  This paper investigates how to apply non-orthogonal multiple access (NOMA) as
an add-on in terahertz (THz) networks. In particular, prior to the
implementation of NOMA, it is assumed that there exists a legacy THz system,
where spatial beams have already been configured to serve legacy primary users.
The aim of this paper is to study how these pre-configured spatial beams can be
used as a type of bandwidth resources, on which additional secondary users are
served without degrading the performance of the legacy primary users. A joint
beam management and power allocation problem is first formulated as a mixed
combinatorial non-convex optimization problem, and then solved by two methods
with different performance-complexity tradeoffs, one based on the branch and
bound method and the other based on successive convex approximation. Both
analytical and simulation results are presented to illustrate the new features
of beam-based resource allocation in THz-NOMA networks and also demonstrate
that those pre-configured spatial beams can be employed to improve the system
throughput and connectivity in a spectrally efficient manner.
