  This paper investigates the application of a unified non-orthogonal multiple
access framework in beam hopping (U-NOMA-BH) based satellite communication
systems. More specifically, the proposed U-NOMA-BH framework can be applied to
code-domain NOMA based BH (CD-NOMA-BH) and power-domain NOMA based BH
(PD-NOMA-BH) systems. To satisfy dynamic-uneven traffic demands, we formulate
the optimization problem to minimize the square of discrete difference by
jointly optimizing power allocation, carrier assignment and beam scheduling.
The non-convexity of the objective function and the constraint condition is
solved through Dinkelbach's transform and variable relaxation. As a further
development, the closed-from and asymptotic expressions of outage probability
are derived for CD/PD-NOMA-BH systems. Based on approximated results, the
diversity orders of a pair of users are obtained in detail. In addition, the
system throughput of U-NOMA-BH is discussed in delay-limited transmission mode.
Numerical results verify that: i) The gap between traffic requests of
CD/PD-NOMA-BH systems appears to be more closely compared with orthogonal
multiple access based BH (OMA-BH); ii) The CD-NOMA-BH system is capable of
providing the enhanced traffic request and capacity provision; and iii) The
outage behaviors of CD/PD-NOMA-BH are better than that of OMA-BH.
