On the design of scalar feedback techniques for MIMO broadcast scheduling

MIMO broadcast channels with partial channel knowledge at the transmitter are considered, obtained via limited- rate feedback. Given a maximum orthogonality factor e between transmit beamforming vectors (e = 0 if orthogonal beamforming) and variable number of active beams, the design of scalar feedback metrics for user scheduling is studied. These metrics provide an estimate of the received signal-to-noise plus interference ratio (SINR) and can take the form of either an upper or lower bound on the SINR. A closed-form expression for the sum-rate is provided, showing the performance of each metric in different scenarios. Analytical and simulation results are shown in order to identify the regions where SDMA provides higher rates than TDMA.