Low-density parity-check (LDPC) code

Low-density parity-check (LDPC) code[1–3]

Alternative names: Sparse graph code.

Description

A binary linear code with a sparse parity-check matrix. Alternatively, a member of an infinite family of \([n,k,d]\) codes for which the number of nonzero entries in each row and column of the parity-check matrix are both bounded above by a constant as \(n\to\infty\).

An LDPC code is \((j,k)\)-regular if the parity-check matrix has a fixed number of \(j\) nonzero entries in each row and \(k\) entries in each column; otherwise, the LDPC code is irregular. Irregular LDPC codes are characterized by the fractions \(v_j\) of columns of weight \(j\) as well as likewise fractions \(h_j\) for rows of weight \(j\); these are collectively called the degree distribution.

A parity check is performed by taking the inner product of a row of the parity-check matrix with a codeword that has been affected by a noise channel. A parity check yields either zero (no error) or one (error).

In alternative conventions (not used here), LDPC codes are referred to as simple LDPC codes, as opposed to generalized LDPC codes, alternatively named as Tanner codes.

Rate

Some LDPC codes achieve the Shannon capacity of the binary symmetric channel under maximum-likelihood decoding [1,4,5]. Other LDPC codes achieve capacity for smaller block lengths under belief-propagation decoding [6]. Random LDPC codes achieve list-decoding capacity [7].

Encoding

Almost linear-time encoder based on transforming the parity-check matrix into upper triangular form [8].

Decoding

Message-passing algorithm called belief propagation (BP) [2,9,10] (see also [11–13]).Soft-decision Sum-Product Algorithm (SPA) [2,11,14] and its simplification the Min-Sum Algorithm (MSA) [15].Linear programming [16–18].Iterative LDPC decoders can get stuck at stopping sets of their Tanner graphs [19], with decoder performance improving with the size of the smallest stopping set; see [20; Sec. 21.3.1] for more details. The smallest stopping set size can reach the minimum distance of the code [21].Ensembles of random LDPC codes under iterative decoders are subject to the concentration theorem [11,22]; see [20; Thm. 21.7.1] for the case of the BEC.Reinforcement learning [23].Quantum-enhanced BP decoding [24].

Notes

The potential of LDPC codes was noted by Margulis [25], but realized by the broader community [4,26] much later after their discovery by Gallager [1,2].See books [27–29] and reviews [30,31] for introductions to LDPC codes, belief-propagation decoding, and connections to statistical mechanics. Other introductory references include Refs. [32–35] as well as a review of LDPC codes circa 2005 [36].See Kaiserslautern database [37] for explicit representatives of several classes of LDPC codes, including \(q\)-ary, WiMAX, multi-edge, and spatially-coupled.See pretty-good-codes database [38] for explicit representatives and benchmarking.See Encyclopedia of sparse graph codes for explicit representatives [39]LDPC codes have been considered for quantum key distribution [40].Codes have been benchmarked using AFF3CT toolbox [41].LDPC Python software library for decoding LDPC and QLDPC codes [42,43].

Cousins

Tensor-product code— Tensor products of random LDPC codes are robustly testable [44,45].
Low-density generator-matrix (LDGM) code— The dual of an LDPC code has a sparse generator matrix and is called an LDGM code.
Random code— LDPC codes are often constructed non-determinisitically.
Hamiltonian-based code— There are relations between LDPC codes and statistical mechanical models of spin glasses [27–29,46].
Low-rank parity-check (LRPC) code— LRPC codes are rank-metric analogues of LDPC codes [47].
DNA storage code— LDPC codes are potentially relevant for DNA storage [48].
LDPC convolutional code (LDPC-CC)— LDPC-CCs are convolutional analogues of LDPC codes.
Lechner-Hauke-Zoller (LHZ) code— The LHZ code is an LDPC c-q code designed to convert the long-range interactions of a quantum annealer into local constraints.
Quantum LDPC (QLDPC) code
Concatenated cat code— Cat codes have been concatenated with LDPC codes (treated as qubit stabilizer codes) [49].
Self-correcting quantum code— Linear confinement of QLDPC (LDPC) codes implies (classical) self-correction [50].
Spacetime circuit code— There is an equivalence between Clifford circuits and LDPC codes with bit-check symmetry [51].
EA QLDPC code— There exist necessary and sufficient conditions for an EA QLDPC code consuming \(e=1\) ebit that is obtainable from a pair of LDPC codes [52].
Long-range enhanced surface code (LRESC)— LRESCs are constructed constructed using a hypergraph product of two copies of a concatenated LDPC-repetition seed code.

Member of code lists

Primary Hierarchy

Classical Domain

Binary Kingdom

Binary codeECC

Binary group-orbit codeECC

Linear binary codeLinear \(q\)-ary Linear code over \(\mathbb{Z}_q\) ECC

Parents

Linear binary code

\(q\)-ary LDPC codeTanner Linear \(q\)-ary LRC Distributed-storage ECC

Low-density parity-check (LDPC) code

Children

Tornado code

Tornado codes are LDPC codes that use a highly irregular weight distribution for their underlying graphs [53].

Algebraic LDPC code

Multi-edge LDPC code

Regular LDPC code

References

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[2]: R. Gallager, Low-density parity check codes. 1963. PhD thesis, MIT Cambridge, MA.
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Victor V. Albert (2022-08-17) — most recent
Victor V. Albert (2022-04-25)
Armin Gerami (2022-04-23)

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“Low-density parity-check (LDPC) code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/ldpc

Github: https://github.com/errorcorrectionzoo/eczoo_data/edit/main/codes/classical/bits/tanner/ldpc.yml.