Decoding method and decoding device

1. A decoding method of decoding N received branchwords produced by a convolutional encoder using a tail-biting convolutional code, comprising:

storing the N received branchwords in memory;

successively performing Viterbi updates on a sequence of branchwords, the sequence comprising a first block comprising S consecutive branchwords from the N received branchwords, a second block comprising the N received branchwords, and a third block comprising T consecutive branchwords from the N received branchwords, where S and T are less than N, and where the Viterbi updates generate updated path metrics;

determining a first encoder state at the end of the third block most likely to have generated the final branchword in the sequence from the best path metric;

first performing a Viterbi traceback procedure from that first encoder state at the end of the third block to determine a second encoder state at the start of the third block of branchwords;

second performing a Viterbi traceback procedure from that second encoder state at the start of the third block to determine a third encoder state at the start of the second block of branchwords; and

outputting a derived tail-biting path, if the second and third encoder states are identical.

2. The decoding method according to claim 1 , wherein if the second and third encoder states are not identical, then the method further comprises:

replacing the second encoder state with the third encoder state;

repeating the second performing; and

outputting the derived tail biting path.

3. The decoding method according to claim 1 , wherein the sequence of branchwords in the successively performing is formed from a logically circular reading of the N received branchwords stored in the memory.

4. The decoding method according to claim 1 , wherein S equals T.

5. The decoding method according to claim 1 , wherein the first block comprises S consecutive branchwords from the end of the second block of N received branchwords.

6. The decoding method according to claim 1 , wherein the third block comprises T consecutive branchwords from the start of the second block of N received branchwords.

7. A decoding device of decoding N received branchwords produced by a convolutional encoder using a tail-biting convolutional code, comprising:

a memory storing the N received branchwords; and

a data processing unit comprising:

a successive performing unit which successively performs Viterbi updates on a sequence of branchwords, the sequence comprising a first block comprising S consecutive branchwords from the N received branchwords, a second block comprising the N received branchwords and a third block comprising T consecutive branchwords from the N received branchwords, where S and T are less than N, and where the Viterbi updates generate updated path metrics;

a determining unit which determines a first encoder state at the end of the third block most likely to have generated the final branchword in the sequence from the best path metric;

a first performing unit which performs a Viterbi traceback procedure from that first encoder state at the end of the third block to determine a second encoder state at the start of the third block of branchwords;

a second performing unit which performs a Viterbi traceback procedure from that second encoder state at the start of the third block to determine a third encoder state at the start of the second block of branchwords; and

an outputting unit which outputs a derived tail-biting path, if the second and third encoder states are identical.