  For the multiple-input multiple-output (MIMO) broadcast channel with
imperfect channel state information (CSI), neither the capacity nor the optimal
transmission technique have been fully discovered. In this paper, we derive
achievable ergodic rates for a MIMO fading broadcast channel when CSI is
delayed and quantized. It is shown that we should not support too many users
with spatial division multiplexing due to the residual inter-user interference
caused by imperfect CSI. Based on the derived achievable rates, we propose a
multi-mode transmission strategy to maximize the throughput, which adaptively
adjusts the number of active users based on the channel statistics information.
