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For the multiuser multiple-input multiple-output (MIMO) downlink channel, theusers feedback their channel state information (CSI) to help the base station(BS) schedule users and improve the system sum rate. However, this incurs alarge aggregate feedback bandwidth which grows linearly with the number ofusers. In this paper, we propose a novel scheme to reduce the feedback load ina downlink orthogonal space division multiple access (SDMA) system withzero-forcing receivers by allowing the users to dynamically determine thenumber of feedback bits to use according to multiple decision thresholds.Through theoretical analysis, we show that, while keeping the aggregatefeedback load of the entire system constant regardless of the number of users,the proposed scheme almost achieves the optimal asymptotic sum rate scalingwith respect to the number of users (also known as the multiuser diversity).Specifically, given the number of thresholds, the proposed scheme can achieve aconstant portion of the optimal sum rate achievable only by the system whereall the users always feedback, and the remaining portion (referred to as thesum rate loss) decreases exponentially to zero as the number of thresholdsincreases. By deriving a tight upper bound for the sum rate loss, the minimumnumber of thresholds for a given tolerable sum rate loss is determined. Inaddition, a fast bit allocation method is discussed for the proposed scheme,and the simulation results show that the sum rate performances with the complexoptimal bit allocation method and with the fast algorithm are almost the same.We compare our multi-threshold scheme to some previously proposed feedbackschemes. Through simulation, we demonstrate that the proposed scheme can reducethe feedback load and utilize the limited feedback bandwidth more effectivelythan the existing feedback methods.

Feedback Reduction for Random Beamforming in Multiuser MIMO Broadcast Channel