Near-optimal selection of transmit antennas for a MIMO channel based on Shannon capacity

Current wireless MIMO (multiple transmit and receive antenna) systems are designed with the assumption that the fading channel is estimated perfectly at the receiver while the transmitter has no channel knowledge. If even a small amount of information is fed back to the transmitter, the capacity of the resulting channel increases appreciably. We consider a low-scattering, quasistatic environment where the matrix channel is rank deficient. Previous results (Gore et al. 2000, and Nabar et al. 2000) for such a channel indicate that channel capacity can be increased by a judicious choice of fewer transmit antennas. The optimal subset of transmit antennas is computed by the receiver as the subset that induces the highest Shannon capacity of all subsets of the same cardinality. Here we describe a computationally efficient, near-optimal search technique for the optimal subset based on classical waterpouring. We also provide enhanced search techniques based on partial waterpouring and uniform pourer allocation over the strongest channel modes that outperform waterpouring at high signal to noise ratios.