188,982 research outputs found
Towards Endurable, Reliable and Secure Flash Memories-a Coding Theory Application
Storage systems are experiencing a historical paradigm shift from hard disk to nonvolatile memories due to its advantages such as higher density, smaller size and non-volatility. On the other hand, Solid Storage Disk (SSD) also poses critical challenges to application and system designers. The first challenge is called endurance. Endurance means flash memory can only experience a limited number of program/erase cycles, and after that the cell quality degradation can no longer be accommodated by the memory system fault tolerance capacity. The second challenge is called reliability, which means flash cells are sensitive to various noise and disturbs, i.e., data may change unintentionally after experiencing noise/disturbs. The third challenge is called security, which means it is impossible or costly to delete files from flash memory securely without leaking information to possible eavesdroppers.
In this dissertation, we first study noise modeling and capacity analysis for NAND flash memories (which is the most popular flash memory in market), which gains us some insight on how flash memories are working and their unique noise. Second, based on the characteristics of content-replication codewords in flash memories, we propose a joint decoder to enhance the flash memory reliability. Third, we explore data representation schemes in flash memories and optimal rewriting code constructions in order to solve the endurance problem. Fourth, in order to make our rewriting code more practical, we study noisy write-efficient memories and Write-Once Memory (WOM) codes against inter-cell interference in NAND memories. Finally, motivated by the secure deletion problem in flash memories, we study coding schemes to solve both the endurance and the security issues in flash memories. This work presents a series of information theory and coding theory research studies on the aforesaid three critical issues, and shows that how coding theory can be utilized to address these challenges
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
When Do WOM Codes Improve the Erasure Factor in Flash Memories?
Flash memory is a write-once medium in which reprogramming cells requires
first erasing the block that contains them. The lifetime of the flash is a
function of the number of block erasures and can be as small as several
thousands. To reduce the number of block erasures, pages, which are the
smallest write unit, are rewritten out-of-place in the memory. A Write-once
memory (WOM) code is a coding scheme which enables to write multiple times to
the block before an erasure. However, these codes come with significant rate
loss. For example, the rate for writing twice (with the same rate) is at most
0.77.
In this paper, we study WOM codes and their tradeoff between rate loss and
reduction in the number of block erasures, when pages are written uniformly at
random. First, we introduce a new measure, called erasure factor, that reflects
both the number of block erasures and the amount of data that can be written on
each block. A key point in our analysis is that this tradeoff depends upon the
specific implementation of WOM codes in the memory. We consider two systems
that use WOM codes; a conventional scheme that was commonly used, and a new
recent design that preserves the overall storage capacity. While the first
system can improve the erasure factor only when the storage rate is at most
0.6442, we show that the second scheme always improves this figure of merit.Comment: to be presented at ISIT 201
Trade-offs between Instantaneous and Total Capacity in Multi-Cell Flash Memories
The limited endurance of flash memories is a major
design concern for enterprise storage systems. We propose a
method to increase it by using relative (as opposed to fixed)
cell levels and by representing the information with Write
Asymmetric Memory (WAM) codes. Overall, our new method
enables faster writes, improved reliability as well as improved
endurance by allowing multiple writes between block erasures.
We study the capacity of the new WAM codes with relative levels,
where the information is represented by multiset permutations
induced by the charge levels, and show that it achieves the
capacity of any other WAM codes with the same number of
writes. Specifically, we prove that it has the potential to double
the total capacity of the memory. Since capacity can be achieved
only with cells that have a large number of levels, we propose a
new architecture that consists of multi-cells - each an aggregation
of a number of floating gate transistors
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
Write-Once-Memory Codes by Source Polarization
We propose a new Write-Once-Memory (WOM) coding scheme based on source
polarization. By applying a source polarization transformation on the
to-be-determined codeword, the proposed WOM coding scheme encodes information
into the bits in the high-entropy set. We prove in this paper that the proposed
WOM codes are capacity-achieving. WOM codes have found many applications in
modern data storage systems, such as flash memories.Comment: 5 pages, Proceedings of the International Conference on Computing,
Networking and Communications (ICNC 2015), Anaheim, California, USA, February
16-19, 201
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