CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2018/021970 filed Jun. 8, 2018.

TECHNICAL FIELD

The present disclosure relates to a decoding apparatus, a decoding method, and a program.

BACKGROUND ART

In the operations of digital data communication systems and storage systems, countermeasures against bit errors, which may be caused by various factors, are taken. Among typical examples of such countermeasures, an error correction encoding technique is known, in which redundant data is added, so that bit errors can be corrected.

The error correction encoding technique is composed of an encoding process (encoding) for generating a transmission signal sequence while adding redundant data to information data on the transmitting side, and a decoding process (decoding) for inferring a transmission signal sequence from a reception signal sequence containing noises received on the receiving side. In general, the decoding process requires a larger amount of calculation than that in the encoding process. Therefore, improving the efficiency of the calculation of the decoding process together with how to realize this improvement is often considered to be a problem to be solved.

As an ordinary decoding processing method, a Viterbi decoder using a Viterbi algorithm is well known. In this method, a transmission signal sequence is regarded as a path in a graph called a trellis diagram, and a Viterbi algorithm is used as means for efficiently selecting a path having the highest likelihood for a given reception sequence. The Viterbi decoder adopts a method for inferring a transmission signal sequence by using the Viterbi algorithm.

Several methods for improving such a Viterbi decoder in order to improve the error correction performance thereof have been known. Each of Patent Literature 2 and Non-patent Literature 1 discloses a list output Viterbi decoder that selects and outputs a predetermined number of paths in descending order of their likelihoods for a reception sequence, rather than selecting only one path having the highest likelihood therefor. Further, Patent Literature 3 discloses a soft-output Viterbi decoder that, instead of outputting a path having a high likelihood, calculates and outputs a reliability (a posterior probability) for each bit of a transmission signal sequence determined by the path. It has been known that all of these methods are useful as a decoding processing method for a concatenated encoding scheme in which a plurality of error correction codes are used in combination as shown in Patent Literature 1.

Further, Patent Literature 4 discloses a configuration of a Viterbi decoder in a MIMO (Multiple-Input Multiple-Output) transmission system including a plurality of transmission antennas and a plurality of reception antennas. The Viterbi decoder disclosed in Patent Literature 4 calculates a transmission signal sequence from one reception signal sequence received from the plurality of reception antennas.

CITATION LIST

Patent Literature

Patent Literature 1: U.S. Pat. No. 5,446,747

Patent Literature 2: U.S. Pat. No. 5,537,444

Patent Literature 3: U.S. Pat. No. 5,933,462

Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2011-239107

Non Patent Literature

Non-patent Literature 1: N. Seshadri and C.-E. W. Sundberg, “List Viterbi decoding algorithms with applications,” IEEE Transactions on Communications, vol. 42, no. 2/3/4, February/March/April 1994.

Non-patent Literature 2: E. Arikan, “Channel polarization: A method for constructing capacity-aligning codes for symmetric binary-input memoryless channels,” IEEE Transactions on Information Theory, vol. 55, no. 7, pp. 3051 -3073, July 2009.

Non-patent Literature 3: I. Tal and A. Vardy, “List decoding of polar codes,” IEEE Transactions on Information Theory, vpl. 61, no. 5, pp. 2213 -2226, May 2015.

SUMMARY OF INVENTION

Technical Problem

As described above, all of the ordinary Viterbi decoders and their improved versions calculate, for one given reception signal sequence, a transmission signal sequence corresponding to the reception signal sequence, a list of candidates of the transmission signal sequence, or a reliability thereof. That is, the purpose of the ordinary Viterbi decoders and their improved versions is to improve the accuracy of the result of the decoding processing by increasing the amount of information related to the transmission signal sequence corresponding to the one given reception signal sequence. As an advanced form of this technique, it has been required to, when a plurality of reception signal sequences, i.e., inputs, are given, calculate a list of transmission signal sequences having high likelihoods for at least one of the reception signal sequences.

An object of the present disclosure is to provide a decoding apparatus, a decoding method, and a program capable of, when a plurality of reception signal sequences, i.e., inputs, are given, calculating a list of transmission signal sequences having high likelihoods for at least one of the reception signal sequences.

Solution to Problem

A decoding apparatus according to a first aspect of the present disclosure includes: a multi-input branch metric calculation unit configured to calculate, by using a branch label corresponding to a path extending toward a state S (S is an integer no less than zero) at a time point N (N is an integer no less than zero) in a trellis diagram and a plurality of reception signal sequences, a branch metric in the state S; a path metric calculation unit configured to calculate a path metric in the state S at the time point N by adding a branch metric in the state S at the time point N to a path metric in each of states at a time point N−1, each of which forms a path with the state S at the time point N; and a surviving path list memory configured to store path labels corresponding to L path metrics (L is an integer no less than one) among a plurality of calculated path metrics, in which the path metric calculation unit generates a path label in the state S at the time point N by combining the branch label with a path label in each of the states at the time point N−1, each of which forms a path with the state S at the time point N, and the surviving path list memory outputs, as a transmission signal sequence, path labels corresponding to L path metrics selected at an end point in the trellis diagram.

A decoding method according to a second aspect of the present disclosure includes: calculating, by using a branch label corresponding to a path extending toward a state S (S is an integer no less than zero) at a time point N (N is an integer no less than zero) in a trellis diagram and a plurality of reception signal sequences, a branch metric in the state S; calculating a path metric in the state S at the time point N by adding a branch metric in the state S at the time point N to a path metric in each of states at a time point N−1, each of which forms a path with the state S at the time point N; storing path labels corresponding to L path metrics (L is an integer no less than one) among a plurality of calculated path metrics; generating a path label in the state S at the time point N by combining the branch label with a path label in each of the states at the time point N−1, each of which forms a path with the state S at the time point N; and outputting, as a transmission signal sequence, path labels corresponding to L path metrics selected at an end point in the trellis diagram.

A program according to a third aspect of the present disclosure causes a computer to: calculate, by using a branch label corresponding to a path extending toward a state S (S is an integer no less than zero) at a time point N (N is an integer no less than zero) in a trellis diagram and a plurality of reception signal sequences, a branch metric in the state S; calculate a path metric in the state S at the time point N by adding a branch metric in the state S at the time point N to a path metric in each of states at a time point N−1, each of which forms a path with the state S at the time point N; store path labels corresponding to L path metrics (L is an integer no less than one) among a plurality of calculated path metrics; generate a path label in the state S at the time point N by combining the branch label with a path label in each of the states at the time point N−1, each of which forms a path with the state S at the time point N; and output, as a transmission signal sequence, path labels corresponding to L path metrics selected at an end point in the trellis diagram.

Advantageous Effects of Invention

According to the present disclosure, it is possible to provide a decoding apparatus, a decoding method, and a program capable of, when a plurality of reception signal sequences, i.e., inputs, are given, calculating a list of transmission signal sequences having high likelihoods for at least one of the reception signal sequences.

