A Multi-threshold Scheme for Feedback Load Reduction in Multiuser MIMO Broadcast Channel

In a multiuser system, independent time-varying channels among different users can be exploited to provide multiuser diversity (MUD) gain and increase the system throughput. For the downlink channel, this requires the users to feedback their channel state information (CSI) to the base station (BS), which incurs a large aggregate feedback load, especially when the number of users is large. In this paper, we propose a novel scheme to reduce the feedback load in a downlink orthogonal space division multiple access (SDMA) system by allowing the users to dynamically determine the number of feedback bits to use according to multiple thresholds. These thresholds are derived from the statistics of the signal-to-interference-plus-noise ratio (SINR). Through simulation, we demonstrate that the proposed scheme is more efficient than the existing feedback methods, and it achieves the same sum rate growth as the full CSI feedback scheme. Furthermore, a fast bit allocation method is also proposed for the multi-threshold feedback scheme. Simulation results show that this method achieves almost the same sum rate as that achieved by the the optimal bit allocation algorithm.