Soft-decision compensation for flash channel variation
View Patent ↗In an SSD controller reading from flash memory, subsequent to failure of an initial soft-decision decoding attempt based on a nominal LLR, soft-decision re-decoding attempts are made using compensated LLR soft-decision information sets, pre-calculated at respective read-equilibrium points corresponding to mean shifts and variance change in the actual charge-state distributions of the flash memory channel. According to embodiment, soft-decision re-decoding attempts are performed without a retry read, or overlapped with one or more retry reads. By overlapping re-decoding with one or more retry reads, the probability of successful decoding increases, the need for further retry reads diminishes, and throughput is improved. The LLR compensation becomes very effective over a large number of retry reads, improving decoding reliability and achieving close to optimal bit error rates, even in the presence of large channel variation.
1. A method, comprising:
performing an initial read and an initial attempted ECC decoding, for each of a plurality of flash memory locations;
performing up to a first plurality of ECC soft-decision redecoding attempts based on the initial read, subsequent to failure of the initial attempted ECC decoding, for at least some of the locations;
wherein each of the acts of performing are at least in part via a controller of a solid-state disk (SSD); and
wherein each ECC soft-decision redecoding attempt uses a selected one of a plurality of pre-calculated soft log likelihood ratio (LLR) soft-decision information sets, each of the sets corresponding to a respective voltage distribution scenario.
2. The method of claim 1 , further comprising:
for the at least some of the locations, overlapped in time with the first plurality of ECC soft-decision redecoding attempts, performing a second plurality of retry reads and respective retry ECC decoding attempts.
3. The method of claim 1 , wherein the initial attempted ECC decoding is an ECC soft-decision decoding attempt using an initial LLR soft-decision information set selected from the plurality of log likelihood ratio (LLR) soft-decision information sets.
4. The method of claim 3 , wherein at least some of the selected LLR soft-decision information sets are selected in accordance with a read threshold-tracking algorithm and a predetermined retry strategy.
5. The method of claim 4 , wherein the threshold-tracking algorithm comprises evaluating a zero-one balance for each of a corresponding plurality of reads.
6. The method of claim 5 , wherein the initial LLR soft-decision information set corresponds to an initial read threshold voltage predicted to be associated with a lowest disparity of the zero-one balances evaluated.
7. The method of claim 6 , further comprising selecting subsequent LLR soft-decision information sets corresponding to read threshold voltages alternating on either side of the initial read threshold voltage in incrementally increasing amounts.
8. The method of claim 7 , further comprising at least sometimes updating at least some of the LLR soft-decision information sets on a periodic basis.
9. The method of claim 7 , further comprising:
wherein the LLR soft-decision information sets comprise a first one of a plurality of LLR soft-decision information groups; and
wherein at least a portion of the flash memory is organized into a plurality of redundancy groups, each of the redundancy groups being associated with a corresponding one of the LLR soft-decision information groups.
10. The method of claim 9 , wherein at least some of the LLR soft-decision information groups used for soft-decision-decoding and soft-decision-redecoding are specific to each of a plurality of redundancy groups in which the flash memory is organized.
11. The method of claim 10 , further comprising:
streaming data from each read and at least some of the redundancy-group-specific corresponding LLR soft-decision information groups to a soft-decision processing unit to produce soft-decision information; and
wherein the redecoding attempts are performed on the soft-decision information.
12. A device, comprising:
means for performing an initial read and an initial attempted ECC decoding, for each of a plurality of flash memory locations;
means for performing up to a first plurality of ECC soft-decision redecoding attempts based on the initial read, subsequent to failure of the initial attempted ECC decoding, for at least some of the locations; and
wherein each ECC soft-decision redecoding attempt uses a selected one of a plurality of pre-calculated soft log likelihood ratio (LLR) soft-decision information sets, each of the sets corresponding to a respective voltage distribution scenario.
13. The device of claim 12 , further comprising:
means for performing a second plurality of retry reads and respective retry ECC decoding attempts, overlapped in time with the first plurality of ECC soft-decision redecoding attempts, for the at least some of the locations.
14. The device of claim 12 , wherein the initial attempted ECC decoding is an ECC soft-decision decoding attempt using an initial LLR soft-decision information set selected from the plurality of log likelihood ratio (LLR) soft-decision information sets.
15. The device of claim 14 , wherein at least some of the selected LLR soft-decision information sets are selected in accordance with a read threshold-tracking algorithm and a predetermined retry strategy.
16. The device of claim 15 , wherein the threshold-tracking algorithm comprises evaluating a zero-one balance for each of a corresponding plurality of reads.
17. The device of claim 16 , wherein the initial LLR soft-decision information set corresponds to an initial read threshold voltage predicted to be associated with a lowest disparity of the zero-one balances evaluated.
18. The device of claim 17 , further comprising means for selecting subsequent LLR soft-decision information sets corresponding to read threshold voltages alternating on either side of the initial read threshold voltage in incrementally increasing amounts.
19. The device of claim 18 , further comprising means for at least sometimes updating at least some of the LLR soft-decision information sets on a periodic basis.
20. The device of claim 18 , further comprising:
wherein the LLR soft-decision information sets comprise a first one of a plurality of LLR soft-decision information groups; and
wherein at least a portion of the flash memory is organized into a plurality of redundancy groups, each of the redundancy groups being associated with a corresponding one of the LLR soft-decision information groups.
21. The device of claim 20 , wherein at least some of the LLR soft-decision information groups used for soft-decision-decoding and soft-decision-redecoding are specific to each of a plurality of redundancy groups in which the flash memory is organized.
22. The device of claim 21 , further comprising:
means for streaming data from each read and at least some of the redundancy-group-specific corresponding LLR soft-decision information groups to a soft-decision processing unit to produce soft-decision information; and
wherein the redecoding attempts are performed on the soft-decision information.