class galois.BCH

A general \(\textrm{BCH}(n, k)\) code over \(\mathrm{GF}(q)\).

A \(\textrm{BCH}(n, k)\) code is a \([n, k, d]_q\) linear block code with codeword size \(n\), message size \(k\), minimum distance \(d\), and symbols taken from an alphabet of size \(q\).

Shortened codes

To create the shortened \(\textrm{BCH}(n-s, k-s)\) code, construct the full-sized \(\textrm{BCH}(n, k)\) code and then pass \(k-s\) symbols into encode() and \(n-s\) symbols into decode(). Shortened codes are only applicable for systematic codes.

A BCH code is a cyclic code over \(\mathrm{GF}(q)\) with generator polynomial \(g(x)\). The generator polynomial is over \(\mathrm{GF}(q)\) and has \(d-1\) roots \(\alpha^c, \dots, \alpha^{c+d-2}\) when evaluated in \(\mathrm{GF}(q^m)\). The element \(\alpha\) is a primitive \(n\)-th root of unity in \(\mathrm{GF}(q^m)\).

\[g(x) = \textrm{LCM}(m_{\alpha^c}(x), \dots, m_{\alpha^{c+d-2}}(x))\]

Examples

Construct a binary \(\textrm{BCH}(15, 7)\) code.

In [1]: bch = galois.BCH(15, 7); bch
Out[1]: <BCH Code: [15, 7, 5] over GF(2)>

In [2]: GF = bch.field; GF
Out[2]: <class 'galois.GF(2)'>

Encode a message.

In [3]: m = GF.Random(bch.k); m
Out[3]: GF([1, 1, 0, 0, 1, 1, 0], order=2)

In [4]: c = bch.encode(m); c
Out[4]: GF([1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1], order=2)

Corrupt the codeword and decode the message.

# Corrupt the first symbol in the codeword
In [5]: c[0] ^= 1; c
Out[5]: GF([0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1], order=2)

In [6]: dec_m = bch.decode(c); dec_m
Out[6]: GF([1, 1, 0, 0, 1, 1, 0], order=2)

In [7]: np.array_equal(dec_m, m)
Out[7]: True

Instruct the decoder to return the number of corrected symbol errors.

In [8]: dec_m, N = bch.decode(c, errors=True); dec_m, N
Out[8]: (GF([1, 1, 0, 0, 1, 1, 0], order=2), 1)

In [9]: np.array_equal(dec_m, m)
Out[9]: True

Constructors

BCH(n: int, k: int | None = None, d: int | None = None, ...)

Constructs a general \(\textrm{BCH}(n, k)\) code over \(\mathrm{GF}(q)\).

String representation

__repr__() → str

A terse representation of the BCH code.

__str__() → str

A formatted string with relevant properties of the BCH code.

Methods

decode(codeword: ArrayLike, ...) → FieldArray
decode(...) → tuple[galois._fields._array.FieldArray, int | numpy.ndarray]

Decodes the codeword \(\mathbf{c}\) into the message \(\mathbf{m}\).

detect(codeword: ArrayLike) → bool | numpy.ndarray

Detects if errors are present in the codeword \(\mathbf{c}\).

encode(message: ArrayLike, ...) → FieldArray

Encodes the message \(\mathbf{m}\) into the codeword \(\mathbf{c}\).

Properties

property d : int

The minimum distance \(d\) of the \([n, k, d]_q\) code.

property extension_field : type[FieldArray]

The Galois field \(\mathrm{GF}(q^m)\) that defines the BCH syndrome arithmetic.

property field : type[FieldArray]

The Galois field \(\mathrm{GF}(q)\) that defines the codeword alphabet.

property k : int

The message size \(k\) of the \([n, k, d]_q\) code. This is also called the code dimension.

property n : int

The codeword size \(n\) of the \([n, k, d]_q\) code. This is also called the code length.

property t : int

The error-correcting capability \(t\) of the code.

Attributes

property is_narrow_sense : bool

Indicates if the BCH code is narrow-sense, meaning the roots of the generator polynomial are consecutive powers of \(\alpha\) starting at 1, that is \(\alpha, \dots, \alpha^{d-1}\).

property is_primitive : bool

Indicates if the BCH code is primitive, meaning \(n = q^m - 1\).

property is_systematic : bool

Indicates if the code is systematic, meaning the codewords have parity appended to the message.

Matrices

property G : FieldArray

The generator matrix \(\mathbf{G}\) with shape \((k, n)\).

property H : FieldArray

The parity-check matrix \(\mathbf{H}\) with shape \((n - k, n)\).

Polynomials

property alpha : FieldArray

A primitive \(n\)-th root of unity \(\alpha\) in \(\mathrm{GF}(q^m)\) whose consecutive powers \(\alpha^c, \dots, \alpha^{c+d-2}\) are roots of the generator polynomial \(g(x)\) in \(\mathrm{GF}(q^m)\).

property c : int

The first consecutive power \(c\) of \(\alpha\) that defines the roots \(\alpha^c, \dots, \alpha^{c+d-2}\) of the generator polynomial \(g(x)\).

property generator_poly : Poly

The generator polynomial \(g(x)\) over \(\mathrm{GF}(q)\).

property parity_check_poly : Poly

The parity-check polynomial \(h(x)\).

property roots : FieldArray

The \(d - 1\) roots of the generator polynomial \(g(x)\).