This paper considers multi-mode transmission for multiple-input multiple-output (MIMO) broadcast systems employing per user unitary and rate control (PU2RC). We present a multi-mode transmission scheme in which the number of users for simultaneous transmission is adaptively chosen to maximize the ergodic sum rate of the system. Due to the difficulty in evaluating the ergodic sum rate, we derive an upper bound of the ergodic sum rate and use it as the metric for transmission mode selection. Numerical results show that the proposed multimode transmission scheme outperforms the conventional PU2RC scheme.
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