Multi-Antenna Multicasting with Opportunistic Multicast Scheduling and Space-Time Transmission

Physical layer multicasting with opportunistic user selection and optimized space-time transmission is examined in this work. We consider a multi-antenna downlink scenario where a multi-antenna base-station transmits common information to a given set of users, each with a single receive antenna. In the past, most existing multi-antenna multicasting solutions have been restricted to spatial multiplexing and transmit beamforming. Here, we adopt the opportunistic multicast scheduling (OMS) scheme, where messages are encoded using fountain codes and an optimal subset of users is targeted for reception in each time slot. Capitalizing on extreme value theory, we derive analytical expressions for the average system throughput and utilize this result to obtain the optimal user selection ratio for both spatial multiplexing and transmit beamforming scenarios. To better utilize the channel state information available at the transmitter, we further consider an optimized space-time transmission (OST) scheme where the statistics of the space-time codeword is chosen to maximize the rate of the worst selected user. For reduced complexity, we utilize a semi-orthogonal user selection algorithm to determine the spatial dimensions of the signal and derive optimal power allocation across these dimensions. The proposed OST is a generalization of transmit beamforming and spatial multiplexing and, thus, outperforms both conventional schemes.