  In this paper, a unified framework of non-orthogonal multiple access (NOMA)
networks is proposed, which can be applied to 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 considered. To characterize
the performance of this unified framework, the exact and asymptotic expressions
of outage probabilities as well as delay-limited throughput for CD/PD-NOMA with
ipSIC/pSIC are derived. Based on the asymptotic analysis, the diversity orders
of CD/PD-NOMA are provided. It is confirmed that due to the impact of residual
interference (RI), the outage probability of the n-th user with ipSIC for
CD/PD-NOMA converges to an error floor in the high signal-to-noise ratio (SNR)
region. Numerical simulations demonstrate that the outage behavior of CD-NOMA
is superior to that of PD-NOMA.
