bchgenpoly

Generator polynomial of BCH code

Syntax

genpoly = bchgenpoly(n,k)
genpoly = bchgenpoly(n,k,prim_poly)
genpoly = bchgenpoly(n,k,prim_poly,outputFormat)
[genpoly,t] = bchgenpoly(...)

Description

genpoly = bchgenpoly(n,k) returns the narrow-sense generator polynomial of a BCH code with codeword length n and message length k. The codeword length n must have the form 2m-1 for some integer m between 3 and 16. The output genpoly is a Galois row vector that represents the coefficients of the generator polynomial in order of descending powers. The narrow-sense generator polynomial is LCM[m_1(x), m_2(x), ..., m_2t(x)], where:

• LCM represents the least common multiple,

• m_i(x) represents the minimum polynomial corresponding to αi, α is a root of the default primitive polynomial for the field GF(n+1),

• and t represents the error-correcting capability of the code.

Note

Although the bchgenpoly function performs intermediate computations in GF(n+1), the final polynomial has binary coefficients. The output from bchgenpoly is a Galois vector in GF(2) rather than in GF(n+1).

genpoly = bchgenpoly(n,k,prim_poly) is the same as the syntax above, except that prim_poly specifies the primitive polynomial for GF(n+1) that has Alpha as a root. prim_poly is either a polynomial character vector or an integer whose binary representation indicates the coefficients of the primitive polynomial in order of descending powers. To use the default primitive polynomial for GF(n+1), set prim_poly to [].

genpoly = bchgenpoly(n,k,prim_poly,outputFormat) is the same as the previous syntax, except that outputFormat specifies output data type. The value of outputFormat can be ‘gf' or 'double' corresponding to Galois field and double data types respectively. The default value of outputFormat is 'gf'.

[genpoly,t] = bchgenpoly(...) returns t, the error-correction capability of the code.

Examples

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Create two BCH generator polynomials based on different primitive polynomials.

Set the codeword and message lengths, n and k.

n = 15;
k = 11;

Create the generator polynomial and return the error correction capability, t.

[genpoly,t] = bchgenpoly(15,11)

genpoly = GF(2) array.

Array elements =

1   0   0   1   1
t = 1

Create a generator polynomial for a (15,11) BCH code using a different primitive polynomial expressed as a character vector. Note that genpoly2 differs from genpoly, which uses the default primitive.

genpoly2 = bchgenpoly(15,11,'D^4 + D^3 + 1')

genpoly2 = GF(2) array.

Array elements =

1   1   0   0   1

Limitations

The maximum allowable value of n is 65535.

References

 Peterson, W. Wesley, and E. J. Weldon, Jr., Error-Correcting Codes, 2nd ed., Cambridge, MA, MIT Press, 1972.