Algebraic geometry code

Algebraic geometry codes, often abbreviated AG codes, are a type of linear code that generalize Reed–Solomon codes. The Russian mathematician V. D. Goppa constructed these codes for the first time in 1982.[1]

History

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The name of these codes has evolved since the publication of Goppa's paper describing them. Historically these codes have also been referred to as geometric Goppa codes;[2] however, this is no longer the standard term used in coding theory literature. This is due to the fact that Goppa codes are a distinct class of codes which were also constructed by Goppa in the early 1970s.[3][4][5]

These codes attracted interest in the coding theory community because they have the ability to surpass the Gilbert–Varshamov bound; at the time this was discovered, the Gilbert–Varshamov bound had not been broken in the 30 years since its discovery.[6] This was demonstrated by Tfasman, Vladut, and Zink in the same year as the code construction was published, in their paper "Modular curves, Shimura curves, and Goppa codes, better than Varshamov-Gilbert bound".[7] The name of this paper may be one source of confusion affecting references to algebraic geometry codes throughout 1980s and 1990s coding theory literature.

Construction

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In this section the construction of algebraic geometry codes is described. The section starts with the ideas behind Reed–Solomon codes, which are used to motivate the construction of algebraic geometry codes.

Reed–Solomon codes

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Algebraic geometry codes are a generalization of Reed–Solomon codes. Constructed by Irving Reed and Gustave Solomon in 1960, Reed–Solomon codes use univariate polynomials to form codewords, by evaluating polynomials of sufficiently small degree at the points in a finite field  .[8]

Formally, Reed–Solomon codes are defined in the following way. Let  . Set positive integers  . Let  The Reed–Solomon code   is the evaluation code 

Codes from algebraic curves

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Goppa observed that   can be considered as an affine line, with corresponding projective line  . Then, the polynomials in   (i.e. the polynomials of degree less than   over  ) can be thought of as polynomials with pole allowance no more than   at the point at infinity in  .[6]

With this idea in mind, Goppa looked toward the Riemann–Roch theorem. The elements of a Riemann–Roch space are exactly those functions with pole order restricted below a given threshold,[9] with the restriction being encoded in the coefficients of a corresponding divisor. Evaluating those functions at the rational points on an algebraic curve   over   (that is, the points in   on the curve  ) gives a code in the same sense as the Reed-Solomon construction.

However, because the parameters of algebraic geometry codes are connected to algebraic function fields, the definitions of the codes are often given in the language of algebraic function fields over finite fields.[10] Nevertheless, it is important to remember the connection to algebraic curves, as this provides a more geometrically intuitive method of thinking about AG codes as extensions of Reed-Solomon codes.[9]

Formally, algebraic geometry codes are defined in the following way.[10] Let   be an algebraic function field,   be the sum of   distinct places of   of degree one, and   be a divisor with disjoint support from  . The algebraic geometry code   associated with divisors   and   is defined as  More information on these codes may be found in both introductory texts[6] as well as advanced texts on coding theory.[10][11]

Examples

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Reed-Solomon codes

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One can see that

 

where   is the point at infinity on the projective line   and   is the sum of the other  -rational points.

One-point Hermitian codes

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The Hermitian curve is given by the equation considered over the field  .[2] This curve is of particular importance because it meets the Hasse–Weil bound with equality, and thus has the maximal number of affine points over  .[12] With respect to algebraic geometry codes, this means that Hermitian codes are long relative to the alphabet they are defined over.[13]

The Riemann–Roch space of the Hermitian function field is given in the following statement.[2] For the Hermitian function field   given by   and for  , the Riemann–Roch space   is where   is the point at infinity on  .

With that, the one-point Hermitian code can be defined in the following way. Let   be the Hermitian curve defined over  .

Let   be the point at infinity on  , and  be a divisor supported by the   distinct  -rational points on   other than  .

The one-point Hermitian code   is

 

References

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  1. ^ Goppa, Valerii Denisovich (1982). "Algebraico-geometric codes". Izvestiya Rossiiskoi Akademii Nauk. Seriya Matematicheskaya. 46 (4): 726–781 – via Russian Academy of Sciences, Steklov Mathematical Institute of Russian.
  2. ^ a b c Stichtenoth, Henning (1988). "A note on Hermitian codes over GF(q^2)". IEEE Transactions on Information Theory. 34 (5): 1345–1348 – via IEEE.
  3. ^ Goppa, Valery Denisovich (1970). "A new class of linear error-correcting codes". Probl. Inf. Transm. 6: 300–304.
  4. ^ Goppa, Valerii Denisovich (1972). "Codes Constructed on the Base of (L,g)-Codes". Problemy Peredachi Informatsii. 8 (2): 107–109 – via Russian Academy of Sciences, Branch of Informatics, Computer Equipment and.
  5. ^ Berlekamp, Elwyn (1973). "Goppa codes". IEEE Transactions on Information Theory. 19 (5): 590–592 – via IEEE.
  6. ^ a b c Walker, Judy L. (2000). Codes and Curves. American Mathematical Society. p. 15. ISBN 0-8218-2628-X.
  7. ^ Tsfasman, Michael; Vladut, Serge; Zink, Thomas (1982). "Modular curves, Shimura curves, and Goppa codes better than the Varshamov-Gilbert bound". Mathematische Nachrichten.
  8. ^ Reed, Irving; Solomon, Gustave (1960). "Polynomial codes over certain finite fields". Journal of the Society for Industrial and Applied Mathematics. 8 (2): 300–304 – via SIAM.
  9. ^ a b Hoholdt, Tom; van Lint, Jacobus; Pellikaan, Ruud (1998). "Algebraic geometry codes" (PDF). Handbook of coding theory. 1 (Part 1): 871–961 – via Elsevier Amsterdam.
  10. ^ a b c Stichtenoth, Henning (2009). Algebraic function fields and codes (2nd ed.). Springer Science & Business Media. pp. 45–65. ISBN 978-3-540-76878-4.
  11. ^ van Lint, Jacobus (1999). Introduction to coding theory (3rd ed.). Springer. pp. 148–166. ISBN 978-3-642-63653-0.
  12. ^ Garcia, Arnoldo; Viana, Paulo (1986). "Weierstrass points on certain non-classical curves". Archiv der Mathematik. 46: 315–322 – via Springer.
  13. ^ Tiersma, H.J. (1987). "Remarks on codes from Hermitian curves". IEEE Transactions on Information Theory. 33 (4): 605–609 – via IEEE.