Quasi-cyclic codes. Jay A. Wood. Algebra for Secure and Reliable Communications Modeling Morelia, Michoacán, Mexico October 12, 2012

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1 Quasi-cyclic codes Jay A. Wood Department of Mathematics Western Michigan University jwood/ Algebra for Secure and Reliable Communications Modeling Morelia, Michoacán, Mexico October 12, 2012

2 Introduction This will be an elementary introduction to cyclic and quasi-cyclic codes from the point of view of ring theory. At least I hope so! JW (WMU) Quasi-cyclic codes October 12, / 27

3 Cyclic codes Let A be an alphabet. The shift operator on A n is the map T : A n A n given by (a 0, a 1,..., a n 2, a n 1 ) (a n 1, a 0, a 1,..., a n 2 ). A linear code C A n is a cyclic code if T (C) C. That is, the shift of any codeword is again a codeword. JW (WMU) Quasi-cyclic codes October 12, / 27

4 Examples A = F 2, n = 7 Let C be the code spanned by the row vectors: T of row 1 is row 2; T of row 2 is row 3. T of row 3 is the sum of rows 1 and 3. Use linearity in general. JW (WMU) Quasi-cyclic codes October 12, / 27

5 Viewing codewords as polynomials Suppose the alphabet is a finite commutative ring R. View a codeword as a polynomial: (a 0, a 1,..., a n 2, a n 1 ) a 0 + a 1 x + a 2 x a n 1 x n 1 JW (WMU) Quasi-cyclic codes October 12, / 27

6 Shift as multiplication by x In the 1950s, Prange observed: if we consider the polynomials modulo x n 1, then the shift operator corresponds to multiplication by x. x(a 0 + a 1 x + a 2 x a n 1 x n 1 ) = a 0 x + a 1 x a n 2 x n 1 + a n 1 x n a n 1 + a 0 x + a 1 x a n 2 x n 1 mod (x n 1) (a n 1, a 0, a 1,..., a n 2 ) JW (WMU) Quasi-cyclic codes October 12, / 27

7 Cyclic codes as ideals This establishes an isomorphism R n = R[x]/(x n 1) as (free) R-modules such that the shift operator T corresponds to multiplication by x in the ring structure of R[x]/(x n 1). Linear cyclic codes in R n correspond to ideals in R[x]/(x n 1). JW (WMU) Quasi-cyclic codes October 12, / 27

8 Structure of R[x]/(x n 1) For R a finite commutative ring, about all we can say about R[x]/(x n 1) is that it too is a finite commutative ring, of order R n. Every finite commutative ring splits (as rings) as the direct sum of local rings. If R[x]/(x n 1) = R i, with R i local, then the ideals of R[x]/(x n 1) are direct sums of ideals of the R i. Structure of ideals of local rings is worth studying in general. We look at a special example. JW (WMU) Quasi-cyclic codes October 12, / 27

9 Finite fields Let R = F q, a finite field. F q [x] is a principal ideal ring. The ideals of F q [x]/(x n 1) correspond to the ideals of F q [x] that contain the ideal (x n 1). An ideal (g) F q [x] contains (x n 1) iff the polynomial g divides x n 1. JW (WMU) Quasi-cyclic codes October 12, / 27

10 Factoring x n 1 In the principal ideal ring F q [x] there is unique factorization into irreducibles. Factor x n 1 = f s 1 1 f s 2 2 f s k k, where the f i are distinct monic irreducible polynomials. The s i are positive integers. JW (WMU) Quasi-cyclic codes October 12, / 27

11 Chinese remainder theorem There is a natural ring homomorphism F q [x] k (x n 1) i=1 F q [x] (f s i i ), given by reduction mod f s i i. The Chinese remainder theorem (CRT) says that this homomorphism is an isomorphism. (Exercise.) Ideals on the left (cyclic codes) are sums of ideals from the right. JW (WMU) Quasi-cyclic codes October 12, / 27

12 Examples Let q = 2, so that = +. x 2 1 = (x + 1) 2 x 3 1 = (x + 1)(x 2 + x + 1) x 4 1 = (x + 1) 4 x 5 1 = (x + 1)(x 4 + x 3 + x 2 + x + 1) x 6 1 = (x + 1) 2 (x 2 + x + 1) 2 x 7 1 = (x + 1)(x 3 + x + 1)(x 3 + x 2 + 1) x 8 1 = (x + 1) 8 JW (WMU) Quasi-cyclic codes October 12, / 27

13 Multiplicity one When does x n 1 factor over F q into distinct irreducibles, all of multiplicity one? This happens when n, q are relatively prime. q is a unit in Z/nZ, so q l 1 mod n for some smallest positive integer l. Then n (q l 1). There is a cyclic n-subgroup in the multiplicative group of F q l, so x n 1 splits into distinct linear factors over F q l. Multiply factors in Frobenius orbits to get distinct factors over F q. (Cyclotomic cosets.) JW (WMU) Quasi-cyclic codes October 12, / 27

14 Relatively prime case (a) When gcd(q, n) = 1, x n 1 factors as x n 1 = f 1 f 2 f k, distinct irreducibles. Chinese remainder theorem gives F q [x] k (x n 1) i=1 F q [x] (f i ). The rings on the right are all field extensions of F q, because the f i are irreducible. JW (WMU) Quasi-cyclic codes October 12, / 27

15 Relatively prime case (b) The only ideals in a field are 0 and the field itself. Ideals on the left (cyclic codes) are generated by g of the form g = f δ 1 1 f δ 2 2 f δ k k, where each δ i = 0 or 1. There are 2 k such cyclic codes. Write then down for q = 2, n = 7 (k = 3). JW (WMU) Quasi-cyclic codes October 12, / 27

16 General case for fields The Chinese remainder theorem gives F q [x] k (x n 1) i=1 F q [x] (f s i i ). The rings F q [x]/(f s i i ) are chain rings, because the ideals of F q [x]/(f s i i ) correspond to ideals of F q [x] that contain (f s i i ). That is, to (g) where g f s i i. Since f i is irreducible, g = f j i i for j i s i. There are k i=1 (s i + 1) such cyclic codes. JW (WMU) Quasi-cyclic codes October 12, / 27

17 Examples q = 2, n = 4 Over F 2, x 4 1 = (x + 1) 4. For g = (x + 1) j, here are the first rows of the cyclic codes. j (x + 1) j first row x x x + x 2 + x x JW (WMU) Quasi-cyclic codes October 12, / 27

18 Examples q = 2, n = 6 Over F 2, x 6 1 = (x + 1) 2 (x 2 + x + 1) 2. There are now 3 2 = 9 cyclic codes. First rows: jk first row JW (WMU) Quasi-cyclic codes October 12, / 27

19 Quasi-cyclic codes Work over F q, and suppose n = lm. A linear code C F n q = F lm q is quasi-cyclic of index l or l-quasi-cyclic if T l (C) C. Example: q = 2, l = 2, m = 4, n = 8. All the codewords (left column is a quasi-cyclic subcode): JW (WMU) Quasi-cyclic codes October 12, / 27

20 Quasi-cyclic codes as codes over a ring Set R = F q [x]/(x m 1). Label a vector in F lm q a = (a 00, a 01,..., a 0,l 1, a 10, a 11,..., a 1,l 1,..., a m 1,0, a m 1,1,..., a m 1,l 1 ). by Set A j = m 1 i=0 a ijx i F q [x]. Map F lm q R l by a (A 0, A 1,..., A l 1 ). Then l-quasi-cyclic codes correspond to R-linear codes in R l. JW (WMU) Quasi-cyclic codes October 12, / 27

21 Work of Ling and Solé As for cyclic codes, the ring R can be decomposed via the Chinese remainder theorem. This allows R-linear codes in R l to be decomposed into codes over local rings (fields and chain rings, here). Ling and Solé, in a series of papers, , describe the structure of quasi-cyclic codes with coefficients in F q or in chain rings. They describe the dual codes and characterize self-dual codes. JW (WMU) Quasi-cyclic codes October 12, / 27

22 Another direction The ring R = F q [x]/(x m 1) is isomorphic to F q [C m ], the group algebra of the cyclic m-group with coefficients in F q. Write C m multiplicatively, as C m = {e, g, g 2, g 3,..., g m 1 }, with g m = e. An element a F q [C m ] has the form a = m 1 i=0 a ig i, with a i F q. F q [C m ] = F q [x]/(x m 1) by sending g to x. JW (WMU) Quasi-cyclic codes October 12, / 27

23 F 2 + uf 2 Multiply in the ring F 2 + uf 2, with u 2 = 0, by (a 0 + a 1 u)(b 0 + b 1 u) = a 0 b 0 + (a 0 b 1 + a 1 b 0 )u. Set v = 1 + u. Notice that v 2 = 1 + u 2 = 1. Use 1, v as basis instead. Then (c 0 + c 1 v)(d 0 + d 1 v) = c 0 d 0 + (c 0 d 1 + c 1 d 0 )v + c 1 d 1 v 2 = (c 0 d 0 + c 1 d 1 ) + (c 0 d 1 + c 1 d 0 )v. JW (WMU) Quasi-cyclic codes October 12, / 27

24 F 2 + uf 2 = F2 [C 2 ] Compare this with the multiplication in the group algebra F 2 [C 2 ]: (c 0 e + c 1 g)(d 0 e + d 0 g) = c 0 d 0 e + (c 0 d 1 + c 1 d 0 )g + c 1 d 1 g 2 = (c 0 d 0 + c 1 d 1 )e + (c 0 d 1 + c 1 d 0 )g We see that F 2 + uf 2 = F2 [C 2 ]. The same proof works for q = 2 t. (Not true for odd q.) JW (WMU) Quasi-cyclic codes October 12, / 27

25 Maschke s theorem (a) The fact that F q [x]/(x n 1) splits into a sum of fields when gcd(q, n) = 1 is a special case of Maschke s theorem in group representation theory. Suppose k is a field of characteristic p and G is a finite group. If p does not divide the order of G (always true for characteristic zero), then the group algebra k[g] is a semisimple ring (a sum of matrix rings over division algebras over k). JW (WMU) Quasi-cyclic codes October 12, / 27

26 Maschke s theorem (b) For G = C n, the group is abelian. Then F q [x]/(x n 1) = F q [C n ] is a commutative ring. If gcd(q, n) = 1, then Maschke s theorem applies, and F q [x]/(x n 1) splits as a sum of matrix rings. In order to be commutative and finite, the matrix rings must be 1 1, hence just fields (extensions of F q ). JW (WMU) Quasi-cyclic codes October 12, / 27

27 Codes over group algebras This leads one to contemplate codes over group algebras. Compare to group codes in the literature. Even more generally: codes over algebras. By fixing a vector space basis for an algebra R over F q, one can view R-linear codes C R n as F q -codes of length n dim Fq R, with additional symmetry coming from the R-module structure. This area should be wide open for investigation. JW (WMU) Quasi-cyclic codes October 12, / 27

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