Distributed transmit mode selection for MISO broadcast channels with limited feedback: Switching from SDMA to TDMA

We consider a multi-antenna broadcast channel with more single-antenna receivers than transmit antennas and partial channel state information at the transmitter (CSIT). An adaptive transmit mode switching scheme, in which the base station selects between single-user (TDMA) and multiuser transmission (SDMA) is proposed. In particular, we formulate a framework for distributed mode selection and sum-rate optimization with limited feedback. By assuming instantaneous local CSIT and statistical knowledge of non-local information, we derive a distributed algorithm in which each receiver feeds back scalar channel quality information (CQI) as well as its preferred mode decision. Low-complexity variants, which exhibit negligible decrease in sum rate, are also investigated. Performance analysis and simulations show that this scheme provides significant throughput gains for low to moderate number of users and linear capacity growth in the interference-limited region.