Random Beamforming with Multi-beam Selection for MIMO Broadcast Channels

Previous work has shown that the capacity region of the Gaussian MIMO broadcast channels is achieved by dirty paper coding (DPC). However, due to high computation complexity of DPC and infeasibility of perfect channel state information (CSI) at the transmitter in many applications, this paper focuses on a reduced complexity transmission scheme named orthonormal random beamforming (ORBF) [16], which only requires partial CSI feedback at the transmitter. Different from the previous work, we analyze the performance of ORBF with moderate number of users and total transmit power constraint. The analysis results show that ORBF scheme is efficient under low SNR. Then we propose a multi-beam selection (MBS) scheme, which selects only the best subset of all the beams to maximize the sum-rate capacity under low SNR. The simulation results show that the proposed MBS scheme achieves great performance improvement when the SNR is low and the number of users is not very large.