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 a plane‐wave distribution, but is unknown a priori. The mechanisms responsible for the distortion, whether simple multipath or more complex propagation conditions, do not play important roles in the scheme. Rather, the emphasis is on estimation of the complex sensor responses using frequency domain processing and subsequent use of the resulting response vector to steer the array and thus maximize its output power for the signal sector of interest. The procedure exploits the spatial structure of the source field as it manifests itself in the eigenstructure of the sample covariance matrix. Whatever the physical mechanism causing the signal field distortion, the eigenstructure is generally such that estimation can be carried out in a unitary space of dimension significantly smaller than the number of sensors in the array. Clearly, temporal stability of the field distribution and signal‐to‐noise ratio are important parameters. Their influence on the detector has been studied and numerical results are presented. Monte Carlo simulation has shown that for threshold signals and under conditions for which a conventional beamformer is degraded by as much as 5 dB, adaptive focusing can perform within a fraction of 1 dB of an ideal beamformer for which the signal field was known a priori. [This work has been supported by the Naval Electronics Systems Command, Code ELEX‐320.]