Investigation on Beamforming Control Methods in Base Station Cooperative Multiuser MIMO Using Block-Diagonalized Beamforming Matrix

This paper investigates a beamforming control method in base station (BS) cooperative multiuser multiple-input multiple-output (MIMO) transmission assuming a block-diagonalized unitary beamforming matrix. More specifically, we comprehensively compare open-loop type random (opportunistic) beamforming control to codebook-based closed-loop type beamforming control. The proportional fair (PF)-based scheduling is assumed, and the number of users, codebook size, and delay time of the feedback signal from the user terminals are taken into account in the comparison. Based on numerical results, codebook-based beamforming control with a large codebook size achieves higher system throughput than random beamforming when the number of users is small and the feedback delay time is short. However, we show that when the number of users is large and the feedback delay time is long, random beamforming achieves higher system-level throughput than codebook-based beamforming. This implies that the transmission-rate control with random beamforming is more robust against channel variation than that with codebook-based beamforming. These results suggest that random beamforming, which requires a lower feedback overhead than the codebook-based beamforming, is a promising beamforming strategy in future radio access.