Single-User MIMO Vs. Multiuser MIMO in the Broadcast Channel With CSIT Constraints

This paper considers fundamental limits on adaptive Multiple-Input Multiple-Output (MIMO) transmission with imperfect Channel State Information at the Transmitter (CSIT). First, the achievable ergodic rates are derived for both single-user and multi-user MIMO systems with different CSIT assumptions, which shows that single-user MIMO is robust to imperfect CSIT while multiuser MIMO loses spatial multiplexing gain in proportion to delay and inverse with channel quantization codebook size. A single-user/multiuser mode switching rule is then proposed based on the average SNR, the amount of delay, and the quantization codebook size. The operating regions for single-user and multiuser modes are determined, which can be used to select the preferred mode.