  Non-orthogonal multiple access (NOMA) is a potential candidate to further
enhance the spectrum utilization efficiency in beyond fifth-generation (B5G)
standards. However, there has been little attention on the quantification of
the delay-limited performance of downlink NOMA systems. In this paper, we
analyze the performance of a two-user downlink NOMA system over generalized
{\alpha}-{\mu} fading in terms of delay violation probability (DVP) and
effective rate (ER). In particular, we derive an analytical expression for an
upper bound on the DVP and we derive the exact sum ER of the downlink NOMA
system. We also derive analytical expressions for high and low signal-to-noise
ratio (SNR) approximations to the sum ER, as well as a fundamental upper bound
on the sum ER which represents the ergodic sum-rate for the downlink NOMA
system. We also analyze the sum ER of a corresponding time-division-multiplexed
orthogonal multiple access (OMA) system. Our results show that while NOMA
consistently outperforms OMA over the practical SNR range, the relative gain
becomes smaller in more severe fading conditions, and is also smaller in the
presence a more strict delay quality-of-service (QoS) constraint.
