Enhanced Unitary Beamforming Scheme for Limited-Feedback Multiuser MIMO Systems

Manuscript received July 1, 2008. The associate editor coordinating the review of this letter and approving it for publication was R. Blum. The authors are with the School of Electrical Engineering and INMC, Seoul National University (SNU), Seoul, Korea (e-mail: sohnis@mobile.snu.ac.kr).In this letter, we consider practical downlink multiuser multiple-input multiple-output systems where each user has multiple antennas and codebook-based limited channel feedback is available at base station. We propose a preprocessing scheme for downlink transmission where a unitary beamforming matrix is used. Firstly, we propose how to construct the unitary matrix that maximizes sum rate. Secondly, we propose a codebook-based channel feedback method for the proposed beamforming method. Numerical results show that the proposed scheme outperforms conventional zero-forcing beamforming scheme in terms of sum rate.This work was supported in part by the Samsung Advanced Institute of Technology (SAIT) and in part by the Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. (NRL) Program funded by the Ministry of Science and Technology (No. M10300000316-06J0000-31610)