Sum Capacity of Block-Diagonalized Multiuser MIMO Downlink with Channel Estimation and Finite-Rate CSI Feedback Link

A multiuser multi-input multi-output (MU-MIMO) downlink system with block diagonalization-based precoding is presented with a study of the impact of limited bandwidth resource for the feedback-links on the sum capacity. The channel estimation is acquired at each user via downlink training, and the linear quantized form of estimated channels is fed back to the base station (BS) via noisy, finite-rate channel state information (CSI) feedback-links. Two architectures for CSI feedback are considered: parallel feedback where there is a feedback-link for each user; and serial feedback where a single feedback-link is used for all the users. The sum capacity is formulated as a function of the link parameters, enabling optimization of the number of training symbols and required feedback bits with the criterion of maximum sum capacity. The approach allows a performance comparison for different feedback architectures, the number of transmit antennas, and the number of users. The effect of time variations of the channel is analyzed via simulation, quantifying the loss of sum capacity relative to the case with perfect CSI for different Doppler frequencies.