Feedback Reduction for Multiuser MIMO Broadcast Channel with Zero-Forcing Beamforming

A multi-user multiple-input multiple-output (MU-MIMO) downlink system with zero-forcing beamforming and semi-orthogonal user selection (SUS) scheduling algorithm is considered in this paper. It is well known that in practical implementation of MU-MIMO, the channel state information (CSI) feedback overhead becomes a limiting factor as the number of users to be supported increases. A novel scheme to reduce the feedback load, in the event of large number of users, by using a threshold-based feedback strategy is proposed and studied in this paper. The key feature of the proposed approach uses the concept of order statistics to construct multiple thresholds on the CSI. This paper also proposes a new modified formulation of the expected signal to interference plus noise ratio (SINR) to address the mismatch of analytical sum rate and simulation results. Simulation results show a significant reduction of feedback load when the scheme is used, while the sum rate performance is not compromised.