Description
An \((n,2^k,d)\) linear code is denoted as \([n,k]\) or \([n,k,d]\), where \(d\) is the code's distance. Its codewords form a linear subspace, i.e., for any codewords \(x,y\), \(x+y\) is also a codeword. A code that is not linear is called nonlinear.
Linear codes can be defined in terms of a generator matrix \(G\), whose rows form a basis for the \(k\)-dimensional codespace. Given a message \(x\), the corresponding encoded codeword is \(G^T x\). The generator matrix can be reduced via coordinate permutations to its standard form or systematic form \(G = [I_k~A]\), where \(I_k\) is a \(k\times k\) identity matrix and \(A\) is a \(k \times (n-k)\) binary matrix.
The automorphism group of a linear binary code is the largest subgroup of coordinate permutations that maps the code onto itself. Two linear binary codes are equivalent if the codewords of one code can be mapped into those of the other under a permutation [1; Ch. 8][2; Ch. 3].
Protection
Distance \(d\) of a linear code is the number of nonzero entries in the (nonzero) codeword with the smallest such number. Corrects any error set for which no two elements of the set add up to a codeword.
Linear codes admit a parity check matrix \(H\), whose columns make up a set of parity checks, i.e., a maximal linearly independent set of vectors that are in the kernel of \(G\). It follows that \begin{align} G H^{\text{T}} = 0 \mod 2~. \tag*{(1)}\end{align}
The decision problem corresponding to finding the minimum distance is also \(NP\)-complete [3], and approximating the weight enumerator is \(\#P\)-complete [4].
There are several standard procedures for increasing or decreasing the length of an \([n,k,d]\) code [1; Ch. 1]:
- (1)
Puncturing: removing a coordinate to yield a code whose length is shorter by one and whose distance is \(\geq d-1\).
- (2)
Expurgating: removing odd-weight codewords of a non-even-weight code to yield a code whose dimension is \(k-1\).
- (3)
Augmenting: adding the all-ones codeword to a code without it to yield a code whose dimension is \(k+1\).
- (4)
Lengthening: adding a the all-ones codeword and then adding a parity check to yield a code whose size and dimension increase by one.
- (5)
Shortening: keeping only codewords which have a zero in a fixed coordinate and removing that coordinate to yield a code whose length is shorter by one.
Rate
A family of linear codes \(C_i = [n_i,k_i,d_i]\) is asymptotically good if the asymptotic rate \(\lim_{i\to\infty} k_i/n_i\) and asymptotic distance \(\lim_{i\to\infty} d_i/n_i\) are both positive. Nearly all good linear binary codes for the asymmetric channel are also good for the symmetric channel [5]; this is not the case for non-binary codes [6].
Binary linear codes on \(D\)-dimensional Euclidean lattices are limited by the classical Bravyi-Poulin-Terhal (BPT) bound [7,8], which states that \(d = O(n^{1-1/D})\) and that \(k d^{1/D} = O(n)\) (using asymptotic notation). This bound is the classical analogue of the BPT bound for qubit stabilizer codes and the subsystem BT bound for subsystem qubit stabilizer codes.
Decoding
Decoding an arbitary linear binary code is \(NP\)-complete [9].Slepian's standard-array decoding [10].Recursive maximum likelihood decoding [11].Deep learning [12] and a transformer graph neural net (GNN) for soft decoding [13].Chase decoding, which uses channel measurement information [14].Notes
Tables of bounds and examples of linear codes for various \(n\) and \(k\), extending code tables by A. E. Brouwer [15], are maintained by M. Grassl at this website.Cousins
- Frustration-free Hamiltonian code— Parity-check constraints defining a binary linear code can be encoded into a classical Ising model Hamiltonian, a commuting-projector model whose terms contain produts of Pauli \(Z\) matrices participating in each parity check. Such Ising models are also frustration-free since the codewords satisfy all parity checks.
- Commuting-projector Hamiltonian code— Parity-check constraints defining a binary linear code can be encoded into a classical Ising model Hamiltonian, a commuting-projector model whose terms contain produts of Pauli \(Z\) matrices participating in each parity check. Such Ising models are also frustration-free since the codewords satisfy all parity checks.
- Construction A code— Every binary linear code yields a lattice under Construction A.
- \([n,n-1,2]\) Single parity-check (SPC) code— Any \([n,k,d]\) code with odd distance can be extended to an \([n+1,k,d+1]\) code by adding a bit storing the sum of codeword coordinates.
- Lexicographic code— Binary lexicodes are linear [16].
- Slepian group-orbit code— Any length-\(n\) binary linear code can be used to define a diagonal subgroup of \(n\)-dimensional rotation matrices with \(\pm 1\) on the diagonals via the antipodal mapping \(0\to+1\) and \(1\to-1\). The orbit of this subgroup yields the corresponding Slepian group-orbit code; see [17; Thm. 8.5.2].
- Binary PSK (BPSK) code— Concatenating binary linear codes with BPSK yields a standard way of digitizing the analog AGWN channel [18; Ch. 29].
- Spacetime circuit code— The set of measurement outcomes of a Clifford circuit can be made into a classical binary linear code. Error syndromes of the spacetime circuit code can be used to obtain the parity checks of the outcome code.
- EA qubit stabilizer code— Any linear binary code can be used to construct an EA qubit stabilizer code [19–21].
- Majorana stabilizer code— When constructing a Majorana stabilizer code from a self-orthogonal classical code with an odd number of bits and generator matrix \(G\), a more complex procedure must be applied to ensure that the fermion code has an even number of Majorana zero modes, and thus a physical Hilbert space [22,23]. Rather than taking \(G\) to be the stabilizer matrix as in the even case, we take \(G\oplus G\). This is a concatenation of classical codes as in the CSS construction and it yields a mapping \([2n-1,k,d]\rightarrow [[2n-1,2n-1-k,d^\perp]]_f\). This procedure may be further generalized by concatenating two different self-orthogonal classical codes with an odd number of bits, as is often done in the CSS construction.
- Coherent-parity-check (CPC) code— The CPC Construction uses two binary linear codes.
- Quantum data-syndrome (QDS) code— The QDS code construction employs a particular binary linear code to provide protection against syndrome measurement errors.
- CSS-T code— CSS-T codes are constructed from a pair of linear binary codes via the CSS construction, with the pair satisfying certain conditions [24].
- Hypergraph product (HGP) code— Hypergraph product codes are constructed out of two linear binary codes.
- XYZ product code— The XYZ product code is constructed using a particular hypergraph product of three linear binary codes
- Qubit CSS code— The CSS construction uses two related binary linear codes, \(C_X\) and \(C_Z\).
- Qubit stabilizer code— Qubit stabilizer codes are the closest quantum analogues of binary linear codes because addition modulo two corresponds to multiplication of stabilizers in the quantum case. Any binary linear code can be thought of as a qubit stabilizer code with \(Z\)-type stabilizer generators [26][25; Table I]. The stabilizer generators are extracted from rows of the parity-check matrix, while logical \(X\) Paulis correspond to rows of the generator matrix. States close to the equal superposition of all bitstrings within Hamming distance \(b\) of a binary linear code can be prepared efficiently [27]. Binary linear codes can be used for error-corrected entanglement distillation protocols [28].
- 2D color code— As CSS codes, variants of the 2D color code are constructed out of self-dual classical codes on cubic planar graphs [29].
- Fractal surface code— The fractal product code is a hypergraph product of two classical codes defined on a Sierpinski carpet graph [30].
- Subsystem lifted-product (SLP) code— SLP codes are constructed from a subsystem hypergraph product of a lifted binary linear code.
Primary Hierarchy
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Page edit log
- Victor V. Albert (2022-09-16) — most recent
- Victor V. Albert (2022-03-21)
Cite as:
“Linear binary code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/binary_linear
Github: https://github.com/errorcorrectionzoo/eczoo_data/edit/main/codes/classical/bits/binary_linear.yml.