Rank-two beamforming for QoS-DiffServ multi-group multicast system based on Alamouti space-time coding

For wireless multicast channels, it is prospective to provide quality of service (QoS) differentiation at physical layer. In this paper, we propose a rank-two transmit beamforming mechanism to support differentiated services (Diffserv) in multi-group multicasting scenarios based on Alamouti space-time coding (ASTC). In QoS-Diffserv multi-group multicast systems, two kinds of users exist, named high-priority users with guaranteed QoS and low-priority users with limited QoS. First the original QoS-Diffserv issue is formulated as a non-convex quadratically constrained quadratic programming (QCQP) program. With the aid of semi-definite relaxation (SDR) technique, we then relax the QCQP problem to a convex semi-definite programming (SDP) problem, which can be easily handled by basic convex optimization tools. To obtain approximate solutions to the original problem which should meet the rank-two constraints, a Gaussian randomization based process and a power control problem are employed together as the last step. Relationship between the QCQP and SDP problems and computational complexity of the proposed algorithm are analysed. Simulation results demonstrate that the proposed rank-two beamforming algorithm could provide near-optimal transmission power under different QoS constraints.