# Bluestein's FFT algorithm

Bluestein's FFT algorithm (1968), commonly called the chirp-z algorithm (1969), is a fast Fourier transform (FFT) algorithm that computes the discrete Fourier transform (DFT) of arbitrary sizes (including prime sizes) by re-expressing the DFT as a linear convolution. (The other algorithm for FFTs of prime sizes, Rader's algorithm, also works by rewriting the DFT as a convolution.)

In fact, Bluestein's algorithm can be used to compute more general transforms than the DFT, based on the (unilateral) z-transform (Rabiner et al., 1969).

## Algorithm

Recall that the DFT is defined by the formula

[itex] f_j = \sum_{k=0}^{n-1} x_k e^{-\frac{2\pi i}{n} jk }

If we replace the product jk in the exponent by the identity jk = –(jk)2/2 + j2/2 + k2/2, we thus obtain:

[itex] f_j = e^{-\frac{\pi i}{n} j^2 } \sum_{k=0}^{n-1} \left( x_k e^{-\frac{\pi i}{n} k^2 } \right) e^{\frac{\pi i}{n} (j-k)^2 }

This summation is precisely a linear convolution of the two sequences ak and bk of length n (k = 0,...,n–1) defined by:

[itex]a_k = x_k e^{-\frac{\pi i}{n} k^2 }[itex]
[itex]b_k = e^{\frac{\pi i}{n} k^2 },[itex]

with the output of the convolution multiplied by n phase factors bj*. That is:

[itex] f_j = b_j^* \sum_{k=0}^{n-1} a_k b_{j-k} \qquad j = 0,\dots,n-1. [itex]

This convolution, in turn, can be performed with a pair of FFTs (plus the pre-computed FFT of bk) via the convolution theorem. The key point is that these FFTs are not of the same length n: such a linear convolution can be computed exactly from FFTs only by zero-padding it to a length greater than or equal to 2n–1. In particular, one can pad to a power of two or some other highly composite size, for which the FFT can be efficiently performed by e.g. the Cooley-Tukey algorithm in O(n log n) time. Thus, Bluestein's algorithm provides an O(n log n) way to compute prime-size DFTs, albeit several times slower than the Cooley-Tukey algorithm for composite sizes.

The use of zero-padding for the convolution in Bluestein's algorithm deserves some additional comment. Suppose we zero-pad to a length N ≥ 2n–1. This means that ak is extended to an array Ak of length N, where Ak = ak for 0 ≤ k < n and Ak = 0 otherwise—the usual meaning of "zero-padding". However, because of the bjk term in the linear convolution, both positive and negative values of k are required for bk (noting that bk = bk). The periodic boundaries implied by the DFT of the zero-padded array mean that –k is equivalent to Nk. Thus, bk is extended to an array Bk of length N, where B0 = b0, Bk = BNk = bk for 0 < k < n, and Bk = 0 otherwise. A and B are then FFTed, multiplied pointwise, and inverse FFTed to obtain the linear convolution of a and b, according to the usual convolution theorem.

## z-Transforms

Bluestein's algorithm can also be used to compute a more general transform based on the (unilateral) z-transform (Rabiner et al., 1969). In particular, it can compute any transform of the form:

[itex] f_j = \sum_{k=0}^{n-1} x_k z^{jk}

for an arbitrary complex number z and for differing numbers n and m of inputs and outputs. Given Bluestein's algorithm, such a transform can be used, for example, to obtain a more finely spaced interpolation of some portion of the spectrum (although the frequency resolution is still limited by the total sampling time), enhance arbitrary poles in transfer-function analyses, etcetera.

The algorithm was dubbed the chirp z-transform algorithm because, for the Fourier-transform case (|z| = 1), the sequence bk from above is a complex sinusoid of linearly increasing frequency, which is called a (linear) chirp in radar systems.

## References

• Leo I. Bluestein, "A linear filtering approach to the computation of the discrete Fourier transform," Northeast Electronics Research and Engineering Meeting Record 10, 218-219 (1968).
• Lawrence R. Rabiner, Ronald W. Schafer, and Charles M. Rader, "The chirp z-transform algorithm and its application," Bell Syst. Tech. J. 48, 1249-1292 (1969).
• D. H. Bailey and P. N. Swarztrauber, "The fractional Fourier transform and applications," SIAM Review 33, 389-404 (1991). (Note that this terminology for the z-transform is nonstandard: a fractional Fourier transform conventionally refers to an entirely different, continuous transform.)

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