A Simple Multiuser and Single-User Dual-Mode Downlink System with Limited Feedback

Limited feedback systems that simultaneously transmit to multiple users are noise power limited at low SNR and interference power limited at high SNR. Specifically, there is a sum rate upper bound such that increasing the transmit power alone cannot surpass. In this paper, we propose a dual-mode system that selects between single user mode and multiuser mode. The mobile determines its preferred transmission mode by a simple threshold rule of its SINR. Judiciously combining mobiles´ feedback, the base station with M_T transmit antennas chooses the better communication mode. It is shown that this proposed system asymptotically achieves unbounded capacity growth when increasing the number of mobiles or when increasing the SNR, without requiring extra feedback bits. We show the asymptotically preferable mode as the number of users and/or SNR increases. Lastly, we use computer simulations to verify our findings.