6 research outputs found
Rewriting Flash Memories by Message Passing
This paper constructs WOM codes that combine rewriting and error correction
for mitigating the reliability and the endurance problems in flash memory. We
consider a rewriting model that is of practical interest to flash applications
where only the second write uses WOM codes. Our WOM code construction is based
on binary erasure quantization with LDGM codes, where the rewriting uses
message passing and has potential to share the efficient hardware
implementations with LDPC codes in practice. We show that the coding scheme
achieves the capacity of the rewriting model. Extensive simulations show that
the rewriting performance of our scheme compares favorably with that of polar
WOM code in the rate region where high rewriting success probability is
desired. We further augment our coding schemes with error correction
capability. By drawing a connection to the conjugate code pairs studied in the
context of quantum error correction, we develop a general framework for
constructing error-correction WOM codes. Under this framework, we give an
explicit construction of WOM codes whose codewords are contained in BCH codes.Comment: Submitted to ISIT 201
Error correction and partial information rewriting for flash memories
This paper considers the partial information rewriting problem for flash memories. In this problem, the state of information can only be updated to a limited number of new states, and errors may occur in memory cells between two adjacent updates. We propose two coding schemes based on the models of trajectory codes. The bounds on achievable code rates are shown using polar WOM coding. Our schemes generalize the existing rewriting codes in multiple ways, and can be applied to various practical scenarios such as file editing, log-based file systems and file synchronization systems
Asymmetric Error Correction and Flash-Memory Rewriting using Polar Codes
We propose efficient coding schemes for two communication settings: 1.
asymmetric channels, and 2. channels with an informed encoder. These settings
are important in non-volatile memories, as well as optical and broadcast
communication. The schemes are based on non-linear polar codes, and they build
on and improve recent work on these settings. In asymmetric channels, we tackle
the exponential storage requirement of previously known schemes, that resulted
from the use of large Boolean functions. We propose an improved scheme, that
achieves the capacity of asymmetric channels with polynomial computational
complexity and storage requirement.
The proposed non-linear scheme is then generalized to the setting of channel
coding with an informed encoder, using a multicoding technique. We consider
specific instances of the scheme for flash memories, that incorporate
error-correction capabilities together with rewriting. Since the considered
codes are non-linear, they eliminate the requirement of previously known
schemes (called polar write-once-memory codes) for shared randomness between
the encoder and the decoder. Finally, we mention that the multicoding scheme is
also useful for broadcast communication in Marton's region, improving upon
previous schemes for this setting.Comment: Submitted to IEEE Transactions on Information Theory. Partially
presented at ISIT 201
Polar coding for noisy write-once memories
We consider the noisy write-once memory (WOM) model to capture the behavior of data-storage devices such as flash memories. The noisy WOM is an asymmetric channel model with non-causal state information at the encoder. We show that a nesting of non-linear polar codes achieves the corresponding Gelfand-Pinsker bound with polynomial complexity
Algorithms and Data Representations for Emerging Non-Volatile Memories
The evolution of data storage technologies has been extraordinary. Hard disk drives
that fit in current personal computers have the capacity that requires tons of transistors
to achieve in 1970s. Today, we are at the beginning of the era of non-volatile memory
(NVM). NVMs provide excellent performance such as random access, high I/O speed, low
power consumption, and so on. The storage density of NVMs keeps increasing following
Moore’s law. However, higher storage density also brings significant data reliability issues.
When chip geometries scale down, memory cells (e.g. transistors) are aligned much closer
to each other, and noise in the devices will become no longer negligible. Consequently,
data will be more prone to errors and devices will have much shorter longevity.
This dissertation focuses on mitigating the reliability and the endurance issues for two
major NVMs, namely, NAND flash memory and phase-change memory (PCM). Our main
research tools include a set of coding techniques for the communication channels implied
by flash memory and PCM. To approach the problems, at bit level we design error
correcting codes tailored for the asymmetric errors in flash and PCM, we propose joint
coding scheme for endurance and reliability, error scrubbing methods for controlling storage
channel quality, and study codes that are inherently resisting to typical errors in flash
and PCM; at higher levels, we are interested in analyzing the structures and the meanings
of the stored data, and propose methods that pass such metadata to help further improve
the coding performance at bit level. The highlights of this dissertation include the first
set of write-once memory code constructions which correct a significant number of errors,
a practical framework which corrects errors utilizing the redundancies in texts, the first
report of the performance of polar codes for flash memories, and the emulation of rank
modulation codes in NAND flash chips