User:Richard Pinch/sandbox-codes

Reed–Solomon code

A family of linear codes defined over finite fields. Let $F = \mathbf{F}_q$ and put $n = q-1$. Let $\beta$ be a primitive element of $F^*$. For an integer $k \le n$, let $L_k$ denote the vector space of polynomials over $F$ of degree $\le k-1$, and let $E$ be the evaluation map $e : L_k \rightarrow F^n$ given by $$ E : f \mapsto \left({ f(\beta), f(\beta^2), \ldots, f(\beta^n) }\right) \ . $$

The image $E[L_k]$ is a subspace of $F^n$ and constitutes the Reed–Solomon code $\mathrm{RS}(q,k)$. It is a primitive BCH code.

The map $E$ is injective, as a non-zero polynomial of degree $k<n$ cannot be zero at $n$ distinct values. The rank of the code is thus $k$. The minimum weight is $n-k+1$, corresponding to a polynomial with $k-1$ zeroes all in $F$. The Reed–Solomon code thus meets the singleton bound and is an MDS code.

More generally, we may prescribe a set of $n$ evaluation points, $k \le n \le q-1$, distinct elements $\mathbf{x} = (x_1,\ldots,x_n)$ of $F$, and non-zero weights $ \mathbf{y} = (y_i)$. The generalised Reed–Solomon code is defined by the evaluation map $$ E : f \mapsto \left({ y_1 f(x_1), \ldots, y_n f(x_n) }\right) \ . $$ The image is of rank $n$ and minimum weight $n-k+1$: it is denoted $\mathrm{GRS}_k(\mathbf{x}, \mathbf{y})$. The dual code is $\mathrm{GRS}_{n-k}(\mathbf{x}, \mathbf{y'})$ for some $ \mathbf{y'}$.

References

• C.M. Goldie, R.G.E. Pinch, Communication theory, London Mathematical Society Student Texts. 20 Cambridge University Press (1991) ISBN 0-521-40456-8 Zbl 0746.94001
• J.H. van Lint, "Introduction to coding theory" (2nd ed.) Graduate Texts in Mathematics 86 Springer (1992) ISBN 3-540-54894-7 Zbl 0747.94018
• F.J. MacWilliams, N.J.A. Sloane, "The Theory of Error Correcting Codes", (Elsevier, 1977). Zbl 0369.94008
• H. Stichtenoth, "Algebraic function fields and codes", Universitext, Springer (1993) ISBN 3-540-58469-6 Zbl 0816.14011

Singleton bound

A constraint on the parameters of a linear block code. If a code has length $n$, rank $k$ and minimum distance $d$, then $$ k+d \le n+1 \ . $$ The bound is obtained by repeatedly shortening a code $C$: selecting the subcode consisting of all words with the symbol $0$ in a specific position and then deleting that position from all words.

A code for which equality holds is a maximum distance separable or MDS code. Examples of MDS codes are the Reed–Solomon codes, which show that if $n \le q+1$ there are MDS codes over $\mathbf{F}_q$ of rank $k$ for all $k< n$. The dual code to a linear MDS code is again an MDS code.

The MDS conjecture states that an MDS code with rank $k$ over the field of $q$ elements has length $n \le q+1$, except in the case $q$ a power of 2 and $k = 3$ or $q-1$ when $n \le q+2$. These bounds are achieved by Reed--Solomon codes.

References

• C.M. Goldie, R.G.E. Pinch, Communication theory, London Mathematical Society Student Texts. 20 Cambridge University Press (1991) ISBN 0-521-40456-8 Zbl 0746.94001
• J.H. van Lint, "Introduction to coding theory" (2nd ed.) Graduate Texts in Mathematics 86 Springer (1992) ISBN 3-540-54894-7 Zbl 0747.94018
• F.J. MacWilliams, N.J.A. Sloane, "The theory of error-correcting codes. Parts I, II" (3rd repr.) North-Holland Mathematical Library 16 Elsevier (1985) ISBN 0-444-85193-3 Zbl 0657.94010
• H. Stichtenoth, "Algebraic function fields and codes", Universitext, Springer (1993) ISBN 3-540-58469-6 Zbl 0816.14011

BCH code

A cyclic code over a finite field. Fix length $n$ and ground field $\mathbf{F}_q$ and a design distance parameter $\delta$. Let $\beta$ be a primitive $n$-th root of unity in a suitable extension of $\mathbf{F}_q$. The generator of the cyclic code is the least common multiple $g$ of the minimal polynomials (over $\mathbf{F}_q$) of the elements $\beta^1, \beta^2, \ldots, \beta^{\delta-1}$.

The minimum distance of the BCH code generated by $g$ is at least $\delta$: this is the BCH bound.

As an example, let $q=2$ and $\beta$ be a primitive $7$-th root of unity in $\mathrm{F}_{8}$: we may take $\beta$ to satisfy the polynomial $x^3 + x + 1$. Choose $\delta = 3$. The minimal polynomial for $\beta^2$ is the same as that of $\beta$, so that the cyclic code is generated by the word $1101000$. This is in fact the Hamming [7,4] code.

References

• C.M. Goldie, R.G.E. Pinch, Communication theory, London Mathematical Society Student Texts. 20 Cambridge University Press (1991) ISBN 0-521-40456-8 Zbl 0746.94001

Hamming code

A binary block code capable of error correction: see Error-correcting code.

The Hamming $[7,4]$ code has generator matrix with first row $1101000$ and the others being its three right shifts. It is a cyclic code, and indeed a BCH code with design distance $3$. It is perfect, because the balls of radius $1$ about the codewords have eight elements and the 16 balls precisely fill out the 7-dimensional space.

References

• C.M. Goldie, R.G.E. Pinch, Communication theory, London Mathematical Society Student Texts. 20 Cambridge University Press (1991) ISBN 0-521-40456-8 Zbl 0746.94001

Cyclic code

A block code over a finite field for which the code words have cyclic symmetric: any cyclic permutation of a code word is again a code word. A linear code of length $n$ over the field $k$ may be identified with polynomials of degree $< n$ over $k$ in an indeterminate $X$: the cyclic symmetry condition states that the code is invariant under multiplication by $X$ modulo $X^n-1$, so that the code may be identified with an ideal of the quotient ring $k[X]/\langle X^n-1 \rangle$. Since the polynomial ring $k[X]$ is a principal ideal domain, the codes are all principal ideals, and hence are determined by their generators, which must be factors of $X^n-1$.

References

• C.M. Goldie, R.G.E. Pinch, Communication theory, London Mathematical Society Student Texts. 20 Cambridge University Press (1991) ISBN 0-521-40456-8 Zbl 0746.94001