Memory system and method for controlling nonvolatile memory

1. A memory system comprising:

a nonvolatile memory; and

a memory controller configured to

convert a received value read from the nonvolatile memory into first likelihood information by using a first conversion table,

execute decoding on the first likelihood information and output a posterior value,

output an estimated value of the received value obtained on the basis of the posterior value in a case where the decoding is successful,

generate a second conversion table on the basis of the posterior value in a case where the decoding fails,

convert the received value into second likelihood information using the second conversion table in a case where the second conversion table has been generated, and

execute decoding on the second likelihood information and output a posterior value.

2. The memory system according to claim 1 , wherein the memory controller is configured to:

calculate likelihood information corresponding to the received value by using a value obtained by inputting the posterior value to a function in which output values for at least three input values monotonically increase, and

generate the second conversion table associating the received value with the calculated likelihood information.

3. The memory system according to claim 2 , wherein the function is a sigmoid function.

4. The memory system according to claim 2 , wherein the function is a function that includes a monotonically increasing linear function.

5. The memory system according to claim 2 , wherein the memory controller is configured to:

calculate likelihood information corresponding to the received value by using a value obtained by inputting the posterior value to the function and using a value for correcting a bias of a transmitted value stored in the nonvolatile memory, and

generate the second conversion table associating the received value with the calculated likelihood information.

6. The memory system according to claim 1 , wherein the memory controller is configured to:

generate a new second conversion table on the basis of a posterior value obtained by decoding that has resulted in a failure every time the decoding of the second likelihood information fails,

convert the received value into the second likelihood information by using the newly generated second conversion table every time the second conversion table is generated, and

decode the second likelihood information every time the received value is converted into the second likelihood information.

7. The memory system according to claim 1 , wherein the memory controller is configured to:

modify the generated second conversion table, and

convert the received value into the second likelihood information by using the modified second conversion table.

8. The memory system according to claim 1 , wherein the memory controller is configured to:

convert a sequence of the received value into a sequence of likelihood information including a predetermined number of pieces of the first likelihood information or the second likelihood information in units of the sequence of a received value including a predetermined number of the received values, and

execute the decoding in units of the likelihood information sequence.

9. A method for controlling a nonvolatile memory, the method comprising:

converting, by a memory controller, a received value read from the nonvolatile memory into first likelihood information by using a first conversion table,

executing, by the memory controller, decoding on the first likelihood information and outputting a posterior value,

outputting, by the memory controller, an estimated value of the received value obtained on the basis of the posterior value in a case where the decoding is successful,

generating, by the memory controller, a second conversion table on the basis of the posterior value in a case where the decoding fails,

converting, by the memory controller, the received value into second likelihood information by using the second conversion table in a case where the second conversion table has been generated, and

executing, by the memory controller, decoding on the second likelihood information and outputting a posterior value.

10. The method according to claim 9 further comprising:

calculating, by the memory controller, likelihood information corresponding to the received value by using a value obtained by inputting the posterior value to a function in which output values for at least three input values monotonically increase; and

generating, by the memory controller, the second conversion table associating the received value with the calculated likelihood information.

11. The method according to claim 10 , wherein the function is a sigmoid function.

12. The method according to claim 10 , wherein the function is a function that includes a monotonically increasing linear function.

13. The method according to claim 10 further comprising:

calculating, by the memory controller, likelihood information corresponding to the received value by using a value obtained by inputting the posterior value to the function and using a value for correcting a bias of a transmitted value stored in the nonvolatile memory, and

generating, by the memory controller, the second conversion table associating the received value with the calculated likelihood information.

14. The method according to claim 9 further comprising:

generating, by the memory controller, a new second conversion table on the basis of a posterior value obtained by decoding that has resulted in a failure every time the decoding of the second likelihood information fails,

converting, by the memory controller, the received value into the second likelihood information by using the newly generated second conversion table every time the second conversion table is generated, and

decoding, by the memory controller, the second likelihood information every time the received value is converted into the second likelihood information.

15. The method according to claim 9 further comprising:

modifying, by the memory controller, the generated second conversion table; and

converting, by the memory controller, the received value into the second likelihood information by using the modified second conversion table.

16. The method according to claim 9 further comprising:

converting, by the memory controller, a sequence of the received value into a sequence of likelihood information including a predetermined number of pieces of the first likelihood information or the second likelihood information in units of the sequence of a received value including a predetermined number of the received values, and

executing, by the memory controller, the decoding in units of the likelihood information sequence.