  In this paper, we present a finite-block-length comparison between the
orthogonal multiple access (OMA) scheme and the non-orthogonal multiple access
(NOMA) for the uplink channel. First, we consider the Gaussian channel, and
derive the closed form expressions for the rate and outage probability. Then,
we extend our results to the quasi-static Rayleigh fading channel. Our analysis
is based on the recent results on the characterization of the maximum coding
rate at finite block-length and finite block-error probability. The overall
system throughput is evaluated as a function of the number of information bits,
channel uses and power. We find what would be the respective values of these
different parameters that would enable throughput maximization. Furthermore, we
analyze the system performance in terms of reliability and throughput when
applying the type-I ARQ protocol with limited number of retransmissions. The
throughput and outage probability are evaluated for different blocklengths and
number of information bits. Our analysis reveals that there is a trade-off
between reliability and throughput in the ARQ. While increasing the number of
retransmissions boosts reliability by minimizing the probability of reception
error, it results in more delay which decreases the throughput. Nevertheless,
the results show that NOMA always outperforms OMA in terms of throughput,
reliability and latency regardless of the users priority or the number of
retransmissions in both Gaussian and fading channels.
