  This paper provides a comprehensive framework to analyze the performance of
non-orthogonal multiple access (NOMA) in the downlink transmission of a
single-carrier and multi-carrier terahertz (THz) network. Specifically, we
first develop a novel user pairing scheme for the THz-NOMA network which
ensures the performance gains of NOMA over orthogonal multiple access (OMA) for
each individual user in the NOMA pair and adapts according to the molecular
absorption. Then, we characterize novel outage probability expressions
considering a single-carrier and multi-carrier THz-NOMA network in the presence
of various user pairing schemes, Nakagami-m channel fading, and molecular
absorption noise. We propose a moment-generating-function (MGF) based approach
to analyze the outage probability of users in a multi-carrier THz network.
Furthermore, for negligible thermal noise, we provide simplified
single-integral expressions to compute the outage probability in a
multi-carrier network. Numerical results demonstrate the performance of the
proposed user-pairing scheme and validate the accuracy of the derived
expressions.
