Long-Term General Rank Multiuser Downlink Beamforming with Shaping Constraints Using QOSTBC

Abstract

This paper addresses multiuser downlink beamforming with shaping constraints under the assumption that the transmitter has long-term covariance based channel state information (CSI). Beamformers are designed to maximize the minimum average signal-to-interference-plus-noise ratio (SINR) of users subject to a total transmit power constraint and additional shaping constraints. We combine beamforming with full-rate quasi-orthogonal space time block coding (QOSTBC) to increase the number of beamforming weight vectors and associated degrees of freedom much beyond the limits achieved by the Alamouti code in the beamformer design. The use of QOSTBC destroys the full-orthogonality structure of the corresponding equivalent channel matrix such that generally maximum-likelihood (ML) pairwise decoding has to be applied for optimal decoding. As an alternative to the pairwise decoding, we propose a simple phase rotation scheme on beamformers at the transmitter side that enables simplified symbol-wise decoding. The original beamforming problem is transformed to a semidefinite programming (SDP) problem which can be solved optimally for a massive number of shaping constraints. Simulation results demonstrate a significant performance improvement over the existing approaches.