User-Wise Asymmetric Beamforming for Millimeter-Wave MIMO Systems

In this paper, a beamforming scheme that aims to support user-wise asymmetric uplink-downlink (UL-DL) traffic in a more energy-efficient manner is proposed for time-division duplex (TDD) millimeter-wave massive multiple-input-multiple-output (MIMO) systems. Assuming that proper data links have been established during the initial access stage for DL or UL traffic, we consider the beamforming problem for UL or DL whose payload traffic is much lighter than the other link. Such asymmetric traffic allows part of the massive MIMO array to be deactivated in order to achieve a higher energy efficiency while still meeting the spectral efficiency (SE) requirements for UL or DL. To deal with such a problem, we propose the corresponding phase shifter (PS) deactivation strategies based on different SE constraints for each mobile stations or user independently, which select the PSs to be deactivated accordingly with low computational complexity. Simulation results demonstrated the superiority of the proposed energy efficient PS deactivation approach and the corresponding codebook design.