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Unformatted text preview: 88 Chapter 7 Codes over Subfields In Chapter 6 we looked at various general methods for constructing new codes from old codes. Here we concentrate on two more specialized techniques that result from writing the field F as a vector space over its subfield K . We will start with linear codes over F and finish with linear codes over K . Of particular practical importance is the case with K = F 2 . Our work on generalized Reed Solomon codes over F has given us many powerful codes, but by Theorem 5.1.1 their length is bounded by  F  . Binary generalized ReedSolomon codes are rendered trivial. 7.1 Basics Let dim K ( F ) = m , and choose e 1 ,...e m to be a Kbasis for F . We define the map φ : F→ K m given by φ ( α ) = ( a 1 ,...a m ) where α = a 1 e 1 + ··· + a m e m . For brevity, we shall write ˆ α for the 1 × m row vector φ ( α ) and ˇ α for its transpose φ ( α ) > = ( a 1 ,...a m ) > , an m × 1 column vector. We extend this notation to any p × q matrix A ∈ F p,q , with i,j entry a i,j by letting b A ∈ K p,mq be the matrix ˆ a 1 , 1 ˆ a 1 , 2 ··· ˆ a 1 ,j ··· ˆ a 1 ,q ˆ a 2 , 1 ˆ a 2 , 2 ··· ˆ a 2 ,j ··· ˆ a 1 ,q . . . . . . . . . . . . . . . . . . ˆ a i, 1 ˆ a i, 2 ··· ˆ a i,j ··· ˆ a 1 ,q . . . . . . . . . . . . . . . . . . ˆ a p, 1 ˆ a p, 2 ··· ˆ a p,j ··· ˆ a p,q 89 90 CHAPTER 7. CODES OVER SUBFIELDS and ˇ A ∈ K mp,q be the matrix ˇ a 1 , 1 ˇ a 1 , 2 ··· ˇ a 1 ,j ··· ˇ a 1 ,q ˇ a 2 , 1 ˇ a 2 , 2 ··· ˇ a 2 ,j ··· ˇ a 1 ,q . . . . . . . . . . . . . . . . . . ˇ a i, 1 ˇ a i, 2 ··· ˇ a i,j ··· ˇ a 1 ,q . . . . . . . . . . . . . . . . . . ˇ a p, 1 ˇ a p, 2 ··· ˇ a p,j ··· ˇ a p,q For our constructions, A might be a spanning or control matrix for a linear code over F . Then the matrices b A and ˇ A can be thought of as spanning or control matrices for linear codes over K . It must be emphasized that these maps are highly dependent upon the choice of the initial map φ , even though φ has been suppressed in the notation. We shall see below that a careful choice of φ can be of great help. (The general situation in which φ is an arbitrary injection of F into K p , for some field K and some p , is of further interest. Here we will be concerned with the linear case, but see the Problem 7.2.3 below.) 7.2 Expanded codes If C is a code in F n , then the code b C = { ˆ c  c ∈ C } in K mn is called an expanded code . expanded code (7.2.1) Theorem. If C is an [ n,k,d ] code, then b C is an [ mn,mk, ≥ d ] code....
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This note was uploaded on 06/13/2011 for the course CRYPTO 101 taught by Professor Na during the Spring '11 term at Harding.
 Spring '11
 NA

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