Efficient power and beam allocation methods for orthogonal random beamforming in multiuser MIMO systems

Abstract

The orthogonal random beamforming (ORBF) method, which is a multiuser MIMO scheme with partial channel state information (CSI) in downlink transmission, can achieve the sum capacity proportional to the the number of transmit antennas when the number of users is infinity. However, the performance of the ORBF with the finite number of users is degraded due to the multiuser interference. In this dissertation, we propose efficient power and beam allocation methods to improve the performance of ORBF with finite number of users. In the first and second one, using the magnitude information of effective channel gain, transmit power and beam subset are optimally determined as well as selection of optimal user set. In the third one, ORBF is extended to the multiple receive antennas case and the linear decoders are designed to suppress interference in the mobile receivers. First, we propose the optimal transmit power allocation method to maximize the sum throughput under total transmit power constraint. In the optimal method, the transmit power allocation and user set selection are jointly determined. A given user set, transmit power to be assigned to each beam is determined using Lagrange multiplier method. In the suboptimal method, transmit power allocation and user set selection are performed separately to reduce the computational complexity. The transmit power is assigned by the iterative power allocation algorithm. Second, we find the optimal number of random beams by considering the trade-off between spatial multiplexing and multiuser interference. In addition to this, we find the optimal beam subset and user set to obtain the additional selection diversity gain. Moreover, we propose two suboptimal methods to reduce the computational complexity and the feedback overhead of the optimal method. Finally, we extend the ORBF to the multiple receive antennas case. We find the transmitter precoder and the receiver decoder using the property of orthogonal random ...