  Imperfect channel state information degrades the performance of
multiple-input multiple-output (MIMO) communications; its effect on single-user
(SU) and multi-user (MU) MIMO transmissions are quite different. In particular,
MU-MIMO suffers from residual inter-user interference due to imperfect channel
state information while SU-MIMO only suffers from a power loss. This paper
compares the throughput loss of both SU and MU MIMO on the downlink due to
delay and channel quantization. Accurate closed-form approximations are derived
for the achievable rates for both SU and MU MIMO. It is shown that SU-MIMO is
relatively robust to delayed and quantized channel information, while MU MIMO
with zero-forcing precoding loses spatial multiplexing gain with a fixed delay
or fixed codebook size. Based on derived achievable rates, a mode switching
algorithm is proposed that switches between SU and MU MIMO modes to improve the
spectral efficiency, based on the average signal-to-noise ratio (SNR), the
normalized Doppler frequency, and the channel quantization codebook size. The
operating regions for SU and MU modes with different delays and codebook sizes
are determined, which can be used to select the preferred mode. It is shown
that the MU mode is active only when the normalized Doppler frequency is very
small and the codebook size is large.
