  We characterize the accuracy of analyzing the performance of a NOMA system
where users are ranked according to their distances instead of instantaneous
channel gains, i.e., product of distance-based path-loss and fading channel
gains. Distance-based ranking is analytically tractable and can lead to
important insights. However, it may not be appropriate in a multipath fading
environment where a near user suffers from severe fading while a far user
experiences weak fading. Since the ranking of users in a NOMA system has a
direct impact on coverage probability analysis, impact of the traditional
distance-based ranking, as opposed to instantaneous signal power-based ranking,
needs to be understood. This will enable us to identify scenarios where
distance-based ranking, which is easier to implement compared to instantaneous
signal power-based ranking, is acceptable for system performance analysis. To
this end, in this paper, we derive the probability of the event when
distance-based ranking yields the same results as instantaneous signal
power-based ranking, which is referred to as the accuracy probability. We
characterize the probability of accuracy considering Nakagami-m fading channels
and three different spatial distribution models of user locations in NOMA. We
illustrate the impact of accuracy probability on uplink and downlink coverage
probability.
