Spatial - Temporal Subband Beamforming for Near Field Adaptive Array Processing

This thesis investigates broadband adaptive beamforming for signal targets located in the near field of an array. The primary application of this research is hands-free sound pickup and speech enhancement for wideband telephony. The technical challenges are three-fold. Broadband beamformers are difficult to design due to large frequency dependent beampattern variations and reduced performances for low frequencies. Near field curvature prohibits the simplified far field assumption and many established far field beamforming techniques are not applicable to near field beamforming. Conventional adaptive beamformers experience desired signal cancellation in reverberant environments where coherent interference is dominant. As a compromise solution to the three problems encountered in near field broadband adaptive beamforming, a Spatial-Temporal Subband (STS) adaptive beamforming structure has been proposed in this thesis. It incorporates a spatial subband array with temporal subband multirate filters and employs a near field adaptive beamformer in each subband. It enables parallel processing of the subband systems, improves the computational efficiency and enhances the performances of the near field broadband beamformers. Three specific STS adaptive beamformers are developed, namely (1) the Nested Array Quadrature Mirror Filter (NAQMF) beamformer which uses a nested array with critically sampled QMF banks and near field Generalized Sidelobe Canceler (GSC) adaptive beamformers, (2) the Nested Array Multirate Generalized Sidelobe Canceler (NAM-GSC) which uses a nested array with non-critically sampled multirate filter banks and near field GSC adaptive beamformers, and (3) the Nested iii Array Switched Beam Adaptive Noise Canceler (NASB-ANC) which incorporates a nested array with non-critically sampled multirate filter banks and near field DelayFilter-and-Sum beamformers followed by adaptive noise cancelers. The three STS systems are shown, via computer simulation and experimental evaluation, to reduce the frequency dependent beampattern variations to the extent which occurs within an octave frequency band. They can achieve higher noise reduction using less adaptive weights than the fullband beamformers. They can improve the convergence of adaptation and reduce the computational complexity. The use of near field beamforming also improves the de-reverberation performance of the STS systems. Several new algorithms are also proposed in the thesis. A simplified implementation is developed for GSC adaptive beamformers to reduce the computational load by 80%. A robust near field GSC design method is developed to improve the robustness of the near field adaptive beamformer against the location errors. A near field Spatial Affine Projection (SAP) algorithm is proposed for adaptive beamformers to suppress coherent interferences and combat desired signal cancellation.