U.S. patent application number 16/516110 was filed with the patent office on 2021-01-21 for soft-input soft-output component code decoder for generalized low-density parity-check codes.
The applicant listed for this patent is SK hynix Inc.. Invention is credited to Meysam Asadi, Xuanxuan Lu, Chenrong Xiong, Fan Zhang.
Application Number | 20210019224 16/516110 |
Document ID | / |
Family ID | 1000004217600 |
Filed Date | 2021-01-21 |
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United States Patent
Application |
20210019224 |
Kind Code |
A1 |
Lu; Xuanxuan ; et
al. |
January 21, 2021 |
SOFT-INPUT SOFT-OUTPUT COMPONENT CODE DECODER FOR GENERALIZED
LOW-DENSITY PARITY-CHECK CODES
Abstract
Disclosed are devices, systems and methods for improved decoding
of a binary linear code. An example method includes receiving a
noisy codeword; computing a syndrome based on the noisy codeword;
identifying N error patterns that correspond to the syndrome;
selecting M error patterns from the N error patterns, wherein
M.ltoreq.N are positive integers, wherein a distance between a
codeword corresponding to each of the M error patterns and the
noisy codeword is less than a distance between a codeword
corresponding to any other error pattern and the noisy codeword,
and wherein the distance excludes a Hamming distance; modifying the
noisy codeword based on each of the M error patterns one-at-a-time;
and decoding the modified noisy codeword one-at-a-time until a
successful decoding is achieved.
Inventors: |
Lu; Xuanxuan; (San Jose,
CA) ; Zhang; Fan; (San Jose, CA) ; Xiong;
Chenrong; (San Jose, CA) ; Asadi; Meysam; (San
Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SK hynix Inc. |
Icheon-Si |
|
KR |
|
|
Family ID: |
1000004217600 |
Appl. No.: |
16/516110 |
Filed: |
July 18, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03M 13/1125 20130101;
G06F 11/1068 20130101; G11C 29/52 20130101 |
International
Class: |
G06F 11/10 20060101
G06F011/10; G11C 29/52 20060101 G11C029/52; H03M 13/11 20060101
H03M013/11 |
Claims
1. A method for improved decoding of a binary linear code,
comprising: receiving a noisy codeword; computing a syndrome based
on the noisy codeword; identifying N error patterns that correspond
to the syndrome; selecting M error patterns from the N error
patterns, wherein N and M are positive integers, wherein M is less
than or equal to N, wherein a distance between a codeword
corresponding to each of the M error patterns and the noisy
codeword is less than a distance between a codeword corresponding
to any other error pattern and the noisy codeword, and wherein the
distance excludes a Hamming distance; modifying the noisy codeword
based on each of the M error patterns one-at-a-time; and decoding
the modified noisy codeword one-at-a-time until a successful
decoding is achieved.
2. The method of claim 1, wherein the binary linear code is a (n,
k, d) Hamming code that is a component code of a generalized
low-density parity-check (G-LDPC) code.
3. The method of claim 2, wherein n=7, k=3 and d=3.
4. The method of claim 1, wherein the distance is a Euclidean
distance.
5. The method of claim 1, wherein selecting the M error patterns
from the N error patterns comprises: calculating a reliability
metric for each of the N error patterns; and selecting an error
pattern of the M error patterns based on a comparison of the
reliability metric of the error pattern and a first threshold.
6. The method of claim 5, wherein the reliability metric is a
log-likelihood ratio.
7. The method of claim 5, wherein the reliability metric is a
probability.
8. The method of claim 5, wherein selecting the M error patterns
from the N error patterns is further based on a comparison of a sum
of the reliability metric of each of the M error patterns and a
second threshold.
9. The method of claim 1, wherein achieving the successful decoding
corresponds to a successful read operation in a non-volatile
memory.
10. The method of claim 9, wherein the non-volatile memory is a
NAND flash memory.
11. A system for improved decoding of a binary linear code,
comprising: a processor and a memory including instructions stored
thereupon, wherein the instructions upon execution by the processor
cause the processor to: receive a noisy codeword; compute a
syndrome based on the noisy codeword; identify N error patterns
that correspond to the syndrome; select M error patterns from the N
error patterns, wherein N and M are positive integers, wherein M is
less than or equal to N, wherein a distance between a codeword
corresponding to each of the M error patterns and the noisy
codeword is less than a distance between a codeword corresponding
to any other error pattern and the noisy codeword, and wherein the
distance excludes a Hamming distance; modify the noisy codeword
based on each of the M error patterns one-at-a-time; and decode the
modified noisy codeword one-at-a-time until a successful decoding
is achieved.
12. The system of claim 11, wherein the binary linear code is a (7,
4, 3) Hamming code that is a component code of a generalized
low-density parity-check (G-LDPC) code.
13. The system of claim 11, wherein the instructions upon execution
by the processor further cause the processor, as part of selecting
the M error patterns from the N error patterns, to: calculate a
reliability metric for each of the N error patterns; and select an
error pattern of the M error patterns based on a comparison of the
reliability metric of the error pattern and a first threshold.
14. The system of claim 13, wherein the reliability metric is a
probability or a log-likelihood ratio.
15. The system of claim 13, wherein selecting the M error patterns
from the N error patterns is further based on a comparison of a sum
of the reliability metric of each of the M error patterns and a
second threshold.
16. The system of claim 11, wherein the distance is a Euclidean
distance.
17. A non-transitory computer-readable storage medium having
instructions stored thereupon for improved decoding of a binary
linear code, comprising: instructions for receiving a noisy
codeword; instructions for computing a syndrome based on the noisy
codeword; instructions for identifying N error patterns that
correspond to the syndrome; instructions for selecting M error
patterns from the N error patterns, wherein N and M are positive
integers, wherein M is less than or equal to N, wherein a distance
between a codeword corresponding to each of the M error patterns
and the noisy codeword is less than a distance between a codeword
corresponding to any other error pattern and the noisy codeword,
and wherein the distance excludes a Hamming distance; instructions
for modifying the noisy codeword based on each of the M error
patterns one-at-a-time; and instructions for decoding the modified
noisy codeword one-at-a-time until a successful decoding is
achieved.
18. The storage medium of claim 17, wherein the binary linear code
is a (7, 4, 3) Hamming code that is a component code of a
generalized low-density parity-check (G-LDPC) code.
19. The storage medium of claim 17, wherein the instructions for
selecting the M error patterns from the N error patterns comprises:
instructions for calculating a reliability metric for each of the N
error patterns; and instructions for selecting an error pattern of
the M error patterns based on a comparison of the reliability
metric of the error pattern and a first threshold.
20. The storage medium of claim 19, wherein the reliability metric
is a probability or a log-likelihood ratio.
21. The storage medium of claim 19, wherein selecting the M error
patterns from the N error patterns is further based on a comparison
of a sum of the reliability metric of each of the M error patterns
and a second threshold.
22. The storage medium of claim 17, wherein achieving the
successful decoding corresponds to a successful read operation in a
NAND flash memory.
Description
TECHNICAL FIELD
[0001] This patent document generally relates to non-volatile
memory devices, and more specifically, to error correction in
non-volatile memory devices.
BACKGROUND
[0002] Data integrity is an important feature for any data storage
device and data transmission. Use of strong error-correction codes
(ECCs) is recommended for various types of data storage devices
including NAND flash memory devices.
[0003] Solid-state drives (SSDs) use multi-level NAND flash devices
for persistent storage. However, the multi-level NAND flash devices
can be inherently unreliable and generally need to use ECCs to
allow dramatic increase in data reliability at the expense of extra
storage space for ECC parity bits. There is a demand for
increasingly efficient ECCs control mechanisms that provide the
most data protection for the least parity and complexity
requirements.
SUMMARY
[0004] Embodiments of the disclosed technology relate to methods,
devices and systems for soft-input soft-output (SISO) decoding of
component codes of a generalized low-density parity-check (GLDPC)
code. The methods and devices described in the present document
advantageously, among other features and benefits, enable
low-complexity implementations of the SISO algorithm for the
component codes of the GLDPC.
[0005] In an example aspect, a method for improved decoding of a
binary linear code includes receiving a noisy codeword; computing a
syndrome based on the noisy codeword; identifying N error patterns
that correspond to the syndrome; selecting M error patterns from
the N error patterns, wherein M.ltoreq.N are positive integers,
wherein a distance between a codeword corresponding to each of the
M error patterns and the noisy codeword is less than a distance
between a codeword corresponding to any other error pattern and the
noisy codeword, and wherein the distance excludes a Hamming
distance; modifying the noisy codeword based on each of the M error
patterns one-at-a-time; and decoding the modified noisy codeword
one-at-a-time until a successful decoding is achieved.
[0006] In another example aspect, the above-described method may be
implemented by a video encoder apparatus or a video decoder
apparatus that comprises a processor.
[0007] In yet another example aspect, these methods may be embodied
in the form of processor-executable instructions and stored on a
computer-readable program medium.
[0008] The subject matter described in this patent document can be
implemented in specific ways that provide one or more of the
following features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates an example of a memory system.
[0010] FIG. 2 is an illustration of an example non-volatile memory
device.
[0011] FIG. 3 is an example diagram illustrating the cell voltage
level distribution (V.sub.th) of a non-volatile memory device.
[0012] FIG. 4 is another example diagram illustrating the cell
voltage level distribution (V.sub.th) of a non-volatile memory
device.
[0013] FIG. 5 is an example diagram illustrating the cell voltage
level distribution (V.sub.th) of a non-volatile memory device
before and after program interference.
[0014] FIG. 6 is an example diagram illustrating the cell voltage
level distribution (V.sub.th) of a non-volatile memory device as a
function of the reference voltage.
[0015] FIG. 7 is a high-level block diagram of a generalized
low-density parity-check (G-LDPC) error correcting system.
[0016] FIG. 8 is an example performance plot comparing the codeword
failure rate (CFR) for a GLDPC code in a binary symmetric channel
(BSC).
[0017] FIG. 9 illustrates a flowchart of another example method for
improved decoding of a binary linear code.
DETAILED DESCRIPTION
[0018] Generalized low-density parity-check (GLDPC) codes are
constructed by replacing some or all of the single parity-check
(SPC) constraint nodes in the Tanner graph of a conventional LDPC
code by more powerful generalized constraint nodes, where the
generalized constraint nodes can correspond to any linear block
code with block length K. The sub-code associated with each
generalized constraint node is referred to as a component code.
GLDPC codes are suitable for low complexity message passing
decoding like conventional LDPC codes, and have many potential
advantages including a large minimum distance, good iterative
decoding performance, and fast decoding convergence. Examples of
component codes that are widely used include Hamming codes.
[0019] FIGS. 1-6 overview a non-volatile memory system in which
embodiments of the disclosed technology may be implemented.
[0020] FIG. 1 is a block diagram of an example of a memory system
100 implemented based on some embodiments of the disclosed
technology. The memory system 100 includes a memory module 110 that
can be used to store information for use by other electronic
devices or systems. The memory system 100 can be incorporated
(e.g., located on a circuit board) in other electronic devices and
systems. Alternatively, the memory system 100 can be implemented as
an external storage device such as a USB flash drive and a
solid-state drive (SSD).
[0021] The memory module 110 included in the memory system 100 can
include memory areas (e.g., memory arrays) 102, 104, 106, and 108.
Each of the memory areas 102, 104, 106, and 108 can be included in
a single memory die or in multiple memory dice. The memory die can
be included in an integrated circuit (IC) chip.
[0022] Each of the memory areas 102, 104, 106, and 108 includes a
plurality of memory cells. Read, program, or erase operations can
be performed on a memory unit basis. Thus, each memory unit can
include a predetermined number of memory cells. The memory cells in
a memory area 102, 104, 106, or 108 can be included in a single
memory die or in multiple memory dice.
[0023] The memory cells in each of memory areas 102, 104, 106, and
108 can be arranged in rows and columns in the memory units. Each
of the memory units can be a physical unit. For example, a group of
a plurality of memory cells can form a memory unit. Each of the
memory units can also be a logical unit. For example, the memory
unit can be a bank, block, or page that can be identified by a
unique address such as bank address, block address, and page basis
address. During a read or write operation, the unique address
associated with a particular memory unit can be used to access that
particular memory unit. Based on the unique address, information
can be written to or retrieved from one or more memory cells in
that particular memory unit.
[0024] The memory cells in the memory areas 102, 104, 106, and 108
can include non-volatile memory cells. Examples of non-volatile
memory cells include flash memory cells, phase change memory (PRAM)
cells, magnetoresistive random-access memory (MRAM) cells, or other
types of non-volatile memory cells. In an example implementation
where the memory cells are configured as NAND flash memory cells,
the read or write operation can be performed on a page basis.
However, an erase operation in a NAND flash memory is performed on
a block basis.
[0025] Each of the non-volatile memory cells can be configured as a
single-level cell (SLC) or multiple-level memory cell. A
single-level cell can store one bit of information per cell. A
multiple-level memory cell can store more than one bit of
information per cell. For example, each of the memory cells in the
memory areas 102, 104, 106, and 108 can be configured as a
multi-level cell (MLC) to store two bits of information per cell, a
triple-level cell (TLC) to store three bits of information per
cell, or a quad-level cells (QLC) to store four bits of information
per cell. In another example, each of the memory cells in memory
area 111 can be configured to store at least one bit of information
(e.g., one bit of information or multiple bits of information), and
each of the memory cells in memory area 112 can be configured to
store more than one bit of information.
[0026] As shown in FIG. 1, the memory system 100 includes a
controller module 120. The controller module 120 includes a memory
interface 121 to communicate with the memory module 110, a host
interface 126 with communicate with a host (not shown), a processor
124 to executes firmware-level code, and caches and memories 122
and 123 to temporarily or persistently store executable
firmware/instructions and associated information. In some
implementations, the controller unit 120 can include an error
correction engine 125 to perform error correction operation on
information stored in the memory module 110. Error correction
engine 122 can be configured to detect/correct single bit error or
multiple bit errors. In another implementation, error correction
engine 125 can be located in the memory module 110.
[0027] The host can be a device or a system that includes one or
more processors that operate to retrieve data from the memory
system 100 or store or write data into the memory system 100. In
some implementations, examples of the host can include a personal
computer (PC), a portable digital device, a digital camera, a
digital multimedia player, a television, and a wireless
communication device.
[0028] In some implementations, the controller module 120 can also
include a host interface 126 to communicate with the host. Host
interface 126 can include components that comply with at least one
of host interface specifications, including but not limited to,
Serial Advanced Technology Attachment (SATA), Serial Attached Small
Computer System Interface (SAS) specification, Peripheral Component
Interconnect Express (PCIe).
[0029] FIG. 2 illustrates an example of a memory cell array
implemented based on some embodiments of the disclosed
technology.
[0030] In some implementations, the memory cell array can include
NAND flash memory array that is partitioned into many blocks, and
each block contains a certain number of pages. Each block includes
a plurality of memory cell strings, and each memory cell string
includes a plurality of memory cells.
[0031] In some implementations where the memory cell array is NAND
flash memory array, read and write (program) operations are
performed on a page basis, and erase operations are performed on a
block basis. All the memory cells within the same block must be
erased at the same time before performing a program operation on
any page included in the block. In an implementation, NAND flash
memories may use an even/odd bit-line structure. In another
implementation, NAND flash memories may use an all-bit-line
structure. In the even/odd bit-line structure, even and odd
bit-lines are interleaved along each word-line and are
alternatively accessed so that each pair of even and odd bit-lines
can share peripheral circuits such as page buffers. In all-bit-line
structure, all the bit-lines are accessed at the same time.
[0032] FIG. 3 illustrates an example of threshold voltage
distribution curves in a multi-level cell device, wherein the
number of cells for each program/erase state is plotted as a
function of the threshold voltage. As illustrated therein, the
threshold voltage distribution curves include the erase state
(denoted "ER" and corresponding to "11") with the lowest threshold
voltage, and three program states (denoted "P1", "P2" and "P3"
corresponding to "01", "00" and "10", respectively) with read
voltages in between the states (denoted by the dotted lines). In
some embodiments, each of the threshold voltage distributions of
program/erase states has a finite width because of differences in
material properties across the memory array.
[0033] In writing more than one data bit in a memory cell, fine
placement of the threshold voltage levels of memory cells is needed
because of the reduced distance between adjacent distributions.
This is achieved by using incremental step pulse program (ISPP),
i.e., memory cells on the same word-line are repeatedly programmed
using a program-and-verify approach with a stair case program
voltage applied to word-lines. Each programmed state associates
with a verify voltage that is used in verify operations and sets
the target position of each threshold voltage distribution
window.
[0034] Read errors can be caused by distorted or overlapped
threshold voltage distribution. An ideal memory cell threshold
voltage distribution can be significantly distorted or overlapped
due to, e.g., program and erase (P/E) cycle, cell-to-cell
interference, and data retention errors, which will be discussed in
the following, and such read errors may be managed in most
situations by using error correction codes (ECC).
[0035] FIG. 4 illustrates an example of ideal threshold voltage
distribution curves 410 and an example of distorted threshold
voltage distribution curves 420. The vertical axis indicates the
number of memory cells that has a particular threshold voltage
represented on the horizontal axis.
[0036] For n-bit multi-level cell NAND flash memory, the threshold
voltage of each cell can be programmed to 2.sup.n possible values.
In an ideal multi-level cell NAND flash memory, each value
corresponds to a non-overlapping threshold voltage window.
[0037] Flash memory P/E cycling causes damage to a tunnel oxide of
floating gate of a charge trapping layer of cell transistors, which
results in threshold voltage shift and thus gradually degrades
memory device noise margin. As P/E cycles increase, the margin
between neighboring distributions of different programmed states
decreases and eventually the distributions start overlapping. The
data bit stored in a memory cell with a threshold voltage
programmed in the overlapping range of the neighboring
distributions may be misjudged as a value other than the original
targeted value.
[0038] FIG. 5 illustrates an example of a cell-to-cell interference
in NAND flash memory. The cell-to-cell interference can also cause
threshold voltages of flash cells to be distorted. The threshold
voltage shift of one memory cell transistor can influence the
threshold voltage of its adjacent memory cell transistor through
parasitic capacitance-coupling effect between the interfering cell
and the victim cell. The amount of the cell-to-cell interference
may be affected by NAND flash memory bit-line structure. In the
even/odd bit-line structure, memory cells on one word-line are
alternatively connected to even and odd bit-lines and even cells
are programmed ahead of odd cells in the same word-line. Therefore,
even cells and odd cells experience different amount of
cell-to-cell interference. Cells in all-bit-line structure suffer
less cell-to-cell inference than even cells in the even/odd
bit-line structure, and the all-bit-line structure can effectively
support high-speed current sensing to improve the memory read and
verify speed.
[0039] The dotted lines in FIG. 5 denote the nominal distributions
of P/E states (before program interference) of the cells under
consideration, and the "neighbor state value" denotes the value
that the neighboring state has been programmed to. As illustrated
in FIG. 5, if the neighboring state is programmed to P1, the
threshold voltage distributions of the cells under consideration
shift by a specific amount. However, if the neighboring state is
programmed to P2, which has a higher threshold voltage than P1,
that results in a greater shift compared to the neighboring state
being P1. Similarly, the shift in the threshold voltage
distributions is greatest when the neighboring state is programmed
to P3.
[0040] FIG. 6 illustrates an example of a retention error in NAND
flash memory by comparing normal threshold-voltage distribution and
shifted threshold-voltage distribution. The data stored in NAND
flash memories tend to get corrupted over time and this is known as
a data retention error. Retention errors are caused by loss of
charge stored in the floating gate or charge trap layer of the cell
transistor. Due to wear of the floating gate or charge trap layer,
memory cells with more program erase cycles are more likely to
experience retention errors. In the example of FIG. 6, comparing
the top row of voltage distributions (before corruption) and the
bottom row of distributions (contaminated by retention error)
reveals a shift to the left.
[0041] In some embodiments, the retention errors described in FIGS.
5 and 6 (due to, for example, P/E cycling) require using the GLDPC
to ensure that the bits that are stored are read correctly.
However, implementing the component codes of a GLDPC using SISO
algorithms typically entails a high level of complexity.
Embodiments of the disclosed technology reduce the complexity of a
SISO Hamming decoder, while maintaining good error correction
performance, by selecting the decoding output candidates using both
the syndrome and the channel information.
[0042] FIG. 7 illustrates an example high level block diagram of an
error correcting system 700, in accordance with some embodiments of
the present disclosure. In the example, GLDPC codes are described
in connection with data storage. However, the embodiments of the
present disclosure are not limited as such. Instead, the
embodiments similarly apply to other usage of GLDPC codes
including, for example, data transmission.
[0043] As illustrated in FIG. 7, a GLDPC encoder 710 receives
information bits that include data which is desired to be stored in
a storage system 720. GLDPC encoded data (e.g., GLDPC codewords) is
output by the GLDPC encoder 710 and is written to the storage
720.
[0044] In various embodiments, the storage 720 may include a
variety of storage types or media such as (e.g., magnetic) disk
drive storage, Flash storage, etc. In some embodiments, the
techniques are employed in a transceiver and instead of being
written to or read from storage, the data is transmitted and
received over a wired and/or wireless channel. In this case, the
errors in the received codeword may be introduced during
transmission of the codeword.
[0045] When the stored data is requested or otherwise desired
(e.g., by an application or user which stored the data), a detector
730 receives data from the storage system 720. The received data
may include some noise or errors. The detector 730 performs
detection on the received data and outputs decision and/or
reliability information. For example, a soft output detector
outputs reliability information and a decision for each detected
bit. On the other hand, a hard output detector outputs a decision
on each bit without providing corresponding reliability
information. As an example, a hard output detector may output a
decision that a particular bit is a "1" or a "0" without indicating
how certain or sure the detector is in that decision. In contrast,
a soft output detector outputs a decision and reliability
information associated with the decision. In general, a reliability
value indicates how certain the detector is in a given
decision.
[0046] The decision and/or reliability information is passed to a
GLDPC decoder 740 which performs GLDPC decoding using the decision
and reliability information. A soft input decoder utilizes both the
decision and the reliability information to decode the codeword. A
hard decoder utilizes only the decision values in the decoder to
decode the codeword. In many situations, the decoding is iterative
and the GLDPC decoder 740 includes a plurality of constituent
decoders, each of which may be a hard decoder or a soft decoder.
For example, a BCH decoder can be used for hard decoding each
constituent codeword when such constituent codewords are BCH codes.
In addition or in the alternative, a maximum a posteriori
probability (MAP) decoder can be used for soft decoding each of the
constituent codewords. The decoded bits generated by the GLDPC
decoder 740 are passed to the appropriate entity (e.g., the user or
application which requested it). With proper encoding and decoding,
the information bits match the decoded bits.
[0047] In an example, consider a Hamming code with code length n,
information length k, and minimum distanced. The parameter
satisfies [n, k, d]=[2.sup.m-1, 2.sup.m-1-m, m], where m is a
positive integer. In the context of FIG. 7, a Hamming codeword is
denoted as c, which the transmitter sends as the modulated codeword
x over the additive white Gaussian noise (AWGN) channel. The
received signal is y=x+n, where n is the Gaussian noise. The
receiver then estimates the bit sequence "{tilde over (c)}" from
the received signal y. In some embodiments, this may be achieved by
calculating the the syndrome based on y. For a fixed syndrome, all
the possible error patterns are collected, and then the most M
possible error patterns which are closest neighbors of the received
signal y are chosen.
[0048] In an example, a (7,4,3) Hamming code is used to elucidate
certain features and aspects of the disclosed technology. It is
assumed that the syndrome of a distorted bit sequence of a
(7,4,3)-Hamming code is z=(0 0 1). Thus, the possible error
patterns are collected in matrix E with element e.sub.i,j:
0 0 0 0 0 0 1 1 0 1 0 0 0 0 0 1 0 0 0 1 0 0 0 0 1 1 0 0 1 1 0 0 1 0
0 1 0 0 1 0 1 0 0 1 1 1 0 0 0 0 0 1 0 1 1 0 ##EQU00001##
[0049] Then, for all the error patterns (each row in the error
pattern matrix E), the nearest M neighbors of the received signal y
are chosen instead of choosing the error patterns with the least
hard error numbers. In an example, the M neighbors with the
smallest non-Hamming distance are selected. In an example, any
distance metric that excludes the Hamming distance may be used to
select the nearest M neighbors. If the channel (e.g., 720 in FIG.
7) between the encoder and the decoder is the AWGN channel, then
the distance used to select the nearest M neighbors is the
Euclidean distance. If the channel is non-Gaussian, then the
distance is a non-Euclidean distance but excludes the Hamming
distance.
[0050] In some embodiments, the reliability for each error pattern
in the matrix is calculated. In an example, the reliability may be
the probability, log likelihood rate (LLR), and so on. If the
reliability is larger than a first threshold Ts, it is chosen as a
decode-output candidate, and its corresponding error patterns in
matrix E is marked. In an example, when the sum of the reliability
of all the candidates is larger than a second threshold Ta, the
search is terminated, and the set of M error patterns to be
analyzed has been ascertained.
[0051] In an example, let P.sub.i denote the probability values
multiplied column-wise for the given error pattern of every row i.
Among these M error patterns, the probability is calculated as:
P . i = j { P ( c .about. j = c j | y j ) if e i , j = 0 1 - P ( c
.about. j = c j | y j ) if e i , j = 1 . ##EQU00002##
[0052] And after normalization, the probability is computed as:
P i = P . i / ( i ' P . i ' ) . ##EQU00003##
[0053] For the j-th bit, the probability P.sub.i of x=1 of all the
patterns is summed, and for the i-th row, the following probability
is computed:
P ^ ( x j = + 1 | y ) = i e i , j = c ~ j P i . ##EQU00004##
[0054] And subsequently, the log likelihood ratio (LLR) can be
computed as:
L L R = log ( P ^ ( x j = 1 ) 1 - P ^ ( x j = 1 ) ) .
##EQU00005##
[0055] FIG. 8 is an example performance plot comparing the codeword
failure rate (CFR) for a GLDPC code in a binary symmetric channel
(BSC). In the example illustrated in FIG. 8, the GLDPC component
code is a (63, 57) Hamming code with M=652. As seen therein, the
performance of the disclosed technology (with circle markers) with
quadratic complexity approaches the performance of the MAP
algorithm (with triangle markers) with exponential complexity. The
GLDPC that selects error patterns with the least hard errors (with
square markers) has the same complexity as some of the described
herein, but worse performance.
[0056] FIG. 9 illustrates a flowchart of a method 900 for improved
decoding of a binary linear code. The method 900 includes, at
operation 910, receiving a noisy codeword. In an example, a noisy
codeword is a codeword that has been affected circuit-level noise,
which may be additive white Gaussian noise (AWGN) or non-Gaussian,
or a combination thereof. In another example, the noisy codeword is
a codeword that has been contaminated with errors, thereby
resulting in a value of one of the bits flipping from "1" to "0",
or vice versa.
[0057] The method 900 includes, at operation 920, computing a
syndrome based on the noisy codeword.
[0058] The method 900 includes, at operation 930, identifying N
error patterns that correspond to the syndrome.
[0059] The method 900 includes, at operation 940, selecting M error
patterns from the N error patterns, wherein M.ltoreq.N are positive
integers, wherein M is less than or equal to N, wherein a distance
between a codeword corresponding to each of the M error patterns
and the noisy codeword is less than a distance between a codeword
corresponding to any other error pattern and the noisy codeword,
and wherein the distance excludes a Hamming distance.
[0060] The method 900 includes, at operation 950, modifying the
noisy codeword based on each of the M error patterns
one-at-a-time.
[0061] The method 900 includes, at operation 960, decoding the
modified noisy codeword one-at-a-time until a successful decoding
is achieved.
[0062] In some embodiments, the binary linear code is a (n, k, d)
Hamming code that is a component code of a generalized low-density
parity-check (G-LDPC) code. In an example, n=7, k=3 and d=3.
[0063] In some embodiments, the distance is a Euclidean distance.
In other embodiments, the distance is a non-Euclidean distance that
is different from the Hamming distance.
[0064] In some embodiments, selecting the M error patterns from the
N error patterns comprises the operations of calculating a
reliability metric for each of the N error patterns; and selecting
an error pattern of the M error patterns based on a comparison of
the reliability metric of the error pattern and a first threshold.
In an example, the reliability metric is a log-likelihood ratio. In
another example, the reliability metric is a probability. In yet
another example, selecting the M error patterns from the N error
patterns is further based on a comparison of a sum of the
reliability metric of each of the M error patterns and a second
threshold.
[0065] In some embodiments, M is a predetermined value. In an
example, if two codewords have the same reliability metric and only
one of them can be included in the M error patterns, one of the two
codewords may be chosen randomly. Alternatively, the codeword with
the smaller (or greater) index may be selected. In other
embodiments, M can be increased to select both codewords with the
same reliability metric.
[0066] In some embodiments, the value of M may be selected to
trade-off performance for complexity, and vice versa. For example,
using a higher value of M improves performance since more candidate
error patterns are analyzed, but this increases complexity.
Alternatively, using a smaller M will reduce the computational
complexity, but may results in slightly degraded performance if the
correct error pattern is not selected.
[0067] In some embodiments, achieving the successful decoding
corresponds to a successful read operation in a non-volatile
memory. In an example, the non-volatile memory is a NAND flash
memory.
[0068] Implementations of the subject matter and the functional
operations described in this patent document can be implemented in
various systems, digital electronic circuitry, or in computer
software, firmware, or hardware, including the structures disclosed
in this specification and their structural equivalents, or in
combinations of one or more of them. Implementations of the subject
matter described in this specification can be implemented as one or
more computer program products, i.e., one or more modules of
computer program instructions encoded on a tangible and
non-transitory computer readable medium for execution by, or to
control the operation of, data processing apparatus. The computer
readable medium can be a machine-readable storage device, a
machine-readable storage substrate, a memory device, a composition
of matter effecting a machine-readable propagated signal, or a
combination of one or more of them. The term "data processing unit"
or "data processing apparatus" encompasses all apparatus, devices,
and machines for processing data, including by way of example a
programmable processor, a computer, or multiple processors or
computers. The apparatus can include, in addition to hardware, code
that creates an execution environment for the computer program in
question, e.g., code that constitutes processor firmware, a
protocol stack, a database management system, an operating system,
or a combination of one or more of them.
[0069] A computer program (also known as a program, software,
software application, script, or code) can be written in any form
of programming language, including compiled or interpreted
languages, and it can be deployed in any form, including as a
stand-alone program or as a module, component, subroutine, or other
unit suitable for use in a computing environment. A computer
program does not necessarily correspond to a file in a file system.
A program can be stored in a portion of a file that holds other
programs or data (e.g., one or more scripts stored in a markup
language document), in a single file dedicated to the program in
question, or in multiple coordinated files (e.g., files that store
one or more modules, sub programs, or portions of code). A computer
program can be deployed to be executed on one computer or on
multiple computers that are located at one site or distributed
across multiple sites and interconnected by a communication
network.
[0070] The processes and logic flows described in this
specification can be performed by one or more programmable
processors executing one or more computer programs to perform
functions by operating on input data and generating output. The
processes and logic flows can also be performed by, and apparatus
can also be implemented as, special purpose logic circuitry, e.g.,
an FPGA (field programmable gate array) or an ASIC (application
specific integrated circuit).
[0071] Processors suitable for the execution of a computer program
include, by way of example, both general and special purpose
microprocessors, and any one or more processors of any kind of
digital computer. Generally, a processor will receive instructions
and data from a read only memory or a random access memory or both.
The essential elements of a computer are a processor for performing
instructions and one or more memory devices for storing
instructions and data. Generally, a computer will also include, or
be operatively coupled to receive data from or transfer data to, or
both, one or more mass storage devices for storing data, e.g.,
magnetic, magneto optical disks, or optical disks. However, a
computer need not have such devices. Computer readable media
suitable for storing computer program instructions and data include
all forms of nonvolatile memory, media and memory devices,
including by way of example semiconductor memory devices, e.g.,
EPROM, EEPROM, and flash memory devices. The processor and the
memory can be supplemented by, or incorporated in, special purpose
logic circuitry.
[0072] While this patent document contains many specifics, these
should not be construed as limitations on the scope of any
invention or of what may be claimed, but rather as descriptions of
features that may be specific to particular embodiments of
particular inventions. Certain features that are described in this
patent document in the context of separate embodiments can also be
implemented in combination in a single embodiment. Conversely,
various features that are described in the context of a single
embodiment can also be implemented in multiple embodiments
separately or in any suitable subcombination. Moreover, although
features may be described above as acting in certain combinations
and even initially claimed as such, one or more features from a
claimed combination can in some cases be excised from the
combination, and the claimed combination may be directed to a
subcombination or variation of a subcombination.
[0073] Similarly, while operations are depicted in the drawings in
a particular order, this should not be understood as requiring that
such operations be performed in the particular order shown or in
sequential order, or that all illustrated operations be performed,
to achieve desirable results. Moreover, the separation of various
system components in the embodiments described in this patent
document should not be understood as requiring such separation in
all embodiments.
[0074] Only a few implementations and examples are described and
other implementations, enhancements and variations can be made
based on what is described and illustrated in this patent
document.
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