  Terahertz (THz)-band communications are currently being celebrated as a key
technology that could fulfill the increasing demands for wireless data traffic
in the upcoming sixth-generation (6G) of wireless communications. Many
challenges, such as high propagation losses and power limitations, which result
in short communication distances, have yet to be addressed for this technology
to be realized. Ultra-massive multiple-input, multiple-output (UM-MIMO) antenna
systems have emerged as practical means for combatting this distance problem,
thereby increasing system capacity. Towards that end, graphene-based
nano-antennas have recently been proposed, as they can be individually tuned
and collectively controlled in compact UM-MIMO array-of-sub-arrays
architectures. In this paper, we present a holistic overview of THz UM-MIMO
systems. We assess recent advancements in transceiver design and channel
modeling, and discuss the major challenges and shortcomings of such designs by
deriving the relationship between communication range, array dimensions, and
system performance. We further highlight several research advances that could
enhance resource allocation at the THz band, including waveform designs,
multi-carrier configurations, and spatial modulations. Based on this
discussion, we highlight prospective use cases that can bring THz UM-MIMO into
reality in the context of sensing, data centers, cell-free systems, and
mid-range wireless communications.
