Adaptive Orthogonal Beamforming for the Mimo Broadcast Channel

This paper addresses both the user selection and the linear precoding design of a MIMO broadcast channel where the number of users is assumed to be greater than the number of transmit antennas. The optimization criterion considered is the sum-rate. A greedy algorithm tackling jointly the two issues is developed. It sequentially adds new users so as their beamformers build up an orthonormal basis which is optimized with the help of the channel knowledge assumed to be exhaustive. This solution benefits asymptotically (in the limit of a large number of users) from the full multiplexing and multiuser diversity gains alike the capacity achieving dirty paper coding. Moreover, it exhibits better results than existing schemes and offers some advantages resulting from the choice of an orthogonal structure.