  Millimeter-wave and terahertz technologies have been attracting attention
from the wireless research community since they can offer large underutilized
bandwidths which can enable the support of ultra-high-speed connections in
future wireless communication systems. While the high signal attenuation
occurring at these frequencies requires the adoption of very large (or the
so-called ultra-massive) antenna arrays, in order to accomplish low complexity
and low power consumption, hybrid analog/digital designs must be adopted. In
this paper we present a hybrid design algorithm suitable for both mmWave and
THz multiuser multiple-input multiple-output (MIMO) systems, which comprises
separate computation steps for the digital precoder, analog precoder and
multiuser interference mitigation. The design can also incorporate different
analog architectures such as phase shifters, switches and inverters, antenna
selection and so on. Furthermore, it is also applicable for different
structures namely, fully connected, arrays of subarrays (AoSA) and dynamic
arrays of subarrays (DAoSA), making it suitable for the support of
ultra-massive MIMO (UM-MIMO) in severely hardware constrained THz systems. We
will show that, by using the proposed approach, it is possible to achieve good
trade-offs between spectral efficiency and simplified implementation, even as
the number of users and data streams increases.)
