Optimized Nonuniform FFTs and Their Application to Array Factor Computation

We deal with developing an optimized approach for implementing nonuniform fast Fourier transform (NUFFT) algorithms under a general and new perspective for 1-D transformations. The computations of nonequispaced results, nonequispaced data, and Type-3 nonuniform discrete Fourier transforms are tackled in a unified way. They exploit “uniformly sampled” exponentials to interpolate the “nonuniformly sampled” ones involved in the nonuniform discrete Fourier transforms (NUFDTs), so as to enable the use of standard fast Fourier transforms, and an optimized window. The computational costs and the memory requirements are analyzed, and their convenient performance is assessed also by comparing them with other approaches in the literature. Numerical results demonstrate that the method is more accurate and does not introduce any additional computational or memory burden. The computation of the window functions amounts to that of a Legendre polynomial expansion, i.e., a simple polynomial evaluation. This is convenient in terms of computational burden and of the proper arrangement of the calculations. A case study of electromagnetic interest has been carried out by applying the developed NUFFTs to the radiation of linear regular or irregular arrays onto a set of regular or irregular spectral points. Guidelines for multidimensional extension of the proposed approach are also presented.