  A multiantenna multiuser broadcast channel with transmitter beamforming and
user selection is considered. Different from the conventional works, we
consider imperfect channel state information (CSI) which is a practical
scenario for multiuser broadcast channels. We propose a robust regularized
zero-forcing (RRZF) beamforming at the base station. Then we show that the RRZF
outperforms zero-forcing (ZF) and regularized ZF (RZF) beamforming even as the
number of users grows to infinity. Simulation results validate the advantage of
the proposed robust RZF beamforming.
