79 research outputs found
Design and Analysis of Graph-based Codes Using Algebraic Lifts and Decoding Networks
Error-correcting codes seek to address the problem of transmitting information efficiently and reliably across noisy channels. Among the most competitive codes developed in the last 70 years are low-density parity-check (LDPC) codes, a class of codes whose structure may be represented by sparse bipartite graphs. In addition to having the potential to be capacity-approaching, LDPC codes offer the significant practical advantage of low-complexity graph-based decoding algorithms. Graphical substructures called trapping sets, absorbing sets, and stopping sets characterize failure of these algorithms at high signal-to-noise ratios. This dissertation focuses on code design for and analysis of iterative graph-based message-passing decoders. The main contributions of this work include the following: the unification of spatially-coupled LDPC (SC-LDPC) code constructions under a single algebraic graph lift framework and the analysis of SC-LDPC code construction techniques from the perspective of removing harmful trapping and absorbing sets; analysis of the stopping and absorbing set parameters of hypergraph codes and finite geometry LDPC (FG-LDPC) codes; the introduction of multidimensional decoding networks that encode the behavior of hard-decision message-passing decoders; and the presentation of a novel Iteration Search Algorithm, a list decoder designed to improve the performance of hard-decision decoders.
Adviser: Christine A. Kelle
Topology-Aware Exploration of Energy-Based Models Equilibrium: Toric QC-LDPC Codes and Hyperbolic MET QC-LDPC Codes
This paper presents a method for achieving equilibrium in the ISING
Hamiltonian when confronted with unevenly distributed charges on an irregular
grid. Employing (Multi-Edge) QC-LDPC codes and the Boltzmann machine, our
approach involves dimensionally expanding the system, substituting charges with
circulants, and representing distances through circulant shifts. This results
in a systematic mapping of the charge system onto a space, transforming the
irregular grid into a uniform configuration, applicable to Torical and Circular
Hyperboloid Topologies. The paper covers fundamental definitions and notations
related to QC-LDPC Codes, Multi-Edge QC-LDPC codes, and the Boltzmann machine.
It explores the marginalization problem in code on the graph probabilistic
models for evaluating the partition function, encompassing exact and
approximate estimation techniques. Rigorous proof is provided for the
attainability of equilibrium states for the Boltzmann machine under Torical and
Circular Hyperboloid, paving the way for the application of our methodology.
Practical applications of our approach are investigated in Finite Geometry
QC-LDPC Codes, specifically in Material Science. The paper further explores its
effectiveness in the realm of Natural Language Processing Transformer Deep
Neural Networks, examining Generalized Repeat Accumulate Codes,
Spatially-Coupled and Cage-Graph QC-LDPC Codes. The versatile and impactful
nature of our topology-aware hardware-efficient quasi-cycle codes equilibrium
method is showcased across diverse scientific domains without the use of
specific section delineations.Comment: 16 pages, 29 figures. arXiv admin note: text overlap with
arXiv:2307.1577
Information-Coupled Turbo Codes for LTE Systems
We propose a new class of information-coupled (IC) Turbo codes to improve the
transport block (TB) error rate performance for long-term evolution (LTE)
systems, while keeping the hybrid automatic repeat request protocol and the
Turbo decoder for each code block (CB) unchanged. In the proposed codes, every
two consecutive CBs in a TB are coupled together by sharing a few common
information bits. We propose a feed-forward and feed-back decoding scheme and a
windowed (WD) decoding scheme for decoding the whole TB by exploiting the
coupled information between CBs. Both decoding schemes achieve a considerable
signal-to-noise-ratio (SNR) gain compared to the LTE Turbo codes. We construct
the extrinsic information transfer (EXIT) functions for the LTE Turbo codes and
our proposed IC Turbo codes from the EXIT functions of underlying convolutional
codes. An SNR gain upper bound of our proposed codes over the LTE Turbo codes
is derived and calculated by the constructed EXIT charts. Numerical results
show that the proposed codes achieve an SNR gain of 0.25 dB to 0.72 dB for
various code parameters at a TB error rate level of , which complies
with the derived SNR gain upper bound.Comment: 13 pages, 12 figure
Spatially-Coupled QDLPC Codes
Spatially-coupled (SC) codes is a class of convolutional LDPC codes that has
been well investigated in classical coding theory thanks to their high
performance and compatibility with low-latency decoders. We describe toric
codes as quantum counterparts of classical two-dimensional spatially-coupled
(2D-SC) codes, and introduce spatially-coupled quantum LDPC (SC-QLDPC) codes as
a generalization. We use the convolutional structure to represent the parity
check matrix of a 2D-SC code as a polynomial in two indeterminates, and derive
an algebraic condition that is both necessary and sufficient for a 2D-SC code
to be a stabilizer code. This algebraic framework facilitates the construction
of new code families. While not the focus of this paper, we note that small
memory facilitates physical connectivity of qubits, and it enables local
encoding and low-latency windowed decoding. In this paper, we use the algebraic
framework to optimize short cycles in the Tanner graph of 2D-SC HGP codes that
arise from short cycles in either component code. While prior work focuses on
QLDPC codes with rate less than 1/10, we construct 2D-SC HGP codes with small
memory, higher rates (about 1/3), and superior thresholds.Comment: 25 pages, 7 figure
Analysis and Design of Partially Information- and Partially Parity-Coupled Turbo Codes
In this paper, we study a class of spatially coupled turbo codes, namely
partially information- and partially parity-coupled turbo codes. This class of
codes enjoy several advantages such as flexible code rate adjustment by varying
the coupling ratio and the encoding and decoding architectures of the
underlying component codes can remain unchanged. For this work, we first
provide the construction methods for partially coupled turbo codes with
coupling memory and study the corresponding graph models. We then derive
the density evolution equations for the corresponding ensembles on the binary
erasure channel to precisely compute their iterative decoding thresholds.
Rate-compatible designs and their decoding thresholds are also provided, where
the coupling and puncturing ratios are jointly optimized to achieve the largest
decoding threshold for a given target code rate. Our results show that for a
wide range of code rates, the proposed codes attain close-to-capacity
performance and the decoding performance improves with increasing the coupling
memory. In particular, the proposed partially parity-coupled turbo codes have
thresholds within 0.0002 of the BEC capacity for rates ranging from to
, yielding an attractive way for constructing rate-compatible
capacity-approaching channel codes.Comment: 15 pages, 13 figures. Accepted for publication in IEEE Transactions
on Communication
A Combinatorial Methodology for Optimizing Non-Binary Graph-Based Codes: Theoretical Analysis and Applications in Data Storage
Non-binary (NB) low-density parity-check (LDPC) codes are graph-based codes that are increasingly being considered as a powerful error correction tool for modern dense storage devices. Optimizing NB-LDPC codes to overcome their error floor is one of the main code design challenges facing storage engineers upon deploying such codes in practice. Furthermore, the increasing levels of asymmetry incorporated by the channels underlying modern dense storage systems, e.g., multi-level Flash systems, exacerbates the error floor problem by widening the spectrum of problematic objects that contributes to the error floor of an NB-LDPC code. In a recent research, the weight consistency matrix (WCM) framework was introduced as an effective combinatorial NB-LDPC code optimization methodology that is suitable for modern Flash memory and magnetic recording (MR) systems. The WCM framework was used to optimize codes for asymmetric Flash channels, MR channels that have intrinsic memory, in addition to canonical symmetric additive white Gaussian noise channels. In this paper, we provide an in-depth theoretical analysis needed to understand and properly apply the WCM framework. We focus on general absorbing sets of type two (GASTs) as the detrimental objects of interest. In particular, we introduce a novel tree representation of a GAST called the unlabeled GAST tree, using which we prove that the WCM framework is optimal in the sense that it operates on the minimum number of matrices, which are the WCMs, to remove a GAST. Then, we enumerate WCMs and demonstrate the significance of the savings achieved by the WCM framework in the number of matrices processed to remove a GAST. Moreover, we provide a linear-algebraic analysis of the null spaces of WCMs associated with a GAST. We derive the minimum number of edge weight changes needed to remove a GAST via its WCMs, along with how to choose these changes. Additionally, we propose a new set of problematic objects, namely oscillating sets of type two (OSTs), which contribute to the error floor of NB-LDPC codes with even column weights on asymmetric channels, and we show how to customize the WCM framework to remove OSTs. We also extend the domain of the WCM framework applications by demonstrating its benefits in optimizing column weight 5 codes, codes used over Flash channels with soft information, and spatially-coupled codes. The performance gains achieved via the WCM framework range between 1 and nearly 2.5 orders of magnitude in the error floor region over interesting channels
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