Fast Walsh–Hadamard transform

The fast Walsh–Hadamard transform applied to a vector of length 8

File:1010 0110 Walsh spectrum (fast WHT).svg

Example for the input vector (1,0,1,0,0,1,1,0)

In computational mathematics, the Hadamard ordered fast Walsh–Hadamard transform (FWHT_h) is an efficient algorithm to compute the Walsh–Hadamard transform (WHT). A naive implementation of the WHT would have a computational complexity of O($N^2$). The FWHT_h requires only $N \log N$ additions or subtractions.

The FWHT_h is a divide and conquer algorithm that recursively breaks down a WHT of size $N$ into two smaller WHTs of size $N/2$. This implementation follows the recursive definition of the $2N \times 2N$ Hadamard matrix $H_N$:

$H_N = \frac{1}{\sqrt 2} \begin{pmatrix} H_{N-1} & H_{N-1} \\ H_{N-1} & -H_{N-1} \end{pmatrix}.$

The $1/\sqrt2$ normalization factors for each stage may be grouped together or even omitted.

The sequency ordered, also known as Walsh ordered, fast Walsh–Hadamard transform, FWHT_w, is obtained by computing the FWHT_h as above, and then rearranging the outputs.

See also

• Fast Fourier transform

External links

• Charles Constantine Gumas, [1]