Adaptive beamforming for multipath environments

Abstract

Coherent multipaths present a significant challenge to most adaptive beamformers because they violate the common assumption of a rank-one plane wave or geometrically focused signal. When the multipath arrivals that characterize shallow water propagation are resolvable by the array’s aperture, the mismatch between the assumed and the true signal spatial structure causes signal suppression. If the amplitude and phase relationships among the various multipaths were known, they could, in principle, be included in a matched field beamforming approach. This is usually impractical due to inadequate knowledge of propagation parameters, especially bottom characteristics, and source/receiver motion. A generalization of the standard MVDR approach, called multirank MVDR, assumes only that the signal lies in a subspace of multiple rank rather than the usual rank-one assumption. An example of a subspace is a small fan of beams that cover the potential multipath directions. The signal may be rank one corresponding to fully coherent multipath or higher rank corresponding to incoherent or partially coherent multipath. The multirank approach is applied to the shallow water multipath problem and compared with the related technique of employing multiple linear constraints. Realistic simulations of alternative beamforming approaches for a large horizontal array are presented. [Work supported by ONR.]