Mobility-Aware Transmit Beamforming for Ultra-Dense Networks With Sparse Feedback

The network densification brings favored channel gain. However, its performance is limited by the increased interference and feedback overhead. In this paper, we focus on the channel state information (CSI) measure error induced by feedback delay and study the effect of the uplink feedback on the downlink transmission performance in ultra-dense networks (UDNs). Using a mobility-aware sparse feedback scheme (MSFS), we analyze the feedback transmission delay and feedback waiting delay separately, considering the impact of CSI feedback probability and measurement range. To capture the effect of proposed MSFS, the coverage probability for a typical mobile user is derived, where the imperfect-CSI-based zero-forcing precoding is employed to mitigate the severe inter-cell interference in UDNs. Numerical results reveal that MSFS succeeds in providing the effective CSI for downlink precoding while keeping a lower feedback load, and the optimal feedback probability and measurement range are provided for diverse mobility cases and different network densities.