Active LFM Mixer Adaptive (ALMA) Beamforming.

Abstract : This report describes the ALMA beamforming approach in detail. Results of simulations have been presented that demonstrate the algorithms can adapt to each range-resolution annulus (without time-averaging) and suppress sidelobe interference, while preserving signals in the intended look direction. Methods of implementing spatial-smoothing for both element- and beam-based adaptive algorithms have been covered, as well as methods of applying the approach to long arrays. Extensions of the approach to other array shapes and to replica-correlation-output beamforming have been discussed. ALMA beamforming reduces the signal from each range-resolution cell at each array element to a single phase and amplitude (accomplished by the mixer) and applies minimum- energy adaptive methods to these quantities to adapt (without time- or range- averaging) to the scatter distribution in each range-resolution annulus, This approach is an unique contribution to the field of adaptive beamforming for active signals. Based on results seen to date, ALMA beamforming offers the promise of increasing the effective array gain of modest-sized active receiving arrays to produce an effect equivalent to making the array several time larger