Wideband, frequency-domain beamforming for coherent signal processing

Abstract

Coherent detection processors for sonar (e.g., matched filters for active systems) require continuous time series as input and have conventionally been associated with delay-and-sum time-domain beamformers. As an alternative, we have extended frequency-domain beamforming techniques to retain the coherence of wideband signals and have demonstrated efficient software for computing continuous time-series beams for flexible postprocessing. Using computer-generated “acoustic-array” data, the beamformer was shown to preserve the envelope, spectrum, and correlation properties of signals while yielding near theoretical performance in reducing beam side-lobe levels. Advantages over a conventional delay-and-sum beamformer are: (1) elimination of the need for high input sampling rates to achieve acceptable beam patterns—the frequency-domain approach is insensitive to the sampling rate provided it exceeds the (band-pass) Nyquist rate; (2) reduction of high-beam side-lobe levels caused by malfunctioning array elements—the response of missing array elements is readily estimated in the frequency domain by interpolating from the spectra of neighboring sensors; (3) minimization of hardware requirements—all stages of signal processing (bandpass filtering, holefixing, beamforming, time-line reconstruction, and matched filtering) can be efficiently performed in a general purpose array processor.