Adaptive focusing: A spatial distortion-adaptive receiver

Abstract

Adaptive focusing is an array processing scheme in which the receiver is designed for adaptive detection of localized sources under conditions where the signal field distribution across the array aperture is distorted from that of a plane wave, but is unknown a priori. Complex propagation conditions, including slow medium fluctuations, coupled with source motion render the signal field non-Gaussian. This property is brought into play in the design of the detector as a maximum-likelihood receiver which actually exploits the slowness of the acoustic channel changes. The problem is to detect the signal in a spatially white Gaussian noise field. The receiver utilizes the spatial structure of the signal as it manifests itself in the maximum eigenvalue of the data sample covariance matrix. Temporal stability of the field distribution and signal-to-noise ratio are important parameters. These are incorporated into a Monte Carlo simulation which shows that for stable signals at threshold level in the output of a conventional beamformer degraded by 5 dB, adaptive focusing can perform within a fraction of 1 dB of a hypothetical ideal beamformer for which the signal field is known a priori. Decreased temporal stability coming about as a result of, say, increased source motion necessarily has an adverse affect on performance.