Rank-two beamforming for single-group multicasting networks using OSTBC

This paper addresses the problem of transmit beamforming for single-group multicasting, where common information is transmitted to a group of users. Beamformers are designed to minimize the total transmitted power subject to quality-of-service (QoS) constraints for all users. In the conventional approach, a single weight vector is computed to jointly satisfy all requirements. However, in the case of a large number of users, the performance of the conventional beamforming methods usually degrades severely due to the limited degrees of freedom for designing spatially selective beamformers. Motivated by this, we propose a novel approach with increased flexibility in which the idea of single-group beamforming is combined with orthogonal space-time block coding (OSTBC). The proposed approach is particularly attractive in combination with the semi-definite relaxation (SDR) technique for which our approach is optimal in the case of rank-one and rank-two SDR solutions whereas the conventional approach is only optimal for rank-one solutions.