Orthogonal Beamforming in SDMA Downlink with Limited Feedback

On a multi-antenna downlink channel, separation of multiple users by transmit beamforming enables simultaneous transmission from the base station to the users, resulting in very high throughput. This paper proposes and analyzes a practical scheme for joint orthogonal beamforming and scheduling, which is enabled by feedback of quantized channel state information. In this approach, the base station generates multiple sets of orthogonal beamforming vectors and associates each vector with a specific user. Subsequently, the base station selects one set of beamforming vectors and the associated users for downlink transmission such that the throughput is maximized. Unlike the optimal approach, the proposed scheme has computational complexity that increases only linearly with the number of users since it does not require an exhaustive search over all possible subsets of users.