JP4520861B2 - Turbo decoder and turbo decoding method - Google Patents

Description

  The present invention relates to a turbo decoder and a turbo decoding method for decoding a data signal that has been error correction encoded by a turbo code.

  As a soft output decoding algorithm used in turbo code decoding, it is said that the MAP (Maximum Aposterior Probability) decoding method, which is a maximum a posteriori probability decoding method, is currently the best. When the circuit is realized, the MAX-LOG-MAP method that approximates the MAP method by logarithmic calculation may be used.

  In turbo decoding in MAP decoding, a probability (forward probability) calculated sequentially from the start end of the input data to the end according to a transition relationship (trellis lattice) that is not so large, and the end of the input data according to a similar transition relationship The likelihood of the original data is estimated from the probability (backward probability) calculated sequentially from the start point to the start end and the transition occurrence probability of the transition relation calculated based on the input data, and the output data To do. By using this output data as one of the input data and calculating the likelihood using the same process, it becomes possible to obtain high decoding characteristics.

  By the way, when the turbo code is decoded by the MAP decoding or the MAX-LOG-MAP method, in order to derive the likelihood (the calculation result in the decoding process) at a certain time point n, from the time point 0 which is the start position of the data. Corresponding to the forward probability corresponding to each transition state of the trellis lattice determined from the received data up to the time point n-1, and to each transition state of the trellis lattice determined from the received data from the time point N to the time point n + 1 that is the terminal position of the data. A backward probability is required. That is, in order to obtain the likelihood at the time point n, it is necessary to perform numerical calculation using data from the start end to the end.

Therefore, conventionally, for the purpose of shortening the number of decoding processing operations, one of the forward probability calculated from time 0 to time N and the backward probability calculated from time N to time 0 is secured in the memory. Method is used.
3GPP TS25.212

  However, in the conventional method, it is necessary to store all the probabilities corresponding to the transition states of the trellis lattice. Therefore, the larger the likelihood calculation range (N in the above example) or the number of states of the trellis lattice. The more you have, the more memory you need. An increase in memory capacity leads to an increase in circuit area. In addition, the large number of data to be stored in the memory means that the number of accesses to the memory is also large, which is a factor that increases the power consumption required for read / write access to the memory and the amount of access delay that occurs during the access. But there is.

  The present invention has been made in view of such a point, and when decoding a turbo code by MAP decoding or the MAX-LOG-MAP method, the memory capacity used for internal computation is reduced without degrading the decoding capability. It is an object of the present invention to provide a turbo decoder and a turbo decoding method that enable the above.

  In order to solve this problem, the turbo decoder according to the present invention allows the likelihood calculation means to access the backward probability calculated by the backward probability calculation means when the turbo encoded data is decoded by the MAP method. In the turbo decoder for storing in the calculation result storage means, the calculation content storage means for storing information generated in the process of calculating the backward probability necessary for the likelihood calculation by the sequential calculation by the backward probability calculation means, and the likelihood Backward probability reproduction means for reproducing the backward probability of the previous stage used at the time of calculation from the backward probability of the next stage using information stored in the calculation content storage means and the state transition probability, and the backward probability calculation means Adopts a configuration in which a backward probability that cannot be reproduced by the backward probability reproduction means among the calculated backward probabilities is stored in the calculation result storage means.

  According to this configuration, since the backward probability value is reproduced from the backward probability at a certain time point using the information generated when calculating the backward probability stored in the calculation content storage means, all the backward probability is stored in the memory. There is no need to store, and the memory capacity can be reduced. Further, since the backward probability is not “estimated” but “reproduced” using back calculation, the decoding performance does not deteriorate.

  Note that the information stored in the calculation content storage means here is information used to reproduce the backward probability in the backward probability reproduction means, in addition to the information indicating the numerical value used in the backward probability calculation means, It also includes information for correcting fluctuations in the state probability value caused by normalization processing and bit deletion.

  When the turbo decoder according to the present invention decodes turbo-encoded data by the MAX-LOG-MAP method, the likelihood calculation means accesses the backward probability calculated by the backward probability calculation means by the sequential calculation. In the turbo decoder stored in the storage means, path information indicating the backward probability necessary for calculating the likelihood calculated by the backward probability calculating means in association with the transition probability used for the calculation is stored. Selected path information storage means, and backward probability reproduction means for reproducing the previous stage backward probability used in likelihood calculation from the subsequent stage backward probability using the path information stored in the selected path information storage means and the state transition probability The backward probability calculation means stores the backward probability that cannot be reproduced by the backward probability reproduction means among the calculated backward probabilities in the calculation result storage means. That.

  According to this configuration, in the MAX-LOG-MAP method, paying attention to the fact that if there is path information used when calculating the backward probability, some of the backward probability of the previous stage can be derived from the backward probability of the next stage by back calculation, Path information at the time of backward probability calculation is stored at the same time, and some of the backward probability of the previous stage is calculated from the backward probability of the next stage at the time of likelihood calculation, so the memory capacity for backward probability without degrading the decoding performance Can be reduced.

  The turbo decoder according to the present invention is the turbo decoder according to the above invention, wherein the backward probability calculation means calculates the backward probability of the L states at the time point n−1 from the backward probability of the M states at the time point n. The number of backward probabilities stored per time in the calculation result storage means is ML, and the information stored per time in the selected path information storage means is information indicating ML paths.

  According to this configuration, since the forward probability and the backward probability are calculated according to the unique trellis lattice in turbo decoding, paying attention to the fact that the next-stage state related to all the previous states of the trellis lattice can be specified, If the next-stage state related to all the states can be expressed by L, L states for calculating M probabilities in the previous stage are specified in advance, and stored in the probability memory and the path information memory for each time point. Since the amount of data to be reduced is reduced by a certain amount, the number of accesses to the memory is reduced, and the memory capacity can be reduced.

  In the turbo decoder according to the present invention, in the above invention, the backward probability reproduction means uses M specific backward probabilities at the time point n, where the number of states of the backward probability at the time point n is M. M / 2 backward probabilities at the time point n + 1 are calculated, and the backward probability calculation means calculates the backward probability and the number of paths to be stored per time point in the calculation result storage means and the selected path information storage means. A configuration of / 2 is adopted.

  In this configuration, the number of states of backward probabilities that can be reproduced by the backward probability reproducing means according to the trellis lattice is M / 2 or more per time point. By fixing the number of backward probability states calculated by the backward probability reproducing means to M / 2, the backward probability used by the backward probability reproducing means can also be fixed at M / 2 per time point. In the Max-Log-MAP method for obtaining the backward probability of one state at time n from the two states at time n + 1, the set of states at time n + 1 for obtaining the backward probability of the state at time n is fixed according to the trellis lattice. The specific M / 2 states can always be handled as the state values for the backward probability reproduction means processing. Further, since the selected path is different from the unselected path among the two states at the time point n + 1, it is easy to obtain M / 2 unselected paths from the M / 2 selected paths. is there.

  Therefore, according to this configuration, the M / 2 state probabilities used in the backward probability reproduction means can be fixed, and the number of path information stored in the selected path information storage means and the calculation result storage means Since the number of state probabilities to be stored can be fixed to M / 2, the number of memory accesses can be reduced and the memory capacity can be reduced without relatively increasing the amount of calculation.

  The turbo decoder according to the present invention, in the above invention, determines whether or not the backward probability calculated by the backward probability calculation means by sequential calculation is reproducible from the backward probability of the next stage and the transition probability used for the calculation. The backward probability is selected based on the determination result and stored in the calculation result storage unit, and the information indicating the state corresponding to the backward probability stored in the calculation result storage unit is stored in the selected path information storage unit. The structure which comprises an availability judgment means is taken.

  According to this configuration, the same state may be selected as the previous stage probability selected at the time of calculating the next stage probability, paying attention to the fact that the number of probabilities that cannot be actually reproduced is variable at each time point, Since the number of probabilities that need to be stored at an arbitrary time, and thus stored at an arbitrary time, are stored in association with each other, the total number of stored probabilities can be minimized for each calculation.

  The turbo decoder according to the present invention stores the forward probability calculated by the forward probability calculation means by the sequential calculation in the calculation result storage means accessed by the likelihood calculation means when decoding the turbo-encoded data by MAP decoding. In the turbo decoder, the forward probability calculation means stores calculation content storage means for storing information generated in the process of calculating the forward probability necessary for likelihood calculation by sequential calculation, and the preceding forward probability used in the likelihood calculation Forward probability reproduction means for reproducing from the forward probability of the next stage using the information stored in the calculation content storage means and the state transition probability, and the forward probability calculation means includes the calculated forward probability. Of these, the forward probability that cannot be reproduced by the forward probability reproduction means is stored in the calculation result storage means.

  According to this configuration, since the forward probability value is reproduced from the forward probability at a certain point in time using the information generated when calculating the forward probability stored in the calculation content storage means, all the forward probabilities are stored in the memory. There is no need to store, and the memory capacity can be reduced. Further, since the forward probability is not “estimated” but “reproduced” using back calculation, the decoding performance does not deteriorate.

  Note that the information stored in the calculation content storage means here is information used to reproduce the forward probability in the forward probability reproduction means, and besides the information indicating the numerical value used in the forward probability calculation means, It also includes information for correcting fluctuations in the state probability value caused by normalization processing and bit deletion.

  When the turbo decoder according to the present invention decodes turbo-encoded data by the MAX-LOG-MAP method, the likelihood calculation means accesses the forward probability calculated by the forward probability calculation means by the sequential calculation. In the turbo decoder stored in the storage means, path information indicating the forward probability necessary for calculating the likelihood calculated by the forward probability calculating means in association with the transition probability used for the calculation is stored. Selected path information storage means, and forward probability reproduction means for reproducing the previous stage forward probability used in likelihood calculation from the next stage forward probability using the path information stored in the selected path information storage means and the state transition probability The forward probability calculation means stores a forward probability that cannot be reproduced by the forward probability reproduction means among the calculated forward probabilities in the calculation result storage means. That.

  According to this configuration, in the MAX-LOG-MAP method, paying attention to the fact that, if there is path information used when calculating the forward probability, some of the forward probabilities of the previous stage can be derived by back calculation from the forward probability of the next stage, Since the path information at the time of forward probability calculation is stored at the same time and some of the forward probabilities of the previous stage are calculated from the forward probability of the next stage at the time of likelihood calculation, the memory capacity for the forward probability without degrading the decoding performance Can be reduced.

  The turbo decoder according to the present invention may be configured so that, in the above invention, the forward probability calculation means calculates the forward probability of the L states at the time point n + 1 from the forward probability of the M states at the time point n. The number of forward probabilities stored per time in the result storage means is ML, and the information stored per time in the selected path information storage means is information indicating M-L paths.

  According to this configuration, since the forward probability and the backward probability are calculated according to the unique trellis lattice in turbo decoding, paying attention to the fact that the next-stage state relating to all the previous states of the trellis lattice can be specified, If the next-stage state related to all the states can be expressed by L, L states for calculating M probabilities in the previous stage are specified in advance, and stored in the probability memory and the path information memory for each time point. Since the amount of data to be reduced is reduced by a certain amount, the number of accesses to the memory is reduced, and the memory capacity can be reduced.

  In the turbo decoder according to the present invention, in the above invention, the forward probability reproducing means uses M specific forward probabilities at the time point n, where the number of states of the forward probability at the time point n is M. M / 2 backward probabilities at the time point n−1 are calculated, and the forward probability calculating means stores the forward probability and the number of paths to be stored per time point in the calculation result storage means and the selected path information storage means. Is set to M / 2.

  In this configuration, the number of forward probability states that can be reproduced by the forward probability reproduction unit according to the trellis lattice is M / 2 or more per time point. By fixing the number of states of forward probabilities calculated by the forward probability reproducing means to M / 2, the forward probabilities used by the forward probability reproducing means can also be fixed at M / 2 per time point. In the Max-Log-MAP method for obtaining the forward probability of one state at time n from the two states at time n + 1, the set of states at time n + 1 for obtaining the forward probability of the state at time n is fixed according to the trellis lattice. Specific M / 2 states can always be handled as state values for the forward probability reproduction means processing. Further, since the selected path is different from the unselected path among the two states at the time point n + 1, it is easy to obtain M / 2 unselected paths from the M / 2 selected paths. is there.

  Therefore, according to this configuration, the M / 2 state probabilities used in the forward probability reproduction means can be fixed, and the number of path information stored in the selected path information storage means and the calculation result storage means Since the number of state probabilities to be stored can be fixed to M / 2, the number of memory accesses can be reduced and the memory capacity can be reduced without relatively increasing the amount of calculation.

  The turbo decoder according to the present invention, in the above invention, judges whether or not the forward probability calculated by the forward probability calculation means by sequential calculation is reproducible from the forward probability of the next stage and the transition probability used for the calculation. In addition, a forward probability is selected based on the determination result and stored in the calculation result storage unit, and information indicating a state corresponding to the forward probability stored in the calculation result storage unit is stored in the selected path information storage unit. The structure which comprises an availability judgment means is taken.

  According to this configuration, the same state may be selected as the previous stage probability selected at the time of calculating the next stage probability, paying attention to the fact that the number of probabilities that cannot be actually reproduced is variable at each time point, Since the number of probabilities that need to be stored at an arbitrary time, and thus stored at an arbitrary time, are stored in association with each other, the total number of stored probabilities can be minimized for each calculation.

  In the turbo decoding method according to the present invention, when turbo-encoded data is decoded by MAP decoding, a forward probability and a backward probability necessary for calculating the likelihood are respectively calculated by sequential calculation, and at least one probability is calculated. In the case of storing in a memory, the step of storing information generated in the process of calculating the one probability in the memory, and the information stored in the memory of the previous stage probability in the one probability used in the likelihood calculation, A step of reproducing from the probability of the next stage in the one probability using the state transition probability, and a step of storing in the memory a probability that cannot be reproduced in the reproducing step among the calculated one probability. It was made to have.

  According to this method, the probability value of the previous stage is reproduced from the forward probability or backward probability at a certain time point using the information generated when calculating the forward probability or backward probability stored in the memory. It is not necessary to store all the probabilities in the memory, and the memory capacity can be reduced. In addition, since the forward probability or backward probability is not “estimated” but “reproduced” using back calculation, the decoding performance does not deteriorate.

  In the turbo decoding method according to the present invention, when turbo-coded data is decoded by the MAX-LOG-MAP method, forward probabilities and backward probabilities necessary for likelihood calculation are respectively calculated by sequential calculation, and at least When storing one probability in the first memory, storing the path information instructing the one probability calculated by the sequential calculation in association with the transition probability used in the calculation in the second memory; Reproducing the previous stage probability in the one probability used in the likelihood calculation from the next stage probability in the one probability using the path information stored in the second memory and the state transition probability, and calculating A step of storing, in the first memory, a probability that cannot be reproduced in the reproducing step among the one probability.

  According to this method, in the MAX-LOG-MAP method, if there is path information used when calculating the forward probability and backward probability, some of the previous stage probabilities can be derived from the next stage probability by back calculation. Since path information at the time of probability calculation is stored at the same time and some of the probabilities of the previous stage are calculated from the probability of the next stage at the time of likelihood calculation, the memory capacity for probability storage is reduced without degrading the decoding performance can do.

  In the turbo decoding method according to the present invention, in the above invention, the state probabilities stored in the first memory and the second memory per time point are all M states at one time point n, When L state probabilities of the next stage are obtained from M states, there are M−L pieces of information, and the information stored in the second memory is not used for calculation of the L state probabilities of the next stage. It is assumed that the information indicates ML state probabilities at time n.

  According to this method, since the forward probability and the backward probability are calculated according to the unique trellis lattice in turbo decoding, paying attention to the fact that the state of the next stage related to all the states of the previous stage of the trellis lattice can be specified. If the next stage state relating to all the states can be expressed by L, L states for calculating some of the M probabilities in the previous stage are specified in advance, and the probability memory and path information for each time point are specified. Since the amount of data stored in the memory is reduced by a certain amount, the number of accesses to the memory is reduced, and the memory capacity can be reduced.

  In the turbo decoding method according to the present invention, in the above invention, the state probability and the number of passes stored in the first memory and the second memory per time point are M in one of the states at the time point n. In some cases, it was M / 2.

  This method is based on the condition that the number of states with a reproducible probability is M / 2 or more per time point in a turbo code using a specific trellis lattice. Under this condition, since the number of probability states calculated in the process of reproducing the probability is fixed at M / 2, the probability used is also fixed at M / 2 per time point. In the Max-Log-MAP method, a set of states required for obtaining a state probability at a certain time is uniquely fixed by a trellis lattice, and therefore, specific M / 2 states are handled as input of calculation processing. I can do it. Further, since the path selected in this calculation is different from the path not selected, it is easy to obtain M / 2 unselected paths from the M / 2 selected paths.

  Therefore, in this method, the M / 2 state probabilities used can be fixed, and the number of path information stored in the second memory and the number of state probabilities stored in the first memory are also M / 2. Since the number of memory accesses can be fixed, the number of times of memory access can be reduced and the memory capacity can be reduced without relatively increasing the amount of calculation.

  In the turbo decoding method according to the present invention, in the above invention, the step of determining whether the calculated one probability is reproducible from the one probability in the next stage and the transition probability used for the calculation, and the calculated One of the probabilities selected based on the determination result is stored in the first memory, and information indicating a state corresponding to the one stored in the first memory is stored in the second memory. And a step of performing.

  According to this method, the same state may be selected as the previous stage probability selected at the time of calculating the next stage probability, paying attention to the fact that the number of probabilities that cannot actually be reproduced is variable at each time point, Since the number of probabilities that need to be stored at an arbitrary time, and thus stored at an arbitrary time, are stored in association with each other, the total number of stored probabilities can be minimized for each calculation.

  According to the present invention, when a turbo code is decoded by MAP decoding or the MAX-LOG-MAP method, it is possible to reduce the memory capacity used for the internal calculation without deteriorating the decoding capability.

  The essence of the present invention is that the probability of each state obtained by calculating from the forward or backward based on the transition probability when a data signal error-encoded by a turbo code is decoded by MAP decoding or MAX-LOG-MAP method. Is stored in the memory, and the probability that is not stored in the memory is recalculated and reproduced from the rear or front, thereby eliminating the need to store all the probabilities in the memory.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1 is a block diagram showing a configuration of an element decoder of a turbo decoder according to Embodiment 1 of the present invention. A decoder 100 shown in FIG. 1 is an element decoder of an error correction decoder that decodes turbo-encoded data by MAP decoding, and includes an input data memory 101, a transition probability calculation unit 102, a backward probability calculation unit 103, A backward probability calculation result storage memory 104, a calculation content storage memory 105, a backward probability reproduction unit 106, a forward probability calculation unit 107, a likelihood calculation unit 108, and an output data memory 109 are provided.

  As a conventional turbo decoder, one using three types of data obtained from the transition probability calculation unit 102, the forward probability calculation unit 107, and the backward probability calculation result storage memory 104 is known as likelihood calculation. At that time, an increase in the capacity of the backward probability calculation result storage memory 104 was a problem.

  Element decoder 100 of the turbo decoder according to Embodiment 1 performs likelihood calculation using three types of data obtained from transition probability calculation section 102, forward probability calculation section 107, and backward probability calculation result storage memory 104. In a conventional turbo decoder using the above, a calculation content storage memory 105 and a backward probability reproduction unit 106 are additionally provided, and likelihood calculation is performed using a transition probability calculation unit 102, a forward probability calculation unit 107, and a backward probability calculation result storage. This is performed by using four types of data obtained from the memory 104 and the backward probability reproduction unit 106 so that the increase in the capacity of the backward probability calculation result storage memory 104 can be suppressed.

  The transition probability calculation unit 102 calculates the occurrence probability of each transition according to the transition relationship (trellis lattice) from the turbo encoded data stored in the input data memory 101, and the calculated transition probability is a backward probability calculation unit. 103, backward probability reproduction unit 106, forward probability calculation unit 107, and likelihood calculation unit 108, respectively.

  The forward probability calculation unit 107 sequentially calculates the forward probability from the start end to the end of the input data using the transition probability from the transition probability calculation unit 102 and the forward probability obtained in the previous stage, and calculates the calculated forward probability. This is given to the likelihood calculation unit 108.

  The backward probability calculation unit 103 sequentially calculates backward probabilities from the end of the input data to the start using the transition probability from the transition probability calculation unit 102 and the backward probability obtained in the previous stage, and calculates the calculated backward probabilities. This is stored in the backward probability calculation result storage memory 104. At that time, the backward probability calculation unit 103 stores information generated when each backward probability is calculated in the calculation content storage memory 105.

  In this case, the backward probability calculation unit 103 does not store all the calculated backward probabilities in the backward probability calculation result storage memory 104, but the backward probability reproduction unit 106, for example, stores the calculation contents at the time point n-1. When the backward probability at the time point n can be calculated backward from the information stored in the memory 105, the backward probability that can be calculated backward is not stored in the backward probability calculation result storage memory 104.

  The likelihood calculation unit 108 reads the backward probability from the backward probability calculation result storage memory 104 at the time of execution, but the backward probability at the time point n not stored in the backward probability calculation result storage memory 104 at the time point n is The probability reproduction unit 106 calculates using the information in the calculation content storage memory 105, the transition probability obtained from the transition probability calculation unit 103, and the backward probability at the time point n−1 obtained from the backward probability calculation result storage memory 104. The likelihood calculating unit 108 is provided.

That is, the backward probability reproduction unit 106 reproduces the backward probability of the previous stage as follows, for example. The backward probability is obtained by sequential calculation. For example, in the MAX-LOG-MAP method, when the next stage state is obtained, one of the calculation results using the two previous stages is selected. More specifically, the backward probability in the state a of the next stage is β _n-1 (a), and the two backward probabilities required to obtain the backward probability β _n-1 (a) of the next stage are states. Assuming that the backward probabilities β _n (b) and β _n (c) of b and c and the calculation formula using the backward probability of the previous stage is f, the backward probability β _n−1 (a) of the next stage is It is obtained from the following equation (1).
β _n-1 (a) = f (β _n (b)) or β _n-1 (a) = f (β _n (c)) (1)

Here, if an argument can be obtained from the result of the calculation formula f, one of the backward probabilities β _n (b) and β _n (c) of the previous stage is derived from the backward probability β _n−1 (a) of the next stage. It can be calculated backwards. For example, in the MAX-LOG-MAP system, the backward probability update formula is expressed by the following formula (2) using γ _ab , which is the transition probability of the path from the previous stage b to the next stage a. Is done.
f (β _n (b)) = β _n (b) + γ _ab (2)

Therefore, the backward probability calculation unit 103 does not store the backward probability β _n (b) of the previous stage that can be calculated backward as described above in the backward probability calculation result storage memory 104. Then, the backward probability reproduction unit 106 performs a reverse calculation of the calculation formula f to derive the backward probability β _n (b) of the previous stage from the backward probability β _n−1 (a) of the next stage, and gives it to the likelihood calculation unit 108.

However, if the backward calculation of the calculation formula f is possible, even if the backward probability update formula is other than that represented by the formula (2), the backward probability β _n-1 (a) of the next stage is used as the previous stage. that is possible to reproduce the backward-probability beta _n of (b), is not limited to MAX-LOG-MAP method.

  As described above, according to the first embodiment, the relationship between the backward probabilities sequentially calculated by the sequential calculation is used, and the backward probability of the previous stage is reproduced from the backward probability of the next stage. There is no need to memorize backward probabilities. Further, since the backward probability of the previous stage is not estimated, the amount of data stored in the memory for storing the backward probability calculation result can be reduced without deterioration of the decoding characteristics. At this time, when the amount of data to be reduced is larger than the capacity of the calculation content storage memory, the total memory capacity per decoding circuit can be reduced.

(Embodiment 2)
FIG. 2 is a block diagram showing the configuration of the element decoder of the turbo decoder according to Embodiment 2 of the present invention. In FIG. 2, components that are the same as or equivalent to the components shown in FIG. 1 (Embodiment 1) are assigned the same reference numerals. Here, the description will be focused on the portion related to the second embodiment.

  As shown in FIG. 2, in the element decoder 200 of the turbo decoder according to the second embodiment, in the configuration shown in FIG. 1 (the first embodiment), instead of the calculation content storage memory 105, the selected path An information storage memory 201 is provided.

  Element decoder 200 of the turbo decoder according to the second embodiment is an error correction decoder that decodes turbo-encoded data by the MAX-LOG-MAP method. The selected path information storage memory 201 includes: Information indicating that the backward probability calculated by the backward probability calculation unit 103 and the transition probability used for the calculation are associated with each other (that is, information on a path selected at the time of calculation or information on a path not selected) is stored. The

As described above, in the MAX-LOG-MAP method, the backward probability update expression is expressed by Expression (2). Therefore, in Expression (1), β _n−1 (a) = f (β _n (b) ) Is selected, the backward probability β _n (b) of the previous stage can be derived as the following equation (3).
β _n (b) = β _n−1 (a) −γ _ab (3)

If the transition probability γ _ab can be prepared in this way, the backward probability β _n (b) of the previous stage can be derived from the backward probability β _n−1 (a) of the next stage. The transition probability γ _ab can be arbitrarily derived from the path information and the input and output data strings of the element decoder.

  That is, the backward probability reproduction unit 106 stores the path information by storing binary information indicating which of the two expressions shown in the expression (1) is selected for each state probability calculation. From the transition probability and the backward probability of the next stage, some of the backward probability of the previous stage can be reproduced, but it cannot be reproduced.

  Therefore, in the second embodiment, the backward probability calculation result storage memory 104 stores backward probabilities that cannot be reproduced by the backward probability reproduction unit 106. In the likelihood calculation, the backward probability reproduction unit 106 reproduces and uses backward probabilities that are not stored in the backward probability calculation result storage memory 104 as in the first embodiment.

  As described above, according to the second embodiment, the information on the selected path is stored as information generated in the backward probability calculation process, and the backward probability that cannot be reproduced using the information is stored for backward probability calculation result storage. Since the data is stored in the memory, the capacity of the backward probability calculation result storage memory can be reduced without degrading the decoding performance. In the Max-Log-MAP method, since one path is selected from two paths in updating one state, the information amount is at most one. Therefore, as in the first embodiment, the total memory capacity per decoding circuit can be small.

  Note that there is a method of adding correction terms, normalization, and offsets for the purpose of improving decoding characteristics during probability calculation. In this case, the selected path information and values representing the correction terms and offset values can be stored. That's fine.

(Embodiment 3)
The turbo decoder according to the third embodiment of the present invention is an error correction decoder that decodes turbo-encoded data by the MAX-LOG-MAP method as in the second embodiment, and is shown in FIG. In the above configuration, the backward probability calculation unit 103 controls the backward probability stored in the backward probability calculation result storage memory 104 and the path information stored in the selected path information storage memory 201 as follows.

  That is, the backward probability calculation unit 103 in the turbo decoder according to the third embodiment uses all the backward probabilities of all states at the time point n without overlapping, and the number of all the states at the time point n is M. When the number of backward probabilities at the time point n−1 calculated in this way is L, the number of backward probabilities stored per time point in the backward probability calculation result storage memory 104 is M−L, and the selected path information is stored. Information stored in the memory 201 per time point is information indicating M−L paths.

  Hereinafter, a specific description will be given with reference to FIGS. FIG. 3 is a diagram illustrating an example of a trellis lattice of turbo codes used for explaining a backward probability calculation process in the turbo decoder according to Embodiment 3 of the present invention. FIG. 4 is a flowchart for explaining backward probability update processing in the turbo decoder according to Embodiment 3 of the present invention. FIG. 5 is a flowchart for explaining backward probability reproduction processing in the turbo decoder according to Embodiment 3 of the present invention.

FIG. 3 shows a trellis lattice of turbo codes described in Non-Patent Document 1. In the trellis lattice shown in FIG. Here, the backward probability of being in state 0 at time n is β _n (0), the backward probability of being in state 1 is β _n (1), and the transition probability of the path from state a to state b is γ Assuming that _n (a, b), the backward probability β _n−1 (0) in the state 0 at the time point n−1 is expressed by the following equation (4).
β _n-1 (0) = β _n (0) + γ _n (0,0) or β _n (1) + γ _n (1,0) (4)

The backward probability calculation unit 103 performs backward probability update processing using Equation (4) according to the procedure shown in FIG. In FIG. 4, when the selection formula (4) of the backward probability β _n−1 (0) in the state 0 at the time point n−1 is obtained (step ST401), the selection formula (4) is calculated in the calculation of the backward probability of the next stage. When the right-hand left expression is selected (step ST402: Yes), “1” is written in the selected path information storage memory 201 (step ST403), and the backward probability β _n (1 ) Is written (step ST404).

On the other hand, in step ST402, when the right-hand side of the selection formula (4) is selected in the calculation of the backward probability in the next stage (step ST402: No), “0” is written in the selected path information storage memory 201 ( In step ST405, the backward probability β _n (0) is written in the backward probability calculation result storage memory 104 (step ST406).

The backward probability reproduction unit 106 performs backward probability reproduction processing according to the procedure shown in FIG. FIG. 5 shows a process for reproducing the backward probability β _n (0) and backward probability β _n (1) of the preceding stage. In FIG. 5, in the backward probability reproduction unit 106, when the path information read from the selected path information storage memory 201 is “1” (step ST501: Yes), the transition probability calculation unit 102 calculates the selection formula (4). receive transition probability gamma _n on the right side left formula (0,0) (step ST 502), retrograde calculation _{β n (0) = β n} -1 (0) preceding backward probability by performing-gamma _n a (0,0) β _n (0) is reproduced (step ST503). On the other hand, since the backward probability β _n (1) is stored in the backward probability calculation result storage memory 104, it is read from the backward probability calculation result storage memory 104 (step ST504).

  In this way, in the turbo code trellis lattice shown in FIG. 3, the backward probability of state 0 at time n-1 is calculated from the backward probability of states 0 and 1 at time n. From the backward probability of state 0, either the backward probability of state 0 or 1 at time n can be derived by back calculation. Similarly, the backward probabilities of states 1 to 3 at time n-1 can be calculated from the backward probabilities of states 2 to 7 at time n. This indicates that the backward probability of the four states at the time point n−1 can be calculated from the backward probability of the eight states that do not overlap at the time point n. This indicates that the backward probabilities for the four states that do not overlap among the backward probability of the eight states at the time point n can be obtained by back calculation from the backward probability of the specific four states at the time point n−1. It also indicates that there is a possibility that the backward probability of the non-overlapping 8-4 state at time n may not be obtained from the n-1 state by back calculation.

  Therefore, the backward probability calculation result storage memory 104 stores backward probabilities of 8-4 states that may not be reproduced by the backward probability reproduction unit 106, and the selected path information storage memory 201 stores the backward probability reproduction unit. Information representing an 8-4 state that may not be reproduced in 106 is stored.

  Further, when the trellis lattice shown in FIG. 3 is used concatenated as shown in FIG. 6, the next stage state calculated from the backward probabilities of eight states that do not overlap is two states and cannot be reproduced by the backward probability reproduction unit 106. Since the number of possible states is 8-2 = 6, the backward probability calculation result storage memory 104 stores backward probabilities of 6 states, and the selected path information storage memory 201 stores 6 states. By storing the information representing, backward probabilities of all states can be reproduced.

  Therefore, according to the third embodiment, when the backward probabilities of the L states at the time point n−1 can be calculated from the backward probabilities of the M states that do not overlap at the time point n, the calculation result storage memory is stored per time point. The backward probabilities to be stored are ML, and the information stored in the memory for storing selected path information is information representing these ML states, so that the backward probabilities of all the states can be reproduced, and the calculation result The memory for storage can be reduced to (ML) / M.

  Note that the information indicating the ML states stored in the memory for storing the selected path information may be any information that can indicate a state probability that cannot be reproduced, and state transition between the time point n and the time point n−1. It may be all path information indicating the relationship, or may be information that directly indicates the ML state that cannot be reproduced. Further, since the state that cannot be reproduced is a state other than the state that can be reproduced, information that indicates L states that can be reproduced may be used.

  As described above, according to the third embodiment, the information on the number of paths stored at an arbitrary time can be fixed to a certain number of information smaller than the number of states of the trellis, and the calculation at an arbitrary time can be performed. Since the number of times the backward probability calculation result storage memory is accessed can be reduced, the memory capacity for storing the backward probability calculation result can be reduced more than before without increasing the amount of calculation so much.

In the trellis lattice of the turbo code shown in FIG. 3, any four probabilities of the backward probabilities of the states 0 to 7 at the time point n can be derived by back calculation from the backward probabilities of the states 0 to 3 at the time point n-1. Pay attention. Since either the backward probability β _n (0) or β _n (1) can be calculated from the backward probability β _n-1 (0), the backward probability β _n (0) and the backward probability β _n (1) Information stored in the backward probability calculation result storage memory 104, and the path information selected by the calculation of the backward probability in state 0 at time n-1 and the calculation of state 0 at time n-1. Thus, the backward probabilities in states 0 and 1 at time n can be uniquely reproduced.

  Similarly, by storing the backward probabilities not selected in the backward probability calculation result storage memory 104, the backward probabilities in the states 1, 2, 3 at the time point n-1 to the state 2, 3 or 4, at the time point n, Any of three values of backward probabilities at 5 or 6, 7 can be reproduced, and all backward probabilities of states 0 to 7 at time point n from the backward probability at states 0 to 3 at time point n-1 at the time of likelihood calculation Can be derived. That is, four path information selected when calculating backward probabilities of specific four states (half of the total number of states, in this example, states 0 to 3) at the time of backward probability calculation are stored in the selected path information storage memory 201. Then, by storing the backward probabilities of the four states not selected in the backward probability calculation result storage memory 104, all backward probabilities can be reproduced.

  Thus, in the third embodiment, by using a specific trellis lattice, by preparing a memory for storing path selection information that is half the number of states, the required capacity of the memory for storing the backward probability calculation results is halved. Can be reduced. In addition, since the state probability reproduction process applied in the third embodiment only needs to be performed in a specific state, the circuit configuration can be further easily realized.

(Embodiment 4)
FIG. 7 is a block diagram showing the configuration of the element decoder of the turbo decoder according to Embodiment 4 of the present invention. In FIG. 7, the same reference numerals are given to components that are the same as or equivalent to those shown in FIG. 2 (Embodiment 2). Here, the description will be focused on the portion related to the fourth embodiment.

  As shown in FIG. 7, in the element decoder 700 of the turbo decoder according to the fourth embodiment, in the configuration shown in FIG. 2 (Embodiment 2), the probability reproduction determining unit 701 includes the backward probability calculating unit 103 and A backward probability calculation result storage memory 104 is provided. The backward probability calculation unit 103 gives the calculated backward probability and information obtained in the calculation process to the probability reproduction determination unit 701, and the path information to the selected path information storage memory 201 is input from the probability reproduction determination unit 701. It is said.

  The probability reproduction determination unit 701 determines whether the backward probability derived by sequential calculation in the backward probability calculation unit 103 is reproducible from the backward probability of the next stage and the transition probability used for the calculation, and according to the determination result A control is performed to select the backward probability and store it in the backward probability calculation result storage memory 104.

  Further, the probability reproduction determination unit 701 stores in the selected path information storage memory 201 information in addition to the path information, information that can determine which state's backward probability is written in the backward probability calculation result storage memory 104. Is supposed to do.

  In the fourth embodiment, since the probability of the same state may be selected as the previous stage probability selected when calculating the next stage probability, the number of probabilities that cannot actually be reproduced is variable at each time point. The number of probabilities that need to be stored at an arbitrary time, and that is stored at an arbitrary time, is associated and stored. Therefore, in this embodiment, the total number of stored probabilities can be minimized for each calculation.

  Specifically, the number of backward probabilities at time n that are actually selected and used to determine the backward probability at a certain time point n−1 is M / 2 or more, where M is the number of states of backward probability per time point. M or less. Therefore, since the number of backward probabilities to be stored in the present invention is M / 2 to 0 per time point, the number of data to be stored based on the determination result of whether or not each state can be reproduced by the backward probability reproduction unit 106 The total storage capacity can be reduced by determining.

  In this method, it is possible not to write all backward probabilities that can be reproduced by the backward probability reproducing unit to the memory, and the backward operation performed by the backward probability reproducing unit is the forward of the sequential calculation performed by the backward probability calculating unit. Since it is a sequential calculation with the direction reversed, it is particularly effective when taking a method of storing data in a LIFO area such as a stack as a calculation result storage memory.

  As described above, according to the fourth embodiment, since the number of data written to the backward probability calculation result storage memory can be reduced as much as possible, the total memory capacity can be suppressed. In addition, since the number of accesses to the backward probability calculation result storage memory can be reduced, processing delay due to memory access can be suppressed. In addition, the number of accesses to the memory can be suppressed as much as possible, and the power consumption associated with the memory access can be suppressed at the same time.

  In each of the embodiments described above, likelihood calculation is applied to a turbo decoder that uses three types of data obtained from a transition probability calculation unit, a forward probability calculation unit, and a backward probability calculation result storage memory. Although an example is shown, the present invention is not limited to this, and likelihood calculation is performed by, for example, three types of data obtained from a transition probability calculation unit, a backward probability calculation unit, and a forward probability calculation result storage memory. The same can be applied to a turbo decoder using. Needless to say, the forward probability and the backward probability are calculated according to the situation and can be similarly applied to a turbo decoder that stores the same in a memory.

  INDUSTRIAL APPLICABILITY The present invention is useful for reducing the memory capacity used for internal computation without degrading the decoding capability when turbo codes are decoded by the MAP method or the MAX-LOG-MAP method.

FIG. 2 is a block diagram showing a configuration of an element decoder of a turbo decoder according to Embodiment 1 of the present invention. FIG. 9 is a block diagram showing a configuration of an element decoder of a turbo decoder according to Embodiment 2 of the present invention. The figure which shows an example of the trellis grating | lattice of the turbo code used in order to demonstrate the calculation process of the back probability in the turbo decoder which concerns on Embodiment 3 of this invention. Flowchart for explaining backward probability update processing in the turbo decoder according to Embodiment 3 of the present invention. Flowchart for explaining backward probability reproduction processing in a turbo decoder according to Embodiment 3 of the present invention. The figure which shows an example of the trellis grating | lattice of the turbo code used in order to demonstrate the calculation process of the back probability in the turbo decoder which concerns on Embodiment 3 of this invention. FIG. 9 is a block diagram showing a configuration of an element decoder of a turbo decoder according to Embodiment 4 of the present invention.

Explanation of symbols

100, 200, 700 Element decoder 101 Input data memory 102 Transition probability calculation unit 103 Back probability calculation unit 104 Back probability calculation result storage memory 105 Calculation content storage memory 106 Back probability reproduction unit 107 Forward probability calculation unit 108 Likelihood Calculation unit 109 Output data memory 201 Selected path information storage memory 701 Probability reproduction determination unit

Claims (15)

  When decoding turbo-encoded data by MAP decoding, in the turbo decoder for storing the backward probability calculated by the backward probability calculating means in the sequential calculation in the calculation result storage means accessed by the likelihood calculating means, the backward probability Calculation content storage means for storing information generated in the process of calculating backward probabilities necessary for likelihood calculation by sequential calculation, and previous stage backward probabilities used in likelihood calculation are stored in the calculation content storage means. And a backward probability reproducing means for reproducing from the backward probability of the next stage using the information and the state transition probability. The backward probability calculating means reproduces the backward probability of the calculated backward probability by the backward probability reproducing means. A turbo decoder characterized by storing a backward probability that cannot be stored in said calculation result storage means.   In a turbo decoder that stores the backward probability calculated by the backward probability calculation means by the sequential calculation in the calculation result storage means accessed by the likelihood calculation means when decoding the turbo encoded data by the MAX-LOG-MAP method A selection path information storage means for storing path information indicating the backward probability necessary for calculating the likelihood calculated by the backward probability calculation means and the transition probability used for the calculation in association with each other, and the likelihood A backward probability reproduction means for reproducing the backward probability of the previous stage used in the calculation from the backward probability of the next stage using the path information stored in the selected path information storage means and the state transition probability, and the backward probability The calculation means stores a backward probability that cannot be reproduced by the backward probability reproduction means among the calculated backward probabilities in the calculation result storage means.   When the backward probability of the L states at the time point n-1 is calculated from the backward probability of the M states at the time point n in the backward probability calculating means, the backward probability stored in the calculation result storage means per time point 3. The turbo decoder according to claim 2, wherein the number is ML, and the information stored in the selected path information storage means per time is information indicating ML paths.   The backward probability reproduction means calculates M / 2 backward probabilities at time point n + 1 using M / 2 backward probabilities at time point n, where the number of states of backward probability at time point n is M. 3. The backward probability calculation means sets M / 2 as the backward probability and the number of paths stored per time in the calculation result storage means and the selected path information storage means. Turbo decoder.   It is determined whether the backward probability calculated by the backward probability calculation means by sequential calculation is reproducible from the backward probability of the next stage and the transition probability used for the calculation, and based on the determination result, the backward probability is selected and the calculation is performed. A reproducibility determining means for storing in the selected path information storage means information indicating a state corresponding to a backward probability stored in the calculation result storage means, as well as being stored in the result storage means. 2. The turbo decoder according to 2.   When decoding turbo-encoded data by MAP decoding, in the turbo decoder for storing the forward probability calculated by the forward probability calculation means by sequential calculation in the calculation result storage means accessed by the likelihood calculation means, the forward probability Calculation content storage means for storing information generated in the process of calculating forward probabilities necessary for likelihood calculation by sequential calculation, and previous forward probabilities used in likelihood calculation are stored in the calculation content storage means. Forward probability reproduction means that reproduces from the next-stage forward probability using the information and state transition probability, and the forward probability calculation means reproduces the calculated forward probability by the forward probability reproduction means. A turbo decoder characterized by storing a forward probability that cannot be stored in the calculation result storage means.   In a turbo decoder that stores forward probabilities calculated by sequential calculation by forward probability calculation means in calculation result storage means accessed by likelihood calculation means when decoding turbo-encoded data by the MAX-LOG-MAP method A selection path information storage means for storing path information indicating the forward probability necessary for calculating the likelihood calculated by the forward probability calculation means and the transition probability used for the calculation in association with each other, and the likelihood Forward probability reproduction means for reproducing the forward probability of the previous stage used in the calculation from the forward probability of the next stage using the path information stored in the selected path information storage means and the state transition probability, and the forward probability. The calculation means stores a forward probability that cannot be reproduced by the forward probability reproduction means among the calculated forward probabilities in the calculation result storage means.   When the forward probability of the L states at the time point n + 1 is calculated from the forward probability of the M states at the time point n in the forward probability calculation means, the number of forward probabilities stored per time point in the calculation result storage means is 8. The turbo decoder according to claim 7, wherein M-L is used, and information stored per time in the selected path information storage means is information indicating M-L paths.   The forward probability reproduction means has a forward probability state number M at time n, and M / 2 forward probabilities at time n-1 using a specific M / 2 forward probability at time n. The forward probability calculation means calculates M / 2 as the forward probability and the number of paths stored per time in the calculation result storage means and the selected path information storage means. The described turbo decoder.   It is determined whether the forward probability calculated by the forward probability calculation means by sequential calculation is reproducible from the forward probability of the next stage and the transition probability used for the calculation, and the forward probability is selected based on the determination result and the calculation is performed. A reproducibility determining means for storing in the selected path information storage means information indicating a state corresponding to a forward probability stored in the result storage means and stored in the calculation result storage means. 8. The turbo decoder according to 7.   When decoding turbo-encoded data by MAP decoding, forward probabilities and backward probabilities required for likelihood calculation are calculated by sequential calculation, and at least one probability is stored in a memory. Storing the information generated in the process of calculating the probability in the memory, and using the information stored in the memory and the state transition probability as the previous stage probability in the one probability used in the likelihood calculation A turbo decoding comprising: a step of reproducing from a probability of the next stage in one probability; and a step of storing in the memory a probability that cannot be reproduced in the step of reproducing among the calculated one probability Method.   When decoding turbo-encoded data using the MAX-LOG-MAP method, forward probabilities and backward probabilities required for likelihood calculation are calculated by sequential calculation, and at least one probability is stored in the first memory. And storing the path information instructing the one probability calculated by the sequential calculation in association with the transition probability used in the calculation in the second memory, and the one probability used in the likelihood calculation. Reproducing the probability of the previous stage from the probability of the next stage of the one probability using the path information stored in the second memory and the state transition probability, and reproducing the one of the calculated probabilities Storing a probability that cannot be reproduced in the process in the first memory.   The state probabilities to be stored in the first memory and the second memory per time point are such that the number of all one states at the time point n is M, and the L state probabilities from the M state to the next stage are When required, there are M−L pieces of information, and the information stored in the second memory is information indicating M−L state probabilities at time n that is not used in the calculation of the next stage L state probabilities. The turbo decoding method according to claim 12, wherein:   The state probability and the number of passes stored in the first memory and the second memory per time point are M / 2 when all of one state at the time point n are M. Item 13. The turbo decoding method according to Item 12.   Determining whether the calculated one probability is reproducible from one probability in the next stage and the transition probability used for the calculation, and selecting one of the calculated probabilities based on the determination result The method includes: storing a probability in the first memory; and storing information indicating a state corresponding to one of the probabilities stored in the first memory in the second memory. 12. The turbo decoding method according to 12.

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