The example shown in FIG.
This is a case where the constraint length of the convolutional code is k, the total number of states is 2 ^k , and the traceback depth is n.

The maximum likelihood determining section 1-6 includes a path metric memory 1
Compare the path metrics 1-11 of each state stored in -4,
Select the most likely path metric 1-11. An information sequence 1-12 corresponding to the path metric 1-11 is selected from the path memory 1-5. For the selected information series 1-12, the information one time earlier is stored in the information memory 1 (1-7-1),..., The information n times earlier is stored in the information memory n (1-7-
The data is divided and stored for each time, for example, the data is stored in n).

FIG. 3 shows an information memory. The size of each information memory depends on the number of information memories. Information memory 1
In the case of, the size is n, and in the case of the information memory n, the size is 1. The memory size required for all information memories is (n × (n + 1)) / 2. That is, when the constraint length of the convolutional code is k, the conventional trace-back method, whereas it is necessary path memory size of ^{(2 k × k × 6)} , in this embodiment, (2 ^k × n +
(N × (n + 1)) / 2). When the constraint length k is 7, if n is 36 or less, the memory can be reduced.

As described above, by providing n information memories, that is, information memories 1 (1-7-1) to n (1-7-n), the information sequence 1-12 is divided for each time. And memorize.
The information sequences that have been subjected to maximum likelihood determination at different times can be compared and determined by the output determination unit 1-8 as information at the same time, and errors included in the received sequences can be corrected.

A method of making a decision by majority decision will be described. For each information sequence at the same time stored in each of the information memories 1 to n, the output determination unit 1-8 selects an information sequence by majority decision to obtain a decoding result. If the number of information memories is odd, the majority will not be the same, but if the number is even, the number will be the same and the number will not be determined. If the majority is the same, the information sequence obtained by performing the maximum likelihood determination on the latest (or oldest) information sequence is used as the decoding result.

A method of making a determination by weighting will be described.
The time at which the maximum likelihood determination was performed by the maximum likelihood determination unit 1-6 and the output determination unit 1
Each information sequence is weighted according to the time difference between the times at which the information sequence is estimated at -8. The weighted values are “0” and “1”
, And a decoding result is obtained by a method employing the information sequence having the larger value. When the calculation results are equal, the information sequence obtained by performing the maximum likelihood determination on the latest (or oldest) information sequence is set as a decoding result.

Referring to FIG. 4, FIG. 5, FIG. 6, and FIG. 7, a case where an error is corrected in the case where the constrained codeword has a constraint length k = 2 and the number of information memories n = 3 will be described. 4 to 7, the vertical axis represents a state, and the horizontal axis represents time. The line connecting the states is an information sequence corresponding to the state transition. The dotted line indicates “0” and the solid line indicates “1”.

FIG. 4 shows the result of the maximum likelihood determination at time t.
FIG. 14 is a diagram illustrating a case where a path that transits from state 2 to state 1 is selected as a maximum likelihood path. This path is in state 0 at time (t-3), transitions to state 1 at time (t-2), transitions to state 2 at time (t-1), and transitions to state 0 at time t. Indicates that you have done. The information sequence corresponding to the path selected here is (0, 1, 0) in order from the oldest information, and is stored in the order of the information memory 3, the information memory 2, and the information memory 1. At this time t, the information at the time (t-3) three times before was all stored in the information memory 1, the information memory 2, and the information memory 3. Information "0" is output as a decoding result by majority decision of information stored in each information memory. Decoding is performed by repeating the same operation.

FIG. 5 shows the result of the maximum likelihood determination at time (t + 2). Here, (0,
1, 0), and information “0” is output as a decoding result at time (t−1).

FIG. 6 shows a case where an error occurs in the received sequence at the same time and an erroneous information sequence is selected. As a result of performing the maximum likelihood determination at the time (t + 2), the path that transits to the state 0 is selected as the maximum likelihood path, and the information sequence corresponding to this erroneous path is (1, 1, 1)
Is stored in each information memory. This time (t +
In 2), the information at the time (t-1) three times before is all stored in the information memory 1, the information memory 2, and the information memory 3. Information "0" is output as a decoding result by majority decision of information stored in each information memory. It can be seen that the error information stored in the information memory 3 is corrected by majority decision, and correct information "0" is output.

FIG. 7 shows the result of performing the maximum likelihood determination at time (t + 3). Here, (1,0,
1), and information “1” is output as a decoding result at time t.

As described above, in the embodiment of the present invention,
The Viterbi decoder divides the information sequence corresponding to the path metric selected by the maximum likelihood determination unit in the information memory for each time and stores the divided information sequence, and compares the information sequences subjected to the maximum likelihood determination at different times as the same time information. Then, since the information sequence is selected by majority decision, it is possible to correct the errors included in the received sequence while reducing the path memory while reducing the deterioration due to the truncation error.

[0024]

As is apparent from the above description, according to the present invention, the Viterbi decoder compares the received sequence with the codeword sequence to determine the maximum likelihood and the maximum likelihood determining unit. A plurality of information memories that store the determined information sequence of the maximum likelihood path at different times and an output determination unit that determines the information sequence based on the contents of the plurality of information memories and obtains a decoding result So without having a huge path memory,
The information sequence corresponding to the maximum likelihood path can be stored in the information memory for each time, and an error can be corrected and decoded by comparing and determining a plurality of information sequences corresponding to the same time. .

[Brief description of the drawings]

FIG. 1 is a functional block diagram of a Viterbi decoder according to an embodiment of the present invention;

FIG. 2 is a configuration diagram of a path memory of the Viterbi decoder according to the embodiment of the present invention;

FIG. 3 is a configuration diagram of an information memory of a Viterbi decoder according to the embodiment of the present invention;

FIG. 4 is a state transition diagram of an operation example 1 of the Viterbi decoder according to the embodiment of the present invention;

FIG. 5 is a state transition diagram of an operation example 2 of the Viterbi decoder according to the embodiment of the present invention;

FIG. 6 is a state transition diagram of an operation example 3 of the Viterbi decoder according to the embodiment of the present invention;

FIG. 7 is a state transition diagram of an operation example 4 of the Viterbi decoder according to the embodiment of the present invention;

FIG. 8 is a functional block diagram of a conventional Viterbi decoder,

FIG. 9 is a configuration diagram of a path memory of a conventional Viterbi decoder.

[Explanation of symbols]

 1-0 Viterbi decoder 1-1 Received sequence 1-2 Branch metric calculation unit 1-3 ACS 1-4 Path metric memory 1-5 Path memory 1-6 Maximum likelihood determination unit 1-7-1 to 1-7- n Information memory 1 to Information memory n 1-8 Output decision unit 1-9 Decoding output 1-10 Branch metric 1-11 Path metric 1-12 Information sequence 8-0 Viterbi decoder 8-1 Receive sequence 8-2 Branch metric Calculation unit 8-3 ACS 8-4 Path metric memory 8-5 Path memory 8-6 Maximum likelihood determination unit 8-7 Decoding output 8-8 Branch metric 8-9 Path metric 8-10 Information sequence

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Claims (5)

[Claims] 1. A maximum likelihood determining unit that compares a received sequence with a codeword sequence and performs a maximum likelihood determination, and a plurality of maximum likelihood path information sequences that are determined by the maximum likelihood determining unit at different times. A Viterbi decoder comprising: an information memory; and an output determination unit that determines an information sequence based on the contents of the plurality of information memories to obtain a decoding result. 2. The apparatus according to claim 1, wherein the output determination unit is provided with means for selecting an information sequence to obtain a decoding result by performing a majority decision for each information sequence at the same time stored in each of the information memories. The Viterbi decoder according to claim 1. 3. The Viterbi decoding according to claim 2, wherein said output judging unit is provided with means for setting the latest or oldest information sequence as a decoding result when the result of the majority decision is the same. vessel. 4. A means for selecting an information sequence based on a result obtained by weighting and calculating the information sequence according to a time difference between a maximum likelihood determination time and an output determination time, and providing a decoding result. The Viterbi decoder according to claim 1, wherein the decoder is provided. 5. The Viterbi decoder according to claim 4, wherein said output judging unit is provided with means for setting the latest or oldest information sequence as a decoding result when the result of the weighting operation is equal.