Cyclotomic fast Fourier transform
The cyclotomic fast Fourier transform is a type of fast Fourier transform algorithm over finite fields.[1] This algorithm first decomposes a DFT into several circular convolutions, and then derives the DFT results from the circular convolution results. When applied to a DFT over , this algorithm has a very low multiplicative complexity. In practice, since there usually exist efficient algorithms for circular convolutions with specific lengths, this algorithm is very efficient.[2]
Background
The discrete Fourier transform over finite fields finds widespread application in the decoding of error-correcting codes such as BCH codes and Reed–Solomon codes. Generalized from the complex field, a discrete Fourier transform of a sequence over a finite field GF(pm) is defined as
where is the N-th primitive root of 1 in GF(pm). If we define the polynomial representation of
as
it is easy to see that is simply
. That is, the discrete Fourier transform of a sequence converts it to a polynomial evaluation problem.
Written in matrix format,
Direct evaluation of DFT has an complexity. Fast Fourier transforms are just efficient algorithms evaluating the above matrix-vector product.
Algorithm
First, we define a linearized polynomial over GF(pm) as
is called linearized because
, which comes from the fact that for elements
Notice that is invertible modulo
because
must divide the order
of the multiplicative group of the field
. So, the elements
can be partitioned into
cyclotomic cosets modulo
:
where . Therefore, the input to the Fourier transform can be rewritten as
In this way, the polynomial representation is decomposed into a sum of linear polynomials, and hence is given by
.
Expanding with the proper basis
, we have
where
, and by the property of the linearized polynomial
, we have
This equation can be rewritten in matrix form as , where
is an
matrix over GF(p) that contains the elements
,
is a block diagonal matrix, and
is a permutation matrix regrouping the elements in
according to the cyclotomic coset index.
Note that if the normal basis is used to expand the field elements of
, the i-th block of
is given by:
which is a circulant matrix. It is well known that a circulant matrix-vector product can be efficiently computed by convolutions. Hence we successfully reduce the discrete Fourier transform into short convolutions.
Complexity
When applied to a characteristic-2 field GF(2m), the matrix is just a binary matrix. Only addition is used when calculating the matrix-vector product of
and
. It has been shown that the multiplicative complexity of the cyclotomic algorithm is given by
, and the additive complexity is given by
.[2]
References
- ↑ S.V. Fedorenko and P.V. Trifonov, Fedorenko, S. V.; Trifonov, P. V.. (2003). "On Computing the Fast Fourier Transform over Finite Fields" (PDF). Proceedings of International Workshop on Algebraic and Combinatorial Coding Theory: 108–111.
- 1 2 Wu, Xuebin; Wang, Ying; Yan, Zhiyuan (2012). "On Algorithms and Complexities of Cyclotomic Fast Fourier Transforms Over Arbitrary Finite Fields". IEEE Transactions on Signal Processing 60 (3): 1149–1158. doi:10.1109/tsp.2011.2178844.