Multiuser eigenmode transmission for MIMO broadcast channels with limited feedback

We consider the problem of communicating in a MIMO broadcast channel (a point-to-multipoint system where both the transmitter and receivers employ multiple antennas) where the transmitter has imperfect knowledge of the MIMO channels due to limited feedback from the receivers. Our proposal is based on a low-complexity technique known as multiuser eigenmode transmission (MET) which has been shown to achieve near-optimum performance using a zero-forcing type of beamforming under perfect channel state information at transmitter. In the case of limited feedback, each user receives residual interbeam interference, and we propose to mitigate its effect at each receiver with spatial MMSE processing. We compare the sum rate performance of the proposed MET with partial channel state information (CSI) technique with a baseline and with the ideal MET that has perfect channel knowledge at the transmitter. We show that the proposal provides performance gains over die baseline for a moderate number of feedback bits and differently from the baseline, when the feedback rate increases, it approaches the ideal MET sum rate over the entire range of SNR conditions considered.