  In this paper, we propose a new hybrid analog and digital combining
architecture for millimeter wave (mmWave) multi-user multiple-input
multiple-output (MU-MIMO) systems. The proposed structure employs antenna
subset selection per radio frequency (RF) chain based on active/inactive
switches and uses constant phase shifters (CPS) to control the phases of
signals in the RF circuit. In this scheme, for each RF chain, a subset of
receive antennas that contribute more to the desired signal power than the
interference power is chosen for signal combining in the analog domain, whereas
other receive antennas are excluded from signal combining, thereby enhancing
sum-rates. Simultaneously, the proposed structure reduces power consumption in
the RF circuit by exclusively activating switches that correspond to the
antennas selected for each RF chain. We also develop three low-complexity
algorithms for per-RF chain antenna subset selection. Finally, through
numerical simulation, we show that the proposed structure provides higher
spectral efficiency and higher energy efficiency than conventional hybrid
analog and digital combining schemes for mmWave MU-MIMO systems.
