Robust adaptive beamforming in software defined radio with adaptive diagonal loading

Adaptive beamforming techniques, in which the radiation pattern of array of elements can be adaptively changed to reject interference signals, may provide significant performance advantages in tactical radio communication systems. Adaptive beamforming algorithms may be very sensitive to array characteristics and RF/IF imperfections. Differences between receiver channels and antenna element positioning errors cause amplitude and phase imbalances between channels. In a software defined radio where a signal-to-noise-ratio (SNR) estimator is available, an adaptive diagonal loading (ADL) can be used to mitigate these problems. In this paper a minimum variance distortionless response (MVDR) beamformer with the ADL is presented. The harmful effects of amplitude and phase imbalances as well as signal-of-interest (SOI) nulling problem known with MVDR beamformer when direction-of-arrival (DOA) estimation error has occurred are effectively mitigated. Also the problem with small and coherent interference signals can be avoided. Extensive simulation results are presented using an eight-element uniform circular array (UCA).