Differential Feedback of Channel Gram Matrices for Block Diagonalized Multiuser MIMO Systems

This paper proposes a differential quantization strategy to be used in the feedback link of a multi-input-multi-output (MIMO) multiuser broadcast communication system. This strategy is based on an algorithm presented for single user MIMO, and is applied to broadcast systems where the transmitter design is based on block diagonalization, i.e., the transmissions are optimized so as not to produce multiuser interference at the receive terminals, which are using more than one antenna each. The algorithm is applied to the channel Gram matrices exploiting geodesic curves and the intrinsic geometry of positive definite Hermitian matrices. Furthermore, the differential nature of the algorithm exploits the correlation in time present in real channels. Simulation results in the paper show that the proposed algorithm outperforms other techniques based on the direct quantization of the channel matrix.