A frequency-domain beamforming algorithm for wideband, 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 coupled to delay-and-sum time-domain beamformers. As an alternative, we have extended the FFT beamforming technique to be functionally equivalent to a time-domain beamformer. The procedure utilizes the ‘‘analytic’’ representations of the hydrophone signals as input and produces continuous beam time series as output, thereby allowing for complete flexibility in post-beamformer processing. The method will be applicable when the array elements are uniformly spaced, and when a large fraction of the possible beams are to be computed. In such cases, the algorithm offers the following advantages relative to a conventional time-domain beamformer: (1) elimination of the need for a high input sampling rate to achieve acceptable beam patterns—the frequency-domain approach is insensitive to the sampling rate provided this exceeds the (bandpass) Nyquist rate; (2) reduction of high beam sidelobe levels due to dead or malfunctioning array elements—the response of missing array elements is readily estimated in the frequency domain by interpolating from the spectra of neighboring sensors; and (3) potential reductions in the computational load when the array is large or the center-frequency/bandwidth ratio of the signal is high. Using a general purpose array processor we have demonstrated near real-time evaluation of a complete beam set for a 128-channel linear array. Computer-generated ‘‘acoustic’’ data were used to confirm that the software preserves the envelope, spectrum, and temporal correlation properties of signals while yielding near-theoretical performance in reducing beam sidelobe levels.