Antenna arrays are widely used in radar systems and mobile networks in the physical layer to mitigate interference as well as to enhance the reliability and the capacity of the system. The performance of beamforming algorithms depends on the channel realisation and its associated micro-parameters (angle of arrivals, multi-path components, fading) plus the array geometry. The multiplicity of these micro-parameters makes it difficult to establish a tractable characterisation of the overall performance. In this Letter, a new parameter, an algebraic angle cosine between the signal subspace and the interference subspace, is introduced to characterise the interference impact. It allows the derivation of a lower bound on the signal-to-interference-plus noise ratio of the Capon beamformer. Outside two regions where the interferers are quasi-orthogonal or quasi-aligned with the desired user, the lower bound decreases linearly with this parameter.
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