In this correspondence, we propose a coherent source model to precisely describe the variations of output powers of beamformers. Based upon these output powers, we propose an effective signal-to-interference-and-noise-ratio (SINRt) to analyze the performance of the beamformer for the case of a single interference. For the finite number of data snapshots, we employ Fisher's Z transformation to derive an effective correlation of the sources. We found that the performance of the beamformer versus the number of snapshots can be approximately estimated by this effective correlation. Monte Carlo simulations are demonstrated to support the proposed theoretical developments.
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