4 (i) Let C be a linear code in IF. Explain what is meant when we say that C is cyclic. Give also the algebraic characterisation of cyclic codes using the ring
(ii) Explain why the cyclic codes in R are in 1-1 correspondence with the monic polynomials in IFq[xJ that divide ? 1. Give the definition of the generator polynomial of a cyclic code in R.
(iii) Let g E lFq[x] be the generator polynomial of a cyclic code C c R. Show that
is a basis of C, where r is the degree of g and, for f E Fq[xJ, f denotes the residue class of f in R.
(iv) Let C and g be as in part (ii). Find a generator matrix of C in terms of the coefficients of g.
(v) Given that
in IF2 [x], write down a generator matrix-and & parity check matrix fo a binary cyclic code of length 7 and dimension 4.
END OF PAPER
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