  Hybrid digital analog (HDA) beamforming has attracted considerable attention
in practical implementation of millimeter wave (mmWave) multiuser
multiple-input multiple-output (MU-MIMO) systems due to the low power
consumption with respect to its fully digital baseband counterpart. The
implementation cost, performance, and power efficiency of HDA beamforming
depends on the level of connectivity and reconfigurability of the analog
beamforming network. In this paper, we investigate the performance of two
typical architectures that can be regarded as extreme cases, namely, the
fully-connected (FC) and the one-stream-per-subarray (OSPS) architectures. In
the FC architecture each RF antenna port is connected to all antenna elements
of the array, while in the OSPS architecture the RF antenna ports are connected
to disjoint subarrays. We jointly consider the initial beam acquisition and
data communication phases, such that the latter takes place by using the beam
direction information obtained by the former. We use the state-of-the-art beam
alignment (BA) scheme previously proposed by the authors and consider a family
of MU-MIMO precoding schemes well adapted to the beam information extracted
from the BA phase. We also evaluate the power efficiency of the two HDA
architectures taking into account the power dissipation at different hardware
components as well as the power backoff under typical power amplifier
constraints. Numerical results show that the two architectures achieve similar
sum spectral efficiency, while the OSPS architecture is advantageous with
respect to the FC case in terms of hardware complexity and power efficiency, at
the sole cost of a slightly longer BA time-to-acquisition due to its reduced
beam angle resolution.
