  Beamspace millimeter-wave (mmWave) and terahertz (THz) massive MIMO
constitute attractive schemes for next-generation communications, given their
abundant bandwidth and high throughput. However, their user and beam selection
problem has to be efficiently addressed. Inspired by this challenge, we develop
low-complexity solutions explicitly. We introduce the dirty paper coding (DPC)
into the joint user and beam selection problem. We unveil the compelling
properties of the DPC sum rate in beamspace massive MIMO, showing its monotonic
evolution against the number of users and beams selected. We then exploit its
beneficial properties for substantially simplifying the joint user and beam
selection problem. Furthermore, we develop a set of algorithms striking unique
trade-offs for solving the simplified problem, facilitating simultaneous user
and beam selection based on partial beamspace channels for the first time.
Additionally, we derive the sum rate bound of the algorithms and analyze their
complexity. Our simulation results validate the effectiveness of the proposed
design and analysis, confirming their superiority over prior solutions.
