Performance metrics for windows used in real-time DFT-based multiple-tone frequency excision

The capability of direct-sequence spread-spectrum receivers to reject narrowband interference can be significantly improved by eliminating narrowband energy in the received signal through a real-time discrete Fourier transform (RT-DFT) process. However, the loss in received signal strength due to this frequency excision process can be significant. In this paper, we present a theoretical framework for evaluating the performance of alternative combinations of time weighting functions (windows), fractions of overlap in overlap-and-add architectures, and frequency-domain mapping algorithms. The sensitivity loss due to time weighting is presented for variable overlaps and several different windows. A set of window metrics is defined that provides a means of calculating distortion losses for an arbitrary number of interfering tones with uniformly distributed center frequencies. Theoretical results are confirmed by simulation. These results can be used to compare sensitivities of alternative RT-DFT frequency excision direct-sequence spread-spectrum systems and to calculate the standoff range of a finite number of in-band tone emitters