Degrees of Freedom in the MISO BC with delayed-CSIT and finite coherence time: Optimization of the number of users

Most techniques designed for the multi-input single-output (MISO) Broadcast Channel (BC) require accurate current channel state information at the transmitter (CSIT). A different approach has been proposed by Maddah-Ali and Tse (MAT), in which significant increase of the multiplexing gain can be obtained by solely relying on perfect but outdated CSIT. This approach is proven to yield the optimal multiplexing gain when the curent channel state is completely independent of the fed back channel state. Recent work focused on an intermediate case: channels exhibiting some temporal correlation, or equivalently in the block fading model, feedback delays being smaller than the coherence time. We propose modified versions of different schemes for the MISO BC with delayed CSIT in which the number of users to serve is optimized in order to maximize the multiplexing gain when accounting for (CSI acquisition) overhead. Indeed, as the coherence time decreases, CSI acquisition overhead starts to dominate and a reduced number of active users (and hence active Base Station antennas) leads to a better compromise. We consider the MAT scheme, the traditional Zero Forcing (ZF) beamforming scheme, and combinations such as MAT-ZF and TDMA-ZF.