01-05-2013, 02:01 PM
Conventional and Adaptive Beamformers
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This example illustrates how to apply digital beamforming to a narrowband signal received by an antenna array. Three beamforming algorithms are illustrated: the phase shift beamformer (PhaseShift), the minimum variance distortionless response (MVDR) beamformer, and the linearly constrained minimum variance (LCMV) beamformer.
Phase Shift Beamformer
A beamformer can be considered a spatial filter that suppresses the signal from all directions, except the desired ones. A conventional beamformer simply delays the received signal at each antenna so that the signals are aligned as if they arrive at all the antennas at the same time. In the narrowband case, this is equivalent to multiplying the signal received at each antenna by a phase factor. To define a phase shift beamformer pointing to the signal's incoming direction
MVDR Beamformer
To overcome the interference problem, we can use the MVDR beamformer, a popular adaptive beamformer. The MVDR beamformer preserves the signal arriving along a desired direction, while trying to suppress signals coming from other directions. In this case, the desired signal is at the direction 45 degrees in azimuth.
Self Nulling Issue in MVDR
On many occasions, we may not be able to separate the interference from the target signal, and therefore, the MVDR beamformer has to calculate weights using data that includes the target signal. In this case, if the target signal is received along a direction slightly different from the desired one, the MVDR beamformer suppresses it. This occurs because the MVDR beamformer treats all the signals, except the one along the desired direction, as undesired interferences. This effect is sometimes referred to as "signal self nulling".