Rank Modulation Encoding for NAND Flash Reliability
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Solution Overview
Problem
NAND flash memory reliability decreases with increasing density due to noise and process variations, leading to shortened device life and inefficiencies in error correction, necessitating a fundamental change in data representation to enhance reliability.
Innovation Solution
Implementing rank modulation (RM) in existing NAND flash devices using the read-retry feature, which represents data using the relative order of cell voltages, allowing for more resilient data storage against noise and errors without requiring modifications to flash chips or internal access.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If NAND flash memory density is increased, then storage capacity is improved, but reliability deteriorates due to noise and process variations
Solution Approach 1:
The patent changes the fundamental parameter of data representation from absolute voltage levels to relative voltage ordering (rank modulation). This parameter transformation makes the storage system immune to noise and process variations that cause absolute voltage drift, thereby maintaining reliability while enabling higher density through multi-level cells.
Solution Approach 2:
Instead of storing data as absolute voltage levels (conventional approach), the patent inverts the approach by storing data as the relative order or rank of voltage levels among cells. This inversion transforms the problem from one affected by noise-induced voltage drift to one that is inherently robust against such variations.
2Reliability
If traditional error correction methods are used in high-density NAND flash, then device complexity increases, but reliability improvement is insufficient
Solution Approach 1:
The patent inverts the conventional error correction approach by preventing errors through rank modulation encoding rather than detecting and correcting them afterward. This inversion eliminates the need for complex error correction codes and algorithms, achieving high reliability with minimal additional complexity.
Solution Approach 2:
The patent converts the harmful effect of noise and voltage drift into a benefit by using relative voltage ordering that is inherently immune to such variations. What was previously a source of errors (voltage drift) becomes irrelevant to the stored information, effectively converting a harmful factor into a benign one.
3Ease of operation
If read-retry feature is used to implement rank modulation, then ease of operation is improved, but manufacturing precision requirements are maintained
Solution Approach 1:
The patent enables the NAND flash device to perform its own rank determination using the existing read-retry mechanism. The device reads cell voltages multiple times with different thresholds and uses the retry outcomes to determine relative ordering, eliminating the need for external precision voltage control or additional hardware.
Solution Approach 2:
The patent makes the read-retry feature, originally designed for error correction, serve a dual purpose: both error correction and rank modulation encoding. This multi-functionality allows the same hardware mechanism to perform multiple tasks without requiring additional specialized components or precision control circuits.
Data Source
AI summary
The reliability of NAND flash memory decreases rapidly as density increases, preventing the wide adoptions of flash-based storage systems. A novel data representation scheme named rank modulation (RM) is discussed for improving NAND flash reliability. RM encodes data using the relative orders of memory cell voltages, which is inherently resilient to asymmetric errors. For studying the effectiveness of RM in flash, RM is adapted to make it simple to implement with existing flash memories. The implementation is evaluated under different types of noise of 20 nm flash memory. Results show that RM offers significantly lower cell error rates compared to the current data representation in flash at typical P/E cycles. RM is applied to flash-based archival storage and shows that RM brings up to six times longer data retention time for 16 nm flash memory.