Error correcting code decoding device, decoding method, and mobile station apparatus

An error correcting code decoding method comprising:

• executing a first decoding process a first predetermined number of times for obtaining first external values of a plurality of bits included in a bit string which has been subjected to error correction encoding in accordance with a word string which corresponds to the bit string and which includes a plurality of words each of which has a bit length at least equal to or larger than two bits and first prior probabilities which are determined for individual bits and which represent probabilities that the bits have a predetermined value, the first external values representing increments of degrees of reliabilities of facts that values of the corresponding bits are the predetermined value, and each of the first external values having a first fixed bit length at least equal to or larger than two bits;
• reducing bit lengths of the first external values corresponding to the plurality of bits obtained after the first decoding process is performed a first predetermined number of times, from the first fixed bit length to a second fixed bit length that is shorter than the first fixed bit length, and to reduce bit lengths of the words included in the word string, from the first fixed bit length to the second fixed bit length;
• and
• decoding the bit string by executing a second decoding process a second predetermined number of times for calculating second external values and posterior values of the bits included in the bit string in accordance with the word string including the words having the second fixed bit lengths using the first external values having the second fixed bit lengths as second prior probabilities that corresponding bits among the plurality of bits are the predetermined value, the second external values representing increments of degrees of reliabilities of facts that values of the corresponding bits are the predetermined value, each of the second external values having a bit length at least equal to or larger than two bits, the posterior values representing increments of degrees of reliabilities of facts that values of the bits being the predetermined value, and the posterior values representing posterior probabilities that values of the bits being a predetermined value.

A mobile station apparatus, comprising:

• an antenna configured to receive a wireless signal including a bit string which has been subjected to error correction encoding and which converts the wireless signal into an electric signal;
• a baseband processing unit configured to extract a word string which corresponds to the bit string which has been subjected to the error correction encoding and which includes a plurality of words each of which has a bit length at least equal to or larger than two bits;
• and
• a decoding unit configured to decode the bit string which has been subjected to the error correction encoding in accordance with the word string,
• wherein the decoding unit includes
• a first decoder configured to execute a first decoding process a first predetermined number of times for obtaining first external values of a plurality of bits included in a bit string which has been subjected to error correction encoding in accordance with a word string which corresponds to the bit string and which includes a plurality of words each of which has a bit length at least equal to or larger than two bits and first prior probabilities which are determined for individual bits and which represent probabilities that the bits have a predetermined value, the first external values representing increments of degrees of reliabilities of facts that values of the corresponding bits are the predetermined value, and each of the first external values having a first fixed bit length at least equal to or larger than two bits;
• a word-length reduction unit configured to reduce bit lengths of the first external values corresponding to the plurality of bits obtained after the first decoding process is performed by the first decoder a first predetermined number of times, from the first fixed bit length to a second fixed bit length that is shorter than the first fixed bit length, and to reduce bit lengths of the words included in the word string, from the first fixed bit length to the second fixed bit length;
• and
• a second decoder configured to decode the bit string by executing a decoding process a second predetermined number of times for calculating second external values and posterior values of the bits included in the bit string in accordance with the word string including the words having the second fixed bit lengths using the first external values having the second fixed bit lengths as second prior probabilities that corresponding bits among the plurality of bits are the predetermined value, the second external values representing increments of degrees of reliabilities of facts that values of the corresponding bits are the predetermined value, each of the second external values having a bit length at least equal to or larger than two bits, the posterior values representing increments of degrees of reliabilities of facts that values of the bits being the predetermined value, and the posterior values representing posterior probabilities that values of the bits being a predetermined value.