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A subspace method for channel estimation is proposed for asymmetric antenna array systems. The so-called pilot contamination problem reported in is found to be due to the linearity of channel estimation in. The authors show that it does not occur in cellular systems with power control and power-controlled handoff when the nonlinear channel estimation method proposed in this paper is used. Power-control hand-off is needed to guarantee separability between signal and interference subspaces. They derive the transmission conditions for subspace separability based on free probability and perturbation theory.
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