Comparing Traditional FFT Based Frequency Domain Excision with Poly-Phase Transform Excision

Hana Abusalem, Fred Harris

Abstract:	This work is part of an ONR program to imbed DSP in next generation GPS receivers to mitigate GPS vulnerabilities. We address and compare a number of frequency domain methods for suppressing stationary and slowly sweeping narrow-band interferes. Frequency-domain interference excision techniques use the FFT to identify and excise undesired spectral components. The technique is known as hole-punching, a spectral gating operation that sets to zero those spectral components ex-ceeding specified threshold levels. The FFT of a signal often exhibits high spectral side-lobes due to the finite aperture processed interval. These side-lobes represent spectral splatter and behave much like a broad band interference source by contributing energy to all FFT spectral bins. Suppressing energy in the main-lobe spectral bins in not adequate due to the residual interfer-ence distributed through the spectrum by these side-lobes. Spectral splatter by high side-lobes is traditionally called spectral leakage. We demonstrate that the spectral leakage that survives the hole punching operation severely de-grades the performance of the excision process. Excision performance is improved substantially with a set of algorithms designed to reduce spectral side-lobe levels. We control the spectral side-lobes of the FFT by pre-processing the collected signal prior to the transform. In the spectral analysis community this control is accom-plished by the use of data windows. Windows suppress spectral side-lobes at the expense of main-lobe widening and the rejection of data near the boundaries of the data collection interval. We compensate for this signal rejec-tion by overlapping and processing successive intervals. For windows with good side-lobe rejection, the required overlap is 75%. This overlap seems to compound the processing task by requiring us to perform transforms four times as often. We avoid this increase in workload by processing all four overlapped windows simultaneously. The resulting process is called a poly-phase filter bank, and the combination of the poly-phase preprocessor with the FFT is termed a poly-phase transform.
Published in:	Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999) June 27 - 30, 1999 Royal Sonesta Hotel Cambridge, MA
Pages:	625 - 634
Cite this article:	Abusalem, Hana, Harris, Fred, "Comparing Traditional FFT Based Frequency Domain Excision with Poly-Phase Transform Excision," Proceedings of the 55th Annual Meeting of The Institute of Navigation (1999), Cambridge, MA, June 1999, pp. 625-634.
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