  This paper proposes a unified framework of non-orthogonal multiple access
(NOMA) networks. Stochastic geometry is employed to model the locations of
spatially NOMA users. The proposed unified NOMA framework is capable of being
applied to both code-domain NOMA (CD-NOMA) and power-domain NOMA (PD-NOMA).
Since the detection of NOMA users mainly depend on efficient successive
interference cancellation (SIC) schemes, both imperfect SIC (ipSIC) and perfect
SIC (pSIC) are taken into account. To characterize the performance of the
proposed unified NOMA framework, the exact and asymptotic expressions of outage
probabilities as well as delay-limited throughput for CD/PD-NOMA with
ipSIC/pSIC are derived. In order to obtain more insights, the diversity
analysis of a pair of NOMA users (i.e., the $n$-th user and $m$-th user) are
provided. Our analytical results reveal that: i) The diversity orders of the
$m$-th and $n$-th user with pSIC for CD-NOMA are $mK$ and $nK$, respectively;
ii) Due to the influence of residual interference (RI), the $n$-th user with
ipSIC obtains a zero diversity order; and iii) The diversity order is
determined by the user who has the poorer channel conditions out of the pair.
Finally, Monte Carlo simulations are presented to verify the analytical
results: i) When the number of subcarriers becomes lager, the NOMA users are
capable of achieving more steep slope in terms of outage probability; and ii)
The outage behavior of CD-NOMA is superior to that of PD-NOMA.
