Opportunistic Beamforming with Precoder Diversity in Multi-User MIMO Systems

In this paper, we propose a dynamic multi-user MIMO beamforming and scheduling scheme, where co- scheduled users are served by mutually orthogonal beams. Our scheme differs from the traditional opportunistic beamforming and scheduling scheme in two ways. Firstly, multiple sets of orthogonal beams are employed as candidate beamforming directions (instead of one as in the traditional opportunistic beamforming case) and only users whose preferred beams are within the same set of orthogonal beams can be co-scheduled. This can effectively improve co-scheduled users´ SINR. Secondly, the number of co-scheduled users is variable, which will change as the SNR and/or the total number of users in the system change in order to achieve a higher throughput. In addition, some analytical results are provided for the interference limited regime (i.e. at high SNR). In particular, for a reasonably large number of users, the sum-rate of our scheme grows logarithmically as the total number of users increases and by introducing multiple sets of orthogonal beams (or what we call precoder diversity), a gain in sum rate can be achieved.