Patent Number: 
Section: claims

1. A decoding method of maximum a posteriori probability for receiving and decoding encoded data obtained by encoding information of length N, characterized by comprising: performing a backward repetitive operation which is a repetitive operation from back to front in terms of time in order starting from the end of encoded data and, while this operation is being performed, saving first to m 1 th results of the backward repetitive operation that correspond to first to m 1 th items of encoded data;  performing a forward repetitive operation which is a repetitive operation from front to back in terms of time in order from first to m 1 th items of encoded data and, while this operation is being performed, outputting first to m 1 th operational results as results of decoding;  again performing a backward repetitive operation in order starting from the end of encoded data and, while this operation is being performed, saving (m 1 +1)th to m 2 th results of the backward repetitive operation;  performing a forward repetitive operation in order from (m 1 +1)th to m 2 th items of encoded data and, while this operation is being performed, outputting (m 1 +1)th to m 2 th operational results as results of decoding; and  subsequently outputting all results of decoding up to an Nth result in similar fashion. 2. A decoding method of maximum a posteriori probability for calculating a kth forward probability using first to kth items of encoded data obtained by encoding information of length N, obtaining a kth backward probability using Nth to kth items of encoded data, and outputting a kth decoded result using these probabilities, characterized by comprising: calculating backward probabilities in a reverse direction from an Nth backward probability to a first backward probability and saving an m 1 th backward probability to the first backward probability;  calculating first forward probabilities, obtaining a first decoded result using the first forward probabilities and the first backward probabilities that have been saved and similarly obtaining second to m 1 th decoded results;  subsequently calculating backward probabilities in the reverse direction from the Nth backward probability to an (m 1 +1)th backward probability and saving an m 2 th backward probability to the (m 1 +1)th backward probability;  calculating (m 1 +1)th forward probabilities, obtaining an (m 1 +1)th decoded result using the (m 1 +1)th forward probabilities and the (m 1 +1)th backward probability that has been saved, and similarly obtaining (m 1 +2)th to m 2 th decoded results; and  subsequently obtaining (m 2 +1)th to Nth decoded results. 3. A decoding method of maximum a posteriori probability for receiving and decoding encoded data obtained by encoding information of length N, characterized by comprising: performing a backward repetitive operation which is a repetitive operation from back to front in terms of time in order starting from the end of encoded data and, while this operation is being performed, saving first to m 1 th, m 2 th, m 3 th . . . results of the backward repetitive operation that correspond to first to m 1 th, m 2 th, m 3 th . . . items of encoded data;  performing a forward repetitive operation which is a repetitive operation from front to back in terms of time in order from first to m 1 th items of encoded data and, while this operation is being performed, outputting first to m 1 th operational results as results of decoding;  subsequently saving results while performing m 2 th to (m 1 +1)th backward repetitive operations utilizing said m 2 th result of the backward repetitive operation;  performing a forward repetitive operation in order from (m 1 +1)th to m 2 th items of data and, while this operation is being performed, outputting (m 1 +1)th to m 2 th operational results as results of decoding; and  subsequently outputting all results of decoding up to an Nth result in similar fashion. 4. A decoding method of maximum a posteriori probability for calculating a kth forward probability using first to kth items of encoded data obtained by encoding information of length N, obtaining a kth backward probability using Nth to kth items of encoded data, and outputting a kth decoded result using these probabilities, characterized by comprising: calculating backward probabilities in a reverse direction from an Nth backward probability to a first backward probability, discretely saving an m s th backward probability, m (sxe2x88x921) th backward probability, . . . , m 2 th backward probability and continuously saving an m 1 th backward probability to the first backward probability;  calculating first forward probabilities, obtaining a first decoded result using the first forward probabilities and the first backward probabilities that have been saved and similarly obtaining second to m 1 th decoded results;  subsequently calculating and saving backward probabilities up to an (m 1 +1)th backward probability starting from the m 2 th backward probability that has been saved;  calculating (m 1 +1)th forward probabilities, obtaining an (m 1 +1)th decoded result using the (m 1 +1)th forward probabilities and the (m 1 +1)th backward probability that has been saved, and similarly obtaining (m 1 +2)th to m 2 th decoded results; and  subsequently obtaining (m 2 +1)th to Nth decoded results. 5. A decoding method of maximum a posteriori probability according to  claim 4 , characterized in that the number of backward probabilities saved continuously is about N xc2xd . claim 4 6. A decoder for calculating a kth forward probability using first to kth items of encoded data obtained by encoding information of length N, obtaining a kth backward probability using Nth to kth items of encoded data, and outputting a kth decoded result using these probabilities, characterized by comprising: a backward probability calculation unit for calculating backward probabilities;  a backward probability saving unit for saving backward probabilities that have been calculated;  a forward probability calculation unit for calculating forward probabilities;  a decoded result calculation unit for obtaining a kth decoded result using a kth forward probability and a kth backward probability that has been saved; and  a controller for controlling operation timings of said backward probability calculation unit, said forward probability saving unit and said decoded result calculation unit; wherein  (1) said backward probability calculation unit calculates backward probabilities in a reverse direction from an Nth backward probability to a first backward probability and saves an m 1 th backward probability to first backward probability in said backward probability saving unit; said forward probability calculation unit calculates a first forward probability to an m 1 th forward probability; and said decoded result calculation unit calculates a kth decoded result using the kth forward probability (k=1 to m 1 ) that has been calculated and the kth backward probability that has been saved; (2) said backward probability calculation unit subsequently calculates backward probabilities in the reverse direction from the Nth backward probability to an (m 1 +1)th backward probability and saves an m 2 th backward probability to the (m 1 +1)th backward probability in said backward probability saving unit; said forward probability calculation unit calculates an (m 1 +1)th forward probability to an m 2 th forward probability; and said decoded result calculation unit calculates a kth decoded result using the kth forward probability (k=m 1 +1 to m 2 ) that has been calculated and the kth backward probability that has been saved and (3) subsequently similarly obtains (m 2 +1)th to Nth decoded results. 7. A decoder for calculating a kth forward probability using first to kth items of encoded data obtained by encoding information of length N, obtaining a kth backward probability using Nth to kth items of encoded data, and outputting a kth decoded result using these probabilities, characterized by comprising: a backward probability calculation unit for calculating backward probabilities;  a backward probability saving unit for saving backward probabilities that have been calculated;  a forward probability calculation unit for calculating forward probabilities;  a decoded result calculation unit for obtaining a kth decoded result using a kth forward probability and a kth backward probability that has been saved; and  a controller for controlling operation timings of said backward probability calculation unit, said forward probability saving unit and said decoded result calculation unit; wherein  (1) said backward probability calculation unit calculates backward probabilities in a reverse direction from an Nth backward probability to a first backward probability, discretely saves an m s th backward probability, m (sxe2x88x921) th backward probability, . . . , m 2 th backward probability in said backward probability saving unit and continuously saves an m 1 th backward probability to the first backward probability; said forward probability calculation unit calculates a first forward probability to an m 1 th forward probability; and said decoded result calculation unit calculates a kth decoded result using the kth forward probability (k=1 to m 1 ) that has been calculated and the kth backward probability that has been saved; (2) said backward probability calculation unit subsequently calculates, and saves in said backward probability saving unit, backward probabilities up to an (m 1 +1)th backward probability starting from the m 2 th backward probability that has been saved; said forward probability calculation unit calculates an (m 1 +1)th forward probability to an m 2 th forward probability; and said decoded result calculation unit calculates a kth decoded result using the kth forward probability (k=m 1 +1 to m 2 ) that has been calculated and the kth backward probability that has been saved and (3) subsequently similarly obtains (m 2 +1)th to Nth decoded results. 8. A decoder according to  claim 7 , characterized in that the number of backward probabilities saved continuously is about N xc2xd . claim 7